## 3 SiN for Coarse Wavelength Division Multiplexing (CWDM) Optical Transceivers in the O-Band
### SiN-Si Hybrid Grating Fiber Coupler  
The transmission characteristics of single polarization grating couplers (SPGC) are often summarized in terms of the minimum insertion loss and the \(-\)1 dB (or \(-\)3 dB) bandwidth (BW). While these figures of merit provide a useful comparison between different designs, for SPGCs destined for use in WDM applications, it may be more practical to consider the range of insertion loss values directly over the entire utilized spectrum. Typical values for the insertion loss/\(-\)1 dB BW of a standard silicon O-band SPGC are 2-4 dB/20-30 nm. Such grating couplers, centered at 1310 nm, have insertion losses of at least 10 dB at the extremes of the CWDM band, ruling out their use in CWDM systems and even rendering reliable wafer-scale characterization of CWDM devices difficult. While it may be difficult to envisage the deployment of grating coupler-based fiber couplers in CWDM  
Figure 3: Optical (**a**) and scanning electron microscope (SEM) (**b**) images of the CWDM echelle grating (de)-multiplexer realized on 200-mm SiNOI platform. Close-up SEM pictures of the DBR-assisted facets used to improve the grating reflectivity. (**c**) Transmission spectrum of the 20-nm-spaced 4-channels echelle grating de-multiplexer across the O-band. Raw data (dashed line) and fitted plot (solid lines). (inset) Wavelength red detuning between 30 \({}^{\circ}\)C and 80 \({}^{\circ}\)C reveals a thermal dispersion coefficient of 13 pm/\({}^{\circ}\)C.  
systems in preference to non-diffractive coupling methods (particularly in the receiver circuit, due to complications relating to the unknown incoming polarization state in the fiber), large-bandwidth SPGCs would be an invaluable tool for the development of the CWDM device library. It can be generally said that, for a given fiber type, a wider transmission band will result from a reduction in the effective index of the grating. While using materials with lower optical indices (such as SiN) to make fiber grating couplers tends to lead to wider transmission spectra, these gains are often associated with increased insertion losses. The inclusion of the SiN layer in this process allowed us to compare Si, SiN and hybrid SiN/Si SPGCs for their use in CWDM applications and device testing. The standard Si SPGC is a partially etched (300/150 nm nominal Si thicknesses), apodised design. The SiN grating design is based on fully etched SiN lines and trenches, optimized for a peak transmission at 1310 nm. The BOX thickness is 2 \(\upmu\)m, but with the SiN being 200 nm above the Si layer, the total substrate-SiN distance is 2.5 \(\upmu\)m. Figure 4a,b shows a hybrid SiN/Si design as introduced by [PERSON] et al. [PERSON] et al. (2018). The SiN waveguide is connected to a primary, fully etched grating, beneath which is a secondary Si grating. The secondary Si grating, while having the same period, may be displaced with respect to the primary SiN grating (denoted as \(\Delta\)xSi in Figure 4a, which allows a lithographic control over the grating directionality. This is not possible with a conventional single-layer grating, where the directionality is fixed by the thickness of the etched and unetched layer thicknesses. As was also demonstrated in [PERSON] et al. (2018), allowing the fiber angle to be a free parameter allows further optimization of the grating directionality for a particular BOX thickness. In this study, however, the fiber angle was fixed to the typical standard value of \(8^{\circ}\) (in the SiO\({}_{2}\) cladding) to allow compatibility with standard test and packaging procedures. Figure 4c shows the results of 2D-FDTD simulations of the transmission spectra of the Si-only (black line) SPGC (into an optimally placed SMF28 fiber). This grating has a simulated group index (ng) value that varies from 3.0 to 2.88, a \(-1\) dB BW of 30 nm and an insertion loss of 1.6 dB to 13 dB over the CWDM band. The SiN-only grating coupler (blue line) has a ng value of 1.56, a \(-1\) dB bandwidth of 54 nm, and an insertion loss of 6.8 dB to 10.0 dB over the CWDM band.