Chinese researchers recently developed a novel method to improve the performance of the gas sensor based on off-axis integrated cavity spectroscopy (OA-ICOS) using a DFB laser operating in the near-infrared at 1653 nm, which relies on implementation of wavelength modulation and radio frequency (RF) white noise perturbation to diode laser current.

This work was done by GAO Xiaoming‘s team in Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science and was detailed in Optics Letters.

Off-axis integrated cavity output spectroscopy (OA-ICOS) is a suitable high-sensitivity spectroscopic technique for fast and continuous real-time monitoring of species concentration change. Noise related to residual cavity-mode structure results in fluctuation in the spectra baseline and is the ultimate sensitivity limiting factor for the OA-ICOS measurement technique.

In their recent work, RF white noise was added to a wavelength-modulated OA-ICOS system (WM-OA-ICOS). The current of a 1653.7 nm DFB laser involved in the OA-ICOS set-up was simultaneously modulated by three current components, namely, low-frequency triangular ramp for; laser wavelength scan, high-frequency sine-wave for wavelength modulation to eliminate 1/f noise and RF white noise was used for suppressing noise level resulted from residual cavity-mode fluctuation.

The RF noise perturbation may effectively smooth the noise and improve the SNR without damage to the linearity between the peak height of the 2f signal and the concentrations of CH₄. The LoD of such RF white noise perturbed WM-OA-ICOS was improved by a factor of six over the unperturbed OA-ICOS approach.

The team believed the promising result showed the high potential of this simple, cost-effective, compact cavity enhanced high-sensitivity gas sensor for various field applications.

This work was supported by the State's Key Project of Research and Development Plan and Key Project of the National Natural Science Foundation of China.

Second harmonic signals of 2.0 ppmv CH₄ absorption, recorded without and with RF noises perturbations at different powers (Image by Wang Jingjing)

The linear relationship between CH₄ concentration and the amplitude of the 2f signals (Image by Wang Jingjing)

Allan deviation analysis of OA-ICOS, WM-OA-ICOS, and RF-WM-OA-ICOS approaches (Image by Wang Jingjing)