Proceedings Article | 4 March 2022
Kevin Lee, Antoine Rolland, Peng Li, Jie Jiang, Martin Fermann
KEYWORDS: Optical clocks, Fiber Bragg gratings, Frequency metrology, Interferometry, Fiber amplifiers, Clocks, Ytterbium, Strontium, Frequency combs, Femtosecond pulse shaping, Supercontinuum generation, Pulse shaping
We control the spectrum of supercontinuum generation by phase shaping the driving pulse, greatly increasing power at wavelengths of interest. Previous work generally optimized one or two wavelength ranges, with spectral broadening below an octave, using spatial light modulators. We optimize a strongly nonlinear octave-spanning supercontinuum for six wavelengths, and we use much a more practical fully fiberized system, using temperature-controlled chirped fiber Bragg gratings. Our particular application is optical frequency transfer with an Er fiber frequency comb. We aim to replace multiple supercontinuum branches for f-2f interferometry and clock wavelengths with a single shaped branch that provides all the needed wavelengths, removing multiple path phase noise. We generated an optimized spectrum with strong outputs at the half harmonics of Yb, Ca, and Sr transitions: 1157, 1315, and 1397 nm, 1050 and 2100 nm for f-2f interferometry, and 1556 nm for c-band referencing. Without optimization, some target lines might be tens of nW, but we increase them to about 100 nW or more. Optimizing only one or two clock lines can further increase output by hundreds of nW. We confirmed the coherence of the shaped spectrum by interference with a second, unshaped supercontinuum branch with a 70 MHz acousto-optical frequency shift. The interference had strong contrast, and the beat notes had over 40 dB signal to noise at 100 kHz resolution bandwidth, showing suitability for high precision frequency comparison and locking.
