Communications Technology Lab 6: Fibre-Optics Communications

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Corey Hubbard
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1 Communications Technology Lab 6: Fibre-Optics Communications Your report for this lab is to be a description of what was done. An optical fibre is a strand of glass or plastic with special optical properties that enable light to travel a large distance down its length. By converting electrical signals into light at a transmitter, sending this light down a length of optical fibre, and reconstituting the electrical signals in a receiver, a communications link is formed. We will use the Fibre-Optics Educator set to see fibre-optics communications at work. See the last page of your handout for a Block Diagram of a Fibre-Optics Transmission System. Transmitter: The Coder converts the original electrical signal into a format suitable for transmission. The Driver circuit performs current amplification as well as controlling the optical output. The Driver feeds the emitting device. This device can be an LED or a laser. The light signal is then launched into the fibre. Along the route there may be a number of Repeaters to boost the optical signal. Receiver: After the signal has passed through the optical cable, it is detected by a receiving diode and then amplified and decoded to regenerate the original signal.

2 Part 1 Optical Cables - Point one end of the fibre-optic cable to a light source, such as room lighting or window light. Notice that the light passes through the cable and can be seen at the other end. - Pass your finger repeatedly across the end of the fibre where the light is entering and view the interrupted beam coming from the fibre. - Turn on the torch and place the bulb close to one end of the cable. The light coming from the fibre is yellow. This is because the fibre absorbs some colours more than others and the yellow light is absorbed least. Part 2 - Analog Transmission When the Educator is used in Analog mode, the output light intensity at the transmitter is directly proportional to the input voltage signal. Part Radio Signal Over 'Free Space' - Switch on both the transmitter and the receiver and switch both to analog - Turn on the FM radio and tune in to a clear signal. - Using the lead provided, connect the earphone output socket of the radio to the 'low Z' socket of the transmitter. - Set the transmitter analog gain control to minimum. Put the transmitter output power onto maximum - Turn on the loudspeaker/low Z switch - Place the receiver so that the receive diode scoket is facing the emitting diode socket of the transmitter. - Adjust the analog gain of the receiver until an adequate output is heard from the loudspeaker. If the sound is distorted, then turn down the radio's volume control until the distortion disappears. - The receiver and transmitter units may be separated by a few metres while still maintaining transmission. - In this demonstration the signal being transmitted comes mainly from the infra-red LED. This can be shown by placing a finger over the high radiance red diode and noting that the level of the received signal is hardly affected. - To show that infra-red radiation behaves in a very similar way to visible light, place the transmitter and receiver at right-angles and position a mirror to

3 reflect the radiation into the receiving diode when sound will again be produced at the loudspeaker. Part Radio Signal Over Optical Fibre - Connect the radio to the transmitter and set up the signal levels as in part Take a length of optical cable and connect it into the sockets at both the transmitter and the receiver. - Reduce the output power of the transmitter until the received signal sounds clear and undistorted. Then adjust the analog gain of the receiver for the most suitable loudspeaker sound level. - Demonstrate the fact that the audio signal is really passing down the fibre by placing the transmitter and receiver at right angles to each other and noting that the audio signal is still received clearly. - Once again, use a mirror to reflect the radiation. Connect one optical cable to the transmitter and another to the receiver. Position a mirror to reflect the radiation from the transmitter cable into the receiver cable when sound will again be produced at the loudspeaker. Part Using the Output Indicator as the Transmitting Device - The output indicator diode at the transmitter emits the same signal as the main emitting diodes although at a lower intensity. - This can be demonstrated by setting up the transmitter and receiver for analog transmission of the radio signal. - Turn the transmitter output power to maximum and the receiver analog gain to maximum. - Connect a length of fibre to the receiver input socket and position the other end of this fibre close to the transmitter output indicator. The radio signal will be heard at the loudspeaker. Part Voice Signal Over System - Parts 2.1 and 2.2 can be carried out using the microphone connected into the 'high Z' socket of the transmitter with the radio disconnected.

4 - In order to prevent the high-pitched whistle caused by feedback, separate the loudspeaker and microphone by as much as the cables will allow. Reduce the analog gain in the receiver and/or the transmitter's output power until the whistling stops. The transmitter analog gain should be at maximum. Part 'Listening to Light' - Turn the loudspeaker on, switch the receiver to analog and turn its analog gain up to maximum. - Connect an optical cable to the receiver and point the other end in the direction of an electric light. - The 100Hz mains 'hum' will be heard at the loudspeaker. This effect shows that the light from a light source that is run from the mains supply is in fact varying in intensity at 100Hz, a frequency too fast for the eye to detect. - The frequency is 100Hz rather than the mains frequency of 50Hz since the power output of a mains source is related to the square of the voltage, which has a frequency of 100Hz. - Another example of 'listening' to light is the 'hiss' that torch light or daylight produces. - If the torch is knocked, a high-pitched sound is heard at the loudspeaker, this is caused by the torch bulb's filament vibrating at a high frequency. Part 3 - Digital Transmission When the Educator is used in the digital mode, the optical output of the transmitter is either ON or OFF, depending on the state of the input voltage signal. Part Communications Using Internal Digital Signal Generator - Set the transmitter to digital operation, turn the rotary switch to Pseudo- Random Signal and turn down the Signal Generator Frequency control to a minimum. Set the output power to maximum. - At the receiver, set the analog/digital switch to digital, put the digital threshold sensitivity control at a midway position and turn the buzzer switch on. - Connect an optical cable between the transmitter and the receiver.

THE FIBRE-OPTICS EDUCATOR MANUAL

THE FIBRE-OPTICS EDUCATOR MANUAL The FIBRE-OPTICS EDUCATOR contains the following items: FIBRE-OPTICS EDUCATOR TRANSMITTER UNIT FIBRE-OPTICS EDUCATOR RECEIVER UNIT 5m. LENGTH OF POLYMER OPTICAL CABLE 1