# O-Band Grating Couplers Using Silicon Nitride Structures
## 1 Introduction  
Grating couplers (GC) based on silicon (Si\({}_{3}\)N\({}_{4}\)) are now widely used to couple the light between a single-mode fiber (SMF) and photonic integrated circuits (PICs) [1; 2]. The use of GC in the field of PICs has brought significant advantages by enabling the avoidance of expensive processing and packaging steps [3]. GCs are devices that couple light into and out of waveguides on PICs [4; 5]. They supply a proficient and budget-friendly resolution, achieving an efficient interface between the optical fiber and waveguides on the PICs without the need for complex alignment techniques [6; 7; 8]. Traditionally, the optical waveguides on chips required polishing to achieve good coupling efficiency with optical fibers [9]. This process was time-consuming and costly, as it involved the precise polishing and alignment of the waveguide facets [10]. Similarly, lensed fibers were used to focus light into the waveguides, which required careful alignment and often resulted in additional expenses [11; 12]. GCs can be useful in addressing this challenge by enabling efficient optical signal coupling and transmission [13]. A GC based on Si\({}_{3}\)N\({}_{4}\) has become a preferred choice for PICs because of its excellent optical properties, such as low absorption and low back reflection (BR) in the O-band spectrum [14; 15; 16]. These properties are crucial in integrated photonic systems as they help prevent signal degradation, interference, and increased noise levels, ensuring high-quality and efficient signal transmission [17]. A study [18] was conducted by employing an Si\({}_{3}\)N\({}_{4}\) on a silicon (Si) bi-layer configuration. This O-band design was developed through an optimization-driven strategy and realized on a 200 mm wafer. These designs exhibited resilience to fabrication variations, maintaining a consistently high performance across the entire wafer. Particularly noteworthy is the attainment of a remarkable peak coupling efficiency of \(-2.2\) dB and a substantial1-dB wavelength range of 72.9 nm in a representative device. Additionally, an innovative accomplishment is presented in a study [19], introducing a trim Si\({}_{\text{in}}\) platform that enables a fiber-chip featuring a dual-band grating coupler with single-state polarization in the transverse electric (TE) mode. This coupler efficiently couples with both O- and C-band wavelengths. In the field of photonics, testing the performance of photonic chips is a crucial step in the development and characterization of integrated photonic devices [20]. A study [21] proposed a design optimization approach to enhance the efficiency of GC using Si material. The focus of their work was to improve the coupling efficiency by utilizing Si on insulator (SOI) technology, which involved a top Si layer with a buried oxide layer. The GC was designed to operate in the TE mode. The result of this study was 30% coupling efficiency with a 1 dB bandwidth of 40 nm on standard wafers. Another study [22] aimed to address the bandwidth limitation of GC. The researchers proposed a focusing-curved arrangement utilizing wide band subwavelength grating couplers (SWGCs). The SWGCs are created using electron-beam lithography (EBL) on a conventional SOI platform through a one-stage complete etching process. TE mode light demonstrates a 1 dB bandwidth exceeding 100 nm around the 1550 nm wavelength. This proposed solution enables seamless integration with different optical devices and systems, expanding the capabilities of GC. An additional notable study [23] delves into the application of Si\({}_{3}\)N\({}_{4}\) as a waveguide. This study demonstrates a semiconductor laser's functionality within a structured Si photonics platform, displaying proficient light coupling into a waveguide. The diode laser employed in this research is centered on mid-infrared gallium antimonide, grown directly on a pre-patterned Si photonics wafer featuring Si\({}_{3}\)N\({}_{4}\) waveguides enveloped by Si dioxide.