Generation of Optical Frequency Comb via Giant Optomechanical Oscillation

Optical frequency combs (OFCs) are essential in precision metrology, spectroscopy, distance measurement, and optical communications. Significant advances have been made recently in achieving micro-OFC devices based on parametric frequency conversion or electro-optic phase modulation. Here, we demonstrate a new kind of microcomb using a cavity optomechanical system with giant oscillation amplitude. We observe both optical and microwave frequency combs in a microtoroid resonator, which feature a flat OFC with 938 comb lines and a repetition rate as low as 50.22 MHz, as well as a flat microwave frequency comb with 867 comb lines. To generate such giant oscillation amplitude, we excite an overcoupled optical mode with a large blue detuning that is assisted with the thermo-optic nonlinearity. A new type of nonlinear oscillation, induced by competition between the optomechanical oscillation and thermo-optic nonlinearity, is also observed.

Figure 1

Schematic diagram and principle of the OFC generation with giant optomechanical oscillation. (a) Schematic diagram of optomechanical frequency comb. (b) Illustration of optomechanical frequency comb span expansion assisted by thermo-optic effect. Left: cavity spectrum. I, static cavity; II, oscillating cavity; III, oscillating cavity with thermo-optic effect. Right: corresponding OFC spectra.

Figure 2

Representative curves of the observed optomechanical frequency comb and its experimental characterizations. (a) Typically observed OFC. (b) Typical MFC revealed in rf spectrum with the repetition rate exactly coinciding with the mechanical oscillation frequency ${\mathrm{\Omega }}_m/2\pi$. (c) Time domain periodical optical pulse train. (d) The measured rf spectrum of the oscillated mechanical mode. RBW: resolution bandwidth.

Figure 3

Evolutions of OFC and MFC as a function of the pump frequency. (a) Optical spectra. (b) Radio frequency spectra. (c) Transmission spectra of the probe light mediated by mechanical oscillation. (d) Transmitted optical power versus time. The curves of the same color indicate the results of the same pump frequency.

Figure 4

Measured and simulated number of comb lines. (a) Number of the comb lines as a function of the cavity decay rate. (b) Number of the comb lines as a function of the pump power. The data with solid symbols are measured results and those with opened ones are numerically simulated results.

Figure 5

Time traces of the OMTO. (a),(b) Experimental results (a) in the slow timescale and (b) three typical snapshots in the fast timescale. (c),(d) Numerical results. The calculated time trace with slow timescale is shown in (c), while three fast time traces [corresponding to the marked times in (c)] are shown in (d).