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The utilization of the broad gain bandwidth available from quantum cascade (QC) devices is considered. The performance of homogeneous and heterogeneous QC gain media is explored in an externalcavity configuration. Paradigms for realizing fixed wavelength or broad tuning performance of QC devices are considered. C 2010 Society of PhotoOptical Instrumentation Engineers. IntroductionQuantum cascade (QC) devices have become the standard gain media for lasers where high power, room-temperature operation, and continuous wavelength coverage are required in the midinfrared region of 3 to 12 μm. Recently, a tuning range of 432 cm -1 was demonstrated 1 for a QC device run in external-cavity mode. Such a broad gain bandwidth is comparable to results achieved with other semiconductor gain media: For example, 596 cm -1 of tuning has been demonstrated in near-IR diodes, 2 and 444 cm -1 in cryogenically cooled mid-IR Pb-salt diodes. 3 Utilizing the broad gain bandwidth of QC devices is an important consideration in the commercialization of QC-based lasers.Broad gain bandwidth in a gain medium offers potential advantages in the manufacture of lasers. For fixedwavelength lasers, a larger offering of wavelengths can be made without having to create multiple gain media. 4 Broad gain bandwidth can be converted into tuning range to access wider spectral ranges. 5 In both cases, however, a method is needed to restrict the laser operation to a single mode or subset of modes that are useful for the application of interest.Distributed feedback (DFB) structures have been integrated into the QC waveguide to restrict lasing to a narrow wavelength range. 6 This approach has the advantage that an extremely compact single-mode laser can be made with narrow linewidth and low frequency noise. 7 DFB lasers can be used in an intrapulse mode to perform spectroscopy of rotationally resolved gas molecules over a few inverse centimeters. 8 Ultimately, however, DFB QC lasers can only realize broader tuning by changing temperature, which limits their effective tuning range to less than 20 cm -1 , not nearly enough to utilize the full gain bandwidth available in QC devices. Moreover, from a manufacturing standpoint, different DFB QC devices have to be stocked for each fixed wavelength, negating the advantage of broad gain bandwidth in manufacturing.An alternative method, which allows for manufacturing settability of wavelength and broad tuning if required, is an external-cavity configuration. External-cavity QC lasers have been successfully employed to realize both narrow linewidth and broad tuning. 9, 10 A typical external cavity arrangement with grating feedback in Littrow configuration is shown in

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