[Paper Review] All-optical frequency division on-chip using a single laser

Yun Zhao, Jae K. Jang+5 more|arXiv (Cornell University)|Mar 6, 2023

Advanced Fiber Laser Technologies4 citations

TL;DR

This paper demonstrates on-chip all-optical frequency division using a single continuous-wave laser, leveraging a microresonator-based optical parametric oscillator (OPO) and a Kerr-comb microresonator to achieve passive synchronization and phase-noise reduction. A 630-fold phase-noise reduction is achieved, enabling chip-scale microwave generation with performance rivaling laboratory-grade atomic clocks.

ABSTRACT

The generation of spectrally pure high-frequency microwave signals is a critical functionality in fundamental and applied sciences, including metrology and communications. The development of optical frequency combs has enabled the powerful technique of optical frequency division (OFD) to produce microwave oscillations of the highest quality. The approaches for OFD demonstrated to date demand multiple lasers with space- and energy-consuming optical stabilization and electronic feedback components, resulting in device footprints incompatible with integration into a compact and robust photonic platform. Here, we demonstrate all-optical OFD on a single photonic chip driven with a single continuous-wave laser. We generate a dual-point frequency reference using the beat frequency of the signal and idler fields from a microresonator-based optical parametric oscillator (OPO), which achieves high phase stability due to the inherently strong signal-idler frequency correlations. We implement OFD by optically injecting the signal and idler fields from the OPO to a Kerr-comb microresonator on the same chip. We show that the two distinct dynamical states of Kerr cavities can be passively synchronized, allowing broadband frequency locking of the comb state, which transfers the stability of the OPO frequencies to the repetition rate of the Kerr comb. A 630-fold phase-noise reduction is observed when the Kerr comb is synchronized to the OPO, which represents the lowest noise generated on the silicon-nitride platform. Our work demonstrates a simple, effective approach for performing OFD and provides a pathway toward chip-scale devices that can generate microwave frequencies comparable to the purest tones produced in metrological laboratories. This technology can significantly boost the further development of data communications and microwave sensing.

Motivation & Objective

• To develop a compact, integrated photonic platform for generating spectrally pure, high-frequency microwave signals.
• To overcome the limitations of conventional optical frequency division (OFD) that require multiple lasers and complex stabilization systems.
• To enable passive, feedback-free synchronization between an OPO and a Kerr comb on a single chip.
• To achieve ultra-low phase noise in microwave generation using only a single continuous-wave laser pump.
• To demonstrate a scalable, robust, and integrable solution for high-performance microwave sources in communications and sensing.

Proposed method

• A single continuous-wave laser pumps a silicon nitride microresonator operating in the optical parametric oscillator (OPO) regime, generating a frequency-correlated signal-idler pair as a stable optical reference.
• The signal and idler fields from the OPO are optically injected into a second microresonator operating in the Kerr-soliton regime, which generates a frequency comb with a microwave repetition rate.
• Passive synchronization between the OPO and Kerr-comb states is achieved through inherent dynamical coupling, enabling broadband frequency locking without electronic feedback.
• The system exploits the strong intrinsic frequency correlation between the OPO signal and idler fields to transfer phase stability to the Kerr comb's repetition rate.
• Phase noise is characterized using a delayed self-heterodyne setup with a phase noise analyzer, enabling precise measurement of noise suppression.
• Thermal noise in the microresonators is modeled and characterized using a homodyne interferometric setup with calibrated laser noise injection.

Experimental results

Research questions

• RQ1 Can all-optical frequency division be achieved on a single photonic chip using only one continuous-wave laser?
• RQ2 How can passive synchronization between an OPO and a Kerr-comb microresonator be achieved without electronic feedback?
• RQ3 What level of phase-noise reduction can be achieved in microwave generation using a single-laser, on-chip OFD system?
• RQ4 Can the inherent frequency correlation in OPO-generated signal-idler pairs be leveraged to stabilize a Kerr comb's repetition rate?
• RQ5 To what extent can thermal and thermorefractive noise be suppressed in integrated microresonators for ultra-low-noise operation?

Key findings

• A 630-fold reduction in phase noise is achieved when the Kerr comb is synchronized to the OPO, representing the lowest noise level reported on a silicon-nitride platform.
• The system operates with a single continuous-wave laser, eliminating the need for multiple lasers and complex stabilization electronics.
• Passive synchronization between the OPO and Kerr-comb states is achieved through intrinsic dynamical coupling, enabling broadband frequency locking.
• The OPO generates a dual-point frequency reference via the beat signal of its signal and idler fields, which provides high phase stability due to strong signal-idler frequency correlations.
• Thermal noise characterization confirms that tailored waveguide geometry suppresses thermorefractive noise, with identical resonance shift coefficients at key telecom wavelengths.
• The experimental setup achieves stable microwave generation at 630 GHz with phase noise performance comparable to state-of-the-art metrological systems.