1:30pm - 2:OOpm (Invited)
`ThSl
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`A Compact, Scalable Cross-Connect Switch Using Total Internal
`Reflection Due to Thermally-Generated Bubbles
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`J.E. Fouquet, S. Venkatesh, M. Troll, D. Chen, H.F. Wong* and P.W. Barth
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`Hewlett-Packard Laboratories, 3500 Deer Creek Road 26M9, Palo Alto, CA 94304, (650) 857-31 38
`voice, (650) 81 3-3626 fax, fouquet@hpl.hwom
`* Hewlett-Packard Company, Lightwave Division, 1400 Fountaingrove Parkway 1 US, Santa Rosa, CA
`95403
`
`Cross-connect switches for connecting optical fibers are important for high speed communications,
`particularly where wavelength division multiplexing is employed. Commercial optomechanical switches
`offer low insertion losses but are bulky, expensive and difficult to scale to large numbers of fibers.
`Commercial thermo-optic switches are far more compact by comparison, but still use a relatively large
`amount of wafer area, have relatively high insertion losses (e.g. - 8 dB in a 4x4 switch) and polarization
`dependence (frequently - 1 dB PDL), and are generally difficult to scale. Free-space silicon
`micromachined switches are usually limited to small numbers of fibers due to insertion losses incurred
`from beam divergence. Friction-based wear and difficult optical alignment of all paths simultaneously are
`problems commonly encountered with free-space micromachined designs.
`
`Our novel, compact single mode fiber cross-connect switch diverts light from one waveguide to a crossing
`waveguide in a silica planar lightwave circuit (PLC) using total internal reflection (TIR) off the interface
`between a waveguide and a thermally-generated bubble. Trenches are etched at the waveguide
`intersections, with one sidewall passing through the point where the waveguide axes intersect. When the
`trenches are filled with a liquid whose refractive index matches that of the waveguide (the default
`condition), light is transmitted across a trench into the next colinear waveguide segment as shown in Fig.
`la. When the liquid is displaced by a bubble, the incident light undergoes TIR into the crossing
`P .
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`wave uide as shown in Fig. 1 b. The concept of a PLC using TIR was demonstrated by Jackel, et al. in
`1990 using electrolytically-generated bubbles. The device switched light, albeit with rather poor
`performance. Our devices exhibit much better optical performance since thermal actuation permits use of
`a fluid with a much closer refractive index match to the waveguide. In addition, our actuators are formed
`on a separate substrate, allowing independent optimization of the PLC to minimize insertion loss, and
`improvements in etching technology have yielded much smoother trench sidewalls.
`
`The diamond-shaped feature of Fig. 2 is the PLC portion of a 4x4 device. It contains two 250pm-pitch
`waveguide arrays intersecting at an angle to support TIR. If no crosspoint along the path of one input
`fiber is activated to reflect, then the light passes straight through the switch along the original path as a
`“drop” output. These “drop” outputs, along with the corresponding “add” inputs, allow modular scaling
`architectures to construct larger switches, as shown in Fig. 3. In contrast, optomechanical and polymer
`thermo-optic NxN switches do not scale as readily. The switch design reported here is basically
`polarization-insensitive; only the crosstalk should depend on polarization.
`
`Flame hydrolysis-deposited planar lightwave matrices were fabricated to our specifications by an external
`vendor. Refractive indexes nominally match those of single mode fiber at 1.55 ym. The core layer is 8
`pm high and is centered 25 pm below the top of the upper cladding layer of the as-fabricated PLC.
`Losses within the waveguides are reported to be approximately 0.1 dB/cm by the vendor. The core layer
`has been patterned in order to create waveguides 8pm wide at the edge of the switch. In some
`structures, waveguides adiabatically expand to 16 pm width in order to reduce optical losses while
`traversing the trench. After the upper cladding deposition, verl:ical-sidewalled trenches have been etched
`to a depth of 50 pm. Waveguides approach the trenches at a 160” angle of incidence, which provides for
`TIR of all rays inside standard single mode fiber. The bubbles used to divert the light are generated using
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`0-7803-4947-4/98/$10.00 0 1 998 IEEE
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`169
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`FNC 1025
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`immersible heaters constructed using a process based on proven thermal inkjet technology. 70 pm -
`diameter vertical holes penetrate the silicon heater substrate to supply fluid to the trenches in the PLC.
`The PLC waveguide matrix is inverted top-to-bottom before being bonded to the actuator circuit.
`
`Initial optical tests were made on PLC structures without actuators. The refractive index of the matching
`liquid reduces beam divergence, and thus transmission insertion loss, to low levels for our trenches.
`BeamPROP version 2.1 h predicts a 0.2 dB insertion loss for transmission through a 25 pm-wide single
`trench with 8 pm x 16 pm waveguides. Indeed, fiber-to-fiber losses at 1.55 pm wavelength in test
`structures containing 32 trenches with these dimensions were measured to be 0.2 dB per trench,
`agreeing well with theory. Polarization-dependent loss is 5 0.02 dB for transmission through four
`trenches along the “drop” path.
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`A 4x4 prototype switch was interconnected with single mode fibers in silicon V-groove arrays. Fiber-to-
`fiber insertion loss was 1.9 to 2.5 dB along the “drop” path, implying approximately 0.55 to 0.85 dB loss
`per fibeddevice interconnection. Insertion loss for reflection off the lowermost crosspoint was 5 dB. For
`ideal conditions, reflection insertion loss off of an empty trench should be 0.1 dB, so reflection loss was
`considerably higher than expected, possibly due to nonideal angle (2” from vertical) and placement of the
`etched trench sidewalls in this device. The same value of reflection insertion loss was measured for
`reflection at an empty trench as for reflection off a bubble having an end-to-end length approximately 2.5
`times the trench width located in the same trench. Extinction (on:off) ranged from 45 to 70 dB for the
`different reflected paths. Measurements were made without temperature control, so fluid refractive index
`match and thus extinction were less than ideal. Even so, these results far exceed the single crosspoint
`performance of interference-based switches.
`
`1. Janet L. Jackel, John J. Johnson and W.J. Tomlinson, “Bistable optical switching using
`electrochemically generated bubbles”, Optics Letters, v. 15, n. 24, 1990.
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`index-
`matching fluid
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`silica
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`I
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`gas
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`silica
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`Fig. 1 a transmission
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`Fig. 1 b reflection
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`TIR
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`I
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`V
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`four 4x4 devices
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`Fig. 2
`PLC array with empty trenches
`bonded to silicon heater chip
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`170
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