# Integrated Optical Phased Arrays for Beam Forming and Steering
## 3 Building Blocks
### Laser Sources  
Chip-scale beam forming and steering require high-performance and low-cost integrated OPAs, which can be readily realized with Si photonics technology [11; 12]. For the wavelength dependent OPA, it is necessary to have a tunable laser source [64]. The tuning precision needs to be high enough to avoid degradation in the spectral purity and multibeam projection capability of the OPA [45]. On the other hand, a wavelength-fixed laser is required for the phase modulation-based 2D OPAs. Figure-of-merits of lasers, such as output power, linewidth, and tunability, are important to the OPAs. Due to the absence of a direct-bandgap material, off-chip lasers are widely used in the Si photonic OPAs [7; 9; 10; 11; 14; 41; 59; 65; 70; 71]. Off-chip lasers provide wide tuning capabilities [72]. Light can be coupled to the photonic components through either a grating coupler [40; 45; 59; 62; 70; 73; 74] or an edge coupler [7; 14; 65; 71; 75]. The latter approach allows for the usage of a lensed fiber for highly efficient light coupling [7; 72]. The grating coupler can be placed anywhere on the chip but is sensitive to the polarization and wavelength of incoming light. Realizing an efficient grating coupler is quite challenging, which requires complicated design and optimization. Different from grating couplers, edge couplers  
Figure 4: A typical integrated OPA structure. have high coupling efficiencies, negligible polarization dependences, and broad working bandwidths. The edge coupler can only be used after the chip is diced. To realize high coupling efficiencies (typically >80%), fabrication of optical-quality facets on the chip sides, which only works well at small volumes, are of great significance [76]. Additionally, relatively complex fabrications and packaging processes are required. Expensive benchtop tunable lasers with a large form factor restrict the practical use of the OPAs [77; 78]. It is thus highly desirable to have the laser sources integrated on-chip, which have been implemented on the Si photonics platform [77; 78], the indium phosphide (InP) platform [79; 80], and the hybrid III-V/Si platform [81; 82]. A monolithically integrated OPA with an on-chip erbium-doped laser was demonstrated for its advantage of CMOS compatibility and fabrication simplicity [78]. The first rare-earth-doped laser monolithically integrated OPA system consists of a Si layer with eight doping masks, two Si\({}_{3}\)N\({}_{4}\) layers, three metal and via layers, a dicing trench for smooth edge-coupled facets, and a gain-film trench. The off-chip 980-nm-wavelength pump is coupled into the on-chip laser with a single lasing peak at 1.599 \(\upmu\)m and a side-mode suppression ratio (SMSR) of 30 dB. Cascaded phase shifters controlled by electrical signals are demonstrated to reduce complexity while enabling tuning for phase variations [77; 78]. Based on 500-\(\upmu\)m-long WGANs and 2-\(\upmu\)m pitch, the 49-channel OPA with an aperture size of 0.5 \(\times\) 0.1 mm\({}^{2}\) showed 15\({}^{\circ}\) beam steering and a beamwidth of 0.80\({}^{\circ}\)\(\times\) 0.16\({}^{\circ}\)[77]. The OPA with grouped cascaded phase shifters enabled 1D beam steering with a beamwidth of 0.85\({}^{\circ}\)\(\times\) 0.20\({}^{\circ}\) and a SMSR of 30 dB [78]. We note that here the OPAs with monolithically integrated erbium-doped lasers can only be tuned by the phase shifters. Based on the InP platform, a monolithically integrated OPA with 2D optical beam steering with an FOV of 10\({}^{\circ}\)\(\times\) 6.5\({}^{\circ}\) and a beamwidth of 2\({}^{\circ}\)\(\times\) 0.2\({}^{\circ}\) was achieved. Polar beam steering efficiency of 0.14 \({}^{\circ}\)/nm was demonstrated by a tunable laser. In addition, very fast beam steering (>10\({}^{\circ}\)/s) in both dimensions has been achieved [79]. As illustrated in Figure 5, a hybrid III-V/Si platform can also be used to achieve OPAs with on-chip laser sources.