Liquid Phase Supercontinuum Fiber-loop Cavity Enhanced Absorption Spectroscopy

H2O has long been an elementary compound that we would like to know its concentrations in different kinds of chemical mixtures. There are many kinds of measurements that can do this well, but in this work, we are focusing on building our own system for H2O detection by using a Cavity Enhanced Absorption Spectroscopy (CEAS) method.

A typical CEAS system is usually made up of a light source, a resonant cavity and an optical detector. The development of CEAS starts in gas phase detection, and it has been developed to liquid and solid phases in the recent decades. The detection limit is not able to match up that of gas phase sensing due to higher optical loss, but it makes a new method for condensed phase detection in low concentrations. The sensing device can be simple, and by using a fiber loop in this work, we are able to build a robust liquid phase spectrometer.

A supercontinuum is used as the light source in this project. It is a broadband light source that is generated from a simple narrowband laser beam. The laser we use is a Q-switched 1064 nm infrared laser, and the supercontinuum we get can cover the region between 600 and 1600 nm. A supercontinuum can have the same characteristics as the pump laser such as spatial coherence and high power, and it can cover a wider spectral range at the same time.
Several liquid phase sensors are built to perform sensing of H2O in different liquids, and each sensor is tested for its stability and detection limit. Fiber tapers, side-polished fibers and a mirror reflection device are tested in this project. Every sensor has some unique properties and its own advantages and limitations.

The main liquid samples we test begin with solutions of H2O in D2O. These inorganic samples allow us understand the detection ability of our system and its signal to noise level. After that some organic samples are tested, and here H2O concentration is the main interest as well. Several organics with low H2O solubility are tested and results show a slightly higher noise level for them but some absorbance can also be found in the spectra.