Bookham Technology will describe a new design of 20Gbit/s optical transmitter with high tolerance to chromatic and polarisation-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 unique 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 polarisation 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 fibre, (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(CZehnder modulators (MZMs) arranged within a Mach(CZehnder superstructure. Each MZM is biased for minimum DC transmission and driven with a data signal at half the total bitrate. 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 (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 fibre.
The 52 !A 3.5mm GaAs/AlGaAs chip is co-packaged with a DFB laser to provide a high-performance small-footprint module (Figure 2). On the chip, the optical waveguides consist of ribs etched into the surface of a GaAs/AlGaAs slab-waveguide. The MZMs use a microwave slow-wave technique to achieve the RF/optical velocity match needed to achieve wide bandwidth with low drive voltage. The phase modulator at the output is a travelling-wave structure similar to the Mach(CZehnder electrodes. The Bookham design uniquely exploits the phenomenon of 2-photon absorption (TPA) in GaAs to provide performance monitors integrated monolithically on the chip after the recombine to avoid additional external components and any interruption and degradation of the optical path.
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