To present a paper at next month’s ECOC 02 describing a novel design for a 40-Gbit/s GaAs optical modulator

August 29, 2002

3 Min Read

OXFORDSHIRE, U.K. -- Bookham Technology plc, a leading provider of integrated optical components and subsystems for fiber optic communication networks, will present a paper at next month’s ECOC 02 describing a novel design for a 40Gbit/s GaAs optical modulator to overcome problems of phase alignment with Return-to-Zero (RZ) modulation systems in long-distance DWDM optical communications. Bookham’s development of GaAs modulators, rather than the conventional LiNbO3, allows a highly integrated design that uniquely exploits 2-photon absorption in GaAs to provide a novel method of monitoring and controlling the waveform to give highly precise generation of RZ data. “RZ modulation is important in long-distance DWDM systems because it gives you a greater tolerance of fibre nonlinearity, which means that you can transmit more power in the fibre, and that translates into a longer reach,” says the paper’s lead author, Robert Griffin, Senior Engineer, Bookham Technology. “An integrated modulator is also very attractive to systems vendors as it minimises the number of components and consequently the board space.” The new GaAs modulator use a pair of concatenated electro-optic Mach-Zehnder modulators (MZMs) integrated on a single chip, together with lossless optical RF power monitors to provide a closed-loop control system. The optical waveguides consist of ribs etched into the surface of a GaAs/AlGaAs slab-waveguide. The MZM structures use a microwave slow-wave technique to achieve the RF/optical velocity match needed for a flat frequency response with low drive voltage.The modulator applies Non-Return-to-Zero (NRZ) data to one MZM and a half-rate clock signal to the second MZM to give RZ pulse shaping. Optimum operation of the concatenated modulators requires close control of the bias voltages and drive levels, and additionally control of the phase of the clock signal relative to the data. Phase control is essential because the RZ pulses should be aligned precisely in the centre of the data bits for optimum operation. Bookham’s new modulator overcomes the challenge of providing a practical, simple and robust solution for adaptive phase alignment in field-deployed modules. It monitors both NRZ and RZ outputs and provides adaptive adjustment as required. By using the phenomenon of 2-photon absorption (TPA) in GaAs to provide performance monitors integrated monolithically on the RZ chip, the design avoids additional external components and any interruption and degradation of the optical path. TPA in GaAs provides a powerful technique for monitoring the optical waveform produced by an integrated RZ transmitter. The TPA process provides a photocurrent proportional to the time-averaged instantaneous intensity squared, and is hence equivalent to detection of the optical power with a fast photodiode followed by an RF power detector. The modulator is one of a series of groundbreaking optical transmitters from Bookham that includes commercial devices operating at 10Gbit/s and 40Gbit/s NRZ and RZ, and also prototypes for binary and quadrature phase coding. All the transmitters, regardless of data format, comprise a InP laser chip co-packaged with a single GaAs modulation chip. "This approach really demonstrates the power of integration on GaAs,” says Andy Carter, Vice President of Research and Development, Bookham Technology. “We have all the elements needed to design wavelength-tunable transmitters for virtually any format, all based on a platform approach which enables us to move to volume manufacture with minimal requalification. With 6-in. GaAs wafer technology we can fabricate many chips per wafer, even with the more complex modulation formats." The paper describes trial devices comprising two concatenated MZMs fabricated on a single GaAs chip with dimensions of 52mm × 1.5mm. The GaAs RZ chip included four TPA monitors — on the output and the complementary output of each MZM. The complementary outputs were ‘dumped’ by appropriately terminating the waveguides following the TPA monitors. The GaAs chip was copackaged with a DFB laser in a small-footprint transmitter module, with output pins providing access to the TPA monitor currents. The devices gave excellent phase alignment and stability for the optimum RZ phase.Bookham Technology plc

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