Electronic Military & Defense Annual Resource

5th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

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Technology Fully Integrated Optical Beam Steering Chips Photonic integration has the potential to enable low-cost, low-SWaP beam shaping and steering for a wide range of military applications — but can the technology make the leap from the lab to the field? By Martijn Heck O ptical beam shaping is the generation of a nar- row, low-divergence beam of laser light in the visible or infrared wavelength range. Optical beam steering is the dynamic pointing or scanning of such beams, preferably over a wide angle. Narrow, steerable beams of laser light have major potential in military applica- tions. One example of such an application is secure, free-space optical communication. Since the point-to-point beam path between sender and receiver can be narrow, there is a low probability of interception — hence, a low risk of eavesdrop- ping. Infrared wavelengths are eye-safe, do not suffer from electromagnetic interference, and are part of the spectrum that is not regulated. Moreover, the bandwidth of such sys- tems can be very large, far over 1 THz, depending on the implementation. Dynamic beam steering in the transmitter can track the movement, or displacement, of the receiver. Such free-space optical systems and networks also can be quickly deployed in disaster areas or war zones. Another military application of beam steering is three- dimensional imaging and mapping, such as that used by Lidar systems. In this application, the laser beam is scanned and the distance to a point is measured using the reflection or scattering, thereby creating a three-dimensional image of the environment. Military use of Lidar includes target detection and identification by (semi-)automated weapons. A final example is electronic warfare, where the laser beam can be used to precision guide missiles to their targets. Alternatively, the laser beam can be used for countermea- sures, interfering with or blinding the optical sensors guid- ing such missiles. Ideally, such countermeasure systems can address a wide range of threats by generating steerable opti- cal beams across various parts of the optical spectrum. Systems that can generate, shape, and steer a narrow opti- cal beam tend to be rather bulky, as they often are composed of discrete optical components, including, for example, lasers, collimation optics, and micro-electromechanical mir- rors or spatial light modulators. Typically, this limits such systems' uses to static, ground-based deployment, or as mounted assemblies on large vehicles. However, laser beam scanning speed is rather slow, which prevents, for example, a Lidar system from providing real-time, frame-rate imaging. Scanning speed is limited by the technology, which often is based on opto-mechanical systems or on liquid crystals. The ultimate path to improv- ing beam steering systems is to reduce size, weight, and power consumption (SWaP), so that they can be mounted on unmanned aerial vehicles (UAVs or drones), and be carried by personnel. Photonic integration is the approach that can achieve this goal. Advantages Of Photonic Integration Technology In recent years, there has been a considerable research effort to integrate beam steering systems onto a single photonic integrated circuit, also known as an optical chip. Such opti- cal chips integrate, for example, lasers, modulators, detec- tors, and filters on a single piece of semiconductor, typically silicon or indium phosphide, much like electronic integrated circuits. A schematic of how these components can be real- ized on a silicon substrate is shown in Figure 1. The technology of optical chips is maturing fast, driven by high-bandwidth communications applications, such as long-haul telecommunications and datacenters, and mature fabrication facilities. State-of-the-art commercial optical chips integrate hundreds of elements; experimental, lab-based optical chips integrate thousands of components. Figure 2 on page 16 shows that this growth in complexity follows an exponential trend, with the number of components doubling every two years. This is remarkably like Moore's Law for elec- tronics — the observation that, over the history of computing Electronic Military & Defense Annual Resource, 5th Edition 14 Figure 1: Schematic of components in silicon photonics technology. The components can be fabricated on a silicon substrate, using CMOS-compatible processes and processing infrastructure. The silicon waveguide confines and guides light. Electrical contacts on the modulator waveguide allow for carrier injection or depletion, thereby changing the refractive index of the waveguide. The photodetector waveguide has a germanium layer, which absorbs the light. Carriers are generated upon absorption, which are detected as an electrical current.

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