(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0081070 A1
`
` Tew (43) Pub. Date: Jun. 27, 2002
`
`
`US 20020081070A1
`
`(54) MICROMIRROR WAVELENGTH
`EQUALIZER
`
`(76)
`
`Inventor: Claude E' Tew, Dallas, TX (US)
`Correspondence Address:
`TEXAS INSTRUMENTS INCORPORATED
`P 0 BOX 655474, M/S 3999
`DALLAS, TX 75265
`
`(2]) App]. NO’:
`-
`.
`(22)
`Filed.
`
`10/008’186
`NOV' 13’ 2001
`Related U.S. Application Data
`
`(63) Non-provisional of provisional
`60/250,520, filed on Nov. 30, 2000.
`
`application No.
`
`Publication Classification
`
`(51)
`
`Int. Cl.7 ............................... G02B 6/26; GOQB 6/35
`
`(52) us. Cl.
`
`.................. 385/39; 385/27; 385/15; 385/18
`
`ABSTRACT
`(57)
`Awavelength equalizer and method. The wavelength equal-
`izer comprises an input waveguide (302), an output
`waveguide (322), a wavelength separation device (3 10), and
`a micromirror array (314). The wavelength separation
`device (310) divides the input beam of light into sub-beams.
`A first sub-array of the micromirrors in the micromirror
`arra
`314 are 0 erable between a first and second osition.
`Theyfi(rst positiolii directing light in the sub-bearii to the
`output waveguide (322), and the second position excluding
`the light in the sub—beam from the output waveguide (322).
`The method of equalizing a plurality of components of an
`optical input signal comprises: separating the components,
`directing each component to a sub—array of a micromirror
`array, positioning micromirrors in each sub-array such that
`micromirrors in a first position direct incident light to an
`output waveguide and micromirrors in a second position do
`not, and combining the sub-beams into an output beam of
`light.
`
`3 31 ail
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`Patent Application Publication
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`Patent Application Publication
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`Patent Application Publication
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`US 2002/0081070 A1
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`Jun. 27, 2002
`
`MICROMIRROR WAVELENGTH EQUALIZER
`FIELD OF THE INVENTION
`
`[0008] FIG. 1 is a perspective view of a small portion of
`a micromirror array of the prior art.
`
`[0001] This invention relates to the field of optical sys-
`tems, particularly to fiber optic communication systems.
`BACKGROUND OF THE INVENTION
`
`[0002] Modulated light beams carry information in fiber
`optic communication systems. A single fiber carries several
`light beams, and therefore several separate information
`streams. Each light beam in the fiber has a unique wave—
`length. When necessary, the light beams are separated by
`wavelength and routed to their particular destination.
`Through the course of the network, various wavelengths
`come from different sources and along different paths. This
`typically results in the different wavelengths having different
`amplitudes.
`
`[0003] When the various amplitudes have different wave-
`lengths, optical amplification is diffieult. An unequalized
`signal passing through an optical amplifier remains unequal-
`ized. The disparity between the various signals can become
`even worse upon amplification. EFDA are the amplifier of
`choice because they have gain across the entire spectrum of
`interest. When amplified by an EFDA, the stronger signals
`in the unequalized source are spontaneously matched/am-
`plified, robbing from the weaker signals.
`
`[0004] When unequalized signals pass through cascaded
`amplifiers, the equalization problem cascades as well. The
`worst case result is that the weaker signals become weaker
`and weaker until the information in the weak signals is not
`recoverable. What is needed it a method of equalizing the
`signal strength between the various wavelengths in an
`optical fiber.
`SUMMARY OF THE INVENTION
`
`[0005] Objects and advantages will be obvious, and will in
`part appear hereinafter and will be accomplished by the
`present invention which provides a method and system for
`wavelength equalization. One embodiment of the claimed
`invention provides a wavelength equalizer. The wavelength
`equalizer
`comprises
`an
`input waveguide,
`an output
`waveguide, a wavelength separation device, and a micro-
`mirror array. The wavelength separation device divides the
`input beam of light into sub-beams. A first sub-array of the
`micromirrors in the micromirror array are operable between
`a first and second position. The first position directing light
`in the sub-beam to the output waveguide, and the second
`position excluding the light in the sub-beam from the output
`waveguide.
`
`[0006] According to another embodiment of the wave-
`length equalizer, a method of equalizing a plurality of
`components of an optical
`input signal
`is provided. The
`method comprises: separating the components, directing
`each component to a sub-array of a micromirror array, a
`positioning micromirrors in each sub-array such that micro-
`mirrors in a first position direct incident light to an output
`waveguide and micromirrors in a second position do not,
`and combining the sub-beams into an output beam of light.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0007] For a more complete understanding of the present
`invention reference is now made to the following descrip-
`tions taken in conjunction with the accompanying drawings,
`in which:
`
`[0009] FIG. 2 is an exploded perspective view of a single
`micromirror element from the micromirror array of FIG. 1.
`
`[0010] FIG. 3 is a schematic view of one embodiment of
`a wavelength equalizer according to the present invention.
`
`[0011] FIG. 4 is a schematic view of one embodiment of
`a wavelength equalizer according to the present invention
`using a detector to sense the strength of the input signals.
`
`[0012] FIG. 5 is a schematic view of another embodiment
`of a wavelength equalizer according to the present invention
`that operates without the use of a fixed mirror.
`
`[0013] FIG. 6 is a schematic view of another embodiment
`of a wavelength equalizer according to the present invention
`similar to the embodiment of FIG. 5, using both a detector
`and a light trap.
`
`[0014] FIG. 7 is a schematic view of another embodiment
`of a wavelength equalizer according to the present invention
`that does not require a circulator or other light separation
`device.
`
`[0015] FIG. 8 is a schematic view of another embodiment
`of a wavelength equalizer according to the present invention
`using a retro—reflector and two groups of mirror elements
`and having an output power monitor.
`
`[0016] FIG. 9 is a schematic view of another embodiment
`of a wavelength equalizer according to the present invention
`using a retro-reflector and two groups of mirror elements
`and having an output power monitor.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[0017] A micromirror wavelength equalizer has been
`developed that allows each signal in a DWDM fiber optic
`communication system to be individually attenuated. By
`adjusting the attenuation of each component signal,
`the
`composite signal is equalized allowing simple amplification
`of the signal.
`
`[0018] The equalizer described below typically uses a
`micromirror device to attenuate portions of the device. A
`typical hidden-hinge micromirror 100 is actually an orthogo-
`nal array of micromirror cells, or elements. This array often
`includes more than a thousand rows and columns of micro-
`
`mirrors. FIG. 1 shows a small portion of a micromirror array
`of the prior art with several mirrors 102 removed to show the
`underlying mechanical structure of the micromirror array.
`FIG. 2 is an exploded view of a single micromirror element
`of the prior art farther detailing the relationships between the
`micromirror structures.
`
`[0019] A micromirror is fabricated on a semiconductor,
`typically silicon, substrate 104. Electrical control circuitry is
`typically fabricated in or on the surface of the semiconductor
`substrate 104 using standard integrated circuit process flows.
`This circuitry typically includes, but is not limited to, a
`memory cell associated with, and typically underlying, each
`mirror 102 and digital logic circuits to control the transfer of
`the digital
`image data to the underlying memory cells.
`Voltage driver circuits to drive bias and reset signals to the
`mirror superstructure may also be fabricated on the micro-
`mirror substrate, or may be external to the micromirror.
`
`

`

`US 2002/0081070 A1
`
`Jun. 27, 2002
`
`Image processing and formatting logic is also formed in the
`substrate 104 of some designs. For the purposes of this
`disclosure, addressing circuitry is considered to include any
`circuitry, including direct voltage connections and shared
`memory cells, used to control the direction of rotation of a
`micromirror.
`
`[0020] Some micromirror configurations use a split reset
`configuration which allows several micromirror elements to
`share one memory cell--thus reducing the number of
`memory cells necessary to operate a very large array, and
`making more room available for voltage driver and image
`processing circuitry on the micromirror integrated circuit.
`Split reset is enabled by the bistable operation of a micro-
`mirror, which allows the contents of the underlying memory
`to change without affecting the position of the mirror 102
`when the mirror has a bias voltage applied.
`
`[0021] The silicon substrate 104 and any necessary metal
`interconnection layers are isolated from the micromirror
`superstructure by an insulating layer 106 which is typically
`a deposited silicon dioxide layer on which the micromirror
`superstructure is formed. Holes, or Vias, are opened in the
`oxide layer to allow electrical connection of the micromirror
`superstructure with the electronic circuitry formed in the
`substrate 104.
`
`[0022] The first layer of the superstructure is a metaliza—
`tion layer, typically the third metalization layer and therefore
`often called M3. The first
`two metalization layers are
`typically required to interconnect the circuitry fabricated on
`the substrate. The third metalization layer is deposited 011 the
`insulating layer and patterned to form address electrodes 110
`and a mirror bias connection 112. Some micromirror designs
`have landing electrodes which are separate and distinct
`structures but are electrically connects to the mirror bias
`connection 112, Landing electrodes limit the rotation of the
`mirror 102 and prevent the rotated mirror 102 or hinge yoke
`114 from touching the address electrodes 110, which have a
`voltage potential relative to the mirror 102. If the mirror 102
`contacts the address electrodes 110,
`the resulting short
`circuit could fuse the torsion hinges 116 or weld the mirror
`102 to the address electrodes 110, in either case ruining the
`micromirror.
`
`[0023] Since the same voltage is always applied both to
`the landing electrodes and the mirrors 102, the mirror bias
`connection and the landing electrodes are preferably com-
`bined in a single structure when possible. The landing
`electrodes are combined with the mirror bias connection 112
`by including regions on the mirror bias/reset connection 112,
`called landing sites, which mechanically limit the rotation of
`the mirror 102 by contacting either the mirror 102 or the
`torsion hinge yoke 114. These landing sites are often coated
`with a material chosen to reduce the tendency of the mirror
`102 and torsion hinge yoke 114 to stick to the landing site.
`
`[0024] Mirror bias/reset voltages travel to each mirror 102
`through a combination of paths using both the mirror
`bias/reset metalization 112 and the mirrors and torsion
`beams of adjacent mirror elements. Split reset designs
`require the array of mirrors to be subdivided into multiple
`subarrays each having an independent mirror bias connec-
`tion. The landing electrode/mirror bias 112 configuration
`shown in FIG. 1 is ideally suited to split reset applications
`since the micromirror elements are easily segregated into
`electrically isolated rows or columns simply by isolating the
`
`mirror bias/reset layer between the subarrays. The mirror
`bias/reset layer of FIG. 1 is shown divided into rows of
`isolated elements.
`
`typically called spac-
`layer of supports,
`[0025] A first
`ervias, is fabricated on the metal layer forming the address
`electrodes 110 and mirror bias connections 112. These
`
`spacervias, which include both hinge support spacervias 116
`and upper address electrode spacervias 118, are typically
`formed by spinning a thin spacer layer over the address
`electrodes 110 and mirror bias connections 112. This thin
`spacer layer is typically a 1 gm thick layer of positive
`photoresist. After the photoresist layer is deposited,
`it is
`exposed, patterned, and deep UV hardened to form holes in
`which the spacervias will be formed. This spacer layer and
`a thicker spacer layer used later in the fabrication process are
`often called sacrificial layers since they are used only as
`forms during the fabrication process and are removed from
`the device prior to device operation.
`[0026] A thin layer of metal is sputtered onto the spacer
`layer and into the holes. An oxide is then deposited over the
`thin metal layer and patterned to form an etch mask over the
`regions that later will form hinges 120. A thicker layer of
`metal, typically an aluminum alloy, is sputtered over the thin
`layer and oxide etch masks. Another layer of oxide is
`deposited and patterned to define the hinge yoke 114, hinge
`cap 122, and the upper address electrodes 124. After this
`second oxide layer is patterned, the two metals layers are
`etched simultaneously and the oxide etch stops removed to
`leave thick rigid hinge yokes 114, hinge caps 122, and upper
`address electrodes 124, and thin flexible torsion beams 120.
`
`[0027] Athick spacer layer is then deposited over the thick
`metal layer and patterned to define holes in which mirror
`support spacervias 126 will be formed. The thick spacer
`layer is typically a 2 pm thick layer of positive photoresist.
`A layer of mirror metal, typically an aluminum alloy, is
`sputtered on the surface of the thick spacer layer and into the
`holes in the thick spacer layer. This metal layer is then
`patterned to form the mirrors 102 and both spacer layers are
`removed using a plasma etch.
`[0028] Once the two spacer layers have been removed, the
`mirror is free to rotate about the axis formed by the torsion
`hinge. Electrostatic attraction between an address electrode
`110 and a deflectable rigid member, which in effect form the
`two plates of an air gap capacitor, is used to rotate the mirror
`structure. Depending on the design of the micromirror
`device, the deflectable rigid member is the torsion beam
`yoke 114, the beam or mirror 102, a beam attached directly
`to the torsion hinges, or a combination thereof. The upper
`address electrodes 124 also electrostatically attract
`the
`deflectable rigid member.
`[0029] The force created by the voltage potential is a
`function of the reciprocal of the distance between the two
`plates. As the rigid member rotates due to the electrostatic
`torque, the torsion beam hinges resist deformation with a
`restoring torque which is an approximately linear function of
`the angular deflection of the torsion beams. The structure
`rotates until the restoring torsion beam torque equals the
`electrostatic torque or until
`the rotation is mechanically
`blocked by contact between the rotating structure and a fixed
`component. As discussed below, most micromirror devices
`are operated in a digital mode wherein sufliciently large bias
`voltages are used to ensure full deflection of the micromirror
`superstructure.
`
`

`

`US 2002/0081070 A1
`
`Jun. 27, 2002
`
`[0030] Micromirror devices are generally operated in one
`of two modes of operation. The first mode of operation is an
`analog mode, sometimes called beam steering, wherein the
`address electrode is charged to a voltage corresponding to
`the desired deflection of the mirror. Light striking the
`micromirror device is reflected by the mirror at an angle
`determined by the deflection of the mirror. Depending on the
`voltage applied to the address electrode, the cone of light
`reflected by an individual mirror is directed to fall outside
`the aperture of an optical system, partially within the aper-
`ture, or completely within the aperture.
`
`[0031] The second mode of operation is a digital mode.
`When operated digitally, each micromirror is fully deflected
`in either of the two directions about the torsion beam axis.
`
`Digital operation uses a relatively large voltage to ensure the
`mirror is fully deflected. Since it is advantageous to drive the
`address electrode using standard logic voltage levels, a bias
`voltage, typically a negative voltage, is applied to the mirror
`metal layer to increase the voltage difference between the
`address electrodes and the mirrors. Use of a sulliciently large
`mirror bias voltage—a voltage above what is termed the
`collapse voltage of the device—ensures the mirror will
`deflect to the closest landing electrodes even in the absence
`of an address voltage. Therefore, by using a large mirror bias
`voltage, the address voltages need only be large enough to
`deflect the mirror slightly so that the mirror bias voltage will
`drive the mirror to the correct landing electrode.
`
`[0032] FIG. 3 is a schematic View of one embodiment of
`a wavelength equalizer according to the present invention.
`In FIG. 3, light from an optical fiber or other waveguide 302
`passes through a circulator 304 and on to an optic 306. The
`circulator 304, as will be described below, is operable to
`separate light beams traveling through the circulator 304
`based on the polarization of the light. Any light separation
`device can be used in place of the circulator 304 shown in
`FIG. 3. After exiting the circulator 304, the light passes
`through the optic 306. The optic 306 shown in FIG. 3 is a
`collimator lens, but the choice of optic 306 depends on the
`system.
`
`[0033] Wavelength separation device 310 can be any type
`of device, such as a diffraction grating, prism, or other
`optical component. The grating spatially separates each
`wavelength carried by the optical fiber 302. The light then
`passes through a second optic 312, typically a focussing
`optic. The second optic 312 directs the light to a micromirror
`array 314. The input light beam is spatially separated into
`various sub-beams according to wavelength. While the
`sub-beams are generally considered single-wavelength,
`it
`should be understood that the subbeams may have a narrow
`range of wavelengths, so long as the sub-beams are sepa-
`rated into the various channels used in the fiber optic input.
`
`the micromirror array 314, each sub—beam
`[0034] At
`impinges on several micromirrors. The group of micromir-
`rors on which a single sub-beam impinges is called a
`sub-array. The number of micromirrors receiving a signifi-
`cant amount of light from the sub-beam depends on the
`optical components used by the equalizer, and by the inten-
`sity of the sub-beam. The micromirrors in a sub-array are
`used to deflect the light striking them. If the micromirrors
`deflect light a first direction, the light will travel to a fixed
`mirror 316, and return to the circulator 304 following the
`same path it traveled to the fixed mirror 316. If the micro-
`
`mirrors in a sub-array deflect light a second direction 318,
`the light does not retrace its path. A light trap 320 is often
`used to control the light traveling along the second direction
`318.
`
`[0035] Other spatial light modulators—for example, vari-
`ous grating light valves or micromechanical
`shutter
`devices—may be used in place of the micromirror array 314.
`The other spatial light modulators typically include arrays of
`individually controllable elements that are operable in a first
`position or state and a second position or state to control the
`reflectivity, including the direction of reflection, or trans-
`mittance of the element. A micromirror array 314, however,
`is an optimal device since it provides precise adjustment of
`the power levels, reliable operation, and excellent isolation
`between the first and second mirror positions.
`
`In retracing its path to the circulator 304. The
`[0036]
`return light separated by the circulator 304 exits the equal—
`izer through an output fiber or other waveguide 322.
`
`In FIG. 3, the strength of each sub-beam returning
`[0037]
`to and exiting from the circulator is determined by the input
`strength of thc sub-beam, by the number of mirrors turned
`in the first direction, by the degree to which the mirrors are
`turned in the first direction, and by the location of the mirrors
`turned in the first direction within the sub-array. The mirrors
`typically are operated in a digital mode in which a given
`mirror is either fully rotated in the first direction, or is fully
`rotated in the second direction. When the mirrors are oper—
`ated in an analog mode, the degree to which the mirrors are
`rotated will determine the degree to which the light striking
`the mirror is reflected by the fixed mirror and returned to the
`circulator 304. The mirrors near the center of a sub-array
`receive more of the sub-beam compared to the mirrors
`farther from the center.
`
`[0038] By controlling the mirrors in a given sub—array, the
`exit strength of the sub-beam corresponding to that sub-
`array is altered. Thus, by individually altering the position of
`the mirrors in each sub—array the strength of each exiting
`sub-beam is altered. The mirrors typically are positioned to
`equalize the power between each sub-beam traveling
`through the input and output fibers, but the various sub-
`beams may be adjusted to have other power levels.
`
`[0039] FIG. 4 is a schematic view of a second embodi-
`ment of a wavelength equalizer according to a second
`embodiment of the disclosed invention. In FIG. 4, light from
`an optical fiber 302 passes through a circulator 304 and on
`to an optic 306. Any light separation device can be used in
`place of the circulator 304 shown in FIG. 4. After exiting the
`circulator 304, the light passes through the optic 306. The
`optic 306 shown in FIG. 4 is a collimator lens, but the choice
`of optic 306 depends on the system. The light then strikes the
`wavelength separation device 310.
`
`[0040] Wavelength separation device 310 can be any type
`of device, such as a diffraction grating, prism, or other
`optical component. The grating spatially separates each
`wavelength carried by the optical fiber 302. The light then
`passes through a second optic 312,
`typically a focussing
`optic. The second optic 312 directs the light to a micromirror
`array 314. The input light beam is spatially separated into
`various sub-beams according to wavelength. While the
`sub-beams are generally considered single-wavelength, it
`should be understood that the subbeams may have a narrow
`
`

`

`US 2002/0081070 A1
`
`Jun. 27, 2002
`
`range of wavelengths, so long as the sub-beams are sepa-
`rated into the various channels used in the fiber optic input.
`
`the micromirror array 314, each sub-beam
`[0041] At
`impingcs on a sub-array of micromirrors. The number of
`micromirrors receiving a significant amount of light from
`the sub-beam depends on the optical components used by
`the equalizer, and by the intensity of the sub-beam. The
`micromirrors in a sub-array are used to deflect the light
`striking them. If the micromirrors deflect light a first direc-
`tion, the light will travel to a fixed mirror 316, and return to
`the circulator 304 following the same path it traveled to the
`fixed mirror 316. If the micromirrors in a sub-array deflect
`light a second direction 318, the light is directed to a detector
`324.
`
`In retracing its path to the circulator 304, the return
`[0042]
`light separated by the circulator 304 exits the equalizer
`through an output fiber 322.
`
`In FIG. 4, the strength of each sub-beam returning
`[0043]
`to and exiting from the circulator is determined by the input
`strength of the sub-beam, by the number of mirrors turned
`in the first direction, by the degree to which the mirrors are
`turned in the first direction, and by the location of the mirrors
`turned in the first direction within the sub-array. The mirrors
`typically are operated in a digital mode in which a given
`mirror is either fully rotated in the first direction, or is fiilly
`rotated in the second direction. When the mirrors are oper-
`ated in an analog mode, the degree to which the mirrors are
`rotated will determine the degree to which the light striking
`the mirror is reflected by the fixed mirror and returned to the
`circulator 304. The mirrors near the center of a sub-array
`receive more of the sub-beam compared to the mirrors
`farther from the center.
`
`[0044] By controlling the mirrors in a given sub-array, the
`exit strength of the sub—beam corresponding to that sub—
`array is altered. Thus, by individually altering the position of
`the mirrors in each sub-array the strength of each exiting
`sub—beam is altered. The mirrors typically are positioned to
`equalize the power between each sub-beam traveling
`through the input and output fibers, but the various sub-
`beams may be adjusted to have other power levels.
`
`[0045] The detector 324 of FIG. 4 allows monitoring of
`the light dumped out of the return path by the mirror array
`314. The detector processor 326 reads the signal from the
`detector and provides a signal to controller 328. By knowing
`the number and location of the mirrors from a given sub-
`array rotated in the second direction, and the strength of the
`signal received by the detector, the detector controller 328
`can determine the input and output strengths of the signal
`corresponding to that sub-array. The controller 328 deter-
`mines which mirrors are rotated in the first and second
`
`directions so that a given signal has the proper signal
`strength at the output.
`
`[0046] Alternatively, the controller not only receives sig-
`nals from the detector processor 326, but also from an
`external source. The external source provides various infor-
`mation including, for purposes of illustration, information
`about the input signal strength of one or more sub-beams and
`information of the desired output signal strength for one or
`more sub-beams. Thus, the external source can indicate that
`a given sub-beam should have a particular output signal
`strength suited to its prospective signal path.
`
`[0047] FIG. 5 is a schematic view of another embodiment
`of a wavelength equalizer according to another embodiment
`of the disclosed invention. The embodiment shown in FIG.
`
`instead tilts the
`5 does not require a fixed mirror, but
`micromirror array 314 at the proper angle to direct light
`striking mirrors in a first position along the return path. In
`FIG. 5, as in the prior embodiments, light from an optical
`fiber 302 passes through a circulator 304 and on to an optic
`306. Any light separation device can be used in place of the
`circulator 304 shown in FIG. 5. After exiting the circulator
`304, the light passes through the optic 306. The optic 306
`shown in FIG. 5 is a collimator lens, but the choice of optic
`306 depends on the system. The light then strikes a wave-
`length separation device 310.
`
`[0048] Wavelength separation device 310 can be any type
`of device, such as a diffraction grating, prism, or other
`optical component. The grating spatially separates each
`wavelength carried by the optical fiber 302. The light then
`passes through a second optic 312,
`typically a focussing
`optic. The second optic 312 directs the light to a micromirror
`array 314. The input light beam is spatially separated into
`various sub-beams according to wavelength. While the
`sub-beams are generally considered single-wavelength, it
`should be understood that the subbeams may have a narrow
`range of wavelengths, so long as the sub-beams are sepa-
`rated into the various channels used in the fiber optic input.
`[0049] At
`the micromirror array 314, each sub-beam
`impinges on a sub-array of micromirrors. The number of
`micromirrors receiving a significant amount of light from
`the sub-beam depends on the optical components used by
`the equalizer, and by the intensity of the sub-beam. If the
`micromirrors in a given sub-array are in a first position, the
`light striking the micromirrors is reflected back to optic 312
`and returns to the circulator 304 along the same path it
`traveled to the micromirror array 314. If the micromirrors in
`a sub-array deflect light a second direction 318, the light is
`directed to a light trap 320.
`[0050]
`In retracing its path to the circulator 304, the return
`light separated by the circulator 304 exits the equalizer
`through an output fiber 322.
`[0051]
`In FIG. 5, as in the prior embodiments, the strength
`of each sub-beam returning to and exiting from the circu-
`lator is determined by the input strength of the sub-beam, by
`the number of mirrors turned in the first direction, by the
`degree to which the mirrors are turned in the first direction,
`and by the location of the mirrors turned in the first direction
`within the sub-array. The mirrors typically are operated in a
`digital mode in which a given mirror is either fully rotated
`in the first direction, or is fully rotated in the second
`direction. When the mirrors are operated in an analog mode,
`the degree to which the mirrors are rotated will determine
`the degree to which the light striking the mirror is reflected
`by the fixed mirror and returned to the circulator 304. The
`mirrors near the center of a sub-array receive more of the
`sub-beam compared to the mirrors farther from the center.
`[0052] By controlling the mirrors in a given sub-array, the
`exit strength of the sub-beam corresponding to that sub-
`array is altered. Thus, by individually altering the position of
`the mirrors in each sub-array the strength of each exiting
`sub-beam is altered. The mirrors typically are positioned to
`equalize the power between each sub-beam traveling
`through the input and output fibers, but the various sub-
`beams may be adjusted to have other power levels.
`
`

`

`US 2002/0081070 A1
`
`Jun. 27, 2002
`
`[0053] FIG. 6 is a schematic view of another embodiment
`of a wavelength equalizer according to another embodiment
`of the disclosed invention. The embodiment shown in FIG.
`
`instead tilts the
`6 does not require a fixed mirror, but
`micromirror array 314 at the proper angle to direct light
`striking mirrors in a first position along the return path. In
`FIG. 6, as in the prior embodiments, light from an optical
`fiber 302 passes through a circulator 304 and on to an optic
`306. Any light separation device can be used in place of the
`circulator 304 shown in FIG. 6. After exiting the circulator
`304, the light passes through the optic 306. The optic 306
`shown in FIG. 6 is a collimator lens, but the choice of optic
`306 depends on the system. The light then strikes a wave-
`length separation device 310.
`
`[0054] Wavelength separation device 310 can be any type
`of device, such as a diffraction grating, prism, or other
`optical component. The grating spatially separates each
`wavelength carried by the optical fiber 302. The light then
`passes through a second optic 312, typically a focussing
`optic. The second optic 312 directs the light to a micromirror
`array 314. The input light beam is spatially separated into
`various sub-beams according to wavelength. While the
`sub-beams are generally considered single-wavelength,
`it
`should be understood that the subbeams may have a narrow
`range of wavelengths, so long as the sub-beams are sepa-
`rated into the various channels used in the fiber optic input.
`In FIG. 6, each light beam and sub-beam is shown as a
`single ray. Thus, FIG. 6 shows three separate sub-beams
`after the wavelength separation device.
`[0055] At
`the micromirror array 314, each sub-beam
`impinges on a sub-array of micromirrors, The number of
`micromirrors receiving a significant amount of light from
`the sub-beam depends on the optical components used by
`the equalizer, and by the intensity of the sub-beam. If the
`micromirrors in a given sub-array are in a first position, the
`light striking the micromirrors is reflected back to optic 312
`and returns to the circulator 304 along the same path it
`traveled to the micromirror array 314. If the micromirrors in
`a sub-array deflect light a second direction 318, the light is
`directed to a light trap 320. If the micromirrors in a third
`position, typically a flat position parallel with the plane of
`