Er-fiber femtosecond optical frequency comb covering visible light

Femtosecond optical frequency combs (FOFCs) with output wavelengths covering visible light have potential applications in absolute frequency measureflent of iodine-stabilized lasers and optical clock lasers. Based on optical amplification, frequency doubling and spectrum broadening, a home-made Er-fiber femtosecond optical frequency comb (Er-FOFC) with output wavelengths covering visible light is demonstrated. One path with an average power of 8 mW from Er-FOFC is used as the seed pulse for spectrum broadening to cover the visible light. This path is first amplified to 532 mW by injecting into an Er-doped femtosecond fiber amplifier with combined forward and backward pumping and then frequency doubled with a MgO: PPLN crystal with an output power of 85 mW, frequency-doubling efficiency of 32% and pulse duration of 85fs. The output power of this path can be first amplified to 532 mW through an Er-doped femtosecond fiber amplifier when the forward pumping and backward pumping both turn on. Then the frequency-doubling laser can be generated in a MgO: PPLN crystal. The frequency-doubling efficiency is 32% and the pulse duration is 85 fs; the frequency-doubling light is spectrally broadened from 500 to 1000 nm in a photonic crystal fiber (PCF), with an output power of 85 mW and coupling efficiency of 50%. To verify the performance of the broadened spectrum, the light from the Er-FOFC and a compact iodine-stabilized frequency-doubled Nd: YAG laser at 532 nm is beaten. A beat signal with a signal-to-noise ratio of 30 dB at 100 kHz RBW is obtained, which provides a useful tool for absolute frequency measureflent of visible lasers.