Ch. Harder

Direct amplitude modulation of short‐cavity GaAs lasers up to X‐band frequencies

Experimental and theoretical studies indicate that a high‐frequency laser with bandwidths up to X‐band frequencies (≳10 GHz) should be one having a short cavity with a window structure, and preferably operating at low temperatures. These designs would accomplish the task of shortening the photon lifetime, increasing the intrinsic optical gain, and increasing the internal photon density without inflicting mirror damage. A modulation bandwidth of >8 GHz has been achieved using a 120‐μm laser without any special window structure at room temperature.

Observation of relaxation resonance effects in the field spectrum of semiconductor lasers

Subsidiary maxima are observed in the field spectra of single mode semiconductor lasers. Measurements of their power dependence show they are linked to the relaxation resonance. We attribute these maxima to combined phase and amplitude fluctuations at the relaxation resonance. A theoretical calculation of the field spectrum using the results of a noise analysisincorporating carrier dynamics agrees very well with observations.

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.

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.

Picosecond Carrier Dynamics and Laser Action in Optically Pumped Buried Heterostructure Lasers

We report the observation of picosecond optical pulses and unusual pulse structures from optically pumped buried heterostructure semiconductor lasers. A model which considers the hot electron and hole energy distributions dynamically accounts well for our experimental findings. The results are relevant to the problem of the limiting response time of semiconductor lasers.

Monolithic integration of a GaAlAs buried-heterostructure laser and a bipolar phototransistor

A GaAlAs buried‐heterostructure laser has been monolithically integrated with a bipolar phototransistor. The heterojunction transistor was formed by the regrowth of the burying layers of the laser. Typical threshold current values for the lasers were 30 mA. Common‐emitter current gains for the phototransistor of 100–400 and light responsivity of 75 A/W (for wavelengths of 0.82 μm) at collector current levels of 15 mA were obtained.

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.

Dynamical switching characteristics of a bistable injection laser

The switching characteristics of a bistable injection laser with very large hysteresis is examined. Switch‐on delays are shown to exhibit a ’’critical’’ part and a ’’noncritical’’ part, both of which can be reduced by increasing the overdrive current. It is possible to obtain fairly fast switching time (<20 ns) with a strong overdrive. Nominal delays of 100–200 ns result under moderate overdrives. These long time scales are due to long carrier lifetimes in the carrier‐depleted absorption section, a property intrinsic to these bistable injection lasers.

Interaction of a bistable injection laser with an external optical cavity

Experimental results on interactions of a bistable laser with an external optical cavity are presented. Switching of a bistable injection laser can be done by varying the amount of optical feedback. The optical switching is accompanied by a switching of the voltage across the absorber section. This can be utilized in digital optical disk readout. A bistable laser with an antireflection coating on one facet is more suitable for this task. No pulsations can be observed in a bistable laser with optical feedback if the absorber section is biased with a constant current source; but when it is biased with a voltage source, pulsation occurs at the external cavity round trip frequency. This indicates that even though the intrinsic absorption of the semiconductor material does not saturate easier than the gain, the presence of such absorptions in GaAs lasers can still produce pulsations when the electrical aspect is taken into account.

Superluminescent damping of relaxation resonance in the modulation response of GaAs lasers

It is demonstrated experimentally that the intrinsic modulation response of injection lasers can be modified by reducing mirror reflectivities, which leads to suppression of relaxation oscillation resonance and a reduction of nonlinear distortions up to multi-GHz frequencies. A totally flat response with a 3-dB bandwidth of 5 GHz was obtained using antireflection coated buried heterostructure lasers fabricated on a semi-insulating substrate. Harmonic distortions were below 40 dB within the entire 3-dB bandwidth. These results are in accord with theoretical predictions based on an analysis which include the effects of superluminescence in the laser cavity.