IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3, MAY /JUNE 2004
`
`579
`
`Interference -Based Micromechanical
`Spectral Equalizers
`Joseph E. Ford, Keith W. Goossen, Member, IEEE, James A. Walker, David T. Neilson, Senior Member, IEEE,
`D. M. Tennant, Seo Yeon Park, and J. W. Sulhoff
`
`Abstract -Dynamic gain equalization filters (DGEFs) are
`important for high -performance wavelength division multiplexed
`(WDM) communications. One of the first demonstrated DGEF
`used a micromechanical etalon filter array illuminated with
`free -space spectral demultiplexing optics. Here, we present subse-
`quent research on etalon -based dynamic spectral filters, including
`vertical device structures which linearize and reduce the drive
`voltage from 70 to 40 V, and spatially- segmented etalons which
`allow channelized spectral equalization and further reduce drive
`voltage. We describe a prototype using a simplified cylindrical
`optomechanical package with a 104 -nm broadband spectral
`response, 7.5 -dB insertion loss and less than 16 -V operation
`voltage. Finally, we show the use of a 42 -nm bandwidth DGEF
`prototype with feedback stabilization to more than double the
`number of channels and operating bandwidth of a conventional
`Erbium -doped fiber amplifier while maintaining < 1 -dB power
`uniformity.
`Index Terms -Amplifiers, dynamic gain equalizing filters
`(DGEF), microelectromechanical systems
`(MEMS), MEMS
`components.
`
`I.
`
`.INTRODUCTION
`
`ACTIVE power level stabilization is of great importance
`for high- capacity wavelength division multiplexed
`(WDM) transmission systems. The initial requirement was for
`dynamic gain equalization filters (DGEF) [1], which produce
`a smoothly varying spectral attenuation profile to remove
`variations in channel net gain profiles. The spectral resolution
`required corresponds to the amplifier gain variances, which
`are many wavelength channels wide (typically 0.8 -1.6 nm
`each). A range of attenuation of 20 dB or more is typically
`needed, although specific applications can vary. The various
`solutions proposed and demonstrated for
`this application
`include micromechanical -based systems [2], [3], liquid -crystal
`[4] [7], acousto -optic [8], and waveguide [9] technologies. A
`second generation of spectral equalizers with greater spectral
`
`resolution was demanded in response to dynamic network
`reconfiguration, wherein individual wavelength channels can
`be switched in and out of a WDM transmission switches
`such as photonic cross connects and wavelength add/drop
`switches. Such transparent optical networking demands power
`level equalization of channels that have not only traversed
`components with nonuniform loss or gain, but may have arrived
`from diverse paths. Several technologies have been proposed
`for high resolution and dynamic range channelized equalizing
`including microelectromechanical systems (MEMS)
`filters
`tilt- mirror arrays [10J, [11], [ 18], liquid crystal [5], [6], and
`planar light wave circuits [12].
`We previously demonstrated spectral equalization using free -
`space optics to disperse an input spectrum over a microoptome-
`chanical variable reflectivity etalon mirror [2]. The basic device
`structure, shown in Fig. 1, is a single- cavity etalon stripe with an
`electrically controlled air gap between a silicon substrate and a
`silicon nitride a/4 layer. Each pair of electrodes applies a local
`force to reduce the etalon air gap from 1.1 to 0.9 µm and create
`smoothly varying changes in the mirror reflectivity along the
`length of an approximately 80 x 1500 /em optical window. Me-
`chanical connection between adjacent regions of the membrane
`ensures a smoothly varying reflectivity profile.
`In this paper, we present subsequent research on etalon -based
`dynamic spectral filters, including materials and device struc-
`tures which allow linearized and reduced voltage response,
`spatially- segmented etalons which allow channelized spectral
`equalization, and a simplified and ruggedized optomechan-
`ical package. Finally, we demonstrate the use of the MEMS
`dynamic spectral equalizer to upgrade a conventional Er-
`bium -doped fiber amplifier (EDFA) for increased numbers of
`WDM channels with wider operating parameters.
`
`II. OPTICAL SYSTEM AND OPTOMECHANiCAL
`PACKAGE DESIGN
`
`Manuscript received November 26, 2003; revised April 13, 2004. This re-
`search was performed at Bell Laboratories, Lucent Technologies.
`J. E. Ford is with the Department of Electrical and Computer Engineering,
`the University of California, San Diego, La Jolla, CA 92093 -0407 USA (e -mail:
`jeford @ucsd.edu).
`K. W. Goossen is with the Department of Electrical and Computer Engi-
`neering, University of Delaware, Newark, DE 19711 -3130 USA.
`J. A. Walker is with JayWalker Technical Consulting, Freehold, NJ 07728
`USA.
`D. T. Neilson and D. M. Tennant are with Bell Laboratories, Lucent Tech-
`nologies, Murray Hill, NJ 07974 -0636 USA.
`S. Y. Park is with OptoVia Corporation, Acton, MA 01720 USA.
`J. W. Sulhoff is with Onetta, Inc., Sunnyvale, CA 94089 USA.
`Digital Object Identifier 10.1109/JSTQE.2004.830612
`
`The optomechanical package described in [2] used a skew
`optical path with vertical separation of the collimated input
`and output beams in the pupil to separate output signals. This
`package was functional, with some 4.6 -dB insertion loss and
`<0.2 -dB polarization dependence, but optimization required 17
`separate alignments: tip, tilt, z shift and rotation of the grating,
`tip, tilt (two controls each) and focus of two collimators, x. y. z.
`tip and tilt of the MEMS device, focus of the imaging lens, and
`rotation of the quarterwave plate. As such this package was
`practical only as a laboratory testbed. The new optical design
`
`1077 -260X/04$20.00 © 2004 IEEE
`
`Capella 2027
`Fujitsu v. Capella
`IPR2015-00727
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`580
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`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3, MAY /JUNE 2004
`
`Voltage
`Applied
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`Voltage
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`
`Fig. I. MEMS micro -etalon structure (left) and fabricated device (right).
`
`Fig. 2. Optical layout for equalizing filter, including custom double -Gauss
`achromatic collimation /focus lens. Distance from grating to image is 100 mm.
`(Optical circulator used to separate input and output not shown.)
`
`shown in Fig. 2 considerably simplifies optical alignment. It
`consists of a coaxial imaging system with a diffraction grating
`at the collimated beam plane and an input/output fiber located in
`an image plane adjacent to the MEMS device. Input and output
`signals are separated using an external optical circulator.
`The optical demultiplexing system, illustrated in the ray -
`is constructed as a folded telecentric 4f f
`trace of Fig. 2,
`system imaging system [13], [14]. The system consists of a
`f = 50 mm relay lens, with a planar 600 -1p /mm grating in
`Littrow configuration (A = 28.3 °) at the stop. The lens is a
`full custom four -element achromat designed for diffraction
`limited resolution over a 30 -nm bandwidth, as compared to the
`40 -nm bandwidth of the original triplet lens [3]. Light enters
`an input fiber and passes through an optical circulator (not
`shown) then is emitted from the optical fiber into free space and
`imaged through the diffraction grating and two passes through
`the lens to arrive at the MEMS device spatially dispersed
`by wavelength. The MEMS membrane mirror reflects the
`light to retrace its path through the lens and grating and be
`rermtitiplexed onto the face of the input fiber. Light coupled
`into the fiber propagates back to the circulator, which directs it
`into a separate output fiber.
`This system is attractive because the full system path is 8f,
`i.e., two sequential relay imaging stages. If the MEMS device
`window is slightly oversized relative to the 10 pm optical mode
`diameter, then slight lateral misalignments of the system will
`be compensated on the second pass and returning light will au-
`tomatically realign to the fiber core. Also, since the same lens
`and grating is used on both passes, the multiplexing and demul-
`tiplexing angles are automatically matched. This considerably
`relaxes alignment and focal length tolerances.
`It is essential to minimize polarization dependent loss (PDL)
`in the system. We selected a grating with a blaze wavelength
`
`(Left) Free -space WDM optomechanics, showing header, 50 -mm focal
`Fig. 3.
`length lens, and 600 -1p /mm grating. (Right) Close -up of the device, electrical
`flex -circuit contacts, and I/O fiber ferrule on the cylindrical metal header.
`
`at 1.85 pm wavelength with equal efficiencies (0.8 -dB loss) for
`S and P polarizations at 1.55 pm. To eliminate small residual
`wavelength variance of the PDL, a zero -order quarter wave plate
`is placed between the lens and the grating and orientated to ro-
`tate the polarization of the light by 90° upon two passes, such
`that the total efficiency for arbitrary input polarization light is
`the product of the S and P efficiency of the grating.
`The photographs in Fig. 3 show the new optomechanical
`mounting, which consists of a 150 -mm -long metal tube, a
`lens, a grating on a locking tip /tilt mount, and a cylindrical
`metal header. An 8 -mm- square MEMS die is mounted on
`the 25 -mm- diameter header. The 1.5 -mm -long active MEMS
`etalon stripe is positioned next to an angle -polished ferrule,
`which acts as both input and output from the package. Traces
`on the chip are wire bonded to two identical flexible circuits,
`which connect 40 pairs of active leads and two grounds to the
`drive electronics. The system was designed to maintain the
`focal plane at the MEMS mirror over a 60 °C temperature
`range. The lens and grating mounts, tube, and header are
`fabricated of super -invar metal. Tests indicated a spectral shift
`of around 0.001 nm/ °C, or about 7.5 GHz over 60 °. This optical
`system is appropriate for constructing systems operating from
`1500 to 1630 nm.
`There are just four alignments, all orthogonal: the device sub -
`mount slides axially for focus, the grating is tip /tilt adjusted to
`position the selected center wavelength on the device and ro-
`tated around the tube axis to align the dispersed spectra with the
`device window. The alignment process uses a broad spectrum
`amplified spontaneous emission (ASE) source and an optical
`spectrum analyzer. First, the lens and grating are inserted and
`
`

`
`FORD et al.: INTERFERENCE -BASED MICROMECHANICAL SPECTRAL EQUALIZERS
`
`581
`
`locked in place (to an accuracy of 1 mm) then the device sub -
`mount is inserted and focused to peak output power. Grating tip
`and tilt are used to center the desired spectrum on the device
`window, aided by alignment marks on the die. The grating rota-
`tion is fine -tuned to align the spectral dispersion with the MEMS
`device window. Then, the focus, tip, and tilt are fine -tuned be-
`fore locking. In this prototype, locking was done with screws;
`in a manufactured product, locking would be by laser welding.
`Alignment of the entire system requires about 5 min, compared
`with approximately 4 h for the previous platform.
`The optical performance of the system met initial design
`goals. The lens in an 8.f configuration with mirror at the back
`focal plane has an insertion loss of 2.4 dB at 1550 nm, with a
`0.2 -dB variation from 1500 to 1620 nm. The coupling losses are
`those from the aberration of the optical system and from the 40
`antireflection coated surfaces and the residual glass absorption.
`Loss could be reduced with more stringent specifications,
`especially on lens coatings. The grating has 1.7 -dB loss on
`two passes with alternate polarizations, which could also be
`improved. This gives a 4.1 -dB insertion loss for the optical
`system. Two passes through the circulator adds 1.3 dB, and
`the MEMS membrane 1.5 dB, to give an expected insertion
`loss of 6.9 dB. The assembled device had 7 dB loss, 0.2 ps /nm
`chromatic dispersion, and less than 0.1 ps polarization mode
`dispersion. The measured polarization dependent loss is less
`than 0.25 dB across the entire spectrum. The following ex-
`perimental results were obtained using MEMS devices in this
`improved optomechanical package.
`
`III. LINEARIZED VOLTAGE RESPONSE MEMS DEVICE
`
`The mechanical anti -reflection switch (MARS) microme-
`chanical modulator was originally developed for digital data
`transmission and later used as a high -speed analog variable
`attenuator. The basic structure is a quarter -wave dielectric
`antireflection coating suspended above a silicon substrate [15].
`A silicon nitride layer with 1/4A optical thickness, separated
`from the silicon substrate by a fixed 3/4A spacer, acts as
`a dielectric mirror with about 70% ( -1.5 dB) reflectivity.
`Voltage applied to electrodes on top of the membrane creates
`an electrostatic force and pulls the membrane closer to the
`substrate, while membrane tension provides a linear restoring
`force. When the membrane gap is reduced to a /2, the layer
`becomes an antireflection coating with close to zero reflectivity.
`Like glass, the deformable nitride layer is brittle; the devices
`are defined as membranes because lateral stress is the dominant
`force. Mechanically, the device moves by elastic deformation,
`similar to a tuning fork. Electrically, the device behaves as a
`tiny capacitor, with zero static power dissipation, regardless of
`reflectivity state.
`MARS device fabrication begins with deposition of a spacer
`layer of phospho -silicate glass (PSG) equal in thickness to the
`desired air gap. Next, a film of silicon nitride, with thickness
`set to achieve an optical path delay of one -fourth of the center
`operating wavelength, is deposited on the PSG layer. Both the
`
`PSG and silicon nitride films are deposited using conventional
`low- pressure chemical vapor deposition (LPCVD) techniques.
`Electrodes, comprising a thin layer of adhesion metal (such as
`titanium or chrome) and a 100 -nm -thick layer of gold, are de-
`posited using a liftoff procedure. Finally, reactive -ion -etching is
`used to open etch access holes through the silicon nitride film
`exposing the PSG film. The optical window is formed and made
`mechanically- active by a timed hydrofluoric acid etch and rinse,
`followed by critical point CO9 drying.
`Starting from the MARS modulator structure, a WDM equal-
`izer can be fabricated by forming a stripe membrane lined on
`both sides of the optical window by mated pairs of individu-
`ally- addressable electrodes, as illustrated in Fig. 1. The WDM
`signal is then spectrally dispersed along the length of the op-
`tical window. The 0.24 -pm vertical deflection required to move
`from maximum to minimum reflectivity is three orders of mag-
`nitude smaller than the 200 to 1500 pm width and length of
`the membrane, making them extremely robust for literally tril-
`lions of cycles. The mechanical resonance time of such devices
`ranges from 0.1 to 10 its depending on the surface geometry
`and material parameters, particularly on membrane stress. The
`system response depends on drive electronics and overall pack-
`aged device capacitance.
`Our initial prototype equalizer [2] used an 1150 -nm -thick
`spacer, approximately three -fourths of the operating wave-
`length. The zero -voltage state is highly reflectivity es-,1.5-dB
`loss), decreasing to low reflectivity (> 20 -dB loss) with ap-
`proximately 70 V applied to a single actuator. Fig. 4(a) shows
`the theoretical voltage response of this device as a function of
`normalized bias voltage. The reflectivity depends on illumi-
`nating wavelength, so the figure plots six curves ranging from
`1520 to 1620 nm (at 100 -nm increments). Voltage response is
`highly nonlinear, with most of the reflectivity change occurring
`from 80% to 100% of applied voltage.
`The 3/4\ -gap configuration was used so that the unpowered
`state of the device would be reflective (low throughput loss).
`However, this concern is not relevant to an equalizer used in
`conjunction with an amplifier, because an unpowered ampli-
`fier will absorb and block incident signals. Therefore, we can
`consider the alternative device shown in Fig. 4(b), where the
`initial gap is set at one wavelength, such that the unpowered
`reflectivity is relatively low This A -gap device balances the non-
`linear optical response .of .the optical etalon against the nonlinear
`voltage response of the micromechanical actuation. The reflec-
`tivity with zero bias depends strongly on wavelength, but for all
`wavelengths the voltage response is more linear.
`The device actuation voltage depends on initial gap and the
`membrane stress as determined by the temperature and compo-
`sition of the deposited layers. Using a silicon -rich silicon nitride
`layer, residual tensile stress was reduced from roughly 600 MPa
`to less than 100 MPa. This reduced the actuation voltage of the
`new a -gap device from 70 to 42 V, despite the increased gap.
`Fig. 5 shows the voltage response of the initial 3/4\ -gap de-
`vice and of the low- stress a -gap device measured at 1552 nm.
`At different wavelengths, the operating voltage range is shifted.
`The more gradual and nearly linear change in reflectivity of
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`Fig. 6. Spectral response of a 3/4À device with several drive voltage
`settings. The top trace shows low and uniform insertion loss resulting from
`zero applied voltage. The trace with five deep features shows the output
`(and maximum dynamic range) when five single electrodes are actuated to
`maximum attenuation. The third trace shows the output when voltages are
`adjusted to achieve two uniform attenuation segments.
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`Fig. 4. Calculated microetalon reflectivity as a function of applied bias voltage
`for two initial air -gap spacings. (a) The initial gap is 1 150 nm, approximately
`3/4A. (b) The initial gap is 1550 nm, approximately A. In both cases, the optical
`delay of the nitride membrane is a /4.
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`Fig. 7. Spectral response of a A -gap device. The trace labeled "off' shows
`reflectivity with no applied voltage, while the trace labeled "maximum" shows
`output with voltages across the array set to maximize output at each wavelength.
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`response from minimum ( -32 dB) to maximum ( -7 dB) transmission.
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`illumination of a 3/4a and improved a device, showing greater linearity in
`response to a reduced drive voltage.
`
`the a device produces a more easily controlled response and is
`better suited to feedback stabilization using power monitoring.
`Figs. 6 and 7 show the full spectral response of the 3/4.A-
`and a -gap devices, respectively. Both can achieve high dynamic
`range (> 25 dB) and are able to provide flat profiles, provided
`suitable drive voltage is applied. They also both show a 7 -dB
`
`insertion loss over the 42 -nm design spectra. Since the a de-
`vice has low unactuated reflectivity, it is necessary to apply a
`bias voltage during assembly to measure the insertion loss and
`achieve correct component alignment.
`
`IV. EXTENDED C +L BAND EQUALIZER
`A flat -spectrum white light source was needed for testing
`of C +L band components. It was possible to build this source
`using two separate 42 -nm equalizers. However, since both
`the optomechanical package and MEMS device are capable
`of ultra broadband performance, a 3/4.X membrane equalizer
`with 40 actuators spaced over a 4 -mm length was fabricated
`and mounted in the same package. The insertion loss of the
`resulting 104 -nm bandwidth spectral equalizer is shown in
`Fig. 8. The minimum loss was less than 7 dB over the range of
`1512 -1600 nm, but increased to as high as 8 dB at 1618 nm.
`Fig. 8 also shows the attenuation created by five widely spaced
`features each adjusted for maximum attenuation to demonstrate
`
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`Fig. 8. Equalizer with I04 -nm spectral bandwidth. Maximum drive voltage
`was <16 V, shown applied to 5 of the 40 electrodes.
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`Fig. 9. Output from the same broadband spectral equalizer when used to
`flatten a broad spectrum (C +L band) ASE source. Traces before (black) and
`after equalization (grey) level -flattening are shown.
`
`that the equalizer is capable of greater than 15 -dB attenuation
`over the full spectral range. Low test source power limited
`the accuracy of measurements at the longer wavelengths,
`creating the noise seen in the difference spectra. The spectral
`features, at same attenuation levels, have the same width in
`the 105 -nm -wide device as in the 42 -nm -wide device (see the
`following section), indicating that the feature size results from
`the mechanical membrane coupling and not the optical system
`resolution.
`Fig. 9 shows the resulting white light ASE source output be-
`fore and after spectral equalization. Spectral uniformity across
`the 1528- 1608 -nm band was improved from 9 to + / -1 dB,
`aside from the deep valley between the C and L bands.
`The spectral width of the features could be reduced by using
`a longer MEMS device and a more dispersive grating system so
`the mechanical coupling would occur over a shorter spectral re-
`gion. Compatible compact optical demultiplexing systems, with
`four to ten times larger spatial dispersions, have been demon-
`strated [10], [16], [19], which would enable spectral resolutions
`of < 1 nm to be achieved. Longer devices also require lower
`actuation voltages; the device illustrated in Fig. 8 required less
`than 16 V for full attenuation.
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`Fig. 10. Spectral resolution of the continuous membrane DGEF, showing
`mechanical coupling between channels that prevent discontinuous spectral
`profiles.
`
`V. SEGMENTED "RIBBON" CHANNELIZED EQUALIZER
`The spectral resolution of the equalizer with a 3/4) device
`and a 1.5 -mm -long strip membrane with 40 actuators is illus-
`trated in Fig. 10. One 10 -dB -deep feature is created by applying
`voltage to a single actuator at 1528 nm (Channel #5), then each
`trace shows the interchannel crosstalk as a second voltage is ap-
`plied to successively closer actuators, each time adjusting both
`voltages to maintain a 10 -dB depth on the center of both fea-
`tures. The continuous nitride membrane has a distance- depen-
`dent mechanical cross -coupling between all actuators. For the
`device tested, two individual 10 -dB features create a > 1 -dB
`change from nominal maximum transmission midway between
`the features when separated by 9 nm and > 3 dB when sepa-
`rated by 4.5 nm. The features can only be considered distinct
`when separated by over 20 nm. The equalizer thus imposes a
`smoothing function on any spectral profile created, regardless of
`feedback. This is preferable for gain flattening filters but useless
`for channelized filters, where, by definition, the attenuation on
`each channel is independent of its neighbors. For this applica-
`tion, it is necessary to physically decouple the adjacent channels
`of the etalon equalizer.
`In fact, the first conception of an etalon equalizer was based
`on providing one discrete equalizer device per spectral channel
`[2]. The equalizer stripe was composed of 32 adjacent bands,
`each 70 {.am long by 35 pm wide along the spectrally dispersed
`direction. This equalizer was only marginally functional, as the
`stress in each segmented equalizer element caused the bands
`to curl at the edges (like potato chips). The usable passband
`window was less than 10% of the 200 -GHz channel spacing,
`making it unsuitable for practical systems. The solution for the
`DGEF was to use a continuous membrane, but another approach
`was needed for a channelized equalizer with fully independent
`attenuation at each wavelength data transmission channel.
`One solution is to relieve the lateral stress in the membrane by
`slicing it into multiple thin ribbons with a common actuator. If
`the cuts between ribbons are smaller than the single- frequency
`spot size, then the cluster of ribbons still function as a variable -
`reflectivity etalon mirror. A representative device is shown in
`
`

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`584
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`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3, MAY /JUNE 2004
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`controlling 240 segmented ribbons plus two sets of unactuated ribbons at the
`ends. Top: entire device, showing 1.5 -mm optical window. Bottom: closeup of
`segmented equalizers, showing individual ribbons.
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`1561
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`1561.5
`
`1562
`1563
`1562.5
`Wavelength (nm)
`
`1563.5
`
`1564
`
`Fig. 12. Spectrum of single channel actuation at 0, 5, 8, 10, 11, 12, 13, and
`14 V.
`
`Fig. 11. As for the DGEF, the device structure is a silicon rich sil-
`icon nitride A/4 layer (n = 1.9) suspended over the silicon sub-
`strate with an electrostatically controlled gap (initially 3A/4).
`The individual actuators in this device were on a 36.75 -µm
`pitch, but each actuator controls a sheet of Silicon Nitride cut
`with submicron -width gaps into six ribbons of 6µm each. These
`slits were defined using e -beam lithography and etched into the
`Nitride using reactive ion etching.
`When voltage is applied to one of the gold- coated electrode
`pairs, force is applied uniformly to the set of six ribbons, causing
`them to deflect toward the substrate and reduce etalon reflec-
`tivity from high ( -1.5 dB) to low ( -20 dB). The packaged de-
`vice achieves an insertion loss of 8 dB.
`
`Fig. 14. Amplifier layout, with a computer -controlled dynamic gain
`equalization filter between two gain stages.
`
`The system produces a spectral spacing and, hence, system
`resolution of 1.1 nm for individual actuators, compared with the
`spectral resolution of 5 to 10 nm (depending on definition) for
`the DGEF. Fig. 12 illustrates the response of a single actuator
`with 0 to 14 V applied, producing 20 dB dynamic range. Fig. 13
`illustrates attenuation patterns created by actuating from one to
`seven different channels. The small ripples visible in the spec-
`trum of Fig. 13 are caused by the illuminating 10.5 µm 1/e2
`mode field diameter sampling the ribbons and spaces.
`These preliminary experimental results constitute a proof -
`of- principle for the segmented device; improving the unifor-
`mity across the passband would require further reduction of
`the spacing between ribbons. With physical segmentation of the
`actuators, creating a desired channel passband is achieved by
`selecting a suitable device pitch, limited only by the optical spot
`size illuminating the device.
`
`VI. SELF- EQUALIZED ERBIUM AMPLIFIER
`
`Finally, we show how a straightforward use of the DGEF
`filter can dramatically enhance the operational versatility of a
`conventional erbium fiber amplifier. The amplifier chosen was
`a C -band EDFA originally used in the first MONET transmis-
`sion system [ 17], designed to provide uniform gain for 16 input
`channels with -8 dBm per channel. Our goal was to increase
`the operating bandwidth from 12 to 30 nm, while at the same
`
`

`
`FORD et al.: INTERFERENCE -BASED MICROMECHANICAL SPECTRAL EQUALIZERS
`
`585
`
`2.8d B
`
`/chx36ch
`-13dBm
`
`-
`
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`
`a
`3
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`
`-20'
`-
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`o.
`.,.
`o -30'
`
`o
`
`-10
`
`-20
`
`-30
`
`4 4dB
`
`-18dBm/chx36ch
`
`7. 1 d B
`
`-23dBm/chx36ch
`
`-10-
`
`-20-
`
`-30°
`
`-40-
`1525
`
`wavelength, nm
`
`1565
`
`-40
`1525
`
`wavelength, nm
`
`1565
`
`-40
`1525
`
`wavelength, nm
`
`1565
`
`m
`
`0'T
`co ì -10`
`O á -20-
`a w
`ó -30- =
`
`-40
`1525
`
`1dB
`
`-13dBm /chx36ch
`
`,l
`
`0.8dB.
`
`-18dBm/chx36ch
`
`-23dBm /chx36ch
`
`-10
`
`-20-
`
`-30-
`
`-40
`1525
`
`wavelength, nm
`
`1565
`
`-10-
`
`-20-
`
`-30
`
`wavelength, nm
`
`1565
`
`-40
`1525
`
`wavelength, nm
`
`1565
`
`Fig. 15. Amplifier output as input power drops by 5, 10, and 15 dB from nominal without (top row) and with (bottom row) active spectral equalization.
`
`time increasing the allowable input power level range to 15 dB,
`all while maintaining less than 1 dB power nonuniformity. This
`capability could be used in upgrading the transmission capacity
`of an existing fiber span.
`The original MONET amplifier provided access between
`two gain stages for dispersion -compensating fiber. Instead, a
`42 -nm- bandwidth DGEF filter was inserted between the two
`gain stages, as shown in Fig. 14. A 1% output power tap was
`monitored by an optical spectrum analyzer connected to a PC.
`The PC controlled the DGEF using a simple iterative feedback
`stabilization algorithm incorporating a mathematical model of
`device voltage response.
`The amplifier was tested using 36 lasers distributed over a
`30 -nm optical band and set at approximately equal input powers
`(under 1-dB level variation). The 36 signals passed through a
`common attenuator used to simulate loss from a variable fiber
`span length. The top row of traces shown in Fig. 15 show the
`output spectra for decreasing average input power (increasing
`span length) with equalization turned "off' (set to uniform max-
`imum internal transmission). The average input power levels
`are -13 dBm /channel (top left), -18 dBm /channel (top center),
`and -23 dBm /channel (top right). The respective output inten-
`sities vary by up to 7 dB signifying poor amplifier configura-
`tion. Based on this performance for a single amplifier, the ef-
`fect of cascading several such nonuniform amplifiers would be
`catastrophic.
`However, when the equalizer is turned "on" (using feedback
`to create compensating attenuation profiles) the outputs con-
`verge to 1 dB or lower variation for all cases, as seen in the
`bottom row of traces in Fig. 15. In this case, the feedback signal
`is local to the DGEF, but it is possible to remotely monitor the
`optical power levels and use this signal to control a single equal-
`izer at midspan, thereby providing end -to -end equalization. In
`
`8
`
`6
`
`-23dBm /chx36ch
`-18dBm /chx36ch
`-13dBm/chx36ch
`
`-
`
`iteration number
`
`30 (a)
`
`23dBm /chx36ch
`-18dBm /chx36ch
`-13dBm /chx36ch
`
`5
`1525
`
`wavelength, nm
`
`1565
`
`(b)
`
`(a) Output power variation as a function of time after a sudden change
`Fig. 16.
`in input power level. (b) Resulting gain equalization filter profiles.
`
`practice, the number of equalizers needed would depend on
`the allowed mid -span power divergence before an unacceptable
`level of nonlinear transmission impairment occurred.
`Fig. 16(a) shows output power convergence as a function of
`time following a sudden drop in input intensity. Each feedback
`
`

`
`586
`
`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 10, NO. 3. MAY /JUNE 2004
`
`loop iteration required about 0.5 s for the optical spectrum ana-
`lyzer to collect and transfer the spectrum, but using a fast power
`monitor would reduce this to less than 100 ms.
`Fig. 16(b) shows the steady state attenuation profiles of the
`equalization filter, from which the compensation of the gain tilt
`is apparent. The filter has a uniform minimum insertion loss of
`about 7 dB, and applies an additional loss of up to 10 dB as the
`input power decreases. Since the amplifier is operated in a gain -
`saturated regime, the resulting output power is not significantly
`affected. The noise figure (indicated by the ASE level between
`signa