Continuous linewidth tuning of a laser source from single frequency to over 30GHz using phase modulation amplification using cascaded four-wave mixing

Temporal coherence control is a vital tool to achieve power scaling by overcoming stimulated Brillouin scattering (SBS). Conventional methods through phase modulation, with a sinusoidal drive or white noise source are limited in the achievable linewidth tunability due to the bandwidth of the RF drive and/or power handling of the phase modulator. Linewidth tuning range can be further increased by phase modulation amplification through cascaded four-wave mixing between multiple input lasers. When the input lasers are correlated, linewidth does not change with mixing order. However, in the case of uncorrelated input lasers, the linewidth increases progressively with increase in cascaded order. In this work, continuous linewidth tuning is achieved in two parts. From single frequency to ~7GHz, a single input laser at the required wavelength is line-broadened with phase modulation driven by a filtered white-noise source. Beyond this, two uncorrelated pumps are chosen, similarly broadened, amplified and sent through highly nonlinear fiber which performs cascaded four-wave mixing based phase modulation amplification. A demultiplexer extracts the required cascaded order and the two input lasers are tuned in wavelength appropriately to ensure the output center wavelength is constant. With the two effects together, continuous linewidth tuning at a single C-band wavelength from single frequency to more than 30GHz is achieved. The upper limit can be enhanced with power scaling. These principles can be translated to 1064nm wavelength, relevant to power combining and SBS limited power scaling of Ytterbium lasers using a combination of 1064nm phase modulators and nonlinear PCFs for cascaded four-wave mixing.