The frequency of an AlGaAs DH laser has been stabilized with respect to a Fabry-Perot interferometer by controlling the injection current. The frequency stability of 2.11012 was obtained at τ90 ms, which was better than the free-funning stability by three orders of magnitude.

Timing noise of 160 GHz optical pulses has been evaluated over nine decades of Fourier frequency using the optoelectronic harmonic mixing technique. For down-converting the 160 GHz pulse intensity into a low-frequency IF signal, the fourth order modulation sidebands produced by a Mach-Zehnder intensity modulator have been employed. Phase noise power spectral density and timing jitter for 155.552-GHz optical time-division multiplexed pulses and 160.640-GHz passively mode-locked pulses are measured using the time domain demodulation and time interval analysis techniques, respectively.

The performance of an indium-gallium-arsenide avalanche photodiode serving as a 1550 nm single-photon detector is investigated. Quantum efficiency is evaluated for laser pulses with an average of < 0.015 photons per pulse, which are important for the demonstration of unconditionally secure quantum key distribution [G. Brassard et al.: Phys. Rev. Lett. 85, 6, p.1330 (2000)]. An operating temperature of 243 K is achieved by thermo-electrical cooling, yielding a quantum efficiency of 18% with a dark-count probability per gate of 2.8 10-5. The results obtained here guarantee unconditionally secure fiber-optic quantum key distribution over 50 km.

We demonstrated all-optical demultiplexing of 160-Gb/s signal to 40- and 80-Gb/s by a Mach-Zehnder Interferometric all-optical switch, where the picosecond cross-phase modulation (XPM) induced by intersubband excitation in InGaAs/AlAsSb coupled double quantum wells is utilized. A bi-directional pump configuration, i.e., two control pulses are injected from both sides of a waveguide chip simultaneously, increases a nonlinear phase shift twice in comparison with injection of single pump beam with forward- and backward direction. The bi-directional pump configuration is the effective way to avoid damaging waveguide facets in the case where high optical power of control pulse is necessary to be injected for optical gating at repetition rate of 40/80 GHz. Bit error rate (BER) measurements on 40-Gb/s demultiplexed signal show that the power penalty is decreased slightly for the bi-directional pump case in the BER range less than 10-6. The power penalty is 1.3 dB at BER of 10 - 9 for the bi-directional pump case, while it increases by 0.3-0.6 dB for single pump cases. A power penalty is influenced mainly by signal attenuation at "off" state due to the insufficient nonlinear phase shift, upper limit of which is constrained by the current low XPM efficiency of 0.1 rad/pJ and the damage threshold power of 100 mW in a waveguide facet.