Wavelength-selective crossconnect switch is scalable

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A 72 x 72 wavelength-selective crossconnect switch can be scaled up to much larger sizes using current technology, say Roland Ryf and several coworkers at Bell Laboratories, Lucent Technologies (Holmdel, NJ) and Agere Systems (Murray Hill, NJ).1 The switch was made by assembling wavelength multiplexers with integrated monitoring taps and a MEMS micromirror array in a hybrid three-dimensional beam-steering crossconnect. The switch demonstrated a 20-dB insertion loss, 100-GHz channel spacing, and 30-GHz passbands.

Ryf and coworkers incorporated the added functions to the switch by combining MEMS micromirrors and planar arrayed waveguide gratings (AWGs) that can work as either passive N x N wavelength-selective crossconnects or bidirectional 1 x N multiplexers/demultiplexers. "The result is an overall smaller and more-compact switch with potentially lower total insertion losses, better loss uniformity, and a drastically reduced number of interconnecting fibers," the researchers said.

The group used integrated silica-on-silicon N x N AWGs. On one end of the optical circuit wafer, the waveguides were coupled to fiber ribbons. On the other side, the surface was polished to face a microlens array. The (rectangular) wafers were stacked so that the waveguides matched a 1.25-mm-square grid. The microlens array collimated each beam into and out of each waveguide. The wafers and the lens array were held in place by a single rigid mount.

Once light has been coupled out of a waveguide, it travels through free space to a two-dimensional array of MEMS mirrors and a folding mirror (see figure). Each waveguide is associated to a specific microlens and micromirror. The design allows bidirectional connectivity between any pair of waveguides. Although the coupling conditions are stringent, a side benefit is that crosstalk is very low. The crosstalk rejection ratio was measured as than 50 dB.

The MEMS array was 36 x 36 mirrors, which (using the AWGs as either multiplexers or demultiplexers) allows for an18 x 18-port crossconnect with 36 optical channels per port. The AWGs were designed on a 100-GHz frequency grid from 1532 to 1564 nm. The Gaussian filter for a single path had a full-width half-maximum (FWHM) passband of about 50 GHz with an adjacent crosstalk of less than -27 dB, and nonadjacent crosstalk of less than -32 dB. The total insertion loss for a single device was 5.5 dB. The polarization-dependent loss (PDL) was less than 0.3 dB and the polarization-dependent wavelength shift was compensated for so that it was less than 0.015 nm.

The AWGs included taps that could monitor the power without disturbing the data traffic. The taps use higher diffraction orders of the array grating which, with equalization, provided 40 separate -25-dB channel taps. Crosstalk among the monitoring channels was less than -27 dB.

When the researchers tested the switch by transmitting data at 10 Gbit/s, it operated without penalty. The total insertion loss for a connection from input fiber to output fiber of this switching fabric was 20 dB. In future experiments, the researchers believe they can reduce loss by using optimized microlenses.

The 72 x 72 channel wavelength-selective switch is scalable to 1296 x 1296 with available technology. For more information contact: Roland Ryf at ryf@lucent.com.

Yvonne Carts-Powell

REFERENCE

  1. R. Ryf et al., ECOC 2001, postdeadline paper PD.B.1.5.
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