Broadly tunable mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometer

Abstract

The sensitive spectroscopic technique noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) has been successfully used in a variety of systems; however, no broadly tunable setup has been developed for the mid-infrared. To this end, we have integrated a difference frequency generation system into a NICE-OHMS setup. Initial optimization and characterization was completed with ro-vibrational spectroscopy of methane. Doppler-broadened frequency-modulated NICE-OHMS spectra were recorded at a sensitivity of 2×10(-7) cm(-1) Hz(-1/2). Sub-Doppler saturation signals (Lamb dips) were also observed using wavelength-modulated NICE-OHMS, achieving a sensitivity of ~6×10(-9) cm(-1) Hz(-1/2).

Doppler broadened NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index

“…It was therefore subtracted from the measured signals. [12,[20][21][22], to the authors' knowledge, these are the first demonstrations of Db-NICE-OHMS signals for β exceeding unity. The corresponding fits of Eq.…”

Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range—demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9 × 10^−14 cm^−1

“…In its Doppler-broadened (Db) mode of operation it has repeatedly demonstrated a spectral NEAL in the 10 −12 -10 −13 cm −1 Hz −1∕2 range [9][10][11]. This mode of operation has mostly been used for trace gas applications, e.g., for detection of species such as O 2 [12], N 2 O [13], NO [14,15], C 2 H 2 [10,16], and CH 4 [17,18]. The so far highest (best) detection sensitivity for Db detection is 4 × 10 −13 cm −1 Hz −1∕2 , which recently was achieved by the use of a well-documented system consisting of a high-finesse cavity and an Er-doped fiber laser [16].…”

External cavity diode laser-based detection of trace gases with NICE-OHMS using current modulation

“…Recently, various mid-IR coherent light sources have been combined with NICE-OHMS, e.g. difference frequency generation [7], optical parametrical oscillators [8] or quantum cascade lasers [9]. For most of these approaches, the optical field gets modulated via an electro-optical modulator (EOM).…”