## 5 SiN for Near Infrared LIDAR  
Over the last decade, optical phased arrays (OPAs) have attracted significant attention due to their potential use in a broad range of advanced technological applications, including light detection and ranging (LIDAR), free space communication, and holographic displays [8]. This interest arises from the advantage that OPAs enable non-mechanical, arbitrary beam shaping and steering. Furthermore, it has been shown that OPAs can be implemented using integrated photonics, creating a path for miniaturized low-cost systems [9; 44; 45]. OPA integration has been explored on various photonic platforms, of which silicon photonics is particularly attractive due to its CMOS compatibility and high refractive index contrast [46]. Advanced OPA circuits have been demonstrated at telecommunications wavelengths using silicon, such as a fully integrated device for two-dimensional beam scanning [10], ultra-low divergence beams [11; 34], and coherent LIDAR circuits [47; 48]. However, the use of silicon provides some limitations in terms of wavelength transparency and maximum optical power. Here, the use of (SiN) presents a host of interesting possibilities, as outlined in the following. Compared to Si, which absorbs light at photon energies greater than its bandgap (\(\sim\)1100 nm) the transparency window of SiN is much larger and is suited for applications at wavelengths down to at least 500 nm. Furthermore, weak nonlinear losses allow SiN circuits to operate at higher optical  
Figure 5: Comb generation using annealing-free silicon nitride on insulator. (**a**) An 800-nm-spanning comb generation using a 56-\(\upmu\)m-radius Si\({}_{3}\)N\({}_{4}\) microresonator for \(P_{\text{in}}\sim\) 1 W. (**b**) Optical microscope and (insets) scanning electron microscope images of the ring and coupling region. (**c**) Plot of short-wavelength OPO power with input power in the bus waveguide. powers than Si, which is important for time-of-flight LIDAR. SiN has recently been demonstrated for OPA integration [34; 49; 50]. To allow the construction of proof-of-concept OPA circuits, we designed and characterized (typical measured insertion losses at \(\lambda\) = 905 nm shown in brackets) a new device library. Key components include fiber grating couplers (\(-\)4.9 dB), single-mode straight and bent waveguides (\(-\)1.0 dB/cm, minimum bend radius \(\sim\)15 \(\upmu\)m), 1 \(\times\) 2 multimode interference splitters (0.12 dB), heater-based phase modulators (87 mW/\(\pi\)), ring resonators, and output grating couplers. To demonstrate the use of this platform for LIDAR applications, we used these building blocks to design an optical circuit comprising four small OPA circuits linked to a single optical input via a switching network, with which we demonstrated rudimentary two-dimensional beam steering at a fixed wavelength of \(\lambda\) = 905 nm (see Figure 6) [51]. To demonstrate the functionality of the OPA circuit, we put in place a set-up that allows wafer-level beam calibration of the far field emission. A fiber couples 905 nm light to the circuit via the input fiber grating coupler, a multi-channel electrical probe allows the application of up to 25 individual drive voltages to the circuit and the output light is detected in the far field by a full frame CMOS camera placed above the OPA output. Custom driving electronics work in conjunction with the camera via a hill-climbing optimization algorithm to form a beam at the desired output angle. The normalized cross-sections in \(\phi\) and \(\theta\) of the main lobe of the optimized beam are shown in Figure 7a in black and blue, respectively. A Gaussian curve was fitted to the data in order to obtain the full width at half maximum (FWHM). The measured FWHM are 4.3\({}^{\circ}\) and 0.7\({}^{\circ}\) in \(\phi\) and \(\theta\). Both values are in in good agreement with the expected beam FWHM in \(\phi\) of 4.3\({}^{\circ}\) (calculated from the OPA geometry) and 0.6\({}^{\circ}\) in \(\theta\) (from FDTD simulations of the output grating coupler).