US RE39833 E1
A thermally actuated spectroscopic optical switch including reflective surfaces which are selectively moved into a position intersecting a beam of light by applying electrical or heat energy to a selected composite cantilever beam on which the reflective surface is mounted.
1. A system for directing a selected light beam to at least one light beam receptor, said system comprising:
an array of stationary optical fibers, each one of said stationary optical fibers constructed and arranged to conduct one of a plurality of light beams including the selected light beam;
an optical switch fabricated on a substrate, the switch having an array of movable reflective surfaces, and having a single thermal actuator associated with each reflective surface, each thermal actuator comprising a cantilevered arm having a fixed end attached to the substrate and a free end to which the reflective surface is attached, the arm being made from a material having a thermal expansion property, the arm having a top surface and a bottom surface with a layer of material having a different thermal expansion property on a portion of at least one of these surfaces, and the cantilevered arm having an air gap between its top and bottom surfaces;
a lens at the end of each optical fiber, operable to direct the light beams to the switch or to collect light from the switch;
wherein each reflective surface is attached such that it is substantially perpendicular to the substrate; and
wherein the optical fibers are arranged around the perimeter of the substrate, such that each reflective surface is moveable into the path of one or more of the optical fibers.
2. The system of
3. The system of
4. The system of
5. The system of
6. The system of
7. The system of
8. The system of
9. A thermally operated optical switch for use in directing a beam of light to at least one receptor, said thermally operated optical switch comprising:
an array of reflective surfaces; and
a plurality of cantilever thermal actuators, each cantilever actuator having a fixed end affixed to the substrate and having a free end to which an associated reflective surface is attached such that each reflective surface has a single associated actuator, and wherein each actuator has a cantilevered arm made from a material having first thermal expansion property, each arm having an upper surface and a lower surface and having a layer of material having a second thermal expansion property on a portion of both of these surfaces;
wherein each reflective surface is attached such that it is substantially perpendicular to the substrate.
10. The thermally operated optical switch of
11. The thermally operated optical switch of
12. The thermally operated optical switch of
13. The thermally operated optical switch of
14. The thermally operated optical switch of
15. The system of
16. The system of
17. A thermally operated optical switch for use in directing a beam of light to at least one receptor, said thermally operated optical switch comprising:
an array of reflective surfaces; and
a plurality of cantilever actuators, each actuator having a pair of arms, each arm of the pair having a fixed end affixed to the substrate, and the arms of each pair having a common free end to which an associated reflective surface is rigidly attached, such that each reflective surface has a single associated actuator;
wherein each reflective surface is attached such that it is parallel to the substrate.
18. The switch of
19. The switch of
20. The switch of
21. The switch of
22. The switch of
The present invention generally relates to the switching of light beams carried by different fiber optic cables to a single light beam receptor; more particularly, the present invention relates to an optical switch that is suitable for use in spectroscopic applications.
The use of optical methods for testing, measuring, and system operations has become increasingly important. Optical methods are especially important for use in monitoring system operations, particularly when the monitoring of system operations is conducted by means such as absorption, emission, reflectance, fluorescence, or Raman spectroscopy. All of these monitoring methods require that a light beam be guided, usually by an optical fiber, to a detection device or a receptor, which, in many cases, is a spectrograph. In monitoring situations which include the simultaneous analysis of multiple samples or the analysis of points that are spatially separated, either multiple detectors, multiple receptors, or the imaging of multiple inputs on a single detector or receptor is required.
It has been found that prior art approaches for imaging multiple inputs on a single detection device or a single receptor are not always feasible. For example, if one wants to image a two-dimensional area of a sample, as is done in confocal Raman spectroscopy, typically there is only sufficient imaging space available on the detection device for one image. Hence, in order to obtain two-dimensional images from several samples, the light input source must be switched to enable use of a single spectrograph.
The potential applications for a spectroscopic grade fiber optical switch are many. For example, a single spectrograph could be shared among several investigations, reducing the expense associated with duplicating equipment, and conserving precious rack space.
In yet another potential application, a spectroscopic grade fiber optical switch could be used on a planetary lander 100 (
In yet still another potential application (e.g., Raman spectroscopy), the problem exists that inorganic minerals are better analyzed using an incident wavelength in the visible range (e.g., 532 nm), while organic materials are better analyzed in the near infrared wavelength range (e.g., 1064 nm). A fiber optic switch could be used to switch laser light beams to a single, dual wavelength, imaging spectrograph.
Prior art devices exist for routing optical signals, but these prior art devices have many limitations. One prior art method uses a mechanical fiber switch that relies on motors to physically align several optical fibers. These prior art active optical switching methods tend to be slow, bulky, and expensive. Additionally, these prior art active optical switching methods are not suitable for applications in space travel since the moving parts may cold-weld together, thereby disabling the optical switching mechanism.
Another prior art active method for routing optical signals is electro-optical switching. The devices using electro-optical switching have no moving components and provide their switching action by the application of a voltage that produces a phase-shift in a waveguide to redirect the light beam. The electro-optical switching method is fast, but has a wavelength range which is limited to only a few nanometers.
Several prior art passive optical “switching” methods are available. One example of a prior art passive switching method is a polarizing splitter. Polarizing splitters can only redirect a light beam having a specific polarization. However, the use of polarizing splitters results in the loss of information that may be contained in the polarization state of the light. Additionally, using only polarized light results in a 50% loss in intensity.
Another prior art passive switch method is a fused splitter. Fused splitters can also be used to split or combine optical signals between multiple fibers. Specifically, fused splitters are constructed by fusing and tapering two optical fibers together. Fusing and tapering two optical fibers together provides a simple, rugged, and compact method of splitting and combining optical signals. Typical excess losses in fused splitters are low, while splitting ratios are accurate to within ±5 percent at the design wavelength. Fused splitters are bidirectional and offer low backreflection. However, fused splitters suffer from some disadvantages. Specifically, signal intensity in fused splitters is split between the outputs. This splitting between the outputs results in high loss for larger port counts.
Still another type of prior art passive switching method is a multi-mode fused splitter. Multi-mode fused splitters have a somewhat limited spectral range and are mode dependent. Certain modes within one fiber in multi-mode fused splitters are transferred to the second fiber, while other modes are not. As a result, the splitting ratio in multi-mode fused splitters will depend on what modes are excited within the fiber. In comparison, single mode fused splitters only transmit one mode. Accordingly, single mode fused splitters do not suffer from mode dependency. However, single mode fused splitters are even more highly wavelength-dependent. A difference in wavelength of only 10 nm can cause a significant change in the splitting ratio.
Except for some mechanical active optical switching methods which use motors, no prior art switching technique can achieve the broad wavelength range and the low signal loss required for spectroscopic measurement (e.g., Raman spectroscopy applications.) However, optical switching methods that use motors or gears to mechanically align fibers are prone to problems in space (e.g., cold welding, stiction). Therefore, to achieve the full potential of distributed multi-spectral optical sensing, a small, inexpensive, broadband, reliable, fast, and low-loss optical switch is required.
Accordingly, there remains a need in the art for an optical switch suitable for use in fiber-optic spectroscopy which is small, inexpensive, reliable, has no moving parts (causing friction and possible cold-welding in space applications), and is able to cover a large wavelength range.
The thermally actuated spectroscopic optical switch of the present invention is small, inexpensive, reliable, has no moving parts, and is able to cover a large range of wavelengths. The design of the thermally actuated spectroscopic optical switch of the present invention has a number of advantages over other types of switches. Being a MEMS-based (MicroElectroMechanical Systems) device, the disclosed thermally actuated spectroscopic optical switch benefits from all the advantages of small size and batch fabrication. The disclosed thermally actuated spectroscopic optical switch includes an array of optical fibers and an array of movable reflective surfaces which are actuated by applying energy to the thermal actuator on which the reflective surface is mounted. The actuators do not have any physical contact with the surface (substrate) near the reflector. This absence of physical contact with the reflector eliminates stiction, wear, and cold welding problems.
The amount of deflection in thermal actuators and hence, the position of the reflector can be fully controlled since the amount of deflection is proportional to the applied current. Thermally actuated spectroscopic optical switches built according to the present invention have actuation speeds of about 100 msec or less.
The present invention also includes a method of spectroscopic switching utilizing a micromachined actuator that can select an input probe or switch between different wavelength sources.
A more complete understanding of the structure and operation of the thermally activated spectroscopic optical switch of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:
As shown in
A variety of actuation methods have been utilized for prior art optical switching using micromachined devices. Comb-drives have been used to make a 2×2 switch with bare fibers. Scratch drives have been used for 2×2 switching. Scratch drives have also been used as actuators for an array of fold-up mirrors. Also proposed was an arrayed switch using torsion mirrors. A 1×8 switch has been made using micromotors.
Bimorph thermal actuators provide a more suitable actuation method since their deflection is a linear function of the change in material temperature. As shown in FIG. 2A and
Heating of the composite beam can be accomplished by using the conductive layers as resistive heaters. By applying a specific current to the heated part of the composite beam, a certain amount of deflection can be achieved.
When a composite beam is attached to a mirror, the bending of the composite beam can either angularly orient the mirror (in the horizontal design shown in
Thermally actuated bimorphs have also been investigated for applications in fluidic valves and optical scanning. A thermally actuated cantilever beam has been used for gripping against a substrate, as a micro-tweezer. Optical scanning has been accomplished using a multilayer actuator with a reflective tip. In yet another application, a horizontal thermal cantilever has been made using a single material to drive a micromoter. However, thermally actuated cantilever beams have not found utilization in optical switches.
MEMS micromirrors have been utilized in a variety of applications, most notably in a commercial DLP projection system manufactured by Texas Instruments. The micromachined surfaces of MEMS mirrors exhibit little scattering since they are nearly atomically smooth. With a metallic coating, such as aluminum or gold, MEMS micromirrors can achieve a reflectivity of greater than 95% over a broad wavelength range.
Unlike the present invention, it has been found that all of the electrostatic MEMS devices make contact with the substrate as the actuator travels or slides along the surface of the substrate. It has also been found that such thermal actuators are either limited to a single direction of actuation, manipulate objects that slide along the surface of the substrate, or simply were not used for switching between optical fibers. Also, no thermal actuators used with MEMS devices have been designed for use other than in room temperature environments.
As shown in
The thermally actuated spectroscopic optical switch 10 of the present invention comprises a single chip linear array of micro mirrors 25 positioned on a thermally actuated beam 40. The thermally actuated beam 40, shown in FIG. 4A and
In its simplest version, and as shown in
As shown in
The thermal cantilever actuator 40 used in the present invention has a number of advantages over other types of actuators. Specifically, the thermal cantilever actuator 40 has no friction since it makes no physical contact with any surface. Likewise, the thermal cantilever actuator 40 will not be prone to any stiction or surface bonding that may occur in cold environments. The deflection of the thermal cantilever actuator 40 is fully controllable, unlike electrostatic actuators, since electrostatic actuators have only a small stable range before reaching their pull-in voltage. The thermal cantilever actuator 40 tends to be moderately fast operating, with an actuation speed on the order of 10-100 msec. In addition, a large deflection can be achieved. Specifically, it has been found that deflections on the order of 20% of the cantilever length can be obtained. Such large deflections correspond to an optical reflection angle of <20°.
The typical power consumption of a thermal cantilever actuator such as the one described herein is on the order of ten to one-hundred milliwatts. Finally, the thermal cantilever actuator 40 has no moving parts that can generate wear or breakage. Since the thermal cantilever actuator 40 can be formed using micromachining techniques, the use of a thermal cantilever actuator 40 obtains all the advantages of very compact size and the repeatability of fabrication needed to produce an operative array of movable mirrors for reflecting a beam of light.
In the horizontal mirror system 110 shown in
The second embodiment 210 shown in FIGS. 5A and
The horizontal mirror embodiment shown in FIGS. 4A and
Since the thermally actuated spectroscopic optical switch of the present invention must operate in a wide range of thermal environments, it has been designed so as to minimize its response to changes in ambient temperature. Minimization of response to changes in ambient temperature can be accomplished for both embodiments 110, 210 by creating a thermally balanced actuator.
As shown in
The embodiment 400 shown in
Additional optical elements must be added to the thermally actuated optical switch of the present invention to prevent large losses. The light beam output of a fiber diverges rapidly, so, as shown in
The behavior of a simple bimorph actuator as shown in
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the inventions, will become apparent to persons skilled in the art upon the reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the scope of the invention.
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