# Integrated Optical Phased Arrays for Beam Forming and Steering
## 7 Performances
### Scalability  
Large-scale 1D OPAs have been implemented, such as 512-element OPAs [11; 103; 104] and 1024-element OPAs [16; 65]. However, an expensive precisely tunable laser source is necessary for wavelength-tuning-based OPAs [11; 104]. On-chip integrated tunable lasers are complex [64] and limited by power, linewidths, and tuning ranges [75; 83; 84]. Addi  
Figure 14: (**a**) An 8 \(\times\) 8 OPA structure with 16 phase shifters placed outside of the array aperture. Reprinted with permission from [74] © The Optical Society. (**b**) Cross section of a III–V/Si phase shifter. Reprinted with permission from [126] © The Optical Society. (**c**) A multi-pass photonic structure based on mode multiplexing. Reprinted with permission from [103] © The Optical Society. (**d**) A two-chip solution OPA and die photos of fabricated chips. Reprinted with permission from the author of [143] © The Author(s). tionally, the OPA compromises its spectral purity, multi-beam projection capability [45], and wavelength-dependent performance [64]. 2D OPAs are superior to 1D OPAs without these problems. Scalability is directly related to high-performance OPAs. Scalability of 2D OPAs is limited by huge power consumption [68; 100; 103], narrow FOVs [45], large sizes of phase shifters [68; 100], and complex control circuits [10; 16; 103; 116; 144]. Owing to difficulties in providing electrical and photonic connections to every pixel, previous LiDAR systems have been typically restricted to fewer than 20 pixels [144]. Typically, each element needs a phase shifter [100]. Overall power consumption of an OPA linearly increases with the number of elements. The scalability of OPAs is challenged by overwhelming power consumption and high drive voltages of phase shifters [45; 103; 143]. In addition, the scalability and output power of Si OPAs are limited by nonlinearity losses and phase changes [65]. FOVs are limited by large element spacing for optical feeds, layouts of large-size WGAs, and crosstalk suppression [45]. Large-scale OPAs incur placement/routing congestion [45]. Low power consumption and wide FOVs can be achieved by sparse aperiodic OPAs [114]. Sparse OPAs with enough room for optical feed distribution are attractive for large-scale OPA-based applications [45], in which 128 antennas can be accommodated with crosstalk suppressed by sparse OPAs. Control complexities can also be decreased by adding another chip. Aperiodic OPAs avoid aliasing induced by sidelobes at the cost of increased design complexities [116]. As presented in Figure 14d, a two-chip solution of low-power sparse aperiodic OPA was implemented to solve the problem of scalability. The OPA provided enough room for planar optical feed distribution as well as an adjustable far-field pattern with minimum sidelobes and a narrow beamwidth [143]. The challenge of scalability can also be lightly addressed by serpentine OPAs. A serpentine OPA based on serially interconnected WGAs supported fully passive, wavelength-controlled 2D beam steering. An implemented serpentine OPA produced 16,500 addressable spots only by wavelength tuning [63]. It is challenging to implement very large-scale serpentine OPAs for the high-power handing ability of materials. Additionally, inline architectures with matched phase shifters, antenna pitches, high aperture fill factors, and minimized footprints were inherently scalable to thousands of elements [11]. A Si/Si\({}_{3}\)N\({}_{4}\) dual-layer OPA chip was used to solve the issues of material-limited power and high loss propagation. Evanescent coupling between a pair of adiabatic linear tapers was used to achieve near-lossless vertical transitions between Si\({}_{3}\)N\({}_{4}\) and Si layers [88; 89]. Leveraging recent achievements in the monolithic integration of photonic and electronic circuits, a dense array of heterodyne detectors combined with a readout architecture enabled straightforward scaling to arbitrarily large arrays [144]. A 512-pixel large-scale coherent detector array was demonstrated in a 3D imaging system [144].