José E. Ripper

Proton-bombardment formation of stripe-geometry heterostructure lasers for 300 K CW operation

A method of defining the active region of stripe-geometry junction lasers by proton-bombardment-induced high-resistivity layers is described. The method yields more reproducible mode patterns and lower threshold currents than the previously used oxide insulation. The improved lasers operated continuously at heat-sink temperatures up to 110°C.

Direct modulation of semiconductor lasers

Methods for direct modulation of semiconductor lasers are reviewed with the objective of indicating the advantages and limitations of each method. Techniques for producing amplitude, pulse, and frequency modulation of the optical wave are included. The modulation capabilities of present pulsed lasers are analyzed with special attention given to their operation at room temperature. In addition, several ways of producing analog position or width modulation of microwave-rate optical pulses are described, and the capabilities of optical frequency modulation by acoustic waves are reviewed. A new way of obtaining mode-locked optical pulses with a semiconductor laser is also suggested.

OPTICAL COUPLING OF ADJACENT STRIPE‐GEOMETRY JUNCTION LASERS

The observation of coupling and consequent coherent operation of two adjacent parallel stripe‐geometry GaAs lasers in a monolithic structure is reported. Coupling is obtained when the distance between the lasers is small enough to allow the optical field of one laser to penetrate into the active region of the other.

OPTICAL PULSES FROM cw GaAs INJECTION LASERS

Intensity pulsations of continuously operating GaAs injection lasers have been directly observed for the first time, with fast sampling techniques. The width of the self‐induced pulses has been measured to be approximately 390 psec at a repetition rate of 620 MHz. In addition, this pulsewidth has been reduced to less than 200 psec by locking the pulsations with an external modulation of the injection current.

SEMICONDUCTOR LASERS OPERATING CONTINUOUSLY IN THE ``VISIBLE'' AT ROOM TEMPERATURE

Continuous operation of AlxGa1−xAs–Aly Ga1−yAs–AlzGa1−zAs double heterostructure injection lasers at room temperature has been achieved with diodes whose lasing energy was as high as 1.61 eV (7730 A). This energy was achieved with y = 0.20, and at this energy the far-field radiation pattern was visible. In the pulsed mode, low thresholds have been achieved up to 7450 A (1.66 eV) using both broad-contact and stripe-geometry lasers. The lasing threshold remains relatively unchanged in the range y = 0 to 0.21.

OPTICAL SELF‐PULSING OF JUNCTION LASERS OPERATING CONTINUOUSLY AT ROOM TEMPERATURE

Pulsing of the output light intensity as a result of self‐induced second‐order mode locking is reported for double‐heterostructure (DH) stripe‐geometry junction lasers operating continuously at room temperature. In comparison with previously reported results for homostructure lasers operating at low temperatures, the self‐induced pulsing in DH lasers occurs at a lower frequency and over a greater range of current and temperature. These differences are attributed to the lower dispersion and lower Q of the laser modes due to the increased temperature and not to the difference in laser structure. In addition, new effects associated with transitions of the mode locking from one modal family to another with different transverse spatial distribution are described.

FREQUENCY PULLING AND PULSE POSITION MODULATION OF PULSING cw GaAs INJECTION LASERS

The frequency pulling and locking of intensity pulsations from continuously operating GaAs injection lasers have been studied by varying the frequency of the externally applied locking signal in the vicinity of the self‐induced pulse rate or one of its harmonics. The ability of the laser pulse rate to follow a rapidly varying locking signal has led to the first realization of optical pulse position modulation with microwave repetition rates. Modulation rates attainable with this effect are expected to be as high as one‐half the self‐induced pulse rate.

SELF‐STABILIZATION AND NARROWING OF OPTICAL PULSES FROM GaAs JUNCTION LASERS BY INJECTION CURRENT FEEDBACK

Self‐induced intensity pulsations of continously operating GaAs junction lasers have been frequency‐stabilized and narrowed by regenerative feedback of oscillations in the injection current. The spectral width of the pulse rate has been reduced by a factor of 20 to less than 30 kHz while the optical pulse width was reduced by at least a factor of 2 to less than 180 psec. These results suggest that a well‐stabilized source of short optical pulses can be constructed from GaAs injection lasers without external microwave circuitry.

Bistable operation of CW junction lasers due to saturable absorbing centers

The observation of bistable operation of CW GaAs junction lasers due to the introduction of saturable absorbing centers throughout the active medium of the laser is reported for the first time. Analysis shows that explanation of the observations requires an average differential quantum efficiency greater than one, thus confirming the presence of the saturable absorbers.

Observation of intrinsic quantum fluctuations in semiconductor lasers

The first experimental observation of the excitation of the natural resonance of a semiconductor laser by the quantum fluctuations intrinsic to the laser is reported. Such excitation and the resulting resonant-like peaks in the microwave noise spectrum of the laser intensity were initially predicted by McCumber and later calculated in detail for semiconductor lasers by Haug. In addition, we present experimental and theoretical results that show that high-level excitation of the resonance by internal or forced modulation of the population inversion lowers the resonant frequency due to the nonlinearity present in the rate equations. In the experiments to be described, intensity noise spectra of continuously operating GaAs injection lasers were observed at microwave frequencies with a high-speed photodiode and a microwave spectrum analyzer. Because of the low level of the photocurrent produced by these fluctuations, it was necessary to reduce the intrinsic noise level of the system by using phase-sensitive detection techniques. In this way, the following experimental observations have been made. First, a sharp peak in the intensity noise spectrum has been observed at currents I from 1.5 to 100 percent above threshold ( Ith ). Second, at constant heat-sink temperature, the frequency of the peak varies, with current as (I/I_{th} - 1)^{1/2} . Third, while the intensity fluctuations relative to the square of the laser intensity continuously decrease (by three orders of magnitude) with increasing current, the absolute value of the noise peak increases (by more than two orders of magnitude) to a maximum value that is maintained with further increases in current. Finally, the frequency of the noise peak at constant current level above threshold shows no variation with heat-sink temperatures between 80°K and 150°K. The above observations were made on lasers in which the resonance was not strongly excited by combination tones present in the active medium and consequently for which there were no deep intensity pulsations. For lasers in which the intensity spontaneously pulses, the resonance peak has also been observed at currents very near threshold. However, in these diodes, the frequency at the peak increases with current as predicted by the theory only over a small range near threshold. Beyond this range, the resonance is excited by the combination tones, causing the frequency to decrease to a minimum value before increasing farther as the current is increased. Such behavior can be qualitatively understood in terms of the decrease in the average inversion (and consequently a reduction of the resonant frequency), which accompanies the self-induced pulsing of the laser intensity. Computer calculations based on nonlinear rate equations have confirmed this behavior.