# Integrated Optical Phased Arrays for Beam Forming and Steering
## 3 Building Blocks
### Phase Shifters  
As one of the most important performance parameters of the phase-modulation-based OPAs, steering speed is determined by phase tuning agility [46]. The phase shift typically results from a refractive index change, which can be introduced by temperature tuning or carrier injection, corresponding to TO and EO phase shifters [91]. An ideal high-performance optical phase shifter features a low power consumption, a low insertion loss, and a small crosstalk [45]. MZI structures can be used to test and analyze the characteristics of phase shifters [40,70]. TO phase adjustments can be easily achieved without additional optical losses [72]. Up to now, the majority of the reconfigurable optical phase shifters used on the Si platform are based on the TO effects [92]. TO phase shifters consisting of various materials for integrated OPAs were demonstrated. Most of the TO heaters are metal film or metal film stacks, such as Ti [7], TiN [59,68,93], Pt [41,94], and Cu [95]. Commonly used metal stacks include Cr/Au [9], Ti/Au [35,72], and Ni-Cr-Au [96] for better adhesion, among which the Ti/Au metal stack is often chosen in mature fabrication. The Ti/Au metal stack can easily be replaced by the CMOS-compatible metals, such as TiN or W [72]. Apart from metals, alternative materials such as doped Si [12,45,86], polymer [97,98], graphene [75] can also be used as heaters. The doped Si was used for the convenience of the CMOS processes [86]. Polymers feature high TO coefficients and low thermal conductivity, facilitating low-power driving of the phase shifters and a low thermal crosstalk [98]. Thanks to the high thermal conductivity of graphene, a high operation speed on the order of 0.2 MHz was realized [75]. As presented in Figure 6a, metal heaters are widely used and placed far enough from the waveguides to reduce optical loss due to metal absorption [7; 35; 41; 72; 96]. The separation layer hinders heat transfer, which prolongs heating and cooling time [75]. Usually, a TO phase shifter possesses inherent drawbacks of a large power consumption and a low speed [70], and thus a large thermal time constant limits the maximum steering speed [52] and generally requires a power of \(>\)10 mW for a 2\(\pi\) phase shift [11]. Some novel structures [45; 64] and materials [75; 97; 98] can be used to increase efficiency and speed. An efficient spiral TO phase shifter with a compact form factor was used to achieve a negligible crosstalk [45], while another highly efficient phase shifter was also proposed with a very small power consumption [101]. Impurity-doped contact heaters feature higher heating efficiencies and faster responses compared with non-contact metal heaters [64]. Graphene contact heaters directly on waveguides can be used to enhance heating efficiency [75]. Polymers have high TO effects and low thermal conductivities, which can be used as efficient phase modulators with low drive power [97; 98]. A 16-channel polymer EF OPA with TO modulators and a small P\({}_{\pi}\) of 2.5 mW was implemented [98]. However, polymer-based OPAs occupy a large chip size due to low refractive indices. A substrate functions as a heat sink. Deep etched trenches help to improve thermal isolation, confine heat, and increase phase-shifting efficiencies [45; 72; 75; 81; 93; 102]. EO phase shifters have low crosstalk and high electrical bandwidths [11] and consume lower power than TO phase shifters [11; 103]. A schematic of a p-i-n-junction-based EO phase shifter for OPAs is shown in Figure 6b [43]. The measured power consumption of the EO phase shifter was only 2 uW for a 2\(\pi\) phase shift [11], which is more than 3 orders of magnitude less than TO shifters. A 512-element OPA only required 1.8 W of power [104]. The dense III-V/Si phase-shifter-based OPA consumed less than 3 nW power for a 2\(\pi\) phase shift [42]. Additionally, the switching time of an EO phase shifter based on a p-n or p-i-n [44; 52; 70; 81; 82] junction can be as high as tens nanosecond level [64], which is much faster than a TO phase shifter.