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A Highly Versatile Microwave Photonic Filter Based on an Integrated Optical Frequency Comb Source Jiayang Wu,1 Xingyuan Xu,1 Thach G. Nguyen,2 Sai T. Chu,3 Brent E. Little,4 Roberto Morandotti,5,6,7 Arnan Mitchell,2 and David J. Moss1*
2
1 Centre for Micro-Photonics, Swinburne University of Technology, Hawthorn, VIC 3122, Australia. ARC Centre of Excellence for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), RMIT University, Melbourne, VIC 3001, Australia. 3 Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Hong Kong, China. 4 State Key Laboratory of Transient Optics and Photonics, Chinese Academy of Science, Xi'an, China. 5 INRS-Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. 6 National Research University of Information Technologies, Mechanics and Optics, St. Petersburg, Russia. 7 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China. *
[email protected]
Abstract: We experimentally demonstrate a highly versatile microwave photonic filter (MPF) based on Kerr optical comb generated by an integrated microring resonator (MRR). The MPF features improved Q factors, wideband tunability, and highly reconfigurable filtering shapes.©2018TheAut h o r ( s ) OCIS codes: (060.5625) Radio frequency photonics; (130.0130) Integrated optics; (190.4390) Nonlinear optics.
1. Introduction Microwave photonic filters (MPFs), photonic subsystems that perform equivalent functions to those of ordinary microwave filters in RF systems, are one of the key devices in microwave photonics [1]. MPFs offer several competitive advantages over their electrical counterparts, including low loss, large filter bandwidths, reconfigurable filter shapes, fast tunability, and strong immunity to electromagnetic interference (EMI) [2, 3]. Among the various schemes to implement MPFs, tapped delay-line filters (DLFs) have attracted great interest due to their high reconfigurability in terms of filtering shapes [4]. In a microwave photonic DLF, different taps are implemented by multi-wavelength channels, and improved performance such as better filtering Q factors and time-bandwidth products can be achieved by increasing the number of wavelength channels. In this paper, by employing an integrated micro-ring resonator (MRR), we generate a broadband Kerr optical comb with a large number of comb lines and use them as the delay taps of the microwave photonic DLF, which significantly improves the performance and reduces the size, potential cost, and system complexity. By programming and shaping the optical comb generated by the integrated MRR, we demonstrate advanced filtering functions including improved Q factors, wideband tunability, and a high degree of reconfigurability in terms of filtering shape. Our experimental results agree well with theory, proving the feasibility of our approach as an effective way towards the implementation of high performance MPF with potentially low cost and compact footprint. 2. Theory and experimental results
Fig. 1. (a) Schematic diagram of the MPF with the integrated optical comb source. TLS: tunable laser source. EDFA: erbium-doped fibre amplifier. PC: polarization controller. BPF: optical bandpass filter. TCS: temperature controller stage. MZM: Mach-Zehnder modulator. SMF: single mode fibre. OC: optical coupler. PD: photodetector. OSA: optical spectrum analyser. VNA: vector network analyser. (b) Optical spectrum of the generated Kerr comb from 1400 nm to 1700 nm. Insets show a zoom-in spectrum with a span of ~32 nm and a SEM image of the cross-section of the MRR.
Figure 1(a) shows a schematic diagram of the MPF based on the Kerr optical comb generated by an integrated MRR. The MRR was fabricated with high-index doped silica glass using CMOS compatible fabrication processes [4, 5]. The optical spectrum of the generated comb is shown in Fig. 1(b). Such comb was then amplified and manipulated by the waveshaper to achieve appropriately weighted tap coefficients. To increase the accuracy, we adopted a real-time feedback control path to read and shape the power of the comb lines accurately. The processed comb lines were then
SM1B.5.pdf
CLEO 2018 © OSA 2018
divided into two parts according to the algebraic sign of the tap coefficients, and then fed into a 2×2 balanced MZM biased at quadrature [6]. The modulated signal produced by the MZM went through ~2.122-km of standard SMF, where the dispersion was ~17.4 ps/(nm km), corresponding to a minimum time delay T of ~59 ps between adjacent taps (with the channel spacing of the time delay lines set equal to the FSR of the MRR), yielding a Nyquist frequency of ~8.45 GHz for the MPF. We note that the operational bandwidth of the MPF was determined by the Nyquist frequency, which could be easily enlarged by decreasing the time delay and, owing to the large FSR of the compact MRR, could potentially reach over ~100 GHz. Finally, the weighted and delayed taps were combined upon detection and converted back into RF signals at the output. The experimental results associated to versatile filtering functions of the MPF in Fig. 1(a) are presented in Fig. 2. Figures 2(a) and (b) show the shaped optical comb and measured RF amplitude response of all-ones MPF. One can see that there is an increase in Q factor [7] of the MPF when expanding the tap number from 4 to 20. The shaped optical comb in Fig. 2(c) corresponds to the bandpass MPF in Fig. 2(d) when the centre frequency is fc = 4.899 GHz. As shown in Fig. 2(d), the tunability of the MPF was demonstrated by adjusting the tap weights, resulting in the center frequencies varying from 25.27% to 57.98% of the Nyquist frequency (8.45 GHz). The Nyquist frequency can be further tuned by changing the length of the SMF or the comb spacing to meet diverse requirements in terms of operational bandwidths. The MPF could also yield a range of other transfer functions by programming different tap coefficients, thus achieving a high degree of versatility in terms of center frequency and filter shape. Figures (e)−(h) show four different filter types for microwave signal processing with shapes ranging from a half-band highpass filter, to a half-band lowpass filter, a band-stop filter, and a Nyquist filter, respectively. It can be seen that the RF response of all the four MPFs agrees well with theory, which further confirms the success of the MPF based on our approach.
Fig. 2. (c) Measured optical spectra of the shaped optical comb and ideal tap weights for the all-ones MPF with 20 taps. (d) Measured RF amplitude response of the all-ones MPF with different number of taps. (c) Measured optical spectra of the shaped optical comb and ideal tap weights for the tunable MPF with fc=4.899 GHz. (d) Simulated and measured RF amplitude responses of a tunable MPF with different center frequencies fc. (e)−(h) Measured and simulated RF amplitude responses of halfband highpass, halfband lowpass, bandstop, and Nyquist filters, respectively.
3. Conclusions Advanced functions, including improved Q factors, wideband tunability, and high reconfigurability in terms of filtering shape, have been demonstrated for a MPF based on Kerr optical comb generated by an integrated MRR. 4. Acknowledgements This work was supported by the Australian Research Council Discovery Projects Program and NSERC/CRC Canada. 5. References [1] J. Capmany, B. Ortega, and D. Pastor, “A tutorial on microwave photonic filters,” J. Lightw. Techno. 24, 201−229 (2006). [2] J. Wu, et al., “Passive silicon photonic devices for microwave photonic signal processing,” Opt. Commun. 373, 44−52 (2016). [3] X. Jiang, “Wavelength and bandwidth-tunable silicon comb filter based on Sagnac loop mirrors with Mach-Zehnder interferometer couplers,” Opt. Exp. 24, 2183−2188 (2016). [4] X. Xu et al., "Reconfigurable broadband microwave photonic intensity differentiator based on an integrated optical frequency comb source," APL Photonics 2, 1-10 (2017). [5] D. J. Moss, et al., "New CMOS-compatible platforms based on SiN and Hydex for nonlinear optics," Nat. Photonics 7, 597-607 (2013). [6] J. Wu, et al., “On-chip tunable second-order differential-equation solver based on a silicon photonic mode-split microresonator," J. Lightw. Technol 33, 3542-3549 (2015). [7] J. Wu, et al., “Micro-ring resonator quality factor enhancement via an integrated Fabry-Perot cavity,” APL Photonics 2, 1-7 (2017).
