We present study of polarization properties of the narrow modes, generated in random Raman fiber lasers near the generation threshold. For this purpose, time and polarization resolved spectral measurements based on optical heterodyning technique were implemented, that allow reconstruction of the ratio of vertical and horizontal projections of the electrical field during the mode generation process. We revealed that modes have high degree of polarization, with the slow change of its state during the mode lifetime. Moreover, it appeared that each mode has its own randomly appeared state of polarization, even when the several modes are generated simultaneously.
We present a new design of a random Raman fiber laser that allows to generate narrow spectral peaks with a high rate, based on a multimode gradient fiber as a media composing the cavity. The proposed scheme is simple for implementation and analysis. We carry out spectral measurements using optical heterodyning technique by projecting multimode radiation onto the fundamental mode of a standard single-mode fiber. The measurements confirmed the presence of localized ultra-narrow short-lived modes. Compared with single-mode fiber based random Raman lasers, the number of localized modes observed per unit time in a given spectral range is significantly higher.
The optical feedback in random distributed feedback Raman fiber lasers is due to extremely weak Rayleigh backscattering, nevertheless it is capable of forming localized spectral modes at the generation threshold. Here we present experimental time-resolved observation of such ultra-narrow spectral modes, proving the presence of cascade Brillouin scattering during the generation. We realized a two-band optical heterodyning measurement system that allows studying the evolution of two arbitrary separated spectral regions with microsecond temporal and of-megahertz spectral resolution. Studying the occurrence rate for cascade Brillouin scattering, we found that it plays an important role in shaping the generation.
Random fiber lasers utilizing Rayleigh backscattering possess a stochastic dynamics of the output power right above the threshold with narrow spectral lines appearing in random parts of the generation spectrum. We study the role of the random feedback in formation of these modes. We show that the generation of the localized modes does not require purely random feedback, and those may appear if both Rayleigh backscattering and point-like reflectors with tunable reflectance, based on a Sagnac fiber mirror are included into the laser cavity.
We directly observe ultra-narrow spectral modes in the generation of the random fiber laser by using a technique of optical heterodyning in real-time. These solitary spectral features appear slightly above the threshold of generation. We found that such modes could have a spectral width well below 3 MHz. These modes are spatially extended over the fiber laser length, and remain for tens of roundtrip times.
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