Spectral and Temporal Stability of Cascaded Raman based High Power, Octave Spanning, Continuous-Wave, Supercontinuum Sources

We recently reported the highest average power (70 W) from an octave spanning (880nm to > 1900nm) CW supercontinuum source module constituted of standard telecom fiber and which can be pumped using an Ytterbium laser source operating at any wavelength. Since many applications demand a spectrally stable and repeatable supercontinuum, we have investigated the spectral stability of this supercontinuum source over an extended period of operation (over 15minutes). The overall change in spectral profile was investigated as a function of time and power cycling of the source. This experiment was carried out at 3 different wavelengths of the Ytterbium fiber laser pumping the supercontinuum and at 3 different output power levels. The RMS value for the spectral change was used as the metric for comparison. It was observed that the changes are small (within 1-dB) over the duration of the continuous run. We attribute this change in spectral profile with time, to the rise in temperature of fiber which reduces the nonlinear coefficient of fiber and can be potentially controlled by better heat sinking the fiber spool. By allowing the fiber to cool down to ambient temperature through power cycling tests, the spectral change was observed to be very small at < 0.4dB. The standard deviation of output power fluctuations measured using a fast photodetector (over several seconds of acquisition, at 1 us time interval) was similar to 3%. These results show that our supercontinuum source offers excellent spectral and power stability over an extended period of operation.