Thermally controllable optical devices and system

Abstract: A system for the thermal control of optical devices in­ cludes an optical device having at least one coating of an elec­ trically conductive material formed on a surface in a heat con­ ducting relationship with the material and connected to a con­ trollable current source which passes varying amounts of elec­ trical current through the layer to vary the amount of thermal energy introduced into the device to control its optical re­ sponse characteristics. The electrically conductive layer or lay­ ers can be optically active, that is, function to affect the light energy transmitted into or through the device as well as acting as an electrically conductive thermal element. The use of an electrically conductive coating or coatings allows the rapid and precise control of thermal energy introduced into the device and, accordingly, the rapid and precise control of the optical characteristics of the device as well as providing a thermal in­ terface between the optical device and the ambient environ­ ment that is effective to minimize the adverse affects of tran­ sient temperature changes occurring in the environment.

Description: BACKGROUND OF THE INVENTION The present invention relates to methods and apparatus for controlling the operating characteristics of optical devices. More particularly, it concerns the thermal control and thermal stabilization of optical elements and devices used in optical systems. It is known that the operating and environmental temperature affect various functional characteristics of an optical device including the index of refraction of the particular optical material from which the device is made and the physical dimensions of the device. In an optical resonator, for example, where the resonant frequency is dependent on the physical length of an optical path, a change in resonator temperature will alter the optical path length and, thus, the frequency of the resonator. Unpredictable changes in temperature are undesirable and will usually diminish device or system performance. On the other hand, temperature can be used effectively to control an optical device or system to provide a desired functional result. An effective use of temperature variation is that of tuning an optical resonator to a specific resonator frequency. A Fabry Perot etalon, for example, providing a resonant structure defined by spaced parallel reflective surfaces with an intermediate transmissive medium, can be tuned by controlling the operating temperature of the etalon. A change in temperature increases or decreases the index of refraction of the transmission etalon medium as well as the optical path length of the device between the reflectors. Thus, the resonant response of the etalon can be varied to support one of a plurality of possible resonant modes by a thermal tuning adjustment. An optical device can be controlled or tuned in the aforementioned manner by placement in a thermally controlled chamber, such as an oven, and controlling the interior temperature of the oven to effect the desired control. While precise control of the chamber temperature is possible, the time required to attain a differential in temperature is longer than desirable for the responsiveness of the optical device in many applications. In addition, the physical constraints of an oven-like chamber or similar enclosure can limit the utility of the optical device or system. In addition to ovens, discrete electrical heater elements or heater assemblies have been used for controlling optical devices. Discrete heater elements have included electrically heated metal plates which are placed in contact with the optical device and heater assemblies have included electrical conductors embedded in flexible or semi-flexible plastics or similar materials which are wrapped about or adhesively secured to the optical device. Discrete heaters, by their nature, involve the need to control the temperature of relatively large masses that are spaced a predetermined distance from the optical path so that rapid response to temperature fluctuations is not possible. With discrete heaters, uniform and rapid changes in the characteristics of the optical device are dependent upon the value and uniformity of the surface to surface thermal conductance. Since it can be difficult to achieve a uniform thermal conductance, discrete heaters can cause undesirable non-uniform heating. As can be appreciated from a consideration of the above, a need exists for the efficient and reliable thermal control of optical devices in such a way that their optical characteristics can be rapidly controlled in a repeatable and predictable manner. US-A-4 119 842 discloses a twisted nematic liquid crystal device containing liquid crystal material having dielectric anisotropy. The light transmission of the device is affected by low temperatures and to ensure proper function also with low temperatures a heating device is applied on the surface thereof. There is however no optical device with a resonance cavity which is to be controlled in respect to resonance frequency. The present invention relates to a thermally controllable optical device in accordance with the preamble of claim 1. A device like this is known from DE-A-33 11 808 comprising an analysator of the Fabry-Perot-Resonator-type having a single mode fibre with highly reflective metallic mirrors at both ends thereof. The single mode fibre is coated on the outer surface with an metallic coating which is electrically heatable for increasing the temperatur of the fibre in order to change the resonating wavelenght. The object of the invention is to simplify the structure of the optical device to ensure a more accurate and even heating to optimize the control function. This object is achieved by the characterizing portion of claim 1 as far as the device is concerned and in accordance with the combined features of claim 14 as far as the method is concerned. The layers comprise one or two coatings and defining the resonance cavity perform the double function also for heating the elements. SUMMARY OF THE INVENTION In view of the above, the present invention provides for the thermal control of optical elements in which the control is repeatable, precise, and relatively rapid compared to prior control arrangements. The optical elements are provided with a coating of a conductive material that is in intimate thermal contact with a surface or surfaces of the optical device. A controllable electric current his passed through the coating to develop thermal energy that is conducted, with a minimal time delay, into the optical material to effect the desired change in the optical characteristics of the device. The conductive coating can also function as an optically active surface, that is, as a reflector or light transmissive coating. In accordance with one aspect of the present invention, the conductive coating or coatings are preferably positioned to function as a thermal boundary, that is, a thermally active interface between the optical device and the ambient environment which acts as a thermal buffer to minimize the adverse affects of thermal transients occurring in the ambient environment. In the preferred embodiment of the present invention, a Fabry Perot etalon is formed with electrically conductive partially transmitting and partially reflecting coatings on each end face so that light of a selected wavelength will be transmitted through the etalon. The electrically conductive end face reflectors are connected in circuit with a source of electrical energy to pass current through the electrically conductive reflectors to develop thermal energy which is rapidly and uniformly conducted into the optically transmissive material of the etalon. The operating temperature of the device is accordingly changed to effect a corresponding change in the optical response characteristics of the etalon. The use of a controlled electrical current flowing through a coating in uniform and intimate contact with the optical path of an optical device, including coatings that are optically active in the operation of the device, allows for the relatively rapid introduction of controllable amounts of thermal energy into the optical path within the device to provide rapid and precise control of the optical characteristics of the device. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings, in which like parts are designated by like reference characters. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an isometric projection, in schematic block form, of an optical device control system in accordance with the present invention; Fig. 2 is a schematic block diagram of an alternate embodiment of an optical device control system in accordance with the present invention; Fig. 3 is a front elevational view of an illustrative optical device in accordance with the present invention; Fig. 4 is a side elevational view, in cross section, of the optical device illustrated in Fig. 3; Fig. 5 is a side view, in cross section, of a variation of the optical device illustrated in Fig. 3; and Fig. 6 is a partial detail view, in cross section, of the device of Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENT A system for controlling optical devices in accordance with the present invention is illustrated in schematic block form in Fig. 1 and designated generally therein by the reference character 10. As shown, the system 10 includes a thermally controllable optical device 12, in the form of a Fabry Perot etalon as described more fully below, a preferably monochromatic light source 14 for projecting light along an axis 16 into and through the optical device 12, and a photosensor 18 aligned along the axis 16 to sense one or more characteristics of the light energy passed through the optical device 12. The photosensor 18 provides an electrical output along a circuit path 20 to an amplifier 22 which, in turn, provides its output along a circuit path 24 to a controllable current driver 26. The thermally controllable optical device 12 is connected to the controllable current source 26 through circuit paths 28 and 30 so that electrical current presented from the controllable current driver 26 through the circuit paths 28 and 30 causes an increase or decrease in thermal energy presented to the optical device 12 to change one or more of the device characteristics. The illustrated optical device 12 is in the form of a Fabry Perot resonant cavity or etalon and is illustrated in front elevational view in Fig. 3 and in side elevational cross section in Fig. 4. The optical device 12 is formed generally as a cylindrical body 32 from an optical material having a diameter dimension "D" and a length dimension "L". In the case of the preferred embodiment, the body 32 is formed from an optical glass having an index of refraction of approximately 1.5, a diameter dimension "D" of approximately 18 mm, and a length dimension "L" of approximately 12 mm. The end faces (unnumbered) are flat and sufficiently parallel with one another to perform the desired function. A coating 34 is applied to one end face and is fabricated from an electrically conductive material designed to have partially light transmitting and partially light reflecting properties. A similar coating 36 is formed on the other end face. In the preferred embodiment, the coatings 34 and 36 are formed from gold that is deposited, for example, by vapor deposition, sputtering, electrochemical deposition, or a functionally equivalent process, onto the two end faces and formed with a coating thickness that provides the desired transmissive/reflective characteristics. In the case of the illustrated embodiment, a 500 Angstrom thick electrically conductive gold coating provides 1.5% transmission efficiency at 800 nm. As can be appreciated, different coating thicknesses, generally between 50 and 1000 Angstroms, provide correspondingly lower or higher transmission values. Additionally, other materials, such as metals or metal oxides including silver, copper, aluminum, indium oxide, indium-tin oxide, or the like, can be used separately or in combination to form the coatings 34 and 36 to provide the desired transmission or reflective characteristics. The use of indium tin oxide, for example, can provide preferred low resistivity (e.g., 10 ohms/square) electrically conductive coatings 34 and 36 with transmission efficiencies of 80-90% and can be placed over low optical loss multi-layer dielectric coatings to provide high reflectivity and low optical loss. The coatings 34 and 36, because of their electrical conductivity and in accordance with the present invention, function as thin film heater elements and also cooperate with the body 32 to define a resonant cavity structure. As shown in Figs. 3 and 4, conductive bridge contacts 38 and 40 are formed on diametrically opposite portions of the peripheral surfaces of the body 32. The bridge contacts 38 and 40 can be formed from a conductive epoxy, for example, and can each subtend a peripheral area, such as 70 degrees, which is sufficient to effect good electrical contact with edge portions of each coating 34 and 36 to thus place the coatings 34 and 36 in parallel circuit. The conductive paths 28 and 30, described above, are connected, respectively, to the conductive bridges 38 and 40 using suitable connection techniques. In the preferred embodiment, each of the coatings 34 and 36 has an ohmic resistance, when measured across diametrically opposed points, of approximately 10 ohms. Accordingly, when the coatings 34 and 36 are placed in parallel circuit, as described above, the composite resistance is approximately 5 ohms. If desired and as explained below in relation to Fig. 9, the conductive coatings 34 and 36 can be placed in series circuit with the controllable current driver 26. The photo detector 18 is designed to be amplitude responsive, that is, to provide an output that indicates when the amplitude of the light along the axis 16 introduced into the photo detector 18 at a selected frequency corresponds to a desired amplitude and also provides an indication as to when the amplitude of the light entering the photo detector 18 is above or below the desired frequency value. If desired, a narrow bandwidth optical filter or filters (not shown) can be provided as part of the photo detector 18 arrangement to achieve the desired frequency responsive characteristics. The amplifier 22 may take the form of a differential amplifier connected between positive and negative power sources to provide a settable output relative to a reference input REF when the output of the photo detector 18 indicates that the output of the light passed through the optical device 12 to the photo detector 18 is at a desired amplitude, a lesser value when the amplitude of the light energy is less than that desired, and a greater value when the amplitude of the light is higher than the desired value. For example and as illustrated by the graph in Fig. 1, the amplifier 22 can provide a control CTRL output between zero and +15 volts with a 7.5 volt output representing the desired equilibrium or set point at which the amplitude of the light is one-half the maximum value, this set point representing a stable reference throughput value that occurs at half the amplitude of the maximum spectral transmission at resonance. The current driver 26 responds to the output of the amplifier 22 to decrease or increase the current passed through the conductive coatings 34 and 36 to decrease or increase the thermal energy developed and the temperature of the optical device 12 to change the optical properties in such a way to thermally control the functional characteristics of the optical device 12. The optical device 12 operates as a conventional Fabry Perot etalon with light energy directed along the axis 16 entering the optical device 12 through an entry coating, for example, the coating 34, passing through the optical material of the optical body 32, and exiting the optical device 12 through an exit coating, namely, the coating 36. Since the coatings 34 and 36 are partially reflecting, a portion of the light energy incident upon the exit coating 36 will be reflected internally back toward the entry coating 34 to define forward and reverse direction wave patterns in the optical body between the coatings 34 and 36. At selected frequencies, that is, frequencies corresponding to one of the plurality of resonant modes supported within the cavity, the forward and reverse direction light wave patterns will constructively interfere to increase the amplitude of the light in the cavity and that of the light passed to the photo detector 18 while decreasing the amount of light reflected back toward the source 14. The system 10 of Fig. 1 operates to control the thermal characteristics of the optical device 12 by varying the current passed through the parallel connected coatings 34 and 36 to frequency lock one of the device's resonant modes to the wavelength of the light source 14 to thus function as an optical filter for passing light at the frequency of the desired spectral line and attenuating light at higher and lower frequencies. More specifically, the current driver 26 is designed to pass current through the parallel connected coatings 34 and 36 between minimum and maximum values, for example and as indicated by the graph in Fig. 1, between zero and five Ampères with an intermediate level current, for example, 2.5 Ampères, passed through the coatings 34 and 36 when the system is operating at the desired equalibrium point described above. The intermediate current level is selected to provide enough thermal energy to raise the temperature of the optical device 12 to an equilibrium temperature that is above the local ambient so that the temperature of the device can be lowered by lowering the thermal energy introduced into the device or increased by increasing the thermal energy introduced into the optical device by appropriate control of the current driver 26. Where the photo detector 18 indicates that the amplitude of the light energy exiting the optical device 12 at the desired frequency is less than the set point, the amplifier 22 provides a control signal CTRL to the controllable current driver 26 to increase the current and the thermal energy provided to the optical device 12 to thus lower the operating temperature. As a result of the lower operating temperature, the index of refraction of the optical material is changed and the effective path length is likewise changed, that is, shortened, so that the frequency of the selected resonant mode continues to be passed to the photo detector 18. Conversely, where the photo detector 18 indicates that the amplitude of the light energy exiting the optical device 12 is greater than a desired level, the amplifier 22 provides a control signal CTRL to the controllable current driver 26 to decrease the current and the thermal energy provided to the optical device 12 to thus decrease its operating temperature to again tune the frequency of a resonant mode to provide a desired photodetector 18 output. As can be appreciated, the system of Fig. 1 thus controls the operating characteristics of the optical device 12 in such a way to maintain the desired optical transmission function. The control elements, namely, the photodetector 18, the amplifier 22, and the controllable current source 26 are preferably configured to provide a critically damped or overdamped system to minimize overshoot. Such control may be achieved, for example, by combining an integrator function with the amplifier 22. The system 10 of Fig. 1 has been shown as a closed loop feedback system responsive to an optical characteristic of the light passed through the optical device 12. If desired, open loop control can be provided by interrupting the circuit path 24 and providing an input 24' to the current driver 26 for effecting direct thermal control of the optical device 12. The use of conductive coatings 34 and 36 has a number of functional advantages. Since the conductive coatings 34 and 36 are in uniform and intimate physical contact with the material which defines the optical body 32, thermal energy is directly conducted into the optical path portion of the optical material with a minimal time decay to cause uniform heating and to allow accurate and repeatable thermal control of the optical device 12. Since the conductive coatings 34 and 36 are positioned to define a boundary or interface between the optical material and the ambient environment, changes in the local environment, such as undesired air currents, have minimal effects since they are instantaneously buffered through the thermally active coatings 34 and 36. In the embodiment of Fig. 1, a characteristic of the light energy passed through the conductive coatings 34 and 36 is used to effect control. Other arrangements, such as that illustrated in Fig. 2, in which like parts are designed by like reference characters, are also suitable. In Fig. 2, a thermally responsive device 50, such as a thermocouple or thermistor, is placed in thermal contact with the optical device 12 so as to sense the operating temperature of the device. The output of the thermally responsive device 50, be it a voltage in response to the operation of a thermocouple or a resistance value in response to the operation of a thermister, is provided along circuit path 52 to an evaluation circuit 54, such as a conventional Wheatstone bridge arrangement, with the output of the evaluation circuit 54 indicative of the operating temperature of the optical device 12 relative to a reference value. The output of the evaluation circuit 54 is provided to an amplifier 22 for controlling the controllable current driver 26 in the manner described above. In the embodiment of Fig. 1, the electrically conductive coatings 34 and 36 are also optically active, that is, they affect a characteristic of the optical energy presented through the optical device 12. In some applications, it may be desirable or necessary to use optically active coatings that are electrically insulating in order to achieve the desired optical effect. For example and as shown in Figs. 5 and 6, an optical device 100 can include an optical body 102 having partially transmitting and partially reflecting coatings 104 and 106 that are also electrically nonconductive. Materials which are electrically nonconducting and which exhibit certain desirable optical functions include various types of dielectrics such as silicon dioxide and silicon monoxide. If desired, the coatings 104 and 106 can be fabricated from a plurality of sublayers. The optical device 100 includes separate relatively thin end plates 108 and 110, preferably having a low thermal capacity, to which electrically conductive coatings 112 and 114 are applied as described above. Where the optical function is critical, the electrically conductive coatings 112 and 114 can be fabricated from a material that is relatively transparent to the light transmitted into or through the optical device 100, such materials including indium oxide, indium tin oxide, and related alloys. Additionally, openings or "windows" (not shown) can be provided in the electrically conductive coatings 112 and 114 through which light energy is passed. The end plates 108 and 110 are cemented to the body 102 with the coatings 112 and 114 in engagement, respectively, with the electrically nonconductive coatings 104 and 106 or, in the alternative, with the uncoated faces of the end plates 108 and 110 cemented to the electrically nonconductive coatings 104 and 106. Bridge contacts, as represented by the bridge contact 116 shown in FIG. 6, can be applied to a peripheral surface portion of the optical device 100, in the manner described above, to place the electrically conductive coatings 112 and 114 in parallel circuit. If desired, and as illustrated in schematic fashion in Fig. 5 by the dotted line circuit paths 118, 120, and 122, the electrically conductive coatings 112 and 114 can be placed in series circuit. The operation of the optical device 100 illustrated in Figs. 5 and 6 and as described above, is similar to that described above for the optical devices of Figs. 1 and 2 in that varying amounts of electrical current can be passed through the electrically conductive coatings 112 and 114 to introduce varying levels of thermal energy into the optical device 100 to control device temperature. The electrically conductive coatings 112 and 114, however, are optically transparent to the light controlled by the device 100 and accordingly are optically inactive. As can be appreciated from a consideration of the above, optical devices and systems fabricated in accordance with the present invention allow precise and rapid control of the optical devices without the need for the control of environmental temperatures, for example, through the use of ovens or other enclosed chambers. Additionally, the use of electrically conductive coatings in uniform and intimate contact with the material of the optical device allows uniform and relatively rapid changes in the operating temperature with minimal time delays and also function as a thermal barrier or buffer between the optical device and the ambient environment to minimize the adverse affect of transient thermal conditions, such as undesired convection currents, on the optical device. Thus, it will be appreciated from the above that as a result of the present invention, highly effective thermally controllable optical devices and systems are provided by which the principal objective, above others, is completely fulfilled.

Claims: A thermally controllable optical device (12; 100) comprising: a body (32; 102) of light transmitting material formed with first and second substantially parallel faces, each of said faces having an electrically conductive layer (34, 36; 112, 114, 104, 106) with light transmitting and reflecting properties applied thereto such that said body (32; 102) forms an optical resonant cavity structure for a light beam transmitted through said body in a direction substantially perpendicular to said faces, wherein the material of the body (32; 102) is such that the resonant characteristics of the device vary with the temperature, means (26, 28, 30; 118, 120, 122) comprising an electrical current circuit to develop thermal energy to said body (32; 102) sufficient to vary its resonant characteristics, characterized in that said electrical current circuit (26, 28, 30; 118, 120, 122) is connected to said conductive layers (34, 36; 112, 114, 104, 106) such that the electrical current is passed through said layers (34, 36; 112, 114, 104, 106) and such that said thermal energy is developed in said body by the current passing through said layers (34, 36; 112, 114, 104, 106). The thermally controllable optical device of claim 1, wherein said layers comprise each one single layer (34, 36). (Fig 1-4) The thermally controllable optical device of claim 1, wherein said layers comprise each one transmitting and reflecting insulating coating (104, 106) applied to said faces and each one electrically conductive coating (112, 114) applied to the former coating (104, 106). (Fig 5, 6) The thermally controllable optical device of claim 1, wherein said layer (34, 36) is gold. The thermally controllable optical device of claim 4, wherein said layer (34, 36) has a thickness less than 500 Angstroms. The thermally controllable optical device of claim 1, wherein said layer (36, 34) is indium tin oxide. The thermally controllable optical device of claim 1, wherein said layer (34, 36) is an indium containing material. The thermally controllable optical device of claim 1, wherein said body (32; 102) is formed from a light transmitting material having an index of refraction which varies as a function of temperature. The thermally controllable optical device of claim 1, wherein said circuit connects said one layer (34) and said other layer (36) in parallel circuit. The thermally controllable optical device of claim 1, wherein said circuit connects said one layer (34) and said other layer (36) in series circuit. The thermally controllable optical device of claims 1 - 10 comprising: means (18; 50) for sensing at least one parameter associated with the operation of said body (32); and means responsive to said sensing means and electrically connected to said layers for passing an electrical current through said layers to develop thermal energy sufficient to change the light affecting property of the material of said body and the sensed parameter. The thermally controllable optical device of claim 11, wherein said sensed parameter is light energy affected by said light affecting property of said material of said body. The controllable optical system of claim 11, wherein said parameter is the temperature of said body. A method of thermally controlling an optical device comprising the steps of: forming the thermally controllable optical device according to claim 1; sensing at least one parameter associated with the operation of the body; and passing an electrical current through said layers to develop thermal energy sufficient to change the light affecting property of the material in response to the sensed parameter. The method of claim 14, wherein the sensed parameter is the light energy affected by the light affecting property of the material. The method of claim 14, wherein the sensed parameter is the temperature of the material.

Inventors: DYES WILLIAM A, MCMAHON DONALD H, DYES, WILLIAM A., MCMAHON, DONALD H.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1987
Main classification: G02B7
IPC codes: G02F1, G02B7