`
`JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL.
`
`I'l'. NO. 5. MAY I999
`
`Wavelength Add—Drop Switching
`Using Tilting Micromirtors
`Joseph E. Ford, Vladimir A. Aksyuk, David J. Bishop. and James A. Walker
`
`magnetic field [5]. Planar waveguide switchcs have also been
`integrated with the router onto a single substrate. either as
`separate 3 x 2 switches [6] or using two waveguide grating
`routers connected by waveguides containing phase-shifters [7].
`Mechanical switches based on macroscopic bulk optics
`and electromechanical actuators have the best
`insenion loss
`and crosstalk performance of any switch technology, but
`are larger. slower, and potentially less reliable than solid-
`statc switches. Micromechanical switches may achieve the
`same high performance levels of bulk optics. yet provide
`the compactness and reliability of solid state devices.
`In this
`paper, we demonstrate WAD using surfaccnormal operation
`of microoptotnechanical switch arrays with free-space optical
`interconnection to single mode fiber inputs and outputs [8].
`We describe the dcsign of the wavelength multiplexing Optics
`and the micromcchanical switches, present monochromatic
`and broad Spectrum measurements on the WAD switch. then
`conclude with some comments on the ultimate potential of
`this approach.
`
`It. SWITCH DESlGN
`
`Abstract—'I'his paper describes a single-mode optical fiber
`Switch which roulcs individual signals into and out of a wave-
`length multiplexed data stream without
`interrupting the re-
`nlaining channels. The switch uses free-space optical wavelength
`multiplexing and a column of mieromcchanieal tilt-mirrors to
`switch I6 channels at EDI} CH2 spacing from I53! to l556 nm.
`The electrostafieally actuated till mirrors use an 30 V peak—to-
`peak 30!] KHz sinusoidal drive signal
`to switch between :10”
`with a 20 its response. The total fiber-towtiber insertion loss [or
`the packaged switch is 5 dB for the passed signals and 8 dB for
`added and dropped signals. with 0.2 dB polarization dependence.
`Switching contrast was 30 dB or more for all 16 channels and all
`input and output states. We demonstrate operation by switching
`on Mbts data on eight wavelength channels between the two
`input and output ports with negligible eye closure.
`
`index Terms—Gratings, microelectromechanieal devices. opti-
`cal communications, optical fiber switches, wavelength division
`multiplexing (“‘01“).
`
`l.
`
`lNTRODUCTEON
`
`ONVERTING fiber transmission systems I‘rom single
`wavelength to wavelength division multiplexing (WDM)
`provides inexpensive bandwidth but can sacri Ii cc routing llex-
`ibility. because diverting part of the traffic in a simple WDM
`line system to an intermediate destination requires that all
`or the remaining wavelength signals must be detected and
`rcgcncmtcd. As the number of wavelengths increases to 40
`or more. the cost of providing dense WDM repeaters on the
`transmitted channels becomes prohibitive. These repeaters can
`be eliminated using wavelength add—drop: a transparent optical
`component
`to divert selected wavelength signals out of a
`WDM transmiSSion line and also add new signals to reuse the
`dropped wavelengths [1]. Fixed wavelength add—drop {WAD}
`on a moderate number of channels can be accomplished with
`a set ot‘ notch litters [2]. But fully reconfigurable (electrically
`controlled) WM) allows efficient bandwidth allocation and
`l‘ault
`recovery. Efficient. high-contrast WAD switching has
`become a high priority, cSpccially for metropolitan networks.
`Reconfigurable WAD switches can be assembled from dis-
`crete wavelength multiplexers and switches (cg. connecting
`microoptomcchanical
`| x l switches to arrayed waveguide
`routers
`[3]} or using an optical circulator with reflective
`fiber grating notch fillers tuned by either temperature [4] or
`
`I) shows the
`The block diagram for our WAD switch (Fig.
`four WDM fiber pons:
`IN. PASS. ADD. and DROP. The
`main input
`is connected through an optical citculator to a
`wavelength dcmultiplcxcr and then to a set of individual
`1
`x | switches. each 01‘ which can rcllect or transmit one wave»
`length channel. Reflected signals rclracc their path through the
`wavelength multiplexer and into the circulator, which separates
`the hack—rellccted light
`into the PASS output. Transmitted
`signals are collected by a separate wavelength multiplexer and
`directed into the second port of a second optical circulator to
`the DROP output. The ADD port. connected to the first input
`of the second optical circulator, brings in the new data by
`retracing the same optical path created by dropping the input
`channels. In essence, the WAD creates an individual 2 x 2
`switch for each wavelength channel where the two allowed
`states are ]N to PASS, or IN to DROP and ADD to PASS.
`Data is never routed from ADD to DROP.
`In this switch
`design. wavelength multiplexing of the added and dropped
`channels.
`if' necessary.
`is done by an external router. Our
`implementation at“ this design connects two circulators [9]
`to a separate optomechanical package with the free-space
`wavelength multiplexing and micromechanical switch array.
`Microelectromechanical systems (MEMS) is a device tech-
`nology using lithographic fabrication techniques developed for
`silicon electronics to create miniature mechanical components.
`Elements are partially released from the substrate using a
`Ui33—S‘t24t99510flfl L‘s) I999 IEEE
`
`l998: revised February 1 1. I999.
`Manuscript received December 29,
`J. E. Ford and J. A. Walker arc with Bell Laboralories Lucent Technologies.
`I-Ioirudcl. N10793:]. USA.
`V.
`.-\. Aksyuk and D. J. Bishop are with Bell Laboratories Luccnt Tech-
`nologies. Murray lliil. NJ 01")“ USA.
`Publisher llcm ldcnlllicr S 0733-8724[99}03?99-8.
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 1
`Exhibit 1015, Page 1
`
`
`
`FORD ('r m'
`
`. \\"z\\'lfLF.NG'i'H .-\lJI) UROI‘ SWITCIHNG USING l'll.T|NG MICROMJRRORS
`
`905.
`
`IN
`
`PASS
`
`Single Chant-Id Switched
`{mutton-ism";
`
`
`
`
`ADD -
`
`DROP
`
`Fig.
`
`l. Wavelength add—drop switch configuration.
`
`m use
`
`m PASS DROP
`
`ADD
`
`
`
`Fig. 3.
`
`Tilting micrnmirror switch geometry
`
`
`
`
`(bl
`(a) Free-space wavelength mulliplexirtg oplies Iayoul and lb] op-
`Fig. 3-.
`tonicchanicnl package.
`
`selective etch to remove portions of one or more sacrificial
`lillns. This produces structures which are mechanically active
`yet partially constrained (attached) to the surface [10]. One of
`the earliest commercial MEMS actuators was a display using a
`two—dimensional (2—D) array of tilting nticrornirrors a display
`developed by Texas instruments using [1 I]. These components
`are now driving commercial 800 x 600 pixel projection
`displays. demonstrating that high yield and reliability can be
`achieved with 480 000 ciement micromechanical device arrays.
`Fig. 2 shows the geometry used in our WAD switch. An
`input signal is imaged onto the tilt mirror so that in one switch
`state {PASSl the signal is back reflected and in the other state
`iDROP) the signal
`is
`tilted to reflect
`the input
`toward the
`"add" signal source. so that the original input and add signals
`are counter—propagating. The switch is never required to route
`lighl From the ADD to DROP ports. [Fa switch clement set
`to PASS is illuminated li'orn the "add“ source,
`the reflected
`light beam is tilted away from both PASS and DROP ports {a
`path not shown in the diagram). To complete the WAD, each
`element in a linear array of such switches is illuminated by a
`singie uavelength picked out ol‘the WDM fiber transmission.
`
`A. Flee—Space Wowlengrh Multiplexing
`The wavelength multiplexing optics used in this switch
`were originally designed for a mieromechanieal spectral equal-
`izer |l2|. where a continuous \-ariahle-reflcctivily mirror illu—
`minated by a wevelengtli-dispcrsed signal enabled dynamic
`
`power equalization. Fig. 3(a) shows the optical system layout.
`Light from an input fiber is collimated by a 25 mm I‘ocal
`length doublet lens and illuminates a 600 linestmm diffi'action
`grating blazed at a 34° angle. The diffracted signal is focused
`by a 50 mm l‘ocal
`length triplet
`lens onto the device plane.
`where the broad spectrum input is distributed over a device
`array. The system uses pupil division to separate the input
`and reflected output signals. The focus lens is shined down
`relative to the input illumination. so that the collimated input
`beam illuminates the top halfofthe lens. and the light reflected
`from the device array illuminates the bottom half of the lens.
`The reflected signal diffracts from a second pass off the
`grating, which recotnbines the spectral components. A small
`fold mirror positioned below the main optical axis picks othhe
`reflected signal and directs it into an output collimator which
`l'ocuses the signal into a second optical fiber.
`The grating diffraction efficiency has some polarization
`dependence. The insertion loss at
`|S43 nrn ranges from about
`0.6 to L] dB. ll‘uncon‘cctcd, the double-passed grating would
`produce ] dB or more of polarization dependent loss (PDL).
`HoweverI PDL can be suppressed using a quarter wave plate
`oriented at 45° to the grating lines so that the reflected signal is
`rotated by 90° before the second pass through the grating. This
`way, any input polarization is attenuated by twice the average
`insertion loss of the grating (LT dB for the current grating}.
`Fig. 3(b) shows the connectorized opto-mechanical pack-
`age.
`including electrical connections for device control and
`a number of tipftilti’translation controls for optical alignment.
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 2
`Exhibit 1015, Page 2
`
`
`
`906
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL.
`
`t‘l', NO. 5. MAY I999
`
`The total fiber-to-fiber insertion loss for this package. mca-
`surcd using a simple gold mirror at the device plane,
`is 4.6
`dB. with 0.2 dB PDL. This package is intended for laboratory
`environments. However, once the mechanics are aligned and
`locked down.
`the package can be handled and fibers can
`be attached and removed with minimal (<05 dB) change in
`insertion loss.
`For operation as a switch. the device array must be designed
`so that the two mirror states either back reflect the input into
`the first collimating lens or tilt the beam toward the second
`collimator. Reflections from the fiber to frcc~space transition
`must also be suppressed. so antirellection coated and angle-
`polished FC connectors were used on tibcrs fusion-spliced
`directly to the circulatots.
`
`B. Miemmechonicol Tilt-Mirror Sit-itch
`
`The tilt-minor switch geometry is dictated by the beam path
`through the optical system. The basic requirement to switch
`from a transmissive to a reflective WDM path is to place a
`flat mirror at the location of each monochromatic signal
`in
`the device plane and reflect the incident cone of light back
`toward the focus lens so that
`it either overlaps the original
`input beam area [in the PASS state), or is shifted to the lower
`halt‘ oi‘ the focus lens and imaged to the second optical fiber
`output [in the DROP state}. Given the numerical aperture of'
`the single mode fiber source (about 0.2) and the magnification
`ol‘ the imaging between fibers and device plane [two times).
`the Full tilt angle required to avoid overlap between the two
`switch states is at least 6°. With a 200 GHz {1.59 nm) WDM
`signal spacing, the mirrors must match a 57 pm pitch of the
`signals at the device plane.
`The devices used in this demonstration were fabricated
`through the multiuser MEMS processes (MUMP’S) commer-
`cial MEMS foundry operated by the Microelectronics Center
`of North Carolina {MCNC}.' MUMP‘S is a general purpose
`three-layer polysilicon surface micromachining process us-
`ing polysilicon as the structural material, deposited oxide
`(phosphosilicate glass. PSG) as the sacrificial material, silicon
`nitride For electrical
`isolation from the substrate. and a rep
`layer of metal. The layer structure, from the bottom up,
`is:
`silicon substrate, 0.6 pm of nitride, 0.5 pm of polysilicon. 2
`pm of P86, 2 pm of polysilicon, 0.75 pm of P56, 1.5 pm
`of polysilicon. and 0.5 pm of metal (gold. with a thin chrome
`adhesion layer). In our devices, the moving minor switch is
`made from the top |.5 pm thick polysilicon and the 0.5 pm
`metal
`layers.
`Fig. 4(a) and (b) shows two SEM micrographs of the
`fabricated switch, with a gold-coated polysilicon tilt-mirror
`suspended 2.75 pm above the silicon substrate. The mirror area
`is 30 x 50 microns, with a 57-pm pitch between the 16 de-
`vices. The photographs reveal a 24° angle between the mirror
`tilt axis and the column of devices. This comes from matching
`the skew—ray beam path through the focus lens to exactly,r
`overlap the input and output beams. The mirrors are supported
`
`'MCNC‘: MEMS Technology Applications Center was recently scp-
`aratcd as
`an
`independent
`commercial
`enlitv called Cronos
`Integrated
`MicroSystcms
`lncorporaled.
`For more
`information.
`see HYPERLINK
`hlrpt.’"mcms.nicnc.org:'munipshnnl: http:.’.-'Incmsaucncorgimumpshlml.
`
`
`
`Fabricated micromechanical
`Fig. 4.
`perspeclive view.
`
`Eli)
`tilt—mirror array: {in} lop view and [b]
`
`by zig-zag torsion bars which allow them to rotate around
`a tilt axis defined by a pair of support points at either end.
`Each device has two electrical contacts leading to electrodes
`under the tilt plates, while the mirror plates are connected to
`a common ground. When the voltage on one of the contacts is
`increased (and the other contact grounded), each mirror goes
`through a regime of continuously increasing analog deflection.
`However,
`the switch is designed to use digital positions.
`At about 20 V applied the mirror snaps down to contact
`the substrate, producing a repeatable deflection angle of 5°
`relative to the substrate surface. The edges of the mirror plate
`have landing lips to reduce the contact area and therefore the
`probability of stiction (semipennanent bonding of the mirror
`edge with the substrate). At still higher drive voltages (about
`30 V). the devices apparently snap down into full contact with
`the substrate, becoming parallel to the substrate surface.
`The mirrors were operated with an ac drive voltage to avoid
`electrostatic charging. When operated with a dc voltage for
`several seconds to minutes [depending on humidity). switching
`would lag the drive voitage by longer and longer until the
`devices evenmally stopped. This apparently arises from charg—
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 3
`Exhibit 1015, Page 3
`
`
`
`FORD ct all:
`
`l\"A\r'1".l.E-.\lGTIl ADD-12min" SWITCHING USING TILTING MICROMIRRORS
`
`90?
`
`50 nm
`
`
`
`
`
`Height(rim) 8
`
`Mai..-
`
`0
`
`10
`
`30
`20
`Length {urn}
`
`4O
`
`50
`
`Fig. 5
`
`Surface profile ol'rndividual mtcroniechanleal Iilt—mirror.
`
`ing of the exposed dielectric (silicon nitride) surface beneath
`the mirror electrode. Alter the contacts were grounded to
`dissipate the accumulated shielding charge the devices would
`function normally again. However. these charging effects were
`completely eliminated by replacing the dc drive voltage with
`a high frequency ac drive voltagc. The electrostatic switch
`is
`insensitive to the sign of the drive voltage. When the
`drive oscillates around zero at a Frequency much larger than
`the device‘s mechanical resonant frequency,
`the charging is
`proportional
`to the averaged voltage level (which is zero).
`while the deflection is proportional to the root mean square
`\oltage. We verified this experimentally. and found that a
`sinusoidal drive signal oscillating at 300 kHz with a peak-
`to-peak amplitude of 30 V produced reliable switching for
`extended operation times.
`The current MUMP‘s process is not optimized for optical
`MEMS applications. The best mirrors available in the standard
`process use a [to—pm thick gold layer principally intended for
`wire bond pads. This gold layer has residual stress and can
`cause Curvature of the released polysilicon plates, especially
`over wide thermal excursions. We used an interferometric
`surface profiionietcr to characterixc our devices. The original
`devices. fabricated entirely within the MUMP's process, had
`a 0.1? inn sag across the full 57 pm width of the tilt-mirror
`plate. This can cause the angle ol‘thc reflected beam to vary by
`as much as 0.?” depending on where the mirror is illuminated.
`an angle largc enough to potentially reduce switching contrast.
`For our mirrors, we deposited 3 nm of chrome, followed
`by 50 nm of gold directly onto the top polysilicon layer of
`the MUMPS die. This composition should produce similar
`reflectivity to the original 300 nm metal
`layer. but result in
`significantly lower stress. Fig. 5 shows the mirror surface pro-
`file. The top is a false-color map of'a single mirror. indicating
`
`
`
`(an:- —4O
` TotalTransmissiontoPassOutput
`
`
`1550
`1545
`1540
`1535
`Wavelength {Inn} on 200 GH: grid
`{:1}
`
`1555
`
`153i}I
`
`0
`
`
`
`
`
`TotalTransmissiontoDionOutput[dB]
`
` ‘40
`
`1530
`
`1550
`1545
`$540
`1535
`Wavelength {nm} on 200 GHz grid
`{1)}
`(a) PASS and {13) DROP transmiszaon ul' ASE inpul dropping ciglil
`Fig. 6.
`of l 6 channels.
`
`1555
`
`two cross sections shown below. With no voltage applied, the
`mirror is very slightly tilted relative to the substrate, but the
`curvature of the cross sections indicate that the stress-induced
`sag has been reduced to 0.02 pm [20 nm) across the full 57"
`pm aperture. This produces less than At20 phase variation in
`there reflected optical wavefront. a flatness comparable to a
`polished glass mirror.
`
`”I. PERFORMANCE
`
`Test results for the assembled and aligned WAD switch
`are shown in Figs. 6—] l. The total fiber—to—fiber insertion loss
`was approximately 5 dB for the PASS output and 8 dB for
`the DROP output,
`including the circulators. The difference
`between the two states comes from the slightly larger than
`optimal mirror switching angle {we chose to align the optical
`system to minimize PASS losses). The polarization dependent
`loss ranged from 0.l
`to 0.2 dB. About 1.8 dB of the loss
`comes from two passes through the circulators; L7 dB from
`two passes though the grating. and the remaining 1.5 dB comes
`from residual surface reflections and aberrations. Comparing
`the 5 dB loss to the 4.6 dB loss obtained using a simple gold
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 4
`Exhibit 1015, Page 4
`
`
`
`9th
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY. VOL. 11‘. ND. 5. MAY 199‘?
`
`I
`
`.e D
`
`s
`
`t'act
`
`1530
`
`1550
`1545
`1540
`1535
`Wavelength {rim} on 200 GI-Iz grid
`ta}
`
`555
`
`
`
`
`
` -'bO
`
`
` TotalTransmissiontoPassOulpot[dB]
`
`
`
`
`
`
`
`
`
`
`
`TotalTransmissiontoDropOutput(d9)
`
`1550
`1545
`1540
`1535
`Wavelength (nml on 200 GHz grid
`lb]
`tal PASS and lb] DROP transmissron ul' ASE input when channel 8
`Fig.1.
`is dropped and replaced by light from is [unable laser.
`
`1555
`
`mirror. we see that the surface quality of the micro mirror
`devices is good For this type of' micromechanical structure.
`Broad spectrum measurements of‘ switch transmission were
`performed using an Erbium-doped fiber ASE (amplified spon-
`taneous emission) source. Fig. 6(a) shows the PASS and
`Fig. 6(bJ shows the DROP output with 9 ofthe |6 switches set
`to drop. The original source nonuniformity has been subtracted
`to show absolute transmission. The 3 dB roll all" in both
`PASS and DROP passbands occurs at a full width of0.'r' nm,
`as compared to the |.59 nm pitch between channels. These
`passbands are created by the nonrcfiectivc spaces between the
`mirrors. The slightly flattened Gaussian shape ot‘lhc passbands
`can be calculated by convolving the optical spot size at the
`device plane [full width at halfmaximum ofabout 14 pm] with
`the (roughly) 30 pin wide mirror aperture. Fig.
`7"
`illustrates
`single channel drop-and-replacc: channel 8 is dropped from
`Fig. 7(a) the pass output to Fig. 7(b) the drop output, and
`replaced by a narrow line width signal from a tunable laser
`source connected to the add port.
`The switch is designed to operate on the ITU Frequency
`grid with 200 GHz wavelength channel spacing. We used a
`
`loss
`wavelength tunable source and polarization dependent
`meter to measure the switching contrast at the center of each
`of the 16 wavelength channels and each of the four possible
`switch states. Fig. 8 shows a bar graph of the results. The
`switching contrast for IN to PASS, or IN to DROP and ADD
`to PASS states is at
`least 32 dB. and ranges as high as
`47 dB. Signals are never routed front ADD to DROP. The
`extinction between ADD and DROP terminals was at least 36
`dB,
`in either direction. These measurements were taken when
`switching a single channel. The change in crosstalk created
`by switching the adjacent channels. and by switching various
`combinations of the other 15 channels. was measured to be
`below 1 dB.
`
`Broad spectrum measurements of the switching contrast
`were made by using the ASE source and spectrum analyzer
`to store the “ON“ transmission.
`then divide by the “OFF“
`transmission. The result
`is plotted in Fig. 9 for switching
`from the IN port to both the PASS and DROP outputs. The
`switching contrast measured using a laser tuned to each of
`the 16 channels [shown in Fig. 8} is also plotted, to verify this
`result. The switching contrast is sharply peaked at the center of
`the passband, where the optical spot on the tilt-mirror switch is
`entirely reflected. As the wavelength is shifted toward the edge
`of the passband. some ofthe input Spot falls on to the mirror
`edge and is scattered into a wide angular range. A portion
`of the scattered light couples to the opposite switch output.
`resulting in crosstalk. Defining the operating passband as the
`spectral width which maintains a 30 dB or higher switching
`contrast, we see that the PASS output has a 0.5? nm average
`(0.40 nm minimum} operating passband, and the DROP output
`has a 0.34 nm average (0.28 nm minimum) operating passband.
`The WAD switching response is shown in Fig. 10. The
`vertical lines. separated by 20 Jus.
`indicate a switching time
`which is significantly Faster the millisecond response typically
`required in SONET ting recovery.
`This
`switch was
`intended as a first proof-of—principle
`demonstration, as opposed to a preproduction prototype, so
`reliability was not tested in any systematic manner. However,
`the switch operated as part of a week—long technology
`demonstration in a semi-enclosed (covered but not
`air—
`conditioned} temporary pavilion. The
`switch functioned
`normally, without adjustment, despite the wide swings in
`temperature and humidity characteristic of a New Jersey
`summer.
`
`the ability of the switch to transmit data on mul-
`Finally,
`tiple wavelengths was tested using the arrangement shown in
`Fig. I]. The test used two eight wavelength multifrequency
`lasers with 200 GHz pitch between wavelengths [13]. The
`two lasers were temperature tuned so that the wavelengths
`aligned exactly. creating the worst possible case for coherent
`crosstalk noise. independent data streams from two 622 Mbr's
`(DC-3) word generators was applied to all wavelengths of
`each laser with two external electroabsorption modulators. The
`input signals were connected to the [N and ADD ports of the
`switch, and the transmitted PASS and DROP outputs were
`converted to electrical signals and fed into a 0H2. oscilloscope
`triggered by either of the word generators. The transmission
`eyes for the data are shown in the lower half of Fig. II; at
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 5
`Exhibit 1015, Page 5
`
`
`
`FURL) a uh: \\":’\\-'l-',LEN(j'l'l-i ADD— DROP S\\r'll'fi'i-lll\'t'i USING 'I'II,'I [HG MIL'ROMIRRORS
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`Swilehing dynamics, showing :1 20 its response.
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`Fig. 9.
`
`Swtlclting contrast for broad spectrum and monochromatic input.
`
`left. the data are switched from IN to PASS; at center. the data
`are switched from lN to DROP; at right, the data are switched
`from ADD to PASS. In each case, the source ofcrosstalk noise
`
`(the alternate input signals) were turned oitand on without any
`perceptible effect on the transmission eye, indicating that there
`was little crosstalk present on an),r of the eight channels. This
`result is consistent with the 32-45 dB crosstalk suppression
`measured with a tunable laser on the individual channels.
`
`IV. DISCUSSION AND SUMMARY
`
`is a first "proof
`The prototype described in this paper
`of principle“ of the mieromeehanieal wavelength add-drop
`switch. There is considerable room for improvement in both
`wavelength resolution and switching crosstalk. The ultimate
`performance limitations come from the grating dispersion
`and the diffraction-limits on optical system resolution.
`it
`is
`impractical
`to increase the grating spatial
`frequency mtlch
`beyond the 600 lineshnm used in the prototype (the blaze
`angle would increase from 30° to approach grazing incidence}.
`It is possible. however, to double pass the grating to provide
`double the wavelength resolution (with a 1—3 dB excess loss
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 6
`Exhibit 1015, Page 6
`
`
`
`lIll]
`
`JOURNAL OF LIGHTWAVE 'I‘IZClINOLOGY. VOL |?. N0 5. MAY IWQ
`
`
`
`In: Roll-Tumo-
`Owllowm
`
`
`
`---rwrrm
`-
`- -
`I Data “A”
`Fig. It.
`Sctup and transmission eyes for eight wavcicngth data channels
`(summed)-
`
`pcnalty}. The optical system was not fully optimized for this
`application.
`In particular,
`the use of pupil division at
`the
`focus lens to separate the two data paths meant that we used
`the lens off-axis, with broad spectrum light, and also used a
`factor oftwo magnification between the input fiber and device
`plane to match the lens numerical aperture to the fiber output.
`A full custom design would increase the spatial resolution
`and wavelength range ofthe switch. Micronicchanical devices
`tailored to a new optical system can then provide a wider and
`flutter operating passbund. (lur optical system designs indicate
`that 128 channels at 100 (1H2 spacing is certainly possible.
`The long—lcnn reliability of micromcchanical switches must
`be well established before they are suitable for telecommunica—
`tions applications. Failure modes may include stress-induced
`fatigue. and suction or wear on contacting surfaces. One of
`our tilt—mirror switches has been in continuous operation for
`over a year without failure, a total of 55 million switching
`cycles. However, some devices have shown intermittent delays
`in switching, apparently due to temporary stiction of the mirror
`edge. These types of problem have been extensively studied
`by Texas Instruments [l1] and Sandia National Laboratories
`[l4]. Their results indicate that with proper design of device
`structures and electrical drive signals it is possible to achieve
`highly reliable operation,
`in terms of both the number of
`operating cycles and in switching after extended pauses. Most
`micromechanica] device failure modes come from surface
`contact. Ficxural actuation, where a beam or membrane is
`elastically deflected by an applied force (without
`friction
`or contact) is probably the most reliable approach. This is
`the basis of ultra-reliable Inicromechanical accelerometers
`used as air bag sensors since about 1993. This indicates
`that suitably designed micromcchanical tilt mirrors can meet
`telecom reliability standards.
`The free-space optomcchanical packaging of the switch is
`another concern. The long path lengths raise the possibility of
`failures from thermal expansion or shock. However, a number
`of fiber optic multiplexers based on planar gratings with simi-
`lar free-space optical systems have been developed as products
`for telecom [15]. These systems use transmissive imaging
`of thc diffracted output into an array of output waveguides.
`
`The fundamental design of our switch uses imaging of the
`input fiber onto reflective micronteehanical modulators.
`thcn
`rcimages the reflected signal
`toward the input fibers.
`11’ the
`device aperture is large compared to the fiber mode. the lateral
`alignment tolerance for the long Optical path can be determined
`by the modulator aperture rather than the fiber mode. This can
`greatly Simplify maintaining wavelength and loss tolerances,
`even in relatively hostile operating conditions.
`Using free-space optical
`interconnections between optical
`fibers and surface-normal device arrays presents one additional
`opportunity not present in planar optic devices. The single in~
`put and output fibers used in the prototype could be extended to
`one-dimensional {I-D} fiber arrays, and the micromechanicol
`device extended to a 2—D array of tilt-mirror switches. This
`would make full use of the optical imaging and lithographic
`devicc fabrication capabilities, and distribute the packaging
`costs among multiple fiber inputs. This significantly extends
`the number of channels which can be switched.
`to fact.
`the
`practical
`limit on the number of switched channels will be
`determined by the electrical pin—out of the micromcchanical
`chip, which can number several hundred.
`In summary. we have demonstrated a reconfigurable wave-
`length addvdrop switch with 213 as switching of 16 input and
`output channels on a 200 GHz pitch. The switch is based on
`free—space optical demultiplexing and a microoptoincchanical
`tilt-mirror switch array. The total insertion loss of the passed
`and dropped channels is 5 and 8 dB, respectively, with 0.2
`dB polarization dependence. The switching contrast
`for 16
`channels on the ITU grid was greater than 30 dB for both
`passed and dropped outputs. This approach has the potential to
`scale to larger channel counts, and to operate on multiple input
`llhcrs. creating a cost-effective switch for telecom applications.
`
`AC KNOWLEDG M ENT
`
`Thc authors wish to thank R. Eliard and F. Beisser of Lucent
`Technologies Bell Laboratories,
`for custom machining and
`electronics, C. Doerr for the use of prototype multifrcqucncy
`lasers for parallel switch operation. and C.—C. Chang for help
`with the data transmission tests.
`
`REFERENCES
`
`[l] C. Bracken. n. Acarnpom. J. Sweirzer, G. Tangonan, M. Smith, W.
`Lennon. K. Wong, and R. Hobs. “A scalabic multiwaveicngtb multihop
`optical network: A proposal for research on alleptical networks." J.
`Lighnmre Tecimoiu vol. 11, pp. ?36—?S3, I993.
`J. Lcmairc. W. M.
`2] V. Mizrahi. T. ErdOgan. D.
`J. DiGiovanni. P.
`MacDonald. S. G. Kosinski. S. Cabot. and J E. Sipe. "Four channel fiber
`grating dcmultiplcxcr,“ Ei’ectmit. Lett. vol. 30. no. 10. pp. TSOJBI.
`May 1994.
`[3] C. R. Giles, B. Barber, V. Askyuk. R, Rucl. L. Stub. and D. J. Bishop.
`“Reconfigurable lfi-channet WDM DROP module using silicon MEMS
`optical switches," [SEE Photon. Termini. Lent. 1998.
`[4] T. Et‘timov, M. C. Fairies, S. Huang. N. Duricic, D. Grobnic. B.
`Kcyworth. and I. S. Ohbi. "it-channel tunable drop device with thermal
`tuning for 100 cm: channel spacing,“ in Pine.
`i999? European Confi
`Optic. Commun. Madrid. Spain. pp.
`l37—I28.
`[5] S. Iin. R. P. Espindoin.
`I-i. Mavoori. T. A. Strasscr, and P. J. Lcmairc,
`“Magnetically programmable fiber Bragg grating.” Etc-crimi. Lem. vol.
`34. p. 2158. 1993.
`[6] S. Suzuki. A. Ilimcno, and M. Ishii. “Integrated multichannel optical
`wavelength selective switches incorporating an arrayed-wai-cguidc grat-
`ing multiplexer and lhciTnuoptic switches." J. Lighrwave Helmet. vol.
`I6. pp. 650—655, Apr.
`I998.
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1015, Page 7
`Exhibit 1015, Page 7
`
`
`
`FORD ct m’.‘ “r'AVitlJ-iNG'i II ADD—DROP SWITCHING USING TIL'nNCi MICRDMIRRORS
`
`9|!
`
`[3]
`
`[10}
`
`Ill]
`
`['5] C. R. Docrr, "Proposed WDM cross connect using a planar arrangement
`of waveguide routers and phase shificrs." J5EE Photon. R‘chnot. Let!”
`vol.
`It). pp.
`Iii—119.
`I998.
`.I. E. Ford. V. Aksyuk. J. A. Walker. and D. J. Bishop, “Wavelengdn
`selectable addv'drop with tilting micro mirrors,“ in Prue. {SEE LEGS
`zl'mtrt.
`.Hetering. Nov. 199?, postdoadline paper PDZJ‘.
`[9] JDSr'F'ttcl CR2500 double isolation circulators, with < I dB Iossr’pass
`nnd 3' 50 dB isolation and return loss.
`Integrated Sensors.
`Issue:
`K.D. Wise. Guest Editor.
`Special
`{MEMS}.
`PJ'DC.
`JEEE,
`Mieronctuators.
`and Microsystcms
`Aug.
`I998. Also
`available:
`http:ttmoms.lsi.sdu
`and
`i’l'IIPLl'MWW‘
`osaceaes.harksloyodutvlsitorshlml.
`P. F. Van Kessci. 1..
`.I. Hombcclt, R. E. Meier, and M, R. Douglass. “A
`MEMS-bascd projection display." Pmt‘. JEEE. vol. 36. pp.
`lGS?—1?0~l.
`Aug. 1998.
`J. E. Ford and