`WKl
`
`Construction and performance of a 576x576 single-stage OXC
`
`Herzel Laor, Consultant
`Astart6 Fiber Networks, Inc.
`2555 55* St., Boulder, CO 80301
`Tel: 303-443-8778
`herzel.laor@starswitch. corn
`
`With the explosive growth in data communications, management of fiber networks becomes a crucial
`issue. Optical Cross-Connects (OXCs) are ideal for this application, if they satisfy several critical
`requirements: large fiber count, transparency (to allow hture increase of bit rates), and low-loss
`performance.
`
`One practical way to create such an OXC uses a single-stage, free-space design. In this design, an
`optical unit attached to each fiber creates an expanded beam, which is directed in space. A large
`number or such optical units can interconnect to form a large, transparent, low-loss OXC.
`
`'
`
`MIRROR
`
`Figure 1 : Directable Beams in Space
`
`This basic free-space architecture is used in an existing commercial 72 x 72 multi-mode fiber cross-
`connect. Excellent performance has been demonstrated by these systems in the field. In this design,
`beam manipulation is performed with piezo-electric actuators, which move the fiber in two degrees
`of freedom.
`
`In the new OXC under development, MEMS mirrors are used to direct the beams rather than the
`earlier piezo devices. The mirrors were developed by Texas Instruments, Incorporated. The mirrors
`are fabricated from silicon, and have an active area that measures 3 mm x 4 mm. The mirrors are
`magnetically actuated for rotation with two degrees of freedom. The mirrors have a precision surface
`
`0-7803-5634-9/99/%10.00@1999 IEEE
`
`481
`
`FNC 1027
`
`
`
`that is gold-plated, resulting in diff'raction-limited performance with a 3-mm diameter beam. Each
`mirror is hermetically sealed in a ceramic carrier with a glass window. Since the mirror is directing
`the expanded beam, the lens is always on-axis, allowing the use of a simple lens.
`
`Four control LEDs surround each mirror, emitting short-wave infrared light at 880 nm. These
`control signals are used for servo control of the position of the MEMS mirrors. The LED si;pals are
`detected using a silicon-based detector mounted near the end of the communication fiber. A dichroic
`beam-splitter decouples the communication wavelength from the control wavelength; the
`communication beam is focussed on the fiber end, while the control signals arriving from the
`opposing unit are focussed on the servo detector.
`
`BEAM SPLIllER
`
`LENS
`
`FIXED MIRROR
`
`/
`SERVO DETECTOR
`
`m
`
`LED
`
`Figure 2: Optical Unit
`
`Loss performance of 3-4 dB was measured for several optical units, well in line with the target
`specification of 6dB. These measurements were done at 1550 nm. The specified transmission
`window is 1260-1360 and 1500-1650 nm; at 1310 nm the loss performance was somewhat better
`than at 1550. The optics are being optimized to further improve performance for the 1500-1650 nm
`window. The target for polarization-dependent loss is less than 0.5 dB; it was measured at less then
`dB. Optical improvements should reduce this figure further.
`0.4
`Cross talk was targeted to be -50 dB (optical) below the signal level. While switching, when a beam
`scans over other units not aimed at the scanning beam, the cross talk is doubled for a very short
`while. This is because most of the cross talk originates from scattering in the optics. The doubled
`cross talk was measured at -41.6 dB. From this measurement we extrapolate -80 dB optical cross
`talk, which translates to -160 dB when measured at the electrical level.
`
`482
`
`
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