Controlled generation of harmonic states in mid-infrared quantum cascade laser frequency combs by external cavity optical feedback

Abstract

We demonstrate the implementation of external cavity optical feedback to improve coherence and promote generation of harmonic states by a mid-infrared quantum cascade laser frequency comb. In particular, we present a Vernier-like scheme to realize harmonic comb states that increase the repetition rate of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm-1 to 92 cm-1. Intermode beatnote and dual comb characterization indicate that the coherence of the comb has greatly improved for sub-optimal devices when the comb is operated in these harmonic states. This approach to control the generation of harmonic states and improve comb performance can be readily incorporated to various sensing systems and has great potential in spectroscopic measurements that require high repetition rates and/or broad optical bandwidth.

Broadband mid-infrared Fourier transform photothermal spectroscopy using a quantum cascade laser frequency comb

“…Further improvement of the PTS system can be achieved by stabilization of the QCL-FC to mitigate fluctuations 8 . The PTS scheme can also cover more absorption features by increasing the optical bandwidth of QCL-FC via RF injection 9 or optical feedback injection 10 . In addition, this FTS based PTS method can benefit from the incorporation of sub-nominal-resolution FTS technique 11 that can achieve higher spectral resolution without being affected by the instrumental line shape (ILS) of the FTS, which is currently a subject of our further studies.…”

Harmonic comb generation in mid-infrared quantum cascade laser frequency combs by external cavity optical feedback

“…The order of generated harmonic states can be controlled LEC tuning, which is summarized in the following Vernierlike scheme 13 . If LEC is set to an integer multiple m of LQCL, namely, LEC = mLQCL, the modes supported by the chip cavity have a free spectral range (FSR) in the frequency domain that is m times as large as that of the EC.…”