Optoelectronic Microwave Synthesizer: Combining Frequency Tunability with Low Phase Noise

Academic Background

In modern communication, navigation, and radar systems, frequency-tunable and low-noise microwave sources are critical. Traditional electronic microwave synthesizers offer frequency tunability but exhibit high phase noise, limiting their use in precision applications. In contrast, photonics-based microwave synthesizers, which leverage high spectral purity lasers and optical frequency combs, can generate microwaves with exceptionally low phase noise. However, these photonics approaches often lack frequency tunability and involve bulky systems with high power consumption, restricting their wider applicability.

To address these shortcomings, this paper introduces a hybrid optoelectronic method that combines simplified Optical Frequency Division (OFD) with Direct Digital Synthesis (DDS) to produce frequency-tunable low-phase-noise microwaves across the entire X-band (8–12 GHz). This research not only resolves the limitations of photonics-based methods in frequency tunability but also simplifies the design to make it compatible with integrated photonics, paving the way for potential chip-scale packaging and applications.

Paper Source

This paper was a collaboration among Igor Kudelin, Pedram Shirmohammadi, William Groman, and others from the University of Colorado Boulder, the University of Virginia, and the National Institute of Standards and Technology. It was published online on December 11, 2024, in the journal Nature Electronics, DOI: 10.1038/s41928-024-01294-x.

Research Process and Experimental Design

1. Design of the Optoelectronic Microwave Synthesizer

The research team designed a hybrid optoelectronic microwave synthesizer that integrates optical frequency division with direct digital synthesis. The core of the synthesizer is a low-noise photonic oscillator that generates a 10 GHz microwave signal. Utilizing Two-Point Optical Frequency Division (2p-OFD), the team achieved extremely low phase noise (-156 dBc/Hz at 10 kHz offset) and fractional frequency instability (1×10^-13 at 0.1 seconds).

2. Implementation of Optical Frequency Division

To realize low-noise microwave generation, the team first reduced the linewidth of continuous wave (CW) lasers by locking their frequencies to a high-Q Fabry-Pérot (FP) cavity. This FP cavity plays a key role in providing the phase and frequency reference for the generated microwave signal. A miniature FP cavity measuring 6.3 mm in length and with a free spectral range of 23.6 GHz and a Q-factor of approximately 5 billion was used.

3. Introduction of Direct Digital Synthesis

To extend frequency tunability, the team used the generated 10 GHz low-noise microwave signal as the reference clock for a Direct Digital Synthesizer (DDS). The DDS output was mixed with the original 10 GHz signal to produce frequency-tunable low-noise microwave signals in the range of 8–12 GHz. The DDS achieved microhertz-level tuning resolution and tuning speeds of tens of nanoseconds.

4. Experimental Results

The team validated the synthesizer’s performance through experiments. At a 10 GHz carrier frequency, the synthesizer achieved phase noise of -156 dBc/Hz at a 10 kHz offset. When tuning frequencies to ±500 MHz, ±1 GHz, and ±2 GHz from 10 GHz, the phase noise was -150 dBc/Hz, -146 dBc/Hz, and -140 dBc/Hz, respectively, at a 10 kHz offset. The DDS enabled microhertz-level frequency tuning over the entire X-band with nanosecond-level tuning speeds.

Key Results and Conclusions

1. Low-Noise Microwave Generation

Using the 2p-OFD technique, the team successfully generated a 10 GHz low-noise microwave signal with phase noise of -156 dBc/Hz at a 10 kHz offset and fractional frequency instability of 1×10^-13 at 0.1 seconds. This performance surpasses that of traditional electronic microwave synthesizers while providing greater frequency tunability compared to existing photonics-based methods.

2. Frequency Tunability

With the introduction of DDS, the synthesizer achieved frequency tunability across the entire X-band (8–12 GHz). The DDS output, combined with the original 10 GHz signal, produced low-noise microwave signals. Experimental results showed that within the 9.5–10.5 GHz range, phase noise remained below -150 dBc/Hz at a 10 kHz offset, while at 8 GHz and 12 GHz, phase noise was measured at -140 dBc/Hz at a 10 kHz offset.

3. System Integration

The synthesizer’s design is compatible with integrated photonics, making it suitable for future chip-scale applications. The team suggests that using a bipolar complementary metal-oxide-semiconductor (Bi-CMOS) process, which integrates low-noise heterojunction bipolar transistors and high-speed CMOS for digital operations, could enable a fully on-chip optoelectronic synthesizer. This integration would significantly reduce both the physical size and power consumption of the system while maintaining high performance.

Research Highlights

1. Combination of Low Phase Noise and Frequency Tunability: The proposed synthesizer successfully combines low phase noise and wide frequency tunability, addressing the limitations of traditional photonics-based methods.
2. Simplification and Integration: By simplifying the optical frequency division system, the team significantly reduced the system’s size and power requirements, making it compatible with integrated photonics for future chip-level packaging.
3. High-Performance Microwave Generation: Experimental results show remarkable performance, achieving phase noise of -156 dBc/Hz at a 10 kHz offset and fractional frequency instability of 1×10^-13 at 0.1 seconds for a 10 GHz carrier frequency.

Significance and Value

This research provides a novel solution for low-noise, frequency-tunable microwave generation with broad applicability. In fields such as communication, navigation, radar, and microwave spectroscopy, low phase noise and frequency tunability are essential. The proposed optoelectronic microwave synthesizer not only meets these requirements but also demonstrates potential for integrated applications, making it viable beyond laboratory settings.

Additionally, the research offers new insights into the simplification of optical systems for high-performance microwave generation. These achievements will advance the development of photonics technology in the field of microwave generation and provide a more reliable microwave source for future communication and navigation systems.

Additional Information

The research team has provided detailed experimental data and system design diagrams, all available on the Figshare platform (DOI: 10.6084/m9.figshare.27000427.v1). These provide valuable references for other researchers wishing to validate or improve upon the performance of this synthesizer.

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This study marks a significant breakthrough in combining low noise and frequency tunability in optoelectronic microwave synthesizers, offering a promising new technological pathway for future communication and navigation systems. The success of this research not only highlights the potential of photonics technology in microwave generation but also lays the groundwork for integrated applications in real-world environments.