Optically locked low-noise photonic microwave oscillator

The next-generation sensing and communication applications rely on high-frequency microwave generation with low-noise. The microwave photonic technology is promising by the practical application is limited by its complex architecture so far. Here, we demonstrate an optically locked low-noise photonic microwave oscillator, so that all the optical components are packaged within a small module of 166 mL, and low noise microwave generation is achieved at 10.4 GHz with single-sideband phase noise of -54 dBc/Hz at 10 Hz, -141 dBc/Hz at 10 kHz, and -162 dBc/Hz at 10 MHz offset. Above performance arises from a dual-laser self-injection-locking scheme to a single Fabry-Perot cavity with high Q exceeding 10^8, with over 20 dB common-mode noise suppression. The low-noise nature of such reference is coherently transferred to the X-band through a high-performance TFLN electro-optic comb chip, thereby overcoming long-standing barriers in photonic microwave integration to enable truly field-deployable low-noise microwave generation.

💡 Research Summary

The authors present a compact, low‑phase‑noise photonic microwave oscillator that integrates all optical components into a 166 mL module. The core of the system is a dual‑self‑injection‑locked (dSIL) scheme in which two commercial distributed‑feedback (DFB) lasers at 1548.3 nm and 1553.3 nm are simultaneously locked to a single ultra‑high‑Q (>10⁸) micro‑Fabry‑Pérot (μFP) cavity of only 1 mL volume. Self‑injection locking (SIL) provides passive linewidth compression without any electronic feedback; the measured fundamental linewidths of the two lasers are 9 mHz and 55 mHz, respectively, and the side‑mode suppression ratio exceeds 50 dB. Because both lasers share the same cavity, common‑mode noise from cavity vibrations and thermal drift is largely cancelled, yielding more than 20 dB of common‑mode noise suppression (CMNS) as verified by the beat‑note phase‑noise measurement.

The stabilized optical reference is transferred to the microwave domain using an electro‑optic frequency‑division (e‑OFD) architecture built on a thin‑film lithium‑niobate (TFLN) cascade modulator. The TFLN chip integrates a Mach‑Zehnder modulator (MZM) and three phase modulators (PMs) in series, achieving a low half‑wave voltage (≈1.7 V for the MZM, ≈2 V for the PMs) and requiring only ~35 dBm of RF drive power. Dual electro‑optic (EO) combs are generated from the two SIL lasers; adjacent comb lines are filtered and detected to produce an intermediate‑frequency (IF) signal. This IF is mixed with a 100 MHz local oscillator, generating an error signal that feeds back to a dielectric‑resonator oscillator (DRO) at 10.4 GHz. The frequency‑division factor of ≈35.5 dB suppresses the phase noise of the DRO by the square of the division ratio, resulting in an X‑band microwave signal with single‑sideband (SSB) phase‑noise of –54 dBc/Hz at 10 Hz, –77 dBc/Hz at 100 Hz, –108 dBc/Hz at 1 kHz, –141 dBc/Hz at 10 kHz, –146 dBc/Hz at 100 kHz, –152 dBc/Hz at 1 MHz, and –162 dBc/Hz at 10 MHz offset. The measured microwave phase‑noise closely follows the theoretical prediction based on the division factor, confirming the effectiveness of the e‑OFD scheme.

Packaging is a major achievement: the μFP cavity, the dSIL lasers, and the TFLN modulator are all housed in a modular enclosure occupying 166 mL and weighing 113 g. The optical subsystem (μFP + lasers) occupies only 18 mm × 34 mm × 9 mm, while the remaining volume contains the electronic control board and a protective metal case. Real‑time monitoring of the SIL state is provided by embedded photodetectors, enabling robust automatic lock acquisition and long‑term stability. Frequency tuning of the microwave output is continuous over a 350 kHz range without mode hops, limited only by the electronic oscillator’s tuning range.

The work addresses several longstanding barriers in photonic microwave generation: (1) the need for bulky ultra‑stable cavities, (2) complex active locking loops, and (3) difficulty of integrating the optical reference with microwave circuitry. By using a passive SIL approach and a high‑performance TFLN comb, the authors achieve a field‑deployable, low‑noise microwave source suitable for next‑generation 5G/6G communications, high‑resolution radar, and precision sensing. Remaining challenges include environmental sensitivity of the air‑gap μFP cavity, RF power handling of the TFLN modulator, and extending the tuning range. Future work may explore monolithic integration of the cavity, advanced temperature‑compensation schemes, and multi‑channel e‑OFD for broader frequency coverage. Overall, the paper demonstrates a practical pathway to bring laboratory‑grade photonic microwave performance into compact, real‑world systems.