ECE 455 – EXAMPLES No. 3 Q1. 1.6 Responsivity (A/W) 1.4   1.2  1 0.8  0.6  0.4 0.2 0 0.8 1 1.2 1.4 1.6 1.8 Wavelength (m) Fig.1 Responsivity versus wavelength for photodiodes ,,,,. (a) The responsivity versus wavelength for five different photodiodes is shown in Fig.1. (i) Which device has a bandgap of 1.24 eV? (ii) Which device has a quantum efficiency of 1.2? (iii) Which devices are definitely not PIN photodiodes? (iv)Which device would you recommend for a long distance fibre-optic link? (b) Consider the expression for the SNR of an APD photoreceiver: SNR APD  i p2 2qB( I m  I D ) M x  4kTBFn RL M 2 A certain APD photodetector has unity gain responsivity of 0.5 A/W and 1 nA of dark current. The load resistance is 70 k and the system bandwidth is 100 MHz. The operating temperature is 410C. No electronic amplification is used and there is no excess noise (i.e. no multiplication noise). If the photodiode is illuminated by a constant optical power of – 50 dBm, calculate: (i) (ii) (iii) The SNR for PIN-like operation. Which type of noise is dominant in this case? The SNR in the shot-noise limit. Sketch a diagram of the SNR versus the multiplication factor M. Q2. Consider the expression for the SNR of an APD photoreceiver: SNR APD  i p2 (1) 4kTBFn 2qBI m  I D  M  RL M 2 x (i) Show that the optimum value of M which will maximise SNR is given by:   4kTFn 2 x M optimum     xqRL I m  I D   (2) Q3. A germanium APD (with x = 1) is incorporated into an optical fibre receiver with a 10 k resistance. When operated at a temperature of 120 K, the minimum photocurrent required to give an SNR of 35 dB at the output of the receiver is found to be a factor of 10 greater than the dark current. If the noise figure of the following amplifier at this temperature is 1 dB and the post-detection bandwidth is 10 MHz, determine the optimum avalanche multiplication factor M.