PIEZOELECTRIC DIFRACTION GRATING LIGHT STEERING DEVICE

Abstract: A piezoelectric diffraction grating steering device including a piezoelectric substrate having an attached or integrally formed diffraction grating. When an electric field is applied to the piezoelectric substrate, the piezoelectric actuator stretches the grating, changing the periodicity of the grating, thereby changing the angle of diffraction of the grating. The preferred grating is a high efficiency reflection type grating which diffracts a maximum amount of light into the first order mode, minimizing the amount of light in the zero order and higher order modes. The invention may be used in any application in which fine control over the angle of diffraction of a light beam is required, and may have an adjustment rate in excess of 100 KHz. One application is as a beam steering element in a free-space laser communication system.

Description: PIEZOELECTRIC DIFFRACTION GRATING LIGHT STEERING DEVICE BACKGROUND 1. Technical Field This invention relates to light steering devices, and more particularly to a piezoelectric diffraction grating steering device. 2. Backgro und Information Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. When used for space-based, air-to-air or air-to-ground communications, such systems pose a number of challenging problems. One such problem is fine-steering of a received beacon light beam that may emanate from a remote transceiver situated kilometers away. For example, FIG. 1 is a block diagram showing a pair of free-space atmospheric laser communication transceivers in accordance with the prior art. An "A" transceiver includes a control system 100A, a laser transmitter 102A, and a receiver 104A. A "B" transceiver includes a control system 100B, a laser transmitter 102B, and a receiver 104B. Transceiver A sends a data stream 106A to transceiver B, while transceiver B sends a data stream 106B to transceiver A. The implementation of the respective control systems 100 A, 100B, transmitters 102 A, 102B, and receivers 104A, 104B is conventional. One or both of the transceivers A, B may be mounted on a moving vehicle, such as an airplane. Conventionally, the receivers 104A, 104B have used a combination of a pointing system, such as an azimuth/elevation ball turret structure, and a fine steering system, such as a gimbal, to keep a received beacon laser beam properly targeted on a sensor system. By so doing, the lasers for the transmitters 102A, 102B will be aimed at their respective remote transceivers. FIG. 2 is a block diagram of an improved steering system 200, described in greater detail in co-pending patent application Serial No. , entitled "Six-Axis Gimbal Mount for a Free-Space Laser Communication System", filed - -1998, and assigned to the assignee of the present invention. In this system, an incoming light beam 202 from a beacon laser of a remote transceiver is received through an optical telescope (not shown) and reflected off of a tiltable steering mirror 204 mounted on piezoelectric "push- pull" actuators 206 that can control the azimuth and elevation of the mirror 204. The reflected light is focused onto a sensor array (e.g., a CCD array) 208, the output of which is coupled to a centroider circuit 210 which determines the position of the focused light beam 200 on the sensor array 208. Such position information is compared by a processor 212 to a calibrated reference signal 214, and steering correction signals are passed through an amplifier 216 to appropriate ones of the piezoelectric actuators 206. When the steering mirror 204 is properly aimed, then output from one or more transmission lasers 218 may be reflected off of the steering mirror 204 with assurance that the resulting light beams 220 will be aimed at the remote transceiver. While the steering system 200 works well with most applications, the mass of the mirror element of the steering mirror 204 may be relatively high, thus limiting the adjustment rate of the steering mirror 204. In particular, the fine steering system must be able to steer the communication laser beam at angular rates greater than the highest rate of disturbances that are present on the system's mounting platform; such disturbances are often referred to as "base motion". In general, base motion has a spectrum of frequency components which cause the communication laser beam to be mispointed from an intended target, which is the source of a received beacon beam. The beam steering system must be able to sense the base motion disturbances and provide error correction sufficient to keep the footprint of the communica- tion laser beam positioned on the target. If the base motion has frequency components which produce angular motion disturbances greater than can be corrected, the communication laser beam will be displaced from the location of the beacon source. The base motion frequency that the beam steering device can respond to is limited by both the sensor and the beam steering device. The beam steering device frequency response is generally limited by mechanical parameters of the mirror and actuators that are being moved to steer the beam. The inertia of the mirror and the spring constant of the "push-pull" piezoelectric actuator form a resonant mechanical system for which operation at or near the resonance is generally the bandwidth limit of the system. Reducing the inertia of the mirror and increasing the stiffness of the piezoelectric actuator will increase its resonant frequency and extend the bandwidth of the system. However, for laser communications involving very high levels of base motion, the system bandwidth is ultimately challenged to be greater than can be achieved with the lightest inertia mirrors and stiffest piezoelectric actuators. Accordingly, the inventors have determined that it would be useful to have a fine steering element that was less massive than a piezoelectric-actuator driven mirror and which has a higher adjustment bandwidth. The present invention provides such a device. SUMMARY The invention comprises a piezoelectric diffraction grating steering device including a piezoelectric substrate having an attached or integrally formed diffraction grating. When an electric field is applied to the piezoelectric substrate, the piezoelectric actuator stretches the grating, changing the periodicity of the grating, thereby changing the angle of diffraction of the grating. The preferred grating is a high efficiency reflection type grating which diffracts a maximum amount of light into the first order mode, minimizing the amount of light in the zero order and higher order modes. The invention may be used in any application in which fine control over the angle of diffraction of a light beam is required, and may have an adjustment rate in excess of 100 KHz. One application is as a beam steering element in a free-space laser communication system. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. DESCRIPTION OF DRAWINGS FIG. 1 is a block diagram showing a pair of free-space atmospheric laser communication transceivers in accordance with the prior art. FIG. 2 is a block diagram of a tillable mirror based steering system. FIG. 3 A is a block diagram of a fine steering system in accordance with the invention, shown in a first steering state. FIG. 3B is a block diagram of a fine steering system in accordance with the invention, shown in a second steering state. FIG. 4 is a diagram of a diffraction grating surface, showing the blaze angle. Like reference numbers and designations in the various drawings indicate like elements. DETAILED DESCRIPTION The invention includes a piezoelectric substrate having an attached or integrally formed diffraction grating. When an electric field is applied to the piezoelectric substrate, the spacing of the diffraction grating varies, thus changing the angle of diffraction of the grating. The diffraction grating is most preferably of the reflection type, but may be of the refractive type. The invention may be used in any application in which fine control over the angle of diffraction of a light beam is required, and may have an adjustment rate in excess of 100 KHz. One application is as a beam steering element in a free-space laser communication system. FIG. 3 A is a block diagram of a fine steering system in accordance with the invention, shown in a first steering state. Shown is an actuator 300 having a length D which includes a material 301 (e.g., a quartz crystal) that exhibits a piezoelectric effect. That is, when a suitable voltage is applied to the actuator 300, the length of the actuator 300 changes. The actuator 300 may include a frame or other supporting structure 302 as desired. The actuator 300 is configured to be coupled to a voltage source 304. In a preferred embodiment, a reflective diffraction grating 303 is integrally formed on, or is attached to, one surface of the actuator 300, or is mechanically affixed to the actuator 300 such that the diffraction grating 303 will stretch when the length of the actuator 300 increases. For example, finely-ruled grooves in excess of 3,000 grooves/cm may be cut in one surface of a piezoelectric substrate using, for example, a diamond tool. As an alternative example, a separately formed diffraction grating comprising molded plastic film or a sheet material may be glued or otherwise affixed to one surface of the actuator 300. Alternatively, a diffraction grating may be made using a replicating process, and then mechanically attached to a supporting structure 302 of the actuator 300, as shown in FIGS. 3A and 3B. As is known, application of a suitable electric field (e.g., 100-1000 V) to a piezoelectric material induces the material to expand or contract in certain directions. The diffraction grating 303 is preferably situated so that the diffraction grooves are perpendicular to the axis of greatest elongation and contraction so that the spacing between the centers of such grooves will change as the actuator 300 expands or contracts. The spacing of the grooves of the diffraction grating 303 equals d. An incident light beam 306 is reflected and diffracted by the diffraction grating 303 as a steered light beam 308 at an initial angle θ. FIG. 3B is a block diagram of a fine steering system in accordance with the invention, shown in a second steering state in which an electric field ΔV has been applied to the actuator 300 so as to cause an overall expansion by an amount ΔD, resulting in an increase in the spacing between grooves of the diffraction grating 303. Because the distance between diffraction grating grooves has changed (increased in this instance), the amount of diffraction of the incident light beam 306 will also change, thus changing the angle θ of the steered light beam 308' by an amount Δθ. A similar effect will occur if the actuator 300 contracts (i.e., AD is negative). Alternatively, if the angle θ of the steered light beam 308 is regarded as fixed relative to, for example, a sensor array, a change in the spacing between grooves of the diffraction grating 303 may be regarded as accommodating a change in the angle of the incident light beam 306. By basic physics, diffraction images will be generated according to the formula nλ =ds ' θ , where n is an integer and λ is the wavelength of light. For monochromatic light from a laser, λ may be taken as a constant. With λ as a constant and assuming that n - 1 , which corresponds to the first order (and hence brightest) diffraction image, the angle θ is dependent only on d. The change in diffraction angle, Δθ, as a result of a change in groove spacing, Ad, can be determined by the following relationships: 0 = Δ dsinθ + dcosθ Δθ Eq. (1) Ad Ad Δθ * -tanθ Eq. (2) For a piezoelectric multilayer actuator, Ad/dis typically about 10' 3 . With an angle of 45 ° for the first order Littrow diffraction angle, this value would provide approximately ±1 milliradian deflection angle for the deflected beam. For optimum performance for this application, the diffraction grating design must be carefully specified. The fraction of light that is diffracted from the incident beam into the desired deflection angle must be high for the device to replace a highly reflective conven- tional steering mirror (for which typically ≥98% of the incident light is reflected). By selecting the design of the diffraction grating, the reflection efficiency (ratio of light in the desired diffraction angle to the incident light) can be made essentially equal to that of a conventional steering mirror. One important parameter of choice for the diffraction grating is the blaze angle, Θ B shown in FIG. 4. Diffraction gratings are available in a range of blaze angles: very low (1 °- 5°), low (5°-10°), medium (10°-18°), high (22°-38°) and very high (38°-76°). The selection of blaze angle will set the first order Littrow diffraction angle for which high efficiency diffraction will occur. The efficiency will also be seen to vary dramatically for the type of polarization of the incident light. Selection of the blaze angle is important if a high efficiency grating is desired for both s and polarizations. For example, a blaze angle of 26°, 45' will produce an efficiency of greater than 90% for both the s andp polarizations with the first order Littrow diffraction angle at approximately 20°. Using these same parameters for the piezoelectric actuator 300, this would provide a steering angle of about ±0.364 milliradians. If instead a blaze angle of 46°, 4' were selected, a large range for the first order Littrow diffraction angle could be used with an efficiency near 100%. Selecting a 45° first order Littrow diffraction angle would provide a steering angle of about ±1 milliradians for the piezoelectric actuator 300. However, this will work only for one polarization, the s polarization, at high efficiency. Stretching of the grating can readily be achieved with the same piezoelectric actuators used for deflecting the mirror, but now the system inertia is much less since the motion of the diffraction grating is linear and not rotational, and the mass of the diffraction grating is much less than the mirror. Thus, the resonant frequency of the system can be much higher, thereby giving a larger steering angle bandwidth. A number of piezoelectric materials have a resonant frequency of more than 100 KHz, making for a very fast steering device. In use, a piezoelectric diffraction grating steering device of the type shown in FIGS. 3A and 3B may be substituted for the steering mirror 204 in FIG. 2. The overall system would be calibrated so that incoming light beam 202 from a beacon laser is diffracted at appropriate diffraction angles θ to the sensor array 208 based on closed-loop feedback through the centroider 210, reference 214, processor 212, and any necessary amplifiers 216 to the actuator 300. The diffraction angle of the piezoelectric diffraction grating steering device will then be set to diffract the output from one or more transmission lasers 218 with assurance that the resulting light beams 220 will be aimed at the remote transceiver. A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, a similar piezoelectric diffraction grating steering device may be made from a refractive diffraction grating, since the actuator 300 may be of a transparent material. However, absorption losses of the incoming light may make such a device suitable only for certain applications. Accordingly, other embodiments are within the scope of the following claims.

Claims: WHAT IS CLAIMED IS: L A piezoelectric diffraction grating steering device comprising: (a) a piezoelectric substrate configured to be coupled to an electrical source so as to change dimension upon application of an electric field; and (b) a diffraction grating on a surface of the piezoelectric substrate, the diffraction grating having a plurality of diffraction grooves spaced by an initial distance d; wherein the distance d varies by an amount Ad with dimension changes of the piezoelec- trie substrate, and light incident on the diffraction grating is diffracted at an angle dependant on d and Ad. 2. The device of claim 1, wherein the diffraction grating is reflective. 3. The device of claim 1 , wherein the diffraction grating is refractive. 4. The device of claim 1 , wherein the diffraction grating is integrally formed in the piezoelectric substrate. 5. The device of claim 1 , wherein the diffraction grating is separately formed and attached to the piezoelectric substrate. 6. The device of claim 1, further including a feedback system coupled to the piezoelectric substrate and including a sensing device, for varying the distance d in order to maintain a diffraction image on the sensing device in response to changes in angle of the incident light. 7. A free-space laser communication system steering system comprising: (a) piezoelectric diffraction grating steering device comprising: (1) a piezoelectric substrate configured to be coupled to an electrical source so as to change dimension upon application of an electric field; and (2) a diffraction grating on a surface of the piezoelectric substrate, the diffraction grating having a plurality of diffraction grooves spaced by a distance d; wherein the distance d varies by an amount Ad with dimension changes of the piezoelectric substrate; (b) a sensing array for receiving an incident beacon beam from a remote transceiver diffracted by the piezoelectric diffraction grating steering device; (c) a feedback system, coupled to the piezoelectric substrate and the sensing array, for causing a change in the distance d in order to maintain a diffraction image on the sensing array in response to changes in angle of the incident beacon beam. 8. The device of claim 7, wherein the diffraction grating is reflective. 9. The device of claim 7, wherein the diffraction grating is refractive. 10. The device of claim 7, wherein the diffraction grating is integrally formed in the piezoelectric substrate. 11. The device of claim 7, wherein the diffraction grating is separately formed and attached to the piezoelectric substrate. 12. The device of claim 7, further including at least one transmitting laser positioned to diffract a laser beam off of the diffraction grating to the remote transceiver. 13. A method of steering an incident light beam, comprising the steps of: (a) forming a diffraction grating on a surface of a piezoelectric substrate, the diffraction grating having a plurality of diffraction grooves spaced by a distance d; (b) applying a controlled electric field to the piezoelectric substrate so as to vary the distance d by an amount Ad such that light incident on the diffraction grating is diffracted at an angle dependant on d and Ad. 14. The method of claim 13, wherein the diffraction grating is reflective. 15. The method of claim 13, wherein the diffraction grating is refractive. 16. The method of claim 13, wherein the step of forming the diffraction grating on the surface of the piezoelectric substrate includes the step of integrally forming the diffrac- tion grating on such surface. 17. The method of claim 13, wherein the step of forming the diffraction grating on the surface of the piezoelectric substrate includes the step of separately forming and attaching the diffraction grating to such surface.

Inventors: ARNOLD ROBERT, BLOOM SCOTT, ARNOLD, ROBERT, BLOOM, SCOTT
Applicants: TREX COMMUNICATIONS CORP, TREX COMMUNICATIONS CORPORATION
Application year: 1999
Main classification: H03H
IPC codes: G02B5, H01L41