K. Y. Lau

Bistability and pulsations in cw semiconductor lasers with a controlled amount of saturable absorption

Experimental results of a buried heterostructure cw laser with a controllable amount of saturable absorption introduced by a segmented contact are presented. With no absorption the laser is stable and has a linear output characteristic. Increasing of the saturable absorption by changing the pump current through the control segment causes the light output of the device to pulsate and to show bistable and hysteretical behavior. The introduction of a controllable amount of saturable absorption suggest the usefulness of this device in generating extremely short pulses, for example, by passive mode locking and as a bistable optical device.

High-speed digital modulation of ultralow threshold (<1 mA) GaAs single quantum well lasers without bias

GaAlAs buried heterostructure lasers with submilliampere threshold current fabricated from single quantum well wafers can be driven directly with logic level signals without any current bias. The switch‐on delay was measured to be <50 ps and no relaxation oscillation ringing was observed. These lasers permit fully on‐off multigigabit digital switching while at the same time obviating the need for bias monitoring and feedback control.

Direct modulation and active mode locking of ultrahigh speed GaAlAs lasers at frequencies up to 18 GHz

It is demonstrated that an ultrahigh speed window buried heterostructure GaAlAs laser fabricated on a semi‐insulating substrate can be used as a narrowband signal transmitter in the Ku band frequency range (12–20 GHz). The modulation efficiency can be increased over a limited bandwidth by a weak optical feedback. A stronger optical feedback enables one to actively mode lock the laser diode at a very high repetition rate up to 17.5 GHz, producing pulses ∼12 ps long.

Passive and active mode locking of a semiconductor laser without an external cavity

This letter describes the first attempt to passively and actively mode lock a discrete semiconductor laser, i.e., one not coupled to an external cavity. Beat notes of the longitudinal modes of a 1.97-mm-long GaAlAs laser have been observed at 17.7 GHz. The spectral width of the beat note was approximately 100 kHz. Stable passive mode locking has been observed under appropriate operating conditions. Active mode locking by an externally injected microwave signal was also achieved.

Self-sustained picosecond pulse generation in a GaAlAs laser at an electrically tunable repetition rate by optoelectronic feedback

We demonstrate that applying optoelectronic feedback to a high‐speed, self‐pulsing semiconductor laser is an effective and practical means of generating picosecond optical pulses (∼10–20 ps) at a very high repetition rate, between 1 to 5 GHz, which can be electrically tuned. The optical pulses are very stable both on a short term basis with a frequency stability of one part in 105, and on the long term basis as a result of the absence of critical optical alignment. This laser system is potentially very useful in high‐speed electro‐optic signal processing, optical multiplexing, or laser ranging.

Monolithic optoelectronic integration of a GaAlAs laser, a field‐effect transistor, and a photodiode

A low threshold buried heterostructure laser, a metal-semiconductor field-effect transistor, and a p-i-n photodiode have been integrated on a semi-insulating GaAs substrate. The circuit was operated as a rudimentary optical repeater. The gain bandwidth product of the repeater was measured to be 178 MHz.

Bistability and negative resistance in semiconductor lasers

Experimental results of a buried heterostructure laser with a segmented contact to achieve inhomogeneous gain are presented. Measurements reveal a negative differential resistance over the absorbing section. Depending on the source impedance of the dc current source driving the absorbing section, this negative resistance can lead to (i) bistability with a very large hysteresis in the light-current characteristic without self-pulsation or (ii) a small hysteresis with self-pulsations at microwave frequencies. An analysis, which includes the electrical part of the device, leads to an explanation of self-pulsations in inhomogeneously pumped lasers without having to rely on a sublinear gain dependence on injected carrier concentration.

The effect of lateral carrier diffusion on the modulation response of a semiconductor laser

The effect of lateral carrier diffusion upon the modulation characteristics of the semiconductor laser is investigated. A self‐consistent analysis of the spatially dependent rate equations is performed using a finite element model. The transverse junction stripe laser is treated as an example and a comparison is made between lateral carrier diffusion and spontaneous emission as damping mechanisms for the resonance peak. Experimental results bear out the conclusion that the relaxation resonance in this device is damped mainly by lateral carrier diffusion. In addition, a simple analytic result is presented which illustrates qualitatively the effect of lateral carrier diffusion upon such devices. The conclusion from this result is that lateral carrier diffusion serves to damp the relaxation resonance in the semiconductor laser quite well, but probably will not serve to improve the upper limit on modulation frequency as might have been suspected.

Direct modulation of semiconductor lasers at f>10 GHz by low-temperature operation

Using a 175‐μm‐long buried‐heterostructure laser fabricated on a semi‐insulating substrate operating at −50 °C, a direct amplitude modulation bandwidth in excess of 10 GHz has been achieved.

Reduction of the spectral linewidth of semiconductor lasers with quantum wire effects—Spectral properties of GaAlAs double heterostructure lasers in high magnetic fields

The spectral linewidth of a GaAlAs double heterostructure laser placed in a high magnetic field is measured at 190 K. It is found that the power-dependent spectral linewidth is reduced by a factor of 0.6 in a magnetic field of 19 T. This reduction is believed to result mainly from the reduction of the linewidth enhancement factor alpha due to a quasi-one-dimensional electronic system formed by the high magnetic field (i.e., by quantum wire effects).