Optical Transmitter Doubles DWDM Spectral Efficiency

March 19, 2003
Northampton, UK, March 19, 2003. Bookham Technology will describe a new design of 20Gbit/s optical transmitter with high tolerance to chromatic and polarization-mode distortion and with improved spectral efficiency at the Optical Fiber Communication Conference and Exposition (OFC) in March 2003.

Northampton, UK, March 19, 2003. Bookham Technology will describe a new design of 20Gbit/s optical transmitter with high tolerance to chromatic and polarization-mode distortion and with improved spectral efficiency at the Optical Fiber Communication Conference and Exposition (OFC) in March 2003.

Exploiting a new technique of differential quadrature phase-shift keying (DQPSK) and Bookham's integrated GaAs optical technology, the transmitters can operate at 20Gbit/s, providing parallel transmission of two Sonet OC-192 data streams on a single optical channel. This allows the potential to increase capacity whilst maintaining a standard 10Gbit/s interface. Since transmitting two bits per symbol inherently halves the optical spectral width compared to OOK, DQPSK increases spectral efficiency, potentially allowing 20Gbit/s transmission with 25GHz channel spacing.

This is achieved without the need for polarization interleaving/multiplexing to reduce cross-channel interaction. This is a particularly attractive proposition for carriers seeking to increase the capacity of installed long-haul point-to-point transmission systems using expensive dispersion-managed fiber.

Alternatively, under 10Gbit/s operation, the DQPSK transmitter can operate over spans of 250km on standard non-dispersion-compensated single-mode fiber, (SMF28). This compares with a typical span in metro networks of 120km achieved by conventional transmitters using on/off keying (OOK) at a similar optical signal/noise ratio (OSNR).

The key to the new device is Bookham's integration of an optical DQPSK encoder onto a single GaAs/AlGaAs chip. The encoder comprises several Mach-Zehnder modulators (MZMs) arranged within a Mach-Zehnder superstructure. Each MZM is biased for minimum DC transmission and driven with a data signal at half the total bit-rate. An optical phase difference 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 (PM) after the recombiner. The PM can be driven with a sinusoidal clock signal to provide chirp on the DQPSK signal, and this additional chirp extends the transmission reach for non-dispersion-compensated fiber.

"Because of our underlying GaAs platform technology we can put a lot of functionality onto a single chip," says Robert Griffin, Senior Engineer, Bookham Technology, and lead author of the paper. "This is crucial because there are currently strong industry trends for high bit-rates and long spans driving the development of complex modulation formats requiring multiple functions, and these generate needs for higher levels of component integration to enhance performance and to simplify implementation."

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