Source-linked AI summary
High performance photonic microwave filters based on a 50GHz optical soliton crystal Kerr micro-comb
X. Xu, M. Tan, J. Wu, T. Nguyen, S. T. Chu, B. E. Little, R. Morandotti, A. Mitchell, D. J. Moss
TL;DR
Photonic RF filters are sought to avoid the bandwidth bottleneck of electrical filters while providing reconfigurability and selectivity. This paper demonstrates an 80-tap transversal filter using a 49 GHz-FSR Kerr micro-comb, with improved selectivity, tunable rejection and centre frequency, and adjustable bandwidth and shapes.
Problem
Electrical RF filters face an intrinsic bandwidth bottleneck, while advanced RF filters are sought with broad bandwidth, reconfigurability, selectivity, and low cost.
Source: §I · p. 1Method
An integrated 49 GHz-FSR Kerr micro-comb supplies 80 C-band wavelengths as transversal-filter taps, whose weights and delays are configured to shape the RF response.
Source: §I · p. 2, §II · p. 2Results
With 80 taps, QRF reaches 73.7, four times higher than previous demonstrations; Gaussian apodization yields up to 48.9 dB MSSR, and centre frequency tunes from 1.4 to 11.5 GHz.
Source: §III · p. 3Takeaways & Limitations
The demonstrated filter offers reconfigurable RF filtering with tunable centre frequency, bandwidth, and shapes, and the approach is presented as potentially reducing cost and footprint.
Source: §I · p. 2, §III · pp. 3–4, §IV · p. 4
Abstract
from arXiv · showhide
We demonstrate a photonic radio frequency (RF) transversal filter based on an integrated optical micro-comb source featuring a record low free spectral range of 49 GHz yielding 80 micro-comb lines across the C-band. This record-high number of taps, or wavelengths for the transversal filter results in significantly increased performance including a QRF factor more than four times higher than previous results. Further, by employing both positive and negative taps, an improved out-of-band rejection of up to 48.9 dB is demonstrated using Gaussian apodization, together with a tunable centre frequency covering the RF spectra range, with a widely tunable 3-dB bandwidth and versatile dynamically adjustable filter shapes. Our experimental results match well with theory, showing that our transversal filter is a competitive solution to implement advanced adaptive RF filters with broad operational bandwidths, high frequency selectivity, high reconfigurability, and potentially reduced cost and footprint. This approach is promising for applications in modern radar and communications systems.
I. INTRODUCTION
Photonic RF filters offer bandwidth and reconfigurability beyond electrical filters, while integrated Kerr micro-combs could supply more taps with reduced footprint and complexity. This work uses a 49 GHz comb spacing to provide 80 C-band taps and reports improved selectivity and versatile filter responses.
I. INTRODUCTION
- Electrical RF filters face an intrinsic electronic bandwidth bottleneck, whereas photonic RF filters can offer higher RF bandwidth, reconfigurable shapes, rapid tunability, and immunity to electromagnetic interference.These capabilities are sought alongside broad operational bandwidth, high frequency selectivity, and low cost.Source: §I · p. 1
- Transversal filters form weighted, progressively delayed RF replicas and can realize arbitrary transfer functions by changing tap weights, but discrete sources limit tap count and EO-comb alternatives require high-frequency modulators or RF sources.Integrated photonics is identified as an attractive approach to these implementation challenges.Source: §I · p. 1
- Silicon Kerr devices combine high nonlinearity with substantial two-photon absorption and a 0.3 nonlinear figure of merit in the telecom band, motivating interest in alternative nonlinear platforms.The passage notes that two-photon absorption can be advantageous in some applications, though it is generally a limitation.Source: §I · p. 1
- New nonlinear CMOS platforms enabled integrated micro-combs, and their low linear loss, high nonlinearity, and very low two-photon absorption have supported diverse photonic applications.Reported applications include mode-locked lasers, quantum physics, optical frequency synthesis, and ultrahigh-bandwidth communications.Source: §I · p. 2
- 80 C-band taps from a 49 GHz-FSR integrated Kerr micro-comb yield a QRF factor four times higher than previous results, with tunable filtering and up to 48.9 dB out-of-band rejection.The reported filter also tunes its centre frequency from 0.05×FSR_RF to 0.40×FSR_RF and its 3-dB bandwidth from 0.5 to 4.6 GHz.Source: §I · p. 2
- Earlier micro-comb transversal filters used fewer than 21 taps across the C-band because their roughly 200 GHz spacing limited tap count, constraining selectivity, tuning resolution, and filter-shape versatility.The C-band matters because optical amplifiers and spectral shapers are more readily available there.Source: §I · p. 2
II. THEORY
A photonic transversal filter realizes its RF response by combining weighted, progressively delayed signal replicas. Its tap coefficients determine the response shape, while adjacent-tap delay sets the RF free spectral range.
II. THEORY
- The filter response is formed from N tap coefficients h(n) applied to RF replicas delayed by integer multiples of T, with angular input frequency ω.The relation between response and these quantities is introduced by the transfer-function equation.Source: §II · p. 2
- FSR_RF equals 1/T, where T is the delay between adjacent taps.Thus the inter-tap delay sets the transversal filter’s RF free spectral range.Source: §II · p. 2
- Setting the tap coefficients h(n) appropriately enables reconfigurable filters with arbitrary shapes.The coefficients represent the discrete impulse response of the filter.Source: §II · p. 2
- The 80-tap implementation generates a Kerr comb in a pumped microring, shapes each comb line to set its tap weight, and uses dispersive fiber to create progressive delays before photodetection combines the taps.The comb spacing matches the microring free spectral range.Source: §II · p. 2
III. EXPERIMENTAL RESULTS
The experiments use an integrated Kerr micro-comb to supply 80 C-band taps for a reconfigurable RF transversal filter. They demonstrate enhanced selectivity, tunable rejection and centre frequency, and adjustable bandwidths and filter shapes, while identifying trade-offs and a scope caveat about the comb state.
III. EXPERIMENTAL RESULTS
- The 80-tap source uses a 49 GHz optical FSR and a high-index doped-silica microring with approximately 1.5 million quality factor and 0.06 dB·cm−1 linear loss.The 592 μm-radius ring’s small optical FSR enables 80 wavelengths across the C-band.Source: §III · p. 2
- The authors treat the observed spectra as indicative of soliton-crystal formation, not conclusive proof, because time-resolved pulse autocorrelation was not performed.They report that high performance required low-noise coherent states and avoidance of the chaotic regime, not a specific soliton state.Source: §III · pp. 2–3
- The tap-delay setup uses about 5 km of standard single-mode fiber to provide 34.8 ps between adjacent taps and a 14.36 GHz Nyquist frequency; shorter fiber can enlarge bandwidth at reduced tuning resolution.The comb spacing limits maximum operation, with significant adjacent-channel crosstalk beyond 24.5 GHz.Source: §III · p. 3
- QRF reaches 73.7 with 80 taps, four times higher than previous demonstrations, as the measured sinc-filter 3-dB bandwidth falls from 3.962 to 0.236 GHz when tap count rises from 2 to 80.The measured spectra agree well with theory, and the reported QRF increases linearly with tap number.Source: §III · p. 3
- Gaussian apodization raises the sinc filter’s main-to-secondary sidelobe ratio from 26.4 to 48.9 dB as σ decreases, at the expense of a deteriorated QRF factor.The authors note that simultaneously achieving very high QRF and MSSR would require more taps, potentially over 200 across the C+L bands.Source: §III · p. 3
- The filter centre frequency tunes from 1.4 to 11.5 GHz, or 0.05×FSR_RF to 0.40×FSR_RF, with MSSR above 25 dB.The reported calculation accounts for the standard single-mode fiber’s third-order dispersion of approximately 0.083 ps/nm2/km.Source: §III · p. 3
- Rectangular-filter bandwidth is tunable from 0.5 to 4.6 GHz; for M=9, passband flatness is below 2.9 dB, out-of-band rejection exceeds 15 dB, and roll-off exceeds 15 dB/GHz.The filter also produces positive and negative passband slopes using distinct tap-weight profiles.Source: §III · pp. 3–4
- All-positive taps tune bandwidth from 0.89 to 7.18 GHz but produce spurious baseband passbands that limit overall MSSR, motivating use of both positive and negative taps.An added high-pass filter can suppress the low-frequency passbands, but the passage still identifies MSSR limitations as a general drawback of all-positive-tap filters.Source: §III · p. 4
IV. CONCLUSION
The 49-GHz-FSR integrated micro-comb provides 80 taps across the C-band, supporting improved selectivity, high rejection, and flexible RF filtering. The demonstrated filters tune centre frequency and bandwidth, and include multiple filter shapes.
IV. CONCLUSION
- 80 taps from a 49-GHz-FSR micro-comb yielded QRF factors four times larger than previously demonstrated.Source: §IV · p. 4
- 48.9 dB out-of-band rejection was achieved using Gaussian apodization.Source: §IV · p. 4
- The centre frequency was tunable from 0.05×FSR_RF to 0.40×FSR_RF, or 1.4 to 11.5 GHz.Source: §IV · p. 4
- The demonstrated 3-dB bandwidths ranged from 0.5 to 4.6 GHz, with reconfigurable positive- and negative-slope passbands and rectangular bandpass shapes.Source: §IV · p. 4
- Experimental results agreed well with theory, supporting the filter as a competitive approach for adaptive RF filtering in radar and communications.Source: §IV · p. 4