Abstract:
An apparatus for selective blocking WDM channels comprises a light modulator, a diffraction grating, and a transform lens. The light modulator comprises an array of pixels. Each pixel of the light modulator is selectively operable to direct light into a first mode and a second mode. The first mode directs the light to an output. The second mode directs the light away from the output. The diffraction grating is operable to receive the WDM channels from an input and to disperse the WDM channels into a range of angles. The transform lens couples the diffraction grating to the light modulator. The diffraction grating is operable to transform the range of angles of the WDM channels into a range of spatially distinct positions along the array of pixels of the light modulator without overlap of two of the WDM channels on an individual pixel. In operation, the light modulator directs at least one of the WDM channels into the second mode while directing a remainder of the WDM channels into the first mode. The light modulator is capable of operating with a large dynamic range, thereby enabling equalization of select, transmitted WDM channels as well as blocking any arbitrary channels over the spectral range of operation.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of wavelength division multiplex (WDM) optical communication. More particularly, this invention relates to the field of wavelength division multiplex (WDM) optical communication where there is a need to selectively block at least one WDM channel. 
   BACKGROUND OF THE INVENTION 
   In WDM (wavelength division multiplex) optical communication, multiple wavelengths of light each carry a communication signal. Each of the multiple wavelengths of light forms a WDM channel. In DWDM (dense WDM) optical communication, a subset of the WDM optical communication, the WDM channels are spaced closer together. A typical DWDM application operates at a wavelength band about 1,550 mm, has 90 channels, and has spacing of 0.4 nm between adjacent channels. 
   In the WDM optical communication there is a need to selectively block at least one of the WDM channels. In order to block a WDM channel, a dynamic range between a blocked WDM channel and non-blocked WDM channels must be at least 30 dB. Preferably, the dynamic range between the blocked WDM channel and the non-blocked WDM channels must be at least 40 dB. There is also a need to selectively equalize a power level of each of the non-blocked WDM channels. 
   What is needed is a method of selectively blocking WDM channels, which is fast, which is cost efficient, and which reduces a power level of a blocked WDM channel by at least 30 dB. 
   SUMMARY OF THE INVENTION 
   An embodiment of the present invention is an apparatus for selectively blocking WDM channels. The apparatus for selectively blocking WDM channels comprises a light modulator, a diffraction grating, and a transform lens. The light modulator comprises an array of pixels. Each pixel of the light modulator is selectively operable to direct light into a first mode and a second mode. The first mode directs the light to an output. The second mode directs the light away from the output. The diffraction grating is operable to receive the WDM channels from an input and to disperse the WDM channels into a range of angles. The transform lens couples the diffraction grating to the light modulator. The diffraction grating is operable to transform the range of angles of the WDM channels into a range of spatially distinct positions along the array of pixels of the light modulator without overlap of two of the WDM channels on an individual pixel. In operation, the light modulator directs at least one of the WDM channels into the second mode while directing a remainder of the WDM channels into the first mode. 
   Embodiments of the present invention can also allow equalization of the non-blocked channels by utilizing a variable reflectivity of the light modulator to partially relect and partially diffract those non-blocked WDM channels which are to be equalized to a reference level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the preferred selective blocking filter of the present invention. 
       FIG. 2  illustrates a grating light valve type device of the present invention. 
       FIG. 3  illustrates the grating light valve type device of the present invention in a reflection mode. 
       FIG. 4  illustrates the grating light valve type device of the present invention in a diffraction mode. 
       FIG. 5  illustrates a first alternative selective blocking filter of the present invention. 
       FIG. 6  illustrates an angled facet of a transceiver optical fiber of a circulator of the present invention. 
       FIGS. 7A and 7B  illustrate a second alternative selective blocking filter of the present invention. 
       FIGS. 8A and 8B  graphically illustrate test results from operation of the second alternative selective blocking filter of the present invention. 
       FIG. 9A  illustrates a third alternative selective blocking filter of the present invention. 
       FIG. 9B  illustrates the polarization diversity module included in the third alternative selective blocking filter. 
       FIG. 9C  illustrates a side view of the grating light valve type device operating in first order retro. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments of the present invention selectively block WDM (wavelength division multiplex) channels. In a WDM communication system, various wavelengths of light each carry information. The various wavelengths of light are referred to as WDM channels. The WDM channels are separated by a channel separation. For example, in a telecom C band having WDM channels from 1,527 to 1,563 nm with a channel separation of 0.4 run (or 50 GHz), 90 individual WDM channels each individually carry information. Applying the present invention to such a WDM communication system allows selective blocking of one or more of the 90 individual WDM channels. 
   The preferred selective blocking filter of the present invention is illustrated in FIG.  1 . The preferred selective blocking filter  10  comprises a circulator  12 , a first collimation lens  14 , a first diffraction grating  16 , a first transform lens  18 , a grating light valve type device  20 , and first electronics  22 . The circulator  12  comprises an input optical fiber  24 , a transceiver optical fiber  26 , and an output optical fiber  28 . The first collimation lens  14  couples the circulator  12  to the first diffraction grating  16 . The first transform lens  18  couples the first diffraction grating  16  to the grating light valve type device  20 . Preferably, a transform lens focal length f t  separates the first diffraction grating  16  from the first transform lens  18 . Preferably, the transform lens focal length f t  separates the first transform lens  18  from the grating light valve type device  20 . The first electronics  22  couple to the grating light valve type device  20 . 
   The grating light valve type device  20  of the present invention is illustrated in FIG.  2 . The grating light valve type device  20  preferably comprises elongated elements  32  suspended by first and second posts,  34  and  36 , above a substrate  38 . The elongated elements  32  comprise a conducting and reflecting surface  40 . The substrate  38  comprises a conductor  42 . In operation, the grating light valve type device  20  operates to produce modulated light selected from a reflection mode and a diffraction mode. 
   A cross-section of the grating light valve type device  20  of the present invention is further illustrated in  FIGS. 3 and 4 . The grating light valve type device  20  comprises the elongated elements  32  suspended above the substrate  38 . The elongated elements comprise the conducting and reflecting surface  40  and a resilient material  44 . The substrate  38  comprises the conductor  42 . 
     FIG. 3  depicts the grating light valve type device  20  in the reflection mode. In the reflection mode, the conducting and reflecting surfaces  40  of the elongated elements  32  form a plane so that incident light I reflects from the elongated elements  32  to produce reflected light R. 
     FIG. 4  depicts the grating light valve type device  20  in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements  32  to move toward the substrate  38 . The electrical bias is applied between the reflecting and conducting surfaces  40  of the alternate ones of the elongated elements  32  and the conductor  42 . The electrical bias results in a height difference of a quarter wavelength λ/4 of the incident light I between the alternate ones of the elongated elements  32  and non-biased ones of the elongated elements  32 . The height difference of the quarter wavelength λ/4 produces diffracted light including plus one and minus one diffraction orders, D +1  and D −1 . 
     FIGS. 3 and 4  depict the grating light valve type device  20  in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements  32  of less than a quarter wavelength λ/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D +1  and D −1 . In other words, by deflecting the alternate ones of the elongated elements less than the quarter wavelength λ/4, the grating light valve type device  20  produces a variable reflectivity. 
   It will be readily apparent to one skilled in the art that the conducting and reflecting surface  40  can be replaced by a multilayer dielectric reflector and a conducting element where the conducting element is buried within each of the elongated elements  32  or within just the alternate ones of the elongated elements  32 . 
   While  FIGS. 2 ,  3 , and  4  depict the grating light valve type device  20  having six of the elongated elements  32 , the grating light valve type device  20  preferably includes more of the elongated elements  32 . By providing more of the elongated elements  32 , the elongated elements  32  are able to function as groups, which are referred to as pixels. 
   It will be readily apparent to one skilled in the art that the term “pixel” is used here in the context of an element of a light modulator rather than its more specific definition of a picture element of a display. 
   In operation of the preferred selective blocking filter  10  (FIG.  1 ), an input signal  46  comprising the WDM channels enters the input optical fiber  24  of the circulator  12  and exits the transceiver optical fiber  26 . The first collimation lens  14  collimates the WDM channels. The first diffraction grating  16  disperses the WDM channels into a range of angles. The first transform lens  18  transforms the range of angles of the WDM channels into a range of spatially distinct positions along the grating light valve type device  20 , which comprises an array of pixels. No two WDM channels on the grating light valve type device  20  overlap. 
   The grating light valve type device  20 , driven by the first electronics  22 , directs at least one of the WDM channels into the diffraction mode while directing a remainder of the WDM channels into the reflection mode. The reflection mode returns the remainder of the WDM channels along a reverse path to the transceiver optical fiber  26  of the circulator  12 . The diffraction mode causes the at least one WDM channel to not follow the reverse path to the circulator  12 . The reverse path comprises the first transform lens  18 , the first diffraction grating  16 , and the first collimation lens  14 . The circulator  12  directs the remainder of the WDM channels out of the output optical fiber  28  of the circulator as an output signal  48 . 
   In an alternative mode of operation, the function of the reflection mode and the diffraction mode can be exchanged. For example, the diffraction mode returns the remainder of the WDM channels along the reverse path to the transceiver optical fiber  26  of the circulator  12 , and the reflection mode causes the at least one WDM channel to not follow the reverse path to the circulator  12 . 
   In order to successfully achieve WDM channel blocking, a dynamic range between the at least one WDM channel and the remainder of the WDM channels at the output optical fiber  28  must be at least 30 dB. Preferably, the dynamic range between the at least one WDM channel and the remainder of the WDM channels at the output optical fiber  28  is at least 40 dB. 
   The dynamic range is met by first and second aspects of the present invention. The first aspect is not overlapping any two WDM channels on an individual pixel of the grating light valve type device  20  by spatially separating the WDM channels at distinct positions along the grating light valve type device  20 . This is accomplished by using high resolution optical components for the first collimation lens  14 , the first diffraction grating  16 , and the first transform lens  18 . 
   The second aspect is a modulator dynamic range provided by the grating light valve type device  20 . In the reflection mode, the grating light valve type device  10  reflects the incident light I to form the reflected light R (FIG.  3 ). In the diffraction mode, the grating light valve type device  10  diffracts the incident light I to form the diffracted light including the plus one and minus one diffraction orders, D +1  and D −1  (FIG.  4 ). In the diffraction mode, however, a small amount of light is directed into the reflected mode. The modulator dynamic range is a ratio of a power level of the reflected light in the reflection mode to a power level of the small amount of reflected light in the diffraction mode. The grating light valve type device  20  has a modulator dynamic range that is at least 30 dB. By careful design and fabrication, including maintaining narrow gaps between the adjacent ones of the elongated elements  32  of the grating light valve type device  20 , the grating light valve type device  20  provides a modulator dynamic range of 40 dB. 
   In an alternative operation of the preferred selective blocking filter  10 , the remainder of the WDM channels are equalized to a reference level in addition to blocking the at least one WDM channel. The alternative operation utilizes the variable reflectivity capability of grating light valve type device  20  to partially reflect and partially diffract those WDM channels of the remainder of the WDM channels which must be reduced in power in order to equalize the remainder of the WDM channels to the reference level. 
   A first alternative selective blocking filter of the present invention is illustrated in FIG.  5 . The first alternative selective blocking filter  50  comprises the circulator  12 , a second collimation lens  52 , a second diffraction grating  54 , a mirror  56 , a second transform lens  58 , the grating light valve type device  20 , and second electronics  60 . The second collimation lens  52  couples the circulator  12  to the diffraction grating  54 . The diffraction grating  54  couples to the mirror  56 . The second transform lens  58  couples the diffraction grating  54  to the grating light valve type device  20 . Preferably, the transform lens  58  is positioned so that a virtual pivot  61  of the diffraction grating  54  is located at a transform lens focal length f t . Preferably, the transform lens  58  is also positioned so that the grating light valve type device  20  is located at the transform lens focal length f t . The second electronics  60  couple to the grating light valve type device  20 . 
   In operation of the first alternative selective blocking filter  50 , the circulator  12  directs the WDM channels to the collimation lens  52 , which collimates the WDM channels onto the diffraction grating  54 . The diffraction grating  54  disperses the WDM channels into a first range of angles. The mirror  56  reflects the first range of angles of the WDM channels back to the diffraction grating  54 , which further disperses the WDM channels into a second range of angles. The transform lens  58  transforms the second range of angles of the WDM channels into spatially distinct positions along the grating light valve type device  20 . 
   The grating light valve type device  20 , driven by the second electronics  60 , directs at least one of the WDM channels into the diffraction mode while directing a remainder of the WDM channels into the reflection mode. The reflection mode returns the remainder of the WDM channels along a second reverse path to the transceiver optical fiber  26  of the circulator  12 . The diffraction mode causes the at least one WDM channel to not follow the second reverse path to the circulator  12 . The second reverse path comprises the second transform lens  58 , the second diffraction grating  54 , the mirror  56 , and the second collimation lens  52 . 
   Comparing the first alternative selective blocking filter  50  to the preferred selective blocking filter  10  it is seen that the first alternative selective blocking filter  50  operates similarly to the preferred selective blocking filter  10  with first and second exceptions. The first exception is that the mirror  56  of the first alternative selective blocking filter causes a double pass of the WDM channels on the diffraction grating  54  before the WDM channels reach the grating light valve type device  20 . The second exception is that the mirror  56  causes the double pass of the remainder of the WDM channels on the diffraction grating  54  along the second reverse path from the grating light valve type device  20  to the circulator  12 . The double pass enhances the spatially distinct positions of the WDM channels along the grating light valve type device  20 . This allows for a smaller physical layout for the first alternative selective blocking filter  50  over the preferred selective blocking filter  10 . However, the first alternative selective blocking filter  50  incurs a slight loss in efficiency over the preferred selective blocking filter  10  due to the two reflections from the mirror  56  and due to the double pass of the second diffraction grating  54 . 
   A fiber end of the transceiver optical fiber  26  of the circulator  12  is further illustrated in FIG.  6 . The fiber end  62  of the transceiver optical fiber  26  preferably includes an angled facet having an angle  64  from a cross-cut of the transceiver optical fiber  26 . The angle  64  reduces back reflection in the first alternative selective blocking filter  50 , which enhances performance of the first alternative selective blocking filter  50 . Preferably, the angle  64  is 8°, which provides a 3.6° beam deviation. Alternatively, the angle  64  is larger or smaller. Further alternatively, the angle  64  is zero. 
   A second alternative selective blocking filter of the present invention is illustrated in  FIGS. 7A and 7B . The second alternative selective blocking filter  70  comprises the circulator  12 , a third collimation lens  72 , a third diffraction grating  74 , a third transform lens  76 , the grating light valve type device  20 , a quarter wave plate  78 , a retro lens  80 , a retro mirror  82 , and third electronics  84 . The third collimation lens  72  couples the circulator  12  to the third diffraction grating  74 . The third transform lens  76  couples the third diffraction grating  74  to the grating light valve type device  20 . The third transform lens  76  also couples the grating light valve type device  20  the quarter wave plate  78 . The retro lens  80  couples the quarter wave plate  78  to the retro mirror  82 . The third electronics  84  couple to the grating light valve type device  20 . 
     FIGS. 7A and 7B  depict a plan view of the second alternative selective blocking filter  70  of the present invention.  FIG. 7A  depicts a first ray trace from the circulator  12  to the grating light valve type device  20 .  FIG. 7B  depicts a second ray trace from the grating light valve type device  20  to the retro mirror  82 . 
   It will be readily apparent to one skilled in the art that the third diffraction grating  74  lies in the first ray trace ( FIG. 7A ) and not the second ray trace (FIG.  7 B). 
   In operation of the second alternative selective blocking filter  70 , the WDM channels couple from the circulator  12  to the grating light valve type device  20  via the third collimation lens  72 , the third diffraction grating  74 , and the third transform lens  76  as depicted by the first ray trace in FIG.  7 A. The grating light valve type device  20 , driven by the third electronics  84 , directs at least one of the WDM channels into the diffraction mode while directing the remainder of the WDM channels into the reflection mode. The remainder of the WDM channels are directed to the retro mirror  82  via the third transform lens  76 , the quarter wave plate  78 , and the retro lens  80  as depicted by the second ray trace in FIG.  7 B. The retro mirror  82  reflects the remainder of the WDM channels back to grating light valve type device  20  via the retro lens  80 , the quarter wave plate  78 , and the third transform lens  76 . The grating light valve type device  20  then directs the remainder of the WDM channels back to the circulator  12  via the third transform lens  76 , the diffraction grating  74 , and the third collimation lens  72 . 
   The second alternative selective blocking filter  70  provides a double pass of the grating light valve type device  20 . Because of the double pass of the grating light valve type device  20 , the second alternative selective blocking filter  70  exhibits an ultrahigh attenuation of a blocked WDM channel. 
   Since the remainder of the WDM channels pass through the quarter wave plate  78  twice, the quarter wave plate  78  provides an orthogonal rotation of a polarization of the remainder of the WDM channels. This feature provides a mechanism for compensating for a polarization dependent loss in the second alternative selective blocking filter  70 . By orienting an optic axis of the quarter wave plate  78  at 45° to the polarization that exhibits a worst polarization loss, the worst polarization loss is reduced by half. 
   Thus, advantages of the second alternative selective blocking filter  70  are that it exhibits the ultrahigh attenuation of the blocked WDM channel and it compensates for polarization dependent loss. A disadvantage of the second alternative selective blocking filter  70  is that it is less efficient due to the reflection from the retro mirror  82  and due to the double pass of the grating light valve type device  20 . 
     FIGS. 8A and 8B  graphically illustrate test results from operation of the second alternative selective blocking filter  70  of the present invention.  FIG. 8A  graphically depicts the test results for a control test. In the control test, eight WDM channels on a channel spacing of 0.4 nm were passed through the second alternative selective blocking filter  70 . In the control test, none of the eight WDM channels were blocked.  FIG. 8B  graphically depicts the test results for a blocking test. In the blocking test, a single WDM channel at 1547.72 run was blocked while seven remaining WDM channels were transmitted. In the blocking test, the single WDM channel exhibited a power reduction of 38 dB. 
   A third alternative selective blocking filter of the present invention is illustrated in FIG.  9 A. The third alternative selective blocking filter  90  comprises the circulator  12 , a polarization diversity (PD) module  92 , a fourth diffraction grating  94 , a fourth transform lens  96 , the grating light valve type device  20 , and fourth electronics  98 . The PD module  92  couples the circulator  12  to the fourth diffraction grating  94 . The fourth transform lens  96  couples the fourth diffraction grating  94  to the grating light valve type device  20 . Preferably, a transform lens focal length ft separates the fourth diffraction grating  94  from the fourth transform lens  96 . Preferably, the transform lens focal length f t  separates the fourth transform lens  96  from the grating light valve type device  20 . The fourth electronics  98  couple to the grating light valve type device  20 . The grating light valve type device  20  is positioned perpendicular to the optical axis. 
     FIG. 9B  further illustrates the PD module  92 . The PD module  92  comprises a fourth collimation lens  100 , a polarization splitter  102  and a half-wave plate  104 . The fourth collimation lens  100  couples the optical fiber  26  of the circulator  12  ( FIG. 9A ) to the polarization splitter  102 . The half-wave plate  104  couples an optical path from the polarization splitter  102  to the fourth diffraction grating  94 . 
   In operation of the third alternative selective blocking filter  90 , the circulator  12  directs the WDM channels to the PD module  92 . The WDM channels are received by the PD module  92  as diverging light beams from the optical fiber  26 . The diverging light is received by the fourth collimation lens  100  and directed as collimated light to the polarization splitter  102 . The polarization splitter  102  is preferably a crystal capable of splitting an input light beam into two light beams with orthogonal polarization states to each other. The polarization splitter  102  receives the collimated light from the fourth collimation lens  100  and splits the collimated light into a first split light  101  and a second split light  103 . The first split light  101  and the second split light  103  are orthogonal to each other. Preferably, the first split light  101  is polarized perpendicular to the page of the  FIG. 9B , and the second split light  103  is polarized parallel to the page of the FIG.  9 B. The polarization of the first split light  101  is directed along an upper optical path and the second split light is directed along a lower optical path, as illustrated in FIG.  9 B. The lower optical path is coupled to the half-wave plate  104  such that the half-wave plate  104  receives the second split light  103  from the polarization splitter  102 . The half-wave plate  104  provides an orthogonal rotation of a polarization of the second split light  103 . In this manner, the first split light  101  and the second split light  103  exit the PD module  92  with the same polarization. 
   The first split light  101  and the second split light  103  are coupled to the grating light valve type device  20  via the diffraction grating  94  and the fourth transform lens  96  as depicted by the optical path illustrated in FIG.  9 A. Although two optical paths are illustrated leaving the PD module  92  in  FIG. 9B , only one optical path is illustrated leaving the PD module  92  in FIG.  9 A. This is because  FIG. 9A  shows a top down view of the PD module  92  relative to the view illustrated in FIG.  9 B. As such, the two beams leaving the PD module  92  in  FIG. 9B  are stacked vertically, as viewed in  FIG. 9A , and only one beam is shown. The first split light  101  and the second split light  103  in  FIG. 9B , viewed as the single light beam in  FIG. 9A , propagates from the diffractive grating  94  to the fourth transform lens  96  off-axis to the optical axis, and arrives off-center at the fourth transform lens  96 . The beams are refracted by the transform lens  96  and directed onto the grating light valve type device  20  at an angle approximately one-half the first order diffraction angle. 
   The grating light valve type device  20 , driven by the fourth electronics  98 , directs at least one of the WDM channels into the diffraction mode while directing a remainder of the WDM channels into the reflection mode. Preferably, the third alternative selective blocking filter  90  uses first order retro operation of the grating light valve type device  20 .  FIG. 9C  illustrates a side view of the grating light valve type device  20  operating in first order retro. As previously described in relation to the preferred, first alternative and second alternative selective blocking filters  10 ,  50 ,  70 , an incident light impinges normal to the grating light valve type device  20  and first order diffracted light is diffracted at a first order diffraction angle θ 1 . In the third alternative selective blocking filter  90 , incident light impinges the grating light valve type device  20  off-axis. Preferably, the incident light impinges the grating light valve type device  20  at an angle of about θ 1 /2. In first order retro operation, reflected light is reflected off-axis while the plus first order diffracted light is diffracted at an angle of about θ 1 /2. In other words, the plus first order diffracted light is diffracted at the same angle as the incident light impinging the grating light valve type device  20 . In this third alternative embodiment, the incident light comprises the first split light  101  along the upper optical path and the second split light  103  along the lower optical path. In reference to  FIG. 9C , the first split light  101  angles toward the grating light valve type device  20  from a position above the plane of FIG.  9 C and in the plane of the incident light IN. Similarly, the second split light  103  angles toward the grating light valve type device  20  from a position below the plane of FIG.  9 C and in the plane of the incident light IN. Preferably, the upper optical path and the lower optical path form mirror images of each other about the plane of the grating light valve type device  20  in FIG.  9 C. 
   First split light  101  impinging the grating light valve type device  20  while in the diffraction mode is directed along a reverse path that comprises the lower optical path. In other words, the first split light  101  is directed to the grating light valve type device  20  along the upper optical path and, if the grating light valve type device  20  is in the diffraction mode, then the first split light  101  is directed away from the grating light valve type device  20  along the lower optical path. Similarly, second split light  103  impinging the grating light valve type device  20  while in the diffraction mode is directed along a reverse path that comprises the upper optical path. The reverse path along the upper optical path comprises the fourth transform lens  96 , the fourth diffraction grating  94 , the polarization splitter  102  and the fourth collimation lens  100 . The reverse path along the lower optical path comprises the fourth transform lens  96 , the fourth diffraction grating  94 , the half-wave plate  104 , the polarization splitter  102  and the fourth collimation lens  100 . Once the second split light  103  returns through the half-wave plate  104 , the first and second split light  101 ,  103  are orthogonal to each other. The polarization splitter  102  then recombines the first and second split light  101 ,  103  into an output signal. The output signal is directed to the optical fiber  26  via the fourth collimation lens  100 . 
   The primary purpose of the polarization diversity module  92  is to suppress polarization dependent loss (PDL). Polarization diversity also enables the use of a highly dispersive grating and a fine pitch grating light valve type device, which both allow for a compact optics design. The PD module  92  can be fabricated reliably and economically. 
   In an alternative operation of the alternative selective blocking filters  50 ,  70  and  90 , the remainder of the WDM channels are equalized to a reference level in addition to blocking the at least one WDM channel. The alternative operation utilizes the variable reflectivity capability of grating light valve type device  20  to partially reflect and partially diffract those WDM channels of the remainder of the WDM channels which must be reduced in power in order to equalize the remainder of the WDM channels to the reference level. 
   It will be readily apparent to one skilled in the art that other various modifications may be made to the embodiments without departing from the spirit and scope of the invention as defined by the appended claims.