# Microscope-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System
## 4 Discussion and Conclusions  
We proposed and numerically investigated a simple design of silica microsphere-based OFC generator in the optical C-band producing stable DKS and simulated its implementation in the WDM data transmission system. The optimal microsphere diameter of 328.5 \(\upmu\)m provides an FSR of 200 GHz, according to ITU-T G. 694.1 recommendation. We considered the TE and TM families of fundamental WGMs and found that the FSR values practically coincide. With a slight deviation of the diameter from the optimal value, FSR changes linearly. For example, if a diameter changes by \(\pm 2\%\), FSR changes by \(\pm 2\%\) too (for diameters of 322 \(\upmu\)m and 335 \(\upmu\)m, FSRs are 204 and 196 THz, respectively). We also investigated the influence of thermo-optical effects on WGM shift and FSR. The WGM shift and FSR are almost linear functions of temperature. When the temperature rises by 60 C, the WGM eigenfrequencies decrease by 100 GHz, but FSR decreases by 0.1 GHz. We simulated steady-state temperature distribution originated from partial pump power dissipation. Although the heat source  
Figure 12: Eye diagrams of the received signal: (**a**) after B2B, (**b**) after 20 km, (**c**) after 40 km, (**d**) after 60 km transmission via SMF optical link section for investigated 4-channel 200 GHz spaced IM/DD WDM-PON system operating at 10 Gbit/s per channel. was located in a small volume equal to the effective mode volume, the temperature distribution was fairly uniform over the microsphere. The dispersion as a function of frequency and the nonlinear Kerr coefficient were calculated. In the C-band, the dispersion is anomalous (of the order of \(-10\) ps\({}^{2}\)/km). Both material and waveguide contributions are important here. The zero-dispersion frequency is about 209 THz. Dispersion curves for TE and TM fundamental modes differ slightly. The nonlinear Kerr coefficient is about 3.5 (W\(\cdot\)km)\({}^{-1}\). It was also verified that the change in dispersion with temperature increasing was negligible for the considered heat powers of a few mW. We simulated OFC generation in the DKS regime for the range of normalized detuning \(\Delta\) where stable DKS can exist. When \(\Delta\) changes from 10 to 70, the spectral width at the level of \(-30\) dB grows from \(\sim\)3.8 THz to 8.8 THz, and the DKS duration in the time domain shortened from 379 fs to 169 fs (FWHM). DKS spectra are asymmetric with respect to the pump frequency at 193.1 THz, which is explained by the influence of the Raman nonlinearity and agrees with the results presented in [43]. We assumed that OFC with FSR of 200 GHz could be anchored to 193.1 THz using temperature control. Narrow-band CW pump can be coupled to a microresonator using a fiber taper and generated OFC can be extracted from a microresonator with the same taper [28]. Note that a reliable way to generate DKS and influence of thermal effects on the dynamics of DKS formation is considered in [47]. The experimental features of the DKS generation in a silica microsphere are reported in [27]. For attaining OFCs with lower FSRs, bottle microresonators can be used as predicted in [48; 49]. We have also simulated a 4-channel 200 GHz spaced IM/DD WDM-PON transmission system with operating data rates of 10 Gbit/s per channel over different SMF fiber link lengths up to 60 km. The variation in the BER performance of the OFC-WGMR portable light source is observed and is mainly affected by the power and noise floor variation between carriers. Therefore, the proposed 200 GHz spaced OFC-WCOMB light source-based IM/DD WDM-PON transmission system is fully capable of providing 10 Gbit/s of NRZ modulated signal transmission to NG-PON2 recommendation specified valid optical link distances up to 40 km. Note that the considered system is attractive to service providers for substituting existing central office (CO) architecture.