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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hardware, the number of transistors in a dense integrated circuit has doubled about every two years — and shows the potential for this technology. Optical chip technology promises miniaturized and robust systems. Control is electro-optical, meaning there is no mechanical motion, making such chips insensitive to shocks and acceleration. The monolithically integrated waveguides ensure tight control of the optical field, allowing for high beam quality. The electrical bandwidths of the control ele- ments can be well over a gigahertz, compatible with high- bandwidth communications. Moreover, by integrating laser diodes intimately with control elements, very power-efficient systems can be developed. Thus, it can be stated that optical chips are, in principle, prime candidates for low SWaP and robust optical systems. This article first will present an over- view of the basic concepts of beam steering, and then it will examine the state-of-the-art in this technology, illustrated by recent breakthroughs. Optical Phased Arrays Using optical chip technology, various fully integrated beam- steering demonstrators based on optical phased arrays have been realized over the last few years. Arrays of coherent emitters, including their phase and amplitude control, are integrated on a single chip. This is shown schematically in Figure 3(a). To achieve coherent emitters, these arrays have to be fed by a single laser source. Alternatively, one can envi- sion an array of phase-locked or injection-locked emitters as the array of coherent sources. Optical phased arrays work much like phased array antennas in the microwave and radio wave domain. Their operation and performance can be understood by introduc- ing some basic physics. The electromagnetic field above the phased array can be spatially controlled by the emitter array (or matrix, in a two-dimensional case). Away from the opti- cal chip, this field changes shape as a result of diffraction. By carefully tuning the electromagnetic field right above the emitters, the far-field can be shaped into a narrow beam, pointing in the direction of the user's choice. The relation between the (complex) field above the emit- ters and the field far away (i.e., where the beam is pointed) can be described by a Fourier transform. This helps us to understand the design criteria. Figure 3(b-d) shows schemati- cally the trade-offs one has to make in terms of emitter spac- ing and the number of emitters. If emitters are spaced widely, over one half of an optical wavelength apart, side lobes appear, directing energy away from the main beam. By mov- ing the array elements closer, to less than one half a wave- length spacing, these side lobes are eliminated. However, by decreasing the overall emitter area, the beam becomes wider, which often is unwanted. The solution is to add more emitters to a densely spaced array to produce narrow beams without side lobes, but this increases the system complexity. This design case obviously depends on the application requirements, but it should be noted that, due to the small wavelength of light, on the order of one micrometer, emitter spacing typically is not smaller than half that length; hence, side lobes cannot be avoided. Advanced design tricks, such as nonuniform emitter spac- ing, have to be implemented to suppress the side lobes, if required. Two-Dimensional Integrated Optical Phased Arrays Conceptually, the most straightforward approach to integrate two-dimensional beam shaping and steering is to use a two- dimensional array of coherent emitters. Following the design criteria discussed above, such arrays should be as large as possible, with the emitters closely spaced. High-contrast sili- con photonics is well suited for this. A 64x64 array, consist- ing of a stunning 4,096 emitters in total, was developed by Sun et al. i , and is shown in Figure 4 on page 18. The emitter pitch is 9 µm (i.e., well beyond half the used wavelength of 1.55 µm), so multiple far-field images are formed. In this implementation, the phase is encoded lithographically by tuning the waveguide lengths, which leads to a relative delay. Hence, the output field is static. Active beam shaping and steering was achieved in the same project by adding thermo-optic phase tuners to the arms. This allows for arbitrary phase shaping of the field Technology 16 Figure 2: Evolution of the number of components in optical chips over the last 30 years. Both silicon and indium phosphide based optical chips are included. Figure 3: (a) Schematic of phased array operation, showing a laser feed- ing an array of emitters. The phase of the output of each emitter is shown (dashed red circles). In the far field, these add up to a beam. The design trade-offs are shown for (b) widely spaced emitters (triangles), (c) densely spaced emitters, and (d) increasing the number of emitters. The output beam angle and amplitude are shown schematically in red. Electronic Military & Defense Annual Resource, 5th Edition