Optical feed network for phased array antennas

[0001] Embodiments of the invention may relate to feed networks for phased array antennas, particularly optical feed networks.

[0002] Success in the development of high-quality and ultra-wideband radio frequency (RF) sources has led to the exploration of applications of such technology, such as ultra-wideband phased-antenna arrays. An example of this may be found in G. J. Schneider et al., “Optical Generation of Narrow-Line RF by Injection Locking of Modulated DFB Lasers” CLEO Conference, 2011, incorporated herein by reference. In a proposed approach of this paper, two laser outputs having a frequency difference corresponding to a frequency of an RF signal may be fed into an optical interleaving and feeding network consisting of two 1-to-N beam splitters, N electro-optic phase modulators, N2-to-1 beam combiners and fiber couplers. The beam splitting, phasing, and recombining can be achieved through typical fiber-based components.

[0003] The proposed approach, however, imposes some technical difficulties in the practical implementation. Due to thermal and mechanical stresses in each channel, the relative phases between channels may drift over time, thereby resulting in significant distortion in RF beamforming.

[0004] Various embodiments of the present invention may address the above-noted phase-drift issues. In particular, various embodiments of the invention may present an optical feed network for phased array antennas by providing a phased-based feed network.

[0010] Various embodiments of the invention may include an optically addressed phase-based feed network for, for example, an ultra-wideband RF phased array that may include techniques designed to compensate the phase fluctuations induced in optical fibers, e.g., due to thermal and mechanical stresses. One may use polarization maintaining (PM) fibers to encode two coherent optical tones in two orthogonal states and to transmit them to a high-speed photomixer at the antenna head for RF signal transmission. Co-propagation in the same fiber may enable two optical signals to experience the same environmental impact, thereby significantly minimizing phase fluctuations. To phase the antenna array, a phase-shifting element may be used, which may, e.g., comprise an electro-optic phase shifter and true time delay line (which may be optically-based), in which phase shift may be dominated at one of the polarizations, consequently leading to an electrically addressed phase shift at the mixer. The electro-optic phase shifter may be implemented in the form of an electro-optic modulator. The combination of an electro-optic modulator and true time delay line may serve, to provide both narrow and ultra-wide band instantaneous bandwidth, which may be applicable, e.g., to phased-array radars, multi-beamforming, multi-in-multi-out (MIMO) systems, and/or communication systems. In addition, dynamic polarization control may also be used to compensate for polarization fluctuation that may be induced, e.g., by bending, thermal, and mechanical stresses.

[0011] FIG. 1 shows a block diagram of a feed network according to an embodiment of the invention. Optical signals, labeled ω₁ and ω₂, may be fed into a polarization combiner 11. The optical signals may be generated by coherent optical sources and may be polarized, e.g., horizontally and vertically, respectively, although the invention is not thus limited. The combined output of polarization combiner 11 may be fed, e.g., through a PM fiber, to one or more amplifiers 12. The amplified output may be provided to one or more splitters 13, which may output multiple optical signals that may be used to generate signals to be fed to an antenna array 15, via phase network 14, which may include phase shifting (which may, e.g., by implemented by means of a phase modulator) and/or true time delay components, as discussed, above.

[0012] FIG. 2a shows a more-detailed example of a phase-based feed network, which may be similar to that shown in FIG. 1, according to an embodiment of the invention. Two coherent optical sources 101, 102 may provide optical signals labeled as ω₁ and ω₂. The optical signals may be combined using a polarization combiner 103 into the fast and slow axes of a PM fiber, as shown in FIG. 2b. The PM fiber may feed the signals into a polarization independent power amplifier 104, which may amplify them simultaneously. The resulting amplified signal may then be split into N channels with beam splitters 105, to produce signals that may undergo further processing, and which may eventually be delivered to antenna heads 111 for RF generation and transmission. Beam splitters 105 may include, for example, 1-to-2 and/or 1-to-N beam splitters; but the invention is not necessarily limited to these types of beam splitters. As a result of the initial processing in blocks 103, 104, and 105, phase fluctuations that occur before the polarization combiner 103 on the two optical tones may be equally distributed to each channel, and as a result, the produced RF signal after photo-mixing may be independent of induced phase noise. Also, due to the co-propagation of both optical signals in the same fiber, after the polarization combiner 103, they may generally experience the same phase fluctuations, which may thus result in reduction of relative phase fluctuations between the signals.

[0013] Following splitters 105, the respective channels may be fed into a phase-shifting block, which may, e.g., be implemented by an electro-optical (EO) phase modulation (PM) and/or true time delay line feed network, shown as block 14 in FIG. 1, and which may be used to tune the phase for beam scanning. The EO-PM network may include phase modulators 107. Phase modulators 107 may be used to phase modulate the transverse magnetic (TM) and transverse electric (TE) modes separately, where the TM mode may be phase modulated and the TE mode may have less modulation than the TM mode. An example is shown in FIG. 2c. Phase modulators 107 may be controlled by a phase controller 112. In addition to EO-PM, phase modulation may be achieved using a true time delay line (instead of or in combination with EO-PM or other phase-shifting), in which a large time delay may be used, e.g., in ultra-wide band applications. Phases of the phase modulators 107 and/or true time delay line may be controlled to provide, for example, beamforming, sweeping, scanning, and/or other functions; but the invention is not thus limited. After the phase modulation of ω₂, both signals may be projected to the fast and slow axes by applying a 45° rotational key 108. The two sets of orthogonal signals may then be filtered by linear polarizers 109 before being delivered to photodetectors 110, which may provide respective feed signals to antenna heads 111. An additional role of photodetectors 110 will be discussed further below.

[0014] Many optical components, such as the polarization combiner 105 and EO phase modulators 107, may be polarization sensitive. The state of polarization in the PM fiber may be altered as the light propagates along the fiber due to a variety of reasons, such as alignment mismatch in the fiber connectors, birefringence due to thermal, and mechanical stress, and/or integration and fabrication defects in each component. Furthermore, if polarization states are not preserved, amplitude modulation may be introduced into the system due to coupling of the optical signals into both TE and TM polarizations of the EO phase modulators 107. Polarization management may, therefore, be needed to precisely control the state of polarization of the two optical signals. This may be done, for example, by means of a polarization controller (polarization control module) 113 coupled to control electrically-addressed polarization controllers 106, which may be placed, for example, between the splitters 105 and the EO phase modulators 107. The polarization controller 113 may receive feedback, e.g., in the form of signals from photodetectors 110, which may be controllers 106. In particular, the feedback may be in the form of direct-current (DC) components detected in the outputs of the photodetectors 110.

[0015] It is noted that, while the discussion in the immediately-preceding paragraph focuses on an embodiment using EO phase modulators, the same applies to other phase shifting schemes and/or embodiments using true time delay lines.

[0016] In an example implementation, the inventors have implemented a 1×2 optically addressed phased, array. To demonstrate phase sweeping, a saw-tooth waveform with a period of 1 second and a magnitude of 7 volts (2V_π) was used to drive the phase modulation. A repeatable and steady RF response was observed, as shown in FIG. 3a. With balanced RF power transmitted, from both antennas, a maximum 30 dB extinction ratio was obtained between in-phase mid out-of phase feeding. In addition, angular radiation patterns of the far field were measured with a varied, applied voltage ranging from 2.1 to 9.5 volts as shown in FIG. 3b. The equivalent phase shifts, determined by the applied voltages, indicated a continuous scanning of the main beam from 0° to 3°, which may have been limited by grating lobes due to a large antenna spacing; this may be improved, e.g., by careful design of the antenna array.

[0017] Various embodiments of the invention have now been discussed in detail; however, the invention should not be understood as being limited to these embodiments. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention.