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Microwave Q-band oscillator at 49GHz for broadband frequency conversion based on a Kerr optical soliton crystal micro-comb
Xingyuan Xu, Jiayang Wu, Mengxi Tan, Thach G. Nguyen, Sai T. Chu, Brent E. Little, Roberto Morandotti, Arnan Mitchell, David J. Moss
TL;DR
The paper addresses the size, cost, and frequency limitations associated with external electrical local oscillators in photonic microwave conversion. It uses a coherent 48.9-GHz Kerr micro-comb from an integrated micro-ring resonator as a millimeter-wave local oscillator, experimentally verifying conversion up to 40 GHz with a −6.8 dB output-RF-to-IF power ratio and spurious suppression above 43.5 dB.
Problem
Photonic microwave frequency converters rely on external electrical local oscillators whose cost and size increase substantially for high-frequency operation.
Method
An integrated high-Q micro-ring resonator generates a coherent 48.9-GHz Kerr optical micro-comb that serves as an equivalent millimeter-wave local oscillator.
Results
The converter was experimentally verified for input RF frequencies up to 40 GHz, achieving a −6.8 dB output-RF-to-IF power ratio and spurious suppression above 43.5 dB.
Takeaways & Limitations
The microcomb-based converter provides broadband microwave conversion with reduced system size, complexity, and potential cost.
Abstract
from arXiv · showhide
We report a broadband microwave frequency converter based on a coherent Kerr optical micro-comb generated by an integrated micro-ring resonator. The coherent micro-comb displays features that are consistent with soliton crystal dynamics with an FSR of 48.9GHz. We use this to demonstrate a high-performance millimeter-wave local oscillator at 48.9GHz in the Q-band for microwave frequency conversion. We experimentally verify the microwave performance up to 40 GHz, achieving a ratio of 6.8 dB between output RF power and IF power and a spurious suppression ratio of > 43.5 dB. The experimental results show good agreement with theory and verify the effectiveness of microwave frequency converters based on coherent optical micro-combs, with the ability to achieve reduced size, complexity, and potential cost.
I. INTRODUCTION
Photonic microwave frequency converters address bandwidth and interference limitations of electrical approaches, but conventional designs depend on bulky external electrical local oscillators. This work replaces that source with a coherent integrated Kerr micro-comb to provide broadband, lower-complexity conversion.
- I. INTRODUCTION: Photonic microwave conversion offers large bandwidth, high isolation, and strong immunity to electromagnetic interference compared with electrical approaches.
- I. INTRODUCTION: Conventional photonic converters require external electrical local oscillators, increasing system cost and size as operating frequencies rise.
- I. INTRODUCTION: Kerr micro-combs can generate coherent electrical signals from 10 GHz to 500 GHz while maintaining chip-scale footprint and reduced complexity.
- I. INTRODUCTION: The proposed LO-free converter uses a coherent Kerr micro-comb from an integrated micro-ring resonator as an equivalent millimeter-wave local oscillator.The reported comb has a 48.9-GHz free spectral range.
- I. INTRODUCTION: The demonstrated converter reaches RF frequencies up to 40 GHz, with a −6.8 dB output-RF-to-IF power ratio and spurious suppression above 43.5 dB.
II. PRINCIPLE
The converter selects two adjacent Kerr-comb lines, modulates them with an RF signal, and photodetects their beat to produce an IF at ω_LO ± ω_RF. A 48.9-GHz comb spacing provides millimeter-wave LO operation and broad conversion bandwidth.
- The integrated MRR generates the comb through optical parametric oscillation after a pumped resonance provides sufficient parametric gain.The resonator has a 48.9-GHz FSR, and Fig. 2 reports a resonance near 193.294 THz with approximately 125-MHz FWHM.
- 48.9 GHz comb spacing enables millimeter-wave LO operation and ultra-broadband conversion from the L-band to the U-band.The comb spacing is determined by the MRR free spectral range and can provide higher LO frequencies with multiple-FSR-spaced lines.
- The converter filters two adjacent comb lines, applies RF modulation with a Mach–Zehnder modulator, and converts the resulting optical signal to electricity by photodetection.The selected lines are separated by ω_LO, while the RF input is V_RF · cosω_RF t.
- The Mach–Zehnder modulator is biased at quadrature, with φ set by the DC bias and γ set by the RF voltage relative to the half-wave voltage.The modulation index is γ = πV_RF / V_π, while φ = πV_dc / V_π.
- The output IF appears at ω_IF = ω_LO + ω_RF or ω_LO − ω_RF after optical beating and detection.The same process also generates spurious components, including ω_LO − 2ω_RF.
III. EXPERIMENT
The experiment generates coherent 48.9-GHz-spaced Kerr micro-combs with soliton-crystal-like signatures and uses selected comb lines for broadband microwave frequency conversion. Conversion is demonstrated from 40 to 23 GHz with low noise, 43.5-dB spurious suppression, and a calculated 48.9-GHz local oscillator.
- Micro-comb generation: The generated combs exhibited soliton-crystal-like spectral fingerprints, a transmission step, and reduced RF intensity noise consistent with coherent micro-comb operation.Varying pump power within ±0.5 dB produced multiple low-noise spectral superstructures.
- Microwave frequency conversion: A 48.9-GHz equivalent photonic local oscillator enabled conversion of input RF frequencies from 40 GHz to 23 GHz, with IF power variation below 5 dB.The IF varied from 8.9 GHz to 25.9 GHz across the tested range.
- Microwave frequency conversion: The converter produced a signal-to-noise ratio above 70 dB, an IF-to-RF output power ratio of −6.3 dB, and 43.5-dB spurious suppression at 26 GHz input.The reported conversion efficiency relative to input RF power was −36.4 dB, including −30.3 dB link gain.
- Microwave frequency conversion: The measured IF-to-RF frequency relationship yielded f_LO = f_IF + f_RF = 48.9 GHz, verifying millimeter-wave local-oscillator generation.The authors identify potential operation from the L- to U-bands, while noting experimental limits from the analyzer and RF-source bandwidth.
- Power dependence: The IF power scaled with input RF power at slope 1.02 until saturation at 10 dBm and with received optical power at slope 1.98.These slopes closely matched the theoretical predictions of 1 and 2, respectively.
IV. CONCLUSION
The integrated Kerr-comb converter performs microwave frequency conversion up to 40 GHz using a 48.9-GHz photonic local oscillator, with strong spurious suppression and reduced system requirements.
- −6.8 dB output-RF-to-IF power ratio and >43.5 dB spurious suppression were achieved for input RF frequencies up to 40 GHz.
- The 48.9-GHz coherent micro-comb served as an equivalent millimeter-wave local oscillator for broadband conversion up to the U-band.
- The integrated micro-comb approach reduces system size and complexity while offering potential cost advantages for radar and radio-over-fibre systems.