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Lasers & Sources

Interband cascade lasers are maturing into practical mid-infrared sources

Design and growth improvements have reduced thresholds and efficiencies for mid-infrared interband cascade lasers increasing the maximum continuous-wave operating temperature to 288K for emission at 4.1 microns.
11 December 2007, SPIE Newsroom. DOI: 10.1117/2.1200712.0937

Semiconductor diode lasers operating at midwave-infrared (mid-IR) wavelengths between 3 and 5μm are needed for a variety of industrial and military applications. Trace concentrations of chemical and biological agents can be detected if the laser emits a narrow range of wavelengths and operates at a temperature accessible with a compact and inexpensive thermoelectric (TE) cooler (> about 250K). High-power semiconductor lasers are also expected to become the next-generation jamming sources for infrared countermeasures against heat-seeking missiles.

Conventional interband diode lasers have never reached room-temperature continuous wave (CW) operation at a wavelength beyond λ ≈ 3.1μm. Further extension of the wavelength becomes challenging due to high strains in the device's active region. On the long-wavelength side, considerable strides have been made by the quantum cascade laser (QCL), which generates light via electron intersubband transitions. These take place within conduction-band energy states that are normally empty unless pumped with current. Nevertheless, to date there has been only one report of a QCL with λ<4μm operating CW to room temperature.1

We have been working to develop a promising alternative ‘hybrid’ architecture, the so-called interband cascade laser (ICL).2 Like a conventional interband diode, the ICL generates light via the radiative recombination of electrons and holes. However, it also resembles the QCL in its cascading of multiple active stages, each of which can produce light independently. Any electron injected into the device can emit a photon at each stage, after which it is re-injected into the next stage for further photon emission.

The active regions of our ICLs employ the type-II (spatially indirect) ‘W’ geometry, which takes its name from the profile of the conduction band energy variation with position: each stage contains a single GaInSb hole quantum well (high) surrounded by two InAs electron wells (low), which are in turn surrounded by high AlSb barriers. Ongoing design improvements to the active/injector/clad (the set of three regions that emit light, transfer electrons between active regions, and confine the generated light, respectively) and molecular-beam epitaxial growth of the semiconductor in precise atomic layers have induced higher internal efficiencies and lower internal losses. Consequently, lasing can occur at lower threshold current densities (jth). Recent results at T=78K include >1W of CW power from a broad stripe (>100μm wide electrical contact to the device),3 260mW into a good beam from a narrow ridge (∼eq10μm wide stripe defined by etched sidewalls),4 and wallplug efficiency—optical output power divided by the electrical input power—up to 27%.5 At T=120K, a distributed-feedback device (that uses a grating for wavelength-sensitive feedback to produce a narrower range of wavelengths) emitted up to 41mW of CW power into a single spectral mode.6

Figure 1 illustrates CW threshold current densities vs. temperature for two recent 10-stage narrow-ridge ICLs emitting at short (3.5μm at 78K to 4.1μm at Tmax, blue points) and longer (4.6μm at 78K to 5.1μm at Tmax, red points) mid-IR wavelengths. The maximum CW operating temperature for the short-λ device was 288K, which is considerably beyond the previous best7 of 269K established in 2006 by a Naval Research Laboratory ICL. The longer-λ device operated CW to 229K, which is > 60K higher than the best previous result for an interband laser emitting beyond 5 μm.8 At 78K that device lased at extremely low current density (<5Acm−2), and its maximum CW output power exceeded 80mW.

Figure 1. CW threshold current densities vs. temperature for two recent 10-stage ICLs with longer (red) and shorter (blue and green) emission wavelengths. In both the blue and red cases, the ridge width was 11μm, the cavity length was 3mm, and one facet had a high-reflection coating.

The efficiencies of recent ICLs have also improved. Figure 2 plots pulsed-mode differential slope efficiencies for the best 5-stage ICL (from 2006), the best 10-stage ICL grown in 2006, and the recent short-λ device from Figure 1. Apart from its steeper slope at low T (650mWA−1 at 78K as compared to 450mWA−1 for the 2006 device), the recent ICL also degrades much more gradually with temperature. In fact its 300K efficiency of 180mWA−1 is nearly as high as that for the 5-stage device at 78K. The recent device's slope still exceeds 80mWA−1 at 350K.

Figure 2. Temperature dependencies of the differential slope efficiencies per facet for the best 5-stage (green) and 10-stage (red) ICLs grown in 2006, and the short-λ ICL grown in 2007 (blue). All of the devices were broad-area ridges with 2 mm cavity lengths and no facet coatings.

In summary, mid-IR interband cascade lasers are now capable of operating in CW mode at TE-cooler temperatures. While the present results indicate only a moderate degradation with increasing wavelength, they nonetheless imply the potential for even better performance once designs have been optimized for the λ=3–4μm spectral band at high T. We are now developing distributed-feedback lasers for output with high spectral purity at TE-cooler temperatures. We are also pursuing other strategies to produce multi-Watt CW powers while maintaining good beam quality.

William Bewley, Chadwick Canedy, Chulsoo Kim, Mijin Kim, Ryan Lindle, Jill Nolde, Diane Larrabee, Igor Vurgaftman, Jerry Meyer 
Naval Research Laboratory
Washington, DC 

Jerry R. Meyer received his PhD in physics from Brown University, Providence, RI in 1977. Since then he has carried out basic and applied research in the Optical Sciences Division of the Naval Research Laboratory, where he is head of the quantum optoelectronics section. He co-invented the type-II ‘W’ laser and Quantitative Mobility Spectrum Analysis. He is a fellow of the Optical Society of America and the American Physical Society, and has co-authored more than 260 refereed journal articles and 12 book chapters. His work has given rise to 20 patents (either awarded or pending), and he has delivered 80 invited conference presentations.