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Abstract
A microwave photonic synthetic aperture radar (MWP SAR) is developed and experimentally demonstrated. In the transmitter, microwave photonic frequency doubling is used to generate a linearly-frequency-modulated (LFM) radar signal; while in the receiver, photonic stretch processing is employed to receive the reflection signal. The presented MWP SAR operates in Ku band with a bandwidth of 600MHz, and is evaluated through a series of inverse SAR imaging tests both in a microwave anechoic chamber and in a field trial. Its imaging performance verifies that the proposed MWP SAR works perfect and shows the potential of overcoming the conventional radar bandwidth bottleneck.
© 2017 Optical Society of America
1. Introduction
For the next generation of radars, such as synthetic aperture radars (SARs), in applications like remote sensing, target recognition and area surveillance, acquiring more accurate and detailed information of regions of interest is one of the most attractive features. To meet such requirement, radar must have capabilities of operating at multi-bands over a large frequency span, transceiving and processing signals with ever-increasing bandwidth and large time duration [1]. In conventional electrical radar, however, both digital and analog techniques encounter bandwidth bottleneck. As bandwidth increases, direct digital synthesizers and analog to digital converters (ADCs) would cause quantization distortion and spurious increment due to large time jitter [2]; while analog microwave components, like mixers, frequency multiplier, suffer from limited bandwidth [3, 4]. Therefore, the performance of wideband electrical radar is limited by the electrical measures. To address these problems, photonic solutions, have been developed thanks to the advantages such as low timing jitter, ultra-wide bandwidth and low transmission loss offered by the photonics [5–11], and photonic-based radar systems have been demonstrated. In the world’s first photonic-based radar proposed by a research group in Italy [12, 13], a mode-locked laser (MLL) with ultralow time jitter is employed to generate a transmission signal through photonic frequency up-conversion, and to undersample the echo signal. After going through a photonic serial-to-parallel converter (SPC), the sampled pulses are digitized by a low speed ADC array for further processing. The field trial tests prove that photonics is capable of improving the performance of the conventional radar [14–17]. However, the operation bandwidth of the proposed radar, which would be increased by a MLL with GHz repetition rate, is essentially limited by the speed of the photonic SPC. Thus, in practical scenario [12, 14], the bandwidth is typically tens of MHz. In order to solve this issue, a different microwave photonic architecture has been proposed [18]. The transmission signal is obtained by beating two optical pulses from the same MLL but with different spectra and chirp rate induced by chromatic dispersion (CD). The echo is temporally stretched by CD as well to reduce the bandwidth and then digitized by a low speed electrical ADC. The operation bandwidth could be tens of GHz, however, the time duration of the radar pulse is typically limited to a few nanoseconds due to limited CD, which sets restriction on the operation range of radar. Therefore, implementation of microwave photonic radar, which can enable large time-bandwidth product, long-range, and high-resolution imaging, is a big challenge.
In this paper, we present a novel radar, a microwave photonic SAR, based on photonic signal generation and photonic stretch processing. In the transmitter, microwave photonic frequency doubling is employed to generate a radar transmission signal. In the receiver end, the incoming echo and a reference signal which is a replica of the transmission signal are photonic-mixed to enable photonic stretch processing (PSP). PSP converts echo from a certain range to a corresponding intermediate frequency (IF), significantly reducing the bandwidth of the signal to be digitized. Thus, a much narrower bandwidth IF can be obtained at the output of the coherent receiver. The IF signal is then digitized by low speed and high bits ADCs and processed by a digital signal processor (DSP) to recover the information of the target, which is a microwave image. A MWP SAR works in Ku band and in a continuous-wave (CW) mode with a bandwidth of 600 MHz and a period of 500 us is demonstrated. The system is evaluated through a series of inverse SAR imaging of a pair of trihedral corner reflectors in a microwave anechoic chamber (MAC) and non-cooperative target Boeing 737 in a field-trial.
2. Operation principle
Figure 1(a) shows the setup of the proposed MWP SAR. The transmitter of the radar consists of a continuous wave (CW) laser, a Mach–Zehnder modulator (MZM), a photodetector (PD), a low noise amplifier (LNA1), an IF signal generator (IF SG), a RF coupler and a RF power amplifier (PA). A linearly-polarized light wave from a laser is sent to a Mach–Zehnder modulator (MZM), which is modulated by an IF signal from the IF SG and biased at the null point to suppress optical carrier and even order sidebands. When the modulation index is small, at the output of the MZM, only two first-order sidebands have significant values. Therefore, after heterodyning two sidebands at the PD, a frequency-doubled signal is produced. Assume the IF signal is a LFM pulse, the generated frequency-doubled LFM signal can be written as
where V is the amplitude of the pulse, ωc is the angular frequency of the IF carrier from the IF-SG, k = B/Tp is the sweep rate of the IF LFM pulse, where B is the bandwidth of the IF LFM pulse, Tp is the duration of the pulse, and rect(t/Tp) is a rectangle function. The frequency-doubled signal at the PD is amplified by LNA1 and split into two paths; one is amplified by the PA for transmission, the other, as a reference, is coupled into the receiver end for PSP.
Fig. 1 (a) architechure of the MWP radar. (b) upper: the structure of the DP-DPMZM; lower: the structure of the Pol-demux coherent receiver.
The receiver consists of a CW laser, a dual-polarization dual parallel MZM (DP-DPMZM), an erbium-doped fiber amplifier (EDFA), a customized polarization-demultiplexed (Pol-demux) coherent receiver, a LNA, an ADC and a DSP. A light wave form laser2 is fed into the DP-DPMZM and spitted into two arms inside. In each arm, there is a sub-MZM, as shown in Fig. 1(b). Two sub-MZMs are biased at the null point and modulated respectively by the reference signal and the incoming echo. After modulation, in one arm, a 90-degree polarization rotator (PR) is embedded to ensure the light waves along two arms have orthogonal polarizations, and a polarization beam combiner (PBC) is integrated to create a polarization-multiplexed (Pol-mux) output. The Pol-mux light waves are amplified by the EDFA and routed to the Pol-demux coherent receiver, in which the orthogonally polarized light waves are separated by an integrated polarization beam splitter (PBS) and heterodyned in a coherent detection manner. The output of the coherent receiver which is the product of the reference signal and the incoming echo is digitized by the followed ADC and Fourier transformed by the DSP to yield target information. Pol-mux modulation and Pol-demux demodulation are used to have two light carriers experienced the same disturbance of environment so that phase relationship between two light carriers are maintained and a phase stable output of the coherent receiver can be achieved.
Theoretical analysis on the PSP of the receiver is presented as follows. For simplicity, the reference signal SRef(t) is treated as same as ST(t); the return signal SR(t) is considered as a delayed replica of the transmitted signal weighted by a factor determined by the target reflectivity and the distance between the target and the radar. Thus, SR(t) can be expressed as:
where VR is the factor which is also the amplitude of SR(t), is the traveling time of the radar signal from the transmitter to the target, where r is the distance between the target and the radar, c is the velocity of the light wave in the vacuum. At the output of each sub-MZM, we only consider the first-order sidebands due to small modulation index and carrier suppression. Using Jacobi-Anger expansion, the output of MZM1 and MZM2 can be expressed as where A1 and A2 are the attenuation factors of the corresponding links, ωo is the angular frequency of the optical carrier, βRef = πV/Vπ is the modulation index applied to MZM1, VRef is the amplitudes of the reference signal, βR = π VR /Vπ is the modulation index applied to MZM2; Vπ is half-wave voltages of two sub-MZMs, . The output of the DP-DPMZM, which is polarization-multiplexed and , travels through a segment of polarization maintaining fiber so that the phase relationship between two light waves remains unchanged and are routed to the Pol-demux coherent receiver, as shown in Fig. 1(b). The in-band photo-current of the two sub-PDs of the balanced PD can be expressed as:where n = 1,2. The output of the Pol-demux coherent receiver can be expressed asAs can be seen from Eq. (5), theoretically the output of the Pol-demux is the product of the reference signal and the incoming echo, including both the sum and the difference in frequency of the two signals; however, practically, due to the relatively limited bandwidth of the PD, only the difference frequency is generated and Eq. (5) can be modified aswhere A1 and A2 is the amplitude of the output, ωr = 8πk and = 4ωc-8πk are the range-dependent angular frequency and original phase of the obtained signal. Clearly, after PSP, each range target maps in to a frequency which is determined by the range between the target and the receiver. In a practical scenario, the two-way time Δτr for radar signal to traverse the range swath of the region of interest is much smaller than Tp. Thus, we can haveEquation (7) indicates that the bandwidth of Spsp is much smaller, which could significantly ease the requirement of the ADC. The digitized signal at the output of the ADC could be further processed by the DSP for imaging. Thanks to large bandwidth of photonics, the photonic-assisted receiver is capable of processing tens of GHz signal.In the radar, all the MZMs are biased at the null point; the bias drifting would cause the appearance of the optical carrier and the power reduction of the first-order sidebands. Consequently, when the bias drifts away from the null point the SFDRs of both transmitter and receiver would decrease. To avoid bias drifting, bias controllers can be used to stabilize the bias.
3. Experimental results and discussion
A MWP SAR based on the setup shown Fig. 1 is implemented. In the transmitter, a light wave at 1560 nm with a power of 13dBm from a laser (RIO, orion) is sent to an MZM (EOspace). A LFM CW signal generated from a home-made IF SG with a carrier frequency of 7.3 GHz, a bandwidth of 300 MHz and a temporal duration of 500 μs is applied to the MZM. At the output of a PD (Finisar, HPDV2120R), a LFM CW in Ku band with a bandwidth of 600 MHz is generated and a noise level of −165 dBm/Hz with an input light wave power of 0 dBm is recorded. The in-band spur-free dynamic range (SFDR) in the transmitter, which is the SNR of the signal when the in-band distortion is equal to the noise floor, is measured as 41 dBc. In the receiver, a light wave at 1550.12 nm with a power of 19 dBm from a laser source (Teraxion) is sent to a DP-DPMZM (Fujitsu, FTM7980EDA). The power of a reference signal applied to a sub-MZM is about 18dBm. LNA2 with a gain of 50 dB is used to amplify the echo. The output of the customized Pol-demux coherent receiver (Discovery Semiconductors DSC-R413-W-C) is sent to an ADC with a sampling rate of 100 MS/s and a resolution of 16 bits. The noise level of the radar receiver is determined by the noise of the coherent receiver, which is recorded as −143.8dBm/Hz, and the SFDR is measured as 40 dBc.
A series of imaging demonstrations are conducted to evaluate of the developed MWP-SAR. Firstly, imaging a pair of trihedral corner reflectors (TCRs) is performed in a microwave anechoic chamber, as shown in Fig. 2. The power of the emitted signal is −30 dBm, and a pair of lens antenna with a gain of 27dBi is used. Two TCRs are about 0.45m away in the range direction, and 0.4m away in the cross-range direction. Static imaging of the TCRs is performed first. Through PSP, echoes from the TCRs are converted to an IF output, which are Fourier transformed by a DSP to calculate corresponding spectrum. The spectrum of the IF output is shown in Fig. 2(c). As we can see, the two tones are separated 3.25 kHz away, corresponding to a range distance of 0.40m and matching well with the relative position of two reflectors. Then the two TCRs are placed on a rotating platform with a speed of 10 degree/s for rotational imaging. Two TCRs are about 0.2m away in the range direction, and 0.45m away in the cross-range direction. Figure 2(d) shows the rotational SAR image of the two reflectors, which is calculated in a time windows of 1s. From the SAR image, we can see the distance of the two reflectors in cross-range direction is about 0.43m, which matches well with the real condition.
Fig. 2 (a) the photo of the MWP SAR setup in the MAC; (b) the photo of two static TCRs; (c) spectrum of the stretched echo from two TCRs; (d) SAR image of a pair of rotating TRCs.
We also tested the MWP SAR in a field-trial demonstration through inverse SAR imaging a non-cooperative moving target during landing, Boeing 737. To achieve this purpose, the output of the PA is further amplified to 44.9 dBm to illuminate the target far away from the radar, while the other parameters of the radar are the same as those set in the microwave anechoic chamber test. The reflections are received and digitized after photonic stretch processing. An integration of 2360 waveform durations has been performed which means the coherent integration time has been set to 1.18s, obtaining a Doppler frequency resolution of 0.85 Hz. To recover the microwave image of the target, bi-dimensional fast Fourier transform (FFT) are employed. First, range FFT is carried out to reconstruct the high-resolution range profile of the target, as shown in Fig. 3(a). The target appeared at around 800m. Then by using envelope alignment and cross-range FFT, the inverse SAR image of the Boeing 737 is obtained and shown in Fig. 3(b), with a maximum Doppler shift of 400Hz. As a comparison, a photo of the Boeing 737 is shown in Fig. 3(c) as well. The profile of the target is close to its photo.
Fig. 3 (a) Calculated range profile of the moving airplane; (b) reconstructed inverse SAR image of the Boeing 737; (c) upper: photo of the setup in the field-trial; lower photo of the target.
The range-resolution of the developed MWP SAR is about 0.25 m in theory and angular motion along trajectory is large enough; both range and cross-range resolution are high enough compared to the size of the Boeing 737, therefore an image of the airplane with good range and cross-range resolutions is obtained. Compared with the inverse SAR imaging results in [14], our imaging results not only clearly show the outline of the Boeing 737, but also provide much more details of the Boeing 737. As shown in Fig. 3(b), the two engines, the wings, the body, the tails and four flap track fairings can be easily identified. The red pixels in the wing area are mainly contributed by the strong reflections from the engines and the flap track fairings. Although the implemented SAR only generates and processes signals with a bandwidth of 600 MHz, it could easily operate with wider bandwidth thanks to the bandwidth advantages provided by the photonics.
4. Conclusion
A microwave photonic SAR based on photonic-assisted signal generation and photonic stretch processing has been implemented and demonstrated. The developed MWP SAR works in LFM CW mode and in Ku band with a bandwidth of 600MHz. The performance of the developed MWP SAR has been evaluated by imaging a pair of TCRs in a microwave anechoic chamber and a non-cooperative aerial target Boeing 737 in a field trial. The excellent inverse SAR imaging results verify that microwave photonic radar has potentials of surpassing the performance of conventional microwave radar and illuminate a path to radars of next generation.
Funding
Science and technology innovation and key deployment program of CAS (KGFZD-125-15-017); National Natural Science Foundation of China (NSFC) (6169190011).
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