WO1991019539A1 - Dynamic control of laser energy output - Google Patents

Abstract

A laser ophthalmic surgery apparatus includes an improved control for assuring that the energy output level of the beam delivered to the eye remains within a prescribed tolerance. An energy output sensor (42) receives a small fraction of the laser beam's energy through a beam splitter (36). The sensor is located downstream of all laser pulse shaping parameters associated with the laser cavity (42). During operation of the laser the energy output sensor (42) constantly monitors laser energy as delivered to the target, and in preferred embodiment it monitors pulse-to-pulse energy deviations and calculates a moving average of the pulse energy over a relatively small number of shots. A laser beam light attenuator (22, 20) upstream of the sensor varies the intensity of the laser beam passing through, under the control of a microprocessor (46) which adjust the beam light attenuator in accordance therewith, making compensating adjustements after each shot.

Description

Background of the Invention

This invention relates to a system for controlling laser energy output to maintain it within prescribed limits. The invention provides a simple, accurate and inexpensive system and method for minimizing the pulse-to-pulse energy variations of emissions from a pulsed laser, particularly in a surgical system wherein it is advantageous to maintain energy emission from the system consistently within relatively close tolerance limits.

Lasers have been used in ophthalmology and other surgical specialties for over twenty-five years. Their success to date has been based in part on the ability of the surgeon to accomplish procedures with a laser surgical system in a manner that reduces, or in some cases eliminates, the need for lengthy hospitalization, and partly on the laser system's ability to generate deep lesions within transparent tissues, causing considerably less trauma or damage than conventional non-laser procedures and in a predictable manner.

Ophthalmologists were among the first specialists to use lasers in surgery extensively. They have traditionally avoided using lasers in too close proximity of sensitive tissues where a misplaced laser shot or an overly energetic laser pulse could lead to undesirable, even irreparable damage. One method for reducing inaccuracies associated with a misplaced shot is addressed in U.S. Patent Application

Serial No. 475,657, filed February 6 , 1990, assigned to the assignee of this invention.^"

The present invention is addressed to a method and system for limiting damage associated with undesired pulse- to-pulse energy variations.

Predictability of the degree of tissue disruption from a laser depends to a large extent on the energy amplitude and distribution of a laser pulse. Recognizing the need to monitor and control laser pulse energy variation, the U. S. Center for Devices and Radiological Health (CDRH) and the U. S. Food and Drug Administration (FDA) have established guidelines for laser emissions and tolerances of repeatable emission variations, as well as a testing process of laser emissions prior to commencing each laser surgical procedure. FDA requires that a small number of test shots be triggered and sampled for performance evaluation prior to enabling a laser for a given surgical procedure.

A common technique for characterizing emission is the following. Typically, the total emission of a large number of pulses is measured, and if the average amount of energy and an average deviation fall within a prescribed tolerance of the prescribed level, then the device is considered to meet specifications. In other words these methods look at standard or average deviations as opposed to maximum pulse- to-pulse deviations. A typical standard used in practice is 20% variation of pulse-to-pulse energy.

In photodisruption techniques, the damage to biological tissues or any other kind of material generally is proportionate to how much energy is transferred from the laser pulse to the material.

If the energy level is well above fluence and irradiance thresholds, the difference in lesion size from 20% pulse variations can be as little as 20%, following the general rule of proportionality. However, if the procedure is working at a level just above breakdown threshold, the difference in the size of the damage lesion, even from a 20% or smaller deviation, can be factors of 300% to 400% or more. The shortcoming with strictly averaging-type standards is that low average tolerance levels can be met for a series of laser pulses while there still are objectionably large maximum pulsed energy deviations. As outlined above, large pulse-to-pulse energy fluctuations can cause considerable and undesired lesions in human tissue. Examples of such damage would be the endothelial cell loss resulting from an unusually large single pulse generated by an excimer laser while performing corneal refractive surgery. In this example, the pressure wave propagating posteriorly into the cornea from the ablation front on the corneal anterior surface can become sufficiently powerful to induce endothelial cell dysfunction leading, in some cases, to blindness.

Another example arises in the performance of posterior capsulotomies with lasers. For this ophthalmic procedure, the surgeon aims a laser at that membrane and photodisrupts it. Considering that there are about 1.2 million cataract surgeries done in the United States every year and that nearly 20% of them result in the need for a posterior capsulotomy, there is a need for close and effective control of laser energy in the procedure. Many manufacturers manuals suggest that the surgeon/user fire posteriorly to the posterior capsular membrane, in the eye vitreous, and to gradually approach the targeted membrane from the posterior so as to avoid any possible pitting of the intraocular lens implant introduced during cataract surgery. If the initial laser shots are aimed too far posteriorly to the posterior lens capsule, or if the pulse energy emitted is too far above the prescribed setting, a sufficiently powerful pressure wave can propagate to the vitreoretinal interface and cause enough traction on the retina to result in retinal detachments. Retinal detachment is one of the commonly listed complications following laser treatment for posterior capsulotomies. Summary of the Invention

In accordance with the present invention, it is first recognized that if any of the laser's internal parameters is varied in an effort to control laser energy emissions, the parameter variations often introduce complicating transient variations. Such excursions from stability are frequently non-linear and there is no presumption of iterative convergence to a specified set of pulse-to-pulse repeatable output parameters. Consequently, it is an important aspect of this invention that the laser parameters are not modulated or controlled. Only the laser emission, once beyond the resonator and into the delivery assembly of the laser system, should be sampled -and controlled to a level within prescribed limits.

A conventional method for monitoring and controlling energy emission from a laser which is part of a surgical laser system has been to (1) place energy detectors behind the reflectors in the resonator cavity, (2) take an average of a specified number of the pulses transmitted through the reflectors to the detectors, and (3) based upon this moving average, vary the energy being supplied to the lasing medium to compensate for performance variations. For excimer lasers, the energy change is commonly achieved by varying the voltages being supplied to the electrodes from which a discharge stimulates the lasing medium. Such voltage variation has been shown sufficient for limiting the standard deviation of the energy emission from excimer lasers to within 10% average tolerances under good operational conditions and is the subject of U. S. Patent No. 4,611,270. The duration of such good operational conditions in a surgical environment or the control of the maximum pulse-to- pulse variation remain open questions for excimers and many other laser systems. Since varying the -voltages supplied to the discharge can result in highly non-linear changes in laser emission, it is questionable that such methods can be improved to ever decreasing tolerances. The need for better emission control from lasers in surgery arises from the need for finely controlled tissue damage, and this is particularly true in ophthalmic surgery. For surgical laser systems seeking to operate near dielectric breakdown thresholds, or which rely on an outwardly expanding shock wave generated as a result of laser induced plasma decomposition to create uniform laser lesions, the tightly controlled energy emission from a laser is one of the keys to accurate and predictable, safe surgical results. The sampling of the emission should be continuous, not limited to a few representative pulses prior to a procedure, and dynamically interactive throughout the course of the procedure.

Accordingly, the present invention embraces an apparatus and method of dynamically monitoring and controlling the energy output of a laser based surgical system such that increasingly narrower pulse-to-pulse error tolerances can be satisfied. In a preferred embodiment this is achieved by placing a detector behind a beam splitter mirror of the laser delivery assembly. The detector samples a known small fraction of the energy of each laser pulse being emitted from the laser surgical system, already fully exited from the laser itself. The detector signal is amplified electronically, and compared against a software generated standard pre-selected by the operator as to both prescribed energy levels and acceptable energy variations. The computer program manipulates a variable attenuator to repeatedly adjust the output to required levels. Correction preferably is made after each pulse. In the event tolerances are not satisfied by the system, instructions are relayed to a safety shutter or other device to interrupt laser beam delivery. The system may then be re-enabled to automatically continue with the designated procedure.

The system of the invention provides a more uniform result when using high powered lasers to cause photodisruption lesions in biological tissues and other materials .

It is therefore among the objects of the invention to provide a relatively simple but more accurate, precise, safe and reliable laser energy emission level control system for a laser beam delivery system, particularly in the field of laser surgery. These and other objects, advantages and features of the invention will be apparent from the following description of preferred embodiments, considered along with the accompanying drawings.

Description of the Drawings

Figure 1 is a schematic diagram showing an example of a laser beam delivery system using the energy monitoring and control system of the invention.

Figure 2 is another schematic diagram indicating optics of a portion of the system shown in Figure 1.

Figure 3 is a schematic view showing an example of a type of energy level detector which may be used with the system of the invention.

Figures 4 through 8 are graphs plotting laser energy versus time in a progression of laser shots, under different circumstances. Figures 4 through 6 show laser energy without the control system of the invention. Figures 5 and 7 show control achieved using the system of the invention, and Figure 8 shows cessation of laser firing due to excessive energy despite the imposed control.

Description of Preferred Embodiments

In the drawings, Figure l shows a delivery system 10 for a medical laser surgery system and procedure. In the context of the drawings and the discussion herein, the laser surgery apparatus is described as concerned with ophthalmic surgery; however, other forms of precision microsurgery, as well as precision industrial micromachining, can also be accomplished efficiently using the principles and system of the invention.

As shown in Figure 1, a laser beam 12 emanates from a laser source indicated schematically at 14. The beam 12, after exiting the laser 14 and all laser pulse shaping parameters associated with the laser 14, travels through appropriate optics which may include a mirror as shown at 16. The beam travels along a path which is upward as shown in

Figure 1, ultimately reaching a beam intensity attenuator 18.

The beam intensity attenuator 18 may take any of several forms, so long as it is capable of light attenuation with a quick response time, on the order of less than one millisecond. In one preferred embodiment of the invention, the attenuator 18 comprises a polarization type attenuator which acts on the linearly polarized laser beam 12. Thus, the attenuator 18 may include a polarizer 20 (e.g., an analyzing polarizer) in conjunction with a half-wave retardation plate 22. As indicated schematically, the half- wave retardation plate is mounted on a rotating stage 24, through which it can be repositioned to various degrees of rotation to increase or decrease the degree of light attenuation effected. As the retardation plate is rotated, the plane of polarization of the exiting beam rotates twice as fast as the plate itself. Thus 45° rotation of the plate 22 is required to move from parallel polarization to 90° rotation from parallel, i.e. from full transmission to full attenuation. As is well known, the polarizer 20 when lined up with the polarization of the laser beam 12 from the retardation plate 22 will pass substantially 100% light intensity, and this can be attenuated to any desired degree down to zero or near-zero intensity, by adjusting the angular position of the half-wave retardation plate 22.

Downstream of the attenuator 18, the laser beam 12 may pass through various additional optical components. As indicated in Figure 1, these may include a lens assembly 26 for increasing the volume of the beam to an expanded parallel beam 12a, and mirrors 28, 30 and beam splitters 32 and 34. The mirror 30 may be a tiltable beam steering mirror.

It should be understood that the invention is illustrated in Figure 1 with an optical beam delivery system which is exemplary only. In Figure 3, described below, the principles of the invention are shown with only the rudiments of a delivery system.

As shown in Figure 1, the expanded beam 12a then reaches a further beam splitter 36 which transmits a small portion 12b of the beam while reflecting the remainder toward an objective lens assembly 38, at the upper left of the drawing. This beam is ultimately focussed toward a target, such as the eye 40 indicated in Figure 1.

Figure 2 shows diagrammatically the downstream end of the system of Figure 1, with the beam splitters 34 and 36, and the objective lens assembly 38 positioned to deliver the focussed beam to the eye 40. As indicated on the drawing, the beam splitters 34 and 36 may transmit light to the eye or from the eye, for the purposes of illumination or sensing or imaging of the eye. These functions and equipment do not form a part of the present invention and are indicated only generally.

The very low-energy beam portion 12b transmitted through the beam splitter 36 is representative of the laser beam intensity, i.e. the energy level of the beam as delivered to the eye.

The beam portion 12b is directed to an intensity or energy level sensor 42 indicated schematically in Figure 1. The sensor may take the form of any accurate, fast-responding intensity or energy sensing device, one example of which is described below in conjunction with Figure 3. As indicated schematically in Figure 1 (and also in Figure 3), the sensor 42 emits an electrical signal indicated on a line 44 which is fed to a control device, e.g. a microprocessor 46. The controller or processor 46 is programmed with an appropriate algorithm to receive and respond to variations in sensed energy level, preferably processing pulse-to-pulse variations as further described below. A preselected tolerance level for variation of the energy output from a prescribed optimum or requested level is programmed into the controller 46, and the controller responds to energy level variations from a preset or requested energy level, in formulating and sending signals along a line 48 to the motor-driven rotating stage 24 driving the half-wave retardation plate 22.

Figures l and 3 also show another feature of the invention by which laser emission is shut off if a single laser pulse deviating from the preset level by more than a preselected tolerance limit is detected (and also, preferably, if pulse energy falls too far below the preset level). If such a laser shot outside limits is detected, the microprocessor/controller 46 emits a signal 49 which is sent to a shutter device 50, which immediately blocks off laser emission, before the next pulse occurs. The shutter device 50 can comprise, for example, a Vincent Uniblitz shutter.

Figure 3 shows in schematic form one example of a common type of integrating light intensity detector, as included in block diagram representation of the system of the invention. An integrating sphere 52 has an opening 54 through which the small portion of light is admitted, i.e. a sample of the treatment beam as indicated at 56 and as carried on the line 12b in Figure 1 and Figure 2. Since the beam 12b may be an expanded-volume beam, a positive lens 57 may be positioned in front of the integrating sphere to focus the beam into the sphere.

In accordance with well-known principles, the integrating sphere 52 integrates and diffuses the sample of light, and the intensity of this integrated sample is detected by the energy monitor assembly 42, which includes a photodetector 58 such as a silicon photodiode, receiving light through a band pass filter 60. The band pass filter 60 is a narrow band filter which is specific to the laser beam's wavelength and it filters out any extraneous light which might be introduced to the integrating sphere from illumination sources associated with the system or from any other extraneous, non-laser source which may have found its way into this part of the system. As is well known, the detector receives light from a direction approximately 90° from the entry direction of the focussed light 56, in this case through a second hole 62 in the integrating sphere.

As shown in Figure 3, the signal from the photodetector 58 may be processed in a sample and hold 64, an amplifier 66 and a buffer 68 before the resulting process signal 44 is sent to the microprocessor controller 46.

Figures 4, 5, 6, 7 and 8 are graph plottings of laser energy versus time, or laser energy versus a progression of laser firing shots. Figures 4 and 6 show laser energy in a series of 512 consecutive shots from an Nd.YAG laser, without controlling. The plottings of Figures 4 and 6 represent different instances, and demonstrate different trends of the laser energy. In all cases the preset energy for the Nd:YAG laser was 500 microjoules. The graphs of Figures 4 through 8 are from actual control exercises using the system of the invention, with the control algorithm described below.

One preferred algorithm which may be used in the system of the invention calculates a moving average of laser energy, for purposes of control. Preferably the moving average is recalculated after each shot for purposes of servo control. In one preferred implementation the last ten shots are used, after each shot, to recalculate the moving average. The difference is taken between the moving average and the requested energy or preset energy for the system. After each shot, the beam energy is adjusted (using the attenuator 18 discussed above) by the amount of the difference, to compensate for the excursion represented by the last calculated moving average.

Results achieved with the system of the invention are shown in Figures 5 and 7. As can be seen from these graphs, the output energy level is kept within a close envelope of the requested 500 microjoules, generally not exceeding about 15%.

Figure 8 demonstrates the laser emission shutoff feature described above. In Figure 8 the laser output has been shut down by the shutter device 50 (Figures 1 and 3) due to the occurrence of a single shot in excess of a preset limit for each shot. The single shot occurred immediately before shutdown. In this example the limit was set at 15% variation from the 500 microjoule requested energy.

The above described preferred embodiment is intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.

WE CLAIM:

Claims

1. In a laser beam delivery system including a laser and associated pulse shaping parameters for a pulsed laser beam produced by the laser, the improvement comprising, an energy output sensor receiving a fraction of the laser beam's energy from the laser beam delivery system, the energy output sensor being located downstream of all laser pulse shaping parameters associated with the laser, and the energy output sensor having means for constantly monitoring laser energy during operation of the laser including means for monitoring pulse-to-pulse energy deviations from a preset desired energy output level, a laser beam intensity attenuator in the beam path upstream of the energy sensor, the intensity attenuator having means for varying the intensity of the laser beam light and thus varying the energy output of the laser beam passing through the attenuator, and control means for receiving a signal from the laser energy sensor and for adjusting the laser beam intensity attenuator as to level of attenuation, in response to the sensing of variations in laser energy output from the preset desired energy output level.

2. Apparatus according to claim 1, wherein the control means further includes means for shutting off laser emission in the event the energy sensor determines that laser beam energy level is not being kept within a preselected tolerance via the attenuator and control means.

3. Apparatus according to claim 2, wherein the means for shutting off laser emission includes means for shutting off emission whenever it is sensed that a single laser pulse has varied from the preset energy level by more than the preselected tolerance.

4. Apparatus according to claim 1, wherein the laser beam is linearly polarized and wherein the laser beam intensity attenuator comprises a linear polarizer and a half- wave retardation plate on a rotating stage, with means for rotating the retardation plate to control the amplitude of the laser beam in response to signals from said control means.

5. Apparatus according to claim 1, wherein the control means operates in accordance with an algorithm which calculates a moving average of a selected number of laser shots in a sequence which has just occurred, after each laser shot, and takes a difference between the moving average and the preset desired level and adjusts the beam energy output with the attenuator by the amount of said difference, to compensate for the variation represented by the moving average.

6. Apparatus according to claim 5, wherein the control means further includes means for shutting off laser energy emission in the event the energy sensor detects a single laser pulse which varies from the preset energy level by more than a preselected tolerance.