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above the emitters. Abediasl et al. went a few steps further and presented a silicon chip that includes both amplitude and phase control of an 8x8 array, for full arbitrary dynamic beam shaping 2 . Moreover, they integrated all the control electronics on the same chip, including current drivers and 7-bit digital-to-analog converters for current control. This was achieved by using a commercial IBM 7RF-SOI CMOS process. The integration of 300 optical and 74,000 electrical compo- nents on the same chip also represents the highest level of integration ever achieved for an electronic-photonic inte- grated circuit. A photograph of the chip is shown in Figure 5. This chip can be used as both a receiver and a transmitter for free-space communications. It should be noted, though, that phase-only beam shaping is not necessarily less attractive than having both phase and amplitude control. Amplitude control typically involves attenuating the light at the emitter, leading to an overall decrease in device energy efficiency. Using algorithms typically applied to holography (e.g., for holographic diffraction gratings), phase-only beam shaping still can be used to obtain arbitrary far-field images. One-Dimensional Phased Arrays With Tunable Lasers Although intuitively the most straightforward solution, two- dimensional phased arrays suffer from a severe control prob- lem. Hundreds or even thousands of emitter elements have to be controlled in phase and, possibly, in amplitude. This becomes especially prohibitive when frame rate beam scan- ning is required and the control electronics have to work in the 10s of megahertz regime. Beam width decreases linearly when increasing the size (N) of a linear array of emitters. This means that for a two-dimensional NxN array, the beam width scales inversely with N 2 . An alternative approach makes use of the dispersive nature of the gratings that are used as emitters in silicon photonic chips iii . The direction of light emission in a grating emitter is actually dependent on the wavelength of light. This means that beam steering can be achieved by changing the wave- length of the light using a tunable laser source. This source can be integrated on-chip, using a monolithic or heteroge- neous integration approach. Alternatively, this source can be placed off-chip, using a system-in-package or a hybrid integration approach. An additional advantage of this method is that the grating grooves can be very closely spaced, below half the free-space wavelength. This means that, in the direc- tion along the grating, no side lobes appear. A disadvantage of this method is that the phase relation is fixed and linear for far-field beam forming. However, many applications in communications and Lidar require only a nar- row beam that can be steered and have no need for arbitrary field patterns. The approach is shown schematically in Figure 6. Simply increasing the grating length will narrow the beam width along the direction of the grating. In the perpendicular direction, beam shaping and steer- ing can be achieved by using a phase-controlled array of grating emitters, such as those discussed in the previous section. However, unlike the N 2 scaling of beam width with two-dimensional array size, in the tunable laser approach Technology 18 Figure 5: (a) Micrograph of an 8×8 optical phased array (OPA), monolithically integrated with control electronics in a 180-nm CMOS process. Reprinted with permission of The Optical Society ii . Figure 4: (a) Schematic illustration of a two-dimensional emitter array. Laser light is coupled into the input waveguide on the chip and distributed equally over the array. Grating couplers act as emitters by coupling the laser light out of plane. A scanning-electron microscope picture of the array is shown in (b) and a close-up of the grating emitter is shown in (c). Reprinted with permission from Macmillan Publishers Ltd: Nature i , copyright 2013. Figure 6: (a) By applying and changing a linear phase profile over the phase tuners, the beam is formed and steered in a direction perpendicular to the grating emitters. Red and green bars schematically indicate the amount of phase tuning. (b) In the direction of the grating emitter, the beam is steered by changing the wavelength of the light, using a tunable laser iii . Three different wavelengths are schematically shown in red, green, and blue. Electronic Military & Defense Annual Resource, 5th Edition