Abstract
Random Raman lasers attract now a great deal of attention as they operate in non-active turbid or transparent scattering media. In the last case, single mode fibers with feedback via Rayleigh backscattering generate a high-quality unidirectional laser beam. However, such fiber lasers have rather poor spectral and polarization properties, worsening with increasing power and Stokes order. Here we demonstrate a linearly-polarized cascaded random Raman lasing in a polarization-maintaining fiber. The quantum efficiency of converting the pump (1.05 μm) into the output radiation is almost independent of the Stokes order, amounting to 79%, 83%, and 77% for the 1st (1.11 μm), 2nd (1.17 μm) and 3rd (1.23 μm) order, respectively, at the polarization extinction ratio >22 dB for all orders. The laser bandwidth grows with increasing order, but it is almost independent of power in the 1–10 W range, amounting to ~1, ~2 and ~3 nm for orders 1–3, respectively. So, the random Raman laser exhibits no degradation of output characteristics with increasing Stokes order. A theory adequately describing the unique laser features has been developed. Thus, a full picture of the cascaded random Raman lasing in fibers is shown.
Highlights
As shown recently[12], a high-power non-resonant pumping enables Raman lasing in non-active bulk random materials (e.g. BaSO412) as well, which makes random lasing possible in almost any “white” powder, offering a new direction in the development of devices and diagnostic techniques
We propose and investigate a new scheme of a cascaded random laser based on an all-PM all-fiber configuration with linearly polarized pumping[30] that does not suffer from the discussed drawbacks
The experiment and calculations show that the XPM effect arising from the pump defines the minimum bandwidth of the generated spectrum amounting to 0.17–0.3, 0.48, 0.72 and 1.03 for the 1st, 2nd, 3rd and 4th Stokes waves, respectively, growing nearly proportionally to the pump power at corresponding threshold
Summary
As shown recently[12], a high-power non-resonant pumping enables Raman lasing in non-active bulk random materials (e.g. BaSO412) as well, which makes random lasing possible in almost any “white” powder, offering a new direction in the development of devices and diagnostic techniques. Fiber-based random Raman lasers demonstrate at the moment the highest efficiency of pump-to-Stokes wave conversion exceeding 70% both for the first[15,16,17] and for the second Stokes order[18], with the output beam power up to 200 W19. Such random Raman fiber lasers (RRFLs) generate a quasi-continuous mode-free spectrum with the resulting shape defined by the Schawlow-Townes narrowing near the threshold and nonlinear broadening at high powers[14,20]. The obtained formulae predict with a high accuracy the output power and bandwidth as a function of the pump power and Stokes order, which is useful both in fundamental research and practical applications of random fiber lasers
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