J. Abell

Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption

The interband cascade laser differs from any other class of semiconductor laser, conventional or cascaded, in that most of the carriers producing population inversion are generated internally, at semimetallic interfaces within each stage of the active region. Here we present simulations demonstrating that all previous interband cascade laser performance has suffered from a significant imbalance of electron and hole densities in the active wells. We further confirm experimentally that correcting this imbalance with relatively heavy n-type doping in the electron injectors substantially reduces the threshold current and power densities relative to all earlier devices. At room temperature, the redesigned devices require nearly two orders of magnitude less input power to operate in continuous-wave mode than the quantum cascade laser. The interband cascade laser is consequently the most attractive option for gas sensing and other spectroscopic applications requiring low output power and minimum heat dissipation at wavelengths extending from 3 μm to beyond 6 μm.

Interband cascade laser emitting at λ=3.75μm in continuous wave above room temperature

We report a five-stage interband cascade laser that operates at λ=3.75μm in cw mode up to a maximum temperature of 319K. With gold electroplating, epitaxial-side-up mounting, and one facet coated for high reflectivity, a 3mm×9.2μm ridge emits over 10mW of cw power at 300K.

Mid-infrared interband cascade lasers operating at ambient temperatures

We discuss the state-of-the-art performance of interband cascade lasers emitting in the 3?5??m spectral band. Broad-area devices with five active stages display pulsed threshold current densities as low as 400?A?cm?2 at room temperature. Auger decay rates are extracted from the analysis of threshold current densities and differential slope efficiencies of nearly 30 lasers, and found to be significantly lower than was anticipated based on prior information. New designs also produce ICLs with room-temperature internal losses as low as ?6?cm?1. The combination of these advances with improvements to the processing of narrow ridges has led to the fabrication of a 4.4-?m-wide ridge emitting at 3.7??m that lased to 335?K in continuous mode. This is the highest continuous-wave (cw) operating temperature for any semiconductor laser in the 3.0?4.6??m spectral range. A 10-?m-wide ridge with high-reflection and anti-reflection facet coatings produced up to 59?mW of cw power at 298?K, and displayed a maximum wall-plug efficiency of 3.4%.

Interband Cascade Lasers With Low Threshold Powers and High Output Powers

The midwave infrared interband cascade laser (ICL) can operate at threshold power densities 30 times lower than those of the quantum cascade laser. This is ultimately attributable to the much longer interband carrier lifetime, rather than to specifics of the cavity dimensions and mirror reflectivities. The ICL is therefore an attractive candidate for insertion into the portable, battery-powered chemical sensors now being developed for this spectral region. We review the characteristics of ICLs operating at wavelengths from 2.9 to 5.5 μm, and show that their Auger coefficients vary by less than a factor of 3 throughout this range. Consequently, the ICL performance degrades only modestly with increasing wavelength. We report that an epitaxial-side-down-mounted ICL ridge of width 30 μm and λ = 3.7 μm emits more than 300 mW of continuous wave (CW) output power at room temperature with M2 ≤ 3.1. A distributed-feedback ICL with a fourth-order grating etched into its corrugated sidewalls produces 55 mW of CW power in a single spectral mode at T = 25 °C.

Mid-IR Type-II Interband Cascade Lasers

The interband cascade laser (ICL) concept provides robust and efficient emission in the midwave infrared spectral band. While the geometry is somewhat analogous to that of a quantum cascade laser employing intersubband transitions, the ICL implementation exploits the type-II band alignment of the GaSb-based material system. A semimetallic band overlap at the boundary between the electron and hole injector regions automatically generates carriers with densities tunable by quantum confinement. Electrical injection then replenishes the carriers already present rather than creating the population inversion. In this paper, we describe and analyze the physical principles governing ICL operation, and discuss specific modifications to the active region, electron injector, hole injector, and waveguide designs that demonstrably improve the performance. The pulsed I- V and L-I characteristics of devices processed from over 50 wafers provide a statistically meaningful confirmation of the established trends.

Corrugated-sidewall interband cascade lasers with single-mode midwave-infrared emission at room temperature

We report interband cascade lasers operating in a single spectral mode (λ≈3.6 μm) at −5–30 °C. A corrugated pattern etched into both sidewalls of the 6- and 9-μm-wide ridges serves to suppress higher-order lateral modes by increasing their loss, and also provides a fourth-order distributed-feedback grating for longitudinal mode selection. Despite the grating’s weak coupling strength, the 9 μm ridge produced up to 12 mW per facet of single-mode cw output power at 25 °C, with a side-mode suppression ratio of >30 dB.

Continuous-wave interband cascade lasers operating above room temperature at λ = 4.7-5.6 μm

We have substantially improved the performance of interband cascade lasers emitting at λ = 4.7 and 5.6 μm, by applying the recently-pioneered approach of heavily doping the injector regions to rebalance the electron and hole concentrations in the active quantum wells. Ridges of ≈10 μm width, 4 mm length, and high-reflectivity back facets achieve maximum continuous wave operating temperatures of 60°C and 48°C, respectively. The threshold power density of ≈1 kW/cm2 at T = 25°C is over an order of magnitude lower than for state-of-the-art quantum cascade lasers emitting in this spectral range.

High-power room-temperature continuous-wave mid-infrared interband cascade lasers

We demonstrate cw output powers >290 mW into a nearly diffraction-limited (M² ≈2.2) output beam from an interband cascade laser operating at λ = 3.6-3.7 μm at room temperature. The interband cascade laser was designed for nearly equal electron and hole populations in the active region with heavy electron-injector doping, and was processed into narrow ridges mounted epitaxial side down on a copper heat sink. A 15.7-μm-wide, 4-mm-long ridge with the back facet coated for high reflection (HR) and an anti-reflection-coated front facet produced 253 mW of cw output power at T = 25°C into a beam with M² ≈2.7. Furthermore, corrugating the sidewalls of the ridge leads to a 20% improvement in the brightness. A 15.7-μm-wide, 0.5-mm-long ridge with an HR-coated back facet and an uncoated front facet exhibited a maximum cw wall-plug efficiency of nearly 15% at room temperature.

Pulsed and CW performance of 7-stage interband cascade lasers.

We report a narrow-ridge interband cascade laser emitting at λ ≈3.5 μm that produces up to 592 mW of cw power with a wallplug efficiency of 10.1% and beam quality factor of M(2) = 3.7 at T = 25 °C. A pulsed cavity length study of broad-area lasers from the same wafer confirms that the 7-stage structure with thicker separate confinement layers has a reduced internal loss of ≈3 cm(-1). More generally, devices from a large number of wafers with similar 7-stage designs and wavelengths spanning 2.95-4.7 μm exhibit consistently higher pulsed external differential quantum efficiencies than earlier state-of-the-art ICLs.

Mid-infrared distributed-feedback interband cascade lasers with continuous-wave single-mode emission to 80 °C

We report continuous-wave (cw) distributed-feedback interband cascade lasers operating in a single spectral mode (λ = 3.7–3.8 μm) at temperatures between 20 and 80 °C. The first-order gratings were realized by patterning high-index germanium layers on top of narrow ridges with relatively thin top claddings. One device generated over 27 mW of cw single-mode output at 40 °C, with a side-mode-suppression ratio >30 dB, while at 80 °C it still emitted >1 mW. At 20 °C, a second device lased in a single spectral mode with <100 mW of drive power. The tuning range was 21.5 nm with temperature and 10 nm with current.