Chúng tôi giả định rằng, tất cả các kênh duy nhất có khả năng đơn vị. Tại S nguồn, thông tin được tạo ra và multicast đến các nút khác trên mạng trong thời trang multihop nơi mà tất cả các chuẩn này chỉ ra rằng, đối với chứng ngộ TDD và FDD, đề nghị (nhưng không bắt buộc) | AVALANCHE PHOTODETECTOR 31 drift field. The consequences are a decreased responsivity gain compression and reduced bandwidth. This effect is particularly important in receivers with optical preamplifiers such as erbium-doped fiber amplifiers EDFAs which readily can produce several 10 mW of optical power at the p-i-n detector. Typical values for the saturation current are in the 10 to 76mA range 64 . AVALANCHE PHOTODETECTOR The basic structure of the avalanche photodetector APD is shown in Fig. . Like the p-i-n detector the avalanche photodetector is a reverse biased diode. However in contrast to the p-i-n photodetector it features an additional layer the multiplication region. This layer provides gain through avalanche multiplication of the electron-hole pairs generated in the i-layer also called the absorption region. For the avalanche process to set in the APD must be operated at a fairly high reverse bias of about 40 to 60 V. As we said earlier a p-i-n photodetector can be operated at a voltage of about 5 to 10 V. Light Multiplication Region Absorption Region Fig. Avalanche photodetector schematically . Similar to the p-i-n detector InGaAs commonly is used for the absorption region to make the APD sensitive at long wavelengths and zm . The multiplication region however typically is made from the wider bandgap InP material which can sustain a higher electric field. Responsivity. The gain of the APD is called avalanche gain or multiplicationfactor and is designated by the letter M. A typical value for an InGaAs APD is M 10. The light power p therefore is converted to electrical current ỈAPD as IAPD M HP where 7Ĩ. is the responsivity of the APD without avalanche gain which is similar to the responsivity of a p-i-n detector. Assuming that n A W as in our example for the p-i-n detector and that M 10 the APD generates 8 A W. Therefore we also can say that the APD has an effective responsivity R-APD 8 A W but we have to be careful to avoid .
