Tunable Optical Frequency Comb with a Crystalline Whispering Gallery Mode Resonator

We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the frequency of the pumping laser with respect to the corresponding resonator mode. We also observe a rich variety of optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating tunable narrow band frequency microwave signals is demonstrated.

Figure 1

Frequency spectrum of the SRS ($\sim 9.67\mathrm{THz}$ from the carrier) and hyperparametric oscillations observed in the ${\mathrm{CaF}}_2$ resonator pumped to a mode belonging to the basic mode sequence. The structure of the lines is shown below the spectrum. The loaded quality factor is $Q={10}^9$; the pump power sent to the modes is 8 mW.

Figure 2

Hyperparametric oscillation observed in the resonator pumped with 10 mW of 1550 nm light. Spectra (a) and (b) correspond to different detuning of the pump from the WGM resonant frequency. One can see the result of the photon summation process in (a), when the carrier and the first Stokes sideband, separated by 25 GHz, generate photons at 12.5 GHz frequency. The process is possible because of the high density of the WGMs and is forbidden in the single mode family resonators.

Figure 3

(a) Optical comb generated by the laser with 50 mW power (b) represents the central part of (a). The comb has two definite repetition frequencies equal to one and four FSRs of the resonator.

Figure 4

Modification of the comb shown in Fig. 3 when one changes the level and the phase of the lock.

Figure 5

Microwave signal generated by demodulating the comb with high-frequency photodiode. (a) The signal in the logarithmic scale; (b) the same signal in the linear scale; (c) optical comb used in the experiment. Linear fit of the microwave line gives a 40 Hz linewidth; however, evaluation of the logarithmic scale shows that the signal bandwidth is much smaller.

Figure 6

Harmonics generated by the externally modulated light. We have pumped the resonator with light 1550 nm light modulated at 25 786 kHz and having 50 mW power. The generated spectrum is not broader than the spectrum that was produced with a cw pumped resonator and the modes are not equidistant.