`
`Optical MEMS for Lightwave Communication
`Ming C. Wu, Fellow, IEEE, Olav Solgaard, Member, IEEE. and Joseph E. Ford
`
`Invited Paper
`
`in optical microelectro-
`Abstrat-r—The intensive investment
`mechanical systems (MEMS) in the last decade has led to many
`successful components that satisfy the requirements of lightwve
`communication networks. In this paper, we review the current
`state of the art of MEMS devices and subsystems for lightwave
`communication applications. Depending on the design, these com-
`ponents can either be broadband (wavelength independent) or
`wavelength selective. Broadband devices include optical switches,
`crossconnects, optical attenuators, and data modulators, while
`wavelength-selective components encompass wavelength addfdrop
`multiplexers, wavelength-selective switches and crossconnects,
`spectral equalizers, dispersion compensators, spectrometers, and
`tunable lasers. Integration of MEMS and planar lightwave cir-
`cuits, microresonators, and photonic crystals could lead to further
`reduction in size and cost.
`
`Index Terms—Microelectromechanical devices, optical fiber
`communication, optical signal processing, optical switches.
`
`I.
`
`INTRODUCTION
`
`EARLY three decades ago, Petersen published a paper
`on the micromechanical spatial light modulator (SLM)
`array [l] and another on the silicon torsion mirror [2]. Thirty
`years later, this has become a thriving field known as optical
`microelectromechanical
`systems (MEMS), sometimes also
`called microoptoelectromechanical systems, with several con-
`ferences dedicated to the field. It is a key enabling technology
`for the “dynamic” processing of optical signals. The first mar-
`ket driver of optical MEMS was display [3], [4]. The digital
`micrornirror devices developed by Texas instruments Incorpo-
`rated are one of the most successful MEMS products. They
`are now widely used in portable projectors. large-screen TVs.
`and digital cinemas [3]. The applications of optical MEMS in
`telecommunications started in the 19905 [5], [6]. Early efforts
`
`Manuscript received July 7. 2006: revised Clcloher 2. 2006. This work was
`supported in part by the US. Defense Advanced Research Project Agency
`[DARFA].r‘Ar|ny Research Dllice under Grant WQI iNF-05-1-0359 and DARPA
`under Grant MDA9?2-U2- l-0020.
`M. C. Wu is with the Berl-E13)’ hen.-tor and Actuator Center (BSACl and
`Electrical Engineering and Computer Sciences Department. University of
`California. Berkeley. CA 94?2[) USA (e—mail: wu@cecs.bcrl:clcy.edu].
`0. Solgaard is with the E. L. Ginzton Laboratory. Stanford University.
`Stanford. CA 94305 USA [C-rnail: .~.0lgaard@slan|'ottl.CdlI].
`J. E. Ford is with the Department of Electrical and Computer Engineering.
`University ot'C:ilil'ornia. San Diego. CA 92093-04-(T’ USA (e—mai|: jel'ord@
`ut:.sd.edul.
`|0—|2. I4.
`Color versions of Figs. 3, S.
`available online at hltpzffieeexpltire.ieeeorg.
`Digital Object ldcntilier |0.| |[)9!.lLT.2(lll6.t'l864(l5
`
`ii‘. 18, 20. 22. and 25-28 are
`
`I5.
`
`have focused on the development of optical MEMS devices and
`fabrication technologies [7]—[ 10]. The telecom boom in the late
`1990s and eariy 20005 has accelerated maturation of the tech-
`nology. A wide range of optical MEMS components were taken
`from laboratories to reliable products that meet Telcordia qual-
`ifications. Although not all commercialization endeavors were
`successful due to the market downturn. the technology devel-
`oped is available for new applications in communications and
`other areas [1 1].
`In this paper. we will review the recent developments in
`optical MEMS for communication applications. With the rapid
`expansion of the field and proliferation of literature, it is not
`possible to cover all developments in the last decade. Instead,
`we will focus on a selected set of applications and discuss the
`design tradeoffs in MEMS devices and systems. Topics selected
`in this paper include optical switches, filters. dispersion corn-
`pensators, spectral equalizers, spectrometers.
`tu nabie lasers,
`and other dcnse~wavelength~division-multiplexing (DWDM)
`devices such as wavelength add/drop multiplexers (WADMS),
`wavelength-selective
`switches
`(WSSS),
`and wavelength-
`selective crossconnects (WSXC}. Most of the practical com-
`ponents reported were based on free-space optics. There are
`increasing interests in extending the benefits of optical MEMS
`to guided-wave optics or even nanoscopic photonic structures.
`This new trend will be discussed at the end of this paper.
`Various types of optical switches are needed in telecommuni-
`cation networks. Small l X N and N X N switches are useful
`
`for protection. while optical crossconnect (OXC) offers fast
`provisioning and network management at the wavelength level.
`Nodes in ring networks employ WADMS. As the networks
`evolve toward mesh configuration, WSSS and WSXC become
`important. Dispersion cornpensators and spectral equalizers are
`essential for improving the link performance as the data rates
`approach 40 Gb/s. Spectral filters and tunable lasers increase
`the flexibility of DWDM nodes.
`This paper is organized as follows: Section 1] discusses
`broadband (wavelength-independent) devices.
`including data
`modulators, variable optical attenuators [VOAs), and two-
`dimensional (2-D) and three-dimensional (3-D) MEMS optical
`switches. Section III describes wavelength-selective MEMS,
`including spectral equalizers, WADMS, WSSs, WSXCs, filters,
`dispersion compensators. transform spectrometers, and tunable
`lasers. Section IV focuses on the integration of MEMS and
`planar lightwave circuits (PLC). Section V introduces new de-
`vice concepts based on MEMS-actuated microresonators and
`photonic crystals, and Section VI concludes this paper.
`
`07I'33—8724i’S20.(IJ ® 2006 IEEE
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`Capella 2027
`JDS Uniphase v. Capella
`IPR2015-00739
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`JDURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 24. NO. I2. DECEMBER 2006
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`
`
`Reflective
`Modulator
`
`
`
` Photodiode
`
`l. MEMS etalon modulator used for digital data modulation at over
`Fig.
`1 Mbfs. The circular optical aperture is 22 pm in diameter.
`
`Fig. 2,
`
`Package con figuration for a MEMS data transceiver.
`
`ll. WAVELENGTH-INDEPENDENT MEMS
`
`A. Data Modulators
`
`The first practical application of MEMS devices in fiber
`communications was as an optical data modulator, originally
`intended for a low-cost fiber-to—the-home network. A modulator
`
`is essentially a l x 1 switch, operated in either transmission
`(two fibers) or reflection (single fiber). The optical power is
`provided by a constant—intensity remote source, and the mod-
`ulator imprints a data signal by opening and closing in response
`to an applied voltage. Signaling in DWDM fiber networks
`usually requires an expensive wavelength-controlled laser at
`each remote terminal. Passive data modulators offered a poten-
`tially inexpensive solution, but waveguide modulators were too
`expensive and too narrow in optical spectral bandwidth to be
`practical. MEMS offered a new and practical solution.
`The mechanical antireflection switch (MARS) modulator
`is a variable air-gap etalon operated in reflection. The basic
`structure is a quarter-wave dielectric antirefiection (AR) coating
`suspended above 21 silicon substrate [5]. The quarter—wave layer
`is Inade of silicon nitride with 1/4/\ optical path (index times
`thickness), which is roughly 0.2 pm for the 1550-nm telecom
`wavelength. The mechanically active silicon nitride layer is
`suspended over an air gap created by a 3 /4/\-thick phospho-
`silicate glass sacrificial layer (0.6 pm). Without deformation,
`the device acts as a dielectric mirror with about 70% (-1.5-dB)
`reflectivity. Voltage applied to electrodes on top of the mem-
`brane 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 /\/2. the layer becomes an AR coating with close to zero
`reflectivity. A switching contrast ratio of [0 dB or more was
`readily achieved over a wide (30-nm) spectral bandwidth.
`The initial MARS device shown in Fig.
`1 consisted of a
`22-um optical window supported by X-shaped arms and had
`a resonant frequency of 1.1 MHz. Later devices used a hi gher—
`yield structure with a symmetric “drum head" geometry [12].
`[13]. These devices were capable of relatively high-speed
`operation: by optimizing the size and spacing of the etch.
`access holes provide critical mechanical damping, and digital
`modulation above 16 Mbfs was demonstrated [I4]. While such
`data rates are no longer relevant for telecom. even for fiber-to-
`
`the-home, related modulators are useful for low-power dissi-
`pation telemetry from remote sensors using free-space optical
`communications.
`
`These early devices provided a proving ground for the reli-
`ability and packaging of optical MEMS telecom components.
`Initial skepticism from conservative telecom engineers was
`combated by the parallel testing of device array operated for
`months to provide trillions of operating cycles. The packaging
`of optical MEMS devices provided new challenges for MEMS
`engineers. but the simple end~coupled configuration was rela~
`tively straightforward to implement. Fig. 2 shows the config-
`uration for a duplex modulator incorporating a MEMS etalon,
`where data can be received by a photodiode and transmitted by
`modulating the etalon reflectivity [15].
`
`B. Variable Attenuators
`
`Data modulators are operated with digital signals, but the
`fundamental response of an etalon modulator is analog. Elec-
`trically controlled VOAS at that time were constructed with
`bulk optical components with electromechanical actuation, with
`l0—l00—ms response. Erbium fiber amplifiers can use VOA to
`suppress transient power surges, but
`the time scale required
`was 10 its, much slower than the data modulation rate. MEMS
`provided an attractive replacement for optomechanical VOAS,
`and this turned out to be the first volume application for MEMS
`devices in telecom networks.
`
`The first MEMS VOA was fabricated by sealing the opti-
`cal aperture of a MARS modulator from 25 to 300 am so
`that it could be illuminated with a collimated beam. The re-
`
`flected signal was focused into a separate output fiber, avoiding
`the need for external splitters or circulators to separate the
`output signal [16]. The first such VOA device is shown in
`Fig. 3. The wavelength dependence of a simple etalon was
`reduced using a more complex three—layer dielectric stack as the
`mechanically active structure, where the original 1/4). silicon
`nitride layer is sandwiched between conductive polysilicon top
`(1/2,\ thickness) and bottom (1/4A thickness) layers. This at-
`tenuator provided fast (3 us) response with 30-dB controllable
`attenuation over the 40—nm operating bandwidth, with 0.06—dB
`polarization-dependent loss, and also supported the 100-mW
`power level present in amplifiers. However, the 3-dB insertion
`loss was excessive.
`
`
`
`WU er a.'.: OPTICAL MEMS FOR LIGHTWAVE COMMUNICATION
`
`4435
`
`“ADD” Channels
`
`2D MEMS
`
`Switch
`
`“
`
`H
`
`\JL[LM .:s2:..
`
`
`
`\ \\\
`\ \\\
`
`\ N
`
`.\R\
`
`.,,,,,..
`Channels
`
` H
`
`H
`
`[
`
`H
`
`Arrays
`
`Output
`Channels
`
`Fig. 5. Schematic of 2-D MEMS optical switches.
`
`C. Two-Dimensional MEMS Switches
`
`Protection switches are made of 1 x N or small N X N
`
`switches. This can be realized by a 2-D array of vertical micro-
`mirrors commonly known as a 2-D MEMS switch. Fig. 5 shows
`the generic schematic of such a switch. The optical beams
`are collimated to reduce diffraction loss. The micromirrors are
`
`“digital”: They either direct the optical beams to the orthogonal
`output ports or pass them to the drop ports. Generally, only one
`micromirror in a column or row is in the reflection position
`during operation.
`The first MEMS 2-D switch (2 X 2) was reported in [22]
`and quickly followed by related work [23], [24]. For 2 x 2
`switches, low insertion loss (0.6 dB) can be achieved without
`using collimators, especially when the micromirror is immersed
`in index-matching fluid [25]. Latchable 2 x 2 switches incor-
`porating MEMS bistable structures were later commercialized
`[26], [27]. Larger switches require optical collimators to reduce
`diffraction loss. Switches with 8 x 8 and 16 x 16 ports were
`demonstrated [28], [29]. There are two basic approaches for the
`actuation of the micromirror. The first is based on the rotation
`
`of the micromirror [22], [28]. [30]. [31]. The mirror is initially
`parallel to the substrate (OFF position). When actuated, it is ro-
`tated to the vertical position (ON). The second approach moves
`the vertical micromirrors in and out of the optical paths without
`changing the mirror angle [23]—[25], [29],
`[32],
`[33]. The
`2-D switches have been realized by both bulk-micromachining,
`[22]—[25] and surface-micromachining [28]—[30], [32] technol-
`ogies. Electrostatic actuation is most commonly used [22]—[29],
`[32]. Magnetic actuation has also been demonstrated [23], with
`some in conjunction with electrostatic clamping [30].
`The port count of 2-D switches is determined by several
`factors, including mirror angle, size, fill factor (minor width
`divided by unit cell width), and curvature. The expandabil-
`ity of the 2-D switch has been studied in [34] and [35].
`To minimize optical diffraction loss, a confocal geometry
`is used with the average optical path length equal
`to the
`Rayleigh range, which is proportional
`to the square of the
`optical beam waist. Larger minors are therefore required to
`support longer Rayleigh length in higher port-count switches.
`In an N x N switch,
`the mirror size scales as N, whereas
`
`
`
`Fig. 3. MEMS ctalon variable attenuator using a 0.5-mm diameter drumhead
`geometry. The lighter area covers an air gap between the silicon substrate. The
`hex-agonally distributed spots are etch access holes.
`
`
`
`Fig. 4. Lightconnecl‘s diffractive MEMS VOA.
`
`The most direct possible approach to attenuation is to use
`a MEMS actuator to insert an optical block between the input
`and output fiber. This was implemented with a surface micro-
`machining (MUMPS process) [IT] and with a comb-driven
`silicon-on-insulator (SOI) device [18]. Such VOAs offered
`excellent dynamic range (measurement limited at 90 dB), but
`the polarization-dependent loss could be large {>> 1 dB} at high
`attenuations.
`
`Further improvement was needed and was made. Combin-
`ing the collimated beam geometry with a first-surface torsion
`mirror reflector provided a low-insertion-loss structure with
`excellent spectral and polarization performance. For example.
`the device demonstrated by lsamoto at at. [19] achieved 40—dB
`attenuation with a 600-,um mirror driven with 5 V to tilt up
`to 0.3°. Similar configurations were commercialized, although
`the specific designs have not been published.
`Another commercial MEMS VOA is based on a diffractive
`
`MEMS device [4] also used with a collimated beam. This
`device provides excellent optical perfonnance as well as high
`speed: stable operation with 30-dB contrast and less than
`40-its response time using an 8-V drive. A novel structure with
`circularly symmetric features, shown in Fig. 4, was used to
`suppress the polarization-dependent loss to under 0.2 dB [20].
`This device was one of the first Telcordia-qualified MEMS
`components, with 40 000 units reportedly shipped by 2005 [21].
`
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`JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 24. NO. I2. DECEMBER 2006
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`Ref|e¢;ting
`Scanner
`
`Input
`Collimator
`N73!
`
`Refleeling
`Scanner
`Array
`
`
`
`
`Eb)
`
`(a) SEM of OMM‘s In x 16 switch (reprinted from I29] with
`Fig. 6.
`pennission). (b) Photograph of the packaged switch [reprinted from [36] with
`pennission).
`
`the linear dimension of the chip scales as JV? [35]. Large
`chips are more susceptible to imperfections in mirror angles,
`which cause walkoff of optical beams at the receiving fibers.
`Ultimately,
`the chip size will be limited by the fabrication
`precision of the micromirror:-3. 16 X I6 switches have been
`realized, and 32 X 32 switches are within the capability of
`today‘s technology.
`Fig. 6(a) shows a scanning electron micrograph (SEM) of
`OMM’s 2-D switch [29]. A vertical mirror is attached at the
`tip of a cantilever. The tilted cantilever can be pulled down
`electrostatically. The mirror angle is maintained at 90° during
`switching. The switch is fabricated using a standard three-
`polysilicon-layer surface-micromachining process. The mirrors
`are assembled into vertical position with angular distribution of
`(90 :t 0.1)°. The hermetic switch package is shown in Fig. 6(b)
`[36]. Maximum insertion losses of 1.7 and 3.] dB have been
`obtained for 8 x 8 and 16 x 16 switches, respectively, and
`the crosstalk is less than -50 dB. The switching time is less
`than 7 ms. Packaging is critical to attain long-term reliabil-
`ity and satisfy Telcordia qualification for telecommunication
`applications [36].
`in nonmirror-based
`There were also significant efforts
`MEMS 2-D switches [37], [38]. Both Agilent’s Champaign
`switch 13?] and NTT’s OLIVE switch [38] used microfluidic
`actuation to switch light between intersecting waveguides. The
`Champaign switch used thermally generated bubbles to dis-
`place index—matching fluids at waveguide intersections, causing
`the light to bend by total internal reflection (TIR). The OLIVE
`switch used thermal-capillary force to move trapped bubbles.
`One drawback of these approaches is the cumulative losses
`and crosstalks through multiple waveguide intersections. The
`maximum port counts achieved are 32 x 32 and 16 x 16 for
`the Champaign and the OLIVE switches. respectively.
`
`Fig. 1 Schematic of a 3-D MEMS switch.
`
`D. Three-Dimensional MEMS Switches
`
`A transparent optical crossconnect (OXC) with large port
`count can be realized by 3-D MEMS switches illustrated in
`Fig. 7. The input and output fibers are arranged in 2-D arrays.
`The optical beams are steered in three dimensions by two stages
`of dual-axis micromirrors, directing it toward the desired output
`port. The 3-D MEMS switch has a favorable scaling law with
`respect to port count: Assuming the maximum scan angle of the
`mirror is fixed, the optical path length is proportional to N in
`an N x N switch. To maintain confocal configuration for min-
`imum loss. the beam waist, and therefore the mirror size. needs
`to scale as \/N. As a result, the linear dimension of the mirror
`chip scales as
`\/N : N [39]-[4]]. In addition, it has low
`and uniform insertion loss. The 3-D MEMS OXC is a subject
`of intense interest during the telecom boom around the turn
`of the century [42]—[46]. Early efforts {before 2002) focused
`on OXCs with port count ~1000 x 1000 [47], [48], driven by
`the explosion of Internet data transport. Recently, interest has
`shifted to applications in metropolitan area networks, including
`metro access and metro core networks, which requires OXC
`with medium port count (~10t] x 100), with emphasis on low
`cost. low-power consumption. and small footprint [44]. [49].
`Our discussion here will focus on this trend.
`
`Detailed design tradeoffs and system implementations of the
`3-D MEMS OXC have been reported recently [42]—[46]. Two
`schemes have been proposed to reduce the size of the switch
`and tilt angle of the micromirror. Lucent inserted a Fourier lens
`between the two micromirror chips with the focal length equal
`to the Rayleigh range of the optical beam (Fig. 8) [50]. This
`reduces the required scan angle of the mirror. In addition, the
`mirrors can be placed at the beam waist, resulting in \/§ times
`smaller optical beams. This permits the use of smaller mirrors
`andfor reduction of the crosstalk. Fujitsu used a “rooftop”
`mirror to connect two adjacent micromirror chips (photograph
`show in Fig. 9) [44]. The rooftop minor shifts the optical beams
`laterally, reducing the tilt angle requirement. Folding of the
`optical beam also shrinks the footprint of the switch.
`In the compact switch category, Lucent’s 64 X 64 switch
`has a size of 100 X 120 X 20 mm3, which can be mounted
`on a standard circuit board [49]. The insertion loss is 1.9 dB.
`Fujitsu’s 80 X 80 switch has a packaged size of 77 X 87 X
`53 n-tn-1"‘ [44]. The average insertion loss is 2.6 dB. impressively,
`the switch continues to operate under vibration or 50G shock
`
`
`
`WU er a.'.: OPTICAL MEMS FOR LIOHTWAVE COMMUNICATION
`
`443'?
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`
`
`‘}'_ [dB -23dBmlch1<J6cl1
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`
`Fiber Array
`
`
`
`
`
`""m"'5,.,,,,,3 Fiber Array
`
`Luc-ent‘s optical system layout For OXC (reprinted from [50] with
`Fig. 8.
`permission). A Fourier lens is inserted between the two MEMS chips to reduce
`the required tilt of the mirror and beam size.
`
`1525
`
`waveten gth, nm
`
`1555
`
` -40'
`
`-40
`1525
`
`-
`wavalan nth. nrn
`
`1555
`
`
`
`Fig. 9. Photograph of Fujitsu‘s 80 X 80 OXC with a rooftop relleetor
`connecting the two MEMS chips (reprinted from [44] with permission}. The
`packaged size is 77 x 87 X 53mn1‘5.
`
`without any signal degradation. The total power consumption
`of Fujitsu’s switch is only 8.5 W, thanks to the low operating
`voltage of the mirrors. NTT's 100 x 100 switch has a size of
`80 X 60 X 35 mm3 with an insertion loss of4 dB [43].
`The two-axis micromirror array is the key enabling device
`of the 3-D switch. Important parameters include size, tilt an-
`gle, flatness, fill factor, and resonant frequency of the minor.
`Additionally,
`the stability of the mirror plays a critical role
`in the complexity of the control schemes. Early development
`focused on surface-micromachined two-axis scanners [5]].
`[52]. The residue stress limits the mirror size to approximately
`1 mm, and the different thermal expansion coefficients be-
`tween the mirror and the metal coating also cause the mirror
`curvature to change with temperature. Bulk-micromachined
`single—crystalline silicon micromirrors are often used in high-
`port-count 0XCs that require larger mirror size [46], [53]—[56].
`Electrostatic actuation is most commonly used because of
`its low—power consumption and ease of control. Early devices
`use parallel-plate actuators, which have high actuation voltage
`and limited scan angle due to pull-in instability [57]. Although
`the pull-in effect can be mitigated by nonlinear controllers, it
`
`(b)
`
`(a) Dynamic spectral equalizer package and (b) transmission spectra
`Fig. 10.
`showing the improvement in channel uniformity for a 36—channc1 DWDM
`transmission.
`
`increases the complexity of electronics [58]. Micromirnors with
`vertical comb drive actuators, first reported in [59], offer many
`advantages. They have a much larger torque, which one can use
`to reduce the operating voltage as well as increase the resonant
`frequency. In addition, they are free from the pull-in effect,
`further increasing the stable tilt angles. It should be mentioned
`that lateral pull-in between comb fingers is a potential issue but
`could be mitigated by MEMS design (such as V-shaped torsion
`beam [60] or off—centered combs [61]). Several variations of
`vertical comb drive mirrors have been reported, including self-
`aligned vertical combs [62], [63], angular vertical combs [64],
`[65], electrostatically assembled vertical combs [66], and thick
`vertical combs (I00 um) attached to mirror edges on double-
`sided S01 wafers [44], [60].
`
`III. WAvELENGTH—SELECTlvE MEMS
`
`A. Spectra! Equalizers
`
`The natural extension of a single variable attenuator is to
`provide a VOA for each channel of a DWDM transmission
`system. The surface—normal geometries of the etalon mirror-
`and grating-based attenuators discussed in Section II-B were
`all compatible with a free-space imaging spectrometer. An
`input fiber is imaged through a diffraction grating so that each
`spectral channel is laterally shifted to illuminate one modulator
`in a linear array. The reflected signal, attenuated to the desired
`value, is collected into a single output fiber by a second pass
`through the imaging spectrometer. The first such MEMS spec-
`tral equalizer used a continuous etalon membrane [67]. This
`approach was later implemented in the compact package shown
`in Fig. 10, which located the MEMS device array next to a
`single inputfoutput (U0) fiber. A single lens is to collimate
`the multiwavelength beam onto a blazed reflective grating and
`
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`JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 24. NO. I2. DECEMBER 2006
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`Grating
`
`“Add”
`input
`fiber
`
`Circuiator
`
`2
`
`3:09;
`oglbpu
`er
`
`
`
`Waveplate &
`
`focus lens
`
`Spectrauy
`separated
`channels
`
`f
`
`-,
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`
`1
`
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`
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`
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`Fiber 2
`
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`fold mirror
`
`1
`H0
`Fiber 1
`
`fiber
`
`Input
`fiber
`
`Clrculator
`
`“Pass”
`output
`
`Fig.
`
`I 1. Optical schematic for a 2 X 2 MEMS wavelength addfdrop switch.
`
`refocus the spectrally separated signals with a second pass onto
`the MEMS array. A third and fourth pass through the lens
`reintegrates the signal into the U0 fiber, where it is separated
`by an external optical circuiator. The use of such equalizers is
`illustrated by the before and after spectral traces at the bottom of
`Fig. I0, showing the improvement in uniformity of 36 channels
`sent through a two-stage amplifier. The equalizer setting was
`generated by an iterative algorithm running on the computer
`controller [68].
`Dynamic spectral equalization quickly went from an option
`to a practical requirement as the channel
`transmission rate
`increased from 2.5 to 10 and then to 40 Gbfs. The simplest
`and least expensive dynamic gain equalizers (DGEs) use a
`mid-amplifier filter that can be spectrally uniform (a VOA,
`as discussed above) or provide a constant spectral slope [69].
`Two distinct categories of spectral equalizers emerged. DGEs
`provide a smoothly varying spectral profile used to compensate
`for the varying gain profiles in amplifiers, while dynamic chan-
`nel equalizers (DCEs) provide the discrete channel-by-channel
`power adjustment needed to compensate for nonuniform trans-
`mission source intensity or path-dependent loss. Channel equal-
`izers are preferable in general but require accurate matching
`of the equalizer passband to the transmission grid to avoid
`passband narrowing.
`Channel equalizers were implemented using discrete VOAs
`attached to waveguide spectral multiplexers [70] and using an
`oversampled array of digital tilt mirrors [7 I ]. However, the best
`performance in channel equalizers was achieved by combining
`the type of free-space grating demultiplexer shown in Fig. 10
`with either diffractive MEMS modulators [72] or analog tilt
`mirrors [73]. The optical setup is similar to that in Fig. 11
`except without circulators. These components typically have
`40-80 channels spaced at I00 or 50 GHz with 6- and Tr’-dB
`insertion loss and 20- to 30-dB dynamic range. The most ad-
`vantageous characteristic of MEMS equalizers is the extremely
`fiat passband transmission profile along with low chromatic
`
`dispersion at the edges. This performance was achieved after
`studying the effects of various mirror geometries [74].
`After understanding the effects of mirror profile on disper-
`sion, it became possible to use the same basic component struc-
`ture as the equalizer to provide channel-by-channel dispersion
`compensation, although this functionality has yet to be adopted
`in the deployed network [75].
`
`B. Wavelength Add/Drop Multiplexers
`
`Wavelength switching allows network operators to use opti-
`cally transparent components to pass through a network node
`without detecting and regenerating the data signal, and com-
`ponents that enable this have been the subject of intense re-
`search and development. The most basic wavelength switch is
`the dynamically reconfigurable WADM, which is essentially a
`1 x 2 or 2 x 2 optical switch operating independently on each
`wavelength channel.
`WADM was a natural extension of MEMS equalizers, and the
`first demonstration of a MEMS addfdrop switch based on dig-
`ital tilt mirrors occurred almost simultaneously with the equal-
`izer [76], [T7]. Addldrop requires four ports, twice as many
`as the equalizer, and so, the basic structure is slightly more
`complex (Fig. I 1). The system is still based on a blazed diffrac-
`tion grating, which is now illuminated with an upper and lower
`beam path. The active device is a linear array of 16 digital tilt
`mirrors fabricated with surface micromachinin g in the MUMPS
`process. Each mirror defines a DWDM channel and, in switch-
`ing, directs the reflected signal back along the input direction
`or tilted into a new path. Optical circulators on the two U0
`fibers separate the forward and reverse propagating signals. The
`mirrors in this switch tilted by :l:5" under a 20-V signal, switch-
`ing in 20 ,us. A quarter—wave plate is used to achieve 0.2—dB
`polarization dependence on a total insertion loss of 7.5 dB.
`The DCE is closely related to the WADM, and in fact, it is
`possible to use high-contrast equalizers as 1 x 1 switches in a
`
`
`
`WU er a.'.: OPTICAL MEMS FOR LIGHTWAVE COMMUNICATION
`
`4439
`
`VOA 1x4 Switch
`._..I'
`..
`2/
`
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`-
`
`._
`
`I_'
`
`Fig. 12. Equivalent circuit of a I x 4 WSS. Eight wavelength channels are
`shown in this example.
`
`“broadcast and select” architecture [78]. The primary disadvan-
`tage of this architecture is that it is intrinsically lossy: Signals
`are power split, and then unwanted signals are blocked before
`combining into the output fiber. This does allow multicasting.
`i.e., duplicating signals to multiple output fibers. Broadcast and
`select was actually the first to be implemented in the network
`but is generally expected to be phased out in favor of multiport
`WSSs, which in addition to switching also provide channel
`equalization [79] with no additional cost or complexity.
`
`C. Wr1vefength—Se!ccIive Switches ( WSSs)
`
`As optical networks evolve from a simple ring topology with
`WADM nodes to optical mesh networks, WSSS with more than
`one output port are needed to link the node to three or four
`neighboring nodes with each link carrying two-way traffic. The
`WADM concept can be extended to switches with N output
`ports. where N is larger than 2. This is called 1 x N WSS
`[80]—[82]. Fig. 12 shows the equivalent circuits of a 1 X 4 WSS.
`It consists ofa WDM demultiplexer, N). of 1 x N space divi-
`sion switches {NA is the number of wavelength channels) and
`N WDM multiplexers. The WSS can be realized by a similar
`grating spectrometer configuration as the WADM. with the dig-
`ital micromirrors replace by “analog" ones. A large continuous
`scan angle is required to direct the output beam to any of the N
`output fiber collimators. High fiil factor is desired to minimize
`the gaps between wavelength channels. The mirror size is
`usually several times larger than the focused optical beam to
`attain a wide and flat passband for minimal signal distortion.
`A detailed review paper on WSS was published recently
`[80]. The optical setup for Lucent’s WSS is shown in Fig. 13.
`The first subassembly maps all fiber U05 to a common spot
`(point B), and the second subassembly (resolution lens and
`grating) separates and focuses the wavelengths onto the mi-
`cromirror array at the image plane. Tilting of the mirror changes
`the direction of the reflected beam at point B and sends the
`optical beam into a different output fiber. A refined design
`incorporates anamorphic optics in the input stage to reduce the
`physical size of the switch while maintaining the same spectral
`resolution at the expense of longer micromirrors.
`Experimentally,
`1 x 4 WSSS with 128 channels spaced on
`a 50-GHz grid and with 64 channels spaced on a 100-GHz
`
`Fiber and
`micro-lens Camden“;
`arrays
`
`MEMS
`micromirror
`
`
`
`
`E
`Position to propagation jangle conversion ‘ _ , _ _ _
`
`
`
`
`
`
`Spatial Clt$D9|'S|0D to resolve individual WDM channels
`
`Fig. 13. Schematic optical setup of I X 4 WSS (reprinted from [80] with
`permission).
`
`grid have been demonstrated. The typical optical insertion loss
`ranges from 3 to 5 dB. The channel passband is directly related
`to the confinement factor, which is defined as the ratio of the
`mirror size to the Gaussian beam diameter. A confinement
`
`factor of > 2.7 is needed to produce a fiattop spectral response
`with > 'r'4% passband width measured at -1 dB point. JDSU
`has reported a similar I x 4 WSS with 3.5-dB insertion loss
`[81]. UCLA has reported a similar WSS with excellent open-
`loop stability [82].
`The analog micromirror array plays a key role in the per-
`formance of the WSS. Several
`types of WSS micromirror
`arrays have been reported, including electrostatic [83], [84]
`and electromagnetic [85] actuations. The key parameters are
`large continuous scan angle and high fill factor, with the mirror
`size and pitch matching those of the optical system. Lucent
`employed a fringe—field actuated SOI micromirror array [83]
`and achieved a mechanical tilt angle of 9.2’ at 175 V. The
`resonant frequency is 3.8 kHz for 80-pm-wide mirrors.
`More efficient actuation has been obtained using vertical
`comb drive actuators. Hah at al. reported a low-voltage analog
`micromirror array for WSS [84]. The schematic and the SEM
`of the micromirror are shown in Fig. I4. The mechanical struc-
`tures are completely covered by the mi