Cavity-enhanced optical feedback-assisted photo-acoustic spectroscopy with a 10.4 urn external cavity quantum cascade laser

摘要

An ultra-sensitive photo-acoustic spectrometer using a 10.4 μm broadly tunable mid-IR external cavity quantum cascade laser (EC-QCL) coupled with optical feedback to an optical power buildup cavity with high reflectivity mirrors was developed and tested. A laser optical power buildup factor of 181 was achieved, which corresponds to an intra-cavity power of 9.6 W at a wavelength of 10.4 μm. With a photo-acoustic resonance cell placed inside the cavity this resulted in the noise-equivalent absorption coefficient of 1.9 x 10~(-10) cm~(-1) Hz~(-1/2), and a normalized noise-equivalent absorption of 1.1 × 10~(-11) cm~(-1) W Hz~(-1/2). A novel photo-acoustic signal normalization technique makes the photo-acoustic spectrometer's response immune to changes and drifts in the EC-QCL excitation power, EC-QCL to cavity coupling efficiency and cavity mirrors aging and contamination. An automatic lock of the EC-QCL to the cavity and optical feedback phase optimization permitted long wavelength scans within the entire EC-QCL spectral tuning range.