# Microscope-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System
## 3 Results
### Architecture and Simulation of 4-Channel 200 GHz Spaced IM/DD WDM-PON Transmission System  
The simulation setup of a 4-channel 200 GHz spaced WDM-PON transmission system is depicted in Figure 9.  
The output of an amplified spontaneous emission (ASE) optical light source with high output power of up to 23 dBm and spectrum power density of \(-6\) dBm/nm within the 1528-1630 nm band is connected to the input of the user-defined optical band-pass filter (OBPF) where the above-simulated silica microsphere-based OFC output spectrum is implemented. Afterward, the output of comb spectral lines is filtered out by an optical band-pass filter with 760 GHz 3-dB bandwidth to obtain four optical carrier signals. The spectral lines from OFC-WGMR are filtered and de-multiplexed utilizing arrayed-waveguide-grating (AWG) de-multiplexer (de-MUX), which corresponds to wavelength-routed WDM-PON (WR-WDM-PON) architecture. The 3-dB bandwidth of each AWG channel for 200 GHz channel spacing was set to 87.3 GHz. The spectrum of the optical signal at the output of the user-defined OBPF with implemented microsphere-based OFC, the obtained band-pass filtering for four optical carriers, and the output of AWG de-multiplexer after back-to-back (B2B) transmission are shown in Figure 10a-c, respectively. Note that Figure 10a contains OFC lines simulated in Section 3.2 and presented in Figure 8a. The Lorentzian line shape is assumed. Carriers (comb spectral lines) separated with AWG de-MUX are fed to the optical input of the Mach-Zehnder modulators (MZMs). The electrical data signals are provided by PRBS from PRBS generator through NRZ driver, which encodes the logical data by using the non-return-to-zero (NRZ) technique generating electrical NRZ signals with a bit rate of 10 Gbit/s. Each MZM, having a 3-dB bandwidth of 12 GHz and 20 dB extinction ratio, is driven by NRZ signal \(S_{1}(t)\)[45]. Optical signals from each transmitter's (Tx) MZM are coupled together using an AWG multiplexer (MUX). The combined, modulated optical signals are transmitted over 20 up to 60 km ITU-T G.652 single-mode fiber (SMF) span, with 0.02 dB/km attenuation and 16 ps/nm/km dispersion coefficients at 1550 nm reference wavelength. According to NG-PON2 (ITU-T G.989.2) recommendation, the specified valid optical link distances are up to 40 km, but longer lengths are supported and reach network distance up to 60 km. Therefore, we extend the optical link section to maximal PON transmission distance of 60 km. The receiver (Rx) consists of a PIN photodiode with 3-dB bandwidth of 12 GHz, sensitivity of \(-18\) dBm for BER of \(10^{-12}\), and responsivity of 0.65 A/W [46]. Afterward, the received, modulated signal is filtered by an electrical low-pass filter (LPF) with 7.5 GHz 3-dB electrical bandwidth. The electrical signal analyzer is used to measure the received signal, e.g., showing bit pattern and BER. Figure 9: Simulation model of the 4-channel 200 GHz spaced IM/DD WDM-PON system used for the designed silica microsphere-based OFC comb spectrum implementation and performance assessment where ASE—Amplified spontaneous emission light source, AWG—Arrayed-waveguide-grating, LPF—Low-pass filter, MZM—Mach–Zehnder modulator, OBPF—Optical band-pass filter, OFC-WGMR—Optical frequency comb generator based on whispering gallery mode resonator, PIN—photodiode, SMF—single-mode fiber. The performance indicators as BER and eye diagrams of the received signal verify the feasibility of the designed transmission. The obtained BER results of each optical channel with respect to optical network link section length of up to 60 km over SMF for NRZ modulated 4-channel IM/DD WDM-PON transmission system with 200 GHz spacing is shown in Figure 11.  
We observed that the worst-performing channel in terms of BER performance was the 1 st channel (193.1 THz). In the best scenario, the highest system performance was observed for the 4 th optical channel (193.7 THz), where BER of the received signal after transmission over 60 km SMF fiber link was \(4.5\times 10^{-30}\).