CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation of U.S. patent application Ser. No. 09/999,038, filed Oct. 24, 2001 now U.S. Pat. No. 6,754,431. This application is related to the following U.S. Patent Applications: U.S. patent application Ser. Nos. 09/999,878,  09/999,610, 09/999,705 and 09/779,189, entitled “A Microelectromechanical Mirror,” filed Feb. 7, 2001, all of which are assigned to the assignee of the present invention and incorporated herein by reference in their entirety. 


   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to optical fiber cross-connect switching. More particularly, it relates lo load balancing in Dense Wavelength Division Multiplexing optical cross-connect systems. 
   2. Description of the Related Art 
   Associated with the information revolution is a need to increase by many orders of magnitude the rate of information transfer. This can be accomplished with optical fibers and the method of Dense Wavelength Division Multiplexing (DWDM), in which many wavelength channels, each including a different narrow band of wavelengths of light and each carrying different information, are multiplexed onto a single optical fiber using an optical multiplexer. Optical signals carried on the various wavelength channels may be separated at the output of the optical fiber with an optical demultiplexer. 
   Optical fiber cross-connect switches may be used to direct the optical signals on some or all of the wavelength channels on a particular optical fiber to other optical fibers. Such optical fiber cross-connect switches include those described in U.S. Patent Application Ser. Nos. 09/999,878, 09/999,610, and 10/002,310, now issued U.S. Pat. No. 6,636,656, all of which are incorporated herein by reference in their entirety. Hence, optical signals on the various wavelength channels on an optical fiber may have originated at separate locations and traveled different distances in optical fiber. Since light is attenuated during transmission through optical fiber by an amount typically proportional to the distance traveled in optical fiber, the various wavelength channels on an optical fiber may carry different power levels. 
   Optical amplifiers such as Erbium Doped Fiber Amplifiers (EDFA) can amplify a wide wavelength band (spanning many wavelength channels), and thus compensate for transmission losses in optical fibers. If the power levels on the various wavelength channels carried by the optical fiber are not nearly equal at the input to the optical amplifier, however, the wavelength channel or channels of highest power may saturate the gain. Under such circumstances, the lower power wavelength channels might not be sufficiently amplified. 
   A variable optical attenuator is an optical device with which the amplitude or power level of an input optical signal may be attenuated by a variable amount to provide an output optical signal of a desired amplitude or power level. The power levels of the various wavelength channels on an optical fiber may be substantially equalized in a “load balancing” or “load equalization” process in which each wavelength channel is routed through a separate variable optical attenuator. Variable optical attenuators are described, for example, in U.S. Pat. Nos. 5,864,643 and 6,130,984. These devices require the insertion of additional hardware into an optical network. The additional hardware may be expensive, requires additional physical space, and may introduce unwanted attenuation of the optical signals. 
   It would be desirable to incorporate the function of a variable optical attenuator into an optical network without the insertion of additional optical elements. 
   SUMMARY 
   A method of controllably attenuating a beam of light coupled into a port in accordance with an embodiment of the present invention includes directing the beam of light against a mirror, and controlling an orientation of the mirror such that a predetermined fraction of the beam of light is coupled into the port. The predetermined fraction is less than a maximum fraction corresponding to optimal coupling of the beam of light into the port. In one embodiment, this method is implemented with a variable optical attenuator including a first port, a second port, a mirror located to direct light output by the first port to the second port, and a controller coupled to the mirror to align it such that the predetermined fraction of light is coupled into the second port. The ports may be or include optical fibers. 
   In one implementation, the variable optical attenuator includes a second mirror located to direct to the second port light output by the first port and reflected by the first mirror. The controller is also coupled to the second mirror to align it such that the predetermined fraction of light is coupled into the second port. Use of two controllable mirrors in the optical path of the light beam allows independent control of the position and angle of incidence of the light beam on the second port. 
   Control of the mirror or mirrors in the variable optical attenuator may be accomplished by numerous methods. In one implementation, the power of light coupled into the second port is measured, and an orientation of a mirror is controlled to maintain the power at a predetermined level. In another implementation, an orientation of a mirror corresponding to the predetermined fraction described above is selected from a look-up table. In another implementation, an alignment beam of light is directed against a mirror, and the orientation of the mirror is controlled to direct the alignment beam to a predetermined position on a position sensing detector. The predetermined position corresponds to the predetermined fraction described above. 
   In another embodiment, a variable optical attenuator includes a first plurality of ports, a second plurality of ports, a first plurality of mirrors disposed on a first surface, a second plurality of mirrors disposed on a second surface, and a controller coupled to align each of the first plurality of mirrors and each of the second plurality of mirrors such that predetermined fractions of light output by the first plurality of ports are coupled into separate ones of the second plurality of ports. The predetermined fractions are less than maximum fractions corresponding to optimal coupling of light output by the first plurality of ports into the second plurality of ports. This embodiment may be employed, for example, to load balance DWDM wavelength channels. 
   A method of equalizing the power levels of (load balancing) a plurality of channels multiplexed on an optical fiber in accordance with an embodiment of the present invention includes demultiplexing the channels from the optical fiber to form a plurality of beams of light, with each beam of light formed from a separate channel, measuring the power level of each channel, directing each of the beams of light against a separate one of a plurality of mirrors, and controlling an orientation of one of the mirrors such that a predetermined fraction of the beam of light directed against that mirror is coupled into a port. The predetermined fraction is less than a maximum fraction corresponding to optimal coupling of the beam of light into the port. 
   Variable optical attenuators in accordance with embodiments of the present invention may be implemented in optical cross-connect switches. In such embodiments, the ports and mirrors of the variable optical attenuator may also support switching functions in the optical cross-connect switch. Optical cross-connect switches are typically designed and operated to achieve minimum insertion loss for all optical signals coupled into the switch. The inventors have recognized, however, that variable attenuation can be accomplished by separately controlling the insertion loss for the various optical signals by controllably misaligning mirrors used to switch the optical signals. Hence, the function of one or more variable optical attenuators may be advantageously integrated into an optical network without the insertion of additional optical elements. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates schematically a variable optical attenuator in accordance with an embodiment of the present invention. 
       FIGS. 2A–2C  illustrate schematically a portion of a variable optical attenuator in accordance with an embodiment of  FIG. 1 . 
       FIG. 3  illustrates schematically a variable optical attenuator in accordance with another embodiment of the present invention. 
       FIG. 4  illustrates schematically an optical fiber cross-connect switch in which is implemented a variable optical attenuator in accordance with an embodiment of the present invention. 
       FIG. 5  is a plot showing attenuation of the optical power coupled into an optical fiber versus misalignment of a light beam with respect to the optical fiber in accordance with an embodiment of the present invention. 
   


   Like reference numbers in the various figures denote same parts in the various embodiments. Dimensions in the figures are not necessarily to scale. 
   DETAILED DESCRIPTION 
   A variable optical attenuator in accordance with embodiments of the present invention variably attenuates light coupled into an optical fiber by controlled misalignment of one or more mirrors directing the light to the fiber. A number of embodiments will be described in which one or more optical signals are variably attenuated, and in which controlled misalignment of one or more mirrors is accomplished using, for example, measurements of the power of the attenuated optical signals or measurements of the position of control light beams separate from the optical signals to be attenuated. 
   Referring to  FIG. 1 , optical fiber  2  carries light to be attenuated by a controlled amount in a variable optical attenuator  1  in accordance with an embodiment of the present invention. As is conventional in DWDM, optical fiber  2  may carry light having a plurality of wavelengths. In one implementation, the light carried by optical fiber  2  has wavelengths near about 1310 nanometers (nm) or about 1550 nm. Optical fiber  2  is, for example, a conventional Corning, Incorporated SMF-28 single mode optical fiber having a core diameter of about 8 microns (μm) and a cladding diameter of about 125 μm. Other optical fibers suitable for optical communications applications may also be used. 
   Optical fiber  2  outputs a diverging cone of light which is, for example, collimated or weakly focused by lens  6  to form light beam  8 . Lens  6  is, for example, a conventional plano-convex lens formed from BK 7 optical glass and having a focal length of about 4 millimeters (mm). Light beam  8  is incident on beam splitter  10 , which divides light beam  8  into light beam  8   a  incident on mirror  12  and light beam  8   b  incident on photodetector  14 . Photodetector  14  is, for example, a conventional InGaAs photodiode. Suitable InGaAs photodiodes are available from, for example, Hamamatsu Corporation of Bridgewater, N.J. and Telcom Devices Corporation of Camarillo, Calif. 
   In one implementation, beam splitter  10  is a cube beam splitter formed from BK 7 optical glass and having a dielectric coating with a reflectivity of about 2% at infrared wavelengths of about 1200 nm to about 1700 nm. In another implementation, beam splitter  10  is a dichroic cube beam splitter formed from BK 7 optical glass and having a dielectric coating with a reflectivity of about 2% at infrared wavelengths of about 1200 nm to about 1700 nm and a reflectivity of about 40% to about 60%, preferably about 50%, at wavelengths of about 600 nm to about 850 nm. Such beam splitters are available, for example, from Harold Johnson Optical Laboratories, Inc. of Gardena, Calif. Suitable coatings for the beam splitter may be obtained, for example, from ZC&amp;R Coatings For Optics, Inc. of Torrance, Calif. 
   The reflectivity of such a dichroic beam splitter may be selected, for example, to allow at least partial separation of wavelengths of light used in telecommunications (e.g., 1200 nm–1700 nm) from another range (e.g., 600 nm–850 nm) of non-telecommunication wavelengths used for control light beams used in some embodiments as described below. 
   Referring again to  FIG. 1 , mirror  12  directs light beam  8   a , incident from beam splitter  10 , through (optional) beam splitter  22 , if it is present, to lens  24 . In some implementations, lens  6  focuses light beam  8   a  to a waist at a location along light beam  8   a  between lens  6  and lens  24 . Such focusing can maintain a relatively small diameter of light beam  8   a  throughout variable optical attenuator  1  and thus reduce uncontrolled optical loss from, e.g., diffraction. 
   Mirror  12  is coupled to actuator  16 , which is controlled by control system  18  with electrical signals provided via electrical line  20  to orient mirror  12  in a range of arbitrary directions (dθ, dΦ). This range of orientations allows mirror  12  to direct light beam  8   a  onto lens  24  at a range of controlled angles with respect to optical axis  28  ( FIGS. 2A–2C ) of lens  24  and to a range of controlled positions on surface  25  of lens  24 . 
   In one implementation, mirror  12  and actuator  16  are, respectively, a micro-electro-mechanical systems (MEMS) micro mirror and a MEMS actuator as described, for example, in U.S. patent application Ser. No. 09/779,189 incorporated herein by reference in its entirety. Other micro mirrors may also be used. In such an implementation, mirror  12  may be a freely rotatable MEMS micro mirror actuated by, for example, electrostatic, electromagnetic, piezoelectric, or thermal actuation means. Mirror  12  may be, for example, approximately elliptical with major and minor diameters of about 1.0 mm and about 0.9 mm, respectively. Control system  18  may be, for example, a control system for a MEMS based optical switch such as, for example, control systems disclosed in U.S. Patent Application Ser. No. 09/999,705, now issued U.S. Pat. Nos. 6,888,769, and U.S. Patent Application Ser. No. 10/003,659, now issued U.S. Pat. No. 6,614,954, both of which are incorporated herein by reference in their entirety. 
   In other implementations, mirror  12  may be a conventional mirror having a metal or dielectric coating highly reflective at wavelengths of about 1200 nm to about 1700 nm. Actuator  16  may be a conventional mirror mount actuated by, for example, conventional stepper motors or conventional piezoelectric actuators. Control system  18  may include, for example, a microprocessor and a conventional stepper motor driver or a conventional piezoelectric driver. 
   Lens  24  focuses light beam  8   a , incident from mirror  12 , onto optical fiber  26 . In one implementation, for example, surface  25  of optical fiber  26  is approximately at the focal plane of lens  24 . Referring to  FIGS. 2A–2C , lens  24  is positioned with its optical axis  28  approximately centered on the core  26   a  of optical fiber  26 . Lens  24  may be, for example, a conventional plano-convex lens formed from BK 7 optical glass and having a focal length of about 4 mm. Optical fiber  26  includes cladding  26   b  surrounding core  26   a.    
   One of ordinary skill in the art will recognize that lens  24  may couple light beam  8   a  into a core (e.g., fundamental) optical mode of optical fiber  26  and/or into a cladding mode of optical fiber  26 . The power distribution of light in a core mode of optical fiber  26  is concentrated in core  26   a , although a portion of the power  10  distribution of such a core mode propagates in cladding  26   b . Light coupled into a core mode can propagate long distances with little attenuation. In contrast, the power distribution of a cladding mode of optical fiber  26  is concentrated in cladding  26   b.    
   Only that portion of light beam  8   a  incident on core  26   a  of optical fiber  26  at angles with respect to optical axis  28  less than the acceptance angle (determined by the refractive indices of core  26   a  and cladding  26   b ) of optical fiber  26  will be efficiently coupled into a core mode of optical fiber  26 . Hence, the fraction of light beam  8   a  coupled into a core mode of optical fiber  26  depends on the location at which light beam  8   a  is incident on lens  24  and the angle that light beam  8   a  makes with respect to optical axis  28 . In  FIG. 2A , for example, light beam  8   a  is incident on the approximate center of lens  24  approximately parallel to optical axis  28  and focused entirely onto core  26   a  within the acceptance angle θ A  indicated by dashed lines  27 . Thus, light beam  8   a  is approximately aligned for maximum coupling into a core mode of optical fiber  26 . The acceptance angle of optical fiber  26  in air may be, for example, about 7.5° (numerical aperture of about 0.13). One of ordinary skill in the art will recognize that the maximum optical power coupled into a core mode of optical fiber  26  is typically less than the total optical power of light beam  8   a  as a result of, for example, Fresnel reflection losses at surface  25 . 
   In contrast, in  FIG. 2B , light beam  8   a  is incident on lens  24  at an angle θ with respect to optical axis  28  sufficiently large that light beam  8   a  misses core  26   a  and is focused entirely onto cladding  26   b . Hence, little or none of light beam  8   a  is coupled into a core mode of optical fiber  26 . Light coupled into a cladding mode of optical fiber  26  is subsequently removed, for example, by a conventional cladding mode stripper  30  ( FIG. 1 ). One of ordinary skill in the art will recognize that light coupled into cladding modes of an optical fiber is typically rapidly attenuated during transmission, particularly if the optical fiber is coiled or otherwise bent. Thus, in other implementations cladding mode stripper  30  is not used. Since light coupled into a cladding mode of optical fiber  26  is subsequently removed or otherwise attenuated, light described herein as being coupled into optical fiber  26  refers to light coupled into a core mode of optical fiber  26  rather than into, for example, a cladding mode of optical fiber  26 . 
     FIGS. 2A and 2B  show alignments of light beam  8   a  resulting in, respectively, approximately minimum attenuation and approximately maximum attenuation of the light coupled into optical fiber  26 . Control system  18  may control the orientation of mirror  12  to achieve alignments of light beam  8   a  intermediate between those of  FIGS. 2A and 2B , and thus vary the attenuation of the light coupled into optical fiber  26  between the approximate minimum and approximate maximum levels of attenuation. In  FIG. 2C , for example, light beam  8   a  is incident on lens  24  at an angle θ with respect to optical axis  28  smaller than that of  FIG. 2B  and focused to overlap both core  26   a  and cladding  26   b  of optical fiber  26 . A fraction of light beam  8   a  focused onto surface  25  at angles less than the acceptance angle of optical fiber  26  will be coupled into a core mode of optical fiber  26 . Another fraction of light beam  8   a  may be coupled into a cladding mode of optical fiber  26  and subsequently removed as described above. 
   Referring to  FIG. 5 , curve  29  is a plot, for one implementation, of the attenuation of the optical power coupled into optical fiber  26  as a function of the offset of the center of light beam  8   a  at surface  25  from the center of core  26   a  of optical fiber  26 . In this implementation, lens  24  has a focal length of about 4 mm, light beam  8   a  has a diameter of about 0.8 mm at lens  24 , core  26   a  of optical fiber  26  has a diameter of about 8 μm, and surface  25  is approximately at the focal plane of lens  24 . As curve  29  indicates, an offset of about 18 μm between the center of the focused beam and the center of optical fiber  26  in this implementation results in an attenuation of about 60 decibels (dB). This offset corresponds to a misalignment of light beam  8   a  with respect to optical axis  28  ( FIGS. 2A–2C ) of about 0.25°. Such a misalignment of light beam  8   a  can be achieved with a misalignment of mirror  12  of about 0.125° since the angular displacement of light beam  8   a  is twice that of mirror  12 . 
   In many optical communication applications the maximum optical attenuation required is about 30 dB. The slope of curve  29  at about 30 dB attenuation, represented by line  31 , is about 1 dB of attenuation per 0.22 μm of offset. This corresponds to about 3 dB per 0.01° misalignment of light beam  8   a  with respect to optical axis  28 . Hence, control of the orientation of mirror  12  with a resolution of better than about 0.005° allows control of the attenuation of the power of light coupled into optical fiber  26  with a resolution better than about 3 dB. Such angular resolution may be achieved, for example, in optical fiber cross-connect switches described in U.S. Patent Application Ser. Nos. 09/999,878, 09/999,610, and 10/002,310, now issued U.S. Pat. No. 6,636,656. Of course, the resolution with which the attenuation is controlled improves as the angular resolution with which the mirror is controlled is improved. Attenuation curves for other implementations are similar to curve  29 . 
   Controlled misalignment of mirror  12  to attenuate light coupled into optical fiber  26  may be accomplished by numerous methods. Referring again to  FIG. 1 , control system  18  receives electrical signals corresponding to the optical power in light beam  8   b  (and thus in light beam  8   a ) from photodetector  14  via electrical line  32 . Control system  18  determines from these electrical signals the amount by which light beam  8   a  is to be attenuated. In one embodiment, (optional) conventional fiber coupler  34  directs a portion of the light coupled into optical fiber  26  to (optional) photodetector  36 . Photodetector  36 , which may be a conventional InGaAs photodiode, provides a signal corresponding to the optical power coupled into optical fiber  26  to control system  18  via electrical line  38 . Control system  18  controls the orientation of mirror  12  such that the electrical signals provided by photodetector  36  indicate that the light coupled into optical fiber  26  is attenuated to the desired power level. Hence, in this embodiment control system  18 , actuator  16 , mirror  12 , and photodetector  36  form a feedback loop by which attenuation of the light coupled into optical fiber  26  is controlled. 
   In another embodiment, a look-up table stored in a computer readable medium (memory  18   a ) in control system  18  relates the orientation of mirror  12  to the attenuation of the light coupled into optical fiber  26 . In this embodiment, control system  18  determines from the electrical signals received from photodetector  14  the amount by which light beam  8   a  is to be attenuated, reads the required orientation of mirror  12  from the look-up table (which contains information correlating the amount of attenuation to the mirror&#39;s orientation), and controls actuator  16  to orient mirror  12  accordingly. The look-up table may be generated by measuring, with photodetectors  14  and  36 , for example, the attenuation of light coupled into optical fiber  26  for each of a series of different orientations of mirror  12 . 
   In other embodiments, controlled misalignment of mirror  2  is accomplished using measurements of the position of one or more control light beams separate from the optical signals to be attenuated. In these embodiments, mirror  12  may be controlled, for example, using methods similar or identical to methods for controlling the orientations of mirrors in an optical fiber cross-connect switch disclosed in the following U.S. Patent Applications: U.S. Patent Application Ser. Nos. 09/999,878, 09/999,610, 09/999,878, 99/999,705, now issued U.S. Pat. No. 6,888,769, U.S. Patent Application Ser. No. 10/003,659, now issued U.S. Pat. No. 6,614,954, and U.S. Patent Application Ser. No. 10/002,310, now issued U.S. Pat. No. 6,636,656. 
   In one implementation, for example, laser  40  ( FIG. 1 ) outputs control light beam  42  incident on dichroic beam splitter  10 . In some implementations, the wavelength of control light beam  42  is a wavelength not typically used in telecommunications. In one implementation, for example, laser  40  is a conventional laser diode emitting light having a wavelength of about 660 nm. Dichroic beam splitter  10  reflects light beam  42  to mirror  12 , which directs control light beam  42  to dichroic beam splitter  22 . Dichroic beam splitter  22  reflects control light beam  42  to position sensing detector  44 . Thus, the position at which control light beam  42  is incident on position sensing detector  44  is determined by the orientation of mirror  12 . Dichroic beam splitter  22  is, for example, substantially identical to dichroic beam spliner  10 . Suitable position sensing detectors are available, for example, from UDT Sensors, Inc. of Hawthorne, Calif., and from Pacific Silicon Sensor, Inc. of Westlake Village, Calif. 
   Position sensing detector  44  provides a signal indicating the position at which control light beam  42  is incident on it lo control system  18  via electrical line  46 . A look-up table stored in memory  8   a  in control system  18  relates the attenuation of the light coupled into optical fiber  26  lo the position at which control light beam  42  is incident on position sensing detector  44 . In this implementation, control system  18  determines from the electrical signals received from photodetector  14  the amount by which light beam  8   a  is to be attenuated, determines from the look-up table the corresponding position on position sensing detector  44  to which control light beam  42  is to be directed, and controls the orientation of mirror  12  to direct control light beam  42  to that position. The look-up table used in this implementation may be generated by measuring the attenuation of light coupled into optical fiber  26  and the position at which control light beam  42  is incident on position sensing detector  44  for each of a series of orientations of mirror  12 . 
   Variable optical attenuator  1  of  FIG. 1  includes only one mirror (mirror  12 ) having an orientation controlled by control system  18  in a range of directions (dθ, dΦ). The angle of incidence of light beam  8   a  on surface  25  and the location on surface  25  at which light beam  8   a  is incident ( FIGS. 2A–2C ) cannot be independently controlled with this single controlled mirror. 
   In other embodiments, light beam  8   a  is directed to optical fiber  26  by two or more mirrors having orientations controlled by controller  18 . For example, variable optical attenuator  47  shown in  FIG. 3  includes, in addition to the elements shown in  FIG. 1 , mirror  48  coupled to actuator  50 . Actuator  50  is controlled by control system  18  with electrical signals provided via electrical line  52  to orient mirror  48  in a range of arbitrary directions (dθ, dΦ). Either or both of mirrors  12  and  48  can be controllably misaligned, by the methods described above, to variably attenuate light coupled into optical fiber  26 . The use of two controllable mirrors in the optical path of light beam  8   a  allows independent control of the angle of incidence of light beam  8   a  on surface  25  and the location on surface  25  at which light beam  8   a  is incident. This may result in better control of light coupled into optical fiber  26 . In some embodiments, lens  6  focuses light beam  8   a  to a waist at a location along light beam  8   a  between mirror  12  and mirror  48 . Such focusing can maintain a relatively small diameter of light beam  8   a  throughout variable optical attenuator  47  and thus reduce uncontrolled optical loss. 
   Variable optical attenuators in accordance with embodiments of the present invention may be implemented within optical fiber cross-connect switches such as those described in U.S. Patent Application Ser. Nos. 09/999,878, 09/999,610, and 10/002,310, now issued U.S. Pat. No. 6,636,656. In particular, mirrors  12  and  48  ( FIG. 3 ) may be mirrors in an optical fiber cross-connect switch oriented to couple light from an input port (optical fiber  2 ) to an output port (optical fiber  26 ). Although  FIGS. 1 and 3  show only a single input optical fiber  2  and a single output optical fiber  26 , an optical fiber cross-connect switch within which a variable optical attenuator is implemented in accordance with an embodiment of the present invention typically has a plurality of inputs and a plurality of outputs. In a typical optical path through such a switch, light entering the switch through an input port is incident on a corresponding first micro-mechanical mirror in a first two dimensional array of micro-mechanical mirrors. The first micro-mechanical mirror, which can be oriented in a range of arbitrary directions (dθ, dΦ), is tilted to direct the light to a second micro-mechanical mirror in a second two dimensional array of micro-mechanical mirrors. The second micro-mechanical mirror, which can also be oriented in a range of arbitrary directions (dθ, dΦ), is tilted to direct the light to a corresponding output port and hence out of the switch. 
   The light may be switched from the output port to which it is initially directed to another output port by reorienting the first micro-mechanical mirror to direct the light to another (i.e., a third) micro-mechanical mirror in the second array of micro-mirrors, and orienting the third micro-mechanical mirror to direct the light to its corresponding output port. The micro-mechanical mirrors may be optimally aligned to couple a maximum amount of light into an output port, or controllably misaligned to variably attenuate the light coupled into an output port. Advantageously, a variable optical attenuator function can be thereby added to an optical network without the insertion of additional optical elements, as power sensing functions such as those provided by beam spliner  10  and photodetector  14  ( FIG. 1 ) are typically present in such switches for control and monitoring purposes. In this way, light entering the switch at any input port can be directed to any output port with the proper attenuation. 
   Referring to  FIG. 4 , for example, optical fiber cross-connect switch  53  (substantially similar to those described in U.S. Patent Application Ser. Nos. 09/999,610, 09/999,610, and 10/002,310, now issued U.S. Pat. No. 6,636,656) routes light carried by any one of N input optical fibers  54 -l– 54 -N to any one of N output optical fibers  56 -l– 56 -N and also performs variable optical attenuation functions in accordance with an embodiment of the present invention. In the implementation shown in  FIG. 4 , the number of input optical fibers equals the number of output optical fibers. Other implementations include N input optical fibers and P output optical fibers, with either N&lt;P or N&gt;P. Typically, both N and P are greater than about 1000. In one implementation, for example, N is about 1200 and P=N. 
   Optical fibers  54 -l– 54 -N output diverging cones of light which are collimated or weakly focused by, respectively, lenses  60 -l– 60 -N to form, respectively, light beams  62 -l– 62 -N incident on beam splitter  10 . Although for convenience of illustration optical fibers  54 -l– 54 -N are shown in  FIG. 4  arranged in a single row, typically the ends of optical fibers  54 -l– 54 -N are arranged in a two dimensional array. Lenses  60 -l– 60 -N may be identical to lens  6  of  FIG. 1 . Alternatively, lenses  60 -l– 60 -N may be lenslets (small lenses) arranged in a two dimensional lenslet array sometimes called a microlens array. 
   Beam splitter  10  divides light beams  62 -l– 62 -N into light beams  66 -l– 66 -N and light beams  68 -l– 68 -N. Light beams  66 -l– 66 -N are incident on, respectively, lenses  70 -l– 70 -N which focus them onto separate spots on input sensor  72 . Input sensor  72  detects the intensity of each of light beams  66 -l– 66 -N and provides corresponding electrical signals to control system  18  via bus  74 . Input sensor  72  is, for example, a SU128-1.7RT infrared camera having a 128×128 pixel array available from Sensors Unlimited, Inc. of Princeton, N.J. 
   Light beams  68 -l– 68 -N are incident on, respectively, micro mirrors  76 -l– 76 -N of micro mirror array  76 . Typically, micro mirrors  76 -l– 76 -N are arranged in a two dimensional array corresponding to that of lenses  60 -l– 60 -N and optical fibers  54 -l– 54 -N. In some implementations the pitch of micro mirrors  76 -l– 76 -N in a direction along mirror array  76  parallel to the planes of incidence of light beams  68 -l– 68 -N (defined by light beams  68 -l– 68 -N and axes normal to mirror array  76  at the points at which the light beams intersect mirror array  76 ) is elongated compared to the corresponding pitch of lenses  60 -l– 60 -N such that light beams  68 -l– 68 -N are incident approximately centered on micro mirrors  76 -l– 76 -N. The orientations of micro mirrors  76 -l– 76 -N are individually controllable over a range of arbitrary angles (dθ, dΦ) by control system  18  with electrical signals transmitted via bus  78 . Micro mirror array  76  is, for example, a MEMS micro mirror array described in U.S. patent application Ser. No. 09/779,189. 
   In the illustrated embodiment, micro mirrors  76 -l– 76 -N reflect light beams  68 -l– 68 -N, respectively, onto fold mirror  80 . Fold mirror  80  reflects light beams  68 -l– 68 -N onto micro mirror array  82 . Micro mirror array  82  includes N micro mirrors  82 -l– 82 -N. The orientations of micro mirrors  82 -l– 82 -N are individually controllable over a range of arbitrary angles (dθ, dφ) by control system  18  with electrical signals transmitted via bus  83 . In one implementation, micro mirror arrays  76  and  82  are substantially identical. 
   In the illustrated embodiment each of micro mirrors  76 -l– 76 -N is controllable to reflect a light beam incident on it from the corresponding one of optical fibers  54 -l– 54 -N to any one of micro mirrors  82 -l– 82 -N via fold mirror  80 . Hence, control system  18  can control the orientations of micro mirrors  76 -l– 76 -N to reflect, via fold mirror  80 , any one of light beams  68 -l– 68 -N onto the approximate center of any one of micro mirrors  82 -l– 82 -N. For example,  FIG. 4  shows light beams  68 - 1 ,  68 - 2 , and  68 -N reflected to, respectively, micro mirrors  82 -K.  82 -J, and  82 -I. Micro mirrors  82 -I,  82 -J, and  82 -K, which need not be adjacent to one another, may be any of micro mirrors  82 -l– 82 -N. In other embodiments micro mirrors  76 -l– 76 -N are controllable to reflect light beams  68 -l– 68 -N to any one of micro mirrors  82 -l– 82 -N without the use of a fold mirror such as fold mirror  80 . In some such embodiments, for example, micro mirrors  76 -l– 76 -N may reflect light beams  68 -l– 68 -N directly to any one of micro mirrors  82 -l– 82 -N. 
   Control system  18  controls the orientations of micro mirrors  82 -l– 82 -N to reflect the light beams incident on them from micro mirror array  76  to, respectively, lenses  84 -l– 84 -N.  FIG. 4  shows micro mirrors  82 -I,  82 -J, and  82 -K reflecting, respectively, light beams  68 -N,  68 - 2 , and  68 - 1  to, respectively, lenses  84 -I,  84 -J, and  84 -K. It should be understood, however, that each particular one of micro mirrors  82 -l– 82 -N is controlled to reflect whichever one of light beams  68 -l– 68 -N is incident on it to the lens  84 -l– 84 -N corresponding lo that particular micro mirror. For example, micro mirror  82 - 1  is controlled to reflect whichever one of light beams  68 -l– 68 -N is incident on it to lens  84 - 1 . 
   Lenses  84 -l– 84 -N focus light beams reflected by, respectively, micro mirrors  82 -l– 82 -N onto, respectively, optical fibers  56 -l– 56 -N. Lenses  84 -l– 84 -N may be, for example, substantially identical to lenses  60 -l– 60 -N. 
   Control system  18  determines from the electrical signals provided by input sensor  72  the amount by which light beams  60 -l– 60 -N must be attenuated, and controls the orientation of micro mirrors  76 -l– 76 -N and  82 -l– 82 -N by, for example, the methods disclosed above (e.g., using control light beams and position sensing detectors) to variably attenuate and/or switch the light beams between output optical fibers  56 -l– 56 -N. 
   In one embodiment, variable attenuation functions of optical cross-connect switch  53  are used to substantially equalize (load balance) the power levels of M Dense Wavelength Division Multiplexing wavelength channels on a single optical fiber, where M≦N (N the number of input ports). The wavelength channels are demultiplexed from the optical fiber with a conventional optical demultiplexer, and each coupled onto a separate one of M of the input optical fibers  54 -l– 54 -N. Control system  18  determines the power levels of the M wavelength channels from the electrical signals it receives from input sensor  72 , and controls mirror arrays  76  and  82  to route each of the M light beams corresponding to the various wavelength channels to a separate one of M of the output optical fibers  56 -l– 56 -N. The lowest power wavelength channel is routed to its corresponding output optical fiber with, for example, approximately minimal attenuation. The power levels of the other wavelength channels are attenuated, for example, to approximately that of the lowest power wavelength channel by controllably misaligning the micro mirrors of mirror arrays  76  and  82  as described above. A conventional optical multiplexer coupled to the M output optical fibers then multiplexes the wavelength channels onto a single optical fiber. 
   While the present invention is illustrated with particular embodiments, the invention is intended to include all variations and modifications falling within the scope of the appended claims.
A method of controllably attenuating a beam of light coupled into a port includes directing the beam of light against a mirror, and controlling an orientation of the mirror such that a predetermined fraction of the beam of light is coupled into the port. The predetermined fraction is less than a maximum fraction corresponding to optimal coupling of the beam of light into the port. The method may be implemented with a variable optical attenuator including a first port, a second port, a mirror located to direct light output by the first port to the second port, and a controller coupled to the mirror to align it such that the predetermined fraction of light is coupled into the second port. The method may also be implemented with an optical switch.