# Integrated Optical Phased Arrays for Beam Forming and Steering
## 5 Material Platforms
### Si Photonics Platforms
#### 5.1.1 OPAs Based on the Si Platform  
Si photonic devices consist of many important components of photonic integrated circuits (PICs) because of the availabilities of the large-area Si wafers, the mature CMOS-compatible fabrication technology, the sophisticated assembly processes, and the monolithically integrated Ge photodetectors and modulators [119]. The high refractive-index contrast between Si and SiO\({}_{2}\) offers strong light confinement, thus enabling high-density PICs. As an example, Si photonic devices have been exploited as fast optical interconnectors for CMOS electronics [120, 121]. In addition, Si is an excellent material for its low-loss window extending from 1.1 to 7 \(\upmu\)m [121]. For OPA applications, each closely placed antenna of an OPA should provide a 2\(\upmu\) phase tunability to allow flexible reshaping of the far-field radiation. Versatile phase-tunning mechanisms such as efficient TO effect and fast free-carrier dispersion make Si PICs a promising candidate to address the stringent design and fabrications of OPAs [58]. For typical SOI wafers, the BOX thickness is 1-3 \(\upmu\)m for efficient optical mode confinement in the devices [122]. To enable single-mode optical waveguides at telecommunication wavelengths, the top Si thickness is usually at least 200 nm [122]. Typically, there are two types of SOI wafers. The ones with a top Si layer of 220 nm are typically used for passive devices at 1.55 \(\upmu\)m, and the ones with a thicker top Si layer of 500 nm are used for heterogeneously integrated Si photonic devices at the same wavelength [114]. SOI wafers with different geometrical specifications are commercially available [76]. As a fundamental platform for photonic integrated devices, SOI platforms have been exploited for the realization of various EF OPAs [9, 59, 61] and WGA-based OPAs [7, 8, 14, 15, 16, 17, 87, 96, 117]. The first experimentally demonstrated aperiodic EF OPA was implemented on the SOI platform [9], where an FOV of 31.9\({}^{\circ}\) was achieved with individually controllable TO shifters at a wavelength of 1.55 \(\upmu\)m [9]. The first demonstrated OPA with half-wavelength spacing was achieved by EF structures [61]. The SOI-based OPA shown in Figure 10a incorporated a near-half-wavelength curved waveguide array for crosstalk suppression [59]. The OPA with a 0.8 \(\upmu\)m pitch enabled aliasing-free beam steering of 64\({}^{\circ}\) with a beamwidth of 6.71\({}^{\circ}\). A 1-\(\upmu\)m-long SiO\({}_{2}\) cavity was exploited for plateau envelope tailoring. The steered beam exhibited a peak intensity fluctuation of less than 0.45 dB in the emission direction of \(-30^{\circ}\) to 30\({}^{\circ}\)[59]. The first OPA introduced in Si photonics was implemented by a 1D 16-element WGA. An FOV of 2.3\({}^{\circ}\)\(\times\) 14.1\({}^{\circ}\) was achieved by the OPA with 0.8-\(\upmu\)m-wide waveguides, spaced 2 \(\upmu\)m apart [7]. An FOV of 23\({}^{\circ}\) and a beamwidth of 1.27\({}^{\circ}\) were shown by an OPA of similar configuration, whose waveguides have a wider width of 4 \(\upmu\)m and a separation of 5 \(\upmu\)m [35]. In contrast, 2D OPAs can also be fabricated on the same platform. A 2D WGA-based OPA presented a small FOV of 1.5\({}^{\circ}\)[8]. An individually controlled 125-channel 2D Si OPA enabled a large FOV of 18.5\({}^{\circ}\)\(\times\) 16\({}^{\circ}\) by leveraging both wavelength tuning and phase control. A phase tuning efficiency of 20 mW/\(\pi\) and a narrow beamwidth of 0.15\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) were shown by this OPA with a large aperture size of 0.5 \(\times\) 0.5 mm\({}^{2}\)[14]. The first demonstration of coherent solid-state LiDAR was implemented by a Si OPA with cascaded phase shifters, and simultaneous distance and velocity measurements were performed using triangular frequency modulations [17].