Analysis of the modulation impairments in optical sideband injection locking for millimeter-wave signal generation

Abstract

In optical sideband injection locking (OSBIL) transmitters, the millimeter-wave (mm-wave) signal is generated heterodyning the phase-correlated outputs of two lasers, resulting in mm-wave signals with high frequency purity. Additionally, OSBIL is a cost-effective mm-wave generation technique since it does not require any external broad-bandwidth optical modulator. However, the non-linear distortion and the noise of the directly modulated laser limit its performance. This paper studies for the first time, the modulation impairments in terms of the injection conditions considering the sideband asymmetry that persists in heterodyne generation techniques. Signal-to-noise ratio and nonlinear distortion are analyzed through simulations showing different performance for upper and lower modulation sidebands and its dependence on the intermediate frequency. We show that the noise spectral density is reduced setting up the injected laser at (i) low power injection ratios and using a low intermediate frequency or (ii) higher power injection ratios, a negative frequency detuning, and a higher intermediate frequency. Regarding nonlinearities, we show that its effect on the lower modulation sideband is less significant than in the upper sideband. This impairment analysis is used to optimize the generation of OFDM signals in the 60-GHz band, achieving 2.5 Gbps generation with a spectral efficiency of 3.4 bps/Hz.