Tham khảo tài liệu 'advances in optical and photonic devices 2011 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 164 Advances in Optical and Photonic Devices p - InGaAs contact layer p - InGaAs contact layer 4pm InP 4pm InP 100Ấ thick InGaAs 300Ấ thick InGaAs 3pm InP 3pm InP n InP substrate n InP substrate Type I Type II p - InGaAs contact layer p - InGaAs contact layer 4pm InGaAsP Xg 3pm InGaAsP Xg 500Ấ thick InGaAs 1000Ấ thick InGaAs 3pm InGaAsP Xg n InP substrate Type III 3pm InGaAsP Xg n InP substrate Type IV Fig. 3. Tested WGPD structures for high responsivity operation. PD type Responsivity flat-ended fiber Responsivity lensed fiber I Polarization dependent Polarization dependent - II W W III A W W IV W W Table I Responsivities for four types of WGPDs s on Poincare sphere degree Fig. 4. Polarization dependencies of Type I and Type II . Another drawback of WGPD with 100Ẵ thick absorption layer is low coupling efficiency which is contradictory to the simulated value. For type I the calculated coupling Waveguide Photodiode WGPD with a Thin Absorption Layer 165 efficiency coupled with flat-ended fiber is which corresponds to responsivity of W for the wavelength of 1490nm. Measured responsivity however implies that coupling efficiency of Type I when coupled with flat-ended fiber is and maximum responsivity is W for the input wavelength of 1490nm. To measure the coupling efficiency of Type I responsivities of PDs with different lengths were measured for TE input light. Figure 5 shows responsivity values versus PD length. This data was fitted with Equation 1 . R C 1 - e 1 In Equation 1 R C Â a r and L are responsivity coupling efficiency input wavelength absorption coefficient of absorption layer confinement factor of guided beam within a absorber and PD length respectively. Fitting results indicate that coupling efficiency is and ar is . Low responsivity for Type I was conformed by measuring five PDs. Five PDs show almost same responsivity. Discrepancy between simulated .
