Continuous-Wave Supercontinuum Generation using Cascaded Raman Fiber Lasers
This thesis explains architecture and techniques to generate high power, CW supercontinuum laser sources using standard Silica fibers as the nonlinear medium, pumped using high power CW Yb doped fiber lasers. Unlike the pulsed supercontinuum sources, the CW supercontinuum sources offer high average powers leading to spectacular power spectral densities at the output and can be realized at lower costs, in an all fiber architecture, which makes the system more compact and favorable towards power scaling. We have demonstrated a high power, all-fiber, wavelength tunable, fiber laser source that can operate in the L-band region (1.5-1.6 um), based on 6th order cascaded Raman amplification scheme, which can generate ~24 W of output power and is widely tunable from 1560-1590 nm. Using this laser we could pump the HNLF to generate a supercontinuum of ~700 nm bandwidth to powers as high as 35 W which is nearly double the output power than what has been previously demonstrated. To enhance the bandwidth and scale the output power, we have demonstrated a simple but highly impactful solution which uses the standard telecom fiber as the nonlinear medium. The supercontinuum generation module we demonstrated, is essentially a high efficiency cascaded Raman converter, which can take any CW, high power Ytterbium-doped fiber laser as the input and generate an octave spanning supercontinuum with an average power of ~34W, spanning over 1000nm (>1 octave) from 880nm to 1900nm at an efficiency of ~44%. The supercontinuum source exhibited excellent spectral and temporal stability for an extended duration of operation (>1 hour). The most highlighting result reported in this thesis is the record high output power that has been demonstrated from a CW supercontinuum so far. Using the recently proposed nonlinear power combining architecture we could leverage the output power (limited by available pump power) from the telecom fiber based octave spanning, CW supercontinuum. This involves nonlinear spectral power combining of outputs from multiple, independent, Yb lasers operating at different wavelengths as the pump sources, to generate supercontinuum at an efficiency of ~44% with a spectacular PSD of >3mW/nm from 850nm-1350 nm and an impressive PSD of >100mW/nm from 1350nm-1900nm at an output power of ~72 W.