A mid-IR ICL-based sensor for field measurements of ambient CH 4

Abstract

We are developing a mid-IR ICL-based sensor for field measurements of ambient CH₄. We describe some of the design considerations for this sensor. Our sensor uses a Type II Quantum Cascade Laser (or Interband Cascade Laser, ICL) operating near 3.3 μm to monitor a well-isolated line in the υ₃ fundamental band of CH₄. The ICL operates in cw mode at cryogenic temperature. The sensor consists of two major components, an optical breadboard containing the laser, transfer optics, sample cell, and detectors, and an instrumentation module containing power supplies and system control computer. Light from the laser is collimated using a reflective microscope objective and transported to a multipass cell via a simple optics train. The multipass cell provides an optical path of ~7 meters in an 0.25 m base path. The spectrometer uses TE-cooled InAs detectors along with our Balanced Ratiometric Detection. Our measured precision for CH₄ is 15 ppbv for a 60 sec integration time. We report on additional sensor characterization and data from recent field trials at two facilities maintained by the University of New Hampshire.