John D. Bruno

Self-mixing detector candidates for an FM/cw ladar architecture

The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARLs goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.

Power, efficiency, and thermal characteristics of type-II interband cascade lasers

High-performance mid-infrared type-II interband cascade lasers have been demonstrated under continuous-wave (CW) conditions with record-high wall-plug efficiencies (>14%) and output powers (>100 mW/facet) above 77 K. Device characteristics of these type-II interband cascade lasers are investigated systematically in terms of their output powers and efficiencies. Also, by comparing the temperature dependence of the threshold currents under pulsed and CW conditions, the thermal resistance and maximum heat sink temperature for CW operation are estimated for several mesa sizes. The limiting factors due to device heating for high-power/high-efficiency operation are identified and discussed in connection with device dimensions and packaging for the purpose of assessing further improvements.

Enhanced CW performance of the interband cascade laser using improved device fabrication

Continuous-wave (CW) operation of a mid-infrared type-II interband cascade (IC) laser has been demonstrated at temperatures up to 142 K by improving device processing and fabrication. Also, the IC laser exhibited record-high wall-plug efficiencies (/spl sim/18% at 60 K) with considerable CW output powers. An analysis of the thermal resistance partially explains the still low maximum CW operating temperature and suggests further potential for improvement with continued development of fabrication/packaging techniques.

Effects of the piezoelectric field on quantum-confined Stark effect in (111)B InGaAs quantum-well structure

The quantum-confined Stark effect is studied in a single In.15Ga.85As quantum well embedded within the intrinsic region of a (111)B GaAs p-i-n diode structure. An expression for the density of quasibound quantum well states is derived and used to determine the optical transition energies in the structure. Satisfactory agreement between experimental results and theoretical predictions are obtained only when an In.15Ga.85As piezoelectric constant of e14=−0.11(3)  C/m2 is used in the analysis. This value for e14 is 79% of what one would obtain from a linear interpolation of the binary e14s and is significantly different from results obtained by others. Comparisons are made with other work, and speculations are made regarding the cause for the differences.

Quantum-confined Stark effect modulator based on multiple triple-quantum wells

A GaAs/AlGaAs triple-quantum-well structure is designed to have its lowest energy excitonic transition located significantly above the GaAs band gap, while maintaining large Stark shifts. We present photocurrent measurements of absorption features found in two different Al0.3Ga0.7As p-i-n diode structures: one contains multiple GaAs coupled-triple-quantum wells, and the other, multiple Al0.6Ga0.94As quantum wells. Both systems are designed to have their absorption edges at the same energy. The photocurrent spectra are compared, and preliminary 810 nm modulator results are presented based on the multiple triple-quantum-well approach.

Mid-IR interband cascade lasers: progress toward high performance

Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we review our recent progress in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers . These semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 A/cm2 at 80 K) and power efficiency exceeding 14% were observed from mesa-stripe lasers when operated in cw mode. Also, these lasers were able to operate at temperatures up to approximately 252 K in pulsed mode and approximately 142 K in cw mode. We observed slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%, from devices at temperatures above 80 K. A peak output power of approximately 6 W/facet was observed from an IC laser at 80 K.

High-power mid-IR type-II interband cascade lasers

Type-II interband cascade (IC) lasers take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions for serially connected active regions. Here, we describe recent advances in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometers ; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 /A/cm2 at 80 K) and power efficiency exceeding 9% were observed from a mesa- stripe laser in cw operation. Also, these lasers were able to operate at temperatures up to 250 K in pulsed mode and 127 K in cw mode. We observed from several devices at temperatures above 80 K, slope efficiencies exceeding 1 W/A/facet, corresponding to a differential external quantum efficiency exceeding 600%. A peak optical output power of approximately 6 W/facet was observed from a type-II IC laser at 80 K.

Multiple triple-quantum-well active region for above-GaAs-bandgap reflection modulator

We have investigated a modulator structure based on GaAs/AlGaAs strongly coupled triple-quantum-wells (TQWs). Photocurrent measurements show sharper excitonic features with the TQW structure, while maintaining an equivalent Stark effect, compared to an alternate approach using multiple AlGaAs quantum wells and barriers with Al mole fractions of 0.06 and 0.3, respectively. With the goal of optimizing the speed of the device, we are currently investigating carrier dynamics in the TQW samples by means of picosecond laser pump/probe experiments. We have grown four samples by MBE containing identical intrinsic regions of multiple-TQWs, but with varying values of doping and/or thickness of the p-region for comparison. The results of picosecond pump/probe measurements that monitor the absorption recovery following an excitation pulse are compared to theoretical predictions of a model based on diffusive conduction of the carriers in the conductive layers, modified to include excess carrier transit times.

Mid-IR type-II interband cascade lasers with quantum efficiency exceeding 450%

Summary form only given. We obtain the highest peak power yet reported among any mid-IR semiconductor diode lasers at emission wavelengths beyond 3 /spl mu/m. Note also that the power values reported here have been corrected only for the transmission coefficients of the collection optics and not corrected for beam divergence. If the spatial divergence of the laser beam is taken into account, the actual emitted powers and differential external quantum efficiency (DEQE) would be considerably higher.

Characterizing electric fields in (111)B InGaAs quantum wells using electric field modulated photoluminescence and reflectance techniques

We have performed a series of electroreflectance, photoluminescence, and electric-field-modulated photoluminescence experiments to characterize the strain-induced electric fields in (111)B InGaAs/AlGaAs quantum well p-i-n diode structures. A 180° phase change in the lineshapes of electroreflectance spectra of these samples determines when the quantum well is biased to flatband. Using this bias and a depletion model for the diode, the polarization field in the quantum well can be determined. Contrary to expectations, this polarization field increases significantly with increasing temperature. In addition, at fixed temperature, the quantum well transition energies red-shift with increasing excitation intensity when excited by photons of energy higher than the lowest quantum well transition but lower than the AlGaAs diodes bandgap. When excited with photons of energy greater than the AlGaAs bandgap, the transition energy first red shifts then blue shifts with increasing excitation intensity.