Abstract

The development of compact and low-cost frequency comb sources with large comb line spacings in the gigahertz range is motivated by numerous applications such as precision spectroscopy, frequency metrology or sensing [1,2]. Here, we present the broadest coherent supercontinuum spectra generated to date in a CMOScompatible silicon nitride (Si₃N₄) waveguide and demonstrate the first carrier-envelope offset (CEO) frequency (f_CEO) detection of a modelocked 1-GHz diode-pumped solid-state laser (DPSSL) based on this platform. The SESAM-modelocked 1-GHz DPSSL consists of a 2-mm-long Yb:CaGdAlO₄ (Yb:CALGO) laser crystal pumped with a spatially multimode laser diode (Fig. 1a) and can deliver pulses as short as 63 fs at a center wavelength of 1055 nm with 1.7 W of average output power [3]. The 7.5-mm-long Si₃N₄ spiral waveguide is oxide clad with a cross section of 690 nm by 900 nm, resulting in a group-velocity dispersion (GVD) profile with two zero-GVD points at 860 nm and 1300 nm. Figure 1b shows the spectral broadening obtained when launching pulses with a duration of 92 fs (17 nm FWHM spectral bandwidth) into the waveguide for coupled pulse energies ranging from 0.6 pJ to 36 pJ (coupling efficiency 15%). In order to characterize the spectral coherence of the entire supercontinuum, we perform a full-range spectral interference measurement using a free-space asymmetric Michelson interferometer, where one arm provides a delay corresponding to the pulse period [4]. Interference fringes on the optical spectrum analyzer (OSA) can be observed when the SC generated by subsequent pulses are coherent. The coherence is extracted from the fringe visibility as described in [4] and is near unity for the majority of the spectral bandwidth (Fig. 1c).

© 2015 IEEE