The Design and Performance Characterization of a Tunable External Cavity Quantum Cascade Laser utilizing Thermo-optically Tuned Thin Film Filters

Quantum cascade lasers (QCLs) and Interband cascade lasers (ICLs) are promising new mid-IR sources for spectroscopic applications. Desirable characteristics include extremely high brightness, broad emission, very high resolution, compact size, and modest power consumption. For most spectroscopic applications, it is necessary to tune QCLs over a broad emission wavelength range. The conventional approach for broad tuning is to use an external cavity (EC) which incorporates a mechanically tuned diffraction grating within the laser cavity. In this paper we will describe an alternative approach to EC-QCL tuning which utilizes miniature, thermally tuned, MEMS fabricated filters, allowing for a very compact, simple, mechanically stable package with no moving parts. The system is well suited for discrete measurements at multiple wavelengths as needed by many of the industrial spectroscopic analyzers in use today. An accuracy of 0.02 cm(-1) over the 50 cm(-1) range of the test laser and a precision of 0.002 cm(-1) over a 15 cm(-1) scan has been demonstrated. High resolution mode hop free CW scanning of a 0.5 cm(-1) range at a scan rate of 200 Hz with a wavelength precision of 0.002 cm(-1) has also been demonstrated. This makes the design an attractive alternative to current Distributed feedback (DFB) QCLs for high resolution gas phase measurements due to the added advantage of broad tunability for the detection of multiple gases, and the capability to select multiple gas lines of different intensity to extend the dynamic range.