Spatial and Spectral Effects in Continuous-Wave Intracavity Raman Lasers

Abstract

We develop an analytic model to explore spatial and spectral effects in intracavity Raman lasers. The effects of deterioration of the spatial and spectral properties of the fundamental and Stokes fields on the laser performance are derived. We quantify how the fundamental field strength can be used to diagnose the behavior of the system, and apply this tool to key prior experimental results.

Intracavity Raman conversion of a red semiconductor disk laser using diamond

“…Note that the spectral resolution of the optical spectrum analyser is 0.05 nm, thus this measurement is instrument-limited and the actual linewidths are likely to be narrower. While the fundamental emission did not show substantial spectral broadening as in our previous works [13], the SDL intracavity power did not clamp at its value at the Raman threshold (10 W), but gradually increased with the pump power up to 12 W. This rise of the fundamental intracavity power above the Raman threshold has been observed in other experiments and can be explained by suboptimal and power-varying spatial and spectral overlaps between the fundamental and the Stokes fields along the Raman crystal [13,25].…”

“…The results of these characterization measurements can be used to estimate the effective Raman gain of this laser and compare it with previous estimations of the Raman gain coefficient of diamond. In contrast to the Raman gain coefficient, the effective Raman gain depends on the spatial and spectral properties of the laser system and is therefore indirectly dependent on the pump power [25]. The effective Raman gain can be determined from the model of an intracavity Raman laser, such as those reported in [13] and [25].…”

Diamond‐based concept for combining beams at very high average powers

“…2.3. Finally, the gain for uncorrelated pumps scales according to a correction factor involving the pump, Stokes and Raman linewidths ( respectively): (for Lorentzian lineshapes), which is near unity for the typical linewidths of crystals and when using many free running pumps. As a result, a Stokes wave of linewidth less than the Raman linewidth is amplified with a gain approximately given by the monochromatic Raman gain coefficient and the average intensity of the pump waves integrated along its path.…”

“…For collinear collimated beams A eff is independent of l so that the gain grows exponentially with l . In the case of tightly focused collinear beams (), the effective area becomes proportional to crystal length so that the integrated gain for the Stokes power is maximized and independent of the length and focusing conditions . For practical beams in a compact Raman amplifier, the exponential gain is determined from the solution of Eq.…”

“…(3) with g 0 as a free parameter, the effective Raman gain coefficient was determined. In addition the bandwidths of the pump and Stokes ( ω P and ω S ≈ 0.83 cm −1 ) are significant compared to the Raman linewidth ( ω R = 1.5 cm −1 ) and a correction factor of 0.46 was applied [39][40][41]43]. Collinear pump and seed beams show strong amplification, with fitted parameter g 0 = 10.5 cm/GW (corrected for relative linewidths) with an experimental error of less than 15% based on repeated experiments; a value in good agreement with many reported Raman gain coefficients measurements for diamond as reviewed recently in [48,49].…”