Integrated tandem modulators provide 40-Gbit/s demux

Yvonne Carts-Powell

A compact demultiplexing receiver based on integrated tandem electroabsorption (EA) modulators provided error-free operation at 40 Gbit/s with a receiver sensitivity of -27 dBm. Developers Volkan Kaman and others at the University of California-Santa Barbara believe that the design could allow demultiplexing at rates up to 80 Gbit/s. The developers want to create a simple, efficient, and inexpensive demultiplexer that can operate at these high data rates.1

"The advantage of using an integrated receiver based on EA modulators," Kaman explains, "is that the EA modulator can be used as the demultiplexer operating at 10 GHz and the second device as a simple reverse-biased photodetector at 10 Gbit/s," which provides the advantage of a low-frequency requirement for high-speed optical data input. He adds, "Since EA modulators with bandwidths in excess of 30 GHz have been demonstrated, this scheme can be easily scaled to 160-Gbit/s systems to demultiplex and receive the 40-Gbit/s base rate from an incoming 160-Gbit/s data stream."

The major alternative method of high-speed demultiplexing has two drawbacks compared to the EA method: it uses high-speed 40-Gbit/s electronic and optoelectronic devices that are not yet commercially available, and it may not be able to scale to higher speeds due to the electronic bottleneck.

The researchers integrated the EA modulators and detectors to reduce insertion losses and improve the signal-to-noise ratio compared to discrete bulk devices. The tandem modulators (300 and 400 µm long) are based on a traveling-wave electrode structure with 10 periods of strain-compensated indium-gallium-arsenide-phosphide quantum wells. The longer device acts as the optical demultiplexer to achieve a maximum extinction of 38 dB at -6 V. The shorter device acts as a reverse-biased photodetector with a responsivity of 0.5 A/W. The two modulators were separated by a 20-micron-long optical waveguide, which also extended 50 microns into each modulator to reduce capacitance and microwave crosstalk. These were measured at 50 k-ohm and >30 dB, respectively, while the optical insertion loss into the waveguide was estimated to be 2.3 dB.

The next step, says Kaman, is to demonstrate this idea at a bit rate of 160 Gbit/s. For more information, contact Volkan Kaman at


1. V. Kaman et al., Elec. Lett. 9, 36 (Nov. 2000).

Yvonne Carts-Powell is a science and technology writer based in Belmont, MA.

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