Pulse-picked octave-spanning microresonator-based frequency comb for optical self-referencing

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Summary form only given. Microresonator-based optical frequency comb generation based on four-wave mixing in ultra-high-Q microresonators has attracted significant interest during the past years [1-4]. However, a missing element to make these combs stand-alone tools for metrology applications is the independent stabilization of their carrier envelope offset frequency (e.g. by using an f-2f self-referencing technique). This has not been achieved to date. So far, stabilization of the offset frequency in microresonators required an additional reference frequency comb, which jeopardizes the advantage of having an integrated and small comb generator. Octave spanning microcombs on the other hand have only been demonstrated in microresonators with mode spacings beyond 200 GHz and under partial loss of coherence, which made it difficult to use them for optical self-referencing [5, 6].Here, we present the first generation of an octave-spanning, externally broadened and coherent frequency comb from an optical microresonator. The octave-spanning spectrum is generated by reducing the repetition rate of a microcomb from its initial repetition rate of 25.6 GHz using a pulse picking setup that transmits every n-th pulse with an electro-optic modulator. This generates pulse trains with repetition rates between 20 MHz and ~3 GHz, and subsequent amplification leads to sufficient peak power to broaden the optical spectrum to an octave in highly nonlinear fiber (HNLF). Fig 1(b) shows the achieved optical bandwidth for different pulse picking frequencies and average amplifier powers after the HNLF. The initial microcomb state is similar to the soliton states recently reported in Ref [7]. Fig 1(a) shows the optical spectrum before pulse picking and broadening with 25.6 GHz mode spacing. In order to maximize the pulse peak power, this spectrum has been flattened and phase adjusted using a liquid-crystal-based pulse shaper, leading to a pulse length at the input of the HNLF of around 280 fs. Moreover, we have shown in an independent measurement against a reference frequency comb (similar to the measurement in Ref. [1]) that the employed microcomb is uniform to a level better than 1 mHz per free spectral range.To date, the highest repetition rate of self-referenced optical frequency comb is ~10 GHz [8]. A challenge for generating self-referenced frequency combs at higher mode spacings is the low pulse peak power and as a consequence the loss of coherence during amplification and spectral broadening. In order to test the coherence of the octave-spanning spectrum, we use a CW 1320-nm-YAG laser to beat against the comb. Fig 1(c) shows the corresponding beat note with >60 dB signal to noise at a resolution bandwidth of 100 kHz. In conclusion we demonstrated the first octave-spanning microresonator-based optical frequency comb with electronically accessible mode spacing. First coherence measurements at 1320 nm show that this comb is a promising candidate to achieve a fully self-referenced microresonator-based optical frequency comb.