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Abstract
A photonic-based true time delay (TTD) phased array antenna (PAA) with ultra-fast angle scan is proposed and experimentally demonstrated. A tunable TTD is realized using a wavelength-swept laser and an array of dispersive elements. The key novelty of our work is the ultra-fast angle scan using an ultra-fast wavelength-swept laser source, which is constructed by a gated multi-wavelength laser (MWL) and a dispersion compensation fiber (DCF). In our experiments, a wavelength-sweep time between two adjacent wavelengths is only several nanoseconds for wavelength spacing of 2.4, and 3.2 nm. We successfully realized an ultra-fast angle scan from 0 to 43° with a step of 8.8° in 12.48 ns.
© 2017 Optical Society of America
1. Introduction
Phased array antenna (PAA) has been widely used in modern radar systems thanks to its fast scanning speed and excellent directivity [1]. The required signal delay in a conventional PAA system is usually provided by electronic true time delay (TTD) lines or phase shifters [2–4]. However, the electronic-based PAA is limited by its inherent weaknesses, such as small bandwidth, high power consumption, and high loss. Compared with the electronic counterpart, photonic-based PAA [5–9] attract more interests, due to its broad bandwidth, squint-free beam steering [10–12], electromagnetic interference (EMI) immunity, and low loss [12,13].
In recent years, many wavelength-controlled TTD beamforming systems have been proposed and demonstrated [14–18]. The beam steering angle is controlled by the wavelengths of optical signals. The rate of angle scan is related to the wavelength sweeping speed. Up to now, many wavelength-swept lasers have been demonstrated with wide wavelength sweeping range [19–24]. A wavelength-swept laser source achieved a wavelength sweeping range of 120 nm and a scanning repetition rate of 20 kHz by using a tunable fiber Fabry-Perot filter [19]. The wavelength sweep rate is improved up to 200 kHz thanks to the application of dispersion tuning [20]. In [21], a polygon scanner filter with 28 facets is employed into the wavelength-swept system, which improve the sweep repetition rate to 400 kHz. For those wavelength-swept lasers, the wavelength sweeping repetition rates is limited at kilohertz due to the fact that the change of wavelength are realized based on slowly mechanical movement, which is normally at a level of millisecond. A significant improvement of the sweeping repetition rate is realized by employing the sampled grating distributed Bragg reflector (SGDBR) into the semiconductor laser, and 5 ns wavelength switching time has been demonstrated with a wavelength ranging (64 channels) from 1525 nm to 1565 nm [22]. For the application in a PAA system, a wavelength-switchable fiber laser source which can scan 14 required wavelengths (1.6 nm wavelength spacing) within 40 ms has been demonstrated [23]. Additionally, a submicrosecond angle scanning has been realized in PAA system with a wavelength switching time < 300 ns [24]. However, the scanning speed is still not fast enough.
In this paper, we report a photonic-based phased array antenna with ultra-fast angle scan using an ultra-fast wavelength-swept laser source. The proposed approach has potential to meet the requirement of modern PAA. The key element of this system is the ultra-fast wavelength-swept laser source which is realized using a dispersed gated multi-wavelength laser (MWL). Wavelength-sweep time between two adjacent wavelengths is only several nanoseconds for wavelength spacing of 2.4 and 3.2 nm. Moreover, we incorporate the proposed optical source into a previously constructed TTD-PAA system [25]. An ultra-fast angle scan from 0 to 43° with a step of 8.8° in 12.48 ns is realized. In addition, the great flexibility of the proposed PAA system has been demonstrated. The experimental results indicate the fast scanning speed and high tunability of angle scanning in PAA system. It opens a solid path towards the application in modern radar systems, such as moving target detection and the ability of early-warning of airbone radar.
2. Ultra-fast wavelength-swept laser source
Figure 1 shows the schematic architecture of the ultra-fast wavelength-swept laser source which consists of a MWL, a polarization controller (PC), a Mach-Zehnder modulator (MZM), an arbitrary waveform generator (AWG), a low-noise amplifier (LNA) and a DCF. Next, the principle of this laser source will be analyzed in detail.
Fig. 1 Schematic architecture of the proposed ultra-fast wavelength-swept laser source. MWL: multi-wavelength laser; PC: polarization controller; MZM: Mach-Zehnder modulator; LNA: low-noise amplifier; AWG: arbitrary waveform generator; DCF: dispersion compensation fiber.
The continuous wave (CW) light emitting from an MWL with wavelength from λ1 to λN and a fixed wavelength spacing of Δλ is sent to the MZM via a PC. The multi-wavelength light is truncated into optical pulse, as shown in Fig. 2(a), by the MZM which is driven by a square pulse. The square pulse (pink dashed box) is generated by an AWG and amplified by a LNA. The time-gated optical signal is then dispersed by a length of DCF, resulting in a temporally stretched multi-wavelength signal, as shown in Fig. 2(b). The time interval between two adjacent wavelengths after dispersion, ∆τ, can be expressed as
where DDCF is the dispersion coefficient, L is the length of DCF, Δλ represents the wavelength spacing of MWL.Fig. 2 The schematic optical spectra of the (a) multi-wavelength optical signal gated by an MZM and (b) then dispersed by a length of DCF.
In order to have a wavelength-swept signal without spectrum overlapping and time gap between the adjacent periods, the electrical pulse should be well designed. The time duration ∆t and period T of the square pulse should meet the following conditions
Where Δτ is the time interval between two adjacent wavelengths after dispersion, as shown in Eq. (1), N is the number of optical wavelength.If the above conditions can be well satisfied, we will have a continuously wavelength-swept laser source. Obviously, the wavelength sweep range is determined by the spectrum width of the MWL and the sweep period is controlled by the electrical time gate signal. By adjusting the spectrum of the MWL, the time gate and the dispersion, the wavelength sweep range and the repetition rate of the wavelength-swept laser can be freely tuned.
We also carried out experiments to verify the feasibility of the proposed scheme. An MWL (Optilab) is utilized as initial laser source in our experiment. It includes 40 channels ranging from 1546 nm to 1561.6 nm with a channel spacing of 0.4 nm. In our experiment, two wavelength configurations were employed. The first one has 4 channels ranging from 1546 nm to 1555.6 nm with a wavelength spacing of 3.2 nm. The spectrum was shown in Fig. 3(a). The dispersion of the DCF is measured to be 848.38 ps/nm. To satisfy the conditions described by Eqs. (1)-(2), the time duration and period of the electrical pulse driven to the MZM was set to be ∆τ = 2.72 ns and T = 10.88 ns (Fig. 3(b)), which is mainly determined by the resolution of AWG (50 Gb/s, Tektronix 70001A). The gated optical signal was detected by a high-speed PD (Optilab LR-40) and sampled by a real-time oscilloscope (OSC) (Tektronix DPO73304D). As shown in Fig. 3(c), the output of the PD is a continuous electrical signal rather than a pulse, which means the wavelength-swept signal occupies a full period without time gap between the adjacent periods.
Fig. 3 The measured (a) optical spectrum of the 4-channel wavelength-swept laser source, (b) the waveform of the electrical pulse with a time duration of 2.72 ns, and (c) the detected waveform of the dispersed and gated optical signal.
In order to check that the wavelength-swept signal has no spectrum overlapping, we turned off the MWL channels in turns. When channel#2 (λ2) and channel#4 (λ4) were turned off (see Fig. 4(a)), the output signal is changed from a CW signal to a square pulse with pulse duty ration of 50%, as shown in Fig. 4(b). The pulse duration was measured to be 2.81 ns, showing a good agreement with that of the electrical pulse of 2.72 ns. When channel#3 (λ3) and channel#4 (λ4), were turned off (see Fig. 4(c)), The pulse duration (5.62 ns) and period of the output signal are both doubled (see Fig. 4(d)) comparing with the waveform shown in Fig. 4(b). The experimental results show that we successfully achieved the ultra-fast wavelength-swept source without spectrum overlapping. The sweep time between two adjacent wavelengths is 2.81 ns which is very close to that of the electrical pulse of 2.72 ns. The sweep period is 11.24 ns demonstrates the wavelength-swept laser source with several hundred megahertz repetition rate is achieved.
Fig. 4 The measured (a) optical spectrum and (b) the detected multi-pulse with a time duration of 2.81 ns after removing λ2 and λ4. The measured (c) optical spectrum and (d) the detected multi-pulse with a time duration of 5.62 ns after removing λ3 and λ4.
As indicated by Eq. (1), the sweep time between two adjacent wavelengths can be improved by decreasing the wavelength spacing Δλ. To show the tunable sweep resolution of the proposed scheme, the second configuration ranging from 1546 nm to 1558 nm with wavelength spacing of 2.4 nm was employed in our experiment. The spectrum was shown in Fig. 5(a). The time duration of the corresponding time gates are set to be 2.04 ns. Figure 5(b) shows the measured time gates generated from AWG, with pulse widths of 2.08 ns. Figure 5(c) shows the waveform of the detected wavelength-swept laser source. The results are very similar with that shown in Fig. 3(c), which means a wavelength sweeping without time gap is realized. When the MWL was turned off every other channel, the output signal turned to be a pulse train, as shown in Fig. 5(d), demonstrating a wavelength sweeping without overlapping. The pulse duration of the output signals is 2.08 ns, coinciding with the time gates shown in Fig. 5(b).
Fig. 5 The measured (a) optical spectrum of the 6-channel wavelength-swept laser source, (b) the waveform of the electrical pulse with a time duration of 2.04 ns, (c) the detected waveform of the dispersed and gated optical signal, (d) the detected multi-pulse with a temporal width of 2.08 ns.
According to the above analyzing, the proposed fast wavelength-swept laser source can scan 12 nm in 12.48 ns with a time resolution of 2.08 ns. As mentioned in Eqs. (1) and (2), the time resolution is equal to the time duration of square pulse and determined by the wavelength spacing. An MWL with a smaller wavelength spacing can improve the sweep time resolution of our wavelength-swept laser source. With the decrease of wavelength spacing, a corresponding narrower square pulse is needed, which is the main limit of our system. A technology or device which can provide an ultra-narrow square pulse can improve the wavelength sweep rate.
3. The application of ultra-fast wavelength-swept laser source in the TTD-PAA system
3.1 The principle of the ultra-fast angle scan TTD-PAA system
In our previous work, we demonstrated an 8-channel TTD-PAA system, showing good performance in beamforming and multi-target detection [25]. In that system, a tunable laser source (TLS, Agilent, 81600B) from 1530 nm to 1560 nm was used. However, the tuning speed is very slow (80 nm/s), resulting in a slow angle scan. In this experiment, the TLS in our previous work is replaced by the proposed ultra-fast wavelength-swept laser source, to achieve an ultra-fast angle scan, as shown in Fig. 6. Thus fast angle scan and tunable angle resolution are expected. The output signal from the wavelength-swept laser source was modulated by a radio-frequency (RF) signal. A PC is inserted between the laser source and the modulator to minimize the polarization-dependent loss. The modulated optical signal was amplified by an erbium-doped fiber amplifier (EDFA) and launched into an 8-channel TTD network. As mentioned in [25], the time-delayed optical signals are converted back to microwave signals at PDs. Phase trimmers were employed to compensate for the phase errors. Eight electrical attenuators were utilized to unify the power among different channels.
Fig. 6 Schematic architecture of the proposed photonic-based PAA system. PC: polarization controller; MZM: Mach-Zehnder modulator; LNA: low-noise amplifier; AWG: arbitrary waveform generator; DCF: dispersion compensation fiber; 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.
In the PAA system, the relation between beam steering angle and optical wavelength is given by [25]
where c is the light velocity in vacuum, d is the width of the antenna elements, (Li + 1-Li) presents the length difference of DCFs between two adjacent channels, DDCF and DSMF are the dispersion coefficients of the DCF and the SMF, respectively. λr is the reference wavelength (at 1545 nm), corresponding to the beam direction along the normal direction. By tuning the wavelength of optical carriers, the beam steering angle can be changed. Thus, an ultra-fast angle scan is achieved by employing an ultra-fast wavelength-swept laser source.3.2. Realization of the ultra-fast angle scan TTD-PAA system
To show the performance of the angle scan based on the proposed wavelength-swept laser source, a linearly chirped microwave waveform (LCMW) [26,27], from 9.5 GHz to 10.5 GHz, is used as a microwave feed signal. The time temporal width of the LCMW is equal to the pulse width of time gate. As shown in Fig. 7, the delayed LCMW, emitted from 8 antenna elements, is received by the standard horn antenna and sampled by an OSC. Microwave absorption materials were placed around to reduce microwave signal reflection. The half width of mainlobe in our system is about 8.5° due to the limited number of TTD lines. When the ultra-fast wavelength-swept laser source with a wavelength spacing of 3.2 and 2.4 nm is employed, a clear RF signal without angle overlapping can be obtained because the simulated angle step of TTD-PAA system is 12.8° and 9.6°, respectively. Every time two channels were turned on since only two horn antennas are available in our experiment. Firstly, channel#1 (λ1) of the MWL is turned on. We moved one horn antenna until the signal with the highest power is received. Then, we turned on channel#2 (λ2) and used another horn antenna to receive the corresponding signal. The distances from the transmitter antenna to the two horn antennas were kept the same. It can avoid additional time difference between two received signals. In the following measurement, channel#1 was kept while channel#2 was replaced by other channels one by one. For each measurement, waveforms of channel#1 and another channel were recorded by a real-time OSC synchronously. The distance between two horn antennas was also measured every time using a tapeline. The angle between two horn antennas can be calculated by the isosceles triangle formulas. The corresponding beam steering angle of other channels can be obtained once the angle of channel#1 is known.
Firstly, the wavelength spacing is set to be 3.2 nm. When channel#1 is turned on, the horn antenna receives the signal with the highest power approximately in the normal direction (0°). The distance to the transmitter antenna is 78 cm. The distance between two horn antennas is about 16.3, 32.4, and 50 cm, respectively. The corresponding beam steering angles from channe#1 (λ1) to channel#4 (λ4) are caculated to be about [0°, 12°, 24°, 35°] with an average resolution of 11.7°, which shows good agreement with the simulated value of 12.8°. The angle measurements accuracy is limited by the test condition such as a lack of the distance or angle measurement instruments with high accuracy. Figure 8 shows the received signals in every measurement. In order to intuitionally display the scan features among different channels, the received signal of channe#1 (λ1) was used as a reference in every measurement, which was adjusted to be at the same start time. Each channel displays a good periodicity with a pulse duty ration about 1/4. The total scan time was measured to be about 11.24 ns with 2.81 ns resolution. It can be seen in Fig. 8, the received signals have been separated well. And the negligible temporal overlapping may result from the signal noise and the measurement error.
As mentioned in Eqs. (1) and (3), the angle scan resolution is related the wavelength spacing. To verify the tunable angle resolution of the PAA system, the wavelength spacing of 2.4 nm is employed into the experiment. The position of the first horn antenna stayed unchanged. The distance between two horn antennas was 12.2, 23.1, 33.8, 45.6, and 57.2 cm, respectively. The calculated angles from channel#1 (λ1) to channel#6 (λ6) are [0°, 9°, 17°, 25°, 34°, 43°] with an average angle resolution of 8.8°, which is close to the simulated value of 9.6°. The received signals as shown in Fig. 9, have a scan periodicity about 12.48 ns with a resolution of 2.08 ns, which means the angle scan in the order of nanosecond was achieved. The angle resolution can be improved by decreasing the wavelength spacing of MWL. In addition, a receiver with high sensitivity may be necessary for a faster beam steering PAA system. Those improvements create more possibilities for the application of TTD-PAA system in fast spatial detection.
4. Conclusions
In this paper, we have proposed and experimentally demonstrated a wavelength-controlled TTD-PAA system with ultra-fast angle scan. The key novelty of our work is an ultra-fast wavelength-swept laser source which is constructed by a dispersed and gated MWL. The wavelength sweep time between two adjacent wavelengths is several nanoseconds for wavelength spacing of 2.4 and 3.2 nm. We have successfully realized an ultra-fast beam steering angle from 0 to 43° with a step of 8.8° in 12.48 ns . This system also shows a great tunability. The wavelength spacing, dispersion and electrical time gate are easily tuned, which results in a tunable angle switching speed and angle resolution.
Funding
National Key Research and Development Program of China (2016YFC0800504); Thousand Young Talent Program.
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