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Electronic Imaging & Signal Processing

Optical beamformer for large microwave antenna arrays

Spatial light modulators are used to change the radiation pattern of large microwave antenna arrays by controlling the phase and amplitude of optical signals.
15 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0604

Antenna arrays are used in applications such as radar, telecommunications, and earth observation to name but a few. This is partly due to their ability to change the radiation pattern by controlling signal amplitude and phase at each antenna element. To obtain wide bandwidths, time delays instead of phase shifts between antenna elements are required.

However, the implementation of large tunable time delays in the microwave domain is quite complex, resulting in bulky and heavy beamforming networks. The use of optics was proposed to alleviate this problem.1 Optics offer low weight, immunity to electromagnetic interference, and true time delay (TTD) capability, i.e. the dependence of the beam-steering angle with frequency, known as beam squint, can be avoided by increasing the bandwidth.

There are presently two main approaches used in optical beamforming: TTD systems which provide large bandwidths,1–3 and phase control systems,4,5 which use a single spatial light modulator (SLM) instead of many microwave phase shifters as antenna elements. Since most applications do not require full TTD control, it is possible to combine TTD and phase control to lower costs. This is achieved by providing TTD to subarrays and controlling the relative phase between the elements of each subarray. Thus, the beam squint problem is significantly reduced for a given bandwidth while alleviating beamforming complexity.

We designed an optical beamformer for large antenna arrays based on the combination of fiber-optic and free-space components.6 It exploits the parallelism of SLMs and free-space optics for phase generation and implements TTD using fiber optical delay lines (ODL) to avoid collimation and loss issues and to improve scalability. The beamformer see (Figure 1) is based on providing TTD to subarrays and phase control to the elements of each subarray. A parallel alignment SLM (PAL-SLM) is used to control the phase of the RF signal of each antenna element. If one polarization component of the light impinging on the PAL-SLM is aligned with the axis of its liquid-crystal molecules, the PAL-SLM can change the refractive index experienced by this signal by varying the voltage applied on each pixel. On the other side, light polarized along the orthogonal polarization experiences a constant refractive index.

Figure 1. Block diagram of an optical beamformer combining time delay with phase and amplitude control to optimize scalability and cost. DD-MZM: dual-drive Mach-Zehnder modulator. DGD: differential group delay. ODL: optical delay line. PD: photodetector. fRF: radio frequency. SLM: spatial light modulator.

Present TTD applications do not require full TTD control due to their limited bandwidth, so the number of subarrays (i.e. TTD units) can be quite limited compared to the number of antenna elements. Thus, parallelism has to be focused on phase control. Implementing TTD units with bulk components does not exploit the parallelism of free-space optics. On the contrary, TTD implementation requires quite large free-space lengths that create major collimation, loss, and beam diffraction problems, which limit the number of antenna elements. Fiber-based optical delay lines avoid all these problems, offering low loss and TTD control with a short free-space section, which increases the number of antenna elements that can be controlled with the beamformer. Finally, this approach allows the use of fiber-based components optimized by the telecom industry for broadband modulators and photoreceivers, couplers, etc.

The combination of free space components (SLMs) and optical fiber devices (optical delay lines and other elements optimized to obtain wide microwave bandwidths) is a good solution to implement beamforming networks for large antenna arrays.

This work has been partially funded by the European Space Agency though the OBEFONE project.

Borja Vidal
Nanophotonics Technology Center, Universidad olitecnica de Valencia
Valencia, Spain

Borja Vidal is an assistant professor at the Universidad Politecnica de Valencia. His research topics include microwave photonics and optical signal processing.