Fully integrated 45-GHz harmonically mode-locked ring laser with an intra-cavity Mach-Zehnder filter

Wavelength division multiplexing (WDM) systems can utilize the full capacity of a single optical fiber and thereby keep up with the increasing demand for higher bandwidths within datacenters. A single mode-locked laser diode emits a comb of wavelengths and can thus, in principle, be used to generate all the channels of a WDM system. However, achieving a large channel spacing of much more than 20-30 GHz can be troublesome, since this depends directly on making the cavity smaller. To circumvent this, harmonic mode-locking can be utilized, as this increases the channel spacing while keeping the cavity size fixed. In this work, we show that a monolithically integrated 45-GHz harmonically mode-locked ring laser based on an intra-cavity Mach-Zehnder filter is feasible on a generic integration platform. True harmonic mode-locking was achieved with no measurable RF peak at the fundamental frequency. The pulse train exhibits an autocorrelation trace width of similar to 2.5 ps FWHM, RF linewidth of similar to 0.44 MHz, and 3-dB comb bandwidth of similar to 240 GHz. (C) 2021 Optical Society of America