In the optical fiber communication receiver, avalanche photodiode (APD) is used when high sensitivity is required. Avalanche multiplication is useful in overcoming the amplifier noise, thus improving sensitivity over that available from PIN photodiode.
In this thesis, the effect of APD on the sensitivity of the receiver is studied experimentally, and theoretical calculations based on the Gaussian approximation of noise statistics are made to compare with experimental results.
For the experiment, a simple optical fiber communication system link has been built employing light emitting diode, step index fiber, reach-through structure type APD, transimpedance amplifier as pre-amplifier main amplifier, decision circuit, and error detector.
For the specification of the receiver, various characteristics of APD such as gain, excess noise factor, leakge-current, junction capacitance, and temperature dependence of gain are measured, while frequency response and equivalent input noise current are measured for the amplifier.
With digital communication in mind, the sensitivity is defined as minimum detectable power which must be coupled to the APD to achieve $10^{-9}$ bit error rate (BER) at the given bit rate (50Mbps in this experiment). To determine the minimum detectable power and optimum avalanche gain, BER is measured as a function of optical power coupled to the APD and APD gain.
Experimental results show that Gaussian approximation of noise statistics predicts the minimum detectable power with reasonable accuracy, while the optimum avalanche gain is predicted too high.
APD를 사용한 광섬유 통신수신기에서는 APD의 내부이득이 증폭기의 잡음의 영향을 크게 줄임으로써 감도가 개선된다.
수신기의 감도를 나타내는 minimum detectable power는 Bit rate, bit error rate, APD 의 excess noise factor와 최적이득, 증폭기의 잡음의 함수가 된다.
본 논문에서는 APD의 이득과 excess noise factor, 증폭기의 잡음을 측정하고 bit rate $5M_bps$에서 APD의 이득과 입력 광 power를 변화시키면서 bit error rate를 측정하여 $10^{-9}$ bit error rate의 경우에 실효입력 광power를 minimum detectable power로 삼았다.
잡음의 진폭분포가 Gaussian이라고 가정하여 구한 minimum detectable power와 그때의 APD의 최적이득을 측정치와 비교한 결과 minimum detectable power는 이론치가 - 56.2 dBm, 실험치가 -56.6dBm으로서 0.4dBm의 오차를 나타내었으나 APD의 최적이득은 이론치가 198, 실험치가 80으로서 상당한 차이가 있었다.
