2 um Raman laser based on CO₂-filled hollow-core silica fiber

Here, we present a high pulse energy Raman laser at 1946 nm wavelength pumped with a 1533 nm linearly polarized fiber laser, with ∼92 μJ pulse energy, ∼60 pm linewidth, 8 kHz repetition rate, and 7 ns pulse duration. The Raman laser is based on the stimulated Raman scattering (SRS) effect in an 8-meter carbon dioxide (CO2) filled nested anti-resonant hollow-core fiber (ARHCF).

The nested structure contributes to the significant reduction of the fiber loss caused by light leakage, surface scattering and bend, therefore allowing coiling the gas-filled ARHCF with a relatively small bend radius of just ~5 cm. When the pressure in the CO2-filled ARHCF increases from 1 to 17 bar, the pulse energy first reaches the maximum pulse energy level of 16.3 μJ (corresponding to 28 % quantum efficiency) at only 1.2 bar, and then rapidly decreases due to the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at 2 μm spectral range.

The relative intensity noise (RIN) of the Raman laser reaches a minimum level (4%) when the pulse energy exceeds ∼8 µJ. Due to the low amount of heat release during the SRS process, the laser has a good long-term stability without significant drift. Our results constitute a novel and promising technology towards high-energy 2 μm lasers.