Contents I. Introduction
Silicon photonics has become a full fledged technology which is used for large scale integration of optical devices such as modulators, photo detectors, waveguide couplers, arrayed waveguide etc on a single photonic chip [1]–[3]. Silicon photonics technology has found applications in next generation short reach data center interconnect, light detection and ranging (LiDAR), biosensing and various other technologies [4]–[6]. It has enabled low cost and mass scale producti, C. Lacava, A. Khokhar, X. Chen, I. Cristiani, D. Richard-tion of integrated photonic circuits and systems. The large index difference between Si and SiO2 enables nanometer scale single mode waveguide configuration thereby enabling integration of large number of optical devices on photonic chip. On the downside the huge difference in mode areas of the fiber and waveguide makes the chip to fiber coupling a challenging task [7]–[9]. Grating couplers and edge couplers are used to overcome the problem of chip to fiber coupling. The complications in post fabrication process of edge couplers such as correct alignment of optical fiber, clean facets in spite of having bandwidth larger than 100 nm and insertion loss lower than 0.5 dB, makes its ill suited for mass production and testing on wafer scale. Thus vertical grating couplers are a viable method that gives greater tolerance in terms of aligning the fiber on the chip and positioning error thus making chip characterization easier [10], [11].
