Adaptive etalon suppression technique for long‐term stability improvement in high index contrast waveguide‐based laser absorption spectrometers

Abstract

The authors present an adaptive algorithm based on a non‐linear regression model for mitigating time‐varying etalon drifts in line‐scanned optical absorption spectrometers. By dynamically varying the etalon spectral background using physically realistic degrees of freedom, the authors’ dynamic etalon fitting‐routine (DEF‐R) significantly increases the spectral baseline recalibration interval as compared to conventional fringe subtraction models. They provide an empirical demonstration of the efficacy of DEF‐R using an on‐chip 10 cm silicon waveguide for near‐infrared methane absorption spectroscopy at 6057 cm−1, which suffers significant etalon spectral noise due to reflections and multi‐path interference from stochastic line‐edge roughness imperfections. They demonstrate the corresponding improvement in both spectral clean‐up and long‐term stability via Allan‐variance analysis. For the sensor presented here, application of DEF‐R enables Gaussian‐noise limited performance for more than 102 s and provides almost an order‐of‐magnitude improvement in stability time with respect to conventional baseline subtraction. Although DEF‐R is applied here to an on‐chip sensor embodiment, they envision their technique to be applicable to any absorption sensor limited by time‐varying etalon drifts.

Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy

“…Hence, despite comparably stronger light–matter interaction in the waveguide, the sensitivity of the free space has not yet been surpassed. However, we stress that this result was achieved with bulk coupling optics, with no thermal stabilization, and without any fringe removal or spectral filtering algorithms, and it is likely to be improved by more elaborate signal processing methods 14 . Nevertheless, the mere 2.5-fold difference in the LOD and comparable system stability almost close the performance gap between the on-chip and free-space-based MIR TDLAS analyzers.…”

“…Similar low reflections are expected at other fabrication imperfections and material defects. As a result, Fabry–Pérot etalons, which substantially limit the performance of other reported waveguide sensors 3 , 7 , 14 , are almost entirely suppressed. Strong evanescent field confinement also implies a low interaction of the mode with the waveguide material (Supplementary S5 ), which relaxes the requirements on material transparency and allows realization of long waveguides in fairly lossy materials.…”

“…Furthermore, reflections at waveguide facets and defects result in distinct etalon patterns in the transmission spectra. This spectral noise interferes with the signal and drastically reduces the gas detection capability and sensor stability 3 , 8 , 14 . Finally, yet importantly, the evanescent field in conventional air-cladded waveguides limits the light–matter interaction to a fraction of that for free space.…”

“…To date, arguably the most successful realization of an on-chip TDLAS gas sensor has been based on a 10 cm long silicon strip waveguide for methane detection, operating at 1651 nm with Γ = 25.5% 8 , 14 . In this work, a sub-100 ppm limit of detection (LOD) was achieved, mainly limited by etalon fringes and the weak absorption of methane in the NIR.…”

Integrated Nanophotonic Waveguide-Based Devices for IR and Raman Gas Spectroscopy

“…Such noise may interfere significantly with the recorded spectrum. For this purpose, antireflection coatings on the waveguide facet [87], the use of subwavelength gratings on the waveguide facet [88], or the use of appropriate signal processing algorithms [89] have been proposed to reduce the effect of the fringes. Substantial fringe reduction has also been achieved in air-suspended waveguides, characterized by strongly delocalized guided modes with an effective mode index close to unity.…”