Enhanced Optical Comb Generation by Gain-Switching a Single-Mode Semiconductor Laser Close to Its Relaxation Oscillation Frequency

We demonstrate, through experimental results and supporting numerical simulations, the optimum conditions to generate an optical comb via gain-switching of a single-mode DFB semiconductor laser. The DFB laser is biased appropriately and gain-switched with a large amplitude sinusoidal current, modulating the gain that is sufficient to turn the laser ON and OFF. This results in the generation of a train of short optical pulses albeit with a relatively large amount of temporal jitter. The jitter is a result of the varying turn on time delay, which breaks the periodicity of the pulses leading to an optical spectrum with no discernible tones. However, we find that the conditions for creating an optical frequency comb exist when the modulation frequency is greater than or equal to the relaxation oscillation frequency of the laser. Corresponding optical pulses are shown to exhibit lower levels of temporal jitter at these optimum points of operation. The spectral purity of the comb tones is analyzed, and we show that the FM-noise of the comb tones decreases with increasing modulation frequency with the lower limit set by operating the laser in CW conditions. All of the simulation results were obtained using standard laser rate equations.