Precise microwave frequency synthesis, first developed in the 1940s, is a core technology in all modern communications, computing, and navigation. It has been woven into industries that generate hundreds of billions of dollars in economic impact for our country.
Today, we are witnessing a new paradigm in electromagnetic synthesis at frequencies 100,000-times higher than the microwave domain— optical frequency synthesis. Analogous to the microwave revolution, optical frequency synthesis will make possible a new wave of scientific and technological developments, enabling enhanced precision for an ever-expanding array of laser-based applications. Some examples: advanced communications and computing systems; remote sensing and ranging; massively parallel spectroscopy for measuring trace gases, chemicals and explosives; the realization of a new generation of clocks with 100x improved precision; non-invasive medical diagnostics; and significant other advances in quantum science and precision metrology.
The promise of such a broad range of applications has been realized in proof-of-principle laboratory experiments, but widespread deployment of optical frequency synthesis is hindered by present technology because of the large size, power requirements and cost, and sensitivity to environmental conditions. NIST-on-a-Chip (NoaC) research is overcoming these limitations through a remarkable new scientific breakthrough—the use of Kerr micro-resonator combs (aka “microcombs”) that function as chip-scale optical frequency synthesizers.
Microcomb technologies are compatible with low-cost semiconductor processing, providing a path to directly integrate optical synthesis with photonic and electronic components on a silicon chip with dimensions on the centimeter scale. This important advance will provide the basis for mass-produced, comb-based, chip-scale systems to support a wide anticipated range of precision measurements and new applications.
NIST is uniquely positioned to lead this revolution because it has pioneered optical frequency synthesis science and technology, shown continuing leadership in developing new laser-based applications, achieved initial success in early microcomb demonstrations, and gained experience in optical frequency science and metrology necessary to convert early microcomb demonstrations into metrology tools and innovative technologies.
The field of microcombs is in its infancy, and NoaC research aims to resolve challenging problems related to material science, device fabrication, nonlinear physics, optical phase-synchronization, and the fundamental quantum and thermodynamic noise properties of microcomb generation. Success in those efforts will lead to a transformative technology of wide scientific and economic importance.
NoaC researchers have developed extensive technical expertise that enables them to participate in several externally-funded programs, including the following.
Optical Frequency Comb System On-a-Chip for Measurements, Distribution, and Application of Atomic Time Standards. Sponsor: DARPA Quantum Assisted Sensing and Readout (QuASAR)
This effort is focused on developing chip-scale optical frequency combs with performance at levels commensurate with the best optical clocks. Within the QuASAR program, we have been able to explore the generation and frequency control of microcombs at the 10-15 level. As such, we leverage the QuASAR funding to explore the fundamental noise limits of microcombs for optical clocks. This effort also funds the chip-integrated silica disk resonators fabricated in Kerry Vahala’s group at Caltech, which has been supplying these devices to our NIST group.
Portable optical frequency comb systems for the generation of ultralow phase noise microwave signals. Sponsor: DARPA Program in Ultrafast Laser Science and Engineering (PULSE)
This research effort is focused on developing the chip-scale science and technology for low-noise microwave generation. Within this project, NoaC researchers are exploring the use of microcombs for low-noise microwaves as well as chip-scale low-noise lasers and chip-integrated reference cavities. This project also funds partners at Caltech and University of Virginia who are supplying NIST with micro-resonators, chip-integrated spiral reference cavities, and chip-integrated photodiodes. This larger effort also provides the NoaC team with resources for frequency metrology at a level below what is commercially available, which is critical for the further development of low-noise microresonator devices.
Stable modelocking of microresonator frequency combs via spatio-temporal field mapping and control. Sponsor: Air Force Office of Scientific Research
This research effort is focused on the time-domain properties of the signals generated with microcombs. In particular, it allows scientists to explore the generation of ~100 fs waveforms using microcombs and the fundamental nonlinear optics leading to such pulse generation. Additionally, these developments are the basis of nonlinear frequency conversion to regions of the optical and mid-IR spectrum that have not been accessible with microcombs and could be applied to spectroscopic sensing.
Development of an optical microcomb system in support of fundamental physics research on the International Space Station. Sponsor: NASA.
In this program, researchers investigate and develop advanced microresonator optical frequency comb technology in support of planned European Space Agency (ESA) activities on the International Space Station (ISS). Microcombs could provide a link between optical and microwave signals and a means to access the stability of optical clocks at any wavelength. For example, a frequency comb will be required for the Space Optical Clock (SOC) experiment to achieve all its fundamental scientific goals by providing the critical phase coherent optical or microwave link between the atomic frequency standard on the ISS and ground-based clocks. The same frequency comb technology will be a required component of other proposed ESA and NASA fundamental-physics clock experiments, as well as an enabling technology for ESA's planned Quantum Weak Equivalence Principle (QWEP) atom interferometry experiments.
Chip-scale Optical Resonator Enabled Synthesizer (CORES). Sponsor: DARPA Direct On-chip Digital Optical Synthesizer.
This research is aimed at providing direct digital synthesis of optical frequencies in a single integrated chip-scale package. All photonic and electronic elements of CORES will be heterogeneously integrated into a package having volume <1 cm3 and power consumption <1 W. This project connects NoaC researchers to world-leading experts in the integration of active and passive optical components on a single silicon chip. Such techniques are at the very forefront of photonic integration efforts that will have a significant impact on NoaC technologies as well as communications, computing, sensing, and navigation.
2-photon Optical Clock. Sponsor: DARPA Atomic Clocks with Enhanced Stability
This research effort aims to develop a chip-scale two-photon optical clock (2-POC) with much enhanced stability. The 2-POC utilizes advanced vapor-cell technology to realize a stabilized clock laser with fractional frequency repeatability of
< 10-13 per month and a novel chip-based microcomb system for optical frequency division and 10 MHz clock synthesis. These technologies will be packaged with miniaturized electronics into a fully integrated clock with orders-of-magnitude improvement in size, weight and power compared to existing commercial and laboratory clocks having similar performance.