Interband cascade lasers with high CW power and brightness

Abstract

We report corrugated narrow-ridge interband cascade lasers emitting at λ ≈ 3.5 μm that have been fabricated using CH 4 /Cl 2 -and BCl 3 -based inductively coupled plasma reactive ion etch processes, with largely similar results from both types of etches. The highest brightness figure of merit was obtained at intermediate ridge width (28 μm), for which the maximum cw output power at T = 25 °C was 522 mW and the corresponding wallplug efficiency and beam quality factor were 10.3% and M 2 = 3.1, respectively. The high output power may be attributed to a 7-stage design that employs thicker separate confinement layers for lower internal loss. Keywords:Interband cascade laser, mid-infrared, cw output power, wallplug efficiency, high efficiency. INTRODUCTIONInterband cascade lasers (ICLs) 1-6 emitting in the midwave infrared spectral region combine the conduction-tovalence optical transitions of a conventional diode laser with the multiple active stages of a quantum cascade laser (QCL). Since the first room-temperature continuous wave (cw) operation of an ICL was achieved in 2008, 7 the maximum cw output powers (P max ) and wallplug efficiencies (WPEs) have advanced rapidly. 8,9 In 2012, an epitaxial-side-down-mounted ridge with width 25 μm, cavity length 4 mm, and corrugated sidewalls 10 emitted 305 mW cw, with a maximum WPE of 8.7% and beam quality factor of M 2 = 2.2. 11 A tapered ICL subsequently emitted more power (403 mW) with similar beam quality, for a higher net brightness. 12 While all of these previous narrowridge ICLs investigated by NRL have emitted within a standard spectral range of 3.7-3.9 μm, similar performance would have been expected at any wavelength between 3.2 μm and about 4.1 μm, based on the pulsed performance of broad-area devices fabricated from a wide variety of wafers. 6In parallel with the design improvements that have accounted in part for the recent advances in cw performance, other ongoing efforts have sought to refine the methods for processing ICL narrow ridges. 11,13 Whereas the protocols for fabricating GaAs-and InP-based optoelectronic devices have matured as a result of several decades of extensive commercial and research optimization activities, the corresponding methodologies for GaSb-based lasers are far less developed, besides being inherently more challenging due to such factors as much more rapid oxidation.A fundamental aspect of the narrow ridge processing is naturally the choice of etching method, which is constrained by the relevant material constituents as well as the etch depth. Many shorter-wavelength diode lasers benefit from weak index guiding, which can be achieved by etching to a relatively shallow depth that stops above the active quantum wells. However, excessive current spreading in an ICL dictates that the etch must extend completely through the top optical cladding and active regions. Due to both the wavelength scaling of the cladding thickness and the relatively thick injector regions contained within each active stage, this corresponds to a total depth of sev...

The Interband Cascade Laser

“…The ridges are typically defined by photolithography and reactive-ion etching. Although both methane-based and BCl 3 -based inductively coupled plasma (ICP) processes have been employed [102], most NRL devices to date have used the CH 4 -based ICP. The dry etching, which is designed to stop just below the active core of the device in the bottom GaSb SCL, is usually followed by cleaning with a phosphoric-acid-based wet etch.…”