Chinese scientists reported new progress in suppressing residual cavity mode noise of off-axis integrated cavity output spectroscopy. The signal to noise ratio was improved at least 3 times by adding a radio frequency white noise to the laser driver current.

This work was conducted by GAO Xiaoming's group at the Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science. 

The ocean is an important source and sink of various gases in the atmospheric environment, such as CO₂ and CH₄. The detection of dissolved methane (CH₄) in seawater is used to study marine ecology and climate but can also help to detect submarine combustible ice resources.

Previously, off-axis integrated cavity spectroscopy technology has great potential for high-precision in situ measurement of marine methane. Further research on this technology is of great significance in improving the accuracy and response time of dissolved gases in water.

The off-axis integrated cavity output spectroscopy (OA-ICOS) obtains an enhanced absorption signal by injecting an off-axis beam into a high-precision optical cavity. The residual cavity mode of OA-ICOS is a dominant noise that limit the performance improvement of OA-ICOS.

The researchers made a lot of effort to solve this problem.

By actively injecting radio frequency (RF) white noise into the laser, the residual cavity mode noise of the integrated cavity was effectively suppressed and the signal-to-noise ratio was increased by 3 times. By using such method, the performance of the developed OA-ICOS system was effectively improved.

This result could be used to develop a high-sensitivity, high-precision, in-situ, compact and portable trace gas analyzer and has also been applied to in situ, on-line measurement of dissolved methane in deep sea, successfully.

This work was supported by the National Key Research and Development Project of China and published in Optics Express.

The on-axis cavity modes without and with rf noise perturbation with different powers. (Image by WANG Jingjing) 

Off-axis cavity modes without and with RF noise perturbations at different power (Image by WANG Jingjing)