Few-GHz Repetition Rate Optical Frequency Combs in III-V/SiN Hybrid Lasers

Introduction

Optical frequency combs (OFCs) are a rapidly developing area in photonics research, with applications ranging from molecular spectroscopy and LIDAR to optical communications and wavelength division multiplexing systems. Silicon photonics integration offers the potential for low-cost, CMOS-compatible OFC devices with reduced size, weight, and power consumption for out-of-the-lab applications. While on-chip OFC generation has seen great success, these integrated devices often require external driving electronics and optics. A solution is the spontaneous generation of OFCs directly from integrated laser sources, as recently demonstrated experimentally.

This tutorial discusses the theoretical modeling and study of self-generated few-GHz repetition rate OFCs in hybrid integrated III-V/SiN external cavity lasers. The OFC arises from the combined effects of four-wave mixing (FWM), undamped relaxation oscillations, and a non-zero linewidth enhancement factor (Henry factor).

Laser Structure

The hybrid laser comprises a III-V reflective semiconductor optical amplifier (RSOA) edge-coupled to a SiN photonic circuit with two coupled ring resonators, as shown in Figure 1(a). This configuration introduces an effective dispersive mirror reflectivity reff to the right of the RSOA via the Vernier effect, illustrated in Figure 1(b). A phase section tunes the lasing frequency relative to the reff peak, while the output is collected before the ring splitter.

Figure 1: (a) Hybrid laser schematic (b) Effective mirror reflectivities via Vernier effect

Modeling and Results

The laser dynamics are described using a time-domain traveling wave model accounting for the non-uniform electric field and carrier density along the RSOA. Figure 2(a,b) shows the temporal output power and instantaneous frequency exhibiting strong amplitude and frequency modulation for Ibias = 300 mA and reff FWHM of 6 GHz. The resulting OFC spectrum is displayed in Figure 2(c).

Figure 2: (a,b) Output power and frequency dynamics (c) Optical comb spectrum (d) Comb bandwidth and line spacing trends (e) Phase and amplitude noise quantifiers

At low currents, the laser emits in a single mode. Increasing the current causes relaxation oscillation undamping, resonating with the separation between the lowest threshold longitudinal modes, resulting in multimode emission. This instability requires detuning of the lasing frequency from the reff peak and a non-zero Henry factor to convert phase to intensity noise.

Following single-mode instability, OFCs arise from efficient mode proliferation via parametric gain and phase locking by FWM of the newly generated comb lines separated by the longitudinal mode spacing.

The maximum achievable -20 dB comb bandwidth and corresponding line separation are shown in Figure 2(d) for varying reff bandwidth and detuning. Narrow reff leads to strong dispersion and filtering, limiting the comb bandwidth, while broader mirrors allow more longitudinal modes but suffer from increased mode competition hindering phase locking. This trade-off suggests an OFC bandwidth saturation around 100 GHz.

To verify the generated multimode regimes are high-quality OFCs, the differential phase noise quantifier β and intensity noise quantifier γ are calculated for all comb lines within -20 dB of the peak, as shown in Figure 2(e). The low β values well below 0.1 attest to the comb quality.

Key Insights

• Few-GHz repetition rate optical frequency combs can be self-generated in hybrid III-V/SiN external cavity lasers from the combined effects of four-wave mixing, undamped relaxation oscillations, and a non-zero linewidth enhancement factor.

• The effective mirror reflectivity implemented via coupled SiN rings provides key dispersion for comb generation while relaxing requirements for ultra-high Q resonators compared to Kerr combs.

• A trade-off between mirror bandwidth and achievable comb bandwidth exists, with optimal -20dB bandwidths up to ~100GHz predicted.

• Quantitative analysis of differential phase/amplitude noise confirms the high quality of the generated frequency combs within the predicted bandwidth limits.

This integrated hybrid laser approach provides a promising path towards fully integrated, self-starting few-GHz rate optical frequency combs highly suitable for applications like coherent communications without requiring external pulsed driving sources.

Reference

[1] C. Rimoldi, M. Novarese, L. Columbo, M. Gioannini, "Few-GHz Repetition Rate Frequency Combs in III-V/SiN External Cavity Lasers," Department of Electronics and Telecommunications, Politecnico di Torino, Torino, Italy, 2024.