Photonic chip-based low-noise microwave oscillator

Abstract

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low-noise microwave signals are generated by the down-conversion of ultrastable optical references using a frequency comb^1-3. Such systems, however, are constructed with bulk or fibre optics and are difficult to further reduce in size and power consumption. In this work we address this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division^4,5. Narrow-linewidth self-injection-locked integrated lasers^6,7 are stabilized to a miniature Fabry-Pérot cavity⁸, and the frequency gap between the lasers is divided with an efficient dark soliton frequency comb⁹. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc Hz^-1 at 100 Hz offset frequency that decreases to -135 dBc Hz^-1 at 10 kHz offset-values that are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems.

Fig. 1. Concept of 2P-OFD for low-noise microwave generation.

a, Two semiconductor lasers are injection locked to chip-based spiral resonators. The optical modes of the spiral resonators are aligned, using temperature control, to the modes of the high-finesse F-P cavity for PDH locking. b, A microcomb is generated in a coupled dual-ring resonator and is heterodyned with the two stabilized lasers. The beat notes are mixed to produce an intermediate frequency, f_IF, that is phase-locked by feedback to the current supply of the microcomb seed laser. c, An MUTC photodetector chip is used to convert the microcomb’s optical output to a 20 GHz microwave signal.

Fig. 2. Experimental setup.

Two DFB lasers at 1,557.3 and 1,562.5 nm are self-injection-locked to Si₃N₄ spiral resonators, amplified and locked to the same miniature F-P cavity. A 6 nm broad-frequency comb with an approximately 20 GHz repetition rate is generated in a coupled-ring resonator. The microcomb is seeded by an integrated DFB laser, which is self-injection-locked to the coupled-ring microresonator. The frequency comb passes through a notch filter to suppress the central line and is then amplified to 60 mW total optical power. The frequency comb is split to beat with each of the PDH-locked SIL continuous wave references. Two beat notes are amplified, filtered and then mixed together to produce f_IF, which is phase-locked to a reference frequency. The feedback for microcomb stabilization is provided to the current supply of the microcomb seed laser. Lastly, part of the generated microcomb is detected in an MUTC detector to extract the low-noise 20 GHz signal. Photographs of the key photonic components used in low-noise microwave generation are in the lower panels. Scale bars (from left to right), 8 mm; approximately 1.5 cm; 4 mm; 1 mm. ISO, optical isolator; PM, phase modulator; PD, photodetector.

Fig. 3. Microcomb characterization.

a, Single side-band phase noise scaled to 10 GHz of free-running 20 GHz microcomb (blue), locked 20 GHz microwave (red) and locked 20 GHz microwave after regenerative frequency division by two (green). b, Optical spectrum of microcomb (grey) and SIL lasers (green and turquoise). c,d, Radio frequency spectra of 20 GHz signal free running (resolution bandwidth (RBW) 100 Hz; c) and locked (RBW 1 Hz; d).

Fig. 4. Phase noise comparison of microwave generation based on microcombs.

The platforms are all scaled to 10 GHz carrier and categorized based on the integration capability of the microcomb generator and the reference laser source, excluding the interconnecting optical/electrical parts. Filled (blank) squares are based on the OFD (stand-alone microcomb) approach: (i) 22 GHz silica microcomb; (ii) 5 GHz Si₃N₄ microcomb; (iii) 10.8 GHz Si₃N₄ microcomb; (iv) 22 GHz microcomb; (v) MgF₂ microcomb; (vi) 100 GHz Si₃N₄ microcomb; (vii) 22 GHz fibre-stabilized SiO₂ microcomb; (viii) MgF₂ microcomb; (ix) 14 GHz MgF₂ microcomb pumped by an ultrastable laser; and (x) 14 GHz microcomb-based transfer oscillator. SSB, Single side band.

Fig. 5. Schematic design of a photonic microwave oscillator on a single chip.

The integrated system uses the same key photonic elements used in this work. Two spiral resonator SIL lasers are PDH locked to the same micro-F-P cavity with two EOMs in series for each SIL laser—the first for fast phase correction and the second for PDH side bands. The right side of the schematic shows the F-P cavity interface, where the two SIL laser paths are fed through an interferometer with an embedded polarization splitting grating. This serves as a reflection cancellation circuit while also shaping the planar waveguide mode to match the F-P mode. The reflection from the F-P cavity is then detected by the right-most detector. The inset shows a photo of the miniature F-P cavity consisting of microfabricated mirrors, with overall volume of approximately 1 cm³. Scale bar, approximately 1 cm. Illustration reproduced with permission from B. Long.

Extended Data Fig. 1. Noise contributions to the 20 GHz microwave.

Cumulative noise represents the quadrature sum of all the shown noise terms. In-loop laser noise is shown only for a single SIL laser, since the noise of both SIL lasers are the same. Red (black) dotted line shows SNR limits of the 20 GHz microwave (intermediate frequency).

Extended Data Fig. 2. Microwave electronics used for stabilization and measurements.

(a) Electronics used to provide feedback for microcomb stabilization. (b) Phase noise measurement setup of 20 GHz microwave signal. (c) Phase noise of the reference 20 GHz signal used for cross correlation.

Extended Data Fig. 3. Regenerative divide-by-2.

(a) Setup of the regenerative frequency divide-by-2. (b) Experimental setup to measure the phase noise of the divider. (c) Phase noise of the regenerative divider referenced to the output carriers of 8 and 9 GHz, respectively.

Extended Data Fig. 4. Optical power requirements.

Estimated optical power to reach a specified phase noise floor or SNR in (a) the detection of the 20 GHz microcomb, and (b) the beat note between the SIL lasers and microcomb. The dotted line represents the required performance to achieve the results presented in the paper. The calculations assume a photodetector quantum efficiency of 0.5.

Extended Data Fig. 5. Integration and characterization of cavity with micro-fabricated mirrors.

(a) Transmission ringdown measurement of the 1 cm³ cavity. (b) Scheme for coupling the light to the cavity through an integrated grating and GRIN lens. (c) Test setup for measuring the coupling to the micro F-P cavity. (d) Optical power of transmitted and back reflected light through Grating-Fibre-GRIN-Cavity system as a function of different input laser frequency.