Michael K. Connors

175 K continuous wave operation of InAsSb/InAlAsSb quantum‐well diode lasers emitting at 3.5 μm

Multiple quantum‐well diode lasers incorporating compressively strained InAs0.935Sb0.065 wells and tensile‐strained In0.15Al0.85As0.9Sb0.1 barriers are reported. These lasers, grown on InAs substrates by molecular beam epitaxy, have emission wavelengths between 3.2 and 3.55 μm. Broad‐stripe lasers have exhibited pulsed threshold current density as low as 30 A/cm2 at 80 K and the characteristic temperatures between 30 and 40 K. The maximum pulsed operating temperature is 225 K. Ridge‐waveguide lasers have cw threshold current of 12 mA at 100 K, and the maximum cw operating temperature is 175 K.

Quaternary InGaAsSb Thermophotovoltaic Diodes

InxGa1-xAsySb1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of nTPV = 19.7% and PD = 0.58 W/cm2, respectively, for a radiator temperature of Tradiator = 950 degC, diode temperature of Tdiode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are nTPV = 28% and PD = 0.85 W/cm2 at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (VOC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode VOC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that VOC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects

Active coherent beam combining of diode lasers

We have demonstrated active coherent beam combination (CBC) of up to 218 semiconductor amplifiers with 38.5 W cw output using up to eleven one-dimensional 21-element individually addressable diode amplifier arrays operating at 960 nm. The amplifier array elements are slab-coupled-optical-waveguide semiconductor amplifiers (SCOWAs) set up in a master-oscillator-power-amplifier configuration. Diffractive optical elements divide the master-oscillator beam to seed multiple arrays of SCOWAs. A SCOWA was phase actuated by adjusting the drive current to each element and controlled using a stochastic-parallel-gradient-descent (SPGD) algorithm for the active CBC. The SPGD is a hill-climbing algorithm that maximizes on-axis intensity in the far field, providing phase locking without needing a reference beam.

Monolithically series-interconnected GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic devices with an internal backsurface reflector formed by wafer bonding

GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic (TPV) cells were monolithically interconnected in series to build open-circuit voltage Voc. GaInAsSb epitaxial layers were transferred to GaAs by wafer bonding with SiOx/Ti/Au, which provides electrical isolation of individual cells and forms an internal backsurface reflector. This configuration is compatible with monolithic series interconnection of TPV cells; can mitigate the requirements of filters used for front-surface spectral control; and has the potential to improve TPV device performance. Wafer-bonded GaInAsSb TPV cells exhibit nearly linear voltage building. At a short-circuit current density of 0.4 A/cm2, Voc of a single TPV cell is 0.2 V, compared to 0.37 and 1.8 V for 2- and 10-junction series-interconnected TPV cells, respectively.

Single‐frequency GaInAsSb/AlGaAsSb quantum‐well ridge‐waveguide lasers emitting at 2.1 μm

Ridge‐waveguide lasers emitting at ∼2.1 μm have been fabricated from a GaInAsSb/AlGaAsSb quantum‐well heterostructure grown on a GaSb substrate by molecular beam epitaxy. The cw threshold current is as low as 29 mA at room temperature, and the maximum cw output power is 28 mW. The lasers operate in a single longitudinal mode which can be continuously tuned without mode hopping over 1.2 nm by changing the heatsink temperature and over 0.8 nm by changing the current.

High-power high-brightness GaInAsSb-AlGaAsSb tapered laser arrays with anamorphic collimating lenses emitting at 2.05 /spl mu/m

Linear arrays of GaInAsSb-AlGaAsSb tapered MQW lasers emitting at 2.05 /spl mu/m have been fabricated and operated with 1-ms current pulses. Peak power over 3 W was obtained for nine-element arrays at 18.5 A. Up to 1.7-W peak power, within a 65-mrad full-angle cone, was measured in the far field using anamorphic collimating lens arrays, fabricated by mass transport in GaP.

Single-mode instability in standing-wave lasers: The quantum cascade laser as a self-pumped parametric oscillator

We report the observation of a clear single-mode instability threshold in continuous-wave Fabry-Perot quantum cascade lasers (QCLs). The instability is characterized by the appearance of sidebands separated by tens of free spectral ranges (FSR) from the first lasing mode, at a pump current not much higher than the lasing threshold. As the current is increased, higher-order sidebands appear that preserve the initial spacing, and the spectra are suggestive of harmonically phase-locked waveforms. We present a theory of the instability that applies to all homogeneously broadened standing-wave lasers. The low instability threshold and the large sideband spacing can be explained by the combination of an unclamped, incoherent Lorentzian gain due to the population grating, and a coherent parametric gain caused by temporal population pulsations that changes the spectral gain line shape. The parametric term suppresses the gain of sidebands whose separation is much smaller than the reciprocal gain recovery time, while enhancing the gain of more distant sidebands. The large gain recovery frequency of the QCL compared to the FSR is essential to observe this parametric effect, which is responsible for the multiple-FSR sideband separation. We predict that by tuning the strength of the incoherent gain contribution, for example by engineering the modal overlap factors and the carrier diffusion, both amplitude-modulated (AM) or frequency-modulated emission can be achieved from QCLs. We provide initial evidence of an AM waveform emitted by a QCL with highly asymmetric facet reflectivities, thereby opening a promising route to ultrashort pulse generation in the mid-infrared. Together, the experiments and theory clarify a deep connection between parametric oscillation in optically pumped microresonators and the single-mode instability of lasers, tying together literature from the last 60 years.

MBE growth of high-power InAsSbInAlAsSb quantum-well diode lasers emitting at 3.5 μm

Abstract Molecular beam epitaxy (MBE) has been employed for the growth of strained quantum-well laser structures on InAs substrates. These lasers consist of compressively strained InAsSb wells, tensile-strained InAlAsSb barriers, and lattice-matched AlAsSb cladding layers. Broad-stripe lasers, with emission at wavelengths between 3.2 and 3.55 μm, have exhibited cw power of 215 mW/facet at 80 K, pulsed threshold current density as low as 30 A/cm 2 at 80 K, characteristic temperatures ( T 0 ) between 30 and 40 K, and maximum pulsed operating temperature of 225 K. Ridge-waveguide lasers have cw threshold current of 12 mA at 100 K, and a maximum cw operating temperature of 175 K. In this paper we will present some of the key issues regarding the MBE growth of such high-power lasers on InAs and discuss future directions for improved device performance.

Low-threshold GaAs/AlGaAs lasers grown on Si by organometallic vapor phase epitaxy

Low‐threshold double‐heterostructure ridge‐waveguide lasers have been fabricated in GaAs/AlGaAs layers grown on Si by organometallic vapor phase epitaxy. The pulsed threshold current of the best ridge‐waveguide laser is 50 mA, with differential quantum efficiency of about 9% per facet. Broad‐area lasers fabricated on the same wafer have pulsed threshold current densities as low as 300 A/cm2 .

High efficiency coherent beam combining of semiconductor optical amplifiers

We demonstrate 40 W coherently combined output power in a single diffraction-limited beam from a one-dimensional 47-element array of angled-facet slab-coupled optical waveguide amplifiers at 1064 nm. The output from each emitter was collimated and overlapped onto a diffractive optical element combiner using a common transform lens. Phase locking was achieved via active feedback on each amplifiers drive current to maximize the power in the combined beam. The combining efficiency at all current levels was nearly constant at 87%.