A novel fiber-optic intra-cavity sensing network using a mode-locked fiber ring laser

A promising method for high sensitivity gas detection is intra-cavity spectroscopy (V.M. Baev et al., Applied Physics vol. B55, pp. 463-477, 1992), where the gas absorber is placed directly within a laser cavity. The high sensitivity arises from the very large number of passes through the gas cell within the cavity, effectively transforming a short absorption cell into a highly efficient multi-pass system. As a consequence a weak gas absorption line can have an enormous impact on the laser output. Recently intra-cavity spectroscopy with fiber lasers has attracted considerable attention. An all-fiber intra-cavity laser system can use the mature passive and active fiber components from the communications market with compact micro-optic gas cells, allowing safe, remote and continuous gas monitoring. In this paper, we present a system that combines a mode-locked fiber ring laser with intra-cavity spectroscopy to distinguish between different gas cells in a ladder sensing network. The system outputs strong pulsed lasing signals only when certain matching conditions are satisfied (J.S. Wey et al., IEEE Photonics Tech. Lett., vol. 7, no. 2, pp. 152-154, 1995) and individual sensors are addressed by different frequencies of the mode-locked fiber ring laser.