Scientists Create Miniaturized Chips for Transforming Light Into Microwave Signals
Embarking on a path to consolidated microwave-photonic systems, a collaboration led by NIST with various institutions has successfully miniaturized what was previously a bulky, tabletop setup into a compact chip-sized device. This innovation holds potential applications in GPS, radar, and wireless communications technology.
The team of researchers managed to downsize a microwave-photonic oscillator from an exclusively large, tabletop system to a more compact form with the promise of heightened precision for future technologies. The accompanying image is provided courtesy of NIST.
Maintaining synchrony across a vast network of interconnected devices often poses significant challenges. Historically, standalone microwave oscillators have been adequate for this task, but the demands of advanced measurement and communication systems call for more effective control of phase noise and device synchronization. The researchers assert that through microwave photonics, it's possible to generate purer microwave frequencies tailored for these scenarios.
A consortium comprising NIST, NASA Jet Propulsion Laboratory, Yale University, the California Institute of Technology, the University of California Santa Barbara, the University of Virginia, and the University of Colorado Boulder, has introduced a photonic circuit capable of transforming light into microwaves. This development aims to refine the performance of navigational, communicational, and radar equipment.
Optical Finesse Applied to Microwave Technology
Each contributing institution had a pivotal part in crafting the prototype of the photonic circuit. The effort began with NIST, JPL, the University of Virginia, Caltech, and UC Boulder, who engineered a microwave oscillator. This device produced a microwave signal by harnessing the high speed and accuracy inherent in optical technology.
This feat was accomplished by channeling semiconductor lasers into a reference cavity, essentially a compact chamber lined with mirrors, and tuning the laser light to resonate with the cavity's dimensions. They achieved this by aligning the light wave's crests and troughs precisely within the mirrors' confines. Such alignment allowed the light to amplify in those frequencies, resulting in a stable laser frequency. Following this, a frequency comb transformed this stable, high-frequency light into microwave frequencies with a lower pitch.
Caltech devised two continuous wave lasers that generated distinct frequencies, which were then "anchored" through self-injection locking (SIL) with microresonators and Fabry-Perot cavities, crafted by UC Boulder. These lasers provided a reference for generating two distinct beat frequencies, utilizing a third laser and a microcomb to establish a phase-locked frequency comb.
The integrated microwave-photonic setup encompasses a suite of elements, each surpassing the capabilities of traditional microwave-only systems, thereby producing a superior quality output signal. Image courtesy of Nature.
The microcomb stood out as a crucial element of the apparatus, capable of producing the optical frequencies spaced at 20 GHz that eventually formed the microwave signal. This microcomb utilized a dual coupled-ring resonator, a design conceived by the teams at UCSB and Caltech.
This comb's output, an array of optical frequencies each separated by 20 GHz, was channeled through an enhanced unitravelling carrier (MUTC) photodetector. This process resulted in the generation of a microwave signal at 20 GHz. The high Q-factor of the optical resonators was instrumental in crafting a microwave signal characterized by exceedingly low phase noise. Such microwaves play a vital role in ensuring precise timing and synchronization across a variety of technologies, including radar, communication networks, and navigational systems.
Collaborative Progress Towards Compactness
While systems based on the aforementioned rudimentary principle have been around for some time, they were typically confined to sizable, benchtop setups. Consequently, embedding such systems into actual devices was not feasible. However, with the collective endeavor of NIST and its partnering institutions, there is now progress towards integrating optical solutions into microwave-frequency applications.
The microwave-photonic oscillator exhibits a much lower phase noise compared to a free-running microwave oscillator. Image used courtesy of Nature
In contrast to conventional microwave electronic devices, the optical-based microwave oscillator showcased significantly reduced phase noise, achieving levels down to -102 dBc/Hz at a 100-Hz offset and -141 dBc/Hz at a 10-kHz offset. This translates to a dramatic 50-dB reduction in phase noise in proximity to the carrier frequency. Therefore, the utilization of microwave photonics may enhance the performance of high-precision applications.
Making Precision Timing Accessible
The disclosed microwave-photonic oscillator may represent an initial stride in the coalescence of lasers, modulators, detectors, and optical amplifiers into a unified chip platform. With continued advancement, this innovation has the potential to significantly simplify the transmission of sensitive, low-noise signals from the confines of the laboratory to the broader domain of professionals such as radar technicians, astronomers, and cellular network operators.
