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`PROVISIONAL APPLICATION under 37 C.F.R. §1.53(c)
`
`TRANSMITTAL FORM
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`B0X- Pmvlslonal -Pa-tenfipphcauon
`Assistant Commissioner For Patents
`Washington, D.C. 20231
`
`Examiner:
`
`Enclosed application parts are:
`
`“EXPRESS MAILING” Mailing Label No EL356819491US
`_
`Date of De osit' November 30 2000
`p
`'
`'
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`4 W
`
`._L., -, W 1-
`
`Docket Number: TI-30693P O E!--
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`
`_X_ Spec w/ Claims
`Spec W/o Claims
`Formal Drawings
`_X__ Informal Drawings
`Other:
`
`Number of Pages
`Number of Pages
`Number of Sheets
`Number of Sheets
`
`_36_
`
`___9_
`
`
`
`Inventor(s)
`LAST NAME
`
`FIRST NAME
`
`MIDDLE
`INITIAL
`
`RESIDENCE (City, State or Foreign Country)
`
`
`(972) 917-4418
`
`CORRESPONDENCE ADDRESS:
`
`
`
`Charles A. Brill
`
`Texas Instruments Incorporated
`P.O. Box 655474, MS 3999
`Dallas, Texas 75265
`
`PHONE: (972) 917-4379
`FAX:
`
`Was this invention made under a Government contract?
`
`Yes
`
`Identify contract and the Government agency:
`
`Please charge $150.00 to Deposit Account No. 20-0668.
`An original and two copies are enclosed.
`
`Respectfully submitted,
`
`é%4;5;%
`
`Charles A. Brill
`
`Reg. No. 37,786
`
`Date: my‘ 3&2
`
`PROVISIONAL APPLICATION ONLY
`
`FNC 1008
`
`

`
`MICROMIRROR WAVELENGTH EQUALIZER
`
`FIELD OF THE INVENTION
`
`This invention relates to the field of optical systems, particularly to fiber optic
`
`communication systems.
`
`BACKGROUND OF THE INVENTION
`
`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 wavelength. When necessary, the light beams are separated by
`
`wavelength and routed to their particular destination. Through the course of the network,
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`various wavelengths come from different sources and along different paths. This typically
`
`results in the different wavelengths having different amplitudes.
`
`When the various amplitudes have different wavelengths, optical amplification is
`
`difficult. An unequalized signal passing through an optical amplifier remains unequalized. The
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`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/amplified, robbing from the weaker signals.
`
`
`
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`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
`
`20
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`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.
`
`TI-30693 - Pagel
`
`

`
`SUMMARY OF THE INVENTION
`
`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 micromirror 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.
`
`According to another embodiment of the Wavelength 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,
`
`positioning micromirrors in each sub-array such that micromirrors in a first position direct
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`incident light to an output waveguide and micromirrors in a second position do not, and
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`combining the sub-beams into an output beam of light.
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`
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`
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`iii:iiilit;§E;EII"ill?"tiiill‘”
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`TI-30693 — Page2
`
`

`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present invention reference is now made to the
`
`following descriptions taken in conjunction with the accompanying drawings, in which:
`
`FIGURE 1 is a perspective View of a small portion of a micromirror array of the prior art.
`
`FIGURE 2 is an exploded perspective View of a single micromirror element from the
`
`micromirror array of Figure 1.
`
`FIGURE 3 is a schematic View of one embodiment of a wavelength equalizer according
`
`to the present invention.
`
`FIGURE 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.
`
`FIGURE 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.
`
`FIGURE 6 is a schematic View of another embodiment of a wavelength equalizer
`
`according to the present invention similar to the embodiment of Figure 5, using both a detector
`
`and a light trap.
`
`FIGURE 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.
`
`FIGURE 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‘
`
`
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`mafia
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`20
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`TI-30693 - Page3
`
`

`
`FIGURE 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.
`
`
`
`TI-30693 - Page4
`
`

`
`DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
`
`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
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`amplification of the signal.
`
`The equalizer described below typically uses a micromirror device to attenuate portions
`
`of the device. A typical hidden-hinge micromirror 100 is actually an orthogonal array of
`
`micromirror cells, or elements. This array often includes more than a thousand rows and
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`columns of micromirrors. Figure 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. Figure 2 is an exploded View of a single micromirror element of the prior art
`
`further detailing the relationships between the micromirror structures.
`
`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
`
`
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`
`ll"
`
`
`
`may also be fabricated on the micromirror substrate, or may be external to the micromirror.
`
`20
`
`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.
`
`TI-30693 — Page5
`
`

`
`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 micromirror, 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.
`
`
`
`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.
`
`The first layer of the superstructure is a metalization 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 on 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
`
`20
`
`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.
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`TI-30693 - Page6
`
`

`
`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 combined 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
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`tendency of the mirror 102 and torsion hinge yoke 114 to stick to the landing site.
`
`Mirror bias/reset voltages travel to each mirror 102 through a combination ofpaths 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 connection. The landing electrode/mirror bias
`
`112 configuration shown in Figure l 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 Figure
`
`1 is shown divided into rows of isolated elements.
`
`
`
`A first layer of supports, typically called spacervias, 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
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`20
`
`connections 112. This thin spacer layer is typically a 1 pm thick layer ofpositive 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
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`TI-30693 - Page 7
`
`

`
`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.
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`A thin layer of metal is sputtered onto the spacer layer and into the holes. An oxide is
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`then deposited over the thin metal layer and patterned to form an etch mask over the regions that
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`later will form hinges 120. A thicker layer of metal, typically an aluminum alloy, is sputtered
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`over the thin layer and oxide etch masks. Another layer of oxide is deposited and patterned to
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`define the hinge yoke 114, hinge cap 122, and the upper address electrodes 124. After this
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`second oxide layer is patterned, the two metals layers are etched simultaneously and the oxide
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`etch stops removed to leave thick rigid hinge yokes 114, hinge caps 122, and upper address
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`electrodes 124, and thin flexible torsion beams 120.
`
`A thick spacer layer is then deposited over the thick metal layer and patterned to define
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`holes in which mirror support spacervias 126 will be formed. The thick spacer layer is typically
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`a 2 um 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.
`
`
`
`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
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`deflectable rigid member, which in effect form the two plates of an air gap capacitor, is used to
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`20
`
`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.
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`TI-30693 - Page 8
`
`

`
`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
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`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
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`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 sufficiently large bias voltages are
`
`used to ensure full deflection of the micromirror superstructure.
`
`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
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`electrode is charged to a voltage corresponding to the desired deflection of the mirror. Light
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`striking the micromirror device is reflected by the mirror at an angle determined by the
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`
`
`
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`5:13;iillliafiisiiiil;
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`deflection of the mirror. Depending on the voltage applied to the address electrode, the cone of
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`light reflected by an individual mirror is directed to fall outside the aperture of an optical system,
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`partially within the aperture, or completely within the aperture.
`
`The second mode of operation is a digital mode. When operated digitally, each
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`micromirror is fully deflected in either of the two directions about the torsion beam axis. Digital
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`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,
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`20
`
`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 sufficiently 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
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`TI-30693 - Page 9
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`

`
`using a large mirror bias voltage, the address voltages need only be large enough to deflect the
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`mirror slightly so that the mirror bias voltage will drive the mirror to the correct landing
`
`electrode.
`
`Figure 3 is a schematic View of one embodiment of a Wavelength equalizer according to
`
`the present invention. In Figure 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
`
`Figure 3. After exiting the circulator 304, the light passes through the optic 306. The optic 306
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`
`
`shown in Figure 3 is a collimator lens, but the choice of optic 306 depends on the system.
`
`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 sub-
`
`beams may have a narrow range of wavelengths, so long as the sub-beams are separated into the
`
`various channels used in the fiber optic input.
`
`At the micromirror array 314, each sub-beam impinges on several micromirrors. The
`
`20
`
`group of micromirrors on which a single sub-beam impinges is called a sub-array. 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
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`TI-30693 - Page 10
`
`

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`direction, the light will travel to a fixed mirror 316, and return to the circulator 304 following the
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`same path it traveled to the fixed mirror 316. If the micromirrors in a sub-array deflect light a
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`second direction 318, the light does not retrace its path. A light trap 320 is often used to control
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`the light traveling along the second direction 318.
`
`Other spatial light modulators——for example, various grating light valves or
`
`micromechanical shutter devices——may be used in place of the micromirror array 314. The other
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`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 transmittance 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 return light separated by the circulator 304
`
`exits the equalizer through an output fiber or other waveguide 322.
`
`In Figure 3, the strength of each sub-beam returning 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
`
`
`
`ll
`3
`
`is fully rotated in the second direction. When the mirrors are operated in an analog mode, the
`
`20
`
`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.
`
`TI—30693 - Page 11
`
`

`
`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.
`
`Figure 4 is a schematic View of a second embodiment of a wavelength equalizer
`
`according to a second embodiment of the disclosed invention. In Figure 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 Figure 4. After exiting the circulator 304, the
`
`light passes through the optic 306. The optic 306 shown in Figure 4 is a collimator lens, but the
`
`choice of optic 306 depends on the system. The light then strikes the wavelength separation
`
`device 310.
`
`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 sub-
`
`beams may have a narrow range of wavelengths, so long as the sub-beams are separated into the
`
`
`
`20
`
`various channels used in the fiber optic input.
`
`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. The
`
`TI—30693 - Page 12
`
`

`
`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 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 light separated by the circulator 304
`
`exits the equalizer through an output fiber 322.
`
`
`
`In Figure 4, the strength of each sub-beam returning 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 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.
`
`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
`
`20
`
`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.
`
`The detector 324 of Figure 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
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`TI—30693 — Page 13
`
`

`
`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 determines which mirrors are rotated in the
`
`first and second directions so that a given signal has the proper signal strength at the output.
`
`Alternatively, the controller not only receives signals from the detector processor 326,
`
`but also from an external source. The external source provides various information 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.
`
`Figure 5 is a schematic View of another embodiment of a Wavelength equalizer according
`
`to another embodiment ofthe disclosed invention. The embodiment shown in Figure 5 does not
`
`require a fixed mirror, but instead tilts the micromirror array 314 at the proper angle to direct
`
`light striking mirrors in a first position along the return path. In Figure 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 Figure 5.
`
`After exiting the circulator 304, the light passes through the optic 306. The optic 306 shown in
`
`Figure 5 is a collimator lens, but the choice of optic 306 depends on the system. The light then
`
`
`
`20
`
`strikes a wavelength separation device 310.
`
`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
`
`TI—30693 - Page 14
`
`

`
`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 sub-
`
`beams may have a narrow range of wavelengths, so long as the sub-beams are separated into the
`
`Various channels used in the fiber optic input.
`
`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
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`micromirrors in a given sub-array are in a first position, the light striking the micromirrors is
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`reflected back to optic 312 and returns to the circulator 304 along the same path it traveled to the
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`micromirror array 314. If the micromirrors in a sub-array deflect light a second direction 318,
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`the light is directed to a light trap 320.
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`In retracing its path to the circulator 304, the return light separated by the circulator 304
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`exits the equalizer through an output fiber 322.
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`In Figure 5, as in the prior embodiments, the strength of each sub-beam returning to and
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`exiting from the circulator is determined by the input strength of the sub-beam, by the number of
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`mirrors turned in the first direction, by the degree to which the mirrors are turned in the first
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`
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`direction, and by the location of the mirrors turned in the first direction within the sub-array.
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`The mirrors typically are operated in a digital mode in which a given mirror is either fiilly
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`20
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`rotated in the first direction, or is fully rotated in the second direction. When the mirrors are
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`operated in an analog mode, the degree to which the mirrors are rotated will determine the
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`degree to which the light striking the mirror is reflected by the fixed mirror and returned to the
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`TI-30693 - Page 15
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`circulator 304. The mirrors near the center of a sub-array receive more of the sub-beam
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`compared to the mirrors farther from the center.
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`By controlling the mirrors in a given sub-array, the exit strength of the sub-beam
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`corresponding to that sub-array is altered. Thus, by individually altering the position of the
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`mirrors in each sub-array the strength of each exiting sub-beam is altered. The mirrors typically
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`are positioned to equalize the power between each sub-beam traveling through the input and
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`output fibers, but the various sub-beams may be adjusted to have other power levels.
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`Figure 6 is a schematic view of another embodiment of a wavelength equalizer according
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`to another embodiment of the disclosed invention. The embodiment shown in Figure 6 does not
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`
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`require a fixed mirror, but instead tilts the micromirror array 314 at the proper angle to direct
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`light striking mirrors in a first position along the return path. In Figure 6, as in the prior
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`embodiments, light from an optical fiber 302 passes through a circulator 304 and on to an optic
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`306. Any light separation device can be used in place of the circulator 304 shown in Figure 6.
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`After exiting the circulator 304, the light passes through the optic 306. The optic 306 shown in
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`Figure 6 is a collimator lens, but the choice of optic 306 depends on the system. The light then
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`strikes a wavelength separation device 310.
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`Wavelength separation device 310 can be any type of device, such as a diffraction
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`grating, prism, or other optical component. The grating spatially separates each wavelength
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`carried by the optical fiber 302. The light then passes through a second optic 312, typically a
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`20
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`focussing optic. The second optic 312 directs the light to a micromirror array 314. The input
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`light beam is spatially separated into various sub-beams according to wavelength. While the
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`sub-beams are generally considered single-wavelength, it should be understood that the sub-
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`beams may have a narrow range of wavelengths, so long as the sub-beams are separated into the
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`Tl—30693 - Page 16
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`various channels used in the fiber optic input. In Figure 6, each light beam and sub-beam is
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`shown as a single ray. Thus, Figure 6 shows three separate sub-beams after the wavelength
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`separation device.
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`At the micromirror array 314, each sub-beam impinges on a sub—array of micromirrors.
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`The number of micrornirrors receiving a significant amount of light from the sub-beam depends
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`on the optical components used by the equalizer, and by the intensity of the sub-beam. If the
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`micromirrors in a given sub-array are in a first position, the light striking the micromirrors is
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`reflected back to optic 312 and returns to the circulator 304 along the same path it traveled to the
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`micromirror array 314. If the micromirrors in a sub-array deflect light a second direction 318,
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`the light is directed to a light trap 320. If the micromirrors in a third position, typically a flat
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`position parallel with the plane of the micromirror array 314, the light is directed to a detector
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`324.
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`In retracing its path to the circulator 304, the return light separated by the circulator 304
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`exits the equalizer through an output fiber 322.
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`In Figure 6, as in the prior embodiments, the strength of each sub-beam returning to and
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`exiting from the circulator is determined by the input strength of the sub-beam, by the number of
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`mirrors turned in the first direction, by the degree to which the mirrors are turned in the first
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`direction, and by the location of the mirrors turned in the first direction within the sub-array.
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`
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`The mirrors typically are operated in a digital mode in which a given mirror is either fully
`
`20
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`rotated in the first direction, or is fully rotated in the second direction. When the mirrors are
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`operated in an analog mode, the degree to which the mirrors are rotated will determine the
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`degree to which the light striking the mirror is reflected by the fixed mirror and returned to the
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`TI-30693 - Page 17
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`circulator 304. The mirrors near the center of a sub-array receive more of the