Surface-Micromachined Free-Space Fiber Optic Switches
`With Integrated Microactuators for Optical Fiber Communication Systems
`
`1A4.07P
`
`Shi-Sheng Lee, Ed M0tamedi*, 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 91360, 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 and/or 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 [1-3] offers a number of advantages over the
`conventional waveguide approach [4].
`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 [l]. 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 SOI (silicon-on-insulator) wafers and deep
`RIE (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 [5-6]. 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 pul1—in
`spring has been integrate with the micromirror to
`
`0-7803-3829-4/97/$10.00 ©1997 IEEE
`
`85
`
`TRANSDUCERS ’97
`1997 International Conference on Solid-State Sensors and Actuators
`Chicago, June 16- 19, 1997
`
`FNC 1029
`
`

`
`
`
`Thermal
`Actuator
`
`Array
`
`Pu||—Back
`
`Spring
`
`1A4.07P
`
`Optical Fiber Guide
`
`Figure 1." Schematic diagram of the surface-micromachined 2x2 free-space fiber 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 [10].
`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.
`
`fabricated using the three—layer
`The switch is
`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-
`um-thick polysilicon is deposited on the silicon substrate
`
`This layer of
`coated with low—stress silicon nitride.
`polysilicon serves as an electrical contact where it
`is
`needed. Before the deposition of the first structural
`polysilicon layer
`(polyl),
`a 2.0-um-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.
`
`TRANSDUCERS '97
`1997 International Conference on Solid-State Sensors and Actuators
`Chicago, June 16-19, 1997
`
`
`
`Figure 2: SEM ofthe switch
`
`A 0.75-um-thick PSG layer is then deposited before the
`deposition of
`the
`1.5-um-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-um-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
`
`four multimode
`In the switching experiment,
`optical fibers with 62.5 um 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
`
`

`
`pulled back to the center by the pull-in spring. A rise time
`of 6 ms has been achieved.
`
`1A4.07P
`
`SpeedofSDA(mm/sec)
`
`01DO
`
`0
`
`O 5101520253035404550
`
`Applied voltage frequencies (kHz
`
`Figure 4: Plot ofSDA 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
`switch against
`external vibrations.
`In particular,
`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
`
`Tektronix
`
`Digital Oscilloscope
`In
`
`
`
`Hewlett Packard
`Bit Error Tester
`Out
`in
`
`
`
`
` Hewlett Packard
`
`FDDI Transceiver
`Out
`in
`
`
`
`
`Side-Mounted
`
`Switch
`
`Optical Fiber
`
`Vibration
`
`
`
`Testing
`Machine
`
`Figure 5: The experimental setup
`
`87
`
`TRANSDUCERS ’97
`1997 International Conference on Solid-State Sensors and Actuators
`Chicago, June 16-19, 1997
`
`states,
`REFLECTION
`and
`TRANSMISSION
`respectively. The fiber—to-fiber spacing is 80 um 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.
`
`(b)
`
`Figure 3 : Response ofthe photodetector during the
`switchingfor: (a) from THROUGH state to
`CROSS state, (b) 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 mm/sec, 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
`
`

`
`1A4.07P
`MEMS switch in the REFLECTION mode.
`
`Two
`
`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-Dickie
`
`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.
`
`.
`
`, MEMS Switch
`
`K
`
`g Machm;
`
`Vibration
`
`579%;
`
`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 l50Hz, 50g vibration is shown in Figure 5. Clear
`open eyes were observed.
`
`CONCLU$ON
`
`switches with
`A 2x 2 free-space fiber optic
`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
`
`88
`
`
`
`I. '
`
`'2'ci0'mi/'0'
`
`'
`
`'9
`
`'
`
`"M" 2.'0'0n's§ 'ch2'
`
`’
`
`'
`
`'—3'0'm\'/"'
`
`Figure 7: A measured eye diagram ofthe device at
`50 g" and 150 Hz ofan 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 200112 to 10kHz. 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.
`
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