Fiber laser intra-cavity spectroscopy (FLICS)

Compact intra-cavity spectroscopic measurements may be obtained with any ma terial that has an absorption signature under the gain bandwidth of a fiber laser. Experiments have demonstrated that compared with a regular absorpti on scheme, an increase in sensitivity is achieved when using the intra-cavi ty configuration. The practical limit for this enhancement is given by the fiber laser noise. Since intra-cavity spectroscopy is essentially a single beam technique, the application of dual-beam noise reduction techniques is not possible. However, considering that a single-mode fiber can support two modes of polarisation, we have used a polarization beam splitter to create two independent cavities (x and y polarization) with the same noise, one c avity of which contains the absorber. For the first time, this permits the convenient use of Balanced Ratio-metric Detection in conjunction with an in tra-cavity absorption arrangement.