Hisakazu Kurita

Precise operation-frequency control of monolithic mode-locked laser diodes for high-speed optical communication and all-optical signal processing

We describe a method for pulse-repetition-frequency tuning of mode-locked laser diodes (MLLDs) monolithically integrated with a distributed Bragg reflector (DBR). The pulse-repetition frequency, i.e., the cavity-roundtrip time, is tuned through loss-induced change in the effective length of the DBR. The frequency-tuning range as large as the chip-to-chip frequency deviation caused by cavity-length fabrication variation of 10 μm has been confirmed experimentally, and the MLLDs operate at SDH (synchronous digital hierarchy) frequencies of 9.953, 19.906 and 39.813 GHz. Synchronization with an external system-clock through the hybrid mode-locking operation reduces the timing jitter of the optical pulses to less than 0.3 ps. As an optical pulse source for optical communication, error-free 20-Gbps transmission over 3000 km has been demonstrated, confirming that the MLLD properties satisfy the requirements for use in real systems. The novel application of MLLDs to all-optical clock extraction, one of the essential functions required in all-optical signal processing, has been demonstrated at the 40-GHz SDH frequency.

Experimental investigations of harmonic synchronization conditions and mechanisms of mode-locked laser diodes induced by optical-pulse injection

We investigated the detailed characteristics of synchronization of mode-locked laser diodes (MLLDs) by subharmonic optical pulse injection. It was shown that optical harmonic injection mode-locking was not very sensitive to injection pulse shapes. In the time domain, subharmonic injection pulses were multiplexed inside the MLLD and these multiplexed pulses effectively modulated the absorbance of the saturable absorber at the MLLDs repetition frequency. It was also found in the spectral domain that injection pulse trains should have optical spectrum components broader than the MLLDs intermode spacing, and injection spectral components should couple to at least two modes of the MLLD.

Picosecond all-optical gate using a saturable absorber in mode-locked laser diodes

A novel application of mode-locked laser diodes (MLLDs) for all-optical gate is described, A saturable absorber (SA) shows 8-ps-absorption recovery and has a small switching energy of 0.9 pJ. For this SA used as an all-optical gate in combination with the all-optical clock recovery function of MLLDs, all-optical signal processing (demultiplexer and repeater) may be possible with a compact and robust configuration. To confirm the all-optical gate operation in MLLDs, 40 to 10 Gb/s all-optical DEMUX is successfully demonstrated.

Optical sampling measurement with all-optical clock recovery using mode-locked diode lasers

Optical sampling measurement of 160-Gbps signals incorporating all-optical clock recovery has been demonstrated. External-cavity mode-locked semiconductor laser modules, which directly generate coherent 2-ps optical pulses, were used for optical clock recovery and as an optical sampling pulse source. We accomplished the real-time observation of waveform distortion in 160-Gbps optical data signals due to nonlinear optical effect in the transmission fiber.

Highly reliable mode-locked semiconductor lasers and their applications

Monolithic and external-cavity mode-locked semiconductor lasers are highly stabilized by means of electrical or optical signal injection incorporating a compact device-module packaging. Potential applications of these MLLDs are described for high-bit-rate optical communication systems at over 40 Gbps rate, and for ultrahigh-speed optical measurement systems.

All-optical clock extraction at bit rates up to 80 Gbit/s with monolithic modeIocked laser diodes

All-optical signal processing is a key technique for future optical time-division multiplexing systems because of its great potential for ultrahigh-speed operation.

Electrophotonic computer networks with strictly nonblocking and self-routing functions

Three-stage optical interconnection networks for use in massively parallel processors are proposed. Wavelength-division- and space-division-multiplexing switches used in these networks are described, and free-space optics to assist in the construction of networks that are small and provide high throughput are discussed.

All-optical subharmonic clock recovery and demultiplexing

We have achieved a phase stabilized an-optical subharmonic clock recovery through a cascading two-stage optical-pulse-injection synchronization of mode-locked diode lasers. Based on this clock recovery scheme, successful 40 to 10 Gbps optical demultiplexing was also demonstrated.

Noise reduction in optical pulses and bit-error-rate improvement with a semiconductor-waveguide saturable absorber

The technological development of the Er-doped fiber amplifier (EDFA) dramatically changed the scheme of long-haul optical communications. EDFAs have removed electronic 3R (retiming, reshaping, regeneration) function repeaters. However, although the EDFA provides a regeneration function by an optical method, it also causes the accumulation of spontaneous emission noise during transmission and thus induces communication penalty. Therefore, if an optical noise reduction function is realized using a simple optical method, it will bring a great benefit to optical communication systems. In this paper, we report that the optical noise suppression effect is obtained by using a single-chip semiconductor-waveguide saturable-absorber (SWSA). This device can improve the bit error rate in high-speed optical communication. This SWSA has a large advantage for real use in a system because of its simple and compact configuration compared to fiber-based interferometer geometries.

Precisely frequency-controlled mode-locked laser diode for all-optical clock extraction at 40 GHz SDH frequency

We describe a method for precise pulse-repetition-frequency tuning of monolithic mode-locked laser diodes (MLLDs) by means of loss-induced change in the effective length of the distributed Bragg reflector. With this method, 39.8131-GHz (the SDH frequency) operation is achieved in a frequency tuning range of 1 GHz. The novel application of mode-locked laser diodes to all-optical clock extraction, one of the essential functions required in all-optical signal processing, is demonstrated at the 40-GHz SDH frequency.