Thomas L. Paoli

Gain spectra in GaAs double−heterostructure injection lasers

Gain spectra for GaAs double−heterostructure junction lasers have been obtained with high resolution. This is accomplished by using an automated data aquisition system to analyze the Fabry−Perot resonance modulation in the spontaneous emission spectra. For active regions doped with Ge at a level of 4×1017 cm−3, the gain in the TE polarization at a fixed wavelength increases linearly with current, below lasing threshold. However, the peak gain (at a variable wavelength) increases slightly faster than linearly with current. The photon energy at which gain is a maximum increases logarithmically with current. Gain in the TM polarization depicts the same general behavior as that for the TE case, except that it is slightly less than the TE gain. It is concluded that for this particular doping the spectral gain characteristics are intermediate between those for undoped and heavily doped active regions. Above the threshold for lasing in the TE mode the TE gain spectra are well saturated, with new fine details rev...

cw degradation at 300°K of GaAs double‐heterostructure junction lasers. II. Electronic gain

The rapid degradation at 300°K in the cw regenerative output of stripe‐geometry GaAs double‐heterostructure junction lasers is shown to be a result of the formation of a local optical absorber in the laser cavity. Gain measurements performed on diodes before and after degradation show that the optical loss within the cavity increases during degradation. By observing the (predominantly) spontaneous emission from the active region directly through the n‐GaAs substrate, it is confirmed that the increased loss is localized in a region where little or no spontaneous emission takes place at lasing energies. In such diodes, the internal radiative efficiency of the undegraded portion of the optical cavity shows a relatively small decrease compared to the external differential quantum efficiency. When the local absorber extends over a sufficient length of the cavity the electronic gain in the undegraded section is insufficient to overcome the loss and the device ceases to act as a regenerative optical oscillator. ...

Waveguiding in a stripe-geometry junction laser

Measurements of the magnitude and spatial distribution of the optical gain in a stripe-geometry junction laser are related to observations of the intensity profile along the junction plane of the same laser. We find that the width of the fundamental mode is in excellent agreement with a two-dimensional waveguide in which confinement along the junction plane is induced primarily by the optical-gain profile. The remainder of the confinement is consistent with a refractive-index profile established by a temperature distribution in the presence of defocusing by the free carriers. Within the two-dimensional model, we find no evidence of defocusing like that deduced previously for double-heterostructure lasers.

Changes in the optical properties of CW (AlGa)As junction lasers during accelerated aging

Changes occuring in selected optical properties of CW (AlGa)As double-heterostructure junction lasers during the initial stage of accelerated aging are reported. The characteristics selected for investigation are relevant to the operation of the laser in an optical communication system and include the current dependence of the lasing emission and the orthogonally polarized (nonlasing) emission, the symmetry of the emission with respect to the two output faces, the time stability of the light intensity, and the frequency of the internal laser resonance. In spite of the variability among the aging behaviors of individual lasers, characteristic changes did emerge as statistically significant. For example, a substantial decrease in the emission symmetry was commonly observed. A second important effect was the creation of a self-induced intensity pulsation, at a frequency typically between 300 and 600 MHz, in lasers which initially exhibited no intensity modulation. A statistical analysis of the data was made to correlate these instabilities with asymmetries in the lasing or nonlasing emission and/or changes in the emission symmetry. No evidence was found to support previously published conjectures that the pulsations are caused by the formation of dark-line defects or by axial nonuniformities in the pumping current.

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.

Nonlinearities in the emission characteristics of stripe-geometry (AlGa)As double-heterostructure junction lasers

Significant changes in the optical and electrical properties of stripe-geometry (AlGa)As double-heterostructure junction lasers have been observed to accompany a nonlinearity in the current dependence of the lasing emission. Over the nonlinear range, the optical field of the lasing emission shifts continuously with current toward one boundary of the active stripe. Simultaneously, effective gain saturation is lost and additional transverse modes are excited. The increasing gain is believed to occur primarily in spatial regions where the optical intensity is reduced as a result of the transverse motion of the field. Additional observations suggest that this transverse motion results from an interaction between the intense lasing field and the active medium. Consequently, gain depletion is discussed as a possible cause for the transverse instability. Implications of this model for a qualitative understanding of the improved output performance recently obtained from lasers with narrow stripes are also considered.

A new technique for measuring the thermal impedance of junction lasers

A new method is presented to measure the temperature of the optical cavity of a junction laser as a function of the average electrical power supplied to the device. The technique relies upon a null measurement of the exact wavelength of a single Fabry-Perot mode and therefore does not require a preliminary calibration measurement. Because of the small linewidth of the mode, very small (<0.2\degC) temperature differences are easily measured. The method is utilized to obtain the thermal impedance for several different types of bonded stripe-geometry junction lasers.

Saturable absorption effects in the self‐pulsing (AlGa)As junction laser

The steady‐state rate equations for a junction laser containing a saturable optical loss are analyzed in terms of parameters typical of contemporaory stripe‐geometry (AlGa)As lasers. The analysis reveals that under conditions which produce self‐induced intensity pulsations the steady‐state power output may also exhibit a bistability or hysteresis over a small range of current in the vicinity of threshold. Experimental observations made with (AlGa)As double‐heterostructure lasers reveal that a nearly discontinuous behavior in the laser’s output characteristic is, in fact, sometimes associated with the self‐induced pulsations.

STRIPE‐GEOMETRY DOUBLE HETEROSTRUCTURE JUNCTION LASERS: MODE STRUCTURE AND cw OPERATION ABOVE ROOM TEMPERATURE

Double heterostructure GaAs junction lasers have been operated continuously at temperatures up to 355°K by use of the advantageous thermal properties of the stripe‐geometry configuration. Spatial and spectral characteristics of the modes of these lasers are reported. For the direction perpendicular to the junction plane, these observations show that the field distributions are more symmetrical, the focusing is stronger, and operation in high‐order modes is more easily achieved than in homostructure (diffused) junction lasers. For the direction along the junction plane, no significant differences are found between double heterostructure and homostructure lasers.

GaAs–AlxGa1−xAs heterostructure laser with separate optical and carrier confinement

Heterostructure injection lasers in which the GaAs active region is the center layer of a five‐layer dielectric slab waveguide have been investigated. The GaAs active layer is bounded on each side by an AlxGa1−xAs layer to confine the carriers. The two outside layers which are AlyGa1−yAs(y > x) confine the optical field. Lasers with this structure have been fabricated and room‐temperature threshold current densities Jth300 as low as 650 A/cm2 have been obtained for 1‐mm cavity lengths. Differential quantum efficiencies ηD for these separate optical and carrier confinement heterostructure (SCH) lasers were higher than generally encountered for double‐heterostructure (DH) lasers with values as high as 65% for [inverted lazy s]300−μ−long cavities. The external quantum efficiency of several typical units was determined as a function of input current, and for one representative unit a maximum value of 39% was obtained at about four times Jth300. Emission in the fundamental TE mode was obtained for symmetrical ...