## 2 Characteristics of Designed Silica Microsphere WGMR for Kerr-OFC
The pumping source for the Kerr-OFC generation consists of a continuous optical wavelength (CW) laser source (Agilent 81989A) with a linewidth of 100 kHz and +6 dBm optical output power at \(\lambda\) = 1551.737 nm wavelength that is directly connected to fixed output power (up to +23 dBm) erbium-doped fiber amplifier (EDFA). The light coming from the EDFA is passed through an optical isolator and PC1 to prevent back-scattering and to control the amplified signal's polarisation state before coupling it into the microsphere through the tapered fiber, providing further stability of the resulting OFC. An optical spectrum analyzer (OSA-1, Advantest Q8384) with 0.01 nm resolution and 1001 sampling points is used to monitor generated OFC as well as to measure the peak powers of generated (\(-1\)) and (+1) carriers and pumping source depicted as a carrier (0); see Figure 4d. The power instability and power distribution stability over the wavelength of the generated OFC comb carriers was not measured during this research. The generated comb carriers (\(-1\)) and (+1) over 10 h have a different performance within about a 3 dB margin; please see our previous work [9]. Silica microsphere WGMR-based Kerr-OFC light source newly generated carriers (\(-1\)) and (+1) and comb carriers followed by (+1) carrier have additional sub-carriers generated under the influence of stimulated Brillouin scattering (SBS); see Figure 4d. The SBS effect occurs due to a relatively high pumping power launched from EDFA (up to +23 dBm) in the 170 \(\upmu\)m silica microsphere. The SBS effect appears and then disappears in the time interval for both the pumping source and the newly generated carriers on the left or right side with a constant FSR of \(\sim\)10 GHz and various power levels  
Figure 4: The characterization of the tapered fiber and its coupling conditions with the silica microsphere: (**a**) spectrum of the adiabatically tapered fiber recorded for the overall transmittance analysis and (**b**) the calculations of pulled one side fiber length at hot zone from center versus fiber diameter at tapered section. (**c**) Experimentally observed (from the microsphere excited by the tapered fiber) WGM resonances of the Kerr-OFC at the optical C-band used for the Q factor calculation and (**d**) optical spectra of the generated silica microsphere WGMR-based Kerr-OFC comb source. within the time interval of about ~15 min (usually this is observed when the comb has been operated for some time). It is relatively hard to detect and predict those sub-carriers bursts because their phenomenon is unstable and changeable in time-lapse, as captured and shown in Figure 4d. Experimental Setup of the Silica Microsphere WGMR-Based Kerr-OFC Light Source for Its Use in High-Speed IM/DD Short-Reach Optical Interconnects  
The experimental setup is shown in Figure 5. The silica microsphere WGMR-based Kerr-OFC light source (i.e., Kerr-OFC) output 400 GHz spaced optical carriers at wavelengths \(\lambda\) = 1548.485 nm depicted as (\(-\)1), \(\lambda\) = 1554.992 nm depicted as (+1), and the pumping source at \(\lambda\) = 1551.737 nm depicted as (0) and having the highest achieved peak power levels (\(\geq\)\(-\)15 dBm) are used to demonstrate NRZ-OOK modulated data up to 50 Gbps/\(\lambda\) over 2 km SMF link. As shown in Figure 5, the generated OFC carriers are sent to an optical band-pass filter (OBPF, Santec (London, UK), OTF-350), possessing a 3 dB bandwidth of 75 GHz. The OBPF is used to filter out one of the three optical carriers. An optical coupler with a 10/90 coupling ratio is used to capture the output carrier spectrum using OSA-2. It is performed for monitoring purposes and to determine carriers' wavelengths and optical peak powers that are later used for data transmission. The filtered optical carrier is amplified by a pre-amplifier (EDFA2) that fixes the output power level for each OFC carrier.