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 ∼2.5ps FWHM, RF linewidth of ∼0.44MHz, and 3-dB comb bandwidth of ∼240GHz.