Frequency-stepped pulse train generation in an amplified frequency-shifted loop for oxygen A-band spectroscopy

We have demonstrated a frequency-stepped pulse train (FSPT) generation system enabling direct oxygen A-band spectroscopy in the time domain. The FSPT is formed by circulating the initial single-frequency pulses at ~1529.5 nm in an amplified frequency-shifted loop (AFSL). An FSPT with up to 80 equidistant optical frequencies can be generated with a frequency spacing of 200 MHz in this configuration. The fundamental wavelength of the FSPT is then frequency doubled to ~764.75 nm, covering one absorption peak in oxygen A-band, for the proof-of-principle experiment in spectroscopy. The oxygen transmission curves retrieved from FSPT-based differential absorption measurement are in good agreement with the calculated results from HITRAN database. The root-mean-squared errors between the measured and calculated results are 0.68% and 0.55% for clean air at 1 atm. and 1.5 atm., respectively. The wavelength of such an FSPT can be further extended to match the absorption lines of various gases by selecting proper gain media or through nonlinear frequency conversion, thereby making the FSPT-based spectroscopy a promising candidate for trace-gas remote sensing.