Silicon Photonic RF Spectrum Analyzer Achieves 10MHz Resolution and Broadband Analysis

Miniaturising spectroscopic instruments presents a significant challenge for diverse fields, from chemical analysis to medical diagnostics, and is particularly demanding in radio frequency (RF) spectrum analysis. Brandon Redding, Joseph B. Murray, Matthew J. Murray, and colleagues at the U. S. Naval Research Laboratory, alongside Sean Pang from CREOL, The College of Optics and Photonics, University of Central Florida, now demonstrate a silicon photonic integrated circuit that overcomes existing limitations in resolution and bandwidth. Their device functions as a broadband RF spectrum analyser, achieving a record 10MHz spectral resolution across a 10GHz bandwidth, and represents a substantial advance by combining an ultra-high-resolution speckle spectrometer with a novel interferometric encoding scheme. This single-shot spectrometer promises compact, persistent, wideband RF spectrum analysis with the potential to revolutionise applications requiring rapid and detailed spectral monitoring.

Speckle Patterns Enable Compact Spectroscopy

Scientists are developing a new generation of compact spectrometers that utilize the principles of light scattering from disordered materials, creating distinctive patterns known as speckles. These devices offer a significant departure from traditional spectrometers, which rely on bulky components like gratings and prisms. Instead, this technology analyzes speckle patterns formed when light passes through a specifically designed structure, such as a photonic chip or multimode fiber, to reconstruct the spectrum of the input light. This approach promises smaller, lower-cost, and more versatile spectroscopic tools with broad applications.

A key advantage of this technology is its potential for miniaturization through the integration of components onto silicon photonic chips, reducing power consumption and enabling scalability for portable and widespread use. Researchers are exploring increasingly complex two- and three-dimensional photonic structures to optimize performance and are developing programmable photonic circuits that allow for adaptive and versatile spectrometers. Utilizing multimode fibers as the disordered medium offers a robust and cost-effective alternative for building these devices. Advanced algorithms, including compressive sensing and Lasso regression, are crucial for decoding the speckle patterns and efficiently reconstructing the spectra.

This technology aims to overcome the size, cost, and power consumption limitations of conventional spectrometers, opening doors to a wider range of applications in fields like radio frequency analysis, chemical sensing, biomedical diagnostics, and environmental monitoring. The potential applications are diverse, spanning multiple fields. In radio frequency spectrum analysis, this technology can detect and identify signals for electronic warfare, monitor and optimize wireless networks, and ensure efficient use of the radio frequency spectrum. Beyond this, it can identify chemical substances through their spectral signatures, analyze biological samples for disease detection, and monitor environmental pollutants. The technology also holds promise for security applications, space-based remote sensing, and advanced imaging systems.

Silicon Photonics Achieve High-Resolution RF Spectrum Analysis

Scientists have created a silicon photonic integrated circuit (PIC) that functions as a radio frequency (RF) spectrum analyzer, achieving a record-high resolution of 10MHz, corresponding to 0. 8pm at a wavelength of 1550nm. This breakthrough addresses a significant challenge in RF spectrum analysis, which demands both high resolution and large bandwidth alongside a fast update rate, capabilities previously unattainable in chip-scale spectrometers. The research team combined an ultra-high-resolution speckle spectrometer with a novel interferometric RF-to-optical encoding scheme to achieve this performance.

The PIC-based spectrometer is designed to cover a 10GHz bandwidth with a 10MHz resolution, readily achievable as state-of-the-art speckle spectrometers can reconstruct spectra with over 10,000 channels. The design tolerates a compression ratio of 10 while detecting 100 speckle grains, and current photonic foundry processes can integrate approximately 100 photodetectors on a PIC, further supporting the required bandwidth. The update rate is also easily satisfied, with integrated detectors capable of bandwidths exceeding 100kHz. Researchers addressed the challenge of maintaining resolution by employing a path-mismatched multimode interferometer.

This design utilizes inverse-designed splitters to compensate for waveguide loss, directing more power into longer path lengths to counteract signal attenuation. Simulations demonstrate that carefully adjusting coupling coefficients into the optical paths balances power transmission and minimizes attenuation, achieving a significant reduction in spectral correlation width compared to conventional designs. Increasing acceptable loss further enhances resolution, highlighting the potential for overcoming resolution limits.

Silicon Photonics Delivers Record Spectrum Resolution

Scientists have developed a silicon photonic integrated circuit (PIC) that functions as an RF spectrum analyzer, achieving a record-high resolution of 10MHz. This breakthrough addresses a significant challenge in RF spectrum analysis, which demands both high resolution and large bandwidth alongside a fast update rate, capabilities previously unattainable in chip-scale spectrometers. The research team combined an ultra-high-resolution speckle spectrometer with a novel interferometric RF-to-optical encoding scheme to achieve this performance. A key innovation lies in the design’s ability to trade-off optical insertion loss for resolution, utilizing a path-mismatched multimode interferometer.

Simulations reveal that a disordered cavity-based speckle spectrometer with a propagation loss of 1dB/cm exhibits a spectral correlation function with a half-width at half-maximum (HWHM) of approximately 350MHz, insufficient for many RF spectrum analysis applications. However, the team’s model demonstrates that a ten-fold reduction in propagation loss could achieve a corresponding ten-fold improvement in resolution. Experiments conducted across a 10GHz band confirmed the analyzer’s performance, demonstrating a path towards low-SWAP-C (size, weight, and power consumption) RF spectrum analysis.

High Resolution On-Chip RF Spectrum Analysis

This work demonstrates a significant advance in on-chip spectrometer design, achieving a resolution of 10MHz for radio frequency spectrum analysis while maintaining a 10GHz bandwidth. Researchers developed a silicon photonic integrated circuit that combines an ultra-high-resolution speckle spectrometer with a novel interferometric radio frequency-to-optical encoding scheme. This approach overcomes limitations of existing chip-scale spectrometers, which struggle to simultaneously deliver high resolution, broad bandwidth, and fast update rates. The key innovation lies in the combination of these two techniques; the speckle spectrometer itself achieves a record-high optical resolution of 100MHz, and the RF encoding scheme further enhances the overall RF resolution.

This allows for the clear separation of radio frequency tones spaced only 10MHz apart, a capability previously unavailable in compact, integrated devices. The demonstrated system operates as a single-shot spectrometer, promising fast data acquisition for applications requiring persistent, wideband RF spectrum monitoring. Future work will focus on integrating components onto the chip and improving the system’s environmental stability through temperature control. Increasing the number of spatial channels within the spectrometer and utilizing lower-loss waveguides are also identified as avenues for enhancing performance and bandwidth. These improvements, the researchers suggest, will pave the way for compact, low-power spectrometers suitable for a wide range of applications beyond radio frequency.