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Optical Fibre Transmission Laboratory
1 Introduction
1.2 Background
This laboratory is designed to demonstrate practically a few of the things that have been discussed in lectures. In particular, it concerns the characteristics of optical fibre systems and the determination of the limitations in their performance.
The basic measurements carried out in the laboratory form the basis of calculations that allow estimation of performance. Given the constraints of academic budgets, the equipment operates in the 800nm wavelength window with graded index fibre rather than in the commercial communications windows around 1300nm and 1550nm using single mode fibre.
1.3 Equipment
• HFBR-1414 transmitter LED module.
• Graded index 62.5ìm core diameter silica multimode fibres (NA 0.275 and effective group refractive index 1.497).
• 1m graded index 62.5ìm core diameter silica multimode fibre patchcords.
• Silicon pin photodiode type HFD3065-002/XBA.
• Waveform generator.
• Two channel laboratory oscilloscope.
• Resistors.
• Laboratory power supply.
Page 1 of 5 OPTICAL COMMUNICATION SYSTEMS
The University of Warwick
2 Transmitter Design
The transmitter and receiver are mounted on a printed circuit board, so that the choice to be made is the value of the resistors to be used. The transmitter circuit is shown in Figure 1 below. Refer to the datasheet for the transmitter module and choose as suitable resistor. R13 HFBR-14122,6,7
Figure 1: LED Module Driver Circuit
3 Receiver Design
The level of power transmitted through the fibres in this laboratory is great enough for a resistive load to suffice, as shown in Figure 2. Empirically determine a suitable load resistance value and connect it. RLHFD3065-002/XBA +5V Vo C1C2
Figure 2: Simple Resistive Load Receiver Page 2 of 5 OPTICAL COMMUNICATION SYSTEMS
The University of Warwick
4 Measurements
4.2 System Test
Ensure that your system is working by sending a square wave (from the 50Ù signal generator output) with a frequency of a few kHz, making any adjustments necessary before proceeding to the next part.
4.3 Attenuation and Delay with Distance
Record the maximum amplitude of received signal, and its delay relative to the input signal using the 1m patchcord. Repeat these measurements for the two fibre reels.
4.4 Estimation of Receiver Noise
Estimate the rms receiver noise amplitude by examining the receiver output for no incident light.
Practical Note: For noise estimation purposes the rms noise voltage is very approximately one fifth of the amplitude of bright band noise observed on an oscilloscope trace.
Page 3 of 5 OPTICAL COMMUNICATION SYSTEMS
The University of Warwick
Questions and Points for Discussion in the Write-Up
The weightings of the various elements are shown in brackets. There is a difference in assignment weightings between the MSc module and the 4th Year module.
MSc students should produce a report addressing all questions.
Fourth Year MEng students should produce concise answers to parts 2, 3 and 5.
1. Discuss the design of your circuits, explaining why you chose the particular resistor values used and the purpose of the capacitors C1 and C2. [10%]
2. Determine the length of the two optical fibres [10%]
3. Find the attenuation per km of the optical fibre reels. [10%]
4. Estimate the received power at your receiver. [10%]
5. Using the noise level results and the fibre attenuation per km measured earlier, calculate the attenuation limited distance for a communication system using the LED transmitter. [20%]
6. Describe the major sources of error in your experiment. [10%]
7. Discuss the differences expected between the basic system that you have constructed and commercial single-mode fibre systems operating in the 1300nm and 1550 nm windows. [30%]
Page 4 of 5 OPTICAL COMMUNICATION SYSTEMS
The University of Warwick
Appendix: Attenuation Limited Distance
For the fibres used in the laboratory a BER of 10-9 is used as the maximum acceptable. 91022erfc21.=……..SNR (A.1)
Solving this numerically (e.g. using MATLAB) we get a value of 12=SNR
The attenuation in the fibre, after a distance L and attenuation coefficient á, is
)exp(Lá. (A.2)
To maintain the required BER, the minimum received power, Pmin, must not fall below 12 times the rms noise voltage. Hence we can say that the maximum length is given by ……..=min10maxlog101PPLiná (A.3)
where Pin is the input power. Alternatively, since the optical power is proportional to the voltage out of the receiver ……..=min10maxlog101VVLiná (A.4)
where Vin is the voltage signal generated in the receiver when it is connected to the transmitter via the patchcord and Vmin is 12 times the rms noise voltage.
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