Surface-Micromachined Free-Space Fiber Optic Switches
`With Integrated Microactuators for Optical Fiber Communication Systems
`
`1 A4.07P
`
`Shi-Sheng Lee, Ed Motamedi*, and Ming C. Wu
`UCLA, Room 63- 128 Engineering 4 Building
`405 Hilgard Avenue, Los Angeles, CA 90095, USA, lee@icsl.ucla.edu
`"Rockwell Science Center, Thousand Oaks, CA 9 13 60, USA
`
`SUMMARY
`
`We report on a novel surface-micromachined free-
`space fiber optic switch with integrated microactuators
`for optical fiber communication systems. The switch
`consists of an out-of-plane micromirror driven by
`integrated scratch drive actuators, and balanced by a
`spring. A fall time and a rise time of 15 ms and 6 ms have
`been achieved, respectively. The switch is equipped with
`the fail-safe feature as required by the FDDI optical
`bypass switch. In addition, a vibration g-test has been
`conducted while the switch is transmitting data. Error
`free operation up to 89 g's has been achieved for
`vibration frequencies from 200 Hz to 10 kHz.
`
`Keywords: Optical Switch, Surface-Micromachining,
`Microactuators
`
`INTRODUCTION
`
`Optical fiber offers many advantages compared with
`electric cables, including high bandwidth, low loss, light
`weight, immunity from lightening strikes and
`the
`resultant current
`surges, and no electromagnetic
`interference.
`Fiber optic networks such as fiber
`distributed data interface (FDDI) are widely accepted and
`supported by the industry as one of the international
`standards for high-speed local area networks (LAN).
`
`Fiber optic switches are used in the network to
`reconfigure the network andlor increase its reliability. For
`example, FDDI fiber optic network employs optional 2x2
`fiber optic switches, called optical bypass switches, to
`bypass the failed nodes. When the FDDI node is powered
`on, the bypass switch routes the incoming signal from
`ring into the station, and directs the transmitted signal
`from the station to the ring. When the FDDI node is
`powered off or failed, the optical bypass switch allows
`the data signals to bypass the node and maintain the ring
`continuity. Since the switch introduces additional optical
`loss, fiber optic switches should be designed to minimize
`the insertion loss. The switch can be realized by free-
`space approach or waveguide approach. The free-space
`approach [I-31 offers a number of advantages over the
`conventional waveguide approach 141.
`It has lower
`
`coupling loss and smaller cross talk. Conventional free-
`space fiber optic switches employ bulk optical elements
`and are very expensive. Recently, there has been a
`growing interest in applying micromachining technology
`to improve the performance and reduce the cost of opto-
`mechanical switches. Bulk-micromachined Si has been
`combined with external actuators to implement 2x2
`switches [I]. A bulk micromachined 2x2 matrix switch
`has also been demonstrated [2]. Deep reactive ion etching
`has been employed to realize a 2x2 switch on silicon on
`insulator wafer [3]. However, they often require unique
`processes and special processing techniques, such as
`extra thick SO1 (silicon-on-insulator) wafers and deep
`R E (reactive ion etching) machine.
`
`Surface-micromachining technique, based on the
`standard CMOS processes, on the other hand, offers
`greater flexibility for realizing free-space optical systems
`on a single chip. Three-dimensional micro-optical
`elements, micropositioners, and microactuators can be
`fabricated by a single unified process [5-61. Previously,
`we have used the surface-micromachined technique to
`demonstrate a 2x2 fiber optic switches [7]. Lower
`insertion loss has been demonstrated. In this paper, we
`report on the performance of a fully actuated 2x2 fiber
`optic switch. A vibration test up to 89'g has also been
`conducted for an active switch, and the experimental
`results will be discussed in the paper.
`
`DESIGN AND FABRICATION
`
`The schematic diagram of the switch is shown in
`Figure 1. It consists of a moveable 3D micromirror, four
`fiber guiding rails, and microactuators. The out-of-plane
`3D micromirror
`is
`realized by
`the micro-hinge
`technology [8] and is integrated on a translation stage. It
`is positioned at the center of the switch and allowed to
`move along the x-axis. The mirror has been coated with a
`500 nm-thick gold layer to increase the reflectivity. The
`switch operates in REFLECTION state without activating
`any actuators. The micromirror can be pulled away from
`the center of fibers by a set of six integrated scratch drive
`actuator (SDA) [9], which will change the switch from
`REFLECTION to TRANSMISSION state. A pull-in
`spring has been integrate with the micromirror to
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`1997 International Conference on Solid-state Sensors TRANSDUCERS and Actuators
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`Chicago, June 16-19, 1997
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`FNC 1028
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`Pull-Back
`
`Optical Fiber Guide
`
`Fzgure I Schematzc dzagram of the surface-mzcromachined Zx2free-spacefiber optic switch
`
`implement the "fail-safe" feature of the FDDI optical
`bypass switch. The micromirror can be held at the
`TRANSMISSION state by applying a DC bias to the
`SDA. When the DC voltage is released, or when power
`failure occurs, the pull-in spring will return the switch to
`the REFLECTION state. Alternatively, the switch can
`also be held at the TRANSMISSION state by locking the
`translation stage into a mechanical latch so that no DC
`power is required. The mechanical latch can be released
`by an integrated thermal actuator array [lo]. In this
`configuration, the actuators are actuated only during the
`switching time, therefore the overall power consumption
`of the switch is very small. The SDAs are driven by a
`sinusoidal voltage source. The minimum amplitude of
`applied voltage is 80V. Thermal actuators can be driven
`by a voltage source as small as 5V in amplitude.
`
`The switch is fabricated using the three-layer
`polysilicon surface-micromachining technology offered
`by MEMS Technology Application Center at North
`Carolina (MCNC) under Defense Advanced Research
`Projects Agency (DARPA) supported Multi-User MEMS
`Processes (MUMPS). Figure 2 shows the scanning
`electron micrograph (SEM) of the mirror with the sliding
`plate and the microactuators. The fabrication process of
`the switch is summarized in the following: First, a 0.5-
`,urn-thick polysilicon is deposited on the silicon substrate
`coated with low-stress silicon nitride. This layer of
`polysilicon serves as an electrical contact wherk it is
`needed. Before the deposition of the first structural
`polysilicon
`layer (polyl), a 2.0-pm-thick sacrificial
`phosphosilicate glass (PSG) layer is deposited. The
`sliding plate, part of the mirror hinge assembly and
`thermal actuators are defined on the polyl layer.
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`TRANSDUCERS '97
`1997 International Conference on Solid-State Sensors and Actuators
`Chicago, June 16-19, 1997
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`86
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`Figure 2: SEM of the switch
`
`A 0.75-pm-thick PSG layer is then deposited before the
`deposition of
`the 1.5-pm-thick
`second
`structural
`polysilicon layer (poly2). The mirror, sliding plate guide
`rail, part of the mirror hinge assembly and SDAs are
`defined on poly2 layer. At the final processing step, a
`0.5-pm-thick gold layer is deposited on the surfaces of
`the mirror and electrical contacts
`to
`increase
`the
`reflectivity and electrical conductivity, respectively.
`
`EXPERIMENTS AND RESULTS
`
`Switching Test
`
`In the switching experiment, four multimode
`optical fibers with 62.5 pm core diameters are attached to
`the Si substrate. The
`insertion
`losses have been
`characterized to be 1.3 dB and 1.9 dB [7] for the
`
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`states,
`REFLECTION
`and
`TRANSMISSION
`respectively. The fiber-to-fiber spacing is 80 ym and the
`fiber
`tip has been melted
`to
`form hemispherical
`microlenses. A commercial optical transceiver from
`Hewlett Packard Company (HP) is used as the light
`source and the receiver. The switching characteristics is
`measured by monitoring the reflected signal on a HP real-
`time digital oscilloscope.
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`1 A4.07P
`pulled back to the center by the pull-in spring. A rise time
`of 6 ms has been achieved.
`
`Applied voltage frequencies (kH;
`
`Figure 4: Plot of SDA speed
`applied voltage
`
`Vibration Test
`
`We have also performed the vibration test of the
`surface-micromachined fiber optic switch at Rockwell
`Science Center. The purpose of this test is to investigate
`the robustness of the surface-micromachined fiber optic
`In particular,
`switch against external vibrations.
`quantitative measurement in terms of the data bit error
`rate (BER) has been obtained.
`To the authors'
`knowledge, this is the first time such measurement was
`ever performed.
`
`The schematic diagram of the experimental setup
`is shown in Figure 5. In this experiment, we prepared the
`
`Figure 5: The experimental setup
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`87
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`TRANSDUCERS '97
`1997 International Conference on Solid-State Sensors and Actuators
`Chicago, June 16-19, 1997
`
`Figure 3: Response of the photodetector during the
`switching for:(a) from THROUGH state to
`CROSS state, Cb) vise versa.
`
`Figure 3(a) shows the fall time of the optical switch when
`the micromirror is pulled away by the SDAs. When the
`SDA is biased at peak voltage of 100 V at 30 kHz, a fall
`time of 15 ms has been achieved. Since the speed of the
`SDA is proportional to the actuating frequency, higher
`switching speed can be achieved by operating the SDA at
`higher frequencies. The speed of the SDA versus applied
`signal frequencies are plotted in Figure 4. We have
`successfully actuated SDAs up to 50kHz (limited by our
`power supply). At 50kHz, the SDAs are moving at a
`speed of 2.5 mrnlsec, which corresponds to a step size of
`25nm per cycle. The switching from TRANSMISSION
`state to REFLECTION state is achieved by actuating the
`thermal actuator to release the latch. The switching
`characteristics is shown in Fig. 3(b). The micromirror is
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`1 A4.07P
`Two
`MEMS switch in the REFLECTION mode.
`multimode optical fibers are attached to the Si substrate
`for in situ monitoring of the optical signals during
`vibration. The switch is mounted in the Unholtz-Diclue
`vibration-testing machine with the hinged micromirror
`facing the axis of the vibration. The photograph of the
`experimental setup is shown in Figure 6. Since the
`hinged micromirror is most sensitive to vibration in this
`direction, this measurement result should be considered
`as the worst case of vibrations in three axes. An Hp bit
`error rate tester is used to drive the optical transceiver
`and measure the error rate, and a Tektronix real-time
`digital oscilloscope is used to record the eye diagram.
`
`zoomvn
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`; M2.00ns C h 2 f
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`-3OmVI
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`J
`Figure 7: A measured eye diagram of the device at
`50 g” and 150 Hz of an external vibration
`micromachined fiber optic switch. This is the first time
`such measurement was ever performed. Error-free
`operation up to 89 g’s has been achieved for vibration
`frequencies from 200Hz to 1OkHz. The robustness and
`the batch fabrication process make this switch an
`attractive candidate
`for
`low cost opto-mechanical
`switches for fiber optic communications.
`
`This project is supported by DARPA.
`
`REFERENCES
`
`M. F. Dautartas, A. M. Benzoni, Y. C. Chen and G. E. Blonder, ‘‘ A
`silicon-based moving-mirror optical
`switch”,
`J. of Lightwave
`Technology, Vol. IO, No. 8, pp. 1078-85, August, 1992.
`H. Toshiyoshi, H. Fujita, “Optical crossconnection by
`silicon
`micromachined torsion mirrors”, Digest. IEEEiLEOS 1996 Summer
`Topical Meetings, pp 63-64, Keystone, CO, 5-9, Aug., 1996
`C. Marser, M.-A. Gretillat, N. F. de Rooji, et al, ” Vertical mirrors
`fabricated by
`reactive
`ion etching
`for
`fiber optical
`switching
`applications”, . IO” IEEE
`Intemational MEMS Workshop, pp. 349-
`54,1997
`L. A. Field, D. L. Burriesci, P. R. Robrish, R. C. Ruby, ”
`Micromachined 1x2 optical-fiber switch”, International Solid-state
`Sensors and Actuators Conference - TRANSDUCERS ‘95, Stockholm,
`Sweden, 25-29 June, 1995. Sensors and Actuators A (Physical), voLA53,
`(no.1-3):31 I-16,May 1996
`L. Y. Lin, S. S. Lee, K. S. J. Pister and M. C. Wu, “Micro-machined
`three-dimensional micro-optics for integrated free-space optical system”,
`IEEE Photonic Technology Letters, Vol. 6 , No. 12, December, 1994.
`S. S. Lee, L. Y. Lin, K. S . J. Pister and M. C. Wu, “ Hybrid integration of
`8x1 micromachined micro-Fresnel lens arrays and 8x1 vertical-cavity
`surface-emitting laser arrays for free-space optical interconnect”, in Tech.
`Dig. of The 40th International Electron Device Meeting (IEDM’94),
`December 11-14, 1994.
`S. S. Lee, L. Y. Lin, and M. C. Wu, “Realization of FDDI optical bypass
`switches using surface micromachining technology,” Proc. SPIE on
`Micromachining and Microfabrication, Austin, TX, Oct. 23-24, 1995
`K. S. J. Pister, M. W. Judy, S. R. Burgett, and R. S. Fearing,
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`3, p. 249(1992)
`T. Akiyama and H. Fujita, “A quantitative aanalysis of scratch drive
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`Workshop,pp. 310 -315, 1995
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`21, I995
`
`Figure 6: The photograph of the switch mounted on the
`vibration testing machine
`
`The performance of the MEMS optical switch
`under vibration is evaluated by measuring the bit error
`rate of the received optical signal in the REFLECTION
`mode. A 223-1 bit long random test patterns at 100 MHz
`clock rate are used to investigate the vibration sensitivity
`at a wide range of frequencies. Error-free operation up to
`89’s (equipment-limited) was observed for vibration
`frequencies from 200 Hz to 10 kHz. Comparison of the
`receiving sensitivity with and without vibration shows
`that there is virtually no effect of vibration of this scale.
`No mechanical failure observed throughout the entire
`test. A measured eye diagram of the received signal
`under 150Hz, 50g vibration is shown in Figure 5. Clear
`open eyes were observed.
`
`CONCLUSION
`
`A 2 x 2 free-space fiber optic switches with
`integrated microactuators have been demonstrated using
`the surface-micromachining technology. The switch has
`achieved fall time and rise time of 15 ms and 6 ms,
`respectively. Higher speed operation is possible by
`operating the scratch drive actuator at higher frequencies.
`We have performed the vibration test of the surface-
`
`TRANSDUCERS ’97
`1997 International Conference on Solid-state Sensors and Actuators
`Chicago, June 16-19, 1997
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`88
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