M. Mittelstein

Second quantized state lasing of a current pumped single quantum well laser

Newly observed features of quantum well lasers are presented and explained with the aid of a simple model. These involve lasing with gain contributions not only from the fundamental (n=1) state, but simultaneously from the second quantized (n=2) state as well. Experimental data for current pumped GaAlAs/GaAs single quantum well lasers are presented. Very high resonator losses (≳100 cm−1) force the lasers to augment their gain with major contributions from the second quantized state. The main signature of n=2 lasing, a sudden and large increase in the lasing photon energy, is observed and explained by the theory.

Enhancement of modulation bandwidth in InGaAs strained‐layer single quantum well lasers

It is shown that the unique properties of strained‐layer quantum well lasers can be identified by measuring the relaxation oscillation frequency as a function of optical gain. These measurements are insensitive to effects due to nonradiative recombinations and leakage currents, which can mask the beneficial effects in terms of a lower threshold current due to a reduced hole mass in strained quantum wells. The conclusion, both theoretically and experimentally, is that strained‐layer quantum well lasers have a higher differential gain but saturate at a lower gain level as compared to regular quantum well lasers. As a consequence, for a strained single quantum well, slightly higher relaxation oscillation frequency results, but only for certain limited ranges of device parameters. A multiple strained‐layer quantum well can in theory take better advantage of the higher differential gain.

Self-stabilized nonlinear lateral modes of broad area lasers

The lateral modes of broad area lasers are investigated theoretically. The nonlinear interaction between optical field and effective refractive index leads to a saturable nonlinearity in the governing field equation, so that self-modulated solutions are found to be stable with increased current injection above saturation intensity. We derive approximate analytical solutions for traveling wave fields within the broad area laser. The field amplitude consists of a small ripple superimposed on a large dc value. Matching fields at the boundary determines the modulation depth and imparts an overall phase curvature to the traveling wave mode. There are multiple lateral modes for a given set of operating conditions, and modes with successively more lobes in the ripple have greater overall phase curvature. In contrast to the linear problem, several lateral modes can achieve the same modal gain, for a given injected current density, by saturating the gain to different extent. Thus, these modes would exhibit slightly different optical powers.

Unstable resonator cavity semiconductor lasers

GaAs heterostructure lasers with unstable resonator cavities were demonstrated for the first time with both curved mirrors fabricated by etching. Typical output powers of 0.35 W were observed in a stable, highly coherent lateral mode. The laser operated stably in a single longitudinal mode over a large range of injection currents. The external quantum efficiency was 70% of that of a similar laser with both mirror facets cleaved implying good output coupling of the energy from the entire region.

Broadband tunability of gain‐flattened quantum well semiconductor lasers with an external grating

Quantum well lasers are shown to exhibit flattened broadband gain spectra at a particular pumping condition. The gain requirement for a grating-tuned external cavity configuration is examined and applied to a semiconductor quantum well laser with an optimized length of gain region. The predicted very broadband tunability of quantum well lasers is confirmed experimentally by grating-tuning of uncoated lasers over 85 nm, with single longitudinal mode output power exceeding 200 mW.

Lateral coherence properties of broad‐area semiconductor quantum well lasers

The lateral coherence of broad-area lasers fabricated from a GaAs/GaAlAs graded index waveguide separate confinement and single quantum well heterostructure grown by molecular-beam epitaxy was investigated. These lasers exhibit a high degree of coherence along the junction plane, thus producing a stable and very narrow far field intensity distribution.

Spectral and dynamic characteristics of buried‐heterostructure single quantum well (Al,Ga)As lasers

We demonstrate that, as predicted, (Al,Ga)As single quantum well (SQW) lasers have substantially narrower spectral linewidths than bulk double-heterostructure lasers. We have observed a further major reduction (>3×) in the linewidth of these SQW lasers when the facet reflectivities are enhanced. This observation is explained theoretically on the basis of the very low losses in coated SQW lasers and the value of the spontaneous emission factor at low threshold currents. We also report on the modulation frequency response parameter of these SQW lasers.

Coherence and focusing properties of unstable resonator semiconductor lasers

The emission characteristics of unstable resonator semiconductor lasers were measured. The output of an 80-µm-wide laser consists of a diverging beam with a virtual source 5 µm wide located 50 µm behind the laser facet. A high degree of spatial coherence of the laser output was measured, indicating single lateral mode operation for currents I ~< 3 Ith.

Tunable active chirped-corrugation waveguide filters

A novel tunable semiconductor waveguide reflection filter is proposed and analyzed. The filter is based on spatially selective gain pumping of a chirped‐corrugation waveguide. This active chirped‐corrugation waveguide filter (ACF) is considered for monolithic broadband tuning of semiconductor lasers.

Half‐ring geometry quantum well GaAlAs lasers

Half‐ring geometry single quantum well GaAlAs lasers have been fabricated. These lasers rely on a single cleave to obtain both resonator end reflectors. Using a buried‐heterostructure waveguide for lateral confinement and a high reflectivity facet coating, threshold currents as low as 14.5 mA and a frequency response extending to 6.5 GHz have been achieved.