Phase Noise Characterization of a Mode-Locked Quantum-Dot Coherent Optical Frequency Comb Source Laser
Modern coherent optical communication systems transmit 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signals on each of the 100 channels in a dense wavelength division multiplexing (DWDM) system. Each of these channels require an individual optical carrier frequency to be generated from a discrete high-performance tunable laser. These arrays of discrete lasers are an expensive component in the transmitter of an optical communication channel. Thus, recent research has investigated an alternative laser source - optical frequency combs (OFCs). OFCs generate numerous spectral modes, referred to as comb lines, from a single photonic device. Each of these comb lines can be used as an optical carrier frequency for a channel in a DWDM system. Thus, the implementation of an OFC as a laser source for coherent optical communications minimizes the number of laser modules in a DWDM system leading to a drastic reduction in system cost and complexity. Additionally, OFCs exhibit an intrinsic coherence among each comb line across the spectrum. This coherence implies that the phase noise exhibited by each comb line in the OFC is related which allows for effective methods of phase noise compensation. Consequently, OFCs are heavily under research and development. In this research, a prototype 25 GHz quantum-dot coherent optical frequency comb source laser is characterized in both the time and frequency domains. The output of the comb source laser is extensively observed and the optical and electrical spectra, spectral flatness, relative intensity noise, phase noise, linewidth and timing jitter are measured. These preliminary results led to the development of a coherent phase noise detection scheme capable of recovering time domain phase noise trajectories for pairs of comb lines. The detection scheme allowed the coherence exhibited by the comb source to be quantified using a correlation coefficient which compared recovered phase noise trajectories for pairs of comb lines. Additionally, the statistical properties of the recovered phase noise trajectories are explored which enables experimental quantification of the relative contribution of amplified spontaneous emission (ASE) noise and timing jitter for each mode. Experimental results are shown to align with theoretical descriptions of phase noise in mode-locked lasers.