# All-Fiber Hyperparametric Generation Based on a Monolithic Fiber Fabry-Perot Microresonator
## 3 Hyperparametric Generation  
The pump laser was set at 1550 nm and tuned into deep resonance of the FFP microresonator from the blue-detuned side. The output spectra were recorded using an optical spectrum analyzer (OSA, AQ6375 YOKOGAWA). The threshold of four-wave mixing was measured to be 280 mW. When the pump power was increased to 1.74 W, Stokes and anti-Stokes Raman peaks were generated at the first and second order (Figure 3a). Note that the Stokes and anti-Stokes spectral lines are asymmetric in power due to the imperfect dielectric mirror coatings, which feature higher reflectivity at longer wavelength, thus the quality factor. The wavelength separation of the pump laser and the first-order Stokes Raman peak was measured to be 109.6 nm. It is worth noting that primary comb lines can also be observed when the pump is just tuned into the cavity resonance. However, due to the low intra-cavity power and the weak thermal lock effect in the fiber resonator, the primary comb state is not stable enough for a complete spectral measurement. Sidebands around the pump and Raman peaks  
Figure 2: (**a**) The cold-cavity transmission signal when scanning a tunable laser across a resonance of around 1550 nm, showing a 667 MHz free spectral range (FSR). Two polarization modes are present, which originate from stress-induced birefringence resulting from the fiber ceramic ferrules, and the frequency detuning of the two modes is 301 MHz. In experiments, one of them can be fully suppressed using a polarization controller to ensure that only one polarization mode is excited in the FFP microresonator. (**b**) The transmission signal of one resonance. The linewidth was measured to be 2.4 MHz, which corresponds to a Q factor of \(8.1\times 10^{7}\). were generated via a four-wave mixing process (Figure 3b) with pump power of 3.5 W, and they started to merge when the pump power was increased to 5.41 W (Figure 3c). When the pump power was further increased to 8.13 W, the spectrum span extended to over 400 nm with the assistance of cascaded stimulated Raman scattering (SRS) (Figure 3d). It is worth noting that cavity dispersion typically limits the bandwidth of the Kerr comb in FFP microresonators to be less than 100 nm [5, 19]. Here, due to the small FSR (667 MHz) of our FFP microresonator, the Raman gain spectrum, which is typically about 100 nm in bandwidth [22], can easily cover over 10,000 cavity resonance modes, resulting in efficient pump energy conversion to Raman peaks. The combination of both SRS and Kerr nonlinearity breaks the limitation of cavity dispersion on the spectral bandwidth. The RF beatnote signal was measured using an electrical spectrum analyzer (ESA, FSVA30 R&S), and the results are shown in Figure 3e,f. A clear peak at 667 MHz was captured with a resolution bandwidth (RBW) of 1 MHz. In order to show the spectral broadening due to the Raman effect more clearly, we performed numerical simulation using the Lugiato-Lefever equation (LLE) [23]. Here, the Raman effect was neglected; thus, pure Kerr nonlinearity was involved in the simulation. According to the manufacturer's published data, the dispersion of HNLF is 1 ps/(nm\(\cdot\)km) and the effective mode area is 12.4 \(\upmu\)m\({}^{2}\)  
Figure 3: (**a–d**) Optical spectra of the FFP microresonator output with pump power set at 1.74 W, 3.5 W, 5.41 W, and 8.13 W, respectively. The resolution of the optical spectrum analyzer (OSA) is insufficient to resolve 667 MHz line-to-line spacing. The small slope of the spectrum baseline results from the uneven wavelength sensitivity of the detector and the Littrow light from blaze grating in the OSA. (**e**) RF beatnote spectrum, showing a repetition frequency of 667 MHz. RBW: resolution bandwidth. (**f**) Zoom-in view of the fundamental beatnote signal, showing a high signal-to-noise ratio (SNR) of about 50 dB. The pump wavelength was set to the deep effective red-detuned regime.