Monolithic Multi-Wavelength Mode-Locked DFB Laser Using Chirped Conventional and Four-Phase-Shifted Sampled Bragg Gratings

Abstract

We report, for the first time, a monolithic multi-wavelength passively mode-locked distributed feedback (DFB) laser operating simultaneously at four wavelengths near $1.55~\mu $ m. The device incorporates two chirped sampled Bragg grating (SBG) designs within a single cavity: 1) chirped conventional SBG (C-SBG) and 2) chirped four-phase-shifted SBG (4PS-SBG). Both configurations achieve uniform 0.96 nm (~120 GHz) wavelength spacing using a single DFB section electrode and monolithically integrated saturable absorber (SA) for synchronized passive mode-locking. This shared-cavity architecture ensures intrinsic stability of the frequency comb against environmental perturbations, as all longitudinal modes experience identical phase variations. The lasers exhibit high spectral purity with side-mode suppression ratios (SMSR) >20 dB, with the 4PS-SBG design offering wider bias current operation. Pulse characteristics include near-transform-limited performance for both designs: C-SBG yields 2.84 ps pulses (time-bandwidth product [TBP] = 0.334), while 4PS-SBG generates 2.79 ps pulses (TBP = 0.337). Fabrication employs a simplified ridge waveguide sidewall grating approach requiring only one metalorganic vapor phase epitaxy (MOVPE) step and a single III–V etch process, enhancing manufacturability. We further demonstrate the design’s versatility by extending operation to six wavelengths using the 4PS-SBG structure. This integrated platform shows strong potential for dense wavelength division multiplexing (DWDM), coherent communications, and photonic sensing applications requiring compact, environmentally stable multi-wavelength sources.