EKT 465: OPTICAL COMM.
SEM 2 2014/2015
Assignment 4
1- Calculate the responsively of a detector with quantum efficiency of 10% at 800 nm.
Ans: 6.45 A/W
2- A detector operating at 800 nm produces an output current of 80  A for an
incident light beam of power 800  W. Calculate the quantum efficiency and
responsivity of the detector.
Ans: 0.1 A/W , 15.5%
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- Determine the SNR of a photodiode at 10 V reverse bias operating at room
temperature with 10 M  load at 100 Hz bandwidth with a total radiant flux of 10-10
W at 800 nm. Consider all sources of noise except that out of 1/f noise region.
Assume the responsivity of the photodetector as 0.5 A/W and dark current as 600
 10 12 A and room temperature is 27oC.
Ans:41.3 dB
5- A silicon photodetector has the following specifications at 27o C.
Minimum radiant input= 2  W
Responsivity at 800 nm= 0.5 A/W
Active area
= 3  10 3 cm2
Dark current at 10 V reverse bias = 0.1 A
Source resistance = 100 
Load resistance = 10 M 
Bandwidth
=500 Hz
Calculate the following:
I) shot noise
II) thermal noise current
mW/cm2 at responsivity of 0.32 A/W
Ans: 4 Na , 9.1e-13, 215.9 dB
III) SNR
IV) Photocurrent at 0.61
6- An APD has a gain of 50, an unamplified bulk dark current of 10 nA and a surface
dark current of 1 nA. the unamplified responsivity is 0.5 A/W.
The excess noise figure is M0.4. Operating into a 100  load resistor at a noise
tempeture of 27o C. The noise bandwidth is 10 MHz. Calculate the SNR of the APD
when irradiated with 10 nW of light.
7- 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.