Optical Fibre Communications Exam Questions

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a) What are the energies in electron volts of light at the first, second and third transmission windows that are used in optical fibre communications?

b) An optical signal after propagating through a long length of single mode fibre is amplified using an optical amplifier before being detected by a typical photodetector at the receiver end. The re-generated electrical signal at the output of the photodetector is composed of the signal and noise currents. State what they are and describe how they are generated.

c) A wave is given by y(t) = 8cos(2t – 0.8z), where the propagation constant is defined in (µm)-1. Find:

(i) The amplitude

(ii) The wavelength

(iii) The angular frequency

(iv) The displacement at t = 0, and z = 4 µm.

(i) Calculate the numerical aperture of a step index fibre having core and cladding refractive indices of 1.48 and 1.46, respectively.

(ii) What is the maximum entrance angle for the fibre in question Q1d(i) if the outer medium is air?

a) In optical fibre communications, three transmission windows have been used since its deployment in 1970s

(i) Identify these transmission windows and state two advantages and two disadvantages for each.

(ii) State the typical light source that are used in each transmission windows and why.

(iii) State which band offer the most in terms of implementing wavelength division multiplexing.

b) A certain single mode optical fibre has an attenuation of 0.3 dB/km at the wavelength of 1550 nm, which is used for data communications. At the transmitter, a laser diode is used to launch an optical signal with a power of 100 µm into the fibre.

(i) Determine the power level in µW at the output of a 20 km long single mode optical fibre.

(ii) The fibre in Q2B(i) is replaced with a multimode optical fibre, which has a typical loss of 2.5 dB/km. Determine the minimum input power level in mW that must be launched into the fibre to maintain the same optical power level achieved in Q2B(i) at the receiving end.

a) Energies of Light in Optical Fibre Communications

c) In optical fibre communication systems, the most widely used detector at the receiver is the positive-intrinsic-negative (PIN)-photodetector. The output of the photodetector is converted into an electrical signal using a voltage amplifier.

(i) Outline five key features of the PIN photodetector including the relationship between the optical power and the photocurrent.

(ii) Determine the receiver bandwidth given that the photodetector has a capacitance of 3 pF, the amplifier has a capacitance and an input resistance of 4 pF and 1 MW, respectively and the load resistor is 1 kW. Is this bandwidth sufficient enough for 40 Mbps data transmission?

a) An engineer wants to construct a 5 km optical fibre communications link operation at a data rate of 20 Mbps using the following components:

• A GaAlAs laser diode at 850 nm that can couple 1mW of power into a fibre.

• Ten sections of optical fibre cables each of 500 m long with an attenuation of 4 dB/km and with connectors on both ends.

• A connector loss of 2 dB/connector.

• A PIN photodetector at the receiver with a sensitivity of -45 dBm.

• An avalanche photodetector (APD) at the receiver with a sensitivity of -56 dBm.

Determine which optical receiver should be used if a 6 dB system safety margin is required.

b) Consider a typical optical receiver composed of a PIN photodetector with a responsivity of 0.6 A/W and a dark current of 2 nA. The other system parameters are:

• Received optical power of 22.5 µW.

• The load resistance of 2 kW.

• An overall system bandwidth is 50 MHz.

• The temperature of 40o C.

Determine:

(i) The shot noise

(ii) The thermal noise

(iii) The power signal-to-noise ratio in dB

c) Using a diagram compare the optical power vs. the current characteristics of a laser diode and a light emitting diode. Comment on if a laser diode can be operated as a LED.

An indoor visible light communications system consists of a LED installed on the middle of the ceiling of the room and an avalanche photodetector-based receiver moving on the room floor. It is designed that the system’s receiver sensitivity is -24dBm at the receiving plane. The distance between the transmitter and the receiver is 1.5m and the receiving coverage area has its diameter of 0.5m. Assume that the received power is uniformly distributed, and the receiver collection area is 30mm2

a) Show the system block diagram and discuss on the functions of each block.

b) Calculate the transmitter full field-of view (FOV), and the required transmit power.

c) A lens is used for LED to increase the transmitter FOV, hence increasing the coverage area’s diameter to 1m. Recalculate the new transmitter full FOV and the new required transmit power.

Comment on the relation of FOV and transmit power.

d) From (b), calculate the new transmit power if the receiving coverage area is double of the one in (b)?

e) If the ambient noise power at the receiver is 40 nW, what is the received Signal-to-Noise ratio (SNR) when the receiver operates at the receiving sensitivity point.

An ultrahigh speed line-of-sight free space optical communications system consists of

- Transmitter with a laser having transmitted power of 10mW and a very narrow full-angle beam divergence α of 0.2º. Data is transmitted at the data rate Rb of 10 Gbit/s.

- The atmospheric link has a range L of 1.2 km. In clear weather the attenuation coefficient of the atmospheric channel is 0.4 km-1. The link also experiences the foggy conditions, which follows the Table Q5.

- The receiver has a concentration lens with a radius r = 30 cm to focus the light to the photodetector. The receiver has a sensitivity of -27 dBm. The receiver noise spectral power density is 0.05 fW/Hz. The bandwidth of the receiver’s electrical low-pass filter is BW = 0.75*Rb.

a) To model the communications in clear weather, determine the followings:

- The link atmospheric loss due to atmosphere

- The link geometry loss

- Link margin

- Actual received power

From the above data, form a link budget table.

b) The link now operates under light fog and moderate fog conditions. Calculate the atmospheric power losses due to these two conditions. Check with your above link budget to determine whether the FSO system have enough link power margin in these conditions? Comment on the results and suggest the solution if the link does not work.

c) Calculate the noise power presenting at the receiver and the received Signal-to-Noise ratio (SNR) under clear weather and light fog conditions. Comment on the results.

d) Given that the link has a good link margin in clear weather condition, calculate the maximum value of the angle α at which the link still works properly?