Enhanced Control of Transient Raman Scattering Using Buffered Hydrogen in Hollow-Core Photonic Crystal Fibers

Abstract

Many reports on stimulated Raman scattering in mixtures of Raman-active and noble gases indicate that the addition of a dispersive buffer gas increases the phase-mismatch to higher-order Stokes/anti-Stokes sidebands, resulting in preferential conversion to the first few Stokes lines, accompanied by a significant reduction in Raman gain due to collisions with buffer gas molecules. Here we report that, provided the dispersion can be precisely controlled, the effective Raman gain in gas-filled hollow-core photonic crystal fiber (PCF) can actually be significantly enhanced when a buffer gas is added. This counterintuitive behavior occurs when the nonlinear coupling between the interacting fields is strong, and can result in a performance similar to that of a pure Raman-active gas, but at much lower total gas pressure, allowing competing effects such as Raman backscattering to be suppressed. We report high modal purity in all the emitted sidebands, along with antiStokes conversion efficiencies as high as 5% in the visible and 2% in the ultraviolet. The results point to a new class of gasbased waveguide device in which the pressure-tunable nonlinear optical response is beneficially adjusted by the addition of other gases.

Sub-40 fs pulses at 1.8 µm and MHz repetition rates by chirp-assisted Raman scattering in hydrogen-filled hollow-core fiber

“…expected in transient SRS 18 , the quantum efficiencies (QE) increased with pressure, reaching 50% (>1.2 W) at 35 bar.…”

“…In this work we use linearly polarized pump pulses, which favours vibrational SRS, although rotational SRS turns out to play a significant role in the observed dynamics, as explained below. In the transient regime, the pump, Stokes and Cw's are able to exchange energy coherently, causing the Stokes light to be generated predominantly towards the trailing edge of the pulse 18 . Interestingly, in this regime the SRS gain does not saturate with increasing gas pressure, in contrast to the steadystate regime.…”

“…In the experiments reported here, 300 fs pulses at 1030 nm from a commercial Yb-based fibre laser are down-shifted to 1.8 µm by vibrational SRS in a H2-filled single-ring HC-PCF. Because the pump pulse duration is less than the lifetime of the Raman coherence, the system operates in the so-called "transient" SRS regime 18 , yielding quantum efficiencies in excess of 50%. Moreover, strong interactions with Raman coherence waves (Cw's), along with nonlinear effects such as SPM, enable temporal compression of the 1.8 µm Stokes pulses to durations of 39 fs (measured using second-harmonic generation frequency-resolved optical gating (SHG-FROG)), without need for post-compression.…”

Efficient Generation of Coherent Stokes Field in Hydrogen Gas-Filled Hollow Core Photonic Crystal Fibres

“…The intersection point between the Stokes efficiency curve and pump one also seem to be on the Stokes efficiency straight line of 50%, like the case of gas pressure change. These give us another simple way to enhance the Raman generation efficiency instead of using other complex Raman generation system with special hollow fibre structure [14,15] or tailor polarization [16].…”

Thresholdless deep and vacuum ultraviolet Raman frequency conversion in hydrogen-filled photonic crystal fiber

“…The mixing pulse energy was kept well below the SRS threshold, ensuring that the dynamics were driven purely by the pre-existing molecular coherence. Since we operated in the "transient" regime (13), the molecular coherence builds up under the pump pulse envelope. As a result, the efficiency could be optimized by tuning the mixing pulse delay; a value of ~1 ns was optimal.…”