An experimental investigation of a semiconductor laser subject to moderate to strong optical feedback shows that sporadic power discontinuities occur everywhere along the boundary separating feedback regimes IV and V. These low-frequency fluctuations (LFF's) do not occur just near the solitary-laser threshold as previously thought. Furthermore, although the LFF's appear as power dropouts near threshold, they manifest as power jump-ups well above threshold. Theoretical modeling shows reasonable agreement with experiments.
There are many different factors to consider when determining the appropriate excitation laser source for continuous wave Raman experiments. The laser characteristics of interest for designing a Raman system fall into two categories. The first category includes parameters associated with the laser output, such as wavelength, linewidth, output power, polarization, and beam quality. The second category includes physical characteristics of the laser system, such as size, reliability, and power consumption. The different characteristics of gas, dye, solid‐state, and semiconductor lasers are discussed. Comparisons of individual lasers for Raman applications illustrate some of the tradeoffs associated with the various laser parameters, and also provide an indication of the signal levels that can be expected in continuous wave Raman measurements from different kinds of materials.
High-power, external-cavity semiconductor lasers with narrow bandwidth and fiber-coupled output are designed and constructed. An output power of 540 mW is coupled out of a 100-mum multimode fiber with coupling efficiency of 72% when the laser is operated at 1.1 A. The emission linewidth is as narrow as 22 GHz, and the wavelength is tunable from 779.7 to 793.0 nm. Application of such lasers to remote real-time Raman sensing of materials is also demonstrated.
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