Precision visible comb from an Er fiber laser

We report a highly coherent visible optical frequency comb, from a doubled 1.5 μm Er fiber laser, spanning 500 nm to 1200 nm. Beatnotes between the frequency comb and CW lasers at 698 nm and 1064 nm exhibits signal-to-noise ratio higher than 40 dB in 100 kHz RBW. We demonstrate the coherence of the comb through a 698/1064 nm frequency ratio measurement with two visible frequency combs. The fractional residual instability is lower than 6 × 10
^-18
at 1s and reaches 3 × 10
^-20
after 1000 s of averaging.