arXiv subjects

Megan L. Kelleher

Publications and source records attributed to Megan L. Kelleher.

5 recordsLinked to original sources

arXiv2024

Photonic Millimeter-wave Generation Beyond the Cavity Thermal Limit

Next-generation communications, radar and navigation systems will extend and exploit the higher bandwidth of the millimeter-wave domain for increased communication data rates as well as radar with higher sensitivity and increased spatial resolution. However, realizing these advantages will require the generation of millimeter-wave signals with low phase noise in simple and compact form-factors. The rapidly developing field of photonic integration addresses this challenge and provides a path toward simplified and portable, low-noise mm-wave generation for these applications. We leverage these advances by heterodyning two silicon photonic chip lasers, phase-locked to the same miniature Fabry-Perot (F-P) cavity to demonstrate a simple framework for generating low-noise millimeter-waves with phase noise below the thermal limit of the F-P cavity. Specifically, we generate 94.5 GHz and 118.1 GHz millimeter-wave signals with phase noise of -117 dBc/Hz at 10 kHz offset, decreasing to -120 dBc/Hz at 40 kHz offset, a record low value for such photonic devices. We achieve this with existing technologies that can be integrated into a platform less than $\approx$ 10 mL in volume. Our work illustrates the significant potential and advantages of low size, weight, and power (SWaP) photonic-sourced mm-waves for communications and sensing.

physics.optics↗

arXiv2024

Tunable X-band opto-electronic synthesizer with ultralow phase noise

Modern communication, navigation, and radar systems rely on low noise and frequency-agile microwave sources. In this application space, photonic systems provide an attractive alternative to conventional microwave synthesis by leveraging high spectral purity lasers and optical frequency combs to generate microwaves with exceedingly low phase noise. However, these photonic techniques suffer from a lack of frequency tunability, and also have substantial size, weight, and power requirements that largely limit their use to laboratory settings. In this work, we address these shortcomings with a hybrid opto-electronic approach that combines simplified optical frequency division with direct digital synthesis to produce tunable low-phase-noise microwaves across the entire X-band. This results in exceptional phase noise at 10 GHz of -156 dBc/Hz at 10 kHz offset and fractional frequency instability of 1x10^-13 at 0.1 s. Spot tuning away from 10 GHz by 500 MHz, 1 GHz, and 2 GHz, yields phase noise at 10 kHz offset of -150 dBc/Hz, -146 dBc/Hz, and -140 dBc/Hz, respectively. The synthesizer architecture is fully compatible with integrated photonic implementations that will enable a versatile microwave source in a chip-scale package. Together, these advances illustrate an impactful and practical synthesis technique that shares the combined benefits of low timing noise provided by photonics and the frequency agility of established digital synthesis.

physics.optics↗

arXiv2023

Photonic chip-based low noise microwave oscillator

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low noise microwave signals are generated by the down-conversion of ultra-stable optical references using a frequency comb. Such systems, however, are constructed with bulk or fiber optics and are difficult to further reduce in size and power consumption. Our work addresses this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division. Narrow linewidth self-injection locked integrated lasers are stabilized to a miniature Fabry-Pérot cavity, and the frequency gap between the lasers is divided with an efficient dark-soliton frequency comb. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc/Hz at 100 Hz offset frequency that decreases to -135 dBc/Hz at 10 kHz offset--values which are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems.

physics.optics↗

arXiv2022

Micro-fabricated mirrors with finesse exceeding one million

The Fabry-Pérot resonator is one of the most widely used optical devices, enabling scientific and technological breakthroughs in diverse fields including cavity QED, optical clocks, precision length metrology and spectroscopy. Though resonator designs vary widely, all high-end applications benefit from mirrors with the lowest loss and highest finesse possible. Fabrication of the highest finesse mirrors relies on centuries-old mechanical polishing techniques, which offer losses at the part-per-million (ppm) level. However, no existing fabrication techniques are able to produce high finesse resonators with the large range of mirror geometries needed for scalable quantum devices and next-generation compact atomic clocks. In this paper, we introduce a new and scalable approach to fabricate mirrors with ultrahigh finesse ($\geq 10^{6}$) and user-defined radius of curvature spanning four orders of magnitude ($10^{-4}-10^{0}$ m). We employ photoresist reflow and reactive ion etching to shape and transfer mirror templates onto a substrate while maintaining sub-Angstrom roughness. This substrate is coated with a dielectric stack and used to create arrays of compact Fabry-Pérot resonators with finesse values as high as 1.3 million and measured excess loss $<$ 1 ppm. Optical ringdown measurements of 43 devices across 5 substrates reveal that the fabricated cavity mirrors -- with both small and large radii of curvature -- produce an average coating-limited finesse of 1.05 million. This versatile new approach opens the door to scalable fabrication of high-finesse miniaturized Fabry-Pérot cavities needed for emerging quantum optics and frequency metrology technologies.

physics.optics↗

arXiv2022

Thermal noise-limited laser stabilization to an 8 mL volume Fabry-Pérot reference cavity with microfabricated mirrors

Lasers stabilized to vacuum-gap Fabry-Pérot optical reference cavities display extraordinarily low noise and high stability, with linewidths much less than 1 Hz. These lasers can expand into new applications and ubiquitous use with the development of compact, portable cavities that are manufacturable at scale. Here we demonstrate an 8 mL volume Fabry-Pérot cavity constructed with mirrors that are fabricated lithographically with finesse near 1 million. A laser locked to the cavity exhibits phase noise limited by the cavity thermal noise for offset frequencies ranging from 1 Hz to $\approx$ 1 kHz, with a fractional frequency stability of 7$\times$10$^{-15}$ at 1 second. Furthermore, the use of microfabricated mirrors allows us to expand the design space of centimeter-scale cavities, and we explore the noise implications of pushing towards cavity volumes of 2 mL or less.

physics.optics↗