Gregory C. Dente

High power, nearly diffraction-limited output from a semiconductor laser with an unstable resonator

The authors describe an on-the-chip design of a nearly diffraction-limited broad-area semiconductor diode laser. The device achieved single lateral mode operation as unstable resonators with magnifications between two and three. The unstable resonators were realized by focused ion beam (FIB) micromachining a diverging mirror at one of the outcoupling facets. The modeling efforts agree well with experimental data and show that an optimum device design exists in which stable nearly diffraction-limited operation is predicted for up to six times threshold. This unstable resonator design has achieved, experimentally, the highest diffraction limited power and best external differential efficiency ever reported for any broad-area device with a curved facet. >

Pseudopotential methods for superlattices: Applications to mid-infrared semiconductor lasers

Many mid-infrared semiconductor laser sources are now being developed with superlattice active regions. Calculations of gain, index of refraction, and intervalence subband absorption for these laser materials require accurate subband energies, wave functions, and radiative matrix elements. We have recently begun using a solution method based on the empirical pseudopotential method (EPM). This method shows particular strength in analyzing structures with short periods or thin layers, for which the standard method, based on k⋅p perturbation theory and the envelope function approximation, may be problematical. We will describe the EPM applied to bulk solids and then demonstrate our direct generalization of the method for applications to superlattice structures. Calculations for recently developed mid-infrared semiconductor lasers using type-II superlattice active regions will be used to illustrate the method.

Chaos in the coherence collapse of semiconductor lasers

The observation of a quasiperiodic route to chaos in the coherence collapse of a single-mode semiconductor laser subjected to back-reflections from an external cavity is discussed. Also, a simple deterministic model that correlates well with the data is presented. >

Filament formation in semiconductor laser gain regions

We have linearized the equations for propagation of the beam of light in a semiconductor optical amplifier about an operating point and have derived the rate of growth of small sinusoidal perturbations of the phase and modulus of the complex field amplitude. The perturbations grow if the spatial frequency is below a critical value that depends on the intensity of the field at the operating point. For spatial frequencies above the critical value, the perturbations die out. The critical spatial frequency decreases as the intensity increases above a certain value. In other words, the tendency to filament becomes weaker as the intensity increases above a certain value. Computer‐generated solutions of the propagation and gain equations are included to illustrate the growth of filaments as the plane‐wave intensity changes in an amplifier.

Spectral blueshift and improved luminescent properties with increasing GaSb layer thickness in InAs–GaSb type-II superlattices

We describe the photoluminescence spectroscopy (PL) and Fourier transform infrared absorbance spectroscopy characterization of a large set of InAs/GaSb type-II strained layer superlattice (SLS) samples. The samples are designed to probe the effect of GaSb layer thickness on the optical properties of the SLS, while the InAs-layer thickness is held fixed. As the GaSb layer thickness is increased, we observe a spectral blue shift of the PL peaks that is accompanied by an increase in intensity, narrower linewidths, and a large reduction in the temperature sensitivity of the luminescence. These effects occur despite a significant reduction in the electron-hole wave function overlap as the GaSb layer thickness is increased. In addition, we compare the results of empirical pseudopotential model (EPM) calculations to the observed blueshift of the primary band gap. The EPM calculations are found to be in very good agreement with the observed data.

Low confinement factors for suppressed filaments in semiconductor lasers

Filament formation through self-focusing of light in semiconductor lasers leads to beam quality degradation, as well as lifetime and facet degradation. In this paper, we rederive an expression for the growth rate of sinusoidal perturbations, or filaments superimposed on the steady-state field in a semiconductor laser. We note that while the mode gain decreases with the confinement factor in a linear manner, the gain for filaments decreases far more rapidly. This agrees with recent observations of improved beam quality in broad-area semiconductor lasers with lowered confinement factors. Finally, we describe design rules for low-confinement-factor semiconductor lasers offering low-filamentation and greatly improved beam quality.

Absorbance spectroscopy and identification of valence subband transitions in type-II InAs/GaSb superlattices

We describe the molecular-beam epitaxy growth, as well as both the structural and optical characterization of a set of InAs/GaSb type-II strained-layer superlattice samples, in which the GaSb layer thickness is systematically increased. Absorbance spectroscopy measurements show well-defined features associated with transitions from the various valence subbands to the lowest conduction subband, and also a significant blueshift of the band edge when the GaSb layers thickness is increased. Empirical pseudopotential method calculations are shown to successfully predict the blueshift and help identify the higher-energy transitions.

Modeling broad-area semiconductor optical amplifiers

The authors report on the wave optics modeling of broad-area semiconductor amplifiers used in a double-pass reflective configuration. The results show excellent agreement with recent reports of 2.5 and 12 W of nearly diffraction-limited output from such systems. Several design issues that must be considered in a practical laser communication system based on this class of amplifier are discussed. >

Matrix methods for bare resonator eigenvalue analysis

Bare resonator eigenvalues have traditionally been calculated using Fox and Li iterative techniques or the Prony method presented by Siegman and Miller. A theoretical framework for bare resonator eigenvalue analysis is presented. Several new methods are given and compared with the Prony method.

Saturation effects in semiconductor lasers

This paper describes a theory for a semiconductor active medium interacting with a laser field. In a semiconductor laser, the charge carrier transitions are inhomogenously broadened, and electron-electron and electron-phonon collisions tend to dephase the laser transitions and maintain thermal equilibrium among the carriers. These properties cause semiconductor lasers to frequency tune as though they are inhomogeneously broadened and to saturate as though they are homogeneously broadened. A theory that contains these two aspects of semiconductor laser behavior is presented. From it, we are able to calculate the loaded gain, efficiency, intensity, and carrier-induced refractive index of a semiconductor active medium.