Mid-infrared microspectroscopic imaging with a quantum cascade laser

摘要

Conventional mid-infrared (mid-IR) Fourier transform infrared (FT-IR) spectroscopic imaging systems employ an incoherent globar source and achieve spectral contrast through interferometry. While this approach is suitable for many general applications, recent advancements in broadly tunable external cavity Quantum Cascade Lasers (QCL) offer new approaches to and new possibilities for mid-IR micro-spectroscopic imaging. While QCL-based devices have yet to achieve the wide spectral range generally employed by spectroscopists for molecular analyses, they are starting to be used for microscopy at discrete frequencies. Here, we present a discrete frequency IR (DFIR) microscope based on a QCL source and explore its utility for mid-IR imaging. In our prototype instrument, spectral contrast is achieved by tuning the QCL to bands in a narrow spectral region of interest. We demonstrate wide-field imaging employing a 128×128 pixel liquid nitrogen cooled mercury cadmium telluride (MCT) focal plane array (FPA) detector. The resulting images demonstrate successful imaging as well as several unique features due to coherence effects from the laser source. Here we discuss the effects of this coherence and compare our instrument to conventional mid-IR imaging instrumentation.