Abstract:
A wireless atmospheric site-to-site full-duplex (i.e., simultaneous transmit and receive) laser communication system, typically for wideband (high-speed) data, voice, and/or video transmission. The system includes at least one laser communication transceiver each having an electro-optical transmitter that includes a laser source for generating laser light to be transmitted site-to-site and at high speed, an electro-optical receiver that includes a baffle assembly for receiving light directly onto a detector, without an intermediate field stop and re-imaging relay optics to reject off-axis light sources. Wideband data are transmitted and received through the atmosphere by the electro-optical transmitter and the electro-optical receiver. The data to be transmitted through each laser communication transceiver is inputted through a fiber-optic receiver and outputted by a fiber-optic transmitter. Outgoing laser light is generated by the laser source after it is triggered by an inputted signal from the fiber-optic receiver.

Description:
BACKGROUND OF THE INVENTION 
   This application claims the benefit of U.S. Provisional Patent Applications Ser. Nos. 60/164,335 and 60/164,336 filed on Nov. 9, 1999, the entire disclosures of which are incorporated herein by reference. 

   1. Field of the Invention 
   The present invention relates to the field of site-to-site atmospheric windband optical communication systems. In particular, the present invention relates to a full-duplex (i.e., simultaneous transmit and receive) laser communication system for point-to-point communications through the atmosphere. 
   Wideband (high-speed) data transfer at rates in excess of 1.5 Mbps may be relatively expensive for dedicated bandwidth (e.g., leased lines) transmission over the existing telecommunications infrastructure. Over modest ranges where an unobstructed line-of-sight exists a laser communication link may provide an alternative means for obtaining dedicated bandwidth transmission at high data rates. 
   2. Description of the Prior Art 
   Microwave line-of-sight systems are known in the art. However, the majority of such systems on the market are not capable of the desired data rates in excess of 1.5 Mbps (e.g., 10, 45, and 155 Mbps). Microwave systems also require FCC licensing, and are susceptible to terrain and building, reflections, typically requiring tall towers which add significantly to the expense. 
   SUMMARY OF THE INVENTION 
   The present invention cost-effectively realizes a wireless atmospheric laser communication system, typically for use over ranges of 0.3-10 km, for high-speed data, voice, and/or video transmission at rates in excess of 1.5 Mbps. The interfaces to the laser communication system are based on computer and telecommunications standards, such as those employed in computer networks or telecom transmission equipment. 
   The system of the present invention enables the full-duplex (i.e., simultaneous transmit and receive) line-of-sight transmission of high-speed data, as well as voice, video, graphics, and images. Data is generally transmitted in binary digital form, but may also be transmitted as an analog waveform; for example, multiple NTSC television signals with video and stereo sound as a pulse frequency modulated waveform. 
   Communication links are established on a point-to-point basis and may be aggregated to form a network interconnecting many nodes, for example in a star or hub topology, or may be used to realize a communications relay function. 
   The approach of the present invention emphasizes a cost-effective implementation for a wideband laser communication system suitable for link ranges up to 10 km. Each laser communication link is line-of-sight and point-to-point between a pair of terminals, and may be integrated into a network of multiple links, as well as interfaced with other communications networks. 
   There are three primary advantages to the system of the present invention:
         1. Ability to mass produce: Because of the design and system implementation described in the preferred embodiment (e.g., cast aluminum housing, ‘soda straw’ receiver baffle, transmitter and receiver tubes fastened to a common bulkhead, with drop-in tolerances for the optical components, etc.), the SupraConnect laser communication terminals are amenable to volume production.   2. Cost: By eliminating costly manufacturing processes (e.g., labor intensive assembly), and by minimizing alignment requirements (e.g., drop-in optics) and employing rapid interferometric and boresight alignment techniques, the SupraConnect design is amenable to significantly lower manufacturing costs in volume production than other systems currently on the market.   3. Performance and reliability: The overall design of the SupraConnect is very rugged and robust. For example, window heating and laser thermal control is more effective and better suited to continuous extended use in extreme environmental conditions than other designs. The design also utilizes a larger collecting aperture and has more link margin than other systems on the market. This enables the SupraConnect to be used in more demanding applications where ruggedness and reliability are important.       

   The system comprises a laser source for generating laser light to be transmitted site-to-site and at high speed, a fiber-optic transceiver, an electro-optical transceiver and at least one of: (a) a baffle assembly for receiving light directly onto the second transceiver, without an intermediate field stop and re-imaging relay optics to reject off-axis light sources, (b) thermoelectric cooler means for actively cooling the laser diode means. While wideband data are received through the atmosphere by the electro-optical transceiver and outputted by the fiber-optic transceiver the user&#39;s equipment when the system is receiving data, a laser light is generated by the laser source after it is triggered by an inputted signal from the fiber-optic transceiver when the system is transmitting data. The system is further optimized by combining a transparent resistive coating, a thermoelectric cooler, an autoranging system with electronic translation, and a steering means yielding a system that to performs within the intended speed and range and in the intended harsh environment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of one embodiment of the present invention. 
       FIG. 2  is a diagram illustrating the major subsystems of the embodiment the present invention shown in FIG.  1 . 
       FIG. 3  is a sectional view of the embodiment the present invention shown in  FIG. 1  with all the components are assembled. 
       FIG. 4  is a block diagram of the five main electronic circuits of the embodiment the present invention shown in FIG.  1 . 
       FIG. 5A  is a cross sectional view of the preferred embodiment of the invention; and  FIG. 5B  is a perspective view of the preferred embodiment shown in FIG.  5 A. 
       FIG. 6A  is a cross sectional view of an optical transceiver employing the preferred embodiment of the invention shown in  FIG. 5A ; and  FIG. 6B  is a perspective view, partially fragmented and in simplified pictorial form, of the optical transceiver shown in FIG.  6 A. 
       FIG. 7A  shows a cross sectional view of an alternate embodiment of the baffle assembly; and  FIG. 7B  is a perspective view, partially fragmented and in simplified pictorial form, of an optical transceiver employing the alternate embodiment of the invention shown in FIG.  7 A. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The preferred embodiment of the SupraConnect laser communication system comprises separate internal optical systems for the transmitter  150  and receiver  101 , packaged in a single cylindrical cast aluminum housing  600  with raised ribs, with a factory-mounted and bore-sighted sighting system  601 .  FIG. 1  is an exploded view of one embodiment of the laser transceiver of the present invention. The present invention was designed to give the most transmission range for the least amount of optical power, namely high-speed transmission in excess of 1.5 Mbps and maximum range to 10 km, and to achieve low cost manufacturing, telecom reliability during severe weather. In particular, the system was optimized by combining the subsystems as shown in FIGS.  2  and  3 : a baffle  10  embedded in the receiver  101 , a window assembly  200 , a thermoelectric cooler  300 , an autoranging system with electronic translation, and a steering means yield the whole system to perform within the intended speed and range in the intended harsh environment. The major environmental issues include scintillation (the mirage effect), visibility penetration through an enhanced system dynamic range (utilizing every last drop of optical power), wind and vibration stability (improved by the steering means), window obscuration from dew and snow (resolved by the transparent coating), and telecom reliability (improved by the thermoelectric cooling of the laser diode). 
   The SupraConnect laser transceiver is packaged in a distinctive aluminum casting  600 , suitable for cost-effective high volume production, which is essentially cylindrical in shape and includes an integral cast hood  603  as a distinctive feature to shield the window from rain and snow, an integral cast mount  604  for a sighting system, and an integral cast heat sink  605 . 
   The internal cylindrical housing  606  for the transmitter assembly  150  is entirely closed, such that any transmitter energy is prevented from scattering into the receiver  101 . The transmitter output aperture  608  is 1″-3″ (nominally 2″) for reasons of laser eyesafety considerations. The receiver collecting aperture  609  is 2″-4″ (nominally 3″) for maximum light collection and aperture-averaging of atmospheric scintillation. 
   The nominal 2″ transmitter optics and nominal 3″ receiver optics  610  are housed in separate cylinders, which are captured in fore and aft bulkhead disks  611  (nominally 6″ in diameter) which permit this composite transceiver assembly ( FIG. 2 ) to slide into the cylindrical castling  600 , as a single unit. The transmit and receive channels are typically operated at the same 785 nm wavelength, where lasers of moderate power are available at reasonable cost. This is possible since the transmit and receive channels are physically separate, and the single transmit/receive wavelength has the advantage of any terminal being, able to communicate with any other terminal. 
   The separate transmit and receive apertures are sealed against moisture and the outside environment with a single piano window  200 , nominally 6″ in diameter, having no optical power. The exterior window surface has a dichroic optical coating which has a distinctive mirror-like appearance which is highly reflective (e.g., 99%) for visible light, but highly transmissive (e.g., 95%) for the near-infrared laser radiation. This exterior window coating reflects nearly all the solar energy in the visible region which otherwise would be trapped and cause heating of the telescope interior, as well as degraded receiver sensitivity. 
   The window is sealed, shock-mounted, and thermally isolated from the SupraConnect housing by an elastomeric gasket  612  around its circumference. The interior surface of the window employs an electrically conductive coating (e.g., indium tin oxide) which provides a resistive heating function  200  to prevent condensation, frost, and icing of the transmit/receive window. This implementation of an optically transparent heater across the entire surface area of the window ensures uniform heating with modest power requirements, compared to less efficient or less uniform heating methods like a heated window mount, an internal heater, or a heated telescope housing. 
   The use of this thin-film heater coating permits superior optical wavefront quality, compared to heater wires or a heater element laminated between (or bonded to) optical substrate(s). The window heater  210  is thermostatically controlled, and the electrically conductive coating is index-matched to air at the operational laser transmit and receive wavelengths to ensure low optical losses. 
   The refractive receiver optics  610 , nominally 3″, provide a larger collecting aperture than other designs (e.g., 3 dB link benefit over a 2″ receiver). Larger apertures are readily available as fresnel lenses, but these optics have significant scatter and a large blur size which adversely impacts detector size, cost, and bandwidth. In addition, fresnel lenses lack the thermo-mechanical stability required for long-life and system operation under extreme environmental conditions (e.g., desert summers and sub-zero winters). 
   For these reasons, the preferred embodiment for the SupraConnect design employs a 3″ refractive lens  610  for the receiver collecting aperture (rather than a fresnel optic), followed by a smaller lens  613  with more optical power near the detector  110  to achieve a fast optical system (e.g., f/1.2). This approach is generally superior to a fire-polished molded glass asphere, which tends to have poorly controlled surface figure and focal length and a spot size which is very unforgiving of receiver pointing error. 
   The fast optical design of the preferred embodiment permits the length of the receiver  101  to be kept short for compactness (circa 5″), and also permits the use of a small detector  110  (e.g., 0.5 mm) with a narrow field of view (e.g., 6 mrad) to help reject off-axis light sources. A 785 nm narrowband optical filter  614 , with short and long wavelength blocking, is used to reject out-of-band background light, and is located just before the smaller lens  613  to minimize the filter size and cost. The filter passband is designed to accommodate the laser center-wavelength manufacturing tolerance and passband shift due to the angle-of-incidence variation represented by the converging cone of energy. 
   The transmitter  150  (typically 2″) comprises of a pair of refractive lenses  607 , either glass or optical grade plastic, with anti-reflection coatings for the near-infrared laser wavelengths of interest (e.g., 770-870 nm, or 1480-1580 nm). The use of a pair of commercially available lenses  607  with spherical curvature results in a cost-effective implementation to reduce the laser beam divergence to the desired value with controlled aberrations. These lenses  607  are operated at about f/2, which sacrifices some energy from the laser diode fast axis  155 . but provides a more uniform illumination at the aperture for greater eyesafety, and reduced ellipticity of the output beam divergence ratio. The resulting transmit beam has a beam divergence ratio of about 2:1, horizontal versus vertical. The transmitter housing  606  is designed for quick factory adjustment of the beam divergence (e.g., 1-20 mrad) by defocusing the lens assembly from the collimated position by means of turning a threaded sleeve  615  in or out, which is then locked in position (e.g., with set screws). 
   The system also requires the electronic circuits to drive higher currents at fast switching rates. Therefore, electronics were custom-designed for the transmitter  150 A, the receiver  101 A, a thermoelectric cooler  300 A, a window heater  200 A, and the input/output board  700 A as shown in  FIG. 4 , since they are not commercially available, to achieve proper optical signal modulation. 
   The wideband, high-current transmitter electronics  150 A include slow-start and transient surge protection features for enhanced laser  155  lifetime. The laser assembly  302  is thermally isolated and actively cooled (or heated) with the thermoelectric cooler  300  (TEC) and a proportional-integral-differential temperature controller  310 . This assembly  302  is directly coupled, via a copper bracket  616  to the massive aluminum casting  600  for optimal heat transfer. The transmitter circuitry  150 A is not activated until the laser  155  has been cooled (or heated) to the operating setpoint temperature  311 . This thermal design offers superior laser lifetime in thermally stressing environments such as desert operation. The long transmission of 0.3-10 km at above 1.5 Mbps requires a significantly high laser output power. And the resulted high electrical current increases the heat generated around the laser diode  155 , preferably the Circulaser Diode PS026-00 (780 nm; 50 mW; GaAlAs) made by Blue Sky Research at 3047 Orchard Parkway, San Jose, Calif. 95134 http://www.blueskyresearch.com/html/product_set.html, or SDL  5420  made by Spectra Diode Labs operated at 200 mW peak power. The increased heat of the lasers increases the junction temperature of the laser diode and drastically decreases the lifetime of the laser diode. In order to achieve high reliability, the heat must be extracted from the laser diode. This was accomplished by custom fitting the thermoelectric cooler  300  to the system. 
   The thermoelectrically cooled laser assembly  302  and drive electronics  300 A are provided for laser and system longevity, with a heat pipe  616  as the preferred embodiment for efficient thermal transfer from the laser assembly  302  to a large-capacity thermoelectric cooler  300  mounted to a massive heat sink  600  (e.g., the casting itself). The transmitter electronics board  150 A utilizes a ground plane, and is shielded on the remaining 5 sides by a metal cover to suppress radiated EMI from the wideband high-current pulses which would otherwise impair the receiver electronics  101 A. 
   The receiver  101  uses an avalanche photodiode  110  (APD) for optimum sensitivity, where the APD  110 , its gain control, and the preamplifier  115  and quantizer integrated circuits  125  have been selected for their combination of characteristics such that, when integrated together, they provide optimal receiver sensitivity and dynamic range. A hybrid receiver designed in-house with a GaAs FET transimpedance preampliflier  115  allows the receiver to achieve a premium sensitivity of 50 nW for 155 Mbaud with background light, which enable the system to outperform other existing systems by a factor of 4. The Anadigics preampliflier  115  sensitivity for 120 MHz BW and ¾ 10-9 BER with a 0.5 pf PIN detector is −37 dBm. The usable dynamic range for the signal is limited by the APD  110  specs to 1.5 μW. When the received light spot size is blurred to 320 μm at the APD  110 , the receiver electronics  101 A can tolerate a laser signal strength  6  as large as 15 μW (15 μW vs. 20 nW=50 dB dynamic range). The preamplifier  115  output is lowpass filtered  120  and ac-coupled into the quantizer  125 , which is input to a clock recovery  126  and data retiming device  127 . The phase-locked-loop characteristics of said device  127  are optimized for use in a fading atmospheric channel  630 . The receiver circuitry  101 A employs differential inputs and outputs for common-mode noise rejection and reduction of EMI emissions. The differential retimed data is input to a fiber-optic transmitter  715 , providing a fiber output interface  3  to the user equipment or an intermediate media converter. Similarly, the user input  4  to the laser modulator  160  is via fiber to the fiber-optic receiver  720 . 
   The interfaces  3  and  4  to the laser communication system are based on conventional computer and telecommunications standards, such as those employed in computer networks or telecom transmission equipment. Data is generally transmitted in binary digital form, but may also be transmitted as an analog waveform; for example, multiple NTSC television signals with video and stereo sound as a pulse frequency modulated waveform. Communication links are established on a point-to-point basis and may be aggregated to form a network interconnecting many nodes, for example in a star or hub topology, or may be used to realize a communications relay function. 
   Baffle Assembly 
   A novel and distinguishing feature of the invention is the use of a unique ‘soda straw’ light baffle for the receiver, nominally 3″ in diameter and 3″ in length for a 3″ receiver, which resembles a bundle of soda straws or a honeycomb, typically with a flat black coating. The baffle component is available from Tenebraex, 362 A Street, Boston, Mass. 02210, http://www.camouflage.com, and is commonly used to minimize reflections and glare from night vision systems. To the applicants&#39; knowledge, it is the first use of the baffles in a laser communication system to provide a means to limit the field of view of the receiver to minimize the effects of a setting or rising sun or moon. Other laser communication systems 1) use a more conventional optical means which increases the cost with additional optics and mechanics; 2) do not use a field stop at all, which does not provide a robust communication link in the environment; or 3) reduces the size of the detector to limit the field of view, which makes the system more susceptible to building and wind vibration. 
   The individual subapertures are typically ⅛″-⅜″ in dimension with a length-to-diameter aspect ratio&gt;6:1 (typically 12:1) to block (i.e., absorb and diffusely scatter) light rays from the sun or background-light sources a couple degrees or more off-axis and prevent them from being imaged onto the receiver detector plane. This light baffle may be implemented in a number of ways (e.g., with subapertures which are circular, square, or hexagonal) and in a variety of materials (e.g., plastic or aluminum), either as an ensemble of discrete subapertures (e.g., plastic straws or metal tubing), an extruded monolithic assembly, or a set of two or more disks with aligned subapertures (i.e., disks with an array of holes). 
   Referring to  FIGS. 5A and 5B , there is shown, in simplified form and in two views, the preferred embodiment of the baffle assembly  10  which consists of an array of hollow cylinders  12  held by a sleeve  14  in accordance with the invention. 
   As a matter of preference, and not of limitation, the entrance aperture  16  and exit aperture  18  of the hollow cylinders  12  are equal in size such that the cross sectional area of the cylinder is constant. The inner diameter, d, of each hollow cylinder  12 , corresponds to the diameter of the smallest aperture of the baffle assembly. Also, as a matter of preference, the lengths of the hollow cylinders  12  are the same. The field of view, θ, of the baffle assembly is equal to the inner diameter, d, of each hollow cylinder  12  divided by the length, L, of the baffle assembly  10 . The inner surface  20  of the hollow cylinders  12  is diffuse so as to absorb off-axis radiation entering the baffle assembly  10  at an angle exceeding its field of view θ. 
   The number of hollow cylinders  12  in the baffle assembly  10  depends on the outer diameter, D, of the hollow cylinders  12 , the cross sectional geometry of the hollow cylinders  12 , and the cross sectional geometry of the baffle assembly  10 , as defined by the system entrance aperture  22 . The array fill factor is the ratio of the transparent cross sectional area within the system entrance aperture  22  to its cross sectional area. The cross sectional geometry, inner and outer diameters, and length of the hollow cylinders is optimized to maximize the array fill factor while satisfying a predetermined field of view. 
   Referring now to  FIGS. 6A and 6B , there is shown, in simplified form and in two views, the application of the preferred embodiment  10  of the present invention, as adapted for use with the associated optical system  24 . The optical system  24  has an optical axis A-B, and compromises: a hollow housing  26 , front end  28  which supports a window  30  with a clear aperture that defines the system entrance aperture  22 , back end  32  with a system exit aperture  34 , with these apertures centered on the optical axis A-B, and with these apertures  22  and  34  sized and shaped to accommodate the field of view of the optical system  24 . The longitudinal axis A′-B′ of the baffle assembly  10  is coincident with the optical axis of the optical system  24 . 
   As a matter of preference, and not of limitation, to demonstrate the scalability and adaptability of the unique baffle assembly  10  to a telescope assembly  38  with a central obscuration  36 , the system entrance aperture  22  is obscured by the secondary mirror  40  at the front end  28 , and the system exit aperture  34  at the back end  32  is sized and shaped in the secondary mirror  40  to accommodate the field of view of the optical system  24 . The corresponding baffle assembly  10  for the optical system  24  comprises: an annular array of hollow cylinders  12  and an inner sleeve  44  whose inner radius is greater than or equal to the marginal ray height at the exit of the baffle assembly  10 , at a distance L from the system entrance aperture  22 , to prevent vignetting of the reflected receive beam  48  off the primary mirror  42 . 
   The manner of operation and use of the preferred embodiment of the baffle assembly  10 , as shown in  FIGS. 5A and 5B , can be easily ascertained by any person of ordinary skill in the art from the foregoing description, coupled with reference to the Figures of the drawings. With reference to  FIG. 5B , any ray of light whose entrance angle exceeds the field of view (of the baffle assembly  10 , will be absorbed by the inner surface  20  of the hollow cylinder  12  after one or more reflections. Rejection of the skew rays of light is achieved with either a diffuse coating applied to the inner surface  20  of the hollow cylinder  12 , constructing the hollow cylinders  12  from a diffuse material, or subsequently treating the extruded cylinder in a manner resulting in a diffuse surface. Rays parallel to the longitudinal axis of the baffle assembly  10  will exit the assembly through the exit aperture  18  to the accompanying optical system  24  as desired. Arrays of hollow cylinders can be easily constructed to operate with any system aperture cross sectional geometry, and likewise, there are no fundamental operational limitations on the cross sectional geometry of the individual hollow cylinders  12 . 
   Multiple Planes of Arrayed Apertures 
   Referring now to  FIG. 7A , there is shown, in a simplified side view, an alternative embodiment of an arrayed aperture baffle  62  which consists of multiple planes  50  of arrayed apertures  52 . As can be seen from  FIG. 7B , the planes are perpendicular to the hollow housing  26 , evenly spaced one behind the other, and oriented such that the centerline of a particular aperture on the first plane is collinear with respect to the centerlines of apertures at the same location of subsequent planes. The arrayed apertures  52  are optically transparent, and the front surface  54  and the back surface  56  of each of the planes  50  are diffuse to absorb off-axis rays whose entrance angle with respect to the longitudinal axis of the arrayed aperture baffle  62  exceeds the assembly&#39;s field of view, defined by the diameter of the arrayed apertures  52  divided by the distance from the front surface  54  of the first plane  58  and the back surface  56  of the last plane  60 . The geometry, diameter, and center-to-center spacing of the apertures as well as the number of planes is optimized to maximize the array fill factor while satisfying a predetermined field of view. 
   As a matter of preference, and not of limitation, for manufacturing simplicity the arrayed apertures  52  are equal in size and each plane is identical in size and shape. Further, the edges of the arrayed apertures  52  are sharp (e.g., a knife edge) to minimize scattering. 
   With reference to  FIGS. 7B and 6A , to demonstrate a modified arrayed aperture baffle assembly  64  to a telescope assembly  38  with a central obscuration  36 , the system entrance aperture  22  is obscured by the secondary mirror  40  at the front end  28 , and the system exit aperture  34  at the back end  32  is sized and shaped in the secondary mirror  40  to accommodate the field of view of the optical system  24 . The modified arrayed aperture baffle assembly  64  consists of a series of planes of arrayed apertures, each with a larger central aperture  66  whose inner radius is greater than or equal to the marginal ray height at the exit of the baffle assembly  10 , at a distance L from the system entrance aperture  22 , to prevent vignetting of the reflected receive beam  48  off the primary mirror  42 . 
   Multiple Planes of Arrayed Apertures-Operation 
   The manner of operation and use of the modified arrayed aperture baffle assembly  64 , as shown in  FIG. 7B , can be easily ascertained by any person of ordinary skill in the art from the foregoing description, coupled with reference to the Figures of the drawings. 
   With reference to  FIG. 7A , any light whose entrance angle exceeds the field of view θ of the arrayed aperture baffle  62 , will be absorbed by the front surface  54  and the back surface  56  of adjacent planes  50  after one or more reflections. Rejection of the skewed rays of light is achieved either by applying a diffuse coating to the front surface  54  and the back surface  56  of each of the planes  50 , constructing the planes  50  from a diffuse material, or subsequently treating planes  50  of arrayed apertures  52  to yield a diffuse surface. Rays parallel to the longitudinal axis of the arrayed aperture baffle  62  will exit the assembly through the exit aperture  18  to the accompanying optical system  24  as desired. Planes of arrayed apertures can be easily constructed to operate with any system aperture cross sectional geometry, and likewise, there are no fundamental operational limitations on the cross sectional geometry of the individual apertures. 
   Accordingly, it can be seen that, according to the present invention described in the foregoing Detailed Description and illustrated in the accompanying Drawings, a baffle assembly has been provided that effectively baffles off-axis radiation while maintaining high throughput of on-axis radiation, permits further reduction in the associated field-of-view, eliminates direction-sensitive system orientations, and thereby increases the system performance and availability of associated optical systems such as an optical receiver for atmospheric laser communications. The baffle assembly is easy to manufacture, light-weight, easy to align, and scalable to any size receiving aperture, regardless of geometry, with or without a central obscuration. 
   Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Various other embodiments and ramifications are possible within its scope. 
   For example, the cross sectional geometry of the hollow cylinders may be circular, square, hexagonal, or any other geometry which proves to be most effective considering volume manufacturability (e.g., extrusion or moulding), assembly, performance, and cost. As such, in the case of moulded assemblies, entire sheets of hollow cylinders or arrayed apertures may be manufactured and stamped, cored, or sawed to match the cross sectional geometry of the entrance aperture of the associated optical system, thereby eliminating individual cylinders and any sleeves, thereby reducing parts count, cost, and assembly. 
   Further, the simplicity and elegance of such an assembly lends itself to all system geometries beyond the conventional circular cross section, with or without an obscuration, described herein. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given. 
   The unique light baffle approach when employed in a laser communication system results in superior system performance compared to designs that image the received light directly onto the detector, without an intermediate field stop and reimaging relay optics to reject off-axis light sources (e.g., the sun, window glints, or street lights). 
   In summary, the SupraConnect design incorporates the following features:
         1) The distinctive cylindrical appearance of the cast aluminum housing with raised ribs which incorporates the following features integral to the casting: an integral hood for transmit/receive window protection, an integral sighting system-mount, and an integral cast heat sink;   2) The full-aperture window (nominally 6″) with the cold-mirror coating and its mirrored appearance on the exterior window surface, and its thermal control and background-light reflecting functions described above;   3) The use of an electrically conductive film on the interior surface of the window as a resistive heater for defogging and deicing;   4) The ‘soda straw’ light baffle for the receiver, described in the preferred embodiment, as a cost-effective means of preventing sunlight and other off-axis and nearly-on-axis light sources from being imaged onto the detector, avoiding the added length and additional expense associated with an intermediate aperture stop and re-imaging optics for the detector;   5) The use of a thermoelectrically cooled laser assembly for laser and system longevity; and   6) The use of separate tubes for the laser and its transmitter optics with zoom adjust for beam divergence, and the detector and its receiver optics, where these tubes are jointly captured and fastened to a common bulkhead, permitting rapid, accurate assembly and alignment procedures.       

   Alternative embodiments of the invention may be realized using optical elements with somewhat different optical prescriptions and spacings. 
   For example, separate transmit and receive wavelengths (e.g., 810 nm and 850 nm) may be used instead of the same wavelength. Such an approach may be advantageous in certain network applications with multiple co-located terminals, since the receiver optical bandpass filter may reject the undesired wavelength(s). It also provides enhanced transmit/receive isolation under foggy conditions where backscattered transmit energy may degrade the receiver sensitivity. 
   Separate transmitter and receiver windows (e.g., 2″ and 3″ instead of a single 6″ diameter window), may be used, with each window separately heated using a transparent heater of the type described in the preferred embodiment. 
   An anti-fog coating may be used on the window(s) to prevent condensation, in lieu of a transparent window heater, for climates where frost, blowing snow, and ice are not an issue. 
   Transmitter beam divergence adjustment may be achieved by defocus of the lens(es) with sliding or rotating tubes (i.e., “trombone action”) rather than a threaded adjustment mechanism. 
   A single 1″-2″ molded plastic asphere may be used, preferably with anti-reflection coatings, instead of the pair of lenses used to collect the laser energy and set the beam divergence. This alternative embodiment is a likely baseline change to the preferred embodiment described above. 
   A full-aperture (e.g., 3″ diameter) receiver optical bandpass filter may be used in front of the receiver lens to realize a narrower filter passband than is possible behind the lens in a conversing cone of rays. This realization could also employ a small diameter (e.g., 10 mm) short wavelength absorptive blocking filter (e.g., Schott RG715) near the detector. This alternative embodiment is likely when separate wavelengths are used for transmit and receive (e.g., 810 nm 850 nm), rather than a universal wavelength (e.g., 785 nm). Also feasible in this configuration is the use of a single f/0.6-f/1.5 aspheric receiver lens (e.g., 3″ diameter plastic or glass asphere), since there is no longer a concern with placing the bandpass filter in this fast cone of light. 
   The addition of closed-loop tracking of the incoming laser energy from a laser communication terminal by means of a position-sensing detector (e.g., quadrant detector) and processing electronics unit which is affixed to the sighting scope eyepiece may be used for the purpose of controlling an internal 2-axis steering device (or an external gimbal, e.g., elevation-overazimuth) which deviates the output laser beam direction in accordance with the time-varying position-sensing detector error signal. The position sensor and electronics may also be incorporated internally, instead of attached to the sighting system, using a beamsplitter cube to sample a small portion of the received energy. 
   The use of forward error correction block codes with deep interleaving (prior to the transmitter electronics, and subsequent to the receiver electronics) may also be applied for correction of burst errors arising from atmospheric scintillation. 
   While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.