Everyday Life, Improved by Light: GRYPHON’s Photonic Discoveries

September 1, 2024

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, fIF, 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. This image is from a study published in the Journal Nature, that is associated with DARPA’s GRYPHON program research. (Image: Nature)

Radio frequency (RF) and microwave signals are integral carriers of information for technology that enriches our everyday life – cellular communication, automotive radar sensors, and GPS navigation, among others. At the heart of each system is a single-frequency RF or microwave source, the stability and spectral purity of which is critical. While these sources are designed to generate a signal at a precise frequency, in practice the exact frequency is blurred by phase noise, arising from component imperfections and environmental sensitivity, that compromises ultimate system-level performance.

This reality drives undesirable tradeoffs between performance, environmental sensitivity, and size that make the simultaneous achievement of stability, precision, and agility in an ultra-compact form factor an elusive feat. However, DARPA’s Generating Radio Frequency with Photonic Oscillators for Low Noise (GRYPHON) program could change all of that, as performers recently demonstrated in the first phase of the program aimed at developing compact, ultra-low-noise microwave frequency oscillators.

The GRYPHON program seeks to develop compact microwave frequency oscillators with extremely low phase noise to enable advanced sensing and communication applications. In the last decade, major advances in oscillator performance have been realized using optical techniques to synthesize high-fidelity microwave signals. Such oscillators employ optical frequency division to reach world-record phase noise levels. The solutions demonstrated to date, however, have sacrificed other important attributes in pursuit of spectral purity. Such trade-offs are problematic, because module size, cost, tunability, and environmental sensitivity are also critical factors that determine the applicability of microwave sources to commercial and military systems.

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 fIF, which is phase-locked to a reference frequency. The feedback for microcomb stabilization is provided to the current supply of the microcomb seed laser. (Image: Nature)

GRYPHON will leverage recent developments in nonlinear photonics and photonic-electronic integration to develop microwave sources with noise performance that meets or exceeds that of the best discrete oscillator modules, yet occupy a compact volume typical of far noisier chip-scale voltage-controlled oscillators (VCOs). Moreover, by program end, GRYPHON microwave sources will operate as synthesizers with the ability to tune to any frequency from 1 to 40+ GHz during operation. This combination of features is unprecedented in today’s state of the art, and will establish a new regime of source technology that is expected to transform the types and capabilities of military and commercial radar and communication systems.

While extremely low phase noise sources do exist, they are expensive, lack tunability, and are impractically large for deployment on mobile platforms that would enable advanced sensing and communication applications. GRYPHON seeks to change this paradigm by realizing viable, small-footprint microwave sources that transcend today’s tradeoffs and far exceed current state of the art. Launched in January 2022, the program builds on advances in optical frequency division, integrated photonics, and non-linear optics – including those from previous DARPA efforts – to establish a new technology regime that transforms military and commercial capabilities.

GRYPHON performers, using different light-based approaches, have made critical progress towards generating high-purity microwaves in significantly reduced form factors. By integrating low-noise lasers with complex optical structures on low-loss photonic platforms, along with high-speed integrated circuits, researchers have established the viability of achieving ultra-low phase noise performance and shrinking these capabilities from conventional table-top sizes down to microchip-size form factors.

“The results and impact from Phase 1 of GRYPHON really show what’s possible. For the first time, we’re seeing how integrated photonics allows us to break from the traditional size vs. performance vs. capability trade space and operate in a regime with exquisite performance that is exponentially better than current state of the art,” said Dr. Justin Cohen, GRYPHON program manager. “Better and faster communications, more accurate sensing, improved detection capabilities – this work could disrupt and advance countless applications.”

The research findings of GRYPHON’s performers were recently featured in Science and Nature journal articles, as well as via the National Institute of Standards and Technology, highlighting the work of contributing NIST researchers and their team. Now in Phase 2, GRYPHON researchers are seeking to further reduce phase noise in their already high-performance sources while introducing tunability and compactifying to targeted form factors, all of which aim to provide systems with unprecedented utility and access to previously unattainable applications.

This article is from material provided by the Defense Advanced Research Projects Agency (Arlington, VA). For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..