DARPA’s GRYPHON programme uses photonics to reduce phase noise promising efficiency improvements for radio frequency applications.
Radios depend on electronic oscillators which convert a Direct Current (DC) into an Alternating Current (AC). A DC current flows in one direction keeping a constant voltage. An AC current changes direction causing the upward and downward motion of a sine wave. This movement is the result of electrons moving either in a positive upwards or a negative downwards direction. AC and DC both have pros and cons: DC has more voltage consistency but cannot travel as far. AC currents travel farther on account of their wavelike motion. An oscillator turns a DC current into an AC one using a material like quartz. When a DC current flows into the crystal it starts to vibrate. This action converts the direct current into an alternating current forming the basis of a radio wave. Electronic oscillators have drawbacks as they produce phase noise. In an ideal world, the electronic oscillator would generate a pure sine wave.
These oscillators produce a carrier wave which move communications traffic between one or several radios. The wave is like a set of train tracks between two or more points with the train and its carriages being the traffic travelling between the radios. Frequency is a measurement of the distance between the peaks and troughs of a radio wave over time. For example, a carrier wave with a one megahertz’ frequency denotes one million cycles-per-second. The cycle is the time it takes for a radio wave to move from a peak to a trough and back again or vice versa.
This one million cycles-per-second is a theoretical aspiration that will not always be consistent. The number of cycles-per-second may change over time giving rise to a phenomenon called jitter. We can imagine frequency depicted along a graph’s x axis and amplitude being shown by the y axis. Ideally there would be a single straight line moving upwards. In reality, the signal’s frequency and amplitude spreads either side of the carrier frequency. The signal will diminish in frequency and power either side of the carrier frequency the further away it is. Areas either side of the signal are known as sidebands and constitute phase noise.
Phase noise is important when working with oscillators and engineers will measure phase noise to determine the cleanliness of the oscillator’s signal. Phase noise is compounded by interference created by the radio’s electronic components.
Gryphon
The US Defence Advanced Research Project Agency’s (DARPA’s) GRYPHON (Generating Radio Frequency with Photonic Oscillators for Low Noise) programme tackles phase noise. DARPA says today’s microwave oscillators significantly reduce phase noise but do so at a cost. Such devices have large size, weight, power and cost demands and frequency restrictions. This “limits their use in advanced defence systems.”
GRYPHON harnesses photonic oscillator technology. In a nutshell, photonic oscillators produce microwaves, but with very little phase noise. Optoelectronic Oscillators combine microwaves and light converting laser radiation into RF (Radio Frequency). The laser is fed into an electrooptical modulator which modifies the laser light by altering its frequency or amplitude for example. The modulated light passes out of the electrooptical modulator entering an optical fibre. The fibre feeds the light to a photodetector converting it into electricity. This current is amplified and fed back into the electrooptical modulator with this process causing the oscillations needed for a microwave signal.
Dr. Gordon Keeler, DARPA’s GRYPHON programme manager, told Armada the project aims to harness photonic oscillator technology to develop compact and tuneable microwave oscillators. It is hoped these will generate radio signals of between one and 40 gigahertz. This is not new territory for DARPA. As Dr. Keeler explains “the generation of stable microwave signals by optical frequency division has been studied for over a decade, and was used to demonstrate world-record performance under DARPA’s PULSE (Programme in Ultrafast Laser Science and Engineering) programme.” DARPA’s DODOS (Direct On-Chip Digital Optical Synthesizer) initiative looked at miniaturised photonic oscillator systems. “GRYPHON is capitalising on these advances to realize compact, high-performance microwave oscillators using precision, wafer-scale manufacturing processes,” says Dr. Keeler.
GRYPHON commenced in January 2022 and is almost one year into its 18-month initial phase during which participants “must demonstrate the viability of their concept for generating RF with exceptionally low phase noise in a compact footprint.” Dr. Keeler says that “each performer will deliver ten synthesizer prototypes that meet programme requirements. (These requirements) specify phase noise, tunability, size, and robustness targets.” This technology is expected to reach Technology Readiness Levels-4/5 (TRLs-4/5). TRL-4 denotes the technology’s validation in a laboratory environment, according to US definitions. TRL-5 denotes the technology’s validation in a relevant environment.
Ultimately, “the technologies developed under GRYPHON are likely to be relevant to a wide variety of applications,” says Dr. Keeler. Specifically, he highlights applicability to military communications and sensors alongside civilian applications like automotive radar.
by Dr. Thomas Withington
