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

Abstract

Expressions for the in-coupling of white noise and etalons into fitted signal strengths are derived. These show that the amount of noise picked up is affected by the scanning range. A methodology for finding the optimum scanning range from a single set of measurements has been developed. This was used to estimate the optimum conditions of a noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup. The methodology was validated by measurements. This resulted in a spectral noise equivalent absorption per unit length of 2.6 × 10 −13 cm −1 Hz −1∕2 and a minimum Allan deviation of 9 × 10 −14 cm −1 at 30 s, which are, to our knowledge, the lowest reported for Doppler-broadened NICE-OHMS.

Shot-noise-limited Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry

“…1, is similar to that in previous works [22], except for a few minor alterations. In short, the light produced by the EDFL is sequentially passed through a fibercoupled acoustic-optic modulator (f -AOM), a fiber-coupled polarizer (f -POL), and a fiber-coupled electro-optic modulator (f -EOM) before it is sent out into free space by a fiber collimator (f -C).…”

“…where F is the finesse of the cavity, L is the cavity length, ffiffi ffi 2 p and γ take into account the facts that only one half of the scan is used to gather the signal and that the laser is not on resonance all of the time, respectively [22], the J 0 and J 1 denote Bessel functions that are functions of the modulation index β, κ is the ratio of the peak-to-peak value of the NICE-OHMS line shape function and the peak value of the direct absorption line shape, e is the electron charge, Δf is the detection bandwidth, η A is the detector responsivity, ρ is the ratio of the powers on the reference and the transmission detectors, and P t is the power impinging onto the detector. Inserting relevant numbers for our system, i.e., L 39.4 cm; η A 1 A∕W; β 1, which implies that J 0 0.77 and J 1 0.44; γ 2.4 (for a scanning range of 3 GHz); κ 1.4; Δf 0.7 Hz; ρ 0.55; and P t 2.0 mW, provides a value for α min SN;bal of 1.6 × 10 −13 cm −1 Hz −1∕2 , which compares well with the measured one.…”

High-resolution trace gas detection by sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry: application to detection of acetylene in human breath

“…This phenomenon is consistent with the data in Fig. 4 in Ref [49], which displays a simulation of the influence of etalons with different FSRs on the NICE-OHMS signal and show that the analytical signal is highly susceptible to etalons whose FSR is similar to the spectral range of the signal. This does not only indicate that a careful design of the optical system (in this case, with respect to the relative distances between the various components) can reduce the influence of etalons, but also that such a strategy cannot, because of the finite dimensions of the optical components and the length of the system, eliminate all etalons in a system.…”

An optical pascal in Sweden

“…Owing to the authors' experience with modulation methodologies to reduce disturbances in spectroscopic measurement systems, primarily the wavelength-modulation [48,49] and the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy [50][51][52][53] techniques, they anticipated that also the field of FP-based refractometry could benefit from an implementation of modulation.…”