We develop a prototype of optically-steered X-band phased array antenna with capabilities of multi-band and multi-beam operations. It exploits high-speed wavelength tunable lasers for optical true-time delays over a dispersive optical fiber link, enabling agile, broadband and vibration-free RF beam steering with large angle.
©2013 Optical Society of America
Phased array antenna (PAA) based on traditional electronic phase-shifters have an intrinsic nature of narrow band, which limits their applications in remote sensing, communications and broadcasting [1–3]. In order to steer wideband beams, photonic beamforming techniques have been proposed to implement optical true-time delays over optical channels for phase shifting of optically-carried RF (radio frequency) signals, enabling the vector sum of fields from the end antenna elements to be independent of frequency [4–9]. The optical true-time delay facilitates small size, light weight, low-power consumption as well as immunity to electromagnetic interference. As such, the photonic PAA technique has significant advantages over electrical counterparts with improved bandwidth and squint-free operation .
Many photonic beamforming schemes have been proposed and demonstrated by using various techniques, including acoustic optics [10,11], Fourier optics [12,13], fiber grating [14–17], waveguide circuits [18,19], photonic crystal fiber , and slow light [21–23]. However, existing beamforming approaches are still far from meeting requirements for viable applications in terms of performances, manufacturability and cost. In addition, most of them require a large number of delay lines consisting of various photonic components including laser sources, optical switches, fibers, gratings, as well as sophisticated electronic components and accessories. Besides their limited operation speed and low optical efficiency, the complexity of these beamforming architectures makes the whole system expensive and difficult to realize .
In this paper, we report a prototype development of a photonic phased array antenna which exploits high-speed optical wavelength tuning based on electro-optic effect through a highly-dispersive optical fiber link for optical true-time delays. The output wavelength of the tunable laser is the only variable for beamforming in the proposed photonic PAA prototype, providing incremental true-time delays for all antenna elements simultaneously. Continuous beam steering is obtained by varying the wavelength of optical waves that propagates through the dispersive optical fiber link. Multi-band and multi-beam operations are realized by using additional tunable wavelengths based on the simple wavelength-division-multiplexing technique.
2. High-speed multi-band multi-beam PAA prototype
Figure 1 shows a schematic of our optically-steered phased array antenna system which can provide continuous variable optical true-time delays for two different RF-tunable channels, capable of auto-cascaded duplications for all antenna elements through a fiber-optic integrated dispersion network. Our recent technological advance in high-speed wavelength tunable lasers based on a novel electro-optic effect  permits the development of this highly-agile PAA system with simplified control scheme for multi-beam wideband operations. More importantly, only one set of fiber-optic true-time-delay beamformer is needed for two different RF channels and all antenna elements, providing fast scanning for simultaneous and independent two-beam operations. It has all the advantages over other approaches while further increasing scanning speed, reducing the system complexity, weight, and power budget by eliminating the complicated optoelectronic controlling and massive optical components such as optical switches, fiber Bragg gratings, and laser diodes.
Based on the design architecture, we fabricated a compact optically-steered X-band PAA prototype as shown in Fig. 2. The prototype dimension is about 7.6 cm × 20.3 cm × 43.2 cm. In our system, there are two RF channels which can work simultaneously or individually depended on the applications. RF-tunable oscillators (Stellex YIG Oscillator 8-10 GHz Tunable) are used to provide two RF channel signals which then drive the associated electro-optic waveguide modulators (Fujitsu FTM7921ER) after amplified by two low-noise RF amplifiers (3.5-11GHz Low-noise Amplifiers). The wavelength tunable lasers are modulated by the electro-optic waveguide modulators to generate optical RF signals in the two RF-optical channels which can work independently to support multiple RF frequencies.
The optically-carried RF signals are combined and then divided into 8 individual optical channels through a 2 x 8 fiber optic splitter. The optical traveling time in the 8 individual optical channels are incrementally delayed by the fiber link which consists of the dispersion compensating fibers. To fabricate such fiber-optic true-time-delay lines with precision time delays, the optical fibers are cut with precision lengths based on the calculation for true-time-delay beamforming. The dispersion compensating fiber produces a wavelength-dependent optical true-time delay , given by
For an 8-element phased array antenna with d = 15 mm, the calculated fiber length increment L is 6.85 m, by assuming the maximum scanning angle of 60°. In the calculation, the dispersion coefficient D = −253 ps/nm/km and wavelength tuning range of Δλ = 25 nm are used. To obtain the required integer multiplication of time delay, the dispersion optical fibers are cut by sequentially increasing the lengths for each antenna element, i.e., 0 m, 6.85 m, 13.69 m, 20.54 m, 27.38 m, 34.23 m, 41.08 m and 47.92 m. The cut dispersion compensating fibers are then spliced with telecommunication grade single-mode optical fibers for linear phase compensation. The length of each conventional single-mode fiber is arranged to equalize the delay time among each element at the center operating wavelength (1550 nm). As such, the 8 spliced optical fibers have equal lengths. Please note we neglect the dispersion of the telecommunication grade fibers since it is much smaller than that of the dispersion compensating fiber. At the central wavelength, the initial emitting angle from the antenna array can be easily set to a direction normal to the antenna array facet by adjusting the phase trimmer array (PTA) as shown in Fig. 1.
High-speed photodetectors (Multiplex MTRX192L) are used to convert optical modulated signals into microwave signals which connect to the RF antenna elements through 8 phase trimmers. The phase trimmers correct the possible phase errors including fiber length cutting errors. The phase-adjusted RF signals are emitted to free space through the 8 antenna elements after amplified by onboard RF amplifiers for each channel. The RF electromagnetic waves constructively interfere to form a RF beam directing at an angle, depended on the time delay between antenna elements.
3. Characterization of the photonic PAA prototype
We first test the RF tuning capability of our multi-band optically-steered X-band PAA prototype. Both RF oscillators are tunable in X-band frequency range from about 8 GHz to 10 GHz by changing their tuning voltages, respectively. The two RF channels are independent and thus can be tuned separately or simultaneously. However, the RF tuning only changes the operation bands rather than steers the RF beams constructed from the end of antenna array.
To steer the RF beam, optical wavelength tuning takes place to generate optical true-time delays through the dispersion fiber optic channels. In other words, the true-time-delay beamforming enables RF-independent broadband beam steering. Figure 3 shows RF beam steered angles as optical wavelength changes. The RF beam direction can be scanned over a large angle by changing the laser wavelength, which agrees well with the calculation result based on Eqs. (1) and (2). The RF beam divergence of full angle is measured about 15 degrees and the beam can be steered to about 40 degrees from the direction normal to the antenna array facet. In addition, with different RF frequencies, we found that the steered angles of RF beam are consistent in tuning the optical wavelength, indicating the broadband RF operation is feasible. The angular deviation at the same optical wavelength arises from measurement errors as well as system errors such as phase adjustment inaccuracy, insertion loss difference between each fiber channels, fiber cutting and splicing errors.
In our prototype, we use 8 elements in the antenna array for feasibility demonstration. Small beam divergence with less angular deviation can be obtained by using large number of elements. In addition, we can further improve the maximal swept angle and beam quality of current antenna by precise calibration of power and phase for each optical and RF channels. Some electronic components in the system, such as photodetectors, amplifier and RF oscillators, also need be precisely calibrated to eliminate noises and errors. In principle, a phased array with multiple faceted segments providing some degree of curvature allows beam steering in a wider angular range .
In the experiments, the electro-optic wavelength tunable laser, developed by us based on high-speed electro-optic scanner technology, are employed to provide wavelength selection with the tuning speed as fast as 107 nm/s, which corresponds to a RF beam steering speed of 15 degree/μs. The new laser also delivers 50 mW average power with near 100 nm swept range and 0.5 nm linewidth. These parameters are necessary for further optimization of the prototype performance.
We have also demonstrated that two RF beams with different frequencies can be operated independently by using two wavelength-tunable lasers. As shown in Fig. 1, two lasers are modulated by separate electro-optic modulators to produce two independent optically-carried RF signals which can be tuned in both optical and RF frequencies. The two signals are first combined and then divided into 8 optical channels by the 2 × 8 fiber splitter. The length increment of dispersion fibers between adjacent optical channels produces an optical true-time-delay which depends on the optical wavelength. When the two lasers are tuned at different wavelengths, their carried RF waves experience different true time delays propagating in the dispersion compensating fibers. Therefore, two independent RF beams which may come with either same or different frequencies are generated from the end of the antenna array, pointing to two directions that are corresponding to the optical wavelengths of two wavelength-tunable lasers. Figure 4 shows experimental results of two generated RF beams detected at far field by a horn antenna receiver that is connected to a microwave spectral analyzer. We first tune the two lasers to the same wavelengths so that the two RF beams with different frequencies (8.58 GHz and 9.11 GHz) are both pointed to the horn antenna receiver. As such, both RF signals are detected and displayed on the spectral analyzer as shown in Fig. 4(a). Then, one laser is tuned about 10 nm away from the initial wavelength, which enables the corresponding RF beam (8.58 GHz) steered completely away from the horn antenna (Fig. 4(b)). Figure 4(c) shows the 9.11 GHz RF beam is steered with a relatively small angle by fine tuning the wavelength of another laser, so that the partial beam are still collected the horn detector. The experiments demonstrate that our prototype can steer two independent RF beams by using two wavelength-tunable lasers.
We develop and demonstrate a prototype of optically-steered X-band phased array antenna by exploiting high-speed wavelength tunable lasers for optical true-time delays. The output wavelength of the tunable laser is the only variable in this prototype for agile, vibration-free RF beam steering with large angle. The true-time-delay beamforming enables RF-independent broadband beam steering with no beam squint effect. In addition, two RF beams with different frequencies can be operated simultaneously and independently by using two wavelength-tunable lasers. Our prototype eliminates the complicated optoelectronic controlling and massive optical components, enabling significant reduction of system complexity, weight, and power consumption. It will have great applications in wireless communications and remote sensing with capabilities of multi-band and multi-beam operations.
This research work was partially supported by NASA SBIR research program (contract number: NNX11CA52C).
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