# Tutorial 5- Noise + Photodetector

```EKT 465: OPTICAL COMMUNICATION SYSTEM
SEM 1 2015/2016
ASSIGNMENT 3
1- A detector operating at 800 nm produces an output current of 80  A for an
incident light beams of power 800  W. Calculate the quantum efficiency and
responsivity of the detector.
Ans: 0.1 A/W , 15.5%
2-Photon at wavelength of 0.85 &micro;m are incident on a photodiode. The numbers of
incident photons are 3&times;1011 .At the terminal 1.4&times;1011 photons are collected. Calculate
the quantum efficiency and responsivity of photodiode.
Ans: 46.7%, 0.3196 A/W
3- A photodetector has a responsivity of 0.5 A/W at a wavelength of interest and a
dark current of 2nA. If the incident power is 100  m and the noise bandwidth is 50
MHz, calculate the mean-square noise current and the RMS noise current due to shot
noise.
Ans: RMS noise current: 29.07 nA.
4- A germanium APD (with x=1) is incorporated into an optical fiber receiver with a
10 kΩ load resistance. When operated at a temperature of 120 K, the minimum
photocurrent required to give a 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.
5- A silicon p-i-n photodiode incorporated into an optical receiver has a quantum
efficiency of 60% when operating at a wavelength of 0.9&micro;m. The dark current in the
device at this operating point is 3 nA and the load resistance is 4 kΩ. The incident
optical power at this wavelength is 200 nW and the post detection bandwidth of the
receiver is 5 MHz. compare the shot noise generated in the photodiode with the
thermal noise in the load resistor at a temperature of 20 oC.
Ans: rms shot noise and thermal noise are: 3.79e-10A, 4.49e-9A.
6- Optical receivers can be classified into THREE (3) types: Low impedance Front
End; High impedance Front end; and Transimpedance Front end. Discuss and
Compare the main advantages and limitations of each type.
```