光探測及測距(Light Detection and Ranging, LiDAR)在自由空間中的光束成形與轉向相當重要，為了在自由空間中完成光通訊、光學投影，光學相位陣列(OPA)是一項調製光束成形與轉向重要的方法，光學相位陣列比起機械方式具有體積小、低功耗、低成本與高掃描速度等優勢。
本論文晶片由IMEC代工，將LiDAR系統佈局於5.15-mm×2.5-mm的範圍內，LiDAR系統分為分光器、相移器與發射器，其中利用1×2 MMI分光器串接6級形成1×64的分光器，量測結果1×2 MMI損耗為2-dB；1×64 MMI損耗為7-dB，量測長度為480-μm的MZI元件V_π為1.125V，光源輸入使用光柵耦合器在1550-nm波段時單邊損耗5-dB。發射器設計2-D天線陣列，對於LiDAR縱向波束轉向需控制波長或光柵週期；橫向波束轉向需控制相位，本論文設計2-D光學陣列天線的間距為1-μm週期為0.702-μm天線長度為100-μm並在天線間加入線寬為0.13-μm矽帶陣列來延長天線陣列間的耦合長度達到預防串擾的目的並經FDTD驗證加入矽帶陣列可降低光瓣與輻射強度分別為2-dB/2%，遠場經由FDTD驗證此設計在橫向轉向角可達±50°其中在50°下SMSR為16-dB；FWHM為1.5°，縱向轉向角在波長為1500~1600-nm調製下可達8°。由於設計2-D天線陣列輻射角度並未考量與光纖輸入的角度相同，導致量測2-D訊號光與反射光重疊，因此量測上述設計之1-D轉向，經由熱相移器改變光在波導內之相位使其達到轉向的目的，量測結果橫向轉向範圍為±50°，並在0°、±20°、±30°、±50°有波束成形，相移器的電壓偏置是使用粒子群演算法來找到，其中在50°下SMSR為 11-dB；FWHM為2°。為了增加縱向轉向範圍，找到設計不同週期的光學陣列天線以分配不同的縱向角度來實現，經FDTD驗證在天線間距為2-μm下週期在610~710-nm之間變化，模擬出橫向/縱向轉向範圍分別為60°/48°，縱向角度的提升可以有效避免訊號光與光纖反射光重疊。

Free-space beam steering and beamforming are essential for light detection and ranging (LiDAR), free-space communications, and optical projection. Optical phased array (OPA) is an effective method to steer the light beam. OPA has a compact size, low power loss, low cost, and high scanning speed, compared with the mechanical-based beam-steering method.
In this paper, the LiDAR chip system with a 5.15 mm x 2.5 mm size was fabricated in IMEC. The LiDAR system is divided into the splitter, phase shifter, and emitter part. The measurement loss of the grating coupler, 1x2 MMI, 1x64 MMI in the 1550-nm band is 5-dB, 2-dB, and 7-dB, respectively. The heater voltage to change the phase of π in the emitter array structure is 1.125 V. The emitter is designed with a 2-D antenna array. In waveguide-based LiDAR, the wavelength or grating period is controlled to steer the beam vertically, while the phase from each phased-array is controlled to steer the beam horizontally. This paper designs 2-D 100-μm length optical array antenna with a pitch of 1-μm, a period of 0.702-μm. Moreover, a silicon strip array with a width of 0.13-μm between the antennas is to extend the coupling length between the antenna arrays to prevent crosstalk. Finite Difference Time Domain (FDTD) verifies that adding a silicon strip array can reduce the grating lobe and radiation intensity to 2-dB/2%.
This design can reach ±50° in the horizontal steering angle with 16-dB Side Mode Suppression Ratio (SMSR) at 50° and 1.5° Full Width at Half Maximum (FWHM). Meanwhile, the vertical steering angle can reach 8° by modulating wavelength of 1500~1600-nm. Since the 2-D designated beam output angle in the antenna array is different from the fiber input angle, the measurement 2-D signal light and the reflected light overlap. We measure the 1-D steering of the above design by controlling the heater in the phase shifter array to change the light phase in the waveguide. The beamforming measurement is taken at 0°, ±20°, ±30°, and ±50°. The voltage biases in phase-shifters are set iteratively using a Particle Swarm Optimization algorithm. The measured beam has an SMSR of 11-dB at 50° with 2° FWHM. The vertical beam steering range of the 2D-LiDAR could be increased using optical array antennas with different periods. Hence, we simulated antenna arrays with a 2-μm pitch, with period variation between 610 and 710-nm through FDTD methods and presented the horizontal and vertical steering ranges as 60° and 48°, respectively.

[1] M. Himmelsbach, A. Müller, T. Luettel, and H.-J. Wünsche, “LIDAR-based 3D object perception,” Proceedings of 1st international workshop on cognition for technical systems 1, 2008.
[2] Y. Bian, Q. Ren, L. Kang, Y. Qin, P. Werner and D. Werner, "Efficient Cross-talk Reduction of Nanophotonic Circuits Enabled by Fabrication Friendly Periodic Silicon Strip Arrays", Scientific Reports, vol. 7, no. 1, 2017.
[3] C. V. Poulton, M. J. Byrd, M. Raval, Z. Su, N. Li, E. Timurdogan, D. Coolbaugh, D. Vermeulen, and M. R. Watts, "Large-scale silicon nitride nanophotonic phased arrays at infrared and visible wavelengths", Optics Letters, vol. 42, no. 1, p. 21, 2016.
[4] H. Yoon ; D.-lS. Lee ; S.-H. Kim ; G. Kang ; J. Shim ; H. Rhee ; N. Kwon ; J.-Y. Kim ; H.-H. Park. “Wide-Angle 2D Beam-Steering with Si-Based 16 × (1 × 16) Optical Phased Arrays.” Electronics Letters, vol. 56, no. 10, 14 May 2020, pp. 501–503, 10.1049/el.2019.3490. Accessed 29 Sept. 2020.
[5] Jie Sun, Erman Timurdogan, Ami Yaacobi, Ehsan Shah Hosseini, Michael R. Watts: ‘Large-scale nanophotonic phased array’, Nature, 2013, 493, pp. 195–199, doi: 10.1038/nature11727
[6] M. Kossey, C. Rizk and A. Foster, "End-fire silicon optical phased array with half-wavelength spacing", APL Photonics, vol. 3, no. 1, p. 011301, 2018.
[7] K. Shang, C. Qin, Y. Zhang, G. Liu, X. Xian, S. Feng and S. B. Yoo, "Uniform emission, constant wavevector silicon grating surface emitter for beam steering with ultra-sharp instantaneous field-of-view", Optics Express, vol. 25, no. 17, p. 19655, 2017.
[8] A. Yaacobi, J. Sun, M. Moresco, G. Leake, D. Coolbaugh and M. Watts, "Integrated phased array for wide-angle beam steering", Optics Letters, vol. 39, no. 15, p. 4575, 2014.
[9] J. C. Hulme, J. K. Doylend, M. J. R. Heck, J. D. Peters, M. L. Davenport, J. T. Bovington, L. A. Coldren, and J. E. Bowers, "Fully integrated hybrid silicon two dimensional beam scanner", Optics Express, vol. 23, no. 5, p. 5861, 2015.
[10] T. Komljenovic, R. Helkey, L. Coldren and J. Bowers, "Sparse aperiodic arrays for optical beam forming and LIDAR", Optics Express, vol. 25, no. 3, p. 2511, 2017.
[11] T. Komljenovic and P. Pintus, "On-chip calibration and control of optical phased arrays", Optics Express, vol. 26, no. 3, p. 3199, 2018.
[12] D. Zhuang, L. Zhagn, X. Han, Y. Li, Y. Li, X. Liu, F. Gao, and J. Song, "Omnidirectional beam steering using aperiodic optical phased array with high error margin", Optics Express, vol. 26, no. 15, p. 19154, 2018.
[13] C. Phare, M. Shin, J. Sharma, S. Ahasan, H. Krishnaswamy and M. Lipson, "Silicon Optical Phased Array with Grating Lobe-Free Beam Formation Over 180 Degree Field of View", Conference on Lasers and Electro-Optics, 2018.
[14] D. N. Hutchison, J. S., Jonathan K. Doylend, R. Kumar, J. Heck, W. Kim, C. T. Phare, A. Feshali, and H. Rong, "High-resolution aliasing-free optical beam steering", Optica, vol. 3, no. 8, p. 887, 2016.
[15] C. Poulton, P. Russo, E. Timurdogan, M. Whitson, M. Byrd, E. Hosseini, B. Moss, Z. Su, D. Vermeulen, and M. Watts, "High-Performance Integrated Optical Phased Arrays for Chip-Scale Beam Steering and LiDAR", Conference on Lasers and Electro-Optics, 2018.
[16] S. Miller, C. Phare, Y. Chang, X. Ji, O. Gordillo, A. Mohanty, S. Roberts, M. Shin, B. Stern, M. Zadka, and M. Lipson, "512-Element Actively Steered Silicon Phased Array for Low-Power LIDAR", Conference on Lasers and Electro-Optics, 2018.
[17] Yuyao Zhai, Qingwen Liu, He Li, Dian Chen, and Zuyuan He. "Non-mechanical beam-steer lidar system based on swept-laser source." Optical Fiber Sensors. Optical Society p. 8427035 of America, 2018.
[18] F. Ito, X. Fan and Y. Koshikiya, "Long-Range Coherent OFDR With Light Source Phase Noise Compensation", Journal of Lightwave Technology, vol. 30, no. 8, pp. 1015-1024, 2012.
[19] J. W. Goodman, “Introduction to fourier optics” (2004, 3rd edition).
[20] Ung, Bora, and Yunlong Sheng. “Optical Surface Waves over Metallo-Dielectric Nanostructures: Sommerfeld Integrals Revisited.” Optics Express, vol. 16, no. 12, 4 June 2008, p. 9073, 10.1364/oe.16.009073.
[21] M. Dakss, L. Kuhn, P. Heidrich and B. Scott, "ERRATA: GRATING COUPLER FOR EFFICIENT EXCITATION OF OPTICAL GUIDED WAVES IN THIN FILMS", Applied Physics Letters, vol. 17, no. 6, pp. 268-268, 1970.
[22] R. Waldhäusl, B. Schnabel, P. Dannberg, E. Kley, A. Bräuer and W. Karthe, "Efficient Coupling into Polymer Waveguides by Gratings", Applied Optics, vol. 36, no. 36, p. 9383, 1997.
[23] F. Van Laere, T. Claes, J. Schrauwen, S. Scheerlinck, W. Bogaerts, D. Taillaert, L. O'Faolain, D. Van Thourhout and R. Baets, "Compact Focusing Grating Couplers for Silicon-on-Insulator Integrated Circuits", IEEE Photonics Technology Letters, vol. 19, no. 23, pp. 1919-1921, 2007.
[24] Shiyang Zhu, Yu Li, Ting Hu, Qize Zhong, Yuan Dong, Zhengji Xu, and Navab Singh, "Silicon Nitride Optical Phased Arrays with Cascaded Phase Shifters for Easy and Effective Electronic Control," in CLEO: Applications and Technology, 2019: Optical Society of America, p. AW4K. 5.
[25] C. V. Poulton, Ami Yaacobi, David B. Cole, Matthew J. Byrd, Manan Raval, Diedrik Vermeulen, and Michael R. Watts, "Coherent solid-state LIDAR with silicon photonic optical phased arrays," Optics letters, vol. 42, no. 20, pp. 4091-4094, 2017.
[26] R. Zhang, T. Lin, W. Liu, S. Hsu, and C. Chang, "Grating Lobe-Free Beam Steering through Optical Phase Array Using Phase-Compensated Two Index-Mismatched Silicon Wires-Based Emitters", Applied Sciences, 10(4), p.1225, 2020.
[27] K.Van Acoleyen, K. Komorowska, W. Bogaerts, and R. Baets, "One-Dimensional Off-Chip Beam Steering and Shaping Using Optical Phased Arrays on Silicon-on-Insulator ", Journal of Lightwave Technology, 29(23), p.3500, 2011.
[28] Chen, Jian, “Grating Lobes Analysis Based on Blazed Grating Theory for Liquid Crystal Optical-Phased Array.” Optical Engineering, vol. 52, no. 9, 10 Sept. 2013, p. 097102, 10.1117/1.oe.52.9.097102.