Effect of optical feedback on a VCSEL TDLAS

This paper describes the effects of optical feedback on the sensitivity of VCSEL tunable-diode laser spectroscopy (TDLS). Three VCSELs, emitting at different wavelengths in the near-infrared, were used. A TDLS system, subjected to optical feedback, exhibited a common signal-to-noise ratio profile for all three lasers. A catastrophic degradation of TDLS sensitivity occurred when feedback exceeded a level which we associate with coherence collapse. The TDLS system had a CH₄ minimum detection limit of 7.5 ppmm without optical feedback. Optical feedback of less than ten percent reduced this sensitivity by two orders of magnitude. This reduction of system sensitivity was accompanied by a second-harmonic absorption signal baseline shift which degraded the system accuracy.