# Wideband E\({}_{00}\)-E\({}_{10}\) Silicon Mode Converter Based on 180 nm CMOS Technology
## 3 Results and Discussion  
The proposed mode converter was fabricated on silicon on insulator (SOI) wafer with 180 nm CMOS technology. The thicknesses of the top silicon and buried silica layers were 220 nm and 3 \(\upmu\)m, respectively. By deep ultraviolet (UV) photolithography, the waveguide patterns were transferred onto the photoresist. After development, the unprotected top silicon was removed by coupled plasma etching, which offers the rectangular shape silicon waveguides. Thereafter, 1 \(\upmu\)m-thick silica was deposited by PECVD onto the SOI wafer as the top cladding. Figure 7 shows the microscope image (\(\times\)1000 times) of the mode converter. The light from the fiber is coupled into the Y-branch through the grating coupler. Thereafter, the split modes pass through the phase shifters and interfere in the MMI section. The light output from the grating coupler is on the right side. To be mentioned, the layout of Y-branch 3 dB splitter is modified to satisfy the design rule of 180 nm CMOS technology. The schematic diagram of measurement setup is shown in Figure 8. Figure 9 shows the picture that the converter is under test. After passing through the polarization controller, the light from the tunable laser source is coupled into the 3 dB splitter by a grating coupler via the single mode fiber (SMF). Since the converted E\({}_{10}\) higher mode is hard to directly observe through the single mode grating coupler, it is first filtered through a tapered waveguide. In this way, only the fundamental E\({}_{00}\) mode can pass and be collected by another SMF via the vertical grating coupler. Through a fiber power splitter, the obtained E\({}_{00}\) mode is coupled into the optical spectrum analyzer (OSA) (90%) and the optical power meter (OPM) (10%), respectively. To find out the loss of the proposed mode converter, referential waveguides with different lengths are fabricated together with the mode converter. These referential waveguides consist of two identical grating couplers and a straight single mode waveguide. The propagation loss single mode silicon waveguide is confirmed to be \(\sim\)1 dB/cm. Two grating couplers-induced loss can be deduced by removing the single mode waveguide-induced loss from the measured insertion loss of referential waveguide. Figure 10 shows the grating coupler-induced loss as the function of the optical wavelength. It can be seen that input/output gratings-induced loss within the wavelength range from 1540 to 1600 nm is lower than \(-\)25 dB. The minimum value of \(-\)25.0  
Figure 8: Schematic diagram of E\({}_{00}\)-E\({}_{10}\) mode converter measurement setup. Figure 7: Microscope image (\(\times\)1000 times) of fabricated silicon E\({}_{00}\)-E\({}_{10}\) mode converter. The measured transmission spectrum and conversion efficiency of the mode converter within the wavelength range from 1540 nm to 1600 nm is shown in Figures 11 and 12, respectively. It can be seen that the highest conversion efficiency of 91.5% occurred at 1575.9 nm, and the corresponding mode crosstalk is \(-\)10.3 dB. The conversion efficiency in percent (\(E_{\mathrm{p}}\)) and dB (\(E_{\mathrm{d}}\)) are calculated by Equation (5) and Equation (6), respectively. Here, \(b\) is the propagation loss of the E\({}_{00}\) mode in the dB scale. \[E_{\mathrm{p}}=(1-10^{0.1\mathrm{b}})\times 100\% \tag{5}\]  
\[E_{\mathrm{d}}=10\lg(1-10^{0.1\mathrm{b}}) \tag{6}\]  
The 3 dB bandwidth is confirmed to be over 60 nm according to the measured E\({}_{10}\) mode spectrum in Figure 11. The fluctuation of transmission is due to the overclose distance between the input/output fiber and the grating coupler, which may lead to the instability caused by the Fabry-Perot cavity. Therefore, the distance between the input/output fibers and the chip surface is supposed to be well controlled to guarantee the accuracy of the measurement. To be noted, the transmission is only characterized at 25 \({}^{\circ}\)C due to the limitation of the measurement setup.