Continuously linewidth tunable, polarisation maintaining, narrow linewidth fiber laser

High power polarised narrow linewidth sources are of immense importance in coherent beam combining (increased path length accuracy), spectral beam combining (reduced angular spread), non-linear frequency conversion (increased parametric gain) and remote detection. Power scaling of narrow linewidth sources is primarily limited by Stimulated Brillouin Scattering which can be overcome by spectral broadening before amplification to higher powers. Various applications have different requirements of spectral purity and power which can be met if the source line width is tunable. We demonstrate a narrow line width polarisation maintaining (PM) laser with continuous linewidth tuning from similar to 2.88 GHz to similar to 9.88 GHz with over 20 W of output power with a polarisation extinction ratio (PER) greater than 20 dB. We achieve continuous linewidth tuning through pure phase modulation of a 1064 nm DBR laser with a white noise source whose bandwidth and power are tuned with a low pass filter bank and variable attenuation of the drive voltage to the modulator. This dual control mechanism enables gapless tuning of the linewidth of the source. A cascade of optimized PM Yb-doped fiber preamplifier and power amplifier is used to scale the output power to over 20W. Combined with the wavelength tuning of the DBR seed, the tunable spectral width makes it versatile for use in a wide range of applications.