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 [l] 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 [2,3,4]. 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. Additionally, fiber laser sources have a broad gain bandwidth, such as the erbium-fiber laser with gain over 1530-1580nm, and thus can be used as a source for a multi-gas sensor avoiding the cost of individual (DFB) lasers for each gas. However the high cost of the active fiber components such as the Erbium-Doped Fibre Amplifier (EDFA) would also limit the commercial application of single point measurements with intra-cavity fiber spectroscopy. Ideally an intra-cavity system would be capable of making multi-point measurement of several gases within the gain spectrum of the erbium doped fibre laser. A multiplexed network would greatly reduce the cost per point by sharing the same fiber source and the same signal-processing unit. 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 [5] and individual sensors are addressed by different frequencies of the mode-locked fiber ring laser