904
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 5, MAY 1999
`
`Wavelength Add—Drop Switching
`Using Tilting Micromirrors
`
`Joseph E. Ford, Vladimir A. Aksyuk, David J. Bishop, and James A. Walker
`
`Abstract— This paper describes a single-mode optical fiber
`switch which routes individual signals into and out of a wave-
`length multiplexed data stream Without
`interrupting the re-
`maining channels. The switch uses free-space optical wavelength
`multiplexing and a column of micromechanical tilt-mirrors to
`switch 16 channels at 200 GHz spacing from 1531 to 1556 nm.
`The electrostatically actuated tilt mirrors use an 80 V peak-to-
`peak 300 KHz sinusoidal drive signal to switch between i10O
`with a 20 [us response. The total fiber-to-fiber insertion loss for
`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
`622 Mb/s data on eight wavelength channels between the two
`input and output ports with negligible eye closure.
`
`Index Terms—Gratings, microelectromechanical devices, opti-
`cal communications, optical fiber switches, wavelength division
`multiplexing (WDM).
`
`1.
`
`INTRODUCTION
`
`ONVERTING fiber transmission systems from single
`wavelength to wavelength division multiplexing (WDM)
`provides inexpensive bandwidth but can sacrifice routing flex-
`ibility, because diverting part of the traffic in a simple WDM
`line system to an intermediate destination requires that all
`of the remaining wavelength signals must be detected and
`regenerated. 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 of notch filters [2]. But fully reconfigurable (electrically
`controlled) WAD allows efficient bandwidth allocation and
`fault recovery. Efficient, high-contrast WAD switching has
`become a high priority, especially for metropolitan networks.
`Reconfigurable WAD switches can be assembled from dis-
`crete wavelength multiplexers and switches (e.g., connecting
`microoptomechanical
`1 x 1 switches to arrayed waveguide
`routers [3]) or using an optical circulator with reflective
`fiber grating notch filters tuned by either temperature [4] or
`
`Manuscript received December 29, 1998; revised February 11, 1999.
`J. E. Ford and J. A. Walker are with Bell Laboratories Lucent Technologies,
`Holmdel, NJ 07733 USA.
`V. A. Aksyuk and D. J. Bishop are with Bell Laboratories Lucent Tech-
`nologies, Murray Hill, NJ 07974 USA.
`Publisher Item Identifier S 0733-8724(99)03799-8.
`
`magnetic field [5]. Planar waveguide switches have also been
`integrated with the router onto a single substrate, either as
`separate 2 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 insertion loss
`
`and crosstalk performance of any switch technology, but
`are larger, slower, and potentially less reliable than solid-
`state 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 surface-normal operation
`of microoptomechanical switch arrays with free-space optical
`interconnection to single mode fiber inputs and outputs [8].
`We describe the design of the wavelength multiplexing optics
`and the micromechanical switches, present monochromatic
`and broad spectrum measurements on the WAD switch, then
`conclude with some comments on the ultimate potential of
`this approach.
`
`11. SWITCH DESIGN
`
`The block diagram for our WAD switch (Fig. 1) shows the
`four WDM fiber ports; IN, PASS, ADD, and DROP. The
`main input is connected through an optical circulator to a
`wavelength demultiplexer and then to a set of individual
`1
`X 1 switches, each of which can reflect or transmit one wave-
`length channel. Reflected signals retrace their path through the
`wavelength multiplexer and into the circulator, which separates
`the back-reflected 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 IN 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 of 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
`
`073378724/99$10.00 © 1999 IEEE
`
`FNC 1004
`
`

`

`FORD el al.: WAVELENGTH ADDiDROP SWITCHING USING TILTING MICROMlRRORS
`
`905
`
`\Grating
`
`A. DeMUX'ed
`
`
`
`Device Plane
`.4 Electrical I/O
`
`
`Gratiny
`
`F 1|
`
`_1m. .
`
`Circulator
`
`IN
`
`ADD
`
`PASS
`
`
`circulator
`
`DROP
`
`Single Channel Switches
`(reflect / transmit)
`
`Fig. 1. Wavelength addidrop switch configuration.
`
`IN PASS
`
` V
`
`T
`
`”V PAss
`
`DROP
`
`ADD
`
`
`
`Fig. 2. Tilting micromirror switch geometry.
`
`(a) Free-space wavelength multiplexing optics layout and (b) op-
`Fig. 3.
`tomechanical package.
`
`(b)
`
`selective etch to remove portions of one or more sacrificial
`films. 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 micromirrors a display
`developed by Texas Instruments using [11]. These components
`are now driving commercial 800 x 600 pixel projection
`displays, demonstrating that high yield and reliability can be
`achieved with 480 000 element 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 (PAS S) the signal is back reflected and in the other state
`(DROP) 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
`light from the ADD to DROP ports. If a switch element set
`to PASS is illuminated from 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
`single wavelength picked out of the WDM fiber transmission.
`
`A. Free-Space Wavelength Multiplexing
`
`The wavelength multiplexing optics used in this switch
`were originally designed for a micromechanical spectral equal-
`izer [12], where a continuous variable-reflectivity mirror illu-
`minated by a wavelength-dispersed signal enabled dynamic
`
`power equalization. Fig. 3(a) shows the optical system layout.
`Light from an input fiber is collimated by a 25 mm focal
`length doublet lens and illuminates a 600 lines/mm diffraction
`grating blazed at a 34° angle. The diffracted signal is focused
`by a 50 mm focal 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 shifted down
`relative to the input illumination, so that the collimated input
`beam illuminates the top half of the 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 recombines the spectral components. A small
`fold mirror positioned below the main optical axis picks off the
`reflected signal and directs it into an output collimator which
`focuses the signal into a second optical fiber.
`The grating diffraction efficiency has some polarization
`dependence. The insertion loss at 1543 nm ranges from about
`0.6 to 1.1 dB. If uncorrected, the double-passed grating would
`produce 1 dB or more of polarization dependent loss (PDL).
`However, 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 (1.7 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 tip/tilt/translation controls for optical alignment.
`
`

`

`906
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 5, MAY 1999
`
`The total fiber-to-fiber insertion loss for this package, mea-
`sured 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 (<0.5 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 free-space transition
`must also be suppressed, so antireflection coated and angle-
`polished FC connectors were used on fibers fusion-spliced
`directly to the circulators.
`
`B. Micromechanical Tilt-Mirror Switch
`
`The tilt-mirror 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
`half of 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
`of 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 um 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).1 MUMP’s is a general purpose
`three-layer polysilicon surface micromachining process us-
`ing polysilicon as the stmctural material, deposited oxide
`(phosphosilicate glass, PSG) as the sacrificial material, silicon
`nitride for electrical isolation from the substrate, and a top
`layer of metal. The layer stmcture, from the bottom up,
`is:
`silicon substrate, 0.6 pm of nitride, 0.5 pm of polysilicon, 2
`pm of PSG, 2 pm of polysilicon, 0.75 pm of PSG, 1.5 pm
`of polysilicon, and 0.5 pm of metal (gold, with a thin chrome
`adhesion layer). In our devices, the moving mirror switch is
`made from the top 1.5 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-pin 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
`overlap the input and output beams. The mirrors are supported
`
`1MCNC’s MEMS Technology Applications Center was recently sep-
`arated as an independent commercial entity called Cronos
`Integrated
`MicroSystems
`Incorporated. For more
`information,
`see HYPERLINK
`http://mems.mcnc.org/mumps.html; http://mems.mcnc.org/mumps.html.
`
`
`
`””5325
`
`35K'U
`
`,xsla
`
`IBHI'I H014
`
`(b)
`
`Fabricated micromechanical tilt-mirror array: (a) top view and (b)
`Fig. 4.
`perspective view.
`
`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 tips to reduce the contact area and therefore the
`probability of stiction (semipermanent bonding of the mirror
`edge with the substrate). At still higher drive voltages (about
`30 V), the devices apparently snap down into fiill 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 voltage by longer and longer until the
`devices eventually stopped. This apparently arises from charg-
`
`

`

`FORD el al.: WAVELENGTH ADDiDROP SWITCHING USING TILTING MICROMIRRORS
`
`907
`
`50 nm
`
`(dB)
`TotalTransmissiontoPassOutput
`
`(dB)
`TotalTransmissiontoDropOutput
`
`
`
`3:f
`
`3i
`
`, in3
`
`i4,
`2§
`
`1530
`
`1550
`1545
`1540
`1535
`Wavelength (nm) on 200 GHz grid
`
`1555
`
`(20
`
`1 540
`1 535
`1545
`Wavelength (nm) on 200 GHz grid
`
`1 550
`
`1555
`
`(b)
`
`(a) PASS and (b) DROP transmission of ASE input dropping eight
`Fig. 6.
`of 16 channels.
`
`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
`um aperture. This produces less than A/ZO phase variation in
`there reflected optical wavefront, a flatness comparable to a
`polished glass mirror.
`
`III. PERFORMANCE
`
`
`section
`
`vertical
`section
`
`horizontal
`
`0
`
`10
`
`20
`
`30
`
`40
`
`50
`
`Length (u m)
`
`Fig. 5.
`
`Surface profile of individual micromechanical tilt-mirror.
`
`ing of the exposed dielectric (silicon nitride) surface beneath
`the mirror electrode. After 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 voltage. 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
`voltage. We verified this experimentally, and found that a
`sinusoidal drive signal oscillating at 300 kHz with a peak-
`to-peak amplitude of 80 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 0.5-/.Lm 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 profilometer to characterize our devices. The original
`devices, fabricated entirely within the MUMP’s process, had
`a 0.17 pm sag across the full 57 pm width of the tilt-mirror
`plate. This can cause the angle of the reflected beam to vary by
`as much as 0.70 depending on where the mirror is illuminated,
`an angle large 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 500 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
`
`Test results for the assembled and aligned WAD switch
`are shown in Figs. 6—11. 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 0.1 to 0.2 dB. About 1.8 dB of the loss
`comes from two passes through the circulators; 1.7 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
`
`

`

`908
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 5, MAY 1999
`
`(dB)
`TotalTransmissiontoPassOutput
`
`
`1 535
`1 540
`1 545
`1 550
`Wavelength (nm) on 200 GHz grid
`
`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 from 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 of the 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 0.57 nm average
`(0.40 nm minimum) operating passband, and the DROP output
`has a 0.34 run average (0.28 nm minimum) operating passband.
`The WAD switching response is shown in Fig. 10. The
`vertical lines, separated by 20 us, indicate a switching time
`which is significantly faster the millisecond response typically
`required in SONET ring 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 fiinctioned
`normally, without adjustment, despite the wide swings in
`temperature and humidity characteristic of a New Jersey
`summer.
`
`Finally, the ability of the switch to transmit data on mul-
`tiple wavelengths was tested using the arrangement shown in
`Fig. 11. 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 Mb/s
`(OC-3) word generators was applied to all wavelengths of
`each laser with two external electroabsorption modulators. The
`input signals were connected to the IN and ADD ports of the
`switch, and the transmitted PASS and DROP outputs were
`converted to electrical signals and fed into a GHz oscilloscope
`triggered by either of the word generators. The transmission
`eyes for the data are shown in the lower half of Fig. 11; at
`
`l1
`
`EM...
`WI
`
` a
`
`
`
`
`
`TotalTransmissiontoDropOutput(dB)
`
`l
`
`-40
`1530
`
`1550
`1545
`1540
`1535
`Wavelength (nm) on 200 GHz grid
`
`1555
`
`(b)
`
`(a) PASS and (b) DROP transmission of ASE input when channel 8
`Fig. 7.
`is dropped and replaced by light from a tunable laser.
`
`mirror, we see that the surface quality of the micro mirror
`devices is good for this type of rnicromechanical 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(b) shows the DROP output with 9 of the 16 switches set
`to drop. The original source nonuniformity has been subtracted
`to show absolute transmission. The 3 dB roll off in both
`
`PASS and DROP passbands occurs at a fiill width of 0.7 nm,
`as compared to the 1.59 nm pitch between channels. These
`passbands are created by the nonreflective spaces between the
`mirrors. The slightly flattened Gaussian shape of the passbands
`can be calculated by convolving the optical spot size at the
`device plane (full width at half maximum of about 14 pm) with
`the (roughly) 30 um wide mirror aperture. Fig. 7 illustrates
`single channel drop-and-replace: 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
`
`

`

`FORD el al.: WAVELENGTH ADDiDROP SWITCHING USING TILTING MICROMIRRORS
`
`909
`
`,. I Mi to miss
`
`I] IN to DROP
`
`D ADD to PASS 3 ADD to DROP ,
`
`
`
`_
`
`I I I ’
`
`I I
`
`
`
`Nwk§§§gContrast/Extinction(dB) § §
`
`”I
`
`1
`
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
`8
`
`9
`
`10
`
`11
`
`12
`
`13
`
`14
`
`16
`
`Channel # (200 GHz pitch on ITU grid)
`
`Fig. 8.
`
`Switching contrast for monochromatic input on a 200 GHz grid.
`
`00->01OOO
`
`(dB)
`SwitchingContrastforPassOutput
`
`
`
`N0
`
`1535
`
`1540
`
`1545
`
`1550
`
`Wavelength (nm) on 200 GHz grid
`
`(20
`
`(dB)
`SwitchingContrastforDropOutput
`
`
`
`1535
`
`1540
`
`1545
`
`1550
`
`1555
`
`Wavelength (nm) on 200 GHz grid
`
`(b)
`
`Fig. 9.
`
`Switching contrast for broad spectrum and monochromatic input.
`
`left, the data are switched from IN to PASS; at center, the data
`are switched from IN to DROP; at right, the data are switched
`from ADD to PASS. In each case, the source of crosstalk noise
`
`
`...PR°..’.’_.....
`
`
`
`
`DROP output
`
`
`.-
`
`
`
` C'i ’ Eio'o'ndil'g'
`' ’
`SOOmVQ M10.0ps CTITIJ'
`106mV
`Switching dynamics, showing a 20 [LS response.
`Fig. 10.
`
`(the alternate input signals) were turned off and on without any
`perceptible effect on the transmission eye, indicating that there
`was little crosstalk present on any 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
`
`The prototype described in this paper is a first “proof
`of principle” of the micromechanical 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 much
`beyond the 600 lines/mm 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—2 dB excess loss
`
`

`

`910
`
`JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 5, MAY 1999
`
`Ghz Real-Time
`Oscilloscope
`622 Mbls
`
`DA TA “A ”
`DATA GENERATOR
`
`SYNC ”A”
`
`a x 200 G":
`E-A
`
`
`
`Multi-Freq. Laser Modulate
`
`
`
`
`622 Mbls
`DATA GENERATOR
`
`
` Data “ ”
`
`
`1
`M may: cu
`m
`w mp;
`r- ;
`rm.
`W M
`fin:
`n 1
`74m
`
`1 1.
`Fig.
`(summed).
`
`Setup and transmission eyes for eight wavelength data channels
`
`penalty). The optical system was not frilly 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 of two 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 of the switch. Micromechanical devices
`tailored to a new optical system can then provide a wider and
`flatter operating passband. Our optical system designs indicate
`that 128 channels at 100 GHz spacing is certainly possible.
`The long-term reliability of micromechanical switches must
`be well established before they are suitable for telecommunica-
`tions applications. Failure modes may include stress-induced
`fatigue, and stiction 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 [11] and Sandia National Laboratories
`[14]. 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
`micromechanical device failure modes come from surface
`
`contact. Flexural 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 micromechanical accelerometers
`
`used as air bag sensors since about 1993. This indicates
`that suitably designed micromechanical tilt mirrors can meet
`telecom reliability standards.
`The free-space optomechanical 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 the diffracted output into an array of output waveguides.
`
`The fundamental design of our switch uses imaging of the
`input fiber onto reflective rnicromechanical modulators, then
`reimages the reflected signal toward the input fibers. If 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-nonnal 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 (l-D) fiber arrays, and the micromechanical
`device extended to a 2-D array of tilt-mirror switches. This
`would make full use of the optical imaging and lithographic
`device fabrication capabilities, and distribute the packaging
`costs among multiple fiber inputs. This significantly extends
`the number of channels which can be switched. In fact, the
`practical limit on the number of switched channels will be
`determined by the electrical pin-out of the micromechanical
`chip, which can number several hundred.
`In summary, we have demonstrated a reconfigurable wave-
`length add—drop switch with 20 us switching of 16 input and
`output channels on a 200 GHz pitch. The switch is based on
`free-space optical demultiplexing and a microoptomechanical
`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
`fibers, creating a cost-effective switch for telecom applications.
`
`ACKNOWLEDGMENT
`
`The authors wish to thank R. Ellard and F. Beisser of Lucent
`
`Technologies Bell Laboratories, for custom machining and
`electronics, C. Doerr for the use of prototype multifrequency
`lasers for parallel switch operation, and C.-C. Chang for help
`with the data transmission tests.
`
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