WO2011152747A1

WO2011152747A1 - Photoacoustic material analysis

Info

Publication number: WO2011152747A1
Application number: PCT/RU2010/000281
Authority: WO
Inventors: Dzhomart Fazylovich Aliev; Anatoly Kravets; Alexandr Sergeevich Pristupnitskiy
Original assignee: H.L Human Laser Limited
Priority date: 2010-06-01
Filing date: 2010-06-01
Publication date: 2011-12-08

Abstract

Disclosed is a method and apparatus for determining the concentration of an interest component in a medium by phase-conjugated photoacoustic spectroscopy with probe and reference light beams having predetermined different wavelengths of equidistant short pulses having variable frequency, number, and power. The wavelength of the probe beam is selected so as to be absorbed by the component of interest. The wavelength of the reference beam is selected so as to be absorbed by the other components, which absorption bands overlapping with absorption band of the interest component. Upon irradiation, acoustic oscillations are generated by the absorbed light in a relatively thin layer of the medium, characterized by a heat-diffusing length. The frequency repetition of the short pulses of the probe beam is chosen equal to be in-phase with a natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam. Measuring of the amplitude and the frequency of the phase-conjugated photoacoustic oscillations determine the concentration of interest component. The method and apparatus are suitable monitoring of blood components, especially glucose.

Description

PHOTOACOUSTIC MATERIAL ANALYSIS

TECHNICAL FIELD

The present invention relates to electro-optics in general, and more particularly to

photoacoustic material analysis.

BACKGROUND ART OF THE INVENTION

Conventional methods of material analysis such as absorption and luminescent spectroscopy, Raman spectroscopy, and measuring polarization and reflectance changes are not sufficiently suitable for a turbid medium such as human tissue due to significant diffuse scattering of the reference light beam.

As an alternative, other material analysis techniques employ photoacoustic spectroscopy, in which a laser beam is used to rapidly heat a sample generating an acoustic pressure wave that can be measured by high-sensitivity ultrasonic detectors such as piezo-electric crystals, microphones, optical fiber sensors, laser interferometers or diffraction sensors.

The laser radiation wavelength is selected so as to be absorbed by the interest component in the medium being analyzed. Thus, laser excitation of a medium is used to generate an acoustic response and a spectrum as the laser is tuned. The use of photoacoustic spectroscopy for glucose testing in blood and human tissue can provide greater sensitivity than conventional spectroscopy. An excellent correlation between the photo-acoustic signal and blood glucose levels has been demonstrated on index fingers of both healthy and diabetic patients.

U.S. Pat. No. 5348002 of Caro discloses a method and apparatus for determining the presence and/or concentration of chemical species, which absorb electromagnetic energy, dependent to a degree upon the chemical species and the wavelength of electromagnetic energy applied to matter including said species. The absorbed electromagnetic energy generates acoustic energy, which is detected and analyzed to determine the presence and/or concentration of the chemical species in the matter.

U.S. Pat. No. 5941821, and U.S. Pat. No. 6049728 of Chou describe method and apparatus for noninvasive measurement of blood glucose by photoacoustic techniques in which an excitation source provides electromagnetic energy at a wavelength corresponding to the absorption characteristics of the analysis. Upon irradiation, acoustic energy is generated in a relatively thin layer of the sample to be measured, characterized by a heat-diffusing length. The acoustic emission is detected with a differential microphone, one end of which is positioned in a measuring cell and the other end of which is positioned in a reference cell. A processor determines the concentration of the substance being measured based upon the detected acoustic signal. In order to determine the concentration of glucose in the bloodstream, the excitation source is preferably tuned to the absorption bands of glucose in spectral ranges from about 1520-1850 nm and about 2050-2340 nm to induce a strong photo-acoustic emission. In these wavelength ranges, water absorption is relatively weak and glucose absorption is relatively strong. Thus, even though tissue may have a high percentage of water at the above-specified wavelength ranges, the electromagnetic radiation is able to penetrate through the tissue to a sufficient depth to allow for accurate measurements. Despite water absorption, the acoustic signal which is generated by the absorption of electromagnetic radiation by glucose is not overwhelmed by that generated by water. The glucose optically absorbs the energy inducing a temperature rise and generating an acoustic emission indirectly in the air. Thus, the photoacoustic intensity is approximately linearly proportional to the glucose concentration.

U.S. Pat. No. 6403944, and U.S. Pat. No. 6833540 of MacKenzie et al. describe a system for measuring a biological parameter, such as blood glucose, the system comprising the steps of directing laser pulses from a light guide into a body part consisting of soft tissue, such as the tip of a finger to produce a photoacoustic interaction. The resulting acoustic signal is detected by a transducer and analyzed to provide the desired parameter.

U.S. Pat. No. 6484044 of Lilienfeld-Toal describes an apparatus for detecting a substance in a sample, particularly for in vivo detecting and measuring glucose in body tissue or blood contains a semiconductor laser for emitting mid-infrared laser light at least two discrete wavelengths, each at a different peak or valley in the absorption spectrum of the substance in the sample. A photoacoustic detector detects acoustic signals originating from absorption of the laser light. An indication unit evaluates the acoustic signals separately for each wavelength and calculates a detection result based on all acoustic signals from the different wavelengths. But due to the very low skin transmission at mid-infrared wavelengths, acoustic signals are excited in very thin epidermis layer of human skin, where glucose concentration is very small. So, above mentioned photoacoustic technique have not measured glucose concentrations in vivo. On the contrary, according to U.S. Pat. No. 5941821, and U.S. Pat. No. 6049728 of Chou, at wavelength ranges of the absorption bands of glucose in spectral ranges about 1520- 1850 nm water absorption is relatively weak and glucose absorption is relatively strong. The light penetration depth in human tissue at these wavelength ranges equals 0.5 - 3 mm of skin dermis layer, where glucose levels in the interstitial fluid (ISF) that surrounds the cells within the tissue are about 10% lower than glucose levels in blood. At the same time, at these wavelength ranges a peak or valley in the absorption spectrum of glucose can not be indicated really, and using the photoacoustic technique of Lilienfeld-Toal U.S. Pat. No. 6484044 is impossible.

U.S. Pat. No. 6921366 of Jeon et al. describes an apparatus and method for non-invasively measuring bio-fluid concentrations using photoacoustic spectroscopy, includes a light source for irradiating an incident light having a predetermined wavelength band to be absorbed into a targeted component of a living body, an acoustic signal generator for generating a first acoustic signal having a similar frequency band as a photoacoustic signal that is generated when the incident light is absorbed into the targeted component.

Unfortunately, all above mentioned photoacoustic techniques are disadvantageous in that they teach the application of energy to a medium without giving consideration to its acoustic oscillation properties, thus requiring relatively high laser power. Consequently, such techniques are energy inefficient, and provide an inadequate level of sensitivity.

A prior art photoacoustic material analysis is described in U.S. Pat. No. 6466806 of Geva et al., in which the concentration of an interest component in a medium is determined by resonant photoacoustic spectroscopy with a light pulse-train comprising equidistant short pulses having variable duration, frequency, number, and power. The light wavelength is selected so as to be absorbed by the component of interest. Upon irradiation, the absorbed light in a relatively thin layer of the medium generates photoacoustic oscillations,

characterized by a heat-diffusing length. The frequency repetition of the light short pulses in the pulse-train is chosen equal to the natural acoustic oscillation frequency of the thin layer of the medium that can be considered as a thin membrane. So, the acoustic oscillation becomes resonant. Measuring of the amplitude and the frequency of the resonant oscillations determine the concentration of interest component. The method and apparatus are suitable for monitoring of blood components, especially glucose. A sensitivity level and a signal-to-noise ratio are increased by a photoacoustic resonance method.

Unfortunately, prior art photoacoustic material analysis techniques are disadvantageous in that they teach the application of energy to a medium without giving consideration to overlapping of absorption bands of different components, and irregularity of elastic properties of a medium, such as human skin. Consequently, such techniques provide an inadequate level of sensitivity and large errors of measuring.

DISCLOSURE OF INVENTION

The primary object of the present invention is to provide a novel method and apparatus of photoacoustic material analysis based on using phase-conjugated photoacoustic spectroscopy with probe and reference light beams having predetermined different wavelengths of equidistant short pulses having variable frequency, number, and power.

The wavelength of the probe beam is selected so as to be corresponded preferably to maximum of an absorption band of an interest component in a medium or may be in the range of the absorption band. The wavelength of the reference beam is selected so as to be corresponded preferably to minimum of the absorption band of the said interest component or may be not far from the minimum.

The both beams are directed to the same testing area of the target. Upon irradiation, acoustic oscillations are generated due to the light absorption in a relatively thin layer of the medium, characterized by a heat-diffusing length.

The amplitude, frequency and relaxation rate of the photoacoustic oscillations excited by probe beam depend on the concentration of interest component and also depend on concentration of the other components, which absorption bands overlapping with absorption band of said interest component in a medium. The amplitude, frequency and relaxation rate of the photoacoustic oscillations excited by the reference beam, only depend on the concentration of components, which absorption bands overlapping with absorption band of interest component, irrespective of concentration of interest component.

The frequency repetition of the short pulses of the probe beam is chosen equal to be in-phase with natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam.

Measuring of the amplitude and the frequency of the phase-conjugated photoacoustic oscillations determine the concentration of interest component.

The second object of the present invention is to provide a novel method and apparatus of photoacoustic material analysis based on detecting frequency, number, and power of light pulses of probe and reference beams and generating representative electrical signals using for feedback operation of the apparatus.

The third object of the present invention is to provide a novel method and apparatus of photoacoustic material analysis based on laser-induced changes of light scattering due to photoacoustic oscillations in a turbid medium such as human tissue and determination of the concentration of an interest component in the medium by measuring of the amplitude and the frequency of the phase-conjugated oscillations. The next object of the present invention is to provide a novel method and apparatus of photoacoustic material analysis based on using novel dual-wavelength pulsed laser source that comprises at least two discrete chips of pulsed laser diodes and at least two chips of photodiodes on ceramic sub-mounts in one package. The laser source is suitable to generate acoustic oscillations and light scattering oscillations in the testing area and determinate the concentration of an interest component in the medium by measuring of the amplitude and the frequency of the phase-conjugated oscillations.

Further consistent with the present invention, there is provided a method of calibrating an electronic-optical apparatus for determining a concentration of an interest component in a medium, comprising the steps of obtaining a sample of a fluid containing an interest component; determining a first concentration of the interest component using a fluid-based apparatus; determining a second concentration of the interest component using the electronic- optical apparatus; and determining if the second concentration is equivalent to the first concentration, wherein if the second concentration is not equivalent to the first concentration, offsetting the electronic-optical apparatus such that the second concentration is equivalent to the first concentration.

The present invention allows to abate an influence of overlapping of absorption bands of different components, and the skin irregularity on determining a concentration of an interest component in a medium and, thus, to increase the signal-to-noise ratio and testing sensitivity. The present invention is suitable for measuring blood components in human tissue, especially glucose.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:

FIG. 1 is a simplified, cross-sectional view of an implementation of an electronic-optical apparatus, constructed and operative in accordance with a preferred embodiment of the present invention.

FIG. 2 is a simplified block diagram of an electronic-optical apparatus of FIG. 1, constructed and operative in accordance with a preferred embodiment of the present invention.

FIG. 3 is a simplified graphical illustration of acoustic oscillations in a medium upon the probe and the reference light beams that to be phase-conjugated in accordance with a preferred embodiment of the present invention.

FIG. 4 is a simplified graphical illustration of resonant curves caused by pulse-train laser- excitation in accordance with a preferred embodiment of the present invention.

FIG. 5 is a simplified, cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using a dual- wavelength pulsed laser diode.

FIG. 6 is a simplified, cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using a bundle of optical wave-guides.

FIG. 7 is a simplified, other cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using a bundle of optical wave-guides.

FIG. 8 is a simplified, cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using an acoustic detector such as a microphone.

FIG. 9 is a flowchart illustrating a method for determining a concentration of an interest component in a medium, consistent with the present invention.

FIG. 10 is a flowchart illustrating a method for calibrating an optical apparatus consistent with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference is now made to FIG. 1, which is a simplified, cross-sectional view of an

implementation of an electronic-optical apparatus constructed and operative in accordance with a preferred embodiment of the present invention. In the implementation of FIG. 1 the apparatus preferably includes at least two chips of pulsed laser diodes as dual-wavelength pulsed laser source and three chips of photodiodes. Ceramic sub-mounts (not shown) for the said chips may be used into the interior of a optical cell 10 with glass window 12 that put on surface of target 14, like human skin testing area 16. The optical cell 10 may be similar to TO- 5 package with thickness of glass window 12 equals 0.3 mm.

The chip 18 of the first pulsed laser diode preferably provides light pulses generation of probe beam 20 with wavelength corresponding to maximum of an absorption band of an interest component in a medium or in the range of the absorption band.

The chip 22 of the second pulsed laser diode preferably provides light pulses generation of reference beam 24 with wavelength corresponding to minimum of the absorption band of the interest component or not far from the minimum.

The both beams 20 and 24 comprise preferably light pulse-trains of equidistant short pulses having variable duration, frequency, number, and power.

Probe beam 20 and reference beam 24 are passed through glass window 12 and directed to the same testing area 16 of the target 14 such as human body to produce in said testing area 16 phase-conjugated acoustic oscillations that define changes of back light scattering 26. One part power of the probe and/or the reference beam excites the acoustic oscillations in testing area due to absorption, and another part of the beams scatters in the testing area 16. As a result, the acoustic oscillations induce changes of light scattering, according to EQ.l - EQ. 21 described further.

The back scattered light 26 of the probe beam 20 and the reference beam 24 are passed through glass window 12 and registered by photosensitive area 28 of the first photodiode 30 and photosensitive area 32 of the second photodiode 34.

Backside of the chip 18 generates beam 36 with power about 1% of probe beam 20. Similarly, backside of the chip 22 generates beam 38 with power about 1% of reference beam 24. The beams 36 and 38 are registered by photosensitive area 40 of the third photodiode 42 to control of frequency and power light pulses generation of probe beam 20 and reference beam 24. It allows feedback operation of the electronic-optical apparatus using pinouts (not shown) for the laser diodes and photodiodes into the interior of the optical cell 10.

In accordance with a preferred embodiment of the present invention, commercial chips 18 and 22 with wavelength radiation in the spectral range of 1550-1750 nm, and preferably 1550- 1625 nm, as the probe beam 20 may be used for noninvasive determination of glucose concentration in human tissue. Additional wavelength radiation in the spectral range of 1300- 1520 nm, and preferably 1480- 1520 nm, may be used as the reference beam 24.

The amplitude, frequency and relaxation rate of the photoacoustic oscillations excited by probe beam 20 depend on the concentration of interest component, such as glucose, and also depend on concentration of the other components, such as water, which absorption bands overlapping with absorption band of said interest component in a medium such as human tissue. The amplitude, frequency and relaxation rate of the photoacoustic oscillations excited by the reference beam 24, only depend on the concentration of components, which absorption bands overlapping with absorption band of interest component, irrespective of concentration of interest component.

The frequency repetition of the short pulses of the probe beam 20 is chosen equal to be in- phase with natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam 24 is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam 20.

Measuring of the amplitude and the frequency of the laser-induced changes of light scattering 26 due to photoacoustic oscillations determine the concentration of interest component.

Reference is now made to FIG. 2, which is a simplified block diagram of an electronic-optical apparatus, constructed and operative in accordance with a preferred embodiment of the present invention and cross-sectional view shown FIG. 1.

As shown in FIG. 2, the electronic-optical apparatus includes an electronics enclosure 44 connected to the optical components enclosure in the optical cell 10 that is shown FIG. 1. Electronics enclosure 44 may be connected to optical components through conductors, wires, wirelessly, or electronics enclosure 44 and optical components may be contained in a single enclosure, with electrical connection there between.

Optical components enclosure in the optical cell 10 may be operable to irradiate target 14 by the probe beam 20 and the reference beam 24. Probe beam 20 and reference beam 24 are passed through glass window 12 to produce in target 14 phase-conjugated acoustic oscillations and back scattered light oscillations. Back scattered light 26 of the probe beam 20 and the reference beam 24 are passed through glass window 12 and registered by the first photodiode 30 and of the second photodiode 34. Backside of the chip 18 generates beam 36 and backside of the chip 22 generates beam 38 that are registered by the third photodiode 42 to control of frequency and power light pulses generation of probe beam 20 and reference beam 24 and feedback operation of the electronic-optical apparatus. Consistent with the present invention, the both beams 20 and 24 comprise preferably light pulse-trains of equidistant short pulses having at least variable frequency, number, and power. The frequency repetition of the short pulses of the probe beam 20 is chosen equal to be in-phase with natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam 24 is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam 20. Consistent with the present invention, the electronic-optical apparatus may be connected to power source 46 for providing power to both electronics enclosure 44 and optical components enclosure in the optical cell 10, and components located therein. Although illustrated as an external AC power source, power source 46 may be included in either of electronics enclosure 44 or optical components enclosure in the optical cell 10, and may be AC or DC. Moreover, if electronics enclosure 44 and optical components enclosure in the optical cell 10 are connected wirelessly, a separate additional power source may be connected to optical components enclosure in the optical cell 10. The electronic-optical apparatus may further be connected to an external processing device 48 for displaying, monitoring, tracking results, and calibrating the optical apparatus. External processing device may comprise a personal computer (PC), a personal digital assistant (PDA), a smartphone, or other such device.

Consistent with the present invention, electronics enclosure 44 may house an array of electronic components suitable for facilitating the determination of a concentration of an interest component in a medium. For example, electronics enclosure 44 may include a processor or CPU 50, a first radiation driver 52, a second radiation driver 54, a first peak detector 56, a second peak detector 58, and third peak detector 60, a multiplexer (MUX) 62, and an analog-to-digital converter (ADC) 64.

Similarly, optical components enclosure in the optical cell 10 may house an array of optical components for use in determining the concentration of an interest component in a medium. Consistent with the present invention, probe beam 20 and reference beam 24 may be emitted from a single radiation source including chips 18 and 22 of the pulsed laser diodes. Consistent with the present invention, a single detector may be configured to receive back-scattered light 26 of the probe beam 20 and the reference beam 24. Further consistent with the present invention, two detectors, for example, first detector 30 and a second detector 34 may be configured to separately receive back-scattered light 26 of the probe beam 20 and the reference beam 24. Optical-electronics enclosures may further include beam splitters 66 and 68 of the probe beam 20 and the reference beam 24 for getting of the beams 36 and 38. The beams 36 and 38 are registered by the third photodiode 42 to control of frequency and power light pulses generation of probe beam 20 and reference beam 24 and feedback operation of the electronic-optical apparatus.

The detectors 30, 34, and 42 may be optical receiving sensors, such as a photodiode, including a P-Intrinsic-N (PIN) photodiode, an avalanche photodiode, a photoelectrical multiplier, a photoresistor, a charge-coupled device (CCD) or other device capable of converting light into electricity. An amplifier (not shown) may further be included in optical- electronics enclosures, for amplifying the power of the back-scattered radiation 26. The detectors convert detected the back-scattered radiation 26 and controlling beams 36 and 38 into electrical signals for processing. Consistent with the present invention, the electrical signals may represent at least one of the amplitude, frequency, or decay time of any transient processes that may be produced in target 14. The electrical signals are then transmitted from first, second, and third peak detectors 56, 58, and 60 to multiplexer 62. Multiplexer 62 combines the electrical signals from the peak detectors and outputs a single combined electrical signal to analog-to- digital converter 64. Analog-to-digital converter (ADC) 64 converts the input analog electrical signal into a digital electrical signal and outputs the digital electrical signal to processor 50. The operation of these components will be discussed further in conjunction with the discussion of FIG. 9. Processor 50 receives the digital electrical signals and executes instructions, which may be stored in an internal memory (not shown), for performing calculations using the digital electrical signals. For example, processor 50 may calculate laser-induced changes of light scattering intensity of radiation 26 caused by photoacoustic oscillations in target 14 and any subsequent transient processes that may occur in target 14 as a result of emitted probe beam 20 and the reference beam 24. The result of emitting depends on variable duration, frequency, number, and power of the beams and variable delay time between pulses of probe beam 20 and the reference beam 24 as described below and shown in FIG. 3 and in FIG. 4.

From the calculated changes in the light scattering intensity, processor 50 will then execute instructions to perform an algorithm for calculating the concentration of an interest component present in testing area 16. Consistent with the present invention, the calculations may also be performed by an external processor, for example, a processor contained in PC 48. The calculated concentration may then be displayed on a display screen attached to electronics enclosure 44, or on computer 48. Moreover, the concentration may also be tabulated in computer 48 for trending and over-time analysis. A system bluetooth (not shown) may be used for intersystem communication between PC 48 and processor 50.

Consistent with the present invention, processor 50 receives the digital electrical signals from the third photodiode 42 to control of frequency and power light pulses generation of probe beam 20 and reference beam 24 and executes instructions for feedback operation of the electronic-optical apparatus.

Processor 50 may be any suitable processor or microprocessor, and must implement conventional amplitude and frequency domain analysis techniques to analyze the amplitude, duration and temporal frequency response of the extracted diffuse light scattering signal in order to improve the signal-to-noise ratio. Conventional chromometric spectral analysis techniques may also be utilized to deduce the observed diffuse light scattering spectrum in order to improve the detection limit and accuracy.

Consistent with the present invention, image analysis techniques may be used in conjunction with the optical apparatus described herein. In particular, image analysis techniques may be used to ensure that first and second radiation beams 20 and 24 are consistently incident on testing area 16, with no variation. Image analysis techniques may include video hardware and software, attached to and/or embedded on optical apparatus, which allows a user to accurately position optical apparatus such that radiation beams 20 and 24 are consistently incident on testing area. Consistent with the present invention, a portable video camera could be installed such that a real time video feed could show user positioning optical apparatus on surface of target 14. Markers could be placed at testing area 16 so that user could reliably, using the video feed, align the optical apparatus with testing area 16 to ensure incidence thereon.

The efficacy of the laser for material analysis is highly dependent on the characteristics of the beam in terms of light amplitude distribution, mode of operation, width of fundamental pulse, instantaneous power within the pulse, wavelength, fine-tuning, and ability to change these and other beam parameters.

First 18 and second 22 radiation sources used in embodiments consistent with the present invention may be selected depending on such factors as the power or wavelength of radiation needed for accurately determining the concentration of an interest component, the periodicity of the radiation needed, size constraints or cost. For example, at least one of first 18 and second 22 radiation sources may be a pulsed tunable laser diode, a fiber-coupled diode laser array, a pulsed tunable fiber optical laser, a flash lamp, or a LED. First 18 and second 22 radiation sources may further include combinations of these types of radiation sources. For example, in one embodiment, at least one of first 18 and second 22 radiation sources may include an erbium (Er)-glass rod or slab laser pumped by additional diode lasers.

In another embodiment, at least one of first 18 and second 22 radiation sources may include a tunable Co:MgF₂ laser. In yet another embodiment, at least one of first 18 and second 22 radiation sources may include a Q-switched neodymium containing optical medium laser. An optical amplifier (not shown) may further be included in electronic-optical apparatus, for amplifying the power of the probe beam 20 and reference beam 24. Consistent with the present invention, optical amplifier may be an optical fiber amplifier. Electronic-optical apparatus may also further include an optical converter (not shown) for converting

wavelengths of probe beam 20 and reference beam 24.

Consistent with the present invention, other embodiment of the electronic-optical apparatus of FIG. 2 using a dual- wavelength pulsed laser diode is shown in FIG. 5.

Consistent with the present invention, optical wave-guides, typically an optical fibers, may be used to inject probe beam 20 and reference beam 24 into target 14 and transmit back scattered light 26 to detectors 30 and 34 as shown in FIG. 6 and FIG. 7.

The dermal or epidermal area of the skin that generates acoustic waves can be considered as a thin membrane. Consistent with the present invention, the amplitude and frequency of the acoustic oscillations may be measured by an acoustic detector such as microphone via air as shown in FIG. 8. The membrane has natural oscillation frequencies that depend on the thickness of the membrane, its elastic constants, and the square of the membrane surface that is equal to the square of the aperture. If the repetition frequency of the light pulses causing the acoustic oscillations equals the oscillation frequency of the membrane, the oscillation becomes resonant as shown in FIG. 4. Under such circumstances the amplitude of the oscillations increases many times, increasing the signal-to-noise ratio and, thus, testing sensitivity.

Reference is now made to FIG. 3, which is a simplified graphical illustration of acoustic oscillations of a medium upon which short mono-pulse laser-excitation has been applied in accordance with a preferred embodiment of the present invention. In the graph of FIG. 3, if the duration τ of the short laser pulse 70 of probe beam 20 or pulse 72 of reference beam 24 is much less than the period T₀ of the natural oscillations 74 of the target membrane, the oscillations will be damping. In this case, displacement of the membrane is

U(t) = A sin (ω₀ΐ - φ), (EQ. 1)

and A = A₀exp (-5 t), (EQ. 2)

where A is amplitude of the membrane oscillation, A₀ is the primary amplitude, δ is damping coefficient, ω₀ is circular frequency of the natural oscillations 74 of the target membrane, φ is the primary phase, and oscillation phase is

ψ = coo t - φ,

ω₀ = 2πί₀ = 2π/Τ₀,

where f₀ is frequency, T₀ is period of the natural oscillations of the membrane.

In general, φ = k x,

where k is wavenumber, k = 2π/Λ = ω/ν,

Λ is wavelength of the acoustic oscillations, v is phase velocity of the acoustic oscillations axial x normal to skin surface.

If Ai is amplitude of the membrane oscillation excited by the short laser mono-pulse 70 of probe beam 20, and A₂ is amplitude of the membrane oscillation excited by the short laser mono-pulse 72 of reference beam 24, then total amplitude of the membrane oscillation equals:

A²= A_! ²+ A₂ ²+ 2A_!A₂cos Δψ, (EQ.3)

were Δψ is phase difference of the membrane oscillation excited by the short laser mono-pulse 70 of probe beam 20, and by the short laser mono-pulse 72 of reference beam 24:

Δψ = α>οΔΐ,

were At is delay time between the laser mono-pulse 70 of probe beam 20 and laser mono- pulse 72 of reference beam 24. According to EQ.3,

A = Ai+ A₂= max,

when Δψ = 2π i, (EQ.4)

and At = T₀, (EQ.5)

were / = 0, 1, 2, 3, ...

In this case the mono-pulses of the probe beam and the reference beam excite phase- conjugated acoustic oscillations in-phase.

A = Ai- A₂= min,

when Δψ = (2i +1)π (EQ.6)

and At = (2/ +1) To/2 (EQ.7)

In this case the mono-pulses of the probe beam and the reference beam excite phase- conjugated acoustic oscillations anti-phase.

Generally, the natural oscillations 74 of the target membrane will be damping and comprise about 10 amplitudes. So, the mono-pulses of the probe beam and the reference beam excite acoustic oscillations independently if the delay time At between the short mono-pulses 70 and 72 equals:

At > 10 T₀. (EQ.8)

The delay time At between the laser mono-pulse 70 of probe beam 20 and laser mono-pulse 72 of reference beam 24 is chosen sufficient to excite phase-conjugated acoustic oscillations so as the short pulses of the reference beam 24 to be anti-phase to acoustic oscillations excited by the probe beam 20 (EQ.7) or independent (EQ.8).

Analyze now pulse-train or QCW radiation of probe and reference beams comprising equidistant short light pulses with repetition period T. When the repetition period T of the equidistant short light pulses equals to the period T₀ of the natural oscillations 74 of the target membrane, the acoustic oscillations exited by this beam become resonant or in-phase conjugated.

In general, according to EQ.5, the acoustic oscillations become in-phase conjugated if

At = T = To,

and repetition frequency f= fo/i, (EQ.9)

where I = 1, 2, 3 ...

In this case, according to EQ.4, difference phases of the acoustic oscillations exciting in the medium by the equidistant short light pulses of the probe beam or the reference beam equals Δψ = 2/π.

According to EQ.7, the acoustic oscillations become anti-phase conjugated if

At = T = (2 +l) To/2,

and f = 2 f ₀/(2 +l), (EQ.IO)

where i = 1, 2, 3 ...

In this case according to EQ.6, difference phases of the acoustic oscillations exciting in the medium by the equidistant short light pulses of the probe beam or the reference beam equals

Δψ = (2i + 1)π.

According to EQ.8, the acoustic oscillations become independent when delay time At between pulse-trains equidistant short pulses of the probe beam 20 and the reference beam 24

At = T > 10 T₀, (EQ.l l)

or repetition frequency f < fo/10.

Usually, T₀ = 0.1-lms, and f₀ = 1/T₀ = 1-lOkHz; the light pulse duration τ = 10-lOOns « T₀. For example, if T₀= 1ms and frequency of the natural oscillations f ₀= 1/T₀ = 1kHz, the acoustic oscillations are independent if T> 10ms and f < 100Hz.

According to EQ.9, the frequency repetition of the short pulses of the probe beam 20 is chosen equal to be in-phase with natural acoustic oscillations in the medium. According to EQ.10, the frequency repetition of the short pulses of the reference beam 24 is chosen equal to be antiphase with acoustic oscillations excited by the probe beam 20. According to EQ.l 1, the frequency repetition of the pulse-trains equidistant short pulses of the reference beam 24 is chosen equal to be independent to acoustic oscillations excited by the probe beam 20.

Reference is now made to FIG. 4, which is a simplified graphical illustration of resonant curves 76 caused by equidistant pulses of laser-excitation. As shown in FIG. 4, the frequency ω of the equidistant short pulses equals the natural oscillation frequency ω₀ of a medium for different damping coefficients δ. It may thus be seen from FIG. 4 that a desirable resonant condition may be expressed by the equation:

δ/ω₀ < 0.1 (EQ. 12).

The natural oscillation frequency ω₀= 2π/Το= 2nf₀, and depends on the elastic constants of the membrane and its thickness and square.

Usually, the resonant oscillation frequency of human skin equals 1 - 3 kHz and number of the natural oscillation of the membrane more than 10.

During the photoacoustic effect, a laser light pulse upon absorption induces an adiabatic temperature rise resulting in a pressure build-up, followed by an acoustic shock wave propagating to the surface. The product of the absorption coefficient and local fluence rate, as well as thermophysical properties of the medium determines the amplitude of the generated photoacoustic signal. The light path of the photon as it is scattered before being absorbed is therefore not relevant. Ultrasonic transduction is preferably used for detection of acoustic oscillations of the surface.

In general, according to EQ.9, the acoustic oscillations become in-phase conjugated if repetition frequency of the equidistant short light pulses f = frfi, where f₀ is repetition frequency of the natural oscillations 74 of the target membrane, and i = 1, 2, 3 ...

The acoustic oscillations exited by this beam become resonant if f = f₀, and i = 1.

Thus, at / = 1 resonant acoustic oscillations is the simplest case of in-phase conjugated acoustic oscillations in a medium.

Using in-phase conjugated acoustic oscillations at > 1 allows to excite resonant oscillations by probe beam 20 having lower average power than it needs for i = 1 exciting resonant acoustic oscillations at the same pulse power of equidistant short pulses in the probe beam. For example, a natural acoustic oscillations equals 2 kHz, and we use pulsed probe light beam having pulse power equals 10 W and pulse duration equals 100 ns. In this case pulse energy equals 1 uJ and average power of the probe light beam equals 2 mW that needs for execting the resonant acoustic oscillations at i = 1. Using in-phase conjugated acoustic oscillations at i = 2 allows to excite resonant acoustic oscillations in a medium by probe beam 20 having lower repetition frequency equals 1 kHz and lower average light power equals 1 mW. In this case, according to EQ.10, the frequency repetition of the short pulses of the reference beam 24 is chosen equal to be anti-phase with acoustic oscillations excited by the probe beam 20. At / = 2 the frequency repetition of the short pulses of the reference beam equals 0.8 kHz. Reference is now made to FIG. 5, which illustrates a simplified, cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using a dual-wavelength pulsed laser diode 10 for determining a concentration of interest component, consistent with the present invention.

In the implementation of FIG. 5 as similar to FIG. 1, the dual-wavelength pulsed laser diode 10 may comprise at least two discrete chips 18 and 22 of pulsed laser diodes. At least one chip of photodiode 42 for feedback operation of the electronic-optical apparatus 44 using pinouts (not shown). All the chips are arranged on ceramic sub-mounts (not shown) in one package. The chip 18 preferably provides light pulses generation of probe beam 20 and giving equidistant short laser pulses having variable frequency and power with wavelength corresponding to maximum of an absorption band of an interest component in a medium or in the range of the absorption band.

The chip 22 preferably provides light pulses generation of reference beam 24 and giving equidistant short laser pulses having variable frequency and power with wavelength corresponding to minimum of the absorption band of the interest component or not far from the minimum.

The both beams 20 and 24 are passed through glass window 12 and directed to the same testing area 16 of the target 14 such as human finger that defines the laser-induced changes of light scattering 26, which is registered by a remote photodetector 30. One part power of the probe and/or the reference beam excites the acoustic oscillations in testing area due to absorption, and another part of the beams scatters in the testing area 16. As a result, the acoustic oscillations induce changes of light scattering, according to EQ.13 - EQ. 21 described below.

A distance "d" between the dual- wavelength pulsed laser diode and remote photodetector 30 may be tuned in the range 1- 10 mm, and preferably 2- 3 mm that is sufficient to detect phase- conjugated acoustic oscillations in a medium such as human finger.

An angle "a" between directions of the probe beam 20 and surface normal of remote photodetector 30 may be tuned in the range 0 - 180 degrees, and preferably 20 - 90 degrees that is sufficient to detect phase-conjugated acoustic oscillations in a medium such as human finger.

If a = 0 degree, the beam 26 is back scattered. If a = 180 degree, the beam 26 is forward scattered.

Backside of the chip 18 generates beam 36 with power about 1% of probe beam 20. Similarly, backside of the chip 22 generates beam 38 with power about 1% of reference beam 24. The beams 36 and 38 are registered by photodiode 42 to control of frequency and power light pulses generation of probe beam 20 and reference beam 24 and feedback operation of the electronic-optical apparatus using pinouts (not shown).

In accordance with a preferred embodiment of the present invention, commercial chips 18 and 22 with wavelength radiation in the spectral range of 1550-1750 nm, and preferably 1550- 1625 nm, as the probe beam 20 may be used for noninvasive determination of glucose concentration in human tissue. Additional wavelength radiation in the spectral range of 1300- 1520 nm, and preferably 1480-1520 nm, may be used as the reference beam 24.

The amplitude and relaxation rate of the photoacoustic oscillations excited by probe beam 20 depend on the concentration of interest component, such as glucose, and also depend on concentration of the other components, such as water, which absorption bands overlapping with absorption band of said interest component in a medium such as human tissue. The amplitude and relaxation rate of the photoacoustic oscillations excited by the reference beam 24, only depend on the concentration of components, which absorption bands overlapping with absorption band of interest component, irrespective of concentration of interest component.

The photoacoustic signal, expressed as a pressure, is determined by the thermo-elastic expansion coefficient β, optical absorption coefficient, μ_α, and distribution of the absorbed photons H (z) as follows:

Ρ (ζ) = β²Η (ζ) μ_β /0_/, (EQ. 13)

where z is depth, and C_p is heat capacity at constant pressure of the medium.

EQ. 13 is strictly valid only when the heating process is instantaneous compared to the medium expansion resulting in instant stress generation. Temporal stress confinement requires laser pulse durations that are much shorter than the time propagation across the light penetration depth in the medium. Laser pulses with duration of 10-100 nanoseconds are an ideal light source for excitation of acoustic oscillations in human tissue.

The pulse of the light applied to the medium causes heating of the local area under absorbed light energy, which is converted into heat. The thermal shock pulse applied to the molecule of a medium causes a rapid change of the molecule's amplitude oscillation, and generation of the damped back scattered light oscillations.

The amplitude of oscillation is proportional to rate of the temperature change or to density of absorption power: Α(λ) ~ dT/dt.

The attenuation of the light back-scattered amplitude is proportional to the density of the light power and absorption coefficient of the medium Α(λ) ~ μ_α(λ) ς_ν (EQ. 14)

where μ_α(λ) - absorption coefficient of the medium, q_v - density of incident light power

where J - energy of the light pulse, τ_/ - duration of the light pulse, V- volume of the illuminated medium area.

The energy of laser beam is absorbed in the volume V = Z S limited by square S of the radiators 18 and 22 of beams 20 and 24, and the light penetration depth Z.

The light penetration depth Z represents distance of light attenuation in e times (e = 2.72) of its original value and is related to the absorption μ_α (λ) and scattering μ₅ (λ) coefficients as a function of the light wavelength: Z = 1/μ_ε/,

were μ_ε/= Λ/(3μ_α (μ_α+μ*)). (EQ. 16)

The light penetration depth represents the rate of decay of intensity of light in scattering media with respect to the path traveled by the light in the same direction as the incident light.

The Beer- Lambert Law describes the absorption of light intensity in a non-scattering medium:

/ = /₀exp(-y_ax), (EQ. 17)

where I₀ is the light intensity incident on medium, / is the light intensity transmitted through the medium, x is optical path that usually equals thickness of sample; and the absorption coefficient:

V- a Q = eC,

were ε - specific extinction coefficient of the absorption component.

The absorption coefficient is the probability of light absorption per unit path length.

The Lambert and the Beer laws are applicable only for transparent not turbid medium.

In general conversion of the light into thermal energy (heat) depends not only on absorption phenomena but also on light scattering.

The scattering coefficient of the medium is expressed:

μ* = ρσ, cm^"1, (EQ. 18)

were p - density of the scattering particles, σ - scattering cross section of particles.

The scattering coefficient is the probability of equivalently isotropic (uniform in all directions) scattering per unit path length.

Scattering of the light in the tissue is caused by chaotic variation of refractive index n =f (λ) at microscopic and macroscopic scale. For an organic material, the refractive index, and hence the reflectance, will be highly wavelength dependent. The scattering of dermis combines the contributions of the Mie scattering by the large cylindrical collagen fibers and Rayleigh scattering by the small-scale cellular structures. Both Mie and Rayleigh scattering are considered to be elastic scattering processes, in which the energy (and thus wavelength and frequency) of the light is not substantially changed. However, electromagnetic radiation scattered by moving scattering centers does undergo a Doppler shift, which can be detected and used to measure the velocity of the scattering centers.

The elastic scattering of light of human tissue may be approximated by an equation that expresses the scattering coefficient for a tissue or a turbid medium as:

μ, = 3.28πα²ρ(2παη„,/λ)°'⁴ (m- 1 )², (EQ. 19) where a represents the average cell diameter, p represents the density of the scattering particles (number concentration of cells), n_m represents the refractive index of interstitial fluid, and λ represents the wavelength. The refractive index mismatch m = n n_m, where ¾ represents the refractive index of the cells, and n_m is the refractive index of the interstitial fluid.

The scattering coefficient changes as cell size a, density of the scattering particles p or refractive index n_m change. The scattering coefficient variation of tissue is dependent on the refractive index mismatch between the interstitial fluid (ISF) and the cellular membranes. So, it depends on the concentration of glucose in the ISF fluid. Glucose levels in ISF are about 10% lower than glucose levels in blood. An increase the glucose concentration, the absorption coefficient and refractive index of the ISF will increase, and refractive index mismatch will decrease. So, elastic scattering of light will decrease too if other parameters will be constant. The wavelength of the light source corresponds to the absorption bands of glucose in two spectral ranges: 1520-1850 nm and 2050-2340 nm. In these ranges, absorption by glucose is relatively stronger then absorption by water. The absorption spectrum of glucose shows the two peaks at about 1600 and 2120 nm. In accordance with a preferred embodiment of the present invention, commercial pulsed or QCW laser diodes with wavelength radiation in the spectral range of 1550-1750 nm may be used as the probe beam 20, and commercial pulsed or QCW laser diodes with wavelength radiation in the spectral range of 1480-1520 nm may be used as the reference beam 24. The range of absorption coefficient μ_α (λ) = (10 - 50) cm^"1 in human tissue at light wavelength 1550 nm, and the light penetration depth Z = (0.5 - 3) mm. The scattering coefficients of human tissues are generally within the range 10-100 mm^"1, roughly (10 - 100) times greater than those for absorption. The most highly scattering tissues include skin dermis, cerebral white matter, and bone.

In order to fully describe scatter of light in tissue, it is necessary to consider the probability of a photon being scattered in a given direction at each interaction. The anisotropy in the probability distribution is commonly characterized in terms of the mean cosine of the scattering angle g. In biological tissue, scatter occurs principally in a forward direction, corresponding to an anisotropy in the range 0.69<g<0.99. Despite the forward scatter, typical values of scattering coefficient ensure that light traveling through more than a few millimeters of tissue loses all of its original directionality, can be treated as effectively isotropically distributed. Thus it is convenient to express characteristic scatter of tissue in terms of a transport scatter coefficient:

μ μ₅ (1-_§), (EQ. 20)

which represents the effective number of isotropic scatters per unit length, and is a

fundamental parameter in diffusion theory.

When a highly scattering medium is considered, the Beer-Lambert (EQ. 17) law must be modified to include an additive term due to scattering losses and a multiplier, to account for the increased optical pathlength due to scattering. Therefore this law cannot be solved to provide a measure of the absolute concentration of an interest component in a medium.

However it is possible to determine a change in concentration of the constituent from a measured change in attenuation. As an alternative, concentration of the interest constituent in the medium may be determined by measuring of difference between light scattering intensity of the probe 20 and reference 24 beams in accordance with the present invention.

The photoacoustic pressure induces change in the density of the scattering particles p, and therefore changes in scattering coefficient according to EQ.13 and EQ.19. Moreover, laser- induced changes of light scattering coefficient μ₅ due to photoacoustic oscillations are proportional to the optical absorption coefficient μ_α, as the photoacoustic pressure, because the density of the scattering particles p change is proportional to the photoacoustic pressure P. So, probe beam 20 and reference beam 24 produce in target 14 phase-conjugated acoustic oscillations and light scattering oscillations 26. Thereby, the acoustic oscillations may be registered by measuring the light scattering oscillations 26 using the first photodiode 30 and of the second photodiode 34. An external pressure effects on light scattering oscillations 26 as well as internal photoacoustic pressure. So, the external pressure must be constant and controlled by a pressure sensor. According to EQ.19 the scattering of light by human tissue depends on the refractive index mismatch. Thereby, decreasing refractive index mismatch between the background and the scattering particles and led consequently to decreased scattering. So, the glucose concentration in the interstitial fluid may be measured if other parameters in EQ.19 are constant. It is possible at low density of incident light power (static/elastic spectroscopy). The photoacoustic pressure induces change in the density of the scattering particles p, and therefore changes of scattering coefficient and the light scattering signal that is proportional to glucose

concentration in the interstitial fluid (dynamic/inelastic spectroscopy). It allows to abate an influence of other different components, and the skin irregularity, to increase the signal-to- noise ratio and, thus, testing sensitivity.

The optical absorption coefficient of the probe beam 20 at the wavelength corresponding to maximum of an absorption band of an interest component equals:

where μ_αι is optical absorption coefficient causing only by the interest component, and μ_α2 is optical absorption coefficient causing only by another components, which absorption bands overlapping with absorption band of said interest component in a medium.

The optical absorption coefficient, and consequently the photoacoustic signal, excited by pulses of the probe beam 20, depends on concentration of the interest component, and also other components, which absorption bands overlapping with absorption band of said interest component in a medium. The photoacoustic signal, excited by pulses of the reference beam 24, is related to optical absorption coefficient μ_α2. So, it is proportionate only to the concentration of the other components, which absorption bands overlapping with absorption band of interest component in a medium, irrespective of said interest component.

If concentration of an interest component much more than concentration of other components, which absorption bands overlapping with absorption band of the interest component in a medium and μ_αι» μ_α2, the photoacoustic signal amplitude, excited by probe beam 20, much more than that of the reference beam 24. In this case the probe beam 20 may be used only to excite resonant acoustic oscillations according to prior art photoacoustic material analysis that described in U.S. Pat. No. 6466806 of Geva et al., in which the concentration of an interest component in a medium is determined by measuring of the amplitude and the frequency of the resonant oscillations. Consistent with the present invention, according to EQ.9, using in-phase conjugated acoustic oscillations at i > 1 allows to excite resonant oscillations by probe beam having lower average power than it needs in prior art. In general case, when concentration of an interest component is covariant or lower than concentration of other components, which absorption bands overlapping with absorption band of the interest component in a medium, using the prior art of U.S. Pat. No. 6466806 is impossible. In this case, consistent with the present invention, the frequency repetition of the short pulses of the probe beam 20 is chosen equal to be in-phase with natural acoustic oscillations in a medium at i > 1 according to EQ.9. The frequency repetition of the short pulses of the reference beam 24 is chosen equal to be anti-phase with acoustic oscillations excited by the probe beam 20 according to EQ.10, or independent to them according to EQ.l 1. In case exciting the acoustic oscillations in a medium by pulse-trains equidistant short pulses of the probe and the reference beams independent, the frequency repetition of the short pulses of the both beams may be chosen equal to be in-phase with natural acoustic oscillations in the medium at i > 1. Measuring of the amplitude and the frequency of the phase-conjugated photoacoustic oscillations determine the concentration of interest component.

Reference is now made to FIG. 6, which illustrates a simplified, cross-sectional view of an embodiment of the electronic-optical apparatus of FIG. 2 using a bundle of optical waveguides.

In the implementation of FIG. 6 the apparatus of FIG. 2 is designed as a sensor head inside a housing 78 comprising the optical wave-guides 80, 82, 84 and 86, typically a bundle of optical fibers. The fiber 80 transmits the probe beam 20 to the testing area 16 of the target 14 such as human skin. The fiber 82 transmits the reference beam 24 to the same testing area 16 of the target 14. The fibers 84 and 86 transmit the back-scattered light 26 from the same testing area 16 of the target 14 to photodetector 30 and 34.

Reference is now made to FIG. 7, which illustrates a simplified, other cross-sectional view of an implementation the embodiment showed in FIG. 6. The apparatus may be designed as a sensor head inside a housing 78 comprising the optical wave-guides 80, 82, 84 and 86, typically a bundle of optical fibers.

Consistent with such an embodiment, the many optical fibers may be arranged as shown in FIG. 7.

FIG. 7 illustrates an optical fiber arrangement, which includes two fibers 80 and 82 for transmitting the probe beam 20 and the reference beam 24 to target 14, and a plurality of pickup fibers 84 and 86 for transmitting scattered radiation 26 from the target 14 to a

photodetector. The photodetector may include one or several photodiodes. FIG. 7 illustrates example of optical fiber arrangements consistent with the present invention, which include four fibers 84 for transmitting scattered radiation 26 from the target 14 to a photodetector 30 and 86 for transmitting scattered radiation 26 from the target 14 to a photodetector 34.

The use of the bundle of optical fibers allows for providing housing 78 for head of the bundle, which is small, lightweight, and easily able to be placed in contact with surface of the target 14. One part power of the probe and/or the reference beam excites the acoustic oscillations in testing area due to absorption, and another part of the beams scatters in the testing area 16. As a result, the acoustic oscillations induce changes of light scattering, according to EQ.l - EQ. 21.

If concentration of an interest component much more than concentration of other components, which absorption bands overlapping with absorption band of the interest component in a medium, the photoacoustic signal amplitude, excited by probe beam 20, much more than that of the reference beam 24. In this case the probe beam 20 may be used only to excite resonant acoustic oscillations according to prior art photoacoustic material analysis that described in U.S. Pat. No. 6466806 of Geva et al., in which the concentration of an interest component in a medium is determined by measuring of the amplitude and the frequency of the resonant oscillations. Consistent with the present invention, using in-phase conjugated acoustic oscillations at / > 1 allows excite resonant oscillations by probe beam having lower average power than it needs in prior art.

In general case, when concentration of an interest component is covariant or lower than concentration of other components, which absorption bands overlapping with absorption band of the interest component in a medium, using the prior art of U.S. Pat. No. 6466806 is impossible. In this case, consistent with the present invention, the frequency repetition of the short pulses of the probe beam is chosen equal to be in-phase with natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam. Measuring of the amplitude and the frequency of the phase-conjugated photoacoustic oscillations determine the concentration of interest component.

Human skin includes an outer epidermis layer and an inner dermis layer. Microscopic roughness of the human skin surface causes the diffraction diffuse reflectance of light in edition to specular reflectance. The specular reflected light is significantly wavelength dependent. It contains information about the complex refractive index of the material and an imaginary term that relates to the absorption coefficient. The epidermis layer contains very little blood, and thus reflected rays contain little information about glucose. Specular reflectance is only useful when the bulk material is adequately represented by surface composition. When this is not the case, such as when performing noninvasive blood analyte measurements, this methodology can give a spurious result. In spite of it, the bundle of optical fibers allows for providing very sensitive acoustic detector using the specular reflected light for measuring skin displacements.

Reference is now made to FIG. 8, which is a simplified, cross-sectional view of an

implementation of the electronic-optical apparatus of FIG. 2 using an acoustic detector such as a microphone, constructed and operative in accordance with an embodiment of the present invention. In the implementation of FIG. 8 the apparatus of FIG. 2 is preferably arranged such that probe beam 20 is injected by wave-guide 88 at incidence angle and reference beam 24 is injected by wave-guide 90 at incidence angle a₂ into the interior of an acoustic cell 92. Acoustic cell 92 may be constructed from any suitable material, preferably ABS plastic material. Acoustic cell 92 transmits the acoustic oscillations 94 from testing area 16 of the target 14 to acoustic detector 96 such as microphone via air. Acoustic cell 92 preferably acts as a housing for acoustic detector 96 and a convex lenses 98, 100. Acoustic cell 92 is designed to be positioned on the surface of a testing area 16 of target 14, such as human skin, and has an aperture 102 to permit laser light to be applied to the testing area. Convex lens 98 serves to focus probe beam 20 and convex lens 100 serves to focus reference beam 24 in the same point 15 of the testing area 16 at certain distance "ε" under target surface 14 that defines the natural acoustic oscillation of the surface. Acoustic detector 96 then detects the acoustic oscillations 94 from target testing area 16 within acoustic cell 92. Tuning an angle between directions of the probe and reference beams provide means sufficient to detect phase-conjugated acoustic oscillations in a medium.

The dermal or epidermal area of the skin that generates acoustic waves can be considered as a thin membrane. The membrane has natural oscillation frequencies that depend on the thickness of the membrane, its elastic constants, and the square of the membrane surface that is equal to the square of the aperture 102. If the repetition frequency of the light pulses causing the acoustic oscillations equals to the oscillation frequency of the membrane, the oscillation becomes resonant. Under such circumstances the amplitude of the oscillations increases many times, increasing the signal-to-noise ratio and, thus, testing sensitivity.

The amplitude and frequency of the acoustic oscillations, excited by the probe beam 20, depend on the concentration of interest component in the human tissue due to absorption of light with a predetermined wavelength. Usually the photo-acoustic signal excited by probe beam 20 also depends on concentration of the other components, which absorption bands overlapping with absorption band of said interest component in a medium. The amplitude and frequency of the acoustic oscillations, excited by the reference beam 24, only depend on the concentration of components, which absorption bands overlapping with absorption band of interest component in a medium, irrespective of said interest component.

Where the component of interest being tested is glucose and the target is human skin, the glucose optically absorbs the light energy of probe beam 20, thereby inducing a temperature rise in target testing area 16 and generating acoustic oscillations 94 indirectly in air. The acoustic wave spectrum depends on the glucose concentration in the interstitial fluid (ISF) that surrounds the cells within the tissue. Glucose levels in ISF are about 10% lower than glucose levels in blood.

If concentration of an interest component much more than concentration of other components, which absorption bands overlapping with absorption band of the interest component in a medium, the photoacoustic signal amplitude, excited by probe beam 20, much more than that of the reference beam 24. In this case the probe beam 20 may be used only to excite resonant acoustic oscillations according to prior art photoacoustic material analysis that described in U.S. Pat. No. 6466806 of Geva et al., in which the concentration of an interest component in a medium is determined by measuring of the amplitude and the frequency of the resonant oscillations. Consistent with the present invention, using in-phase conjugated acoustic oscillations at i > 1 allows to excite resonant oscillations by probe beam having lower average power than it needs in prior art.

Reference is now made to FIG. 9, which is a flowchart illustrating a method for determining a concentration of an interest component in a medium, consistent with the present invention. In an embodiment consistent with the present invention, the method illustrated in FIG. 2 may be performed using the optical apparatus illustrated in FIG. 1. For the purpose of illustrating such an embodiment, the steps of FIG. 9 will be described in conjunction with the operation of FIG. 2.

A head of sensor, which may be the optical cell 10 or an acoustic cell 92, is initially placed in contact with surface of the target 14 (104). Consistent with the present invention, testing area 16 may be at a surface of the target 14, or may be below surface of the target 14. Next testing area 16 is irradiated with a probe beam 20 (106). Testing area 16 is subsequently irradiated with a reference beam 24 (108).

The both beams 20 and 24 define the laser-induced oscillations of light scattering 26, which are registered (110) by photodetectors 30 and 34. The acoustic oscillations 94 from testing area 16 of the target 14 may be registered also using acoustic detector 96 such as microphone via air.

Consistent with the present invention, the frequency repetition of the short pulses of the probe beam is chosen equal to be in-phase with natural acoustic oscillations in the medium. The frequency repetition of the short pulses of the reference beam is chosen equal to be anti-phase or independent to acoustic oscillations excited by the probe beam. Measuring of the amplitude and the frequency of the phase-conjugated photoacoustic oscillations determine the concentration of interest component.

The probe beam 20 and the reference beam 24 may further cause periodic or non-periodic transient processes in the target 14, which may at least partially modulate scatterings of radiation 26. The detector 96 (or photodetectors 30 and 34) converts detected acoustic oscillations 94 or scattered light oscillations 26 into electrical signals for processing.

Consistent with the present invention, the electrical signals may represent at least one of the amplitude, frequency, or decay time of any transient processes that may be produced in the target 14. The electrical signals are then transmitted from first and second peak detectors 56 and 58 to multiplexer 62. Multiplexer 62 combines the electrical signals from first and second peak detectors 56 and 58, and outputs a single combined electrical signal to analog-to-digital converter 64. Analog- to-digital converter 64 converts the input analog electrical signal into a digital electrical signal and outputs the digital electrical signal to processor 50.

Processor 50 receives the digital electrical signals and executes instructions, which may be stored in an internal memory (not shown), for performing calculations using the digital electrical signals. For example, processor 50 may calculate changes in the intensity of scatterings of radiation 26 (112) caused by repeated emission of the probe beam 20 and the reference beam 24. From the calculated changes in intensity, processor 50 will then execute instructions to perform an algorithm for calculating the concentration of an interest component present at testing area 16 (114). Consistent with the present invention, the calculations may also be performed by an external processor, for example, a processor contained in PC 48. The calculated concentration may then be displayed for a user to view (116). Consistent with the present invention, the concentration may be displayed on a display screen attached to electronics enclosure 44, or on computer 48. Moreover, the concentration may also be tabulated in computer 48 for trending and over-time analysis.

Consistent with the present invention, image analysis techniques may be used in conjunction with the optical apparatus described herein. In particular, image analysis techniques may be used to ensure that the probe beam 20 and the reference beam 24 are consistently incident on testing area 16, with no variation. Image analysis techniques may include video hardware and software, attached to and/or embedded on optical apparatus, which allows a user to accurately position optical apparatus such that the probe beam 20 and the reference beam 24 are consistently incident on testing area. Consistent with the present invention, a portable video camera could be installed such that a real time video feed could show user positioning optical apparatus on surface of the target 14. Markers could be placed at testing area 16 so that user could reliably, using the video feed, align the optical apparatus with testing area 16 to ensure incidence thereon.

FIG. 10 is a flowchart illustrating a method for calibrating an optical apparatus consistent with the present invention. If the interest component being tested is glucose, as described above, it is important for the health of the user that the concentrations obtained are accurate, and in conformance with other accepted means of testing glucose concentration. Accordingly, in performing a calibration process, the results of a standard in vitro blood test is compared to the results in vivo of the optical apparatus, and the optical apparatus is offset to match the blood test. Although this calibration process has been summarized with respect to glucose testing, the calibration process described in detail below may also be used when using the optical apparatus consistent with the present invention to determine the concentration of interest component other than glucose.

In the first approximation a calibration curve for calculation glucose concentration in a human body has form:

Y = a + bX, (EQ.22)

where Y is the glucose concentration; X is measuring value, for example amplitude of acoustic oscillations 94 from testing area 16 of the target 14 measured by acoustic detector 96 such as microphone;

"a" and "b" are stable parameters, that should be found through comparing invasive and noninvasive concentration measurements.

"a" is offset that mainly depends on skin properties; "b" is gain that mainly depends on operating mode of an electronic-optical apparatus.

According to EQ.22, at least two independent invasive measuring of different glucose concentration should be used for determining "a" and "b" parameters. A calibrating needs to match in vivo with in vitro measuring within a predetermined degree of accuracy.

As shown in FIG. 10 flowchart illustrating, first, a fluid sample is obtained in vitro (118), and using a fluid concentration determining in vitro means, a first concentration of an interest component is determined (120). This first concentration is recorded, and then the optical apparatus consistent with the present invention is used to take a concentration measurement in vivo (122). The optical apparatus performs a method, such as illustrated in FIG. 2, and determines a second concentration of the interest component in vivo (124). The first concentration and the second concentration are compared to one another to determine if they match within a predetermined degree of accuracy (126). If the first concentration and the second concentration match, no further calibration is needed (128). If, however, the first concentration and the second concentration do not match, the optical apparatus is offset by a predetermined amount such that the second concentration will match the first concentration (130). After this step, the calibration is complete (132). Consistent with embodiments of the present invention, a computer, external to the optical apparatus or on-board the optical apparatus, may perform the recordation of the concentrations, the match determination, and the offset.

While the methods and apparatus disclosed herein may or may not have been described with reference to specific hardware or software, the methods and apparatus have been described in a manner sufficient to enable persons of ordinary skill in the art to readily adapt commercially available hardware and software as may be needed to reduce any of the embodiments of the present invention to practice without undue experimentation and using conventional techniques. In addition, while the present invention has been described with reference to a few specific embodiments, the description is intended to be illustrative of the invention as a whole and is not to be construed as limiting the invention to the embodiments shown. It is appreciated that various modifications may occur to those skilled in the art that, while not specifically shown herein, are nevertheless within the true spirit and scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS What is claimed is:

1. Apparatus for determining a concentration of an interest component in a turbid medium such as human body, comprising:

at least one light source operative to emit probe and reference beams having predetermined different wavelengths of equidistant short pulses having at least variable frequency, number, and power output sufficient to excite phase-conjugated acoustic oscillations in said medium, at that rate, said phase-conjugated acoustic oscillations are excited by said probe beam in-phase with a natural acoustic oscillations in said medium, and said phase-conjugated acoustic oscillations are excited by said reference beam anti-phase or independent to said acoustic oscillations excited by said probe beam;

at least two detectors:

first detector for detecting said phase-conjugated acoustic oscillations and generating electrical signals representative of the amplitude of said acoustic oscillations, and

second detector for detecting at least frequency and power of light pulses of said probe and reference beams and generating electrical signals representative of the said frequency and power of said light pulses suitable for feedback operation of the said apparatus; and a processor for determining the concentration of said interest component in response to said electrical signals.

2. Apparatus according to claim 1 wherein said probe beam has wavelength

corresponding preferably to maximum of an absorption band or may be in the range of the absorption band of said interest component in said medium for exciting of said phase- conjugated acoustic oscillation in said medium due to absorption of light by said interest component and subsequent adiabatic temperature rise in a testing area of said medium.

3. Apparatus according to claim 1 wherein said reference beam has wavelength corresponding preferably to minimum or may be not far from the minimum of said absorption band of said interest component for exciting of said phase-conjugated acoustic oscillation in said medium, irrespective of concentration of said interest component, due to absorption of light by other components, which absorption bands overlapping with said absorption band of said interest component, and subsequent adiabatic temperature rise in a testing area of said medium.

4. Apparatus according to claim 1 wherein said probe and reference beams are directed to the same testing area under the surface of said medium to excite of said phase-conjugated acoustic oscillation in said medium.

5. Apparatus according to claim 1 wherein said probe and reference beams preferably comprise pulse-train equidistant short pulses of said light source radiation having variable frequency, duration, number, and power output sufficient to excite phase-conjugated acoustic oscillations in said medium.

6. Apparatus according to claim 1 wherein said probe and reference beams comprise equidistant short pulses of quasi-continuous wave radiation of said light source having variable frequency, and power output sufficient to excite phase-conjugated acoustic oscillations in said medium.

7. Apparatus according to claim 1 wherein said probe and reference beams comprise short mono-pulses of said light source radiation having variable delay time between them sufficient to excite phase-conjugated acoustic oscillations in said medium.

8. Apparatus according to claim 1 wherein said light source comprises pulsed lasers to emit said probe and reference beams.

9. Apparatus according to claim 1 wherein said light source is preferably dual- wavelength pulsed light source comprising two different chips of pulsed laser diodes to emit said probe and reference beams.

10. Apparatus according to claim 1 wherein said light source comprises tunable pulsed diode lasers to emit said probe and reference beams.

11. Apparatus according to claim 1 wherein said light source comprises fiber coupled diode laser arrays to emit said probe and reference beams.

12. Apparatus according to claim 1 wherein said light source comprises tunable Co:MgF₂ lasers to emit said probe and reference beams.

13. Apparatus according to claim 1 wherein said light source comprises Q-switched neodymium containing optical medium lasers to emit said probe and reference beams.

14. Apparatus according to claim 1 wherein said light source comprises Erbium-doped fiber laser to emit said probe beam.

15. Apparatus according to claim 1 wherein said light source comprises Er-glass rod or slab laser to emit said probe beam.

16. Apparatus according to claim 8 and further comprising optical converters for converting the wavelength radiation of said pulsed lasers is suitable for exciting phase- conjugated acoustic oscillations in said medium.

17. Apparatus according to claim 8 and further comprising an optical amplifier to increase the power output of said pulsed lasers sufficient to excite phase-conjugated acoustic oscillations in said medium.

18. Apparatus according to claim 1 wherein said first detector of said phase-conjugated acoustic oscillations is preferably a photodetector of light scattering changes induced by said phase-conjugated acoustic oscillations in said medium.

19. Apparatus according to claim 1 further comprising preferably a measuring optical cell adapted to enclose to surface of said medium such said probe and reference beams reach said surface within the area enclosed by said cell.

20. Apparatus according to claim 19 wherein said measuring optical cell is suitable for detecting said phase-conjugated acoustic oscillations.

21. Apparatus according to claim 19 further comprising at least two different chips of pulsed laser diodes to emit said probe and reference beams, at least one photodetector of light scattering changes induced by said phase-conjugated acoustic oscillations in said medium, and additional photodetector to control pulse duration, frequency, number, and power of said probe and reference beams is suitable for feedback operation of said apparatus.

22. Apparatus according to claim 19 other comprising at least two different chips of pulsed laser diodes to emit said probe and reference beams, at least one photodetector to control pulse duration, frequency, number, and power of said probe and reference beams is suitable for feedback operation of the said apparatus, and at least one remote photodetector of light scattering changes induced by said phase-conjugated acoustic oscillations in said medium.

23. Apparatus according to claim 19 other comprising a bundle of optical fibers for transmitting said probe and reference beams to said medium, and scattered radiation from said medium to said photodetector, which may include one or several photodiodes.

24. Apparatus according to claim 1 wherein said first detector of said phase-conjugated acoustic oscillations is a microphone.

25. Apparatus according to claim 1 other comprising a measuring acoustic cell adapted to enclose to surface of said medium such that light from said light source reaches said surface within the area enclosed by said cell.

26. Apparatus according to claim 25 wherein said measuring acoustic cell is suitable for detecting said phase-conjugated acoustic oscillations.

27. Apparatus according to claim 1 wherein said first detector of said phase-conjugated acoustic oscillations is a piezoelectric crystal.

28. Apparatus according to claim 1 wherein said first detector of said phase-conjugated acoustic oscillations is an optical fiber sensor.

29. Apparatus according to claim 1 wherein said first detector of said phase-conjugated acoustic oscillations is a laser interferometer.

30. Apparatus according to claim 1 wherein said second detector is at least one

photodetector.

31. Apparatus according to claim 1 wherein a small portions of said probe and reference beams are directed to said the second photodetector by beamsplitters.

32. Apparatus according to claims 9 and 31 wherein said beamsplitters are backside mirrors of said chips of pulsed laser diodes emitting said probe and reference beams.

33. Apparatus according to claim 31 wherein said beamsplitters are remote mirrors for separate off a small portions of said probe and reference beams

34. Apparatus according to claims 1 and 30 wherein said second detector is suitable for detecting pulse light duration, frequency, number, and power of said probe and reference beams.

35. Apparatus according to claims 1 and 30 wherein said second detector is suitable for feedback operation of the said apparatus.

36. Apparatus according to claims 19 and 30 wherein said second detector is placed in said measuring optical cell.

37. Apparatus according to claim 1 wherein the apparatus is enclosed in a portable, handheld or desktop device.

38. Apparatus according to claims 19 and 37 wherein the portable device further comprises a pressure sensor enabling detection of a pressure between said optical cell and surface of said medium.

39. Apparatus according to claim 1, further comprising at least one of image analysis hardware and software for monitoring said testing area to ensure that said probe and reference beams are incident on a predetermined location.

40. Apparatus according to claim 39, wherein at least one of image analysis hardware and software comprises a portable video camera, and imaging software.

41. Apparatus according to claims 1 and 37, wherein said portable device comprises a wireless communication device based on a bluetooth system, between components of the said apparatus.

42. A method of determining a concentration of an interest component in a turbid medium such as human body, comprising the steps of: irradiating a surface of said medium having said interest component by probe and reference beams having predetermined different wavelength of equidistant short pulses having at least variable frequency, number, and power output sufficient to excite phase-conjugated acoustic oscillations in said medium, at that rate, said phase-conjugated acoustic oscillations are excited by said probe beam in-phase with a natural acoustic oscillations in said medium, and said phase-conjugated acoustic oscillations are excited by said reference beam anti-phase or independent to said acoustic oscillations excited by said probe beam;

detecting said phase-conjugated acoustic oscillations and generating electrical signals representative of the amplitude of said acoustic oscillations,

detecting at least frequency and power of light pulses of said probe and reference beams and generating electrical signals representative of the said frequency and power of said light pulses; and

determining the concentration of said interest component in said human body in response to said electrical signals;

43. The method according to claim 42 wherein said irradiating step comprises said probe beam has wavelength corresponding preferably to maximum of an absorption band or may be in the range of the absorption band of said interest component in said medium for exciting of said phase-conjugated acoustic oscillation in said medium due to absorption of light by said interest component and subsequent adiabatic temperature rise in a testing area of said medium.

44. The method according to claim 42 wherein said irradiating step comprises said reference beam has wavelength corresponding preferably to minimum or may be not far from the minimum of said absorption band of said interest component for exciting of said phase- conjugated acoustic oscillation in said medium, irrespective of concentration of said interest component, due to absorption of light by other components, which absorption bands overlapping with said absorption band of said interest component, and subsequent adiabatic temperature rise in a testing area of said medium.

45. The method according to claim 42 wherein further tuning said wavelength of said probe beam to maximum of an absorption band of said interest component for exciting of said phase-conjugated acoustic oscillations in said medium.

46. The method according to claim 42 wherein further tuning said wavelength of said reference beam to minimum of said absorption band of said interest component for exciting of said phase-conjugated acoustic oscillations due to absorption of light by other components, which absorption bands overlapping with said absorption band of said interest component.

47. The method according to claim 42 wherein said irradiating step further directing said probe and reference beams to the same testing area under surface of said medium to excite said phase-conjugated acoustic oscillation.

48. The method according to claim 42 wherein said irradiating step comprises said probe and reference beams preferably comprise pulse-train equidistant short pulses of said light source radiation having variable light pulse duration, frequency, number, and power output sufficient to excite said phase-conjugated acoustic oscillations in said medium.

49. The method according to claim 42 and claim 48, wherein further tuning said frequency, number, and power output for exciting of said phase-conjugated acoustic oscillations in said medium.

50. The method according to claim 42 wherein said irradiating step comprises said probe and reference beams comprise equidistant short pulses of quasi-continuous wave of said light source radiation having variable frequency, and power output sufficient to excite said phase- conjugated acoustic oscillations in said medium.

51. The method according to claim 42 and claim 50, wherein further tuning said frequency and power output for exciting of said phase-conjugated acoustic oscillations in said medium.

52. The method according to claim 42 wherein said irradiating step comprises said probe and reference beams comprise short mono-pulses of said light source radiation having variable pulse power and delay time between said short mono-pulses sufficient to excite said phase- conjugated acoustic oscillations in said medium.

53. The method according to claim 42 and claim 52, wherein further tuning said pulse power and delay time between said short mono-pulses of said probe and reference beams for exciting of said phase-conjugated acoustic oscillations in said medium.

54. The method according to claim 42 wherein said detecting step of said phase- conjugated acoustic oscillations using preferably a photodetector of light scattering changes induced by said phase-conjugated acoustic oscillations in said medium.

55. The method according to claim 42 and claim 54, and further comprising tuning a distance between said light sources radiation and a remote photodetector of said light scattering change sufficient for detecting of said phase-conjugated acoustic oscillations in said medium.

56. The method according to claim 42 and claim 54, wherein further tuning an angle between direction of the probe beam and surface normal of said remote photodetector sufficient for detecting of said phase-conjugated acoustic oscillations in said medium.

57. The method according to claim 42 wherein said detecting step of said phase- conjugated acoustic oscillations via air using a microphone.

58. The method according to claim 42 and claim 57, wherein further tuning an angle between directions of the probe and reference beams sufficient for detecting of said phase- conjugated acoustic oscillations in said medium.

59. The method according to claim 42 wherein further detecting pressure between an optical cell and surface of said medium.

60. The method according to claim 42 wherein further monitoring a testing area of said human body sufficient to ensure that the said probe and reference beams are incident on a predetermined location using a portable video camera, and imaging software.

61. The method according to claim 42, wherein further detecting step comprises a wireless communication device based on a bluetooth system of the apparatus according to claim 41.

62. The method of according to claim 42, wherein further calibrating step comprises: obtaining in vitro a sample of a fluid containing an interest component;

determining a reference concentration of the interest component using a fluid-based invasive apparatus;

determining if in vivo determined concentration is equivalent to said reference concentration, wherein if in vivo determined concentration is not equivalent to said reference concentration, offsetting the detected electrical signals such that in vivo determined concentration is equivalent to said reference concentration.

63. The method according to claim 62, wherein further comprising repeating the steps a plurality of times until in vivo determined concentration is equivalent to said reference concentration

64. The method according to claim 42, wherein further displaying the determined in vivo concentration of the interest component in said medium.