## 3 Experiment Setup and Results  
To determine the performance result of the 1 \(\times\) 4 channel wavelength demultiplexer, first, we have measured the transmission responses of the different passive test devices which were designed on the same chip. The test devices include a 10 cm long Si\({}_{3}\)N\({}_{4}\) waveguide having nine alternating bends of 4 mm bending radius, a spiral shaped 1.5 m long delay waveguide, a heater integrated Mach-Zehnder interferometer, and three test structures for MMI couplers with different core waveguide lengths. The 1.5 m long test delay waveguide has a 4 mm bending radius at the center and routing regions (Figure 7a). Several 10 cm long test structures were designed next to each of the test components. The transmission response of these test devices allowed us to determine the propagation loss of the Si\({}_{3}\)N\({}_{4}\) waveguide and the insertion loss of the MMI couplers. These test results also facilitate assessing the insertion loss of each channel of the demultiplexer. We proceed to measure the BER performance of the demultiplexer. Figure 7 shows the experimental setup used to measure the transmission response of the passive components. A continuous wave (CW) signal from a laser with an optical power of 10 dBm is connected to a polarization controller (PC) and tuned over the wavelength range from 1530 nm to 1580 nm. At the output of the polarization controller (PC), the measured optical power is around \(\sim\)8.8 dBm. The polarized CW optical signal is applied to the input edge coupler (I/P) of the test device. The output optical power is measured for each of the test components and their corresponding 10 cm long test waveguide. The optical power transmitted through the 10 cm long test Si\({}_{3}\)N\({}_{4}\) waveguide is subtracted from the output power of the other test devices such as the MMI couplers and wavelength demultiplexer to find their corresponding insertion loss. The distance between the i/o waveguide edges of the test components and the 10 cm long test waveguide at the polished facet was kept to 60 \(\upmu\)m. This minimizes the error in measuring the test component's insertion loss due to the variation in fiber to chip coupling loss. Figure 8 shows the optical transmission response of the \(\sim\)10 cm long straight waveguide and \(\sim\)1.5 m long spiral delay waveguide. The optical coupling to the chip was implemented using two cleaved fibers at the input and output edge of the chip. It also shows the transmission response of the CW signal after the PC. From this figure it is seen that the total edge coupling in and out loss is around 9.8 dB  
Figure 7: **(a)** Layout of the 1.5 m long spiral waveguide, **(b)** Experimental setup to measure the transmission responses of the different test devices, (10 cm long waveguide, 1.5 m long spiral waveguide and the 1 \(\times\) 4 channel wavelength demultiplexer). (8.8-(-1) dBm) at the wavelength of 1550 nm, considering the propagation loss in the 10 cm long Si\({}_{3}\)N\({}_{4}\) waveguide is negligible compared to the edge coupling loss. The measured coupling loss variation over all the 10 cm long waveguides is less than 0.05 dB. From Figure 8, it is also seen that the 1.5 m long spiral delay waveguide suffers an extra 4.6 dB optical loss compared to the 10 cm long Si\({}_{3}\)N\({}_{4}\) waveguide. From this result, we measured an approximate 3.1 dB/m propagation loss for the 72 nm thick Si\({}_{3}\)N\({}_{4}\) waveguide, where the total fiber to chip coupling loss is assumed to be 9.8 dB. We have experimentally determined the group index of the 72 nm thick Si\({}_{3}\)N\({}_{4}\) waveguide using the transmission response of a test Mach-Zehnder interferometer (MZI). The length difference between the two arms of the MZI is 4.4 mm (the two MZI arms are 12.26 mm and 16.66 mm in length).