Download Optical Fibre Communications Tutorial on Optical Transmitter and

Optical Fibre Communications
Tutorial on Optical Transmitter and Receiver
DR2
1
The minority carrier recombination life time for an LED is 5 ns. When a constant dc drive
current is applied to the device the output power is 350 µW. Determine the optical output
power when the device is modulated with an rms drive current corresponding to the dc
drive current at frequencies (a) 22 MHz; and (b) 98 MHz. Further, determine the 3 dB
optical bandwidth for the device. Briefly outline the advantages and drawbacks of the LED
in comparison with laser diode for use as a source in optical fibre communication.
2
When 3 x 1011 photons each with a wavelength of 0.85 µm are incident on a photodiode,
an average 1.2 x 1011 electrons are collected at the terminals of the device. Determine:(i)
the quantum efficiency;
(ii)
the responsivity;
(iii)
the incident optical power required to obtain a photocurrent of 2.9 µA.
3
A pin photodiode has a quantum efficiency of 50% at a wavelength of 0.9 µm.
Calculate:
(i)
its responsivity;
(ii)
the received optical power of the mean photocurrent is 10-6 A;
(iii)
the corresponding number of received photons at this wavelength.
4
An analogue optical fibre system operating at a wavelength of 0.85 µm has a post-detection
bandwidth of 6 MHz. Assuming an ideal detector and considering only quantum noise on
the signal. Calculate the incident optical power necessary to achieve a signal-to-noise ratio
of 50 dB at the receiver output.
5
A silicon-p-i-n photodiode has a quantum efficiency of 65% at a wavelength of
0.8 µm.
(a)
the mean photocurrent when the detector is illuminated at a wavelength of
0.8 µm with 5 µW of optical power;
(b)
the rms quantum noise current in a post detection bandwidth of 20 MHz;
(c)
the SNR in dB when the mean photocurrent is the signal.
6
The photodiode in problem 5 has a capacitance of 8 pF. Calculate:
(a)
(b)
(c)
the minimum load resistance corresponding to a post-detection bandwidth of 20
MHz;
the rms thermal noise current in the above resistor at a temperature of 25°C;
the SNR in dB resulting from the illumination in problem 5 when the dark current in
the device is 1 nA.
7
A silicon p-i-n photodiode has a quantum efficiency of 65% at a wavelength of
0.8 µm, and a capacitance of 8 pF, which is used in a receiver where it drives an amplifier
with a noise figure of 2 dB and an input capacitance of 7 pF. Determine:(a)
(b)
the maximum amplifier input resistance to maintain a post detection bandwidth of 20
MHz without equalisation;
the minimum incident optical power required to give an SNR of 50 dB.
Prof. Z Ghassemlooy
win\c\fg\tutorial\optfibre\detrecr\dr2.doc