We report on InGaAs/InGaAlAs electro-absorption modulators for ultrafast all-optical signal processing. The structure design is based on the use of small conduction and valence band offsets. Switching windows as short as 5 ps for a 2 V applied reverse bias have been obtained by cross-absorption modulation.

We report here on a novel multiple quantum well (MQW) electroabsorption modulator designed for polarisation insensitivity as well as very fast carrier escape time. Experiments using this novel structure are being carried out at 40 Gb/s. Regeneration of the incident signal that is inherent to the wavelength conversion based on the use of EAMs is also investigated.

Electroabsorption modulators (EAM) have proved to be very attractive both as optical sources (monolithically integrated NRZ transmitters and RZ pulse generators) as well as for very fast signal processing (demultiplexing, regeneration, wavelength conversion,...).Their design criteria, technology and implementation in future networks are reviewed, and the main issues are discussed.

The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-mum fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (+/-1.6 mum and +/-2 mum, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than + 10 dBm and a high output voltage of 0.8 V. These capabilities allow the photo-diode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes.

50GHz bandwidth photodiodes. attaining a compression pint of +12dBm, have been fabricated. The structure design allows an almost complete absorption at 25 mum, corresponding to 0.5A/W responsivity and gives a polarisation dependence as low: as +/-0.1dB. These components have been realised with a low-cost technology including etched waveguide input facet. antireflection coating on wafer and cleaving V-grooves.

Evanescently-coupled photodiodes, integrating double stage taper, have been realized for low cost, high-speed, and high power applications. 50 GHz bandwidth and more over +14 dBm optical power, have been achieved at -3 V bias with 0.8 V output voltage. 0.5 A/W responsivity has been measured for a 25 /spl mu/m long diodes. Polarization dependence is as low as /spl plusmn/0.1 dB. These diodes are suitable for future 40 Gbit/s and radio over fiber applications.

A low cost integration technology has been developed to assemble 2.5 and 10 Gb/s photodiodes. It relies on flip-chipped devices with lensed fibres positioned in V-grooves. For 2.5 Gb/s the photodiode, the fibre and the transimpedance amplifier are integrated on a single Si motherboard compatible with mounting in a mini-dil module. The sensitivity is -23.5 dBm at 10/sup -10/ BER with no error floor.

Side-illuminated photodiodes designed for surface hybrid integration on a silicon optical bench are described. InGaAs PIN photodiode chips fabricated on a 2inch-wafer with an integrated dry etched waveguide input facet with an antireflection coating deposition on the wafer, exhibit very low dark currents of typically 20 pA at -10 V bias voltage and 25/spl deg/C. Fiber coupled responsivity as high as 0.95 A/W at 1.3 /spl mu/m wavelength, and vertical coupling tolerance at -1 dB as wide as /spl plusmn/2 /spl mu/m are demonstrated. Reliability testing under high temperature and bias-stress conditions shows very stable operation.

Simultaneous optical regeneration of four 200 GHz-spaced 40 Gbit/s channels in a single polarisation-insensitive InP Mach-Zehnder modulator is demonstrated by means of a dense WDM loop transmission over 10,000 km, yielding spectral efficiency of 0.27 (bit/s)/Hz.