Abstract:
A method and apparatus for achieving dynamic intensity modulation of the channels in a wavelength-division multiplexed optical communication system is presented. Wavelengths are spatially separated into a plurality of channels, the polarization states of which are individually modulated. The channels can be combined or filtered by polarization states to achieve the desired intensity in the output signal. An exemplary embodiment includes at least a polarization modulator, a birefringent wedge, a lens, and a dispersive element (e.g., diffraction grating) arranged in various order. Each segment of the polarization modulator can be made to rotate the polarization direction of an incident channel by a specified angle. A half-wave plate may be inserted between the second dispersive element and the second birefringent wedge to eliminate polarization-dependent loss. Optionally, a parallel birefringent plate may be inserted after the second birefringent wedge to reduce polarization mode dispersion.

Description:
RELATED APPLICATIONS 
     This application is related to Provisional Application No. 60/209,300 filed on Jun. 5, 2000, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to the field of optical communications, and more specifically, to modulation of optical signals. 
     2. Discussion of Related Art 
     Communication networks increasingly rely upon optical fiber for high-speed, low-cost transmission. Optical fibers were originally envisioned as an optical replacement for electronic transmission media, such as high-speed coaxial cable and lower-speed twisted-pair cable. However, even high-speed optical fibers are limited by the electronics at the transmitting and receiving ends. For switching purposes, operating speeds are generally rated at a few gigabits per second, although 40 Gb/s systems have been prototyped. Such high-speed electronic systems are expensive and still do not fully exploit the inherent bandwidth of fiber-optic systems, which can be measured in many terabits per second. 
     All-optical transmission systems offer many intrinsic advantages over systems that use electronics within any part of the principal transmission path. Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. For example, the use of wavelength division multiplexing, in combination with time division multiplexing, permits a long distance telephone company to carry thousands or even millions of phone conversations on a single fiber. Wavelength division multiplexing makes it possible to effectively use the fiber at multiple wavelengths, as opposed to the costlier option of installing additional fibers. Using wavelength division multiplexing, optical signals can be carried on separate optical channels with each channel having a wavelength within a specified bandwidth. It is advantageous to carry as many channels as possible within the bandwidth where each channel corresponds to an optical signal transmitted at a predefined wavelength. 
     U.S. Pat. No. 4,655,547 to Heritage, et. al., entitled “Shaping Optical Pulses by Amplitude and Phase Masking,” which is herein incorporated by reference, discloses how an input optical signal can be spatially divided into frequency channels, for example with a diffraction grating. Then, the separated channels are independently operated upon by a segmented modulator. U.S. Pat. No. 5,132,824 to Patel et al., entitled “Liquid Crystal Modulator Array,” which is also herein incorporated by reference, discloses using liquid-crystal modulators to manipulate optical pulses. After separating the input optical signal into channels, each channel is separately phase-modulated or amplitude-modulated. The performance of wavelength division multiplexing systems is optimal when signal strength or intensity of each channel is adjusted dynamically. A system and a method for dynamically adjusting the intensity of each channel is needed. 
     SUMMARY 
     In accordance with the present invention, a modulation system is presented that can, in some embodiments, achieve dynamic intensity modulation of each channel in a wavelength-division multiplexed optical communication system. The wavelengths may be spatially separated into channels and individually modulated by changing the polarization state of each channel and using the polarization states to selectively combine or filter channels and achieve the desired intensity modulation. 
     An exemplary embodiment of the present invention includes two birefringent wedges, two lenses, and two dispersive elements (e.g., diffraction gratings) arranged symmetrically at two opposing sides of a segmented polarization modulator. Each segment of the polarization modulator can be made to alter the polarization direction of an incident beam of light by a specified angle. A half-wave plate may be inserted between the second dispersive element and the second birefringent wedge to eliminate polarization-dependent loss. Additionally, a parallel birefringent plate may be inserted after the second birefringent wedge to compensate for any polarization mode dispersion. 
     A more compact embodiment of the invention can use a reflective surface on the polarization modulator to redirect the beams of light through a dispersive element, a lens, and a birefringent wedge which the beams passed through to reach the reflective surface. In some embodiments, two prisms may be placed around the polarization modulator, instead of a reflective surface, to direct the channels back in the direction from which they came. Alternatively, an aperture may be used to prevent all of the output signals from entering the signal transfer medium, thereby achieving the desired attenuation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A shows an embodiment of a wavelength-selective intensity modulator according to the present invention. 
     FIG. 1B shows the embodiment shown in FIG. 1A from the perspective of a y-z plane. 
     FIG. 2 schematically shows an input signal entering and multiple output signals exiting a wavelength-selective intensity modulator according to the present invention. 
     FIG. 3A shows an embodiment of a wavelength-selective intensity modulator according to the present invention which includes two birefringent displacers sandwiching a segmented polarization modulator. 
     FIG. 3B shows a y-z plane perspective of the embodiment shown in FIG.  3 A. 
     FIG. 4 shows a compact version of the embodiment shown in FIG.  3 A and FIG. 3B using a half-wave plate to reduce polarization dependent loss. 
     FIG. 5A shows an embodiment of the wavelength-selective intensity modulator including two prisms and a dispersive element. 
     FIG. 5B shows the embodiment of FIG. 5A from the x-z plane. 
     FIG. 5C shows the polarization modulator used in the embodiment of FIG.  5 A. 
     FIG. 6 shows a wavelength-selective intensity modulator including a polarization modulator, a birefringent beam displacer, and an angled reflector. 
     FIG. 7 shows a wavelength-selective intensity modulator including an aperture and a micro-mirror array modulator. 
    
    
     DETAILED DESCRIPTION 
     A “channel,” as used herein, refers to a beam of light that was either spatially separated by wavelength or combined to form one or more rays of light. In some embodiments, a channel may have a wavelength range of less than one nanometer. A “channel,” therefore, does not indicate a particular state of polarization. A “beam,” as used herein, does not indicate a limited range of wavelength. A “fiber,” as used herein, refers to any medium through which optical signals can be transmitted, including but not limited to an optical fiber. 
     FIG. 1A depicts an embodiment of WDM wavelength-selective intensity modulator  1  including a first birefringent wedge  10 , a first dispersive element  11 , a lens  12 , a polarization modulator  16 , a second lens  17 , a second dispersive element  18 , a half-wave plate  19 , and a second birefringent wedge  20 , which are optically coupled. Birefringent wedge  10  has optic axis  48  lying along the z direction as defined by coordinate system  8 . Thus, when a beam  21  of arbitrarily polarized light travels along the axis as defined by coordinate system  8  and strikes birefringent wedge  10 , the beam is split into a first beam  22  polarized in the y-direction and a second beam  23  polarized in the z-direction. Birefringent wedge  10  may be made of any conventional birefringent material, such as calcite, yttrium vanadate, and yttrium orthovanadate. First dispersive element  11  and second dispersive element  18  may be wavelength-dispersive elements such as diffraction gratings or prisms. Polarization modulator  16  may be, but is not limited to, a liquid-crystal device, and may be substituted by any device that can significantly alter the polarization states of incident channels. Polarization modulator  16  includes a plurality of segments, shown in FIG. 1A as segments  13 ,  14 , and  15  along the x-axis as defined by coordinate system  8 . Each segment can be made to change the polarization state of an incident beam. Half-wave plate  19  rotates the polarization direction of an incident beam by ninety degrees. Second birefringent wedge  20  has optic axis  49  lying substantially in the z-direction. 
     FIG. 1B shows the embodiment of FIG. 1A from the perspective of y-z plane as defined by coordinate system  8 . For clarity of illustration, FIG. 1B does not show first and second dispersive elements  11  and  18 . Input signal  21  is typically a collimated broad-band beam of an arbitrary polarization state. The polarization state of input signal  21  may be time-varying. When input signal  21  from an input optical fiber (not shown) strikes first birefringent wedge  10 , input signal  21  splits into two orthogonally linearly polarized beams, first beam  22  and second beam  23 , along the y-axis as defined by coordinate system  8 . For clarity of illustration, FIG. 1A shows first beam  22  with a solid line and second beam  23  with a dashed line. If first birefringent wedge  10  has a greater index of refraction along the extraordinary axis (e.g., as in yttrium orthovanadate) than along the ordinary axis, first beam  22  will have a polarization direction perpendicular to optic axis  48 , and second beam  23  will have a polarization direction parallel to optic axis  48 . 
     First and second beams  22  and  23  strike dispersive element  11  (not shown in FIG.  1 B), which spatially separates first beam  22  by wavelength into channels  24  and  26  and spatially separates second beam  23  into channels  25  and  27 , along the x-axis as defined by coordinate system  8 . Although only two wavelength channels are shown for clarity, input beam  21  may contain more than two wavelength channels. Lens  12  focuses the incident channels  24 ,  25 ,  26 , and  27  onto different segments of polarization modulator  16 . Lens  12  focuses beams  26  and  27  onto segment  13  of polarization modulator  16  and beams  24  and  25  onto segment  14  of polarization modulator  16 . Each of segments  13 ,  14 , and  15  can be set to change the polarization state of incident beams in a desired manner. After passing through segments  13  and  14 , channels  28  and  30 , which are assumed to be polarized in a direction perpendicular to optic axis  48  as mentioned above, may become elliptically polarized channels  32  and  34 , respectively. Similarly, channels  29  and  31 , which are assumed to be polarized in a direction parallel to optic axis  48 , may become elliptically polarized channels  33  and  35  after passing through segments  13  and  14 . Channels  32 ,  33 ,  34 , and  35  strike second lens  17 , which collimates channels  32 ,  33 ,  34 , and  35  to form channels  36 ,  37 ,  38 , and  39 . Collimation may be achieved by positioning lens  17  a focal length away from polarization modulator  16  in the particular medium between lens  17  and polarization modulator  16 . “Focal length,” as used herein, refers to the focal length of lens  17 . Collimated channels  36 ,  37 , 38 , and  39  strike second dispersive element  18 , which recombines the channels into output beams  40  and  41 . In the embodiment shown in FIG.  1 A and FIG. 1B, channels  36  and  38  are combined into first output beam  40 , and channels  37  and  39  are combined into second output beam  41 . First and second output beams  40  and  41  are generally elliptically polarized. 
     Output beams  40  and  41  each contain two orthogonal polarization components. Of the two polarization components in output beams  40  and  41 , the polarization components that result from rotation by polarization modulator  16  is rotated ninety degrees by half-wave waveplate  19 . Output beams  42  and  43  enter second birefringent wedge  20 , which has optic axis  49  lying substantially in the z-direction as shown in FIG.  1 A and defined by coordinate system  8 . Birefringent wedge  20  splits output beam  42  into output signals  44  and  45  according to polarization states. Similarly, birefringent wedge splits output channel  43  into output signals  46  and  47  according to polarization states. Output signals  45  and  46  propagate in a parallel direction with respect to each other, and are therefore both coupled into an output fiber (not shown). Output signals  44  and  47 , which constitute light having the original polarization states of first and second input beams  22  and  23 , propagate at an angle with respect to output signals  45  and  46 . Output signals  44  and  47  are therefore not coupled into an optical fiber, resulting in the attenuation of the output signal. By controlling the segments of polarization modulator  16 , the intensity of output signals  45 ,  46 ,  47 , and  48  can be modulated. 
     Although the embodiment shown in FIG.  1 A and FIG. 1B results in a non-zero polarization-mode dispersion (PMD) due to the finite thickness of first and second birefringent wedges  10  and  20 , the PMD can be easily compensated by inserting a planar parallel wave plate (not shown) after second birefringent wedge  20 . The parallel wave plate should have the combined thickness of first and second birefringent wedges  10  and  20 , and its optic axis should be perpendicular to optic axis  49 , i.e. lie along the y-direction as defined by coordinate system  8  and as shown in FIG.  1 A. 
     FIG. 2 depicts input signal  21  traveling through a signal transfer medium (e.g., optical fiber) and reaching WDM wavelength-selective intensity modulator  1  of the present invention through port  2 . Input signal  21  passes through input port  2  in substantially the x-direction as defined by coordinate system  8 . Output signals leave WDM wavelength-selective intensity modulator  1  through output port  3 . Electrical signals  5  control the segments of polarization modulator  16 . As previously explained, tuning the segments of polarization modulator  16  modulates the intensity of output signals that pass through output port  3 . 
     FIG. 3A shows an embodiment which includes first birefringent beam displacer  50  and second birefringent beam displacer  51  sandwiching polarization modulator  16  along the direction in which the channels propagate. Collimated input signal  21  strikes dispersive element  11  and spatially separates into channel  52  and channel  53  substantially along the x-axis as defined by coordinate system  8 . FIG. 3A depicts channel  52  with a dashed line and channel  53  with a solid line. Lens  12  focuses channels  52  and  53  onto different segments of polarization modulator  16 . After being focused, channels  52  and  53  are shown as focused channels  54  and  55 , respectively. Although two channels are shown for clarity, input signal  21  may include more than two channels. 
     FIG. 3B illustrates the embodiment of FIG. 3A from the perspective of y-z plane as defined by coordinate system  8 . For clarity of illustration, FIG. 3B omits dispersive elements  11  and  18 . First and second birefringent beam displacers  50  and  51 , which are identical, have optical axes in the y-z plane as defined by coordinate system  8 . In the example illustrated in FIG.  3 A and FIG. 3B, focused channel  54  strikes segment  13  of polarization modulator  16  and focused channel  55  reaches segment  14 . 
     Upon striking first birefringent beam displacer  50 , focused channel  54  separates into beam  56  and beam  58  of orthogonal polarization states. Beam  58  is polarized in the x-direction as defined by coordinate system  8 , and therefore passes through first birefringent beam displacer  50  without being displaced. Beam  56 , however, is polarized in the y-direction as defined by coordinate system  8 , and is therefore displaced along the y-direction as defined by coordinate system  8 . Thus, beams  56  and  58  are focused onto two spots on segment  13  that are separated along the y-axis as defined by coordinate system  8 . Segment  13  is set up so that after passing through segment  13 , at least a portion of beam  56  becomes polarized in the x-axis, forming beam  60 . Beam  60  passes through second birefringent beam displacer  51  without displacement. As for beam  58 , at least a portion of beam  58  becomes polarized in the y-axis by segment  13  and forms beam  62 . Beam  62  is displaced along the y-axis as defined by coordinate system  8 . Beams  60  and  62  combine to form output channel  64 . Output channel  64  is coupled into an output optical fiber (not shown). 
     The portion of beam  56  having a polarization state unaffected by segment  13  is displaced along the y-axis and forms channel  66 , as shown by the dashed line. The portion of beam  58  having a polarization state unaffected by segment  13  passes through second birefringent beam displacer  51  without being displaced, and forms channel  68 , shown by the dashed line. Channels  66  and  68 , unlike output channel, 64 , are not coupled into the output fiber. Therefore, controlling the degree of polarization of beams  56  and  58  through segment  13  of polarization modulator  16  results in the modulation of output signal  64 . 
     Focused channel  55  passes through first and second birefringent beam displacers  50  and  51  and polarization modulator  16  in a manner similar to focused channel  54 . Upon striking first birefringent beam displacer  50 , focused channel  55  separates into beam  57  and beam  59 . Beam  59  is polarized in the x-direction as defined by coordinate system  8 , and therefore passes through first birefringent beam displacer  50  without being displaced. Beam  57 , however, is polarized in the y-direction as defined by coordinate system  8 , and is therefore displaced along the y-direction as defined by coordinate system  8 . Thus, beams  57  and  59  are focused onto two spots on segment  14  that are separated along the y-axis as defined by coordinate system  8 . Segment  14  is set up so that the polarization state of a portion of the incident light is altered. Thus, after passing through segment  14 , a portion of beam  57  becomes polarized in the x-axis, forming beam  61 . Beam  61  passes through second birefringent beam displacers  51  without displacement. As for beam  59 , a portion of beam  59  becomes polarized in the y-axis by segment  14  and forms beam  63 . Beam  63  is displaced along the y-axis as defined by coordinate system  8 . Beams  61  and  63  combine to form output channel  65 , which is coupled into an output fiber (not shown). 
     The portion of beam  57  having a polarization state unaffected by segment  14  is displaced along the y-axis and forms channel  67 , as shown by the dashed line. The portion of beam  59  having a polarization state unaffected by segment  14  passes through second birefringent beam displacers  51  without being displaced, and forms channel  69 , shown by the dashed line. Channels  67  and  69 , unlike output channel  65 , are not coupled into the output optical fiber. Therefore, controlling the degree of polarization of beams  57  and  59  through segment  14  of polarization modulator  16  results in the modulation of output signal  65 . If input signal  21  is composed of more than two channels, a person of ordinary skill in the art would understand that how to achieve attenuation with the other channels in the manner described above with regard to channels  52  and  53 . 
     FIG. 4 shows a compact version of the embodiment shown in FIG. 3A and 3B. The embodiment in FIG. 4 uses reflective polarization modulator  80  instead of polarization modulator  16 , dispersive element  11 , and a birefringent wave plate (not shown). Reflective polarization modulator  80 , which has a reflective surface  81 , is a waveplate with retardation tunable in the range between zero and quarter wavelength. The optic axis of the wave plate lies at a 45-degree angle to both the x-axis and the y-axis as defined by coordinate system  8 . When input channels  52  and  53  strike first birefringent beam displacer  50 , the portion that is polarized in the x-direction as defined by coordinate system  8  (i.e., beams  58  and  59 ) passes through without displacement, while the portion that is polarized in the y-direction (beams  56  and  57 ) is displaced. When retardation for a channel is tuned to zero, reflective polarization modulator  80  does not change the polarization of incident channels. Thus, when retardation is set at zero, channels  52  and  53  (which become focused channels  54  and  55  after passing through lens  12 ) travel through birefringent beam displacer  50  and reflective polarization modulator  80  in the path shown with solid lines, and the channels are not attenuated. On the other hand, when retardation is set at a non-zero value, reflective polarization modulator  80  changes the polarization of incident beams. When the polarization direction is changed, the reflected channels each split into two beams upon passing through birefringent beam displacer  50  on their way to dispersive element  11 . The portion of beam  58  that became polarized in the y-direction as defined by coordinate system  8  is displaced along the y-direction, as shown by dashed line  109   a.  The portion of beam  56  that became polarized in the x-direction as defined by coordinate system  8 , on the other hand, is not displaced when passing through first birefringent beam displacer  50 , as shown by dashed line  109   b . Beams depicted by dashed lines  109   a  and  109   b  propagate in different directions from beams  64  and  65 , and are not coupled into the output fiber. Elimination of the two beams leads to channel attenuation, and the degree of attenuation is controlled by tuning reflective polarization modulator  80 . 
     Half-wave waveplate  71  with an optic axis aligned 45 degrees to both the x- and the y-axes as defined by coordinate system  8  can be used to eliminate polarization dependent loss (PDL). Half-wave waveplate  71  rotates the polarization of an incident channel to balance the diffraction efficiencies of the gratings. Thus, the PDL caused by the grating can be eliminated. Furthermore, PMD can be eliminated with a birefringent plate in the embodiment shown in FIG. 4, as described above with regard to the embodiment shown in FIG.  1 A and FIG.  1 B. 
     FIG. 5A depicts an embodiment of intensity modulator that uses birefringent beam displacers  90  and  91 , a dispersive element  11  (e.g., Echelle grating), a polarization modulator  16 , a lens  104 , and two prisms  106   a  and  106   b.  Input signal  21  passes through birefringent beam displacer  90  which has an optic axis in the x-z plane. Birefringent beam displacer  90  spatially separates input signal  21  along the x-axis as defined by coordinate system  8 , into beam  100  and beam  101  of orthogonal polarization states. In the example provided, beam  100  is polarized perpendicular to the optic axis of birefringent beam displacer  90 , and therefore passes through birefringent beam displacer  90  without displacement. Beam  101 , on the other hand, is polarized in the x-z plane which contains the optic axis of birefringent beam displacer  90 , and is therefore displaced along the x-axis. Upon striking dispersive element  11 , beam  100  separates into channels  102 - 1  through  102 -n along the y-axis as defined by coordinate system  8 , and beam  101  separates into channels  103 - 1  through  103 -n, also along the y-axis. As used herein, channels  102 -i and  103 -i refer to one of beams  102 - 1  through  102 -n and one of beams  103 - 1  through  103 -n, respectively, “n” indicating the total number of channels in each of beam  100  and beam  101 . Channels  102 - 1  through  102 -n and channels  103 - 1  through  103 -n travel through lens  104 , which focuses the channels onto prisms  106 . Lens  104  may include, for example, a cylindrical lens or two conventional, rotationally symmetrical lenses. 
     FIG. 5B depicts the embodiment of FIG. 5A from the x-z plane. In the x-z plane perspective, beams  100  and  101  are shown as separate lines, beam  100  is shown to be “under” channels  102 - 1  through  102 -n, and beam  101  is shown to be “under” channels  103 - 1  through  103 -n. Channels  102 - 1  through  102 -n and channels  103 - 1  through  103 -n do not pass through birefringent beam displacers  90  and  91 . After striking dispersive element  11 , channels  102 - 1  through  102 -n and channels  103 - 1  through  103 -n pass through lens  104  and strike surface  105   a  of first prism  106   a . Channels  102 - 1  through  102 -n reflect off surface  105   a  and pass through polarization modulator  16 . 
     FIG. 5C shows that polarization modulator  16  is segmented along the y-axis as defined by coordinate system  8 . Each segment can be tuned to change the polarization state of a portion of incident channels. Thus, at least a portion of channel  102 -i, which was initially polarized perpendicular to the optic axis of birefringent beam displacer  90 , becomes polarized parallel to the optic axis of birefringent beam displacer  90 . Likewise, at least a portion of channel  103 -i, which was initially polarized in the x-z plane which contains the optic axis of birefringent beam displacer  90 , becomes polarized perpendicular to the optic axis of birefringent beam displacer  90 . After passing through polarization modulator  16 , channels  102 -i and  103 -i reflect off surface  105   b  of second prism  106   b  and return to dispersive element  11  through lens  104 . Dispersive element  11  recombines channels  102 - 1  through  102 -n into beam  107 , and channels  103 - 1  through  103 -n into beam  108 . Both beam  107  and beam  108  travel through birefringent wedge  91 . In the exemplary embodiment of FIG. 5B, the optic axis of birefringent beam displacer  91  is parallel to the optic axis of birefringent beam displacer  90 . In those embodiments, the portion of beam  107  which changed its polarization state when it passed through polarization modulator  16  is indicated as beams  107   a , and is displaced in birefringent beam displacer  91  as shown by line  115 . The portion of beam  107  which did not change the polarization state upon passing through polarization modulator  16  passes through birefringent beam displacer  91  without displacement, and is denoted as beam  107   b . Beam  107   b  is not coupled into an output fiber, and therefore contributes to the attenuation. 
     The portion of beam  108  which did change its polarization state when it passed through polarization modulator  16  is denoted as beam  108   a , and passes through birefringent beam displacer  91  without displacement. Beam  108   a  combines with beam  107   a  to form an output signal which is coupled into an output fiber (not shown). The portion of beam  108  which did not change its polarization state is displaced along line  116  when it passes through birefringent beam displacer  91 , and is denoted as beam  108 -i. Like beam  107   b , beam  108   b  is not coupled into an output fiber. Thus, wavelength-selective attenuation is achieved by using polarization modulator  16  to control the polarization states of channels  102 -i and  103 -i. 
     FIG. 6 depicts an embodiment of wavelength-selective intensity modulator including polarization modulator  16 , first birefringent beam displacer  50 , angled reflector  79 , and a wavelength dispersive element (not shown). Polarization modulator  16  is segmented along the x-axis, and each segment can be tuned to modulate the polarization states of incident beams. Channels  52  and  53 , which are spatially separated along the x-axis as defined by coordinate system  8 , exit wavelength dispersive element  11  and pass through different segments of polarization modulator  16 . Channels  52  and  53  exit polarization modulator  16  with new polarization components. When channel  52  strikes first birefringent beam displacer  50 , the portion of channel  52  that is polarized along the x-direction pass through as beam  58  without displacement, while the portion that is polarized along the y-direction is displaced and becomes beam  56 . Similarly, when channel  53  strikes birefringent beam displacer  50 , it splits into beam  59  polarized in the x-direction, and beam  57  polarized in the y-direction as defined by coordinate system  8 . Beams  58  and  59  enter angled reflector  79  at a point that is separated along the y-axis from the point at which beams  56  and  57  enter angled reflector  79 . Beams  58  and  59  strike reflective surface  78   a  and propagate through first birefringent beam displacer  50  and polarization modulator  16  as beams  72  and  73 , respectively. Beams  56  and  57 , on the other hand, reflect off of reflective surface  78   b  and propagate through first birefringent beam displacer  50  and polarization modulator  16  as beams  64  and  65 , respectively. To achieve attenuation, only beams  64  and  65  (not output channels  72  and  73 ) may be coupled into an output fiber. Tuning the segments of polarization modulator  16  controls how much intensity remains in output channels  64  and  65 . Therefore, the intensity of beams  64  and  65  can be modulated using the tuner for polarization modulator  16 . 
     FIG. 7 depicts an embodiment of WDM wavelength-selective intensity modulator  1  including a micro-mirror array optical router and apertures  110   a  and  110   b.  A micro-mirror array optical router includes a plurality of tunable micro-mirrors  112 - 1  through  112 -n, dispersive element  11 , lens  12 , and at least one aperture  110 . As used herein, micro-mirror  112 -i refers to one of micro-mirrors  112 - 1  through  112 -n. Input signal  21  strikes dispersive element  11  and splits into channels  83 ,  84 , and  85 . Lens  12  focuses each of channels  83 ,  84 , and  85  onto a different micro-mirror  112 -i. Since the angles of each micro mirror is tunable, each micro-mirror  112 -i can be set to reflect the channels in a preselected direction. After being reflected, each channel strikes dispersive element  11  and is coupled into an output fiber. The example in FIG. 7 shows two output channels, channel  120  and channel  121 . Attenuation can be achieved by tuning micro-mirrors  112  to control the intensity of channel  120  and channel  121 , and coupling only one of the two channels into an output fiber or coupling the two channels into different output fibers. Further intensity modulation can be achieved by controlling the size of apertures  110   a  and  110   b,  thereby allowing only a portion of output channel  118  to be coupled into the optical fiber. Further details on the micro-mirror array are provided in pending U.S. patent application Ser. No. 09/794,590 to Ming Li, et al entitled “An Optical Wavelength Router Using Reflective Surfaces to Direct Output Signals.” 
     While the present invention is illustrated with particular embodiments, it is not intended that the scope of the invention be limited to the specific and preferred embodiments illustrated and described.