Quantum-Well Lasers for High-Speed Optical Information Processing
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QUANTUM-WELL LASERS FOR HIGH-SPEED OPTICAL INFORMATION PROCESSING M. C. Wu, Y. K. Chen, T. Tanbun-Ek, and R. A. Logan AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT Monolithic colliding pulse mode-locked (CPM) multiple quantum well lasers generating optical pulses as short as 600 femtoseconds are reported. The CPM laser is built on a single chip of InGaAs/InGaAsP multiple quantum well laser. The pulse repetition rates are synchronized with an rf synthesizer up to 40 GHz in hybrid mode-locking scheme. In passive mode-locking scheme, a record high repetition rate of 350 GHz has been achieved. All the 2 sub-picosecond pulses obtained have pulse shapes of sech and transform-limited timebandwidth products between 0.30 to 0.34. This new optical source is very useful for ultra-high speed optical switching and optical logic in optical fibers, and ultra-long distance optical soliton transmissions.
INTRODUCTION Semiconductor lasers generating ultra-short optical pulses are very important for their applications in ultra-high speed all-optical soliton logic, multi-hundred Gbit/sec time-division multiplexed optical communication/switching systems, high speed electro-optic sampling systems, optical soliton sources for ultra-long distance transmission in optical fibers, and generation of millimeter and sub-millimeter waves. Direct modulation of semiconductor lasers [1], whose intrinsic response is limited by relaxation oscillation to a few tens of GHz, can not satisfy the requirements of these applications. Gain switching of semiconductor lasers has generated optical pulses with duration of a few picoseconds, however, the pulses are not very stable and suffer from jittering because the pulses build up from spontaneous emission [2]. Much shorter and more stable optical pulses can be generated by mode-locking technique, in which case each pulse is seeded by a previous emission. Subpicosecond optical pulses have been demonstrated with mode-locked semiconductor lasers using external cavities and bulk optics [3]. Recently, monolithic mode-locked semiconductor lasers have received a great deal of attention [4-10]. The fully integrated optical cavities not only eliminate the tedious optical alignment processes in external-cavity mode-locked lasers, they also suppress the undesired multiple pulse output generated by residue intra-cavity reflections [3]. Picosecond pulses have been generated in monolithic semiconductor lasers with integrated passive cavities [5], tandem-contact semiconductor lasers [6,81, and hybrid mode-locked semiconductor lasers with active waveguides [7]. The monolithic colliding pulse mode-locked (CPM) multiple quantum well lasers [9,10] are the first monolithic mode-locked semiconductor lasers that generate optical pulses in the femtosecond regime. Semiconductor quantum well lasers have great advantages for applications in high-speed optical information processing, including optical switching and optical transmission. Their unique properties, such as high differential gain, high modulation bandwidth, reduc
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