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Bookham optical transmitter doubles operating distance

Posted: 24 Mar 2003 ?? ?Print Version ?Bookmark and Share

Keywords:bookham technology? optical transmitter? differential qpsk? dqpsk? smdf28?

Bookham Technology plc has developed a 20Gbps optical transmitter that uses a differential QPSK (DQPSK) modulator to double the DWDM spectrum-packing efficiency, as well as double the operating distance on uncompensated standard non-dispersion-compensated single-mode fiber (SMDF28).

The combination of DQPSK and the company's GaAs optical technology enable the transmitters to operate at 20Gbps, providing the parallel transmission of two SONET/OC-192 data streams on a single optical channel. This allows the potential to increase system capacity, while maintaining a standard 10Gbps interface.

By transmitting two bits per symbol, the device halves the optical spectral width compared to OOK, allowing 20Gbps transmission with 25GHz channel spacing, without the need for polarization interleaving/multiplexing to reduce cross-channel interaction.

At 10Gbps, the DQPSK transmitter can operate over spans of 250km on SMF28. Conventional transmitters using OOK have a typical span of 120km in metro networks.

The device integrates an optical DQPSK encoder onto a single GaAs/AlGaAs chip. The encoder is comprised of several Mach-Zehnder modulators arranged within a Mach-Zender superstructure and biased for minimum dc transmission. An optical phase difference of p/2 is maintained between the upper and lower branches to ensure quadrature addition of the optical fields on recombination.

The design also adds a phase modulator after the recombiner, which can be driven with a sinusoidal clock signal to provide chirp on the DQPSK signal.

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