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Abstract
What we believe to be a novel reconfigurable multi-channel microwave photonic (MWP) receiver for multi-band RF signal is demonstrated for the first time, to the best of our knowledge. A reconfigurable MWP signal processing chip based on two cascaded microring filter banks is employed in the proposed receiver, which slices the multi-band RF input into several narrow band signals and selects optical frequency comb lines for frequency converting of each channel. Due to the significant reconfigurability of the signal processing chip, the proposed receiver can flexibly choose the output frequency band of each channel, and thus different frequency components of the multi-band RF input can be down converted to the intermediate frequency (IF) band for receiving or converted to other frequency band for forwarding. A multi-band RF signal composed of a linear frequency modulation (LFM) signal with 2 GHz bandwidth and a quad-phase shift keyed (QPSK) signal with 100 Mbit/s rate is experimentally received and reconstructed by the proposed receiver, where the reconstructed LFM component exhibits a signal to noise ratio (SNR) of 10.2 dB, and the reconstructed QPSK component reaches a high SNR of 26.1 dB and a great error vector magnitude (EVM) of 11.73%. On the other hand, the QPSK component of the multi-band RF signal centered at 13.5 GHz is successfully converted to 3.1 GHz.
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1. Introduction
Radio frequency (RF) receivers with the ability to process multi-band signals are of growing need in a multitude of applications, such like radar system, satellite communication, and electronic warfare [1–5], which brings increasing requirements on reconfigurability to RF receivers. While traditional microwave devices are limited by the bottleneck of working band, tunability, and sampling rate [6,7], microwave photonic (MWP) systems attract increasing attention due to their advantages of high reconfigurability, large bandwidth, and immunity to electromagnetic interference [8–11]. Restricted by the performance of the photodetector (PD) and analog-to-digital converter (ADC) used in down conversion and sampling, MWP receivers with single channel are difficult to achieve broad instantaneous bandwidth to fully cover the multi-band RF signal in frequency domain.
Multi-channel MWP receivers, which have been applied in a variety of RF systems including 3D imaging system, multifunctional radar and electronic warfare [12–14], provide a promising solution for multi-band signal receiving [15–17]. Among the typical schemes of the multi-channel receiver, channelization is an attractive method to realize broad instantaneous bandwidth through slicing the wideband RF signal into a number of narrowband segments within the capacity of common microwave devices [18–20], which meets the requirements of multi-band signal receiving. A variety of studies have been conducted on MWP channelizers based on different methods including slicing the RF spectrum through a bank of optical filters [21,22], employing the vernier effect between an optical frequency comb (OFC) source and a periodic optical filter [23–25], and applying two coherent OFCs with different frequency space serving as the carrier of RF input and local signal for down conversion, respectively [26–28]. These works bypass the restriction of instantaneous bandwidth and are capable of receiving multi-band signal through completely covering the corresponding frequency band. However, due to lack of reconfigurability, the outputs after down-conversion in those schemes are restricted in fixed RF frequency band, which brings extra requirements of sampling rates for ADCs.
We have previously reported a RF front-end converting RF input to the IF band with a single channel [29], which is realized through filtering modulated RF sideband and generated local frequency signal respectively. By extending the single channel RF front-end to multiple channels, a multi-channel reconfigurable MWP receiver can be achieved, each channel of which has a tunable output frequency band including the IF band. Meanwhile, multiple channels can receive wide band RF input through channelization.
In this paper, we demonstrate a novel multi-channel reconfigurable MWP receiver for multi-band RF signal. A reconfigurable MWP signal processing chip composed of two cascaded microring resonator (MRR) filter banks is employed in the receiver. By adjusting the center frequencies of the MRRs, one of the cascaded MRR filter bank slices the RF spectrum, while the other is applied to filter an OFC and select the comb line for down conversion of each channel. The significant reconfigurability brought by the signal processing chip enables the receiver to flexibly determine the output frequency band of each channel. By keeping the output frequency band within the intermediate frequency (IF) band of 0-1.3 GHz, the proposed receiver meets the capacity of common ADCs. A multi-band RF signal composed of a 2 GHz linear frequency modulation (LFM) signal and a 100 Mbit/s quad-phase shift keyed (QPSK) signal is experimentally received and successfully reconstructed, achieving a signal to noise ratio (SNR) of over 10 dB. The error vector magnitude (EVM) of the reconstructed QPSK signal reaches 11.73%. On the other hand, the QPSK component of the multi-band RF signal centered at 13.5 GHz is successfully converted to 3.1 GHz, which shows the capacity for frequency converting of the proposed receiver. The proposed reconfigurable multi-channel MWP receiver provides an attractive solution for future multi-band RF signal receiving and exhibits potential for realizing multi-function RF signal processor.
2. Operation principle
Figure 1 shows the schematic of the proposed multi-channel reconfigurable MWP receiver for multi-band RF signal. Two subcarriers with a frequency difference of 2f0 are generated through carrier suppression modulation and filtering, where subcarrier 1 is centered at fc – f0 and subcarrier 2 is centered at fc + f0. Afterwards, the RF input signal is introduced to subcarrier 1 through a phase modulator, while subcarrier 2 serves as the source of an OFC generator. A reconfigurable MWP signal processing chip with multiple channels is designed to process the signals in the two paths and determines the working band of each channel. The mixed outputs of all channels are down-converted with a PD array and sampled for digital signal processing to reconstruct the RF input signal. Controlled with tunable RF signal generators, the frequency difference between the two subcarriers (2f0) is set to a multiple of the frequency difference between adjacent OFC lines ($\mathrm{\Delta }f$) to realize seamless and non-aliasing slicing of the RF spectrum.
Fig. 1. Schematic of the proposed reconfigurable microwave photonic receiver for multi-band RF signal. CWL: continuous wave laser, MZM: Mach-Zehnder modulator, MRR: microring resonator, PM: phase modulator, OFCG: optical frequency comb generator, PD: photodetector, DSP: digital signal processing
The structure of the reconfigurable MWP signal processing chip is shown in Fig. 2. Two cascaded MRR filter banks are integrated on the chip, which are applied for RF sideband slicing and comb line demultiplexing, respectively. The drop ports of corresponding MRRs are combined by a MMI coupler for mixing and down conversion, and thus the 16 MRRs form 8 channels working independently, where channel i is constructed with MRRi and MRR(i + 8). Additionally, an extra MRR is integrated on the signal slicing path for carrier suppression.
The modulated subcarrier 1 is sliced by MRR1-8, whose center frequencies are set at the middle of two adjacent optical frequency comb lines, as denoted by (1):
On the other hand, the generated OFC lines are demultiplexed by the other cascaded MRR filter bank whose center frequencies are set to:
${k_j}$ is also an integer, which ensures that the center frequency of MRR j equals to the ${k_j}$th comb line. After mixing and down conversion, the RF output of the channel i exhibits a frequency band of:Therefore, the proposed MWP receiver can convert the sliced RF sideband of each channel to other frequency band by properly tuning the center frequencies of the MRRs to choose ki and kj. Meanwhile, the sliced RF inputs can be down converted to the IF band for receiving through setting ${k_i} = {k_j}$, which further decreases the requirements of ADCs. As shown in Fig. 3, each channel can independently determine the frequency band of the output RF signal for different applications due to the high reconfigurability of the signal processing chip. Moreover, the proposed reconfigurable multi-band MWP receiver is capable of receiving wideband RF frequency components through slicing them into narrow-band signals and recombining the corresponding output signals with multiple channels. In this way, the theoretically instantaneous bandwidth of the receiver reaches N$\mathrm{\Delta }f$, where N is the number of channels of the MWP signal processing chip.
Fig. 3. Sketch of the functions of receiving and frequency converting implemented in different channels of the proposed reconfigurable multi-band MWP receiver.
Based on the operation principle of the receiver, all the frequency components of the multi-band RF input signal can be reconstructed. The frequency component with bandwidths less than $\mathrm{\Delta }f$ is received by a single channel and reconstructed directly by loss compensation in frequency domain according to the spectra of the MRRs, while the frequency component with larger bandwidth is received by multiple channels, reconstructed separately and recombined. As shown in Fig. 4, each response of the MRR shows notch peaks at the center frequencies of the MRRs closer to the input port due to the serial structure of the cascaded MRR filter bank, which can be used for crosstalk suppression in both signal slicing and comb line selection processes. As shown in Fig. 4(a), by setting the center frequencies of MRR2 and MRR3 at 9.75 GHz and 8.45 GHz, respectively, the RF signal selected by MRR3 in the frequency band of 7.8∼9.1 GHz exhibits a great crosstalk suppression. This effect can also be used to suppress the power of adjacent combs lines during comb line selection, as shown in Fig. 4(b). Therefore, through setting ki and kj to decrease as i and j increase, it can be ensured that each channel only has crosstalk from the adjacent lower frequency band, and the channel with the lowest frequency band exhibits negligible crosstalk. The RF input can be reconstructed one by one from low frequency band to high frequency band. The spectra of the MRRs and selected comb lines are introduced to the reconstruction process to eliminate the crosstalk by calculating the crosstalk introduced from the reconstructed channel with lower frequency band.
Fig. 4. The settings of the center frequencies of the MRRs for (a) signal slicing and (b) comb lines selection.
3. Experimental setup and results
3.1 Reconfigurable Si3N4 signal processing chip
We fabricate the reconfigurable MWP signal processing chip composed of two cascaded MRR filter banks on Si3N4 waveguide platform, occupying an area of 13.5 × 6 mm, as shown in Fig. 5(a). The low loss Si3N4 waveguide platform has been demonstrated in our previous work [30]. Both of the cascaded MRR filter banks integrate 8 MRRs with the same diameters of 1090 µm and coupling gaps of 1 µm. We design and apply elliptical waveguide crossings on the chip with a negligible insertion loss of ∼0.08 dB per crossing. The drop ports of the corresponding MRRs in the two filter banks are combined through 2 × 2 MMI couplers. Meanwhile, micro-heaters are employed on all MRRs to tune the center frequencies of their responses. The fabricated reconfigurable Si3N4 MWP signal processing chip is packaged with a thermo-electric cooler (TEC), as shown in Fig. 5(b). And the optical coupling loss of the chip is measured to be about 3.2 dB/facet, achieving a low fiber-to-fiber optical insertion loss of 10.6 dB.
Fig. 5. (a) Micrograph of the fabricated reconfigurable Si3N4 MWP signal processing chip. (b) Photograph of the packaged reconfigurable Si3N4 MWP signal processing chip. (c) The optical transmission spectra of the 8 cascaded MRRs. (d) The thermo-optical tuning characteristic of the chip.
Figure 5(c) shows the transmission spectra of the MRRs in one cascaded MRR filter bank measured through an advanced optical spectrum analyzer (OSA, APEX, AP2081B) with a built-in tunable laser. The free spectral range (FSR) and bandwidth are measured to be 260.2 GHz and 1.3 GHz, respectively, which brings a bandwidth of 1.3 GHz of each channel in the system. All the MRRs show high extinction ratios of over 20 dB. Meanwhile, the measured on-chip losses of MRR1∼8 are kept within 4.2∼4.7 dB, which is mainly introduced by the 2 × 2 MMI couplers. Since the transmissions of MRR2∼8 are affected by MRRs closer to the input port, the on-chip losses of them are ∼1.1 dB lower than MRR1 when all the MRRs are at the same working band. On the other hand, we evaluate the center frequency tuning characteristics of the MRRs, as shown in Fig. 5(d), which exhibits a linear relation of about 0.14 GHz/mW. Moreover, each MRR reaches a tuning range of over 40 GHz with a heating power up to 300 mW.
3.2 Experimental setup
In Fig. 6, we show the experimental setup of the proposed reconfigurable multi-channel MWP receiver for multi-band RF signal. A continuous wave laser with 10.1 dBm power centered at 1549.69 nm is modulated by a 6.5 GHz RF signal through a carrier suppression Mach–Zehnder modulator (MZM), and thus two subcarriers with a frequency separation of 13 GHz are generated, as displayed in Fig. 7(a). An integrated tunable MRR filter is applied to divide the subcarriers into two paths, where the 3 dB bandwidth of the MRR filter is 2.6 GHz and the center frequency is set to 1549.64 nm. The subcarriers centered at 1549.64 nm and 1549.74 nm are thus selected by the drop port and the through port of the MRR, respectively. The measured spur suppression ratios of the selected subcarriers in the two paths are over 25 dB.
Fig. 6. Experimental setup of the proposed reconfigurable multi-channel MWP receiver for multi-band RF signal. EDFA: erbium-doped fiber amplifier, PC: polarization controller, TEC: thermo electric cooler, FA: fiber array, DSO: digital storage oscilloscope.
Fig. 7. (a) The spectrum of the generated carrier suppression modulated signal. (b) The spectrum of the generated optical frequency comb.
Afterwards, subcarrier 1 centered at 1549.64 nm serves as the carrier of the RF input signal, and subcarrier 2 centered at 1549.74 nm serves as the pump source of an OFCG. The OFCG is realized by cascading a MZM and a phase modulator, exhibiting an insertion loss of 10.1 dB. By adjusting the power and the frequency of the RF input, the number of the comb lines can be tuned within 10∼19 and the frequency separation between comb lines can be tuned within 0.5∼1.6 GHz. To match the bandwidth of the MRRs on the signal processing chip, a 1.3 GHz RF signal is fed to the OFCG, which brings a frequency separation of 1.3 GHz between adjacent comb lines. The spectrum of the generated OFC is shown in Fig. 7(b), where the power differences of the 10 comb lines near 1549.74 nm are kept below 2 dB.
To evaluate the capacity of multi-band RF signal receiving of the proposed reconfigurable multi-channel MWP receiver, we generate a multi-band RF signal constructed with a LFM signal and a QPSK signal through an arbitrary waveform generator (AWG). Figure 8(a) displays the spectrum of the generated multi-band RF signal, where the LFM component exhibits a wide bandwidth of 2 GHz (8 GHz – 10 GHz) with a period of 100 µs and the QPSK component is centered at 13.5 GHz with a rate of 100 Mbit/s. The frequency-to-time relation of the LFM component of the multi-band RF signal is displayed in Fig. 8(b).
Fig. 8. (a) The spectrum of the multi-band RF input. (b) The frequency-to-time relation of the sampled LFM signal after bandpass filtering.
The generated multi-band RF signal is introduced to subcarrier 1 through a phase modulator, and the modulated signal is then sliced by one cascaded MRR filter bank on the Si3N4 signal processing chip. Meanwhile, the generated OFC lines are demultiplexed by the other cascaded MRR filter bank and mixed with the corresponding sliced RF sideband signals through the 2 × 2 MMIs on the chip. To seamlessly cover the multi-band RF signal in frequency domain, three channels of the receiver are applied, including channel 3 formed by MRR3 and MRR11, channel 2 formed by MRR2 and MRR10, and channel 5 formed by MRR5 and MRR13.
We realize frequency converting of the QPSK signal with channel 5 at first, where the center frequency of MRR5 is set to 13.65 GHz relatively to subcarrier 1 to filter the QPSK component, and the center frequency of MRR13 is tuned to 10.4 GHz relatively to subcarrier 1 to select the comb line for down conversion, as shown in Fig. 9(a) and (b). After photodetection, the spectrum of the output is acquired through a high-resolution signal analyzer (Keysight, N9010A). As shown in Fig. 9(c), the center frequency of the QPSK component is successfully transferred to 3.1 GHz with a high SNR of 31.2 dB. The RF frequency conversion efficiency of the system reaches -25.2 dB. By changing the selected comb line, the 13.5 GHz QPSK signal can be converted to other frequency band within 0.5∼12.2 GHz, where the RF frequency conversion efficiency keeps over -28 dB and the SNR keeps over 28 dB, as shown in Fig. 10.
Fig. 9. (a) The spectrum of the MRR for signal slicing. (b) The spectrum of the MRR for comb line selection. (c) The measured spectrum of the QPSK signal converted to 2.6-3.8 GHz.
Fig. 10. The RF frequency conversion efficiency and SNR of the QPSK signal converting to different frequency.
3.3 RF signal receiving and reconstruction
To receive and reconstruct the multi-band RF input signal, the center frequencies of the MRRs in the three channels are properly set to ensure the sliced RF sidebands are transferred to the IF band at 0-1.3 GHz after down conversion with the PD array, which further decreases the requirements of bandwidth and sampling rate of the ADC, as shown in Fig. 11. The multi-band RF input is sliced into the three channels with frequency bands of 7.8-9.1 GHz, 9.1-10.4 GHz, and 13.0-14.3 GHz, respectively. On the other hand, the center frequencies of MRR9 and MRR12 are also properly tuned to suppress the crosstalk caused by adjacent comb lines.
Fig. 11. (a) The spectrum of the MRRs for signal slicing. (b) The spectrum of the MRRs for comb lines selection.
The output RF signals of the three channels after photodetection are sampled with a 40-GSa/s real-time oscilloscope (Agilent, DSO91204A). The spectra of the sampled RF output of the three channels are displayed in Fig. 12. Afterwards, the LFM and QPSK components of the multi-band RF input are reconstructed with MATLAB, respectively. Since the outputs of channel 3 and 5 exhibit negligible crosstalk while the output of channel 2 suffers from crosstalk of channel 3 due to the spectral lineshape of the response of single MRR filter, we reconstruct part of the LFM signal (8 GHz ∼ 9.1 GHz) received by channel 3 and the QPSK signal received by channel 5 through directly compensating the loss induced by the corresponding MRRs in frequency domain, where the power and phase response are both compensated based on the measured responses of MRRs shown in Fig. 11. Afterwards, the other part of the LFM signal (9.1 GHz ∼ 10 GHz) received by channel 2 is reconstructed through loss compensation and crosstalk suppression based on the reconstructed LFM signal in channel 3 and the measured spectra of the MRRs.
Fig. 12. The spectrum of the sampled RF output from (a) channel 3, (b) channel 2, and (c) channel 5.
Figures 13(a) and (c) show the spectra of the reconstructed LFM and QPSK signals at the IF band. The SNRs of the reconstructed LFM component and QPSK component are 10.2 dB and 26.1 dB, respectively. The reconstructed LFM signal is modulated to a 7.8 GHz RF signal for up-conversion back to 8–10 GHz and the corresponding frequency-to-time relation is displayed in Fig. 13(b). On the other hand, the reconstructed QPSK signal is demodulated and the constellation is shown in Fig. 13(d), where the EVM reaches 11.73%.
Fig. 13. (a) The spectrum of the reconstructed LFM signal. (b) The frequency-to-time relation of the reconstructed LFM signal. (c) The spectrum of the reconstructed QPSK signal. (d) The constellation of the reconstructed QPSK signal.
3.4 RF link performance evaluation
Furthermore, the RF link performance of the proposed reconfigurable multi-channel MWP receiver is evaluated with the method of two-tone test [29]. We generate two tone signals centered at 8.45 GHz with a frequency interval of 10 MHz through two synthesized signal generators. The generated two-tone signals are down converted to the IF band by the receiver through channel 3 and the crosstalk is suppressed by properly setting the center frequencies of other MRRs. We measure the output powers of the fundamental tones and intermodulation distortion (IMD) tones with different RF input powers. By fitting the measured powers and the input RF powers, the RF gain, noise factor (NF) and spur-free dynamic range (SFDR) are evaluated, which is shown in Fig. 14. The RF link performance of the receiver can be further improved by increasing the power of the OFC lines and improving the modulation efficiency of the phase modulator.
4. Conclusion
In conclusion, we have demonstrated a reconfigurable multi-channel MWP receiver for multi-band RF signal. The proposed receiver is capable of receiving and frequency converting by independently deciding the output frequency band of each channel due to its significant reconfigurability. A multi-band RF signal composed of a LFM signal with 2 GHz bandwidth and a QPSK signal with 100 Mbit/s rate is experimentally received by three channels and successfully reconstructed with a SNR of over 10 dB. Moreover, the reconstructed QPSK component reaches a high SNR of 26.1 dB and a great EVM of 11.73%. On the other hand, the QPSK component centered at 13.5 GHz is successfully converted to 3.1 GHz with a high SNR of 31.2 dB and a RF frequency conversion efficiency of -25.2 dB. Comparing to the works before, the output signals in all channels are kept within the frequency band of 0-1.3 GHz for receiving, which further reduces the requirements of the sampling rates and bandwidths of the ADCs. The proposed reconfigurable multi-channel MWP receiver shows a potential for realizing multi-function RF signal processer and is expected to play an important role in future multi-band RF applications.
Funding
National Key Research and Development Program of China (2021YFB2800800).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
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