Optical frequency comb generation from aluminum nitride micro-ring resonator (1307.6761v1)

Abstract: Aluminum nitride is an appealing nonlinear optical material for on-chip wavelength conversion. Here we report optical frequency comb generation from high quality factor aluminum nitride micro-ring resonators integrated on silicon substrates. By engineering the waveguide structure to achieve near-zero dispersion at telecommunication wavelengths and optimizing the phase matching for four-wave mixing, frequency combs are generated with a single wavelength continuous-wave pump laser. The Kerr coefficient (n2) of aluminum nitride is further extracted from our experimental results.

• The paper presents an efficient optical comb generation method by leveraging Kerr nonlinearity in aluminum nitride micro-ring resonators.
• It details achieving near-zero dispersion at 1550 nm through optimized waveguide dimensions, yielding Q factors up to 800,000 as confirmed by simulations and SEM.
• The findings pave the way for integrated on-chip systems in telecommunications and precision metrology with low-power, high-efficiency operation.

Optical Frequency Comb Generation via Aluminum Nitride Micro-Ring Resonators

The paper presents significant advancements in the generation of optical frequency combs utilizing aluminum nitride (AlN) micro-ring resonators. By exploiting the Kerr nonlinearity of AlN and engineering waveguide structures to achieve near-zero dispersion at telecommunication wavelengths, the paper demonstrates the successful generation of frequency combs using a continuous-wave pump laser. This achievement aligns with the broader trend of utilizing high-Q factor micro-resonators on chip-scale platforms, emphasizing their applicability in high-speed telecommunications and frequency references.

Key Contributions

The authors have detailed the methodology employed in designing the AlN waveguide structures, highlighting two principal contributions to managing dispersion: material dispersion, dominant at shorter wavelengths, and geometric dispersion, important at longer wavelengths. By adjusting the waveguide widths, near-zero dispersion is achieved at a wavelength of 1550 nm, critical for effective frequency comb generation. The paper explores the structural design, with a waveguide height set at 650 nm and widths ranging from 2.5 µm to 3.5 µm, ensuring the waveguides support multiple transverse modes.

The paper utilizes scanning electron microscopy and numerical simulations to illustrate the micro-ring resonators' fabricated structure and modal profiles. With resonator Q factors of 800,000 and 600,000 for waveguide widths of 3.5 µm and 2.5 µm, respectively, the authors confirm the high-Q conditions conducive to efficient Kerr-induced comb generation.

Experimental Insights

Notably, the frequency combs generated span 200 nm with a free spectral range (FSR) of 370 GHz, utilizing a 60 µm-radius micro-ring resonator and a coupling waveguide width of 3.5 µm. The power threshold for comb generation is approximately 210 mW, and by analyzing the comb generation under varying detuning conditions, the paper showcases transitions from six FSRs to a single FSR as the laser is tuned closer to resonance.

The paper presents a quantitative analysis of the Kerr coefficient of AlN, estimated to be $$n_2 = (2.3 \pm 1.5) \times 10^{-15} \, \text{cm}^2/\text{W}$$ for the TE-like mode. This is compared to the Kerr nonlinearity of silicon nitride (SiN), indicating AlN's competitive edge in terms of nonlinear optical properties.

Implications and Future Directions

The implications of this work are multifaceted. Practically, the integration of AlN micro-ring resonators on silicon substrates paves the way for innovative on-chip systems capable of low-power, high-efficiency frequency comb generation. Such systems can significantly impact areas like precision metrology, optical communications, and frequency synthesis.

Theoretically, the paper enriches the understanding of Kerr-induced optical phenomena in AlN-based systems, leveraging both second and third-order nonlinearities. Future research directions could involve refining the fabrication processes to enhance Q factors, further minimizing power thresholds, and exploring electrically tunable frequency comb devices using AlN’s electro-optic properties. These enhancements could broaden the applications of on-chip frequency comb technology, potentially leading to new developments in optical clocks and chip-scale high-speed communications.

This paper thus contributes to the ongoing exploration of integrated photonics, showcasing AlN's potential in advancing compact, efficient optical frequency comb generation. Its findings could catalyze future innovations in photonic and optoelectronic applications.