*4.2. Experimental Measurements for the Optical Propagation Loss of an SOI Waveguide*

Among the methods for measuring the OPL of waveguides, the Fabry–Perot (F-P) resonance method is the most appropriate approach when the SOI waveguide chip has a length of around 1 cm due to the ultrahigh fiber-waveguide but coupling loss [18]. With this method, the two ends of the waveguide were polished to form an optical F-P cavity for an optical signal launched into the waveguide. When the optical length of the cavity was continuously changed with either the refractive index of the waveguide material or the wavelength of the optical signal, the optical intensity transmission coefficient of resonance output of the cavity would present a form of the periodic wave. In this method, if the wavelength λ, the waveguide length *lWG*, and the intensity reflection coefficient *Rend* of two waveguide ends were given, the resonance output intensity of the waveguide cavity was defined by Feuchter and Thirstrup in 1994 [19]. Then, with the special transmittance values, *T*max and *T*min, its optical propagation loss α coefficient was obtained. In the experiments, we uniformly changed the refractive index of an SOI waveguide by heating the waveguide with a length of 8.5 mm, then the optical resonant output curve with the temperature was obtained, as shown in Figure 6, and, consequently, the optical propagation loss of α = ∼ 3.1 dB/cm was reached. In the SWR measurement and prediction of the SWR-caused OPL, this result was very agreeable with the numerical simulation results, shown in Figures 4 and 5, in which the average 20.33 nm anisotropic SWR based on the CLSM measurements were exploited and the OPL distributions of 3.0–3.5 dB/cm that were obtained with the 3-D construction in Figure 4b. From the view of the fabrication of SOI waveguides, the OPL of 3.1 dB/cm was relatively lower than the value of 3.6 ± 0.1 dB/cm, published in 2002 [20], and relatively higher than the value of around 1.4 dB/cm, published in 2016 [21]. So, the ICP fabrication of the waveguide samples employed in this work still needs to be improved.

**Figure 6.** Measurement output curve of Fabry–Perot (F-P) resonance of the SOI waveguide vs. temperature.
