## 2 Characteristics of Designed Silica Microsphere WGMR for Kerr-OFC
Side and top view microscope cameras (CAM-1 and CAM-2) with 160\(\times\) zoom lens (see Figure 2e) were used to monitor silica microsphere and tapered fiber positions to control the touchpoint of the resonator with the slightly thicker tapered fiber placed to excite the fundamental mode of the microsphere. Silica microsphere and tapered fibers, including integrated and developed components for fiber tapering and microsphere positioning, are included in an enclosure box to protect it from dust and airflows. Humidity inside the enclosure box is reduced and maintained at levels below 20% using a silica gel desiccant. The enclosure box together with pumping source components is located on a vibration isolation system breadboard table to minimize the impact of external low frequency vibrations. The setup used for the characterization of the taper fiber (see Figure 3) consists of InGaAs switchable gain amplified photo-detector (PD) \(\uplambda=\) 800-1700 nm (Thorlabs (Bergkirchen, Germany), PDA20 CS2) connected with a digital signal analyzer (DSA) used to record the received electrical signal in time and to monitor the transmission spectra from the adiabatically tapered fiber for an overall transmittance analysis used also in further telecom data transmission. A tunable \(\uplambda=\) 1550 nm laser (Thorlabs SFL1550P) with a narrow linewidth of 50 kHz and 10 dBm output optical power is connected via a polarization controller (PC1) and used for transmission spectra measurements. The transmission spectrum was monitored during the tapering process to determine when it returns to the single-mode operation state (see Figure 4a). As shown in Figure 4a, the adiabatically tapered fiber spectrum presents single-mode (SM) operations in the beginning from 0 to 0.5 min mark and after pulling at around 3.3 min mark (represented in the red color) when the signal intensity becomes constant. SM fiber becomes multi-mode (MM) when the signal starts to propagate through the cladding (represented in the blue color). Calculations show that the hot zone's width of exponentially pulled tapered fiber should be at least 7 mm long to satisfy the adiabaticity criteria at the middle of the pulling range and to maintain high final transmission [33]. Please see the calculations of the pulled length of tapered fiber hot zone to one side from center versus fiber diameter at the tapper section in Figure 4b. The setup shown in Figure 3 also was used to determine the Q factor of the silica microsphere and to identify the best coupling condition region between the microsphere and tapered fiber used in OFC generation. The measured Q factor of the microsphere used for OFC comb generation and further telecom data transmission is \(3.7\times 10^{7}\) (5.2 MHz FWHM of the WGM resonance); please see Figure 4c. As shown in Figure 4c, resonances were obtained by scanning the laser over resonances while Kerr-OFC was generated with a single pumping wavelength. The silica microsphere resonator supports many modes inside the cavity, but not all modes in addition to the fundamental mode can support Kerr-OFC generation. Figure 3: Experimental setup used for the characterization of the tapered fiber and silica microsphere. Figure 2: (**a**) Experimental setup illustrating the developed silica microsphere WGMR-based Kerr-OFC as a light source for optical communications. (**b**) Schematic of a Hydrogen generator for pure generation of hydrogen (H\({}_{2}\)) and oxygen (O\({}_{2}\)) by electrolysis of water. (**c**) Captured setup of silica microsphere WGMR-based Kerr-OFC light source. (**d**) Tapered fiber and silica microsphere resonator positions inside of enclosure box for dust and airflow prevention. (**e**) The 3-axis X, Y, and Z micro-positioner stage with a built-in Piezo controller and compact, motorized translation stages together with zoom microscopes used to monitor the position of the WGMR resonator. (**f**) Captured hydrogen generator for pure hydrogen and oxygen production.