Compact intra-cavity spectroscopic measurements may be obtained with any material that has an absorption signature under the gain bandwidth of a fiber laser. Experiments have been carried out with acetylene and the erbium fiber laser. Since intra-cavity spectroscopy is essentially a single beam technique, dual beam noise reduction is not possible. Considering that a single mode fiber can support two modes of polarization, we have used a polarization beam splitter to create two independent cavities (x and y polarization) with the same noise, one cavity of which contains the absorber. For the first time this permits the convenient use of Balanced Ratiometric Detection in conjunction with sensitive intra cavity absorption.
T. F. Morse,
"Fiber laser intra-cavity spectroscopy (FLICS)", Proc. SPIE 3746, 13th International Conference on Optical Fiber Sensors, 37460P (1 September 1999); doi: 10.1117/12.2302015; https://doi.org/10.1117/12.2302015