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Abstract
A novel photonic frequency up/down-converting interface (FCI) with the capability of up-converting an intermediate frequency (IF) signal to a radio frequency (RF) signal and simultaneously down-converting a RF signal to a low IF signal is proposed, and a new application scenario, where both up and down frequency conversion stages of a deramp-on-receive linearly frequency modulated (LFM) continuous wave (CW) radar system are replaced by the FCI, is demonstrated. The five-port photonic FCI can be seen as two ultra-wideband phase-correlated photonic RF mixers incorporated in a single hardware, and the working frequency range of the FCI is up to Ka-band. The FCI is tested by an LFM waveform with 1GHz bandwidth in a deramp-on-receive LFM CW imaging radar system. In the test, the LFM signal can be transmitted and received correctly, and deramp output signals are able to coherently combine among multiple pulses, which generates a clear image of two point-targets with a 3dB range resolution of 15cm.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Frequency conversion as a stage between RF antenna and digital system is vital for radar, communication, and radio astronomy systems which may need to operate in frequency higher than several Gigahertz [1]. The three-port RF mixers, which are widely employed for frequency conversion, are based on nonlinear devices like Schottky diodes, and an LO with a significantly strong RF power is needed to switch the transconductance of the diodes [2]. The nonlinearity of the diodes, which is mainly caused by the inherent imperfect of diode I-V characteristics, generates many undesired mixing spurs, which cast limitations on the instantaneous bandwidth and the spurious-free dynamic range (SFDR) [3]. Consequently, multi-cascaded frequency conversion and filter stages are employed in both transmitter and receiver to avoid mixing spurs and image-frequency at the expense of system complexity.
Photonic techniques with the advantages of wide frequency coverage and low transmission loss have been investigated in frequency down-converting in the 1990s [4,5]. After that, various nonlinear mechanisms based on the materials including semiconductor [6–10], optical fiber [11–14], periodically poled lithium niobate (PPLN) [15], and even more exotic As2S3 planar waveguide [16] have been studied for frequency conversion. At present, conventional functionalities of photonic mixers like in-phase and quadrature (I-Q) mixing [17–21] have been realized and implemented in a deramp-on-receive LFM CW imaging radar system [21]. Distinct characteristics of photonic mixers like reconfigurable [22,23] and simultaneously multi-channel mixing [24] have also been achieved. In terms of RF performance of photonic mixers, superiority has been demonstrated by photonic mixers based on balanced detection in parallel external modulators topology [5,25,26]. However, the noise figure (NF) of photonic mixers is still significantly higher than conventional RF mixers, which could deteriorate the dynamic range of the entire system [27]. As a result, the photonics-based frequency mixing schemes are expected to be implemented in special scenarios where the performance of the whole system could meet or exceed the performance of conventional RF electronic systems, like satellite payload [28], wideband down-conversion RF link spanning tens of meters in length [3], and high-resolution synthetic aperture radars [29,30]. To push the photonic mixing techniques to practical applications, the photonic integration technology and the new scenarios for application need to be studied.
In this paper, a photonic FCI, which can convert an IF signal up to a RF signal while simultaneously converting a RF signal down to a low IF signal, is proposed. The FCI contains a dual-polarization quadrature phase shift keying (DP-QPSK) modulator and a polarization-demux coherent receiver (PDCR). The inputs of the FCI are the four RF ports of the DP-QPSK modulator while the output of FCI is the RF port of the PDCR. At the input of the DP-QPSK modulator, the light wave from a continuous wave (CW) laser is split into two branches, the one is modulated by the IF signal of up-conversion and the RF signal of down-conversion, while the other is modulated by two local oscillator (LO) signals. Since the two modulated light waves are polarization orthogonally multiplexed at the output of the DP-QPSK modulator and travel in two paths of the same polarization-maintaining (PM) link, they experience correlated optical fiber phase noise due to thermodynamic fluctuations. In the PDCR, the correlated phase noises of two paths are cancelled out as a common noise, and the products of the light wave crossing polarizations give up-converted RF and down-converted low IF signal simultaneously. The FCI is implemented in an LFM radar system, and is evaluated via a series of tests.
2. Principle
The architecture of most RF systems today has a canonical block diagram as illustrated in Fig. 1(a), it consists of digital sub-system, frequency synthesizer, transmitter, receiver, and antennas. The frequency up and down conversion stages of the conventional RF system are two sets of hardware with limited bandwidths. The photonic FCI, which is located between antennas and digital sub-system with four RF input ports and one RF output port, is proposed to enhance the bandwidth, SFDR, size, weight, and power (SWaP) performance in multi-band or wide frequency coverage operation scenario by replacing the independent frequency up and down conversion stages with a single ultra-wideband interface as shown in Fig. 1(b).
Fig. 1. (a) The schematic diagram of a conventional RF system; (b) The schematic diagram of a RF system employing the proposed photonic FCI. DAC: digital to analog converter; ADC: analog to digital converter.
The structure of the FCI is shown in Fig. 2(a). It consists of a CW laser, a DP-QPSK modulator which is based on monolithic integration of the parallel modulators, a dual-polarization optical band-pass filter (DP-OBPF), a dual-polarization erbium-doped fiber amplifier (DP-EDFA), and a PDCR which is composed of a polarization beam splitter (PBS), an optical coupler (OC), and a balanced photodiode (BPD). The DP-QPSK modulator contains two parallel branches which are designated as X-branch and Y-branch, and each branch has a QPSK modulator which is composed of two parallel sub-MZMs. The four sub-MZMs are designated as XI, XQ, YI, and YQ, and are null biased.
Fig. 2. (a) The structure of the FCI; (b) The schematic optical spectra at the output of the DP-QPSK modulator; (c) The schematic RF spectrums at the output of the PDCR. PR: polarization rotator; PBC: polarization beam combiner; PBS: polarization beam splitter.
As shown in Fig. 2(a), the IF1 and LO1 of up-conversion are applied on XI and YI respectively, thus the optical sidebands generated by the IF1 and LO1 are in the polarization X and polarization Y respectively. At the output of the DP-QPSK modulator, the optical sidebands in two polarizations are multiplexed, and the optical spectra are principally sketched in Fig. 2(b). Then the polarization multiplexed signals are sent to the PDCR via the DP-EDFA and DP-OBPF. In the PDCR, the polarization multiplexed optical sidebands are separated by the PBS and then coherently heterodyned [30]. The products of the light waves crossing polarizations give the up-converted RF which is designated as RF1. Then the RF2 and LO2 of down-conversion are fed into XQ and YQ respectively, and optical sidebands generated by the RF2 and LO2 are in the polarization X and polarization Y respectively, as shown in Fig. 2(b). Then the beating results of optical sidebands generated by the RF2 and LO2 give the down-converted IF which is designated as IF2.
Since the refraction index of an optical fiber undergoes statistical fluctuations, a special phase noise which is fluctuations in the phase of an optical signal propagating in the medium is introduced [31,32]. For photonic mixers, the phase noise of the optical signal would be converted to RF signal by beating between the optical signal, thus needs to be suppressed for optimum RF performance [5,33]. In the proposed FCI configuration, the two modulated light waves are polarization orthogonally multiplexed at the output of the DP-QPSK modulator and travel in two paths of the same PM link, thus they experience correlated optical fiber phase noise due to thermodynamic fluctuations. In the PDCR, the correlated phase noises of two paths are cancelled out as a common noise, and the products of the light wave crossing polarizations give up-converted RF and down-converted low IF signal simultaneously. The correlation of the phase noise in the two paths of a PM link has been utilized to suppress the phase noise by using optical phase-locked loop [34], and heterodyne detection with digital signal processing [35]. The proposed FCI deals with the correlated phase noise in the same principle as previous work in which correlated phase noise is suppressed by heterodyne detection with phase correlated LO [30].
Note that, the length of the two optical paths of the PDCR need to be matched. These enable the PDCR to obtain the beating results of the electronic fields of light waves crossing polarizations while the beating results of the electronic fields of light waves in the same polarization are suppressed. The output of the PDCR includes four signals as shown in Fig. 2(c), which are LO1 mixing with IF1, LO1 mixing with RF2, LO2 mixing with IF1, and LO2 mixing with RF2. The RF1 and IF2 signal, which are the expected results, can be separated in the frequency domain with RF filters.
When the FCI is implemented in a radar system, the transmitted signal RF1 is the up-converted IF1 signal, the IF2 is the down-converted echo, and the RF2 is the reflected version of RF1. Since the sum frequency of two light waves is out of the response of a photodiode (PD), RF1 is the difference frequency of the optical sidebands corresponding to LO1 and IF1. This means that $L{O_1} > R{F_1}$ and $L{O_1} > I{F_1}$. Note that there are two unwanted signals, which are located at $L{O_1} - R{F_2}$ and $L{O_2} - I{F_1}$, designated as S1 and S2 respectively. To avoid RF1 and IF2 overlap with S1 or S2, the frequency of the IF1, LO1, and LO2 should be carefully designed.
In the work, the FCI is applied in a deramp-on-receive LFM imaging radar system. Mathematically, the IF1 and LO1 signal can be written as:
where ${V_{IF}}$, ${f_{IF}}$, and k represent amplitude, frequency, and chirp-rate of the IF1 signal. ${V_{LO}}$ and ${f_{LO}}$ are amplitude and frequency of the LO1 signal respectively.The incident light wave can be expressed as ${E_0}\exp({j{\omega_0}t} )$ where ${\omega _0}$ is the angular frequency of the incident light, and ${E_0}$ is complex amplitude, which is related to the CW input light power ${P_0}$ by ${P_0} = {|{{E_0}} |^2}/2$. Since all sub-MZMs are biased at the null point, the optical signals generated by the IF1 and LO1 signal can be expressed as:
The two optical signals are polarization orthogonally multiplexed at the output of the DP-QPSK modulator. Then the polarization multiplexed signals are filtered by the DP-OBPF and amplified by the DP-EDFA. At the output of the DP-EDFA, the signals can be expressed as:
In the PDCR, the optical signals in orthogonal polarization are separated and heterodyne detected. The photocurrent of the two sub-PDs can be expressed as ${I_1}(t )= \frac{\mathrm{\Re }}{2}{|{{E_X}(t )+ {E_Y}(t )} |^2}$ and ${I_2}(t )= \frac{\mathrm{\Re }}{2}{|{{E_X}(t )- {E_Y}(t )} |^2}$ respectively, where $\mathrm{\Re }$ is the responsivity of the sub-PDs. At the output of the PDCR, the generated RF signal which is designated as RF1 can be represented as:
When the FCI is implemented in a deramp-on-receive LFM radar system, the RF1 signal is split into two paths. One path is fed into an antenna while the other path is coupled to YQ of the FCI as a reference signal, which means LO2 is a copy of the RF1 and RF2 is an echoed version of the RF1. The transmitted and reference signal can be expressed as:
where ${V_{transmit}}\; $ and ${V_{Ref}}$ are the amplitudes of the transmitted and reference signal.For simplicity, a point target at distance r is assumed, and the echo can be expressed as:
Then, the optical signals are filtered by the DP-OBPF and amplified by the DP-EDFA. At the output of the DP-EDFA, the optical signals when the IF1, the LO1, the echo, and the reference are applied together can be expressed as:
At the output of the PDCR, the output signals can be expressed as:
3. Experiment and result
To test the transmit and receive capability of the LFM radar system, an optical delay line (ODL) test is performed as shown in Fig. 3. In the FCI, the CW light wave from a laser source (Teraxion, PS-TNL) with a wavelength of 1550.32 nm and a power of 13dBm is sent to a DP-QPSK (FUJITSU, FTM7977HQA) modulator. An IF1 signal centered at 3.6GHz with a bandwidth of 1 GHz and a period of 100µs is generated by an arbitrary waveform generator (AWG, Keysight M8190A). The IF1 is fed into IF-Chain1, which is composed of an amplifier (PA) and band-pass filters, then applied on XI with a power of 7dBm. An LO1 signal generated by a signal source (SignalCore SC5511A) with a frequency of 16.1 GHz and a power of 7dBm is applied on YI.
First, the signal generation capability of the LFM radar system is tested, and the transmitted signal is generated by frequency up-conversion. In the up-conversion, the IF1 is applied on XI while the LO1 is applied on YI, and the optical spectra of dual polarizations at the output of the DP-QPSK modulator are measured by an optical spectrum analyzer (OSA, APEX AP2041B) and shown in Fig. 4(a). A DP-OBPF (Optoplex, CF-C2BFAC050) is used to pick up the +1st-order sidebands of both the IF1 and LO1 signal. The optical spectra of dual polarizations at the output of the DP-QPSK modulator, the DP-OBPF, and the DP-EDFA are shown in Fig. 4(b) and Fig. 4(c) respectively. Then, the optical signals are sent into the PDCR (LightPromotech, PDCR-H-40-PM), which is made by integrating of a PBS, a 3dB optical coupler, and a BPD into a self-contained module.
Fig. 4. (a) The optical spectra of dual polarizations at the output of the DP-QPSK modulator with the IF1 applied on XI and the LO1 applied on YI; (b) The optical spectra of dual polarizations at the output of each stage along the optical path in the polarization X; (c) The optical spectra of dual polarizations at the output of each stage along the optical path in the polarization Y.
To evaluate the performance of the generated RF LFM signal, which is designated as RF1, at the output of the PDCR, the spectrum of the signals is measured by an electrical spectrum analyzer (ESA, Keysight N9030A), while the waveforms of the signals are recorded by an oscilloscope (OSC, Keysight DSO-X 92004A) with a sampling rate of 80GS/s. The spectrum of the RF1 which is generated by up-converting the IF1 with LO1 is shown in Fig. 5(a), and in-band interference which is analyzed based on the method of short-time Fourier transform (STFT) is shown in Fig. 5(b).
At the output of the PDCR, the RF1 signal is fed into RF-Chain, which is built by a low noise amplifier (LNA), a power amplifier (PA, Keysight 83020A), and band-pass filters. After that, the amplified RF1 with a power of 30dBm is split into two paths by a 1:10 RF coupler.
Then the reception capability of the LFM radar system is tested. The low-power output of the RF coupler is fed into the input of the ODL with a power of 0dBm to simulate the echo and the output of the ODL is used to drive the electrical input of the XQ. The ODL is comprised of a laser source, an MZM modulator, a section of optical fiber, and a PD. The length of the optical fiber in the ODL is 400m, and the output power of the ODL is -21dBm. The other path of the RF coupler is fed into YQ with a power of 3dBm via a 20dB RF attenuator, acting as the reference signal. The optical spectra of dual polarizations at the output of the DP-QPSK modulator with the IF1 applied on XI, the LO1 applied on YI, the echo applied on XQ, and the reference applied on YQ are shown in Fig. 6(a). The 1st-order optical sidebands of the echo and reference signal are falling into the passband of the DP-OBPF. The optical spectra at the output of each component in each polarization are shown in Fig. 6(b) and Fig. 6(c) respectively. The RF spectrums at the output of the PDCR when the IF1, the LO1, the echo, and the reference are applied are shown in Fig. 7(a), and we can see the four signals are separated in frequency with a large span. The IF2, which is the deramp output, is fed into IF-Chain2, which is comprised of an LNA and low-pass filters, and the detailed spectrum of the IF2 at the output of the IF-Chain2 is shown in Fig. 7(b). A Sinc envelope with a frequency centered at 20.11 MHz can be observed, and the frequency matches well with theory in which $f = k \times t = $ 20.1 MHz, where $k = $ 1×1013Hz/s is the chirp rate and $t = $ 2.01×10−7s is a total delay of 401 m fiber and 1.5 m RF cable. The Sinc envelope has a 3 dB bandwidth of 1.04×104Hz which matches well with theory in which $\Delta B = 1/T = $ 1×104Hz where $T = $ 1×10−4s is the duration of the RF1. The results of the ODL test prove that using the FCI in the radar system, the signal can be transmitted and received correctly.
Fig. 6. (a) The optical spectra at the output of the DP-QPSK modulator with the IF1, the LO1, the echo, and the reference applied in the ODL test; (b) The optical spectra at the output of each stage along the optical path in the polarization X; (c) The optical spectra at the output of each stage along the optical path in the polarization Y.
Fig. 7. (a) The spectrums at the output of the PDCR with the IF1, the LO1, the echo, and the reference applied in the ODL test; (b) The zoom-in spectrum of the IF2 at the output of the IF-Chain2.
As a key element of the FCI, the PDCR is designed to suppress the beating results of the electronic fields of light waves in the same polarization. To evaluate the suppression, a test is performed to compare the PDCR with a PD as shown in Fig. 8. The IF1 is applied on XI, the LO1 is applied on YI, and the reference is applied on YQ. The optical signals at the output of the DP-EDFA are split by a dual-polarization-maintaining 50:50 optical coupler and sent to the PDCR and a PD (Discovery Semiconductors, DSC20H) respectively. The spectrums at the output of the PDCR are shown in Fig. 9(a), and the PD output spectrums are shown in Fig. 9(b). In the comparation, the optical sidebands generated by the LO1 and reference are both in polarization Y, and will beat in the PD generating an IF signal with the same frequency and bandwidth as the IF1, as shown in Fig. 9(b). At the same time, with the PDCR, the same IF signal is suppressed by 35dB, as shown in Fig. 9(a).
Fig. 8. The schematic diagram of the comparation experiment between PDCR and PD. DP-OC: dual-polarization-maintaining optical coupler.
Fig. 9. The spectrums at output of (a) the PDCR and (b) a PD, when the IF1, the LO1, and the reference is applied.
The ODL test does not give the clue on the system coherence from pulse to pulse. To verify pulse-to-pulse coherence, it is useful to perform an inverse synthetic aperture radar (ISAR) imaging experiment in Fig. 10 with a pair of trihedral corner reflectors (TCRs) on a rotating platform.
As shown in Fig. 11(a), the rotating platform is parallel to the horizontal, and two TCRs which are point targets for microwave, are placed on the rotating platform with an initial distance of 36cm in the range direction and 50cm in the cross-range direction as shown in Fig. 11(b) and Fig. 11(c). The platform is rotated at an angular speed of 60 degrees/s, and the ISAR imaging result of the rotational targets is shown in Fig. 12(a), which is calculated in a coherent integration time of 0.1s. Two points can be clearly identified in the image and section spectrums of a point in the image are shown in Fig. 12(b) and Fig. 12(c), where the Sinc profile can be clearly identified in both range and cross-range directions. The spectrum in range direction shows a 3dB resolution of 15cm while bright points in the image are focused well. The result of ISAR shows the deramp output signals are able to coherently combine among multiple pulses, which means the LFM-CW radar system where the FCI is implemented works well.
Fig. 11. (a) The ISAR imaging experimental scene; (b) The range distance of the TCRs; (c) The cross-range distance of the TCRs.
Fig. 12. (a) The ISAR image of two rotating TCRs; (b) The section spectrum in range direction; (c) The section spectrum along cross-range direction.
The measured OIP3 of the down-conversion mixer in the proposed FCI is 10dBm. With an NF of 26dB, the SFDR of the down-conversion mixer of the photonic FCI is 106 dB*Hz2/3. Since the SFDR of the photonic RF mixer is determined by the half-wave voltage of the sub-MZM modulator of the DP-QPSK modulator, the optical power launched into the DP-QPSK modulator, and the responsivity of the PDs of the PDCR, the improvement can be expected with enhanced performance on DP-QPSK modulator and PDs.
4. Conclusion
In this paper, a novel photonic frequency converting interface which is able to up-convert an IF signal to the RF signal and simultaneously down-convert the RF signal back to the low IF signal is proposed. The photonic interface is proposed to enhance the performance of the bandwidth and SWaP in ultra-wideband radar and communication systems, and the interface is also expected to enhance the SFDR performance with higher performance components. At current stage, the working frequency range of the FCI is limited by the bandwidth of DP-QPSK modulator. The proposed FCI is implemented in an LFM imaging radar system with an operating bandwidth of 1GHz, and the radar system works well in a series of tests.
Funding
National Key Research and Development Program of China (2018YFA0701900, 2018YFA0701901); Aerospace Information Research Institute, Chinese Academy of Sciences (E1Z201020F).
Acknowledgments
The authors would like to thank Mr. Fei Luo and Mr. Xinwei Fan of Rohde & Schwarz China for the support in the mixer test.
Disclosures
The authors declare no conflicts of interest.
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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