Effects of optical local oscillator power on the network bit error rate in optical heterodyning of comb lines from a gain-switched VCSEL comb

The photonic technique is a feasible solution for generation of high-frequency RF carrier signals, attracting considerable attention in the past decades. In the laser gain-switching technique for photonic generation of high -frequency RF carrier signals, two optical lines, separated by the desired RF carrier frequency are filtered from the generated optical frequency comb. One of the filtered comb lines is externally modulated with an optical modulator, while the other acts as an optical local oscillator signal. In this paper, we report on the effects of optical local oscillator signal power on the RF carrier signal and the baseband signal of a fronthaul network using a sub-60 GHz RF carrier signal generated from a gain-switched, low-power, VCSEL-based optical frequency comb source. A 56-GHz RF carrier signal was generated by filtering out from the generated optical frequency comb, two optical lines with wavelength difference corresponding to the desired RF carrier frequency. The resultant RF carrier signal was photonically amplitude-modulated with 16-Gbps data before transmitted over 21-km of SSMF. In this study, we investigated the effects of attenuating the optical local oscillator signal power below the modulated optical carrier signal power. It was found that the RF carrier power is reduced when attenuating the optical local oscillator power below the power of the modulated optical carrier signal. However, the performance of the baseband signal is improved. It was found that by optimally establishing a power ratio between the two filtered optical signals, a power gain of up to 18-dB can be achieved to reach an error-free BER of 10-9 for the baseband signal. For the RF signal at the beating frequency, it was found that reducing the power of the modulated optical carrier signal below the power of the optical local oscillator signal could results in power improvements.