Next-generation Infrared Focal Plane Arrays (IRFPAs) are demonstrating ever increasing frame rates, dynamic range,
and format size, while moving to smaller pitch arrays.1 These improvements in IRFPA performance and array format
have challenged the IRFPA test community to accurately and reliably test them in a Hardware-In-the-Loop environment
utilizing Infrared Scene Projector (IRSP) systems. The rapidly-evolving IR seeker and sensor technology has, in some
cases, surpassed the capabilities of existing IRSP technology.
To meet the demands of future IRFPA testing, Santa Barbara Infrared Inc. is developing an Infrared Light Emitting
Diode IRSP system. Design goals of the system include a peak radiance >2.0W/cm2/sr within the 3.0-5.0μm waveband,
maximum frame rates >240Hz, and >4million pixels within a form factor supported by pixel pitches ≤32μm. This paper
provides an overview of our current phase of development, system design considerations, and future development work.
Recent progress and state of GaSb based type-I lasers emitting in spectral range from 2 to 3.5 μm is reviewed. For lasers
emitting near 2 μm an optimization of waveguide core width and asymmetry allowed reduction of far field divergence
angle down to 40-50 degrees which is important for improving coupling efficiency to optical fiber. As emission
wavelength increases laser characteristics degrade due to insufficient hole confinement, increased Auger recombination
and deteriorated transport through the waveguide layer. While Auger recombination is thought to be an ultimate limiting
factor to the performance of these narrow bandgap interband lasers we demonstrate that continuous improvements in
laser characteristics are still possible by increasing hole confinement and optimizing transport properties of the
waveguide layer. We achieved 190, 170 and 50 mW of maximum CW power at 3.1, 3.2 and 3.32 μm wavelengths
respectively. These are the highest CW powers reported to date in this spectral range and constitute 2.5-fold
improvement compared to previously reported devices.
The paper describes the heterostructures and device output parameters of Type-I quantum-well (QW) laser diodes with
InGaAsSb active regions designed for room-temperature operation near 2.3 μm and 3.1 μm. For both designs decrease of
the threshold current density and increase of the room-temperature output power have been achieved with increase of the
QW depth for holes. For the 2.3 μm laser diodes, confinement of holes in the QW embedded into the AlGaAsSb
waveguide was improved with increase of the hole energy level with compressive strain. Arrays of 1-mm-long 100-μmwide
laser diode emitters with a fill-factor of 30 % have been fabricated. A quasi-CW (30 μs, 300 Hz) output power of
16.7 W from a 4-mm-wide array has been obtained with conductive cooling. For the laser diodes designed for roomtemperature
operation above 3 μm, the hole confinement was improved by lowering the valence band energy in the
waveguide. Two approached were implemented: one with increase of the Al composition, and another with utilization of
quinternary InAlGaAsSb waveguide with increased As composition compared to the conventional AlGaAsSb
waveguide. With the quinternary waveguide approach, a room-temperature CW output power in excess of 130 mW and
a threshold current as low as 0.6 A have been obtained at λ = 3 μm from 2-mm-long 100-μm-wide emitters.
High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time.
The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.
Gain in broad area mid-infrared diode W lasers ((lambda) =3- 3.1micrometers ) has been measured using lateral mode spatial filtering combined with the Hakki-Paoli approach. The internal optical loss of approximately equals 19cm-1 determined from the gain spectra was the same for devices with either 10- or 5-period active regions and nearly constant in the temperature range between 80 and 160K. Analysis of the differential gain and spontaneous emission spectra shows that the main contribution to the temperature dependence of the threshold current is Auger recombination, which dominates within almost the entire temperature range studied (80-160K).