Open Access
Add to BibSonomy
Abstract
A flexible channel selection method based on optical combs is proposed for reconfigurable optical channels in this paper. Optical-frequency combs with a large frequency interval are used to modulate broadband radio frequency (RF) signals, and an on-chip reconfigurable optical filter [Proc. of SPIE , 11763, 1176370 (2021). [CrossRef] ] is used to perform periodic carrier separation of wideband and narrowband signals and channel selection. In addition, flexible channel selection is achieved by presetting the parameters of a fast-response programmable wavelength-selective optical switch and filter device. Channel selection only relies on the combs through the Vernier effect of the combs and the passbands for different periods and does not require the use an additional switch matrix. Finally, flexible switching between and selection of specific channels for 13 GHz and 19 GHz broadband RF signals are experimentally verified.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Developments in communication technology have resulted in increases in the complexity and bandwidth of transmission signals, as well as the coexistence of multiple signals in time. Channelized receiving based on microwave photonics is one effective method for the simultaneous reception of broadband multifrequency signals [1,2]. Generally, a photonic channelizer system receives a wideband radio frequency (RF) signal, performs in-band channel processing on the signal, divides the signal into several parallel narrowband signals, and outputs the information of each channel of the received signal. Representative schemes for photonic channelizer systems include channel-independent filtering for direct intensity detection [3] and channelized receiving based on a single optical comb and an optical filter [4–8], as well as dual optical combs and optical filtering [9]. However, these systems have limitations in terms of the information extracted from signals, extremely high requirements for optical filters, and complex compositions. Therefore, limited scene applicability and poor achievability present challenges in using these systems for practical applications.
To meet the requirements for flexible switching between multifrequency signals for electronic reconnaissance systems, a large-bandwidth RF signal received by a microwave photonic channelizer must typically be divided into narrowband signals. In most cases, only one or several channels of the broadband signal are output, and switching between different channels must be performed according to the requirements of the reconnaissance mission. Conventional photonic channelized receiving requires the use of an additional RF switch matrix [10] or optical switch matrix [11] to select a specific channel after channelization. However, both microwave and optical switch matrices have low frequency sensitivities and switching speeds, high control complexity, and require fixed ports to output fixed channels. Therefore, a channel selection method that requires the use of an additional microwave or optical switch matrix after channelization considerably limits the agility of wavelength switching and flexibility of channel selection. The three most prominent adverse factors for practical application of large-scale and good-performance switch matrices are frequent difficulty with realization, high manufacturing costs, and large weights and volumes.
A technical scheme for reconfigurable optical channel selection based on comb switching is proposed in this paper. Optical-frequency combs with a large frequency interval are used to perform broadband RF signal modulation. A highly coherent and reconfigurable on-chip optical filter is used to perform periodic carrier separation and channel selection to reduce the degradation of coherence caused by separate filtering. In addition, differentiated period settings are used for broadband and narrowband channels. Within the range of each optical comb, different channel numbers corresponding to different narrowband passband positions can be obtained through the Vernier effect, thus achieving one-to-one correspondence between the optical combs and channel passbands. In practical applications, a specific channel can be flexibly selected simply by presetting a fast-response Programmable-filter and switching between channels during channelization by switching between combs. Notably, an additional optical switch matrix is not required, and the system has a simple composition and high achievability and operability.
2. Principles and method
2.1 System schematic
Figure 1 is a schematic of the proposed flexible channel selection method for broadband channels based on optical combs. The system is mainly composed of an optical-frequency comb generation unit, an electro-optic modulator, a programmable wavelength-selective optical switch and filter device (abbreviated as Programmable-filter), a unit for on-chip periodic carrier separation and channel selection (abbreviated as Channel-Selection-Chip), and a photoelectric detector.
The optical-frequency comb generation unit is used to generate multicomb optical-frequency signals with the same frequency interval. This unit is composed of a local oscillator source, an electrical phase shifter, an electrical amplifier, a laser, an intensity modulator, and a phase modulator. The electro-optical modulator is used to modulate the broadband RF signal onto the multicomb optical signal. The Programmable-filter is used to select any comb signal. Channel-Selection-Chip is obtained by functional reconstruction of the on-chip reconfigurable optical filter [12] and mainly functions to separate the carrier and sidebands of the signal processed by the filter. The parameter information preset by the filter in practical applications and the corresponding coding relationship between the initial phase and comb are used to extract the signal of any channel from the sideband signal, which is combined with the carrier and output to the photoelectric detector. The photoelectric detector completes the final channel selection and outputs the RF signal of the selected channel.
As Channel-Selection-Chip is obtained by functional reconstruction of the on-chip reconfigurable optical filter [12,13], the filter components are briefly described here. The filter [12,13] is a passive photonic integrated circuit (hereinafter referred to as Passive-PIC) made of silicon nitride. Figure 2 shows a topology diagram and photograph of the filter. The chip has four functional parts: a ring resonator-assisted Mach-Zehnder interferometers (RAMZI), a first-order add-drop optical ring resonator (RR), fifth-order optical RRs (5th-order RRs), a 2 × 4 Hybrid Coupler (HC), and two sets of ports (a two-channel optical input port and a four-channel optical output port).
Fig. 2. On-chip reconfigurable optical filter (a) The topology of the on-chip reconfigurable optical filter(Ref. [12], Fig. 4) (b) The photograph of the on-chip reconfigurable optical filter
2.2 Operating principle
In the proposed flexible wideband channel selection system (shown in Fig. 1), the high-power single-frequency RF signal generated by the local oscillator source is input to the electrical phase shifter and the electrical amplifier and used to drive the intensity modulator and phase modulator. In addition, the single-frequency optical carrier generated by the laser is successively input to the intensity and phase modulators. After parameter adjustment and optimization, a multicomb optical signal with a large frequency interval (assume the frequency interval is X) between combs is output.
The Channel-Selection-Chip is an on-chip reconfigurable optical filter [12,13], and every device of the chip could be reconfigured by the tunable coupler unit, each of which can be independently programmed to operate as a directional coupler or an optical switch in a cross or bar state providing phase-controlled optical routing. Figure 3 shows the layout diagram of the Channel-Selection-Chip and the operating principle of the tunable coupler unit.
Fig. 3. The Channel-Selection-Chip (a)layout diagram (Ref. [13], Fig. 1) (b) operating principle of the tunable coupler unit(Ref. [13], Fig. 2)
When the Channel-Selection-Chip is used for the channel selection, in order to perform periodic carrier separation and channel selection, adaptive and reconfigurable settings are used for the functional part of the Passive-PIC: one optical input port and one optical output port are selected as the optical input and output ports of Channel-Selection-Chip, respectively, and the RAMZI is set to a straight-through state. The first-order add-drop RR and fifth-order cascaded optical RRs are adjusted to perform broadband and narrowband bandpass filtering, respectively, and the 2 × 4 HC is adjusted to serve as a 2 × 1 optical combiner. Figure 4 shows the topology and operating principle of Channel-Selection-Chip.
The multicomb optical signal with a frequency interval X drives the electro-optical modulator to modulate the broadband RF signal onto the optical signal in a double-sideband mode. In this study, for X = 19 GHz, the comb section of the double-sideband-modulated optical-frequency comb is divided into two parts, each of which can be divided into nine subchannel positions. An example of a carrier optical comb is shown in Fig. 5.
The modulated carrier optical signal is first input to the optical filter, which selects a specific channel. As shown in Fig. 4, the selected optical comb signal is made to enter the Channel-Selection-Chip input port by adjusting the frequency response of the on-chip first-order add-drop RR (FSR1 is X), such that all the optical carriers are output from the drop end of the resonator and the remaining modulation sidebands are output from the through end. The signal output from the through end is subjected to narrowband filtering by the fifth-order cascaded RRs (FSR5 is Y) [12,13], passes out the drop end and through the 2 × 1 optical combiner, and is output with the carrier signal output of the first-order add-drop end from the output port of Channel-Selection-Chip.
Equation (1) [14] can be used to express the free spectral range (FSR) in terms of the parameters of the resonant loop:
As the frequency difference between the broadband and narrowband channels is fixed (|XY| = 6 GHz), one-to-one correspondence between each optical comb and a specific channel can be achieved through the Vernier effect. Figure 7 shows an example in which the leftmost optical comb is selected as the initial comb and aligned. The consecutive frequency intervals between the passband of the fifth-order RRs and the corresponding optical combs from the second period onwards are 7, 6, 1, 8, 5, 2, and 9 GHz.
The channel is coded to produce a simple retrieval table, as shown in Table 1. The filter is then set such that the passbands are located at A, B, C, D, E, F, and G (corresponding to different comb numbers, as shown in Fig. 7), and the signals corresponding to the 7, 6, 1, 8, 5, 2, and 9 RF channel numbers are obtained.
Table 1. Simple channel selection retrieval table
The table lookup method is used to obtain the output of a specific channel by quickly switching the passband position of the aperiodic filter. The RF signal of a specific channel can be obtained by using the photoelectric detector to convert the specific optical signal output by Channel-Selection-Chip.
Table 2 summarizes the characteristics of the presented method in channel selection and provides a comparison with other channel selection methods [15,16,19] or channelized applications [17,18]. The presented work is the first demonstration of reconfigurable optical channel selection method to select the RF channels, which exploits the optical-frequency combs and the Vernier effect to achieve channel selection for the periodic broadband signals.
Table 2. A comparison of different channel selection method
3. Experiment and discussion
3.1 Simulation analysis
Figure 8 shows the simulated frequency response spectrum of the carrier signal passing through the on-chip reconfigurable optical filter for a wide frequency band. The periodic frequency response corresponding to the add-drop (FSR1: X = 19 GHz) and fifth-order (FSR5: Y = 13 GHz) RRs are shown in Fig. 8(a) and Fig. 8 (b), respectively, and the passbands of two different frequency responses for the same FSR are compared in Fig. 8(c).
Fig. 8. Frequency response corresponding to the (a) add-drop and (b) fifth-order RRs and (c) passbands of two different frequency responses for the same FSR
Figure 9(a) combines the frequency responses generated by the add-drop and fifth-order RRs, showing the passband positions vary with the periods, that is, each comb corresponds to a specific channel. The correspondence between the optical combs and selected channels (Fig. 7) shows that the simulation results are consistent with the system preset, as shown in Fig. 9(b), thus confirming the effectiveness and achievability of channel selection using combs.
Fig. 9. (a) Superposition of the frequency responses of the add-drop and fifth-order RRs and (b) correspondence between the optical combs and selected channels
3.2 Experiment and results
Figure 10 shows the experimental verification system built according to the schematic shown in Fig. 1. The core components include an optical-frequency comb generation unit, an electro-optical modulator, a Programmable-filter, Channel-Selection-Chip, and a photoelectric detector. The optical-frequency comb generation unit and electro-optical modulator provide a broadband periodic optical signal with a wavelength of 1550 nm and a frequency interval of 19 GHz. The optical signal is connected to the filter through an optical fiber array and then input to Channel-Selection-Chip. After channel selection is completed, the selected signal is connected to the photoelectric detector through the optical fiber array, and finally, the RF signal corresponding to the selected channel is output.
Note that the initial working state of the filter must be set to the all-pass state because the RF signals of all channels need to be transmitted to Channel-Selection-Chip to obtain the channel coding information with one-to-one correspondence (produced by the Vernier effect).
After the initial setup is completed, the frequency response test results of the add-drop and fifth-order RRs are obtained using a broadband light source, as shown in Fig. 11(a) and Fig. 11(b), respectively. The FSR measured by the add-drop RR is 19 GHz, and the 3-dB bandwidth of the passband is 2.4 GHz. The FSR measured by the fifth-order RRs is 13 GHz, and the 3-dB bandwidth of the passband is 480 MHz. The experimental and simulation results are consistent. In addition, Fig. 11(c) shows that the frequency intervals of the passband of the add-drop RR and the center of the passband of the fifth-order RRs vary with the filter period. The filter spacings for the three periods are 1.57 GHz, 4.15 GHz, and 9.73 GHz. The phase shift of one of the filters can be changed to reconstruct the passband spacing.
Fig. 11. Passband test results: passband response of the (a) add-drop and (b) fifth-order RRs and (c) frequency intervals between the channels for the add-drop and fifth-order RRs
The phase is adjusted to align the passbands of the add-drop and fifth-order RRs with the first passband from the left. Figure 12 shows that the specific channels are effectively selected.
Fig. 12. The selected channels (a) Input wideband multi-tone signal (b) optical combs (c) Modulated optical combs (d) Output spectrum of channel 1 (e) Output spectrum of channel 2 (f) Output spectrum of channel 3 (g) Output spectrum of channel 4
Figure12(a) shows the input multi-tone RF signal before modulation, which is used to simulate the RF components of different channels. The multi-tone signal frequencies are 4 GHz, 5 GHz, 6 GHz, 7 GHz with interval △f = 1 GHz; Figure12(b) shows the generated 6 optical frequency combs with an interval △f = 19 GHz; Figure12(c) shows the spectrum after the multi-tone RF signal is modulated over the whole optical frequency comb, and it can be seen that all the RF information is replicated next to each comb tooth.
It is known that the filter period of the FSR5 is 13 GHz, according to the Vernier effect, which makes the center frequency of a certain filter period in the higher order filter correspond to the optical frequency combs, so that the specific sideband information can be obtained in different filter periods. In the experiment, the filtering of the Programmable-filter is still broadband filtering, and its bandwidth and center frequency can be set arbitrarily. By adjusting the filter center and passband to a certain optical comb, the rest of the filtering cycle information can be filtered out. By changing the position of the Programmable-filter passband, the filtered retained sideband information and its corresponding optical comb teeth are obtained. It is known that the input is a multi-tone signal of 4 GHz, 5 GHz, 6 GHz and 7 GHz, and the spectral outputs are shown in Figure12(d)-Fig. 12(f), respectively, after filtering and selecting different signals independently. It can be seen that for each selected output, except for the required signal frequency and optical carrier, the other information is suppressed, and the independent channel selection function is achieved. So the experimental results also support the functional assumption made in this study.
3.3 Discussion
The simulation and experimental results verify the feasibility of the proposed channel selection scheme based on comb switching. A fast-response Programmable-filter can be used to realize flexible selection of any channel. The frequency response of the filter can be adjusted numerically to switch the passband between different combs, so that combs can be switched by adjusting the filter parameters to switch between any channels during channelization, thus achieving flexible channel selection. In addition to the 13 GHz and 19 GHz responses verified in this study, the frequency interval of the input multicomb signal can also be adjusted according to actual needs to reconstruct other different frequency responses. The period range of the optical-frequency comb can be further expanded to flexibly select channels with more comprehensive coverage.
4. Conclusion
A flexible channel selection method based on comb switching is proposed for reconfigurable optical channels in this study. An optical-frequency comb with a large frequency interval is used to modulate broadband RF signals, and periodic carrier separation and channel selection are achieved through Channel-Selection-Chip. Flexible selection of any channel during channelization is achieved through the Vernier effect of the optical comb and the passbands of different periods and by presetting the parameters of the fast-response Programmable-filter. Finally, flexible switching and selection of specific channels is experimentally verified for 13 GHz and 19 GHz broadband RF signals. In addition, the period range of the optical-frequency combs of the system can be extended. The proposed system has comprehensive coverage locations and applicable ranges, a simple composition, and strong achievability and operability.
Funding
Postdoctoral Fellows of “Zhuoyue” Program; National Key Laboratory Foundation of China (61424020305, HTKJ2019KL504013); National Natural Science Foundation of China (61571022, 61801376, 61971022); National Key Research and Development Program of China (2020YFB1807400); National Key Laboratory of Science and Technology on Space Microwave (6142411032201).
Disclosures
The authors declare that they have no conflicts of interest relevant to the study.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. K. Qu, S. Zhao, L. Xuan, Z. Zhu, L. Dong, and D. Y. L. iang, “Ultra-flat and broadband optical frequency comb generator via a single mach–zehnder modulator,” IEEE Photonics Technol. Lett. 29(2), 255–258 (2017). [CrossRef]
2. L. X. Wang, N. H. Zhu, W. Li, H. Wang, J. Y. Zheng, and J. G. Liu, “Polarization division multiplexed photonic radio-frequency channelizer using an optical comb,” Opt. Commun. 286, 282–287 (2013). [CrossRef]
3. C. S. Brès, S. Zlatanovic, and A. O. J. Wiberg, “Parametric photonic channelized RF receiver,” IEEE Photonics Technol. Lett. 23(6), 344–346 (2011). [CrossRef]
4. B. Vidal and M. A. Piqueras, “Tunable and reconfigurable photonic microwave filter based on stimulated Brillouin scatting,” Opt. Lett. 32(1), 23–25 (2007). [CrossRef]
5. S. R. Connor and M. L. Dennis, “Optimal biasing of a self-homodyne optically coherent RF receiver,” IEEE Photonics J. 2(1), 1–7 (2010). [CrossRef]
6. D. Okky, G. J. Liu, and D. Marpaung, “Microwave photonic notch filter with integrated phase-to-intensity modulation transformation and optical carrier suppression,” Opt. Lett. 46(3), 488–491 (2021). [CrossRef]
7. R. Y. Ge, Y. N. Luo, and S. Q. Gao, “Reconfigurable silicon bandpass filters based on cascaded Sagnac loop mirrors,” Opt. Lett. 46(3), 580–583 (2021). [CrossRef]
8. D. Pérez, I. Gasulla, and J. Capmany, “Field-programmable photonic arrays,” Opt. Express 26(21), 27265–27278 (2018). [CrossRef]
9. J. S. Fandiño, P. Muñoz, and D. Doménech, “A monolithic integrated photonic microwave filter,” Nat. Photonics 11(2), 124–129 (2017). [CrossRef]
10. C. Ward and A. J. Stark, “RF switch matrix design and trade study in photonics,” IEEE Avionics, Fiber-Optics and Photonics Conference (2014). [CrossRef]
11. K. Suzuki, R. Konoike, S. Suda, and H. Matsuura, “Low-Loss, Low-Crosstalk, and Large-Scale Optical Switch Based on Silicon Photonics,” J. Lightwave Technol. 38(2), 233–239 (2020). [CrossRef]
12. D. Liang, Q. Tan, X. Li, L. Li, and Y. Liu, “A reconfigurable Si3N4 on-chip filter for programmable photonic processor,” Seventh Symposium on Novel Photoelectronic Detection Technology and Application 2020, Proc. of SPIE 11763, 320 (2021). [CrossRef]
13. D. Liang, J. Capmany, D. Perez, P. Dasmahapatra, and Y. Liu, “Programmable rf receiver related on-chip photonic processor,” IEEE Photonics J. 13(1), 1–11 (2021). [CrossRef]
14. T. Hu, “Study on silicon photonic devices based on the filter characteristics of microring resonators,” Zhejiang University, 29, (2014).
15. T. Sakamoto, G. W. Lu, and T. Kawanishi, “Filter-less Multi-Tone Coherent Orthogonal Detection for Multi-Channel Reception of Super-Channel/OFDM signals,” Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC), (2013).
16. J. Zhang, W. Liu, F. Kong, and J. Yao, “Microwave photonic hilbert transformer based on a single passband microwave photonic filter for simultaneous channel selection and signal processing,” J. Lightwave Technol. 32(17), 2996–3001 (2014). [CrossRef]
17. R. G. Sebastian, M. M. Alvaro, M. Juliana, B. Shen, M. Florian, and W. Jeremy, “Wideband multi-stage crow filters with relaxed fabrication tolerances,” Opt. Express 26(4), 4723–4737 (2018). [CrossRef]
18. M. Tan, X. Xu, J. Wu, A. Boes, T. G. Nguyen, S. T. Chu, B. E. Little, R. Morandotti, A. Mitchell, and D. J. Moss, “Photonic microwave and RF channelizers based on Kerr micro-combs,” Proc. SPIE 11685, 22 (2021). [CrossRef]
19. R. Rady, C. Madsen, S. Palermo, and K. Entesari, “A 20–43.5 GHz wideband tunable silicon photonic receiver front-end for mm-wave channel selection/jammer rejection,” J. Lightwave Technol. 41(5), 1309–1324 (2023). [CrossRef]
