Abstract:
A communication device uses one or two stacks of reflective deflectors to steer the electromagnetic waves carrying signals received and transmitted through a single telescope/aperture device. The signals outside the device may be circularly polarized while inside the device they are linearly polarized most of the time. The deflectors within each stack are transparent to the signals steered by the deflectors behind them. Since the deflecting wave band may shift with the changing angle of incidence of the signals due to steering, the wave bands are sufficiently spaced apart. When the signals impact the deflectors at nearly normal angles, the wave bands can be made more narrow. When more than one stack of deflectors is used, the spacing between the wave bands within one stack may be utilized by another stack. Beam splitters and a variety of other optical devices (such as quarter wave plates, half wave plates, Faraday rotators, and equivalent devices) are used to separate signals for further processing within the device. Instead of reflective deflectors, the device may generally use stacks of transmissive deflectors for similar effect in a similar way.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/422,185, filed on Oct. 29, 2002. The entire teachings of the above application are incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     The invention was supported, in whole or in part, by a grant Contract No. F19628-00-C-002 from USAF (Program 26). The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention is related to communication methods and devices using electromagnetic waves and has particular application to optical communications in free space. The growing interest in dynamic and rapidly deployable free space optical communication links is combined with efforts to increase traffic throughput, implement sophisticated networking capabilities for intelligent routing of data, and widen the utility of optical terminals. Multi-channel approaches can meet these demands, in particular through the use of wavelength division multiplexing (WDM). There are several advantages to WDM that justify its role in free space optical communications. WDM is well established in the 1.5 μm wavelength band, primarily due to availability of erbium amplifiers, and this wavelength range meets eye safety requirements. Furthermore, the necessary components for WDM already exist and growth in terrestrial communications has driven down their prices. In addition, WDM fiber systems are well understood, and this knowledge relates directly to free space WDM systems. 
     SUMMARY OF THE INVENTION 
     A method and communication device embodying this invention allow several deflectors to share the same aperture device, which may be a telescope. These deflectors are used to steer the signals carried by electromagnetic waves into and out of the communication device. These deflectors may be combined into one or more stacks for more efficient use of the hardware. To allow such stacking of deflectors, some deflectors, while deflecting the signals steered by them, are at the same time transparent for signals steered by some other deflectors in the same stack. The deflective properties of the deflectors may vary with the angle at which they are positioned in the course of steering, for this reason rather that a single frequency, each deflector may be assigned a wavelength band. This wavelength band may be made narrower if the electromagnetic waves impact the deflectors at a nearly normal angle. Each deflector may handle both the transmitted and received signals coming to and from the same direction. When two stacks of deflectors are used, it may be advantageous to intermesh the wavelength bands used by the deflectors in each stack. Further separation of signals within the device is achieved with beam splitters in combination with quarter wave plates and/or Faraday rotators. The deflectors may be transmissive or reflective. 
     The present invention provides a communication device, which may comprise an aperture structure and deflectors deflecting respective electromagnetic signals of respective wavelengths at respective angles, the electromagnetic signals passing through the aperture structure. Such device may further comprise an aperture linear/circular polarization device between at least one of the deflectors and the aperture structure. Some deflectors may be movable. The deflectors may form a stack, where at least one deflector in the stack passes a signal deflected by another deflector in the stack. The individual deflectors in the stack may pass signals deflected by other deflectors in the stack. The individual deflectors in the stack may deflect substantially all signals each within its respective non-overlapping wavelength band. Some deflectors in the stack may reflect signals at nearly normal incidence. The deflectors in the stack may be reflectors. The device may also comprise a second stack of deflectors with similar properties sharing the same aperture structure. Second stack deflectors&#39; wavelength bands may be intermeshed with first stack deflectors&#39; wavelength bands, i.e., at least one second stack deflectors&#39; wavelength band is located between two first stack deflectors&#39; wavelength bands and at least one first stack deflectors&#39; wavelength band is located between two second stack deflectors&#39; wavelength bands. Such device may employ a polarization beam splitter coupled to the aperture structure and the first stack, a signal path coupled to the polarization beam splitter, and a linear/circular polarization device or a polarization rotation device, positioned so that electromagnetic signals deflected by at least one of the deflectors in the first stack, are deflected at nearly normal angle, are linearly polarized when leaving and entering the polarization beam splitter, and, before being deflected, first pass through the polarization beam splitter and then pass through the linear/circular polarization device and, after being deflected, first pass through the linear/circular polarization device and then pass through the polarization beam splitter, the deflectors in the first stack being reflectors. Such device may further comprise a transmission path and a reception path, the signal path receiving from the transmission path at least one signal carrying communications transmitted by the device and the reception path receiving from the signal path at least one signal carrying communications received by the device, the electromagnetic signals deflected by at least one of the deflectors in the first stack carrying communications transmitted by the device and communications received by the device. Such device may further comprise a second stack of deflectors deflecting respective electromagnetic signals passing through the aperture structure at respective angles, the individual deflectors in the first stack passing signals deflected by other deflectors in the first stack and individual deflectors in the second stack passing signals deflected by other deflectors in the second stack, the deflectors in the second stack being reflectors; and a second linear/circular polarization device, positioned so that electromagnetic signals deflected by at least one of the deflectors in the second stack, are deflected at nearly normal angle, are linearly polarized when leaving and entering the first polarization beam splitter, and, before being deflected, first pass through the first polarization beam splitter and then pass through the second linear/circular polarization device and, after being deflected, first pass through the second linear/circular polarization device and then pass through the first polarization beam splitter, the direction of polarization of the electromagnetic signals passing through the first linear/circular polarization device being substantially orthogonal, within the signal path, to the direction of polarization of the electromagnetic signals passing through the second linear/circular polarization device. Such device may further comprise a second polarization beam splitter coupled to the signal path; a first transmission path; a first reception path; a second transmission path; a second reception path; a first polarized path coupled to the second polarization beam splitter; the first polarized path receiving from the first transmission path at least one signal carrying communications transmitted by the device and the first reception path receiving from the first polarized path at least one signal carrying communications received by the device; and a second polarized path coupled to the second polarization beam splitter; the second polarized path receiving from the second transmission path at least one signal carrying communications transmitted by the device and the second reception path receiving from the second polarized path at least one signal carrying communications received by the device. In such device electromagnetic signals deflected by at least one of the deflectors may carry communications transmitted by the device and communications received by the device and the aperture structure may be a telescope. 
     An embodiment of this invention may also be a device for deflecting electromagnetic waves comprising a first deflector deflecting electromagnetic waves within a first wavelength band and passing electromagnetic waves within a second wavelength band and a second deflector deflecting electromagnetic waves within a second wavelength band, the second deflector positioned to receive the electromagnetic waves passed through the first deflector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  shows the functioning of a quarter wave plate (QWP). 
         FIG. 2  shows the functioning of a 45 degree Faraday rotator (FR). 
         FIG. 3  shows the functioning of a half wave plate (HWP). 
         FIG. 4  shows the functioning of a directional rotation module (DRM). 
         FIG. 5  is a diagram showing the general functioning of an embodiment of this invention. 
         FIG. 6  shows an embodiment of this invention. 
         FIG. 7  shows the functioning of a single stack of reflective deflectors used in some embodiments of this invention. 
         FIG. 8  shows the functioning of two stacks of reflective deflectors used in some embodiments of this invention. 
         FIG. 9  shows the dependence of the reflective deflectors&#39; passbands on the angle of incidence. 
         FIG. 10  shows the functioning of a holographic optical element. 
         FIG. 11  shows an embodiment of this invention using a single stack of deflectors. 
         FIG. 12  shows an embodiment of this invention using two stacks of deflectors. 
         FIG. 13  shows an embodiment of this invention with additional details. 
         FIG. 14  illustrates some uses of directional rotation modules in some embodiments this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     Some embodiments of the present invention include at least one linear/circular polarization device with two terminals also known in the art as a quarter wave plate (QWP). Such devices may be used to convert incoming circularly polarized electromagnetic waves into linearly polarized electromagnetic waves and convert incoming circularly polarized electromagnetic waves into linearly polarized electromagnetic waves. A variety of such devices is known in the pertinent art. 
       FIG. 1  schematically shows the functioning of a QWP  75  where the directions of linear polarization (S and P) of electromagnetic waves entering and leaving the QWP  75  are shown as  71  and  72  and the directions of circular polarization (left hand circular (LHC) and right hand circular (RHC)), as defined by the Institute of Electrical and Electronics Engineers, are shown, respectively, as  74  and  73 . 
     In some types of quarter wave plates, the correspondence between the directions of linear polarization and of circular polarization is determined by the orientation of so called “slow” and “fast” axes shown in  FIG. 1  as  78  and  79  respectively. 
     The functioning of a QWP  75 , as pertinent to this invention, can be briefly summarized as following: 
     An incoming wave that is linearly polarized within the plane S  71  on QWP terminal A  76  produces on QWP terminal B  77  an outgoing wave that is RHC polarized  74 . 
     An incoming wave that is linearly polarized within a plane P  72  (P being orthogonal to the plane S) on QWP terminal A  76  produces on QWP terminal B  77  an outgoing wave that is LHC polarized  73 . 
     An incoming wave that is RHC polarized  74  on QWP terminal B  77  produces on QWP terminal A  76  an outgoing wave that is linearly polarized within S  71 . 
     An incoming wave that is LHC polarized  73  on QWP terminal B  77  produces on QWP terminal A  76  an outgoing wave that is linearly polarized within P  72 . 
     Some embodiments of the present invention include at least one 45 degree Faraday rotator (FR). Such two terminal devices may be used to rotate the polarization plane of the incoming linearly polarized light by a set amount in the same direction regardless of the terminal the wave enters. A variety of such devices is known in the pertinent art. 
       FIG. 2  schematically shows the functioning of a FR  95  designed to produce 45 degree rotation where the directions of linear polarization of electromagnetic waves entering and leaving the FR  95  are shown as  91 - 94 . 
     The functioning of a FR  95 , as pertinent to this invention, can be briefly summarized as following: 
     An incoming wave that is polarized within any plane PA in    91  on FR terminal A  96  produces on FR terminal B  97  an outgoing wave polarized within a plane PB out    94  which is rotated by 45 degrees to PA in . 
     An incoming wave that is polarized within any plane PB in    93  on FR terminal B  97  produces on FR terminal A  96  an outgoing wave polarized within a plane PA out    92  which is rotated by 45 degrees in the same direction to PB in  as PB out  is to PA in . 
     The principal properties of a FR, as pertinent to this invention, are the following: 
     An increment in the angle of PA in  produces the same increment in the angle of PB out . An increment in the angle of PB in  produces the same increment in the angle of PA out . If PB in  is PB out , PA in  and PA out , are orthogonal. If PA in  is PA out , PB in  and PB out  are orthogonal. 
     Some embodiments of the present invention include at least one half wave plate (HWP). Such two-terminal devices may be used to reflect the polarization plane of an incoming linearly polarized electromagnetic wave about a so-called fast axis. A variety of such devices is known in the pertinent art. 
       FIG. 3  schematically shows the functioning of a HWP  25  with the fast axis  30  where the directions of linear polarization of electromagnetic waves entering and leaving the HWP  25  are shown as  26 - 29 . 
     The functioning of a HWP, as pertinent to this invention, can be briefly summarized as following: 
     An incoming wave that is linearly polarized within a plane  26  (at a 67.5 degrees angle to the fast axis  30 ) on HWP terminal A  31  produces on HWP terminal B  32  an outgoing wave that is polarized within the plane  28  at 45 degrees to the original plane  26 . 
     An incoming wave that is linearly polarized within a plane  28  on HWP terminal B  32  produces on HWP terminal A  32  an outgoing wave that is polarized within the plane  26 . 
     An incoming wave that is linearly polarized within a plane  27  (at a 22.5 degrees angle to the fast axis  30 ) on HWP terminal A  31  produces on HWP terminal B  32  an outgoing wave that is polarized within the plane  29  at 45 degrees to the original plane  27 . 
     An incoming wave that is linearly polarized within a plane  29  on HWP terminal B  32  produces on HWP terminal A  31  an outgoing wave that is polarized within the plane  27 . 
     Some embodiments of the present invention include at least one directional rotation module (DRM).  FIG. 4  shows the operation of a DRM  40  implemented as a combination of a HWP  49  with its fast axis  351  and a FR  50 . Arrows  41 - 48  show the direction of polarization of linearly polarized electromagnetic waves entering and leaving the DRM  40 . Linear polarization planes represented as arrows  41 ,  42 ,  44 , and  47  are at a 67.5 degrees angle to the fast axis  351 . Linear polarization planes represented as arrows  43 ,  45 ,  46 , and  48  are at a 22.5 degrees angle to the fast axis  351 . The DRM  40  allows linear polarization to be rotated 90 degrees when electromagnetic waves travel in one direction and to stay unchanged when they travel in the opposite direction. 
     Some embodiments of the present invention include at least one polarization beam splitting device or polarization beam splitter (PBS). A variety of such devices is known in the pertinent art. The functioning of one type of PBS, as pertinent to this invention, can be briefly summarized as following: PBS reflects at some non-zero angle electromagnetic waves linearly polarized within plane P 1  and is transparent to electromagnetic waves linearly polarized within plane P 2 , P 2  being orthogonal to P 1 . 
       FIG. 5  schematically shows functioning of an embodiment of this invention. The device  3  transmits electromagnetic signals  1  and receives electromagnetic signals  2  via a single aperture structure  4 . The aperture structure  4  allows transmission and receiving of electromagnetic waves  5 ,  6  from and to multiple directions and with multiple wavelengths λ. The directions of electromagnetic waves  5 ,  6  can be changed using steerable deflectors within the device  3 . 
       FIG. 6  shows a specific embodiment of the device  3  of  FIG. 5 . The dotted lines show the path traveled by electromagnetic waves carrying signals. 
     In this embodiment of the invention, the aperture device  4  is a large field of view off-axis afocal telescope. It stands between the optical paths of incoming and outgoing signals outside the device  101  and inside the device  102 . These signals are carried by circularly polarized electromagnetic waves. Along the optical path  103  between QWP  19  and PBS  21 , the signals are linearly polarized. QWP  19  is responsible for the conversion of circularly polarized signals into linearly polarized signals and vice versa, and any linear/circular polarization device performing this function may be used in this position. For left-hand and right-hand circular polarizations on  102 , the corresponding directions of linear polarization on  103  are perpendicular. The PBS  21  is positioned so that it is transparent to signals on  103  polarized along one direction, so that these signals travel along  104 , and reflects the signals with perpendicular polarization so that they travel along  105 . 
     If in positions  14  and  18  the embodiment employs QWPs or any linear/circular polarization devices, the signals on  107  and  108  are circularly polarized before they impact one of the deflectors  61 - 68 , and they are circularly polarized in the opposite direction after the deflection After reentering the QWPs in the opposite direction, these signals are converted back into linearly polarized signals, but the direction of the polarization is orthogonal to the original direction, so that a signal for which the PBS  21  was transparent ends up being reflected on PBS  21 , and vice versa. The functioning of the deflectors  61 - 68  will be explained below. 
     Instead of a QWP, the embodiment shown on  FIG. 6  may employ a FR in positions  14  and/or  18  to the same overall effect. The polarization planes of linearly polarized signals on  107  and  108  are rotated by 45 degrees before the signals impact one of the deflectors  61 - 68 . After reentering the FR in the opposite direction, the polarization planes of these signals are further rotated by 45 degrees and end up being polarized in the direction orthogonal to the original direction, so that a signal for which the PBS  21  was transparent ends up being reflected on PBS  21 , and vice versa. 
     Therefore, the embodiment on  FIG. 6  provides the following paths for signals leaving the device through the aperture  4 : 
     Path segment  106  to PBS  21  to path segment  105  to QWP or FR  14  to path segment  108  to one of the deflectors  61 - 64  to path segment  108  to QWP or FR  14  to path segment  105  to PBS  21  to path segment  103 . 
     Path segment  106  to PBS  21  to path segment  104  to QWP or FR  18  to path segment  107  to one of the deflectors  65 - 68  to path segment  107  to QWP or FR  18  to path segment  104  to PBS  21  to path segment  103 . 
     Both of these paths can also be traversed in the opposite direction for signals entering the device through the aperture  4 . 
     If, as in some embodiments, the deflectors  61 - 68  are used to direct two signals each (received and transmitted), the embodiment on  FIG. 6  then may handle up to 16 independent signals traveling to and from 8 different directions. These reflectors may be movable to allow steering of signals in different directions outside the device or fixed for a set of predetermined directions of communications. The movement of the reflectors may be achieved by placing each detector in a electronic motor-controlled mount that can steer the deflected beam in at least the two directions of pitch and yaw. 
     Note that the path segment  106  carries all the signals handled by the device and therefore it will be referred to as “signal path”. Note also that within the signal path  106  the direction of polarization of signals passing through QWP or FR  18  is substantially orthogonal to the direction of polarization of signals passing through QWP or FR  14 . 
     The signals on the signal path  106  may be separated using the PBS  22  and further separated so that the transmission paths  109  and  111  carry signals T 1,3,5,7  and T 2,4,6,8  for transmission by the device (shown as in  FIG. 5 ) while the reception paths  110  and  112  carry signals R 1,3,5,7  and R 2,4,6,8  received by the device (shown as  2  in  FIG. 5 ). 
     The embodiment shown on  FIG. 6  uses reflective wavelength selective reflectors in the role of deflectors  61 - 68 . The functioning of these reflectors as deflectors is illustrated on  FIGS. 7 and 8 . They may be used to steer the signals as they enter and leave the device and each deflector  61 - 68  may handle signals transmitted to and received from the same direction via the aperture device  4 . The arrangement of deflectors  61 - 68  as two stacks  61 - 64  and  65 - 68  permits sharing of the of the aperture  4  and of other elements within the device by several signals. Deflectors  64  and  68  may be referred to as front deflectors, while deflectors  61  and  65  may be referred to as back deflectors. 
       FIG. 7  shows the properties of wavelength selective reflectors as used in an embodiment of the invention. 
     The horizontal axis for the graphs  161 - 164  represents the wavelength. The vertical axis for graphs  161 - 164  represents the reflectivity of the respective reflector at those wavelengths (high value indicating reflection, low value indicating transmission). Graphs  161 - 164  show, for reflectors  61 - 64 , the reflectivity at normal incidence as solid lines. The dashed lines represent the reflectivity at the maximal angle of incidence used in the shown embodiment of the invention, i.e. when the given reflector is steered to the maximal angle with respect to the path segment  105 . 
     The wavelength selective reflector reflects some electromagnetic waves and is transparent for all wavelengths used by signals steered by the wavelength selective reflectors behind it. If a movable wavelength selective reflector is transparent for different wavelengths depending on the angle of their incidence on the wavelength selective reflectors, the reflector at all its positions should reflect the electromagnetic waves steered by it and stay transparent for the signals that are handled by the reflectors behind it. For example, the wavelength selective reflector  63  at all its positions is transparent for transmitted signals T 4  and T 2  and for received signals R 2  and R 4  handled by reflectors  61  and  62 , respectively. The reflector  61  does not have to be transparent for any wavelength. Note that in this embodiment, the same reflector is used to receive and transmit signals from and to the same direction. 
     As can be seen, the functioning of the stack of reflectors  61 - 64  is unaffected by the motion of the reflectors despite the change in their reflective properties. 
     This design may be extended for two stacks of deflectors as shown on  FIG. 8  which is analogous to  FIG. 7  but shows the allocation of spectrum between two stacks of four deflectors each. 
     The horizontal axis for the graphs  165 - 168  represents the wavelength. The vertical axis for graphs  165 - 168  represents the reflectivity of the respective reflector at those wavelengths (high value indicating reflection, low value indicating transmission). Graphs  165 - 168  show, for reflectors  65 - 68 , the reflectivity at normal incidence as solid lines. The dashed lines represent the reflectivity at the maximal angle of incidence used in the shown embodiment of the invention, i.e. when the given reflector is steered to the maximal angle with respect to the path segment  104 . The wavelengths T 2 , T 4 , T 6 , T 8 , R 2 , R 4 , R 6 , and R 8  together with shaded areas around them correspond to the wavelengths and wave bands allocated to the stack  61 - 64  as shown in  FIG. 7 . 
     The interleaved structure of wave bands used by stacks  61 - 64  and  65 - 68  allows efficient utilization of the available spectrum. The usage of a relatively narrow spectrum for operation of an embodiment of the invention may be beneficial for example in cases when some of the components used in the embodiment (such as HWP or QWP) operate properly only within such a narrow spectrum. 
       FIG. 9  illustrates the properties of bandpass optical filters that may be used as a part of a wavelength selective reflectors  61 - 68  used as deflectors in some embodiments. A conventional narrow bandpass interference filter contains alternating dielectric thin film layers constructed of high and low index media. These filters rely on Fabry-Perot effects, and the optical path length through the individual dielectric layers plays a key role in determining the spectral characteristics of the filter. When the angle of incidence changes, this alters the effective path length through the layers, thus modifying the center wavelength of the filter. Moving away from normal incidence causes such a filter to experience a shift to shorter wavelengths in its center wavelength because the round trip path length for constructive and destructive interference shortens. 
     For a multilayer dielectric structure, an incident plane wave sees an effective index n eff  that can be approximated by the average index within the structure. The effective index can be used to determine macroscopically how a plane wave would interact with the medium under Fresnel reflections and transmissions. This abstraction provides a means of predicting how the center wavelength of the filter, λ(θ), varies as a function of angle of incidence, θ. This angle dependence follows the formula:
 
λ(θ)=λ θ=0  cos(θ/n eff )
 
     For conventional dielectric narrowband filters, n eff  is typically approximately between 1.5 and 1.8. For higher index materials one can obtain values of 3.2 and greater using, for example, III-V semiconductor materials, such as GaAs/AlGaAs or InGaAs/AlAsSb. A plot of the change in center wavelength, δλ, as a function of the angle of incidence is shown in  FIG. 9  for different n eff  values. One can use Snell&#39;s law to understand that a high index material will bend light more towards the normal than a low index material; this concept extends to the case here, where an effective index describes Fresnel interactions. Such high index narrowband filters provide low angle sensitivity for the wavelength selective reflectors thus allowing different signals&#39; frequencies to be spaced as closely as about 1 nm (about 125 Ghz) and at the same time keeping the field of view as wide as approximately 10 degrees. 
       FIG. 10  illustrates the functioning of holographic optical elements, which may be used to implement deflectors (HOE deflectors) as an alternative or in addition to wavelength selective reflectors in embodiments of this invention. 
     Holographic optical elements (HOE) are constructed by photorefractive imprinting of a periodic structure within a glass substrate and can produce chromatically-dependent bending of light. Their basic operation is similar to a diffraction grating with the exception that the holographic element can operate in transmission instead of reflection. HOEs can have extremely narrow spectral features. They may be produced using photothermorefractive glass (PTRG) technology which provides high spectral efficiency and may be used for wavelengths near 1550 nm. 
     The operation of a transmission-mode HOE is shown in  FIG. 10 . Laser beams at two different input wavelengths are incident upon the device in the vicinity of normal incidence. λ 1  (for example, 1550 nm) is Bragg-matched to the grating so that it is diffracted by the HOE, while λ 2  (for example, greater than 1560 nm or less than 1550 nm) is far enough outside of the HOE&#39;s spectral zone of selectivity so that it passes through the HOE undeflected. The diagram shows the case where two HOEs are combined into a single glass substrate thus creating a two-dimensional HOE deflector. A single conventional HOE steers in one plane. 
     The use of the deflectors such as or similar to HOE deflectors in an embodiment of this invention is shown in  FIG. 11 . Each deflector  55 - 57  acts upon a single wavelength channel chosen among λ 1 -λ N , directing it into the common path. There is an incoming and outgoing signal for each wavelength λ 1 -λ N , each signal at the same wavelength. The Faraday rotator  53  and half-wave plate  52 , acting in combination as a directional rotation module, apply different polarizations to the incoming, received (Rx), and outgoing, transmitted (Tx), signals to isolate them, while eliminating any polarization effects within the wavelength steering deflectors  55 - 57  by keeping all wavelengths there at a common linear polarization (denoted as S). A quarter wave plate  51  transforms the linearly polarized waves into right hand circularly (RHC) polarized waves passing through the aperture. The PBS  54  separates the received and transmitted signals which are polarized within orthogonal planes P and S on the right side of the DRM  52 - 53  and have a common polarization S on the signal path  300 . 
     The advantages of this transmission-mode approach over the reflection-mode bandpass filter approach described above include fewer optical components and no direct retroreflections which improves isolation. Furthermore, the transmission-mode operation also allows one to neglect the polarization effects of the HOE within HOE deflectors and operate with undefined, arbitrary polarizations. 
     Another embodiment of this invention using HOE devices to steer wavelengths, as shown in  FIG. 12 , increases the number of channels by using orthogonal polarizations. In this setup, waves with different linear polarizations (denoted as P and S) are steered either by the deflectors  55 - 57  or deflectors  58 - 60 . Each of the deflectors  55 - 60  may contain individual HOEs that deflect the appropriate wavelength. The PBS  83  separates received signals and joins transmitted signals of orthogonal linear polarization. Directional rotation modules composed of HWP  52  and FR  53  and HWP  80  and FR  81  change the polarization of signals depending on their direction and allow use of PBS  82  and PBS  54  for separating and joining transmitted and received signals Tx and Rx before and after they traverse the signal paths  300  and  301 . 
     Deflectors  57  and  60  may be referred to as front deflectors, while deflectors  55  and  58  may be referred to as back deflectors. 
       FIG. 13  shows an embodiment of this invention similar to the embodiment shown in  FIG. 5 . The components absent in  FIG. 5  include the following elements employed for precise aiming and tracking of the signals outside the device: nutating fiber coupling systems,  201  and  202 , mirrors applying point ahead angle to compensate for relative motion between terminals,  203  and  204 , quad-area detectors,  205  and  206 , fast steering mirror for removing mechanical jitter,  207 , wide area detector for acquisition,  208 , large steering mirror pointing to the general theater of operation,  210 . Directional rotation modules  211  and  213  composed of half-wave plates and Faraday rotators change the polarization of signals depending on their direction and allow the use of PBS  212  and PBS  214  for separating and joining transmitted and received signals. 
       FIG. 14  illustrates additional possible uses of directional rotation modules in embodiments of this invention. The directional rotation module composed of HWP  221  and FR  222  gives in combination with QWP  19  opposite (LHC and RHC) circular polarizations to the received and transmitted signals handled by the same reflector (chosen among  61 - 68 ) for the same target. This provides additional isolation between such signals. The directional rotation module  221 - 222  gives orthogonal polarizations on  103 ″ to any two signals with identical linear polarization moving in opposite directions along  103 ′. 
     The directional rotation module composed of HWP  223  and FR  224  may take the role of the directional rotation modules  211  and  213  of  FIG. 13 . 
     As can be seen from the above description, embodiments of this invention may use a single aperture to provide significant size, weight, power, and cost reductions for airborne, spacecraft, and terrestrial applications. The directional rotation module combined with a wavelength steering mirror assembly provides polarization division multiplexing of channels and high optical isolation between transmit and receive channels and reduces hardware requirements by redundant use of components. The optical bandpass filter with low sensitivity to angle-of-incidence allows a high number of arbitrarily pointed WDM channels. The terrestrial network applications of this invention exist in dense urban areas where fiber deployment is impractical and/or expensive. The specific applications of this invention include network nodes, relay links between points of presence, and wavelength and polarization routing switches. In all these cases aperture sharing has the advantages of reduced cost, rapid provisioning, and more efficient utilization of size, weight, and power. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.