Tham khảo tài liệu 'advances in optical and photonic devices part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Waveguide Photodiode WGPD with a Thin Absorption Layer 169 Fig. 8. A measured frequency response of WGPD with a thin absorption layer of 1000Ẵ. 3. Intermodulation distortion properties In some optical communication systems such as fiber-optic community antanna television CATV systems many optical signals with different modulation frequencies are inputted to a PD. In this case non-linearity properties of PD should be supressed to re-generate elctrical signals from optical signals without distortions. When a device shows nonlinear response input-output relation is represented as shown in Figure 9. An output can be expressed as polymomials of input signal. With this nonlinear relations supurious outputs of which frequencies are f2 f1 f2-f1 2f1-f2 2f2-f1. can be generated when sinusoidal inputs of which frequencies are fl f2 . are applied to device. These supurious outputs should be filtered out not to influence on original signals with cos ffl1 t cos 2 t cos ffl3 t V x Nonlinear device al V x a2 V x 2 a3 V x 3 cos rn 1 t cos rn2 t cos 2 rn1 t cos 2 ro2 t cos ro1 ffl2 t cos 2 ro2- ro1 t Fig. 9. Supurious signals from nonlinear devices 170 Advances in Optical and Photonic Devices frequencies of fl f2 . As can be seen in Figure 10 however frequencies of some supurious outputs are close to frequencies of original signal. These supurious signals cannot be filtered out and quality of converted signals from optical to electrical is degraded. The degree of degradations is determined by linearity of PD. The second order intermodulation products of two signals at f1 and f2 occur at f1 f2 f2-f1 2 f1 and 2 f2. The third order intermodulation products of two signals at f1 and f2 would be at 2 f1 f2 2 f1-f2 f1 2 f2 and 2 f2--f1. Among these products signals at f1 f2 2 f1-f2 and 2 f2--f1 are not filtered out. Therefore to obtain high purity signal among many signals signals at f1 f2 2 f1-f2 and 2 f2--f1 should be supressed when optical-to-electrical conversion occurs at PD. Signals
