Linewidth narrowing via low-loss dielectric waveguide feedback circuits in hybrid integrated frequency comb lasers

Abstract

We present an integrated hybrid semiconductor-dielectric (InP-Si 3 N 4) waveguide laser that generates frequency combs at a wavelength around 1.5 µm with a record-low intrinsic optical linewidth of 34 kHz. This is achieved by extending the cavity photon lifetime using a low-loss dielectric waveguide circuit. In our experimental demonstration, the on-chip, effective optical path length of the laser cavity is extended to 6 cm. The resulting linewidth narrowing shows the high potential of on-chip, highly coherent frequency combs with direct electrical pumping, based on hybrid and heterogeneous integrated circuits making use of low-loss dielectric waveguides.

Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits

“…The total fibercoupled output power is 2 mW. (right) Calculated frequency comb spectrum using the experimental parameters as listed in the appendix of [168]. The calculation is based on a transmission line model to represent the spatial, spectral and temporal distribution of light and charge carriers in the gain section, while using an analytically calculated, complex-valued field amplitude reflectivity spectrum to model the feedback circuit [129].…”

“…In order to provide narrower linewidth in diode laser combs we have investigated comb generation with a hybrid integrated InP-Si3N4 laser as shown in Fig. 12 [168]. We use a standard low loss Si3N4 feedback circuit to increase the photon lifetime and thereby decrease the intrinsic linewidth of the individual comb frequencies.…”

High-purity microwave generation using a dual-frequency hybrid integrated semiconductor-dielectric waveguide laser

“…The Si 3 N 4 feedback circuit is, in contrast to previously reported hybrid lasers [23,30,32], fabricated with an asymmetric double stripe (ADS) cross section. Compared to symmetric double stripe (SDS) waveguides, these require less fabrication steps and offer similar or lower propagation loss (< 0.1 dB/cm) [33].…”

“…To enable lasing at two frequencies, which we refer to as 𝑓 1 and 𝑓 2 , the circuit contains two Vernier loop mirrors (M 1 and M 2 ). These loop mirrors have a similar design as in [23,32], with two microring resonators (MRRs) which have slightly different radii (𝑅 1 = 125 𝜇m, 𝑅 2 = 127.5 𝜇m). The power coupling to the bus waveguides was designed to be 1%, to realize significant cavity length enhancement at resonance (enhancement factor 𝐹 = 99), due to multiple passes through the MRRs [34].…”

Recent Advances in the Photonic Integration of Mode-Locked Laser Diodes

“…Another appealing option is to use silicon nitride waveguides, as they allow for dispersion engineering and have no two-photon absorption. No III-V-on-silicon-nitride was demonstrated so far, but recently a MLL based on a buttcoupled silicon nitride cavity was reported [33].…”