Refractive laser communication beam director with dispersion compensation.

Abstract

A co-boresight refractive beam director for a full-duplex laser communication terminal includes a chromatic beam steering element, such as a two- or three-prism Risley prism assembly, and a dispersion compensation mechanism (DCM) inserted in either the transmit or receive path. The DCM adjusts the beam direction of either the transmit or receive laser beam to compensate for the pointing difference introduced by the beam steering element due to the difference between the transmit and receive wavelengths. The DCM can include a tip / tilt mirror and actuator, which can be a commercially available FSM assembly. The beam steering element can be temperature stabilized. A position feedback sensor can increase DCM speed and accuracy. The pointing difference can be calculated and / or interpolated from a pre-established look-up table or fitted curve that relates the pointing difference to the transmit and receive frequencies and the pointing direction of the beam steering element.

Description

[Technical field]

[0001]

[0001] This disclosure relates to laser communications, and more particularly to apparatus and methods for pointing and steering laser communications beams. [Background technology]

[0002]

[0002] The proliferation of both civilian and military wireless communications has created a need for high-speed, reliable, and secure wireless communication of large amounts of data between communication nodes. It should be noted that the term "wireless" is used throughout this disclosure to refer to any communication that does not rely on a physical link between a transmitter and a receiver. Thus, the term "wireless" as used herein excludes optical fiber communication as well as communication over copper wires.

[0003]

[0003] Conventional communication by wireless radio frequencies suffers from several drawbacks, many of which stem from the wide geographical distribution of typical radio wave emissions. Even when directional antennas and antenna arrays are used, radio signals are generally distributed over a wide geographical area, causing rapid attenuation of signal strength with distance and also causing the signals to be relatively easily intercepted by unintended receivers. Because radio communication signals overlap geographically, radio channels usually need to be assigned to specific frequency bands, and these frequency bands are often in limited supply. Furthermore, it is relatively easy for a hostile party to attempt to jam radio communication by transmitting radio signals at high energy covering an area of ​​interest.

[0004]

[0004] Laser communications offers an attractive wireless alternative to radio communications, especially when point-to-point communications are required over long distances, since the non-dispersive, highly directional characteristics of laser communications beams essentially avoid most of the problems associated with radio communications. In particular, laser signals suffer little attenuation as a function of distance, since the signal energy remains tightly collimated within the beam. Also, there is no need to allocate frequency bands to laser communications users, since interference between laser signal beams is avoided unless the two beams are directed to the same receiver with very minimal angular separation. Also, laser communications security is inherently high, since interception of and interference with laser communications requires direct interception of the laser communications beam and / or focusing a jamming beam directly on the intended signal receiver.

[0005]

[0005] Full-duplex (simultaneous, two-way) laser communication can be implemented between terminals in a laser communication network to maximize network connectivity and bandwidth and minimize handshaking overhead. Typically, two laser wavelengths are implemented in each full-duplex link between two communication nodes in the laser communication network, whereby each of the nodes transmits on one of the two wavelengths and receives on the other. The two implemented wavelengths can be generally referred to as a "first" wavelength and a "second" wavelength. However, for simplicity of expression, the two wavelengths implemented in a full-duplex laser communication link are sometimes referred to herein as "red" and "blue" wavelengths, although any two wavelengths can be selected, including wavelengths not in the visible spectrum.

[0006]

[0006] The two wavelengths of a laser communication link are typically selected so that the receiving channel can be separated from the transmitting channel by a factor of 60 to 100 dB or more. In practice, this generally requires a separation between the "red" and "blue" wavelengths of at least 3.2 nm. In some implementations, each of the message-transmitting lasers implemented in a full-duplex laser communication network is tunable between several closely spaced wavelengths, thereby allowing communication links in the laser communication network to be formed using different pairs of "red" and "blue" wavelengths. Generally speaking, however, each of the message-transmitting lasers is limited to a group of closely spaced "red" or "blue" wavelengths, and is therefore either a "red" laser or a "blue" laser.

[0007]

[0007] In many cases, nodes in a full-duplex laser communications network are configured to transmit and receive laser beams having wavelengths within the International Telecommunications Union Dense Wavelength Division Multiplexing (ITU DWDM) 1555 nm band, which ranges from approximately 1540 to 1570 nm, thereby enabling the terminals to implement commercial optical fiber telecommunications devices and infrastructure.

[0008]

[0008] Of course, the highly directional nature of laser beams requires that communication nodes precisely align their lasers with each other and maintain alignment over time to communicate effectively. Typically, it is inconvenient or impossible to align the laser beams by reorienting the laser communication terminal as a whole, for example by reorienting the entire satellite, aircraft, or ground station. Thus, laser communication terminals generally need to implement beam directors that can align the terminal with the remote node so that the transmitted laser beam is precisely aimed at the remote node and the laser light received from the remote node is precisely aimed at the signal detector in the terminal. Typically, the initial pointing must be better than 2 milliradians, corresponding to an initial uncertainty volume for pointing the laser beam, and the pointing is subsequently reduced to only a few microradians by each terminal.

[0009]

[0009] Laser communication terminals often implement a "co-boresight" architecture, whereby both the receive and transmit laser light pass through the same "shared" beam director. In principle, this co-boresight approach simplifies alignment between the laser communication terminal and the remote node, in that aligning the shared co-boresight beam director with the remote node serves to align both the transmitted laser beam and the received laser light.

[0010]

[0010] Referring to Figure 1A, one approach is to implement a mirror 118 in a beam director 110 and mechanically reorient the mirror 118 as needed to provide alignment with the remote node. Figure 1A is a simplified block diagram of a "red" laser communication terminal including a transmitter section 100, a receiver section 114, and a controller 148. The message transmitter section 100 includes a "red" laser 102 and a high power optical amplifier (HOPA) 108 that generate a linearly polarized transmit laser communication beam 134. The transmit beam 134 is directed by a mirror 104 to a polarizing beam splitter or dichroic filter 122 that functions as a transmit / receive diplexer (TX-RX diplexer), and then to a beam director 110 that includes a quarter wave plate 132 that converts the linearly polarized beam to circular polarization, followed by an additional fixed mirror 106 and a mechanically operated beam director mirror 118 that directs the transmit beam 112 to the remote node. “Blue” circularly polarized light 112 received from the remote node follows the same path in reverse through the beam director 110, is converted to linearly polarized light by the quarter wave plate 132, passes through the polarizing beam splitter or dichroic filter 122, and enters the receiver 114, which includes the blue bandpass filter 116, the preamplifier 130, and the photodetector 162.

[0011]

[0011] While reflective beam directors 110 according to this approach can be effective, they typically consume a significant amount of space and can be problematic for aircraft laser communications because they can require mounting the beam directing mirrors in a beam-deflecting turret that protrudes into the airflow, which can introduce drag and boundary layer turbulence.

[0012]

[0012] Another approach is to implement a refractive beam director such as a Risley Prism Assembly (RPA). Figure 1B illustrates the refraction of an incident laser beam 124 into an outgoing beam 126 by a single prism 120, where the two beams 120, 124 differ in direction by an angle α. By rotating 136 the prism 120 about the axis of the incident beam 124, the direction of the outgoing beam 126 can be varied through a family of all possible output beams 124 that form the surface of a cone. When the outgoing beam impinges on a target, the rotation of the prism 120 changes the impact point of the beam around a circle 128 on the target.

[0013]

[0013] Of course, the single rotatable prism 120 of Fig. 1B is not sufficient by itself to align the laser communication terminal with the remote node. Instead, referring to Figs. 1C and 1D, the RPA combines at least two prisms 120, 138, both of which are independently rotatable about the axis of the incident beam 124. In Fig. 1C, the two prisms 120, 138 are aligned such that the refraction of the beam 124 by the two prisms 120, 138 is cumulative, whereby the total refraction angle is the sum of their separate refraction angles, i.e., α+β. In Fig. 1D, the second prism 138 is rotated such that the refraction of the beam 124 by the two prisms 120, 138 is subtractive, whereby the total refraction angle is the difference between their separate refraction angles, i.e., α-β.

[0014]

[0014] By rotating both prisms 120, 138, the impingement point of the output beam 126 on the target can be directed to any point within the annular region bounded by a circle of maximum refraction 140 corresponding to α+β and a circle of minimum refraction 142 corresponding to α-β. Typically, two prisms 120, 138 with the same divergence angle α are implemented in the RPA, so that the circle of minimum refraction is reduced to a central point, making it possible to direct the output beam 126 to a target anywhere within the disk defined by the circle of maximum refraction 140. Figure 1E is a simplified block diagram of a laser communication terminal similar to that of Figure 1A, but with a simple two-prism RPA 144 implemented in the beam director 110.

[0015]

[0015] For aircraft, the RPA 144 may be an attractive choice for the beam director 110 because it can be conformally mounted to the skin or pod surface of the aircraft for optimal aero-optical considerations with minimal boundary layer turbulence. Naturally, it may be desirable to use a high refractive index material to form the two prisms 120, 138 of the RPA 144 to provide a large maximum deflection angle at a minimum size and weight of the RPA. In particular, it may be desirable to form the prisms 120, 138 from optical grade single crystal silicon, which has the highest optical index (approximately 3.48) of any glass that can be used for an RPA. By utilizing this high refractive index of silicon, a beam steering cone of 120 degrees (half angle of 60 degrees) can be practically realized.

[0016]

[0016] However, a simple two-prism RPA 144 as shown in Figure 1E may not be acceptable as a beam steering element in a co-bore sight full-duplex laser communications terminal, especially when the prisms 120, 138 are made from a material having a high refractive index, because the two-prism RPA 144 is inherently "colored" in that the angle at which the incident laser beam 124 is refracted depends on the wavelength of the laser beam. As mentioned above, in a full-duplex co-bore sight laser communications link, both the "red" transmit light and the "blue" receive light (or vice versa) need to be refracted by exactly the same angle by the "shared" co-bore sight beam director 110.

[0017]

[0017] Because the refractive index for virtually all materials depends on the wavelength of light, a simple two-prism RPA 144 will necessarily refract "red" and "blue" light at slightly different angles, thereby creating a "pointing difference" between red and blue wavelengths. Thus, as shown in Figure 1E, if the two-prism RPA 144 in the beam director 110 is configured to precisely direct the transmitted "red" light 112 to the remote node, the received blue light 146 from the remote node will be refracted by the RPA 144 at a slightly different angle (exaggerated in Figure 1E) due to the pointing difference, and will not be properly aligned with the receiver 114 of the laser communication terminal.

[0018] Alternatively, referring to FIG. 1F, an achromatic RPA with three or more prisms can be implemented to achieve a refractive angle that is substantially independent of wavelength. One example is the achromatic RPA disclosed in U.S. Pat. No. 9,140,941, also by the applicant, which is incorporated herein by reference in its entirety for all purposes. In the example of FIG. 1F, the achromatic RPA implements a "triplet pair" 156 of prisms 150, 152, 154, i.e., six prisms, with each of the "triplets" 156 including three prisms 150, 152, 154 made from different materials. Although effective, the achromatic RPA is physically much thicker, larger, and heavier than a comparable single-material two-prism RPA 144 as in FIGS. 1C-1E, resulting in a substantial increase in the size and weight of the beam director 110.

[0019] Therefore, what is needed is a small, lightweight refractive beam director that can be implemented as a co-bore sight beam director in a full-duplex laser communication terminal that uses different TX and RX wavelengths. Summary of the Invention

[0020]

[0020] The present disclosure is a compact and lightweight refractive beam director that can be implemented as a co-bore sight beam director in a full-duplex laser communication terminal implementing different transmit and receive (TX and RX) wavelengths. The disclosed co-bore sight beam director can maintain co-bore sight alignment of the TX and RX laser beams in that the angular difference between the TX and RX laser beams passing between the laser communication terminal and the remote node is maintained to less than 100 microradians throughout the entire beam steering cone of the refractive beam director.

[0021]

[0021] The TX and RX wavelengths may be generally referred to as the "first" and "second" wavelengths. However, for ease of expression, the two wavelengths implemented in a full-duplex laser communication link may be referred to herein as the "red" and "blue" wavelengths, although any two wavelengths may be selected, including wavelengths not within the visible spectrum.

[0022]

[0022] The disclosed refractive beam director implements a simple chromatic refractive beam steering element such as a two-prism FPA in combination with a dispersion compensation mechanism (DCM) that compensates for the transmit red / receive blue (or vice versa) pointing difference of the FPA, i.e., the difference in the refracted angles of red and blue light through the FPA, thereby enabling the disclosed beam director to be implemented as a co-bore sight beam director in a full-duplex laser communication terminal.

[0023]

[0023] For ease of expression, it should be noted that the term "RPA" is used hereinafter to refer to any refractive beam steering device that is chromatic, whether or not it includes a Risley prism assembly, unless otherwise required by context. It should be further noted that although examples and embodiments are presented herein that list a two-prism RPA as the beam steering element, the scope of the present disclosure extends to all beam directors that implement a chromatic refractive beam steering element, regardless of the number of prisms and / or other optical elements included in the beam steering element.

[0024]

[0024] The disclosed DCM implements a small "tip-tilt mirror" whose orientation is mechanically adjusted by a tip-tilt actuator to compensate for pointing differences, i.e., chromatic differences in beam deflection angles between red and blue light passing through the RPA. The tip-tilt mirror is inserted in a beam path segment in the terminal where one, but not both, of the transmit and receive lights pass, e.g., in the transmit or receive path, before the transmit and receive lights are combined by a TX-RX diplexer. The beam that is adjusted by the DCM tip-tilt mirror is referred to herein as the "compensated" laser beam and can be either the transmit or receive laser beam. The other laser beam that is not adjusted by the DCM is referred to as the uncompensated "laser beam."

[0025]

[0025] In embodiments, either a dual-axis voice coil or a piezoelectric drive is implemented for the tip-tilt actuator. Some embodiments implement a commercially available "fast steering mirror" (FSM) assembly as the tip-tilt mirror and actuator in the DCM. Various embodiments provide improved speed and precision control of the tip-tilt mirror by implementing a tip-tilt mirror position feedback sensor, which may include any combination of capacitive sensors, inductive sensors, strain gauges, and / or any other position feedback sensor known in the art.

[0026]

[0026] For any given pair of "red" and "blue" wavelengths, the pointing difference introduced by the RPA depends only on the red and blue wavelengths, the temperature of the RPA, and the material properties, dimensions, and relative positioning of the optical elements of the RPA, e.g., the rotational orientation of the two prisms in a two-prism Risley prism assembly. In some embodiments, the temperature of the RPA is monitored and included as a variable when determining the pointing difference. In other embodiments, the RPA is temperature stabilized, e.g., by maintaining the RPA in a temperature-regulated enclosure, thereby eliminating temperature effects as a factor in determining the pointing difference. Typically, for a given RPA maintained at a constant temperature, the pointing difference depends only on the two wavelengths and the pointing angle, i.e., the angle at which the RPA points the uncompensated laser beam.

[0027]

[0027] According to an embodiment of the disclosed method, each time the RPA is reconfigured to a new pointing direction, the controller determines the red-blue pointing difference for the new pointing direction, and the orientation of the tip-tilt mirror is adjusted to compensate for the pointing difference, thereby providing precise co-bore sight alignment of the transmitted laser beam and the received laser light.

[0028] In some embodiments, a "look-up" table or polynomial curve fit of pointing differences is pre-determined for a given RPA and for each combination of red and blue wavelengths as a function of pointing angles and other factors for multiple beam directions across the field of interest. Based on the look-up table or polynomial fit, the controller can then quickly and accurately determine the applicable pointing difference each time the pointing direction of the RPA is changed by simply interpolating the correct values ​​from the pre-determined look-up table or extracting the correct values ​​from the curve fit. In some embodiments, the look-up table or curve fit is determined entirely by calculation based on the properties and dimensions of the RPA prism. In other embodiments, the values ​​in the look-up table or the points used to create the curve fit are determined or at least verified based on experimental measurements.

[0029] In exemplary embodiments, the difference between the selected "red" and "blue" wavelengths can be 2 nm to 15 nm. In embodiments, the transmit laser beam in the laser communication terminal is about 1 cm in diameter. Also, in some of these embodiments, the DCM has a total size of about 2 to 4 cubic inches.

[0030]

[0030] One general aspect of the present disclosure is a laser communication terminal including a transmit channel configured to generate a transmit laser beam at a transmit wavelength to a remote node, a receive channel configured to receive a receive laser beam from the remote node at a receive wavelength different from the transmit wavelength, a controller, and a co-bore sight beam director configured to direct the transmit laser beam to the remote node and direct the receive laser beam to the receive channel. The co-bore sight beam director includes a beam steering element configured to refract the transmit laser beam and the receive laser beam, the beam steering element being colored, thereby refracting the transmit laser beam and the receive laser beam through a first refraction angle and a second refraction angle, respectively, according to a difference between the transmit wavelength and the receive wavelength, where the difference between the first refraction angle and the second refraction angle is referred to herein as a pointing difference, and a dispersion compensation mechanism (DCM) configured to adjust a direction of a compensated laser beam, one of the transmit laser beam and the receive laser beam, the other of the transmit laser beam and the receive laser beam being an uncompensated laser beam. The controller is configured to cause the DCM to adjust the direction of the compensated laser beam to compensate for the pointing difference, thereby causing the angular difference between the transmit laser beam and the receive laser beam passing between the laser communication terminal and the remote node to be less than 100 microradians.

[0031] In embodiments, the beam steering element is a Risley prism assembly. In some of these embodiments, the Risley prism assembly is a two-lens Risley prism assembly.

[0032] In any of the above embodiments, the beam steering elements may include optical elements made from optical grade single crystal silicon.

[0033] In any of the above embodiments, the DCM can include a tip-tilt mirror in cooperation with a mirror actuator that can reorient the tip-tilt mirror as directed by the controller. In some of these embodiments, the mirror actuator comprises a voice coil and / or a piezoelectric drive. In any of these embodiments, the DCM can be a fast steering mirror assembly. In any of these embodiments, the DCM can include at least one feedback sensor configured to sense a position of the tip-tilt mirror about an axis. Also, in some of these embodiments, the position sensor is one of a capacitive sensor, an inductive sensor, and a strain gauge.

[0034] In any of the above embodiments, the difference between the transmit and receive wavelengths can be from 2 nm to 15 nm. In some of these embodiments, the co-bore sight beam director can cause the transmit and receive laser beams to be parallel to each other within less than 2 microradians while they are passing between the laser communication terminal and the remote node.

[0035] In any of the above embodiments, the beam steering element may be temperature stabilized.

[0036] In any of the above embodiments, the cobore sight beam director can occupy a volume of at most 4 cubic inches.

[0037]

[0037] A second general aspect of the present disclosure is a method of aligning a laser communication terminal with a remote node, the method including providing a laser communication terminal according to the first general aspect, adjusting a beam steering element to direct an uncompensated laser beam in a pointing direction to thereby align the uncompensated laser beam with the remote node, determining a pointing difference, and causing a DCM to adjust a direction of a compensated laser beam to compensate for the pointing difference, thereby causing the transmit laser beam and the receive laser beam to be parallel and substantially collinear during passage between the laser communication terminal and the remote node.

[0038] In some of these embodiments, the method further includes establishing a lookup table including a pointing difference of the beam director as a function of the transmit wavelength, the receive wavelength, and the pointing direction, and determining the pointing difference includes interpolating the pointing difference based on the lookup table. In some of these embodiments, establishing the lookup table includes experimentally verifying at least one entry in the lookup table.

[0039]

[0039] Also, in any of the above embodiments, the method may further include establishing at least one fitted curve relating the pointing difference to the pointing direction as a function of the transmitting wavelength and the receiving wavelength, the fitted curve being a curve fitted to experimentally measured values.

[0040]

[0040] The features and advantages described herein are not all-inclusive, and in particular many additional features and advantages will become apparent to those skilled in the art in light of the drawings, specification, and claims. Moreover, it should be noted that the language used in this specification has been selected primarily for readability and instructional purposes, and not to limit the scope of the subject matter of the present invention. [Brief description of the drawings]

[0041] [Figure 1A]FIG. 1A is a block diagram of a prior art full-duplex, dual-wavelength laser communications terminal. [Figure 1B]

[0042] FIG. 1B is a simplified diagram showing the refraction of a laser beam by a prism, as known in the art. [Figure 1C]

[0043] FIG. 1C is a simplified diagram illustrating the refraction of a laser beam at a maximum refraction angle by two rotatable prisms of a Risley prism assembly, as known in the art. [Figure 1D]

[0044] FIG. 1D is a simplified diagram showing the refraction of a laser beam at the minimum refraction angle by two rotatable prisms of a Risley prism assembly, as known in the art. [Figure 1E]

[0045] FIG. 1E is a block diagram similar to FIG. 1A illustrating the misalignment of the transmit and receive beams resulting from the implementation of a Risley prism assembly as a beam director according to the prior art. [Figure 1F]

[0046] FIG. 1F is a cross-sectional view of a prior art six-prism achromatic Risley prism assembly. [Diagram 2]

[0047] FIG. 2 is a block diagram of a full-duplex, dual-wavelength laser communications terminal according to an embodiment of the present disclosure. [Diagram 3]

[0048] FIG. 3 is a graph showing the dependence of dispersion angle on wavelength for a single 10.8 degree Risley prism assembly wedge made from optical grade single crystal silicon and configured to refract an incident laser beam by a nominal angle of 30 degrees, as known in the art. [Figure 4]

[0049] FIG. 1 is a flow diagram illustrating an embodiment of a method of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042]

[0050] Disclosed is a compact and lightweight refractive beam director that can be implemented as a co-bore sight beam director in a full-duplex laser communication terminal. The disclosed co-bore sight beam director can maintain co-bore sight alignment of the TX and RX laser beams in that the angular difference between the TX and RX laser beams passing between the laser communication terminal and the remote node is maintained to less than 100 microradians throughout the entire beam steering cone of the refractive beam director.

[0043]

[0051] The TX and RX wavelengths may be generally referred to as the "first" and "second" wavelengths. However, for ease of expression, the two wavelengths implemented in a full-duplex laser communication link may be referred to herein as the "red" and "blue" wavelengths, although any two wavelengths may be selected, including wavelengths not within the visible spectrum.

[0044]

[0052] Referring to FIG. 2, the disclosed refractive beam director 210 implements a simple, chromatic refractive beam steering element 144, such as a two-prism FPA 144, in combination with a dispersion compensation mechanism (DCM) 212 that compensates for the red / blue pointing difference, i.e., the difference in the refraction angles of red and blue light, thereby allowing the disclosed beam director to be implemented as a co-bore sight beam director in a full-duplex laser communication terminal. In an embodiment, the RPA 144 is a Risley prism assembly that implements beam steering wedges made from optical grade silicon as the two prisms. This is because silicon has the highest optical index (about 3.48) of any glass that can be used for the RPA 144. The embodiment implements a two-prism RPA 144, where the two silicon prisms of the Risley prism assembly can achieve a 120 degree beam steering cone (60 degree half angle). Other embodiments use optical materials other than silicon that have a lower refractive index, and therefore a correspondingly reduced beam steering cone, as compared to an otherwise identical embodiment that implements a silicon prism.

[0045]

[0053] It should be noted that for ease of expression, the term "RPA" is used hereinafter to refer to any refractive beam steering device that is chromatic, whether or not it includes a Risley prism assembly, unless otherwise required by context. It should be further noted that although examples and embodiments are presented herein that list a two-prism RPA as the beam steering element, the scope of the present disclosure extends to all beam directors that implement a chromatic refractive beam steering element, regardless of the number of prisms and / or other optical elements included in the beam steering element.

[0046]

[0054] The embodiment leverages existing commercial optical fiber telecommunications technology by implementing red and blue wavelengths within the telecommunications ITU DWDM band in the "1555 nm region," which is between approximately 1540 nm and 1570 nm. In this wavelength region, for a silicon wedge with a refractive index of n=3.48, the wedge angle must be 10.8 degrees to provide a 30 degree deflection, thus achieving a maximum steering angle of 60 degrees with a pair of wedges 120, 138. The dispersion of a single 10.8 degree silicon RPA wedge 120 deflecting 30 degrees is 20 microradians / nanometers, as shown in FIG. 3. For a pair of such wedges 120, 138 to achieve a maximum deflection of 60 degrees, the dispersion at that maximum angle is 40 microradians / nm.

[0047]

[0055] In general, it is necessary that the separation between the TX and RX wavelengths (red and blue wavelengths) is at least 3.2 nm to allow adequate spectral discrimination. Thus, the worst-case pointing difference between TX and RX is (40 μrad / nm) × (3.2 nm) = 128 μrad for a perfect refraction of 60 degrees. For an RPA with a nominal aperture size of 50 mm, the refraction-limited beamwidth in the 1555 nm region is about 40 μrad FWHM, i.e., 20 μrad HWHM. Under these exemplary conditions, the DCM residual error must be less than 10 μrad to result in a signal loss of at most 0.8 dB. At smaller RPA off-axis pointing angles, the pointing difference is less than this 10 μrad maximum value.

[0048]

[0056] The disclosed DCM implements a small "tip-tilt mirror" 200 with an orientation 202 that is mechanically adjusted by a tip-tilt actuator 202 to compensate for pointing differences, i.e., chromatic differences in the beam deflection angles between the red and blue light introduced by the RPA 144. The tip-tilt mirror 200 is inserted into a beam path segment in the terminal through which either the transmit or receive light passes, but not both. In the illustrated example, the DCM 212 is inserted into the transmit path 134 before it is combined with the receive path 160 by the TX-RX diplexer 122. The beam adjusted by the tip-tilt mirror 200 is referred to herein as the "compensated" laser beam, and the other beam is referred to as the uncompensated "laser beam." In the embodiment of FIG. 2, the transmit beam 134 is the compensated laser beam, and the receive beam 160 is the uncompensated laser beam.

[0049]

[0057] In embodiments, either a dual-axis voice coil or piezoelectric drive is implemented for the tip-tilt actuator 204. Some embodiments implement a commercially available "fast steering mirror" (FSM) assembly as the tip-tilt mirror 200 and actuator 204 in the DCM. Various embodiments provide improved speed and precision control of the tip-tilt mirror by implementing a tip-tilt mirror position feedback sensor 206, which may include any combination of capacitive sensors, inductive sensors, strain gauges, and / or any other position feedback sensors known in the art.

[0050]

[0058] For example, for a laser communications link implementing red and blue wavelengths separated by 2 to 15 nm, and for a terminal having a transmit laser beam diameter of approximately 1 cm, a reflective tip-tilt DCM212 can be realized having a total volume of only 2 to 4 cubic inches.

[0051]

[0059] For any given pair of "red" and "blue" wavelengths, the pointing difference introduced by the RPA 144 depends only on the red and blue wavelengths, the temperature of the RPA 144, and the material properties, dimensions, and relative positioning of the optical elements of the RPA 144, e.g., the rotational orientation of the RPA prisms (e.g., 120, 138) in the two-prism Risley prism assembly of FIG. 1C and FIG. 1D. In an embodiment, the RPA 144 temperature is monitored and included as a variable when determining the pointing difference. In other embodiments, the RPA 144 is temperature stabilized, e.g., by maintaining the RPA in a temperature-regulated enclosure, thereby eliminating temperature effects as a factor in determining the pointing difference. Typically, for a given RPA 144 maintained at a constant temperature, the pointing difference depends only on the red and blue wavelengths and the pointing angle, i.e., the angle at which the RPA 144 directs the uncompensated laser beam 146.

[0052]

[0060] In some embodiments, a "look-up" table of pointing differences is pre-determined for a given RPA and for each red / blue wavelength pair as a function of pointing angle for multiple beam directions across the field of interest. In other embodiments, a fitted curve is pre-determined that represents the pointing difference as a function of pointing angle. In still other embodiments, a set of mathematical formulas is provided that allows for the calculation of the pointing difference according to the pointing direction and other factors. Based on the look-up table, fitted curve, and / or equation, the controller can then quickly and accurately determine the applicable pointing difference each time the pointing direction of the RPA is changed by simply interpolating and / or calculating the correct values ​​from the pre-determined look-up table, fitted curve, and / or equation. In some embodiments, the look-up table is completely determined by calculation based on the characteristics and dimensions of the RPA prism. In other embodiments, the values ​​in the look-up table and / or the values ​​fitted to obtain the fitted curve are determined or at least verified based on experimental measurements.

[0053]

[0061] 4, an embodiment of the disclosed method includes predetermining the pointing difference as a function of pointing direction for a given RPA 144 and red / blue wavelength pair. These can be calculated 400 and then verified 402 by measurement. Thereafter, each time the RPA 144 is reconfigured 404 to a new pointing direction, the controller 148 determines the pointing difference for the new pointing direction by looking up 406 the previously determined pointing difference, for example as recorded in a lookup table or fitted curve, and then interpolating 408 the pointing difference from the values ​​in the table or fitted curve. In other embodiments, the pointing difference is calculated according to a previously determined formula. The tip-tilt mirrors are then adjusted 410 to compensate the beam directors (210, 212) for the pointing difference, thereby providing precise co-bore sight alignment of the transmitted laser beam 134 and the received laser light 160.

[0054]

[0062] The foregoing description of embodiments of the present disclosure has been presented for purposes of illustration and description. Each and every page of this submission and all of its contents, however characterized, identified, or numbered, regardless of form or location within this application, are considered to be a substantial part of this application for all purposes. This specification is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.

[0055]

[0063] Although the present application is presented in a limited number of forms, the scope of the present disclosure is not limited to these forms, and various changes and modifications can be accepted without departing from the spirit thereof. The disclosure presented herein does not explicitly disclose all possible combinations of features within the scope of the present disclosure. The features disclosed herein for various embodiments can generally be interchanged and combined in any combination that is not self-contradictory without departing from the scope of the present disclosure. In particular, the limitations presented in the following dependent claims can be combined with their corresponding independent claims in any number and in any order without departing from the scope of the present disclosure, as long as the dependent claims are not logically inconsistent with each other. The invention as described in the claims of the original application is set forth below. [C1] A laser communication terminal, a transmit channel configured to generate a transmit laser beam at a transmit wavelength to a remote node; a receive channel configured to receive a receive laser beam from the remote node at a receive wavelength different from the transmit wavelength; A controller; a co-boresight beam director configured to direct the transmit laser beam to the remote node and to direct the receive laser beam to the receive channel; The co-bore sight beam director comprises: a beam steering element configured to refract the transmit laser beam and the receive laser beam, the beam steering element being chromatic, thereby refracting the transmit laser beam and the receive laser beam through a first refraction angle and a second refraction angle, respectively, in accordance with a difference between the transmit wavelength and the receive wavelength, the difference between the first refraction angle and the second refraction angle being referred to herein as a Poynting difference; a dispersion compensation mechanism (DCM) configured to adjust a direction of a compensation laser beam, which is one of a transmission and reception laser beam, the other of the transmission and reception laser beams being an uncompensated laser beam; The controller is configured to cause the DCM to adjust the direction of the compensated laser beam to compensate for the pointing difference, thereby causing an angular difference between the transmitting laser beam and the receiving laser beam to be less than 100 microradians during passage between the laser communication terminal and the remote node. [C2] The laser communication terminal of C1, wherein the beam steering element is a Risley prism assembly. [C3] The laser communication terminal of C2, wherein the Risley prism assembly is a two-lens Risley prism assembly. [C4] The laser communication terminal of C1, wherein the beam steering element includes an optical element made of optical grade single crystal silicon. [C5] The laser communication terminal of C1, wherein the DCM comprises a tip / tilt mirror, the tip / tilt mirror cooperating with a mirror actuator capable of reorienting the tip / tilt mirror as instructed by the controller. [C6] The laser communication terminal of C5, wherein the mirror actuator comprises a voice coil. [C7] The laser communication terminal of C5, wherein the mirror actuator comprises a piezoelectric drive. [C8] The laser communication terminal of C5, wherein the DCM is a high speed steering mirror assembly. [C9] The laser communication terminal of C5, wherein the DCM comprises at least one feedback sensor configured to sense a position of the tip / tilt mirror about an axis. [C10] The laser communication terminal of C9, wherein the position sensor is one of a capacitive sensor, an inductive sensor, and a strain gauge. [C11] The laser communication terminal of claim 1, wherein the difference between the transmission wavelength and the reception wavelength is from 2 nm to 15 nm. [C12] The laser communication terminal described in C11, wherein the co-bore sight beam director can cause the transmitting laser beam and the receiving laser beam to be parallel to each other within less than 2 microradians during passage between the laser communication terminal and the remote node. [C13] The laser communication terminal of C1, wherein the beam steering element is temperature stabilized. [C14] The laser communications terminal of claim 1, wherein the co-bore sight beam director occupies a volume of at most 4 cubic inches. [C15] 1. A method for aligning a laser communications terminal with a remote node, comprising: providing a first and a second laser communication terminal according to C1; adjusting the beam steering element to direct the uncompensated laser beam in a pointing direction, thereby aligning the uncompensated laser beam with the remote node; determining said pointing difference; and causing the DCM to adjust a direction of the compensated laser beam to compensate for the pointing difference, thereby causing the transmit laser beam and the receive laser beam to be parallel and substantially collinear during passage between the laser communication terminal and the remote node. [C16] The method of claim 15, further comprising establishing a lookup table containing pointing differences of a beam director as a function of the transmit wavelength, the receive wavelength, and a pointing direction, and determining the pointing differences comprises interpolating the pointing differences based on the lookup table. [C17] The method of C16, wherein establishing the lookup table includes empirically verifying at least one entry in the lookup table. [C18] The method of claim 15, further comprising establishing at least one fitted curve relating pointing difference to pointing direction as a function of the transmit wavelength and the receive wavelength, the fitted curve being a curve fitted to experimentally measured values.

Claims

1. A laser communication terminal, a transmit channel configured to generate a transmit laser beam at a transmit wavelength to a remote node; a receive channel configured to receive a receive laser beam from the remote node at a receive wavelength different from the transmit wavelength; A controller; a co-boresight beam director configured to direct the transmit laser beam to the remote node and to direct the receive laser beam to the receive channel; The co-bore sight beam director comprises: a beam steering element configured to refract the transmit laser beam and the receive laser beam, the beam steering element being chromatic, thereby refracting the transmit laser beam and the receive laser beam through a first refraction angle and a second refraction angle, respectively, in accordance with a difference between the transmit wavelength and the receive wavelength, the difference between the first refraction angle and the second refraction angle being referred to herein as a Poynting difference; a dispersion compensation mechanism (DCM) configured to adjust a direction of a compensation laser beam, which is one of a transmission and reception laser beam, the other of the transmission and reception laser beams being an uncompensated laser beam; The controller is configured to cause the DCM to adjust the direction of the compensated laser beam to compensate for the pointing difference, thereby causing an angular difference between the transmitting laser beam and the receiving laser beam to be less than 100 microradians during passage between the laser communication terminal and the remote node.

2. The laser communications terminal of claim 1 , wherein the beam steering element is a Risley prism assembly.

3. 3. The laser communication terminal of claim 2, wherein the Risley prism assembly is a two-lens Risley prism assembly.

4. The laser communications terminal of claim 1 , wherein the beam steering element comprises an optical element made of optical grade single crystal silicon.

5. 2. The laser communications terminal of claim 1, wherein the DCM comprises a tip-tilt mirror in cooperation with a mirror actuator capable of reorienting the tip-tilt mirror as instructed by the controller.

6. The laser communications terminal of claim 5 , wherein the mirror actuator comprises a voice coil.

7. The laser communications terminal of claim 5 , wherein the mirror actuator comprises a piezoelectric drive.

8. The laser communications terminal of claim 5 , wherein the DCM is a high speed steering mirror assembly.

9. The laser communications terminal of claim 5 , wherein the DCM comprises at least one feedback sensor configured to sense a position of the tip-tilt mirror about an axis.

10. The laser communication terminal described in claim 9, wherein the feedback sensor is one of a capacitive sensor, an inductive sensor, and a strain gauge.

11. 2. The laser communication terminal according to claim 1, wherein the difference between the transmission wavelength and the reception wavelength is from 2 nm to 15 nm.

12. 12. The laser communications terminal of claim 11, wherein the co-bore sight beam director is capable of causing the transmit laser beam and the receive laser beam to be parallel to each other within less than 2 microradians during passage between the laser communications terminal and the remote node.

13. The laser communications terminal of claim 1 , wherein the beam steering element is temperature stabilized.

14. The laser communications terminal of claim 1 , wherein said coboresight beam director occupies a volume of at most 4 cubic inches.

15. 1. A method for aligning a laser communications terminal with a remote node, comprising: Providing first and second laser communication terminals according to claim 1; adjusting the beam steering element to direct the uncompensated laser beam in a pointing direction, thereby aligning the uncompensated laser beam with the remote node; determining said pointing difference; and causing the DCM to adjust a direction of the compensated laser beam to compensate for the pointing difference, thereby causing the transmit laser beam and the receive laser beam to be parallel and substantially collinear during passage between the laser communication terminal and the remote node.

16. 16. The method of claim 15, further comprising establishing a lookup table containing pointing differences of a beam director as a function of the transmit wavelength, the receive wavelength, and a pointing direction, and determining the pointing differences comprises interpolating the pointing differences based on the lookup table.

17. The method of claim 16 , wherein establishing the lookup table includes empirically verifying at least one entry in the lookup table.

18. 16. The method of claim 15, further comprising establishing at least one fitted curve relating pointing difference to pointing direction as a function of the transmit wavelength and the receive wavelength, the fitted curve being a curve fitted to experimentally measured values.

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