Wei Zhang

The Moon’s permanently shadowed regions (PSRs) are among the coldest places in the Solar System and are expected to become key landing sites for upcoming international space agency missions. Their proximity to peaks of perpetual solar power and potential resource richness makes them prime candidates for lunar exploration and future Moon bases. Here, we propose to deploy a passive, ultrastable optical resonator in these regions that will enable laser systems with unprecedented phase-coherence. The unique physical environment of lunar PSRs greatly benefits the construction of a cryogenic monolithic silicon cavity that exhibits low 10 − 18 thermal noise-limited stability and coherence time exceeding 1 min, more than a decade better than the current best terrestrial system. Such a stable laser will form an enabling infrastructure for quantum technology in space to serve many applications, including establishing a lunar time standard, building long-baseline optical interferometry, distribution of stable optical signals across networks of satellites, testing general relativity and gravitational physics, and forming the backbone for space-based quantum networks.

Astrocombs are highly precise optical reference systems used for astronomical spectrograph calibration, enabling accurate and consistent measurement of stellar absorption line frequencies across a broad spectral range. By comparing a star’s spectrum to the astrocomb spectrum, minute radial velocity shifts indicative of an exoplanet’s presence can be detected. The highest precision and accuracy astrocombs are generated by laser frequency combs. However, in the visible band, frequency comb calibrators have stringent operational lifetime limitations due to degradation of the supercontinuum-generating medium, and are thus typically used to calibrate Fabry–Pérot etalons, which are then used for simultaneous observations with target stars. This work demonstrates a specially designed crystalline whispering-gallery-mode resonator (WGMR) that serves as an astrocomb etalon, featuring a coupling wavelength span of two octaves and a quasi-single-mode family. Several innovations have been incorporated into the system’s compact design to achieve frequency stability, in addition to wide-range wavelength coupling. These include dual-mode thermometric frequency stabilization, in addition to vacuum packaging, effectively minimizing thermal and pressure-induced frequency drift. Long-term frequency stability, measured via a heterodyne RF beat note, exceeds 2×10 ⁻¹² , with a projected long-term modal frequency stability of ≲10 ⁻¹⁰ (equivalently 3 cm/s in Doppler velocity). This level of stability obviates the need for frequent cross-calibration with a laser reference, as was required with other astrocomb etalons. Overall, the architecture of the WGMR etalon provides a new and powerful capability, beyond what has previously been achieved by microresonator-based etalons, as a compact, ultra-wideband spectral reference for detecting small planets in long-period orbits around nearby stars. It also offers a path forward for a low size, weight, and power (SWaP) spectral reference appropriate for spaceflight.

Optically generated microwave signals exhibit some of the lowest phase noise and timing jitter of any microwave-generating technology to date. The success of octave-spanning optical frequency combs in down-converting ultrastable optical frequency references has motivated the development of compact, robust and highly manufacturable optical systems that maintain the ultralow microwave phase noise of their tabletop counterparts. Two-point optical frequency division using chip-scale components and ~1 THz-spanning microcombs has been quite successful, but with stringent requirements on the comb source’s free-running noise and feedback control dynamics. Here we introduce a major simplification of this architecture that replaces feedback control of the frequency comb in favor of electronic feedforward noise cancelation that significantly relaxes the comb requirements. Demonstrated with both a high repetition rate solid-state mode-locked laser and a microcomb, feedforward on a 10 GHz carrier results in more robust operation with phase noise as low as -153 dBc/Hz at offsets ≥10 kHz, femtosecond timing jitter, and elimination of the large “servo bump” noise increase at high offset frequency. The system’s compatibility with a variety of highly manufacturable mode-locked laser designs and its resilience and straightforward implementation represents an important step forward towards a fully chip-scale implementation of optically generated microwaves, with applications in radar, sensing, and position, navigation and timing.

The revolutionary impact of optical frequency combs in fields as diverse as timekeeping and spectroscopy hinges on a way to independently control comb line spacing and spectral position. However, achieving such control in microcombs remains challenging. In this work, we demonstrate separable control of these parameters in a microcomb using a single integrated actuation mechanism. As a proof of concept, we fully frequency-lock a microcomb to a high-stability reference cavity and apply it to spectroscopy and microwave generation. This approach significantly simplifies the implementation of frequency-stabilized microcombs across a broad range of applications.

The remarkable frequency stability of resonant systems in the optical domain (optical cavities and atomic transitions) can be harnessed at frequency scales accessible by electronics using optical frequency division. This capability is revolutionizing technologies spanning time keeping to high-performance electrical signal sources. A version of the technique called two-point optical frequency division (2P-OFD) is proving advantageous for application to high-performance signal sources. In 2P-OFD, an optical cavity anchors two spectral endpoints defined by lines of a frequency comb. The comb need not be self-referenced, which greatly simplifies the system architecture and reduces power requirements. Here, a 2P-OFD microwave signal source is demonstrated with record-low phase noise using a microcomb. Key to this advance is a spectral endpoint defined by a frequency-agile single-mode dispersive wave that is emitted by the microcomb soliton. Moreover, the system frequency reference is a compact all-solid-state optical cavity with a record Q factor. A hybridly packaged version of the system offers excellent longer term stability. The results advance integrable microcomb-based signal sources into the performance realm of much larger microwave sources.

An inch-long monolithic SiO₂resonator enables a laser with 18 Hz integrated linewidth and 7 ×10⁻¹⁴ relative frequency stability. A microwave signal with phase noise below -100 dBc/Hz at 10 kHz offset is generated using this resonator.

Microcombs are advancing optical frequency comb technology towards a chip-integrable form. Here, we characterize a microwave signal source based upon the two-point optical frequency division (2P-OFD) technique. The system uses a frequency microcomb to transfer relative frequency stability of two low-noise optical oscillators to the microcomb repetition rate tone. The two optical oscillators are based on semiconductor lasers jointly stabilized to an ultra-stable Fabry-Pérot cavity. The coherence of the comb spectrum is confirmed through multiple stability comparisons between its repetition rate and comb line spectrum. The results underscore the excellent performance of microcombs as coherent links between optical and microwave frequencies, and how they enable simplified, miniaturized architectures for optical frequency division. ©2024. All rights reserved.

Optical resonators are indispensable tools in optical metrology that usually benefit from an evacuated and highly-isolated environment to achieve peak performance. Even in the more sophisticated design of Fabry-Perot (FP) cavities, the material choice limits the achievable quality factors. For this reason, monolithic resonators are emerging as promising alternative to traditional designs, but their design is still at preliminary stage and far from being optimized. Here, we demonstrate a monolithic FP resonator with 4.5 cm³ volume and 2 × 10⁵ finesse. In the ambient environment, we achieve 18 Hz integrated laser linewidth and 7 × 10⁻¹⁴ frequency stability measured from 0.08 s to 0.3 s averaging time, the highest spectral purity and stability demonstrated to date in the context of monolithic reference resonators. By locking two separate lasers to distinct modes of the same resonator, a 96 GHz microwave signals is generated with phase noise -100 dBc/Hz at 10 kHz frequency offset, achieving orders of magnitude improvement in the approach of photonic heterodyne synthesis. The compact monolithic FP resonator is promising for applications in spectrally-pure, high-frequency microwave photonic references as well as optical clocks and other metrological devices. ©2024. All rights reserved.

We demonstrate an efficient simultaneous self-injection locking of two semiconductor lasers to high-order modes of a standalone monolithic non-confocal Fabry–Perot cavity. The lasers are used to generate a low-noise microwave signal on a fast photodiode. The overall improvement of the laser spectral purity exceeds 80 dB. The observed single-sideband phase noise of X- to W-band signals is at the −110 dBc/Hz level and is limited by the fundamental thermorefractive noise of the cavity. The demonstrated cavity–laser configuration can be tightly packaged and is promising for the generation of high-frequency RF signals as well as for referencing optical frequency combs.