Continuous angle steering of an optically- controlled phased array antenna based on differential true time delay constituted by micro-optical components

Jian Wang; Peipei Hou; Haiwen Cai; Jianfeng Sun; Shunan Wang; Lijuan Wang; Fei Yang

doi:10.1364/OE.23.009432

1. Introduction

Phased array antennas (PAAs) based on traditional electronic phase-shifters intrinsically have a narrow band and introduce beam squint, which limit their applications in broadcasting, communications and remote sensing [1,2]. To steer wideband beams with large instantaneous bandwidth, photonic beamforming techniques have been proposed to perform optical true-time delays (OTTD) for phase shifting of optically-carried radio frequency (RF) signals over optical channels, enabling the vector sum of fields from the end antenna elements to be independent of frequency. Electronic phase shifters generally have a bandwidth of 10%~30% of their center frequency [3], while OTTDs can provide a bandwidth of nearly the whole RF band (up to 100 GHz) [4]. Also, losses of electronic phase shifters are much higher than that of OTTDs. The time responses of the OTTDs are approximately equivalent to that of the electronic phase shifters. In addition, the size and weight of beamforming networks with electronic phase shifters are much larger and heavier than those with OTTDs. In brief, photonic beamforming techniques based on OTTDs have significant advantages over electrical counterparts with ultra-wide instantaneous bandwidth, compact size, lightweight, no electro-magnetic interference, as well as squint-free operations.

Many different optical beamforming architectures have been proposed in a wide variety of implementations such as wave guide devices [5,6], optical switches [7], liquid crystal on silicon [8], silicon based integrated devices [9] and fiber grating [10]. They either have high resolution time delays [5,6] or agile to control [7–9], or largely reduce the size and weight of overall system [10]. All of these OBFNs have certain fixed time delays for fixed steering angles, and are called quasi-continuous OTTD. Continuous OTTD based on tunable lasers and optical fiber dispersion generation techniques such as photonics crystal fiber [11,12], chirped fiber grating [13,14], diffraction grating [15], as well as dispersion compensation fiber [16,17] has also been proposed. OTTD structures adopted in [11–17] make the whole systems compact and less complex. Continuous angle steering is achieved by using a tunable laser source and dispersion related [11,12], or wavelength selected devices [13–15]. In [16,17] multi-band and multi-beam operation with steering angle ranging from −40° to 15°is achieved, in which time delay is generated by dispersion compensation fiber [18], demonstrates a novel tunable true time delay line with separate carrier tuning using a dual parallel Mach-Zehnder modulator and stimulated Brillouin scattering-induced slow light. Continuous OTTD is achieved by a compact electrically tunable liquid-crystal (LC) infiltrated photonic-bandgap (LCPBG) fiber with simple configuration, and the structure is easy to integrate into microwave photonic systems [19]. The time delay can be continuously tuned by electrically changing the effective index of the core mode of the LCPBG fiber near the bandgap edge. Moreover, continuous OTTD based on optical single sideband polarization modulation by controlling the polarization state of each channel has been proposed by Yamei, the structure is simple and suitable for frequency-agile system [20].

In this paper, we propose an optically controlled PAA constructed by an optical beamformer network (OBFN) and wideband antenna array. The OBFN is composed of novel optical differential true time delay lines, which are implemented by micro-optical components, fixed wavelength laser sources and dense wavelength divided multiplication (DWDM). To our knowledge, this study is the first time for OTTD illustration based on micro-optics used in OBFN. The proposed optically-controlled PAA enables RF-independent broadband beam steering, and continuous angle steering. In addition, the optically-controlled PAA accomplishes synchronous multi-beam operation for broadband RF signals.

2. Principle

Figure 1 illustrates the schematic diagram of beam steering using phase shifters. In a PAA, when the beam is steered to a direction with an angle $θ$ , the adjacent distance difference, ΔR, between the beams radiated from each antenna element can be expressed as

Δ R = d \cdot \sin θ

where d is the spacing between adjacent antenna elements. Given that the signals radiated from the antennas have the same phase shifts at the wave front, the phase difference between adjacent antennas is

Δ ϕ = 2 π \cdot Δ R / λ

where

λ

is the wavelength of the radiated microwave signal. From Eqs. (1) and (2), we obtain

Fig. 1 Schematic diagram of beamforming using phase shifters.

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θ = \sin^{- 1} \frac{λ \cdot Δ ϕ}{d \cdot 2 π}

As shown in Eq. (3), the beam steering angle is a function of microwave wavelength. Thus, phase-shifter-based beamforming systems only operate for narrowband signals.

For beamformer systems using time delay lines, the time delay difference between adjacent beams from adjacent antennas can be expressed as

Δ τ = \frac{Δ R}{c}

where, c is the speed of light in free space.

The beam steering angle of the beamformer using time delay lines is given by

θ = \sin^{- 1} \frac{Δ R}{d} = \sin^{- 1} \frac{n_{e f f} \cdot Δ L}{d}

where, n_eff is the effective refractive index of the waveguide, and

Δ L

is the required delay line difference. Theoretically

Δ R = n_{e f f} * Δ L + Δ n * L

, the first item

n_{e f f} * Δ L

is wavelength independent and the second item

Δ n * L

is wavelength dependent. The second item induced phase shift is much less than that of the first item for optical true time delay lines, and can be ignored. Equation (5) illustrates that beamformers using time delay lines have frequency independent beam steering.

Figure 2 illustrates the schematic diagram of the optically controlled PAA based on DWDM for fixed wavelength laser sources and the proposed OTTD configuration, which is composed of N fixed wavelength sources, a RF splitter, N low noise amplifiers (LNA), N electro-optic modulator, a N × 1 DWDM-M, an Er³⁺ doped fiber amplifier (EDFA), a 1 × N DWDM-De, N optical time delay trimmers, N convex lenses, and N photodetectors. Broadband microwave signal is split into N paths and then amplified by N LNAs. The amplified N paths of the microwave signal are fed to N electro-optic modulators (EOM), and then modulated on the N lightwaves of different wavelengths from N distributed feedback lasers (DFB). The N beams output from N EOMs are multiplexed by one N × 1 DWDM-M and then amplified by an EDFA. The amplified signals are de-multiplexed into N signals by 1 × N DWDM-De, and then fed to the proposed N × N scale OTTD network module. N beams from each optical differential true time delay (ODTTD) line group in the vertical are focused on a PD by a lens.

Fig. 2 Schematic diagram of the optically steered PAA DFB: distributed feedback laser; EOM: electro-optic modulator; DWDM-M: dense wavelength division multiplexer-mux; DWDM-DE: dense wavelength division multiplexer-demux; EDFA: erbium-doped optical fiber amplifier; OTDT: optical time delay trimmer; ODTTD: optical differential true time delay.

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The raytrace of the four beams of the ODTTD is demonstrated in Figs. 3(a) and 3(b). Four de-multiplexed laser beams modulated with RF signals are coupled into a 4 × 4 ODTTD network through four vertically aligned fiber collimators. Each of the beams is split into four beams by four cylindrical polarization beam splitters (PBSs) in the horizon as shown in Fig. 3(a). In order to realize large angle steering range with compact size, achieve for symmetrical scanning, and reduce the time required to traverse the whole steering angle range. The time delays of optical paths increase from up to down for the four channels of the first and third group beams, and decrease from up to down for the four channels of the second and fourth group beams. Four channels (with different wavelengths) of each of the four group beams in the vertical constitute one group of OTTD lines. The optical paths of the four beams in the vertical (i.e. the first group beams) reflected by the first PBS (in the propagation direction) are opposite to those of the four beams (i.e. the second group beams) reflected by the second PBS. This condition corresponds to steering at angles that are symmetrical to 0 degree steering angle. The beams reflected by the third and fourth PBSs (i.e. the third and fourth beam groups respectively) are similar to those of the afore-mentioned two group beams. The only difference is the value of d₂. The distance values of $d_{1}$ and $d_{2}$ between prisms groups of the first group beams are the same as that of the second group beams in the horizon. The first and second group beams correspond to steer at angles of $θ$ and $- θ$ . The distance values of $d_{1}$ and $d_{2}'$ between prisms groups of the third group beams are the same as that of the fourth group beams in the horizon. The third and fourth group beams correspond to steer at angles of $θ'$ and $- θ'$ . The values of $d_{2}$ and $d_{2}'$ are changeable, and their variance are achieved by movement of their respective electrically controlled servo motor, then angle scanning of the PAA can be realized. Four beams of different wavelengths in the vertical position are then passed through the zigzagged prism group to generate four channels. The optical path differences of adjacent channels are the same as is shown in Fig. 3(b). The optical true time delay between adjacent channels depends only on the value of d₁-d₂ from Fig. 3(b). The tuning of the differential time delay is obtained by adjusting the distance between the prism groups. Three stack integrated prism groups are fixed on the platform and one prism group is adhered to the movable base. There are four movable bases in all for the four group beams. The first two movable bases are bonded with the same guided rail, and the latter two ones are bonded with another guided rail, and the two guide rails are respectively driven by two servo motors. To realize amplitude control of each beam, a half-wave plate is inserted before passing through the PBS. A rotating dual prism is applied to correct the parallelism of each beam before it enters in the stack integrated prism groups.

Fig. 3 Principle scheme of differential true time delay generation. (a) Propagation of one laser beam in the horizontal direction; (b) Propagation of the four laser beams in the vertical position reflected by the first PBS.

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The time delay between adjacent signals is given by

τ = {\begin{matrix} 2 \cdot (d_{1} - d_{2}) / c For the 1 st group beams \\ - 2 \cdot (d_{1} - d_{2}) / c For the 2 rd group beams \end{matrix} and τ = {\begin{matrix} 2 \cdot (d_{1} - d_{2}^{'}) / c For the 3 rd group beams \\ - 2 \cdot (d_{1} - d_{2}^{'}) / c For the 4 th group beams \end{matrix}

where

d_{1}

is the space interval between the first and second prism groups,

d_{2}

is the space interval between the third and fourth prism groups for the first and second group beams, and

d_{2}'

is the space interval between the third and fourth prism groups for the third and fourth group beams. Based on Eq. (6), the time delay is defined as differential time delay.

The relationship between the corresponding beam steering angle and the distance of adjacent prism groups can be expressed as

θ = {\begin{matrix} \sin^{- 1} [2 \cdot (d_{1} - d_{2}) / d] For the1st group beams \\ - \sin^{- 1} [2 \cdot (d_{1} - d_{2}) / d] For the 2 rd group beams \end{matrix} and θ = {\begin{matrix} \sin^{- 1} [2 \cdot (d_{1} - d_{2}^{'}) / d] For the 3rd group beams \\ - \sin^{- 1} [2 \cdot (d_{1} - d_{2}^{'}) / d] For the 4 th group beams \end{matrix}

where d is the space interval of adjacent antenna elements.

By taking a derivative of the steering angle with respect to the change in distance for $d_{1}, d_{2} (d_{2}')$ , the accuracy of the steering angle with respect to the accuracy of $d_{1}, d_{2} (d_{2}')$ is derived as

| δ θ | = {\begin{matrix} \frac{2 \cdot | δ d_{1} - δ d_{2} |}{d \cdot \cos θ} For the 1st and 2rd group beams \\ \frac{2 \cdot | δ d_{1} - δ d_{2}' |}{d \cdot \cos θ} For the 3rd and 4th group beams \end{matrix}

In Eq. (8), $δ θ$ is defined as the steering accuracy, and $Δ θ / (| Δ d_{1} - Δ d_{2} |) = Δ θ / (| Δ d_{1} - Δ d_{2}^{'} |) =$ $2 / (d \cdot \cos θ)$ is defined as the tuning resolution. We can see from the equation that a larger steering angel will result in lower steering accuracy and tuning resolution. For linear antenna array with space interval of 39 mm, the steering accuracy and tuning resolution are respectively 0.667° (while $δ d_{1} - δ d_{2} = 0.01 mm$ ) and 5.88°/mm at a steering angle of 60°.

3. Experiment demonstration

Based on the design architecture shown in Fig. 2, we fabricate a compact optically controlled 2-6 GHz PAA prototype as shown in Figs. 4(a) and 4(b). A photograph of experiment setup is shown in Fig. 4(c). The key parameters of the devices used in the experiments are as follows. The wavelengths of the four distributed feedback (DFB) lasers (Emcore 1772 DWDM high power CW laser source) are 1549.92, 1551.72, 1552.52, and 1553.33 nm, whereas their power values are the same at approximately 14 dBm. The linear frequency modulation microwave signal transmitted from the microwave signal generator is split into four channels and then amplified by four 2-6 GHz LNAs with a gain of 35 dB to drive four polarization-maintaining (PM) EOMs (SDL-OC-192). The four EOMs are worked on the positive quad point of their transmission curve. ITU PM DWDM-M module with 100 GHz channel spacing is used to multiplex the four wavelengths. PM EDFA (maximum output power 200 mw) is used to amplify the optical power of the four channels. ITU PM DWDM-DE module with 100 GHz channel spacing is used to de-multiplex the signals in a common fiber after amplification.

Fig. 4 (a) Photograph of the proposed 4 × 4 ODTTD network module; (b) OBFN with the proposed ODTTD network module; (c) Experiment with the proposed PAA in an anechoic chamber.

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For precise time delay compensation, an optical time delay trimmer array is applied. The four signals with the same time delay are then input to the proposed 4 × 4 OTTD network. The four channels of each of the four beam groups converge into each pin photodetector (ET3500EXT, EOT Inc.) which has a bandwidth of above 15 GHz and a responsivity of 0.92 A/W on the 1550 nm wavelength band. The radiation patterns are measured in an anechoic chamber.

4. Discussion

By adjusting the values of d₂, $d_{2}^{'}$ , the differential time delay of the RF signal can be continuously tuned. To investigate the feasibility of the proposed PAA for multi-beam operation, radiation patterns with synchronous different pointing angles are measured. As can be theoretically calculated from Eq. (1), time delay differences of 68, −68, 24, and −24 ps are required for steering angles of 30.8°, −30.8°, 10.6°and −10.6° respectively for antenna spacing of 39 mm. From Fig. 5 we can see that experimentally measured radiation patterns of the PAA at 3 and 5 GHz agree well with the simulated ones. The main lobes of the measured radiation pattern of the four beams are very near for RF frequency of 3 and 5 GHz. Slight difference is present in the pointing angle between experimentally steering angles and the simulation, and the slightly asymmetric of the two pair of beams in Fig. 5(a) and 5(b) might be caused by the inconsistence of differential time delays among adjacent channels of the four channels in vertical position for each of the four steering beams caused by manufacturing and assembling. This can induce the non-equality of differential time delay between adjacent channels, and hence the error of the steering angle. Figure 6 shows performance of one of the four steering beams of the optically-controlled PAA as the steering angle moves from −5° to −32° by movement of the guide rail drove by the servo motor. For sake of brevity, we only give continuous angle steering performance of one of the four steering beams while those of the other three steering beams are not shown. As can be seen from this figure, the optically-controlled PAA achieves satisfactory continuous angle steering.

Fig. 5 Simulated and experimented multi-beam radiation patterns of the PAA based on the OBFN for (a) 3 and (b) 5 GHz RF signals when the steering angles of the four beams are −29°, −10°, 10°, and 29°.

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Fig. 6 Performance of one of the beams as the steering angle moves from −5° to −32° continuously.

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Figure 7 illustrates the 3 dB bandwidth performance of two of the four steering beams with the PAA in different RF frequencies of 2, 4, and 6 GHz. We aim to have a comparison of the 3 dB bandwidth for independent steering beams at different steering angles. So it is enough to select two of the four steering beams. We select performances of the two steering beams at −9° (Fig. 7(a)) and 30° (Fig. 7(b)), the other two steering beams are not shown. From the figure, the 3-dB bandwidth of the main lobes at −9° steering angle for 2, 4 and 6 GHz are 52.9°(−16.7° to −36.2°), 31.9°(−6.3° to 25.6°) and 18.9°(17.6° to 1.3°) respectively. For a 30° steering angle, the values are 51.5° (53° to 1.5°), 31°(14.5° to 45.5°) and 18°(20° to 38°). We can see that the 3-dB bandwidths of different steering angles are close to one another for the same RF frequency. The small 3-dB bandwidth deviation at the same frequency arises from measurement errors as well as amplitude and phase error of their respective channels.

Fig. 7 3 dB bandwidth of two of four optically controlled steering beams with steering angles of (a) −9° and (b) 30° for 2, 4, 6 GHz RF signals.

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The 0.69 mm gradual increment of d₂ induces continuous tuning of the steering angle from 29.5° to 32°, hence the tuning resolution of 3.6°/mm is obtained. Theoretically, the tuning resolution of the system at the steering angle of 30° for PAA with distance of 39 mm between adjacent elements is 3.4°/mm. The slight difference between theoretical tuning resolution and experimental one might come from the measurement errors, as well as tiny distance errors between adjacent antenna elements.

5. Conclusion

In conclusion, an optically controlled PAA with continuous angle steering by exploiting differential true time delay based OBFN is proposed and demonstrated. The differential true time delay network is realized by stack integrated micro-optical components. The optically-controlled PAA can achieve continuous time delay tuning and steer the main lobe of the radiation pattern to any desired direction. The optically-controlled PAA for microwave signals of 2-6 GHz is demonstrated. Experimental results reveal that the PAA achieves RF-independent broadband beam steering, continuous angle steering and multi-beam operations as well. The structure of our optically-controlled PAA eliminates complicated optoelectronic control and is easy to assemble. In addition, the structure has a small volume and light weight, aside from being insensitive to temperature changes, making it suitable for the application of practical environment.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 61377004).

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Continuous angle steering of an optically- controlled phased array antenna based on differential true time delay constituted by micro-optical components

Abstract

1. Introduction

2. Principle

3. Experiment demonstration

4. Discussion

5. Conclusion

Acknowledgements

References and links

Cited By

Figures (7)

Equations (8)

Optics Express