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An X-band optically-steered phased array radar is developed to demonstrate high resolution multi-target detection. The beam forming is implemented based on wavelength-swept true time delay (TTD) technique. The beam forming system has a wide direction tuning range of ± 54 degree, low magnitude ripple of ± 0.5 dB and small delay error of 0.13 ps/nm. To further verify performance of the proposed optically-steered phased array radar, three experiments are then carried out to implement the single and multiple target detection. A linearly chirped X-band microwave signal is used as radar signal which is finally compressed at the receiver to improve the detection accuracy. The ranging resolution for multi-target detection is up to 2 cm within the measuring distance over 4 m and the azimuth angle error is less than 4 degree.
© 2016 Optical Society of America
1. Introduction
Phased array radar (PAA) is extensively used in high-spatial-resolution target detection [1, 2] with the advantages of fast scanning, long distance and strong adaptability. The linearly chirp signal [3–8] as the conventional emitted signal of the PAA can increase the performance of the radar in terms of the detection distance, dynamic range and signal-to-noise ratio, which is compressed to the microwave pulse to obtain the high temporal resolution without the need of high peak power short pulse. During a duration time of the chirp signal, the instantaneous frequency is linearly scanning from f0-B/2 to f0 + B/2, where f0 is the center frequency; B is the total bandwidth of the chirp signal. To further improve the ranging resolution, the chirp signal with a large time-bandwidth product is needed. The conventional microwave devices [9–12] using in the PAA suffer from the limited bandwidth, especially the electrical phase shifter. It can only work up to a few gigahertzes. Therefore, the ranging resolution [13] is largely restricted due to a limited time-bandwidth product. In contrast, photonic techniques [9, 11] can provide high precision and ultra-wide bandwidth. In [12], the photonics-based coherent radar is developed and tested in outfield with high effectiveness and expected precision. In the coherent radar, a single mode-locked laser is used for both the photonics-based radio frequency (RF) generation and the analogue-to-digital conversion (ADC). The parabolic antenna is used to transmit/receive baseband signal. In order to realize wideband beam forming, photonics-based beam forming techniques have been proposed to implement phase-shift. In the past few decades, many true-time delays (TTD) techniques have been proposed to implement the photonics-based phased array radars [14–24], which include acousto-optic integrated circuit technique [14], Fourier optical technique [15], bulky optics technique [16], dispersive fiber technique [17–19], and fiber grating technique [23, 24], etc. The acousto-optic integrated circuit technique is considerable compact and integrated. The relatively limited bandwidth is largely restricted for the wide-band application [14]. Fourier optical technique is able to rapid beam steering with high resolution. For a certain steering angle, the optical frequency changes with the RF frequency to maintain a correct phase ramp period. Thus, this method is not a broadband TTD technique [15]. The bulky optics technique requires large space and complicated alignment [16]. The fiber grating technique for beam forming is mainly limited by the phase ripples induced by the multiple reflections in fiber gratings [23, 24].
Of these techniques, the dispersion-based TTD technique [17–19] is considered a promising technique due to the compact, low insertion loss and lightweight. The high-dispersion fiber is a better choice than conventional single-mode fiber (SMF) and it can reduce the overall length of the true time delay (TTD) line. In [17], the photonic crystal fiber (PCF) based on dual-concentric core structure was used to generate the time delay with the dispersion value of −600 ps/nm/km. Although the size and the weight of the overall system is largely reduced, the connecting between the PCF and the SMF is adopted the fiber adapter that increases the difficulty to calibrate the amplitude. This beam forming architecture makes the whole system high loss and low stability. The dispersion compensation fiber (DCF) is other greatly choice to be time delay media. In [19], a prototype optically-steered X-band phased array antenna is designed with the capability of multi-beam operation. The directional angle of beam forming is over from −20 degree to + 20 degree. However, the angle deviation is up to 10 degree within the bandwidth of 1.5 GHz due to the phase adjustment inaccuracy and the magnitude adjustment error. Until now, the optically-steered phased array antennas and target detection have been studied separately and not been test in a common radar system.
In this paper, we present the development and the demonstration of multi-target detection ability of the X-band optically-steered phased array radar. The optically-steered phased array radar is implemented based on DCF-based TTD technique. By precise calibration of magnitude and phase for each optical and RF channels, the RF beam divergence and beam quality are further improved. Therefore, the RF frequency can be extended to the full X-band and the half width of the pattern is up to 8.5 degree. By sweeping the optical wavelength from 1530 nm to 1560 nm, the optically-steered beam forming system has a wide steered angle tuning range of ± 54 degree with a low magnitude ripple of ± 0.5 dB and a small delay error of 0.13 ps/nm. And then, three experiments are carried out to demonstrate single and multiple target detection ability. A linearly chirped X-band microwave signal is used as the emitted signal of the optically-steered phased array radar with a long unambiguous range up to 390 m. It is able to rapidly discovery the target by simple tuning the optical wavelength and the echo signal is finally compressed to improve the ranging accuracy. The ranging resolution for multi-target detection is less than 2 cm within the measurement range of 4 m and the azimuth angle error is less than 4 degree. The experimental results can verify the feasibility of the proposed optically-steered phased array antennas and also provide strong evidence to support its practical applications for multi-target ranging.
2. X-band optical-steered phased array radar for transmitting mode
2.1 Principle of the beam forming
The proposed X-band optically-steered phase array radar is shown in Fig. 1. The optical carrier is emitted from a tunable laser (TLS) with the tunable wavelength from λ1 to λm. The radio-frequency (RF) signal is amplified by the low-noise amplifier (LNA) and modulates the optical carrier in the Mach-Zehnder modulator (MZM). The modulated optical RF signal passes through the erbium doped optical fiber amplifier (EDFA). the amplified optical RF signal is divided into eight individual optical channels in the 1 × 8 optical splitter and send to the DCF-based TTD units. The variable delays are realized by tuning the optical wavelength. After the pre-design time delay lines, 8 element optical signals are converted into the corresponding electrical signals in eight photo-detectors. The electrical signals are fed to the phase trimmers (PTs) which are used to slightly correct the possible phase errors in the normal direction. Then the signal powers are equalized by the electrical attenuators. The RF signals are emitted to free space through the 8-element linear array antennas. The RF electromagnetic waves constructively interfere to form a RF beam directing at an angle which depends on the time delay of each adjacent element.
Fig. 1 Schematic architecture of the proposed optically-steered phased array radar. TLS: tunable laser source; MZM: Mach-Zehnder modulator; LNA: low-noise amplifier; EDFA: erbium-doped optical fiber amplifier; PS: optical power splitter; DCF: dispersion compensation fiber; SMF: single mode fiber; PD: photodetector; PT: phase trimmer; ATT: attenuator.
The TTD lines produce a wavelength-dependent optical true-time delay. When the optical wavelength is tuned from λ0 to λm, the time delay τi corresponding to the i-th element of the PAA is expressed as [17]
where DDCF and DSMF are the dispersion coefficient of the DCF and the SMF, respectively. Li is the length of the DCF and L is the overall length of the delay line. The length difference of DCFs between each adjacent channel is designed as (Li + 1-Li). Therefore, the time delay difference Δτ between two adjacent channels is given byFor the one-dimensional 8-element phase array antennas, only one equivalent time delay difference is required between each adjacent channel. In the Eq. (2), the dispersion impact of SMF is considered to improve delay precision. Hence, the achievable beam steered angle is expressed as [19]where c is the light velocity in vacuum, d is the separation between two adjacent array elements of the antennas. It is obviously seen from the Eq. (3) that the beam steered angle can be continuously changed just by tuning the optical wavelength.2.2. Experimental results of optically-steering beam forming
An experiment is carried out to verify the proposed photonics-based radar system. As shown in Fig. 1, an X-band RF signal comes from an electrical vector network analyzer (ZVA 40 R&S). It passes through the low-noise amplifier (SHF 806E). The amplified RF signal drivers the MZM (Oclaro, AM-20) biased at quadrature point. The tunable laser with output power of 7 dBm is modulated by the MZM and the optical RF signal is amplified by EDFA (KEOPSYS). The output of the EDFA is 16 dBm. It is split into eight paths in the 1 × 8 power splitter with the loss of 9.5 dB. The split optical RF signals are sent to the DCF-based TTD lines to realize the variable time delay by simple tuning optical wavelength.
For the DCF-based TTD lines in the experiment, the DCF and SMF have an experimental dispersion value of −140 ps/nm/km and 17 ps/nm/km, respectively. The wavelength of 1545 nm is chosen as a reference for zero time delay. In order to achieve the steered angle from −60 degree to + 60 degree for 8-element PAA sub-array having 1.5 cm spacing, the time delay changes from −43.3 ps to + 43.3 ps by tuning the optical wavelength from 1530 nm to 1560 nm. To obtain the required time delay, the lengths of DCF are sequentially increased, i.e., 0 m, 18.5 m, 37 m, 55.5 m, 74 m, 92.5 m, 111 m and 129.5 m. The designed DCFs are then spliced with the SMF for linear phase compensation. The phase alignment resolution of multi-channel delay lines determines the accuracy of beam forming. One of the features in our work is that the dispersion value of SMF has also been considered in our system, which will slightly influence the accuracy of angular deviation. As such, eight spliced TTD lines have equal lengths. Eight high-speed photodetectors (EM169-03) are used to convert optical RF signals into RF signals. The phase trimmers (Spectrum) slightly correct the phase errors caused by the fiber length cutting errors and the electrical attentions compensate the magnitude errors resulting from the fiber splicing errors. The adjusted RF signals are sent to the free space by 8 element linear array antennas. One dimensional array antennas used in the experiment can cover the whole X-band with a gain of 6 dB and a standing-wave ratio of 1.8 dB.
The delay of the proposed system is demonstrated by measuring the microwave phase-frequency curve (φ-f curve). In the measurement, the time delays are calculated by the scaled derivative (1/2π)dφ/df, with the results shown in Fig. 2(a). As can be seen, the time delay is linearly proportional to the optical wavelength, and the equivalent time delays among eight channels are obtained on the wavelength of 1545 nm. From the results, the maximum time delay of two adjacent paths is 40.7 ps, and the wavelength step is 2.5 nm corresponding to the 8 element delay steps of 5.7 ps, −2.3 ps, −9.9 ps, −16.7 ps, −25 ps, 32.2 ps, 40 ps and 47.9 ps, respectively. The slope expresses the decrement of the time delay that reveals the time delay difference Δτ between each adjacent channel, as shown in Fig. 2(b). It can be clearly seen that the slope of linear fitting curve is −3.08 ps/nm, which agrees well with the theoretical prediction of −2.95 ps/nm calculated by Eq. (2).
Fig. 2 Time delays of the DCF-based TTD units. (a) The measurement time delays of the TTD. (b) The time delay difference of the TTD.
The magnitude ripples among eight channels will affect the beam forming performance such as the half width of pattern and the sidelobe suppression ratio [25, 26]. The insert loss of each optical delay line is less than 3 dB. To improve the beam forming performance, the magnitudes of eight channels at the wavelength of 1545 nm are carefully adjusted and calibrated using an electrical vector network analyzer. The adjustment results are shown in Fig. 3. As can be seen, the magnitude variation of each path is within ± 0.5 dB and the magnitude unbalance among the eight paths is less than ± 1 dB. These ripples can be attributed to the differences such as the broadband electro-optical conversion efficiency, broadband photo-electric conversion response and splicing loss. By optimizing the device performance, the magnitude ripple can be further decreased.
For a fixed optical wavelength, the difference of time delay between each adjacent channel is only determined by the lengths of DCF and SMF. In other word, each optical wavelength relates to a specific steering angle as given by Eq. (3). Therefore, the pattern of the photonics-based PAA is measured in the far-field in which the Fraunhofer condition ~2d2/λ is satisfied. The received RF signal is measured by the PAA on the accurate positioner by using a standard horn antenna with a bandwidth from 8 GHz to 12 GHz and a gain coefficient of 20 dB. The distance between the photonics-based PAA and the received horn is 50 cm. The photonics-based PAA is placed in a high precision rotary table with an angle resolution of 0.1 degree.
The received RF power in different direction is measured by rotating the high precision rotary table. As shown in the inset of Fig. 4, the beam-forming patterns of an RF signal with a frequency of 10 GHz are measured when the wavelengths of optical carrier are 1545 nm and 1553 nm, respectively. The steering angles are 0 degree and −22.5 degree when the wavelength is tuned to 1545 nm and 1553 nm, respectively. The half width of the pattern is about 8.5 degree. During the implementation of this experiment, a microwave anechoic chamber is not available and the pattern measurement is conducted in an open space. To reduce the environmental influence, a few pieces of microwave absorbing materials are placed around the experimental setup. In addition, the beam steering angle in different RF frequencies is plotted by calculating the peak of the pattern. The steered angle shown in Fig. 4 can be scanned over ± 54 degree from the direction normal to the antenna array facet, which agrees well with the theoretical curve by using Eq. (3). The RF beam steered angle is continuously changed by tuning the optical wavelength, independent of the RF frequency. With different frequency, the beam angle deviations are less than 7 degree in the common wavelength. The errors result from measurement errors as well as system errors, e.g. phase adjustment inaccuracy and insertion loss difference among fiber channels.
3. Multi-target detection ability demonstration
3.1. Principle of target detection
The target detection performance of the optically-steered phased array radar is estimated in the proof-of-concept experiment. It is worth noting that it is an initial experiment and the comprehensive analysis of the target detection capability has not been implemented due to the limited experiment condition. The linear chirped signal as the emitted signal can be written as [27, 28]
where f0 is the initial frequency, R is the chirp rate. It is emitted to free space through optically-steered phased array radar. When the target is placed in the beam direction area, the reflected waveform is received by the horn antenna. Hence, the reference signal ~Sr and the echo signal ~Sb can be expressed as [29, 30]where Δt is the time delay of optical fiber, Δτ is time delay of free space, φ is the initial phase depending on the time delay difference of TTD lines. The reference signal and the echo signal in Eq. (5) are cross-correlation to implement the microwave pulse compression.where xcorr(x,y) denotes a correlation function and the symbol represents the proportionality.Consequently, the received waveform is compressed [26, 28] by off-line processing. The compressed microwave pulse with the time delay of Δτ and the optical wavelength of λ. Hence, the target ranging is D = cΔτ/2 for Δt as a reference time to be zero and the azimuth angle of the target is expressed as: θ = arcsine (0.058(λ-1545)). Therefore, the target in the two-dimensional coordinates can be calculated by analyzing the compression signal and the optical wavelength in the time domain.
3.2. Experimental results of the target detection
The experimental structure is set up to verify the single-target detection ability as shown in Figs. 5(a). In the experimental layout of Fig. 5(b), the transmitter and receiver blocks are positioned approximately side by side. The optically-steered phased array radar illustrated in section 2 is applied as transmitter. The linearly chirp signal is generated by an arbitrary waveform generator (AWG, Tektronix 70001A) with a frequency range from 7.5 GHz to 12.5 GHz and a chirp rate of 5 GHz/μs. The time duration of 1.32 μs is large enough to make sure the same period between the reference signal and the echo signal. Then, it is split into two paths by power splitter (PS). The upper path signal is used for target detection and the lower path signal is used as the reference signal for synchronous clock trigger. One aluminum object with the size of 6 cm × 6 cm is placed within the antenna beam as the target. The reflected chirp signal is detected by a standard horn antenna in the far-field and then is recorded by an electronical oscilloscope with a sampling rate of 40 GSa/s (Tektronix DPO73304D). Therefore, the target location such as distance and horizontal azimuth angle can be calculated by off-line processing. The target ranging experiment is also carried out in an open space. Similar to the pattern measurement experiment, a few pieces of microwave absorbing materials are used to reduce the disturbances due to microwave reflected signal from the surrounding equipment. Moreover, To guarantee the stability of the system, the ambient temperature are kept around 25 °C during the whole experiment.
Fig. 5 Single-target detection experiment. (a) Schematic diagram, AWG: Arbitrary Waveform Generator; PS: power splitter; OSC: oscilloscope. (b) Experimental layout of single-target detection.
As a proof of concept, the experimental structure is set up following the Fig. 5 and the experimental demonstration is carried out for the single-target ranging. The system initial delay is firstly measured with the transmitting/receiving antennas face to face. Then, a single-target is placed 1.25 m, 1.65 m, 2.95 m and 3.95 m far from the transceiver and the experimental measurements are implemented with the transmitting/receiving antennas side to side.
The experimental results of the single-target detection are shown in Fig. 6 when the optical wavelength is 1545 nm corresponding to the beam direction along the normal direction. The 1.32 ns long chirp waveform shown in Fig. 6(a) is used the clock synchronous signal and the waveform of the initial delay system is shown in Fig. 6(b). After cross-correlation based on the Eq. (6), the compressed signal, which shows in the black line of Fig. 6(h), is the initial delay. The distance is 268.412 m as the reference distance which is fixed and related to lengths of optical fibers and electrical cables.
Fig. 6 Measurement results of the single-target detection with the wavelength of 1545 nm. (a) The reference signal. (b) The received signal. (c)-(f) The received echo signal when the target distance is 1.25 m, 1.65 m, 2.95 m, 3.95 m. (g) Normalized cross correlation. (h) The target distance.
The conducting the ranging procedure, experimental results are presented in Figs. 6(c)-6(f). The insets of Figs. 6(a)-6(f) are magnified to show the details of the waveforms with the time span of 0.001 μs. With normalized cross-correlation operation, the compressed microwave pulses are expressed in Fig. 6(g) and the magnification of the peaks in Fig. 6(h) express the target distance. The experimental results are 270.897 m, 271.689 m, 274.293 m and 276.303 m along the normal direction, respectively. Therefore, the round-trip target ranging are 2.485 m, 3.277 m, 5.881 m and 7.891 m, respectively. From the experimental results, it can be seen that the distance error is less than 1.5 cm.
The photonics-based PAA is not only used to detect the target in the normal direction, but also used to detect the target deviated from the normal direction. The experimental structure is set up to measure the azimuth angle of the target. As shown in the Fig. 5(b), the target is placed away from the normal direction of the array antennas. Two-dimensional positional coordinates of the target is a distance of 34 cm and a clockwise rotation of 48 degree along the normal direction. And then, the steering beam angle is changed by tuning the optical wavelength to detect the target.
Figures 7(a)-7(d) are the reflected waveforms from the target when the optical wavelength is 1550 nm, 1554 nm, 1557 nm and 1560 nm, respectively. The insets of the Figs. 7(a)-7(d) are shown the details of received waveforms by magnifying the waveforms around 0.9 μs and 1.3 μs with the time span of 0.001μs. The normalized cross-correlation results based on the Eq. (6) are shown in Figs. 7(e)-7(h). It is clearly seen that Fig. 7(g), with roughly 1 normalized cross-correlation power, is the maximum received signal among Figs. 7(e)-7(h). By magnifying the peak around 0.9 μs in the inset of Fig. 7(g), The absolute round-trip distance of target is calculated by subtracting the reference distance from the detected distance and the azimuth angle of the target is equal to arcsine (0.058(λ-1545)). From the Fig. 0.7(g), the peak position of the compressed signal is 269.162 m and the optical wavelength is 1557 nm. The reference distance is 268.412 m shown in the black line of Fig. 6(h). Therefore, the calculated round-trip distance of the target is 75 cm and the calculated azimuth angle ~θ is 44.2 degree. The ranging error is about 3 cm and the azimuth angle error is within 4 degree compared with the actual position of the target.
Fig. 7 Measurement results with the angle 48 degree. (a)-(d) The received signal when the wavelength is 1550 nm, 1554 nm, 1557 nm and 1560 nm. (e)-(h) The results after matched filtering processing between (a)-(d) and the reference signal when the wavelength is 1550 nm, 1554 nm, 1557 nm and 1560 nm. (i) The target distance ~(g).
The ranging resolution of optically-steered phased array radar is also demonstrated. In Fig. 8(a), the double-target detection structure is set up corresponding to the experimental layout shown in Fig. 8(b). A double-plate is placed side by side but separated a distance along the ranging direction, which consists of two flat sheets B1 and B2 with the size of 4 cm × 4 cm. It is worth noting that two targets cannot overlap with each other along the angular direction. The half width of the pattern, which is shown in the insets (a) and (b) of Fig. 4, can cover an area to make the double-target in the same direction angle. Thus, a part of energy is reflected from target B1 and the other part of energy is reflected from target B2. We gradually increase the separation distance between the two flat sheets B1 and B2 and repeat the ranging procedure. To guarantee the same height, the positions of two targets are appropriately adjusted in the experiment.
Fig. 8 Double-target detection experiment. (a) Schematic diagram of dual-target detection. (b) Experimental layout of dual-target detection.
The reflection signals from the double-plate are measured in Figs. 9(a)-9(d) with the separated distances of 1.9 cm, 3.2 cm, 4.5 cm and 6.0 cm as the details of received waveforms shown in the insets of Figs. 9(a)-9(d). After cross-correlation, the normalized experimental results are shown in Figs. 9(e)-9(h). From the compressed pulse signal in the Fig. 9(e), two targets are unable to distinguish with each other. With the separated distance increase, two peaks are obviously distinguished. The round-trip distances of double-target are 0 cm, 3.8 cm, 5.0 cm and 7.6 cm, respectively. Therefore, the minimum resolution of dual-target is about 2 cm. It also shows from Fig. 9 (e) – 9(h) that the second compressed signal becomes weaker when the target B2 moves far from the target B1. As shown in the Fig. 9(h), the maximum resolvable of the double-target is exceeding to 6 cm. According to the radar equation, it is known that the received power is proportional to the transmitting power of the antenna, the gain of the antenna, effective cross-sectional area of the object and four square of distance. Therefore, the maximum resolvable range depends on the transmitting power when other parameters are unchanged. In other words, maximum resolvable range can be increase with a high transmitting power.
Fig. 9 Experimental results. (a)-(d) The received signals when the distance between dual target is 1.9 cm, 3.2 cm, 4.5 cm and 6 cm. (e)-(h) The cross-correlation results when the separated distances are 1.9 cm, 3.2 cm, 4.5 cm and 6.0 cm.
4. Conclusion
In this paper, we have demonstrated an X-band optically-steered phased array radar for high resolution multi-target detection with large beam steered angle. The low magnitude ripple of ± 0.5 dB and small delay error of 0.13 ps/nm are realized by power equalization and phase compensation. By tuning the optical wavelength from 1530 nm to 1560 nm, the optically steering beam forming system has a wide direction tuning range of ± 54 degree based on high dispersion fiber delay technique. The proposed radar has been used to multi-target detection. With the assistance of linear chirped pulse compression technique, the ranging resolution for multi-target detection is about 2 cm within the ranging distance over 4 m and the azimuth angle error is less than 4 degree.
Acknowledgments
This work was supported by the National Natural Science Foundation of China under 61377002, 61321063, 61522509, 61535012 and 61090391. This work was also partly supported by Beijing Natural Science Foundation 4152052 and in part by the National High-Tech Research and Development Program of China under 2015AA017102. Ming Li was supported in part by the Thousand Young Talent program.
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