Tunable and wavelength-modulated rare earth-doped fiber lasers with applications to trace gas sensing.

摘要

New methods for tuning fiber lasers based on temperature tuning of highly overcoupled couplers are introduced and investigated for single and dual wavelength sources. A wavelength modulation technique is implemented to demonstrate field viable methods for ultrasensitive gas sensing using fiber lasers containing fiber Bragg gratings. The fiber lasers fabricated for this work allow for highly sensitive quantitative spectroscopy when used in conjunction with a balanced detector to cancel out laser amplitude noise. The quantitative effect of laser source broadening was analyzed and probable laser profiles were deduced from experimental data. Various configurations using intracavity techniques were also investigated and the inherent challenges in making a field viable intracavity sensor were addressed. Acetylene was detected in both intracavity and external cavity setups. Intracavity lasers show a threshold type effect which may be useful in some applications, but quantitative results are difficult due to the lack of a true reference and the fact that the laser itself is perturbed when wavelength modulation is attempted. In addition, ammonia was investigated with an Er3+ fiber laser near 1548 nm and nitric oxide was detected with a Tm3+ fiber laser near 1800 nm. Linestrengths could be derived from the data. For strong absorbers, detection limits as low as 125 ppbv-m are predicted. Theoretical predictions were made for the performance of the system for other gases which absorb under the gain bandwidth of the Er3+ and Tm3+ fiber laser systems.