Product category: Light Emitting Diodes (LEDs)
News Release from: Bookham | Subject: 40Gbit/s GaAs optical modulator
Edited by the Electronicstalk Editorial Team on 2 September 2002
ECOC paper looks
at novel GaAs modulator
Bookham Technology will present a paper at next month's ECOC 02 describing a novel design for a 40Gbit/s GaAs optical modulator
Bookham Technology will present a paper at next month's ECOC02 describing a novel design for a 40Gbit/s GaAs opticalmodulator to overcome problems of phase alignment withreturn-to-zero (RZ) modulation systems in long-distance DWDMoptical communications. Bookham's development of GaAsmodulators, rather than the conventional LiNbO3, allows a highlyintegrated design that uniquely exploits two-photon absorption inGaAs to provide a novel method of monitoring and controlling thewaveform to give highly precise generation of RZ data.
This article was originally published on Electronicstalk on 2 September 2002 at 8.00am (UK)
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'RZmodulation is important in long-distance DWDM systems because itgives you a greater tolerance of fibre nonlinearity, which meansthat you can transmit more power in the fibre, and thattranslates into a longer reach', says the paper's leadauthor, Robert Griffin, Senior Engineer, Bookham Technology.'An integrated modulator is also very attractive to systemsvendors as it minimises the number of components and consequentlythe board space'.
The new GaAs modulator use a pair ofconcatenated electro-optic Mach-Zehnder modulators (MZMs)integrated on a single chip, together with lossless optical RFpower monitors to provide a closed-loop control system.
Theoptical waveguides consist of ribs etched into the surface of aGaAs/AlGaAs slab-waveguide.
The MZM structures use a microwaveslow-wave technique to achieve the RF/optical velocity matchneeded for a flat frequency response with low drive voltage.
Themodulator applies non-return-to-zero (NRZ) data to one MZM and ahalf-rate clock signal to the second MZM to give RZ pulseshaping.
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Optimum operation of the concatenated modulatorsrequires close control of the bias voltages and drive levels, andadditionally control of the phase of the clock signal relative tothe data.
Phase control is essential because the RZ pulses shouldbe aligned precisely in the centre of the data bits for optimumoperation.
Bookham's new modulator overcomes the challengeof providing a practical, simple and robust solution for adaptivephase alignment in field-deployed modules.
It monitors both NRZand RZ outputs and provides adaptive adjustment as required.
Byusing the phenomenon of two-photon absorption (TPA) in GaAs toprovide performance monitors integrated monolithically on the RZchip, the design avoids additional external components and anyinterruption and degradation of the optical path.
TPA in GaAsprovides a powerful technique for monitoring the optical waveformproduced by an integrated RZ transmitter.
The TPA processprovides a photocurrent proportional to the time-averagedinstantaneous intensity squared, and is hence equivalent todetection of the optical power with a fast photodiode followed byan RF power detector.
The modulator is one of a series ofgroundbreaking optical transmitters from Bookham that includescommercial devices operating at 10 and 40Gbit/s NRZ and RZ, andalso prototypes for binary and quadrature phase coding.
All thetransmitters, regardless of data format, comprise a InP laserchip copackaged with a single GaAs modulation chip.
'Thisapproach really demonstrates the power of integration onGaAs', says Andy Carter, Vice President of Research andDevelopment, Bookham Technology.
'We have all the elementsneeded to design wavelength-tunable transmitters for virtuallyany format, all based on a platform approach which enables us tomove to volume manufacture with minimal requalification.
With 6inGaAs wafer technology we can fabricate many chips per wafer, evenwith the more complex modulation formats'.
The paperdescribes trial devices comprising two concatenated MZMsfabricated on a single GaAs chip with dimensions of 52 x 1.5mm.The GaAs RZ chip included four TPA monitors - on the output andthe complementary output of each MZM.
The complementary outputswere 'dumped' by appropriately terminating thewaveguides following the TPA monitors.
The GaAs chip wascopackaged with a DFB laser in a small-footprint transmittermodule, with output pins providing access to the TPA monitorcurrents.
The devices gave excellent phase alignment andstability for the optimum RZ phase.
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