P1bL5 Fibre Optics: Digital Signals

Key Words

Angle of Incidence - the angle at which a light ray strikes a surface (measured from the normal line)

Critical Angle - the angle at which total internal reflection starts to occur.

Digital signal - an electrical signal consisting of a series of ON and OFF pulses.

Optical Fibre - fine strands of glass which transmit digital pulses as flashes of light.

Pulse - an electric current flowing for a short time.

Total internal reflection - light rays reflecting off the inner surface of a glass strand.

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Homework
Physics GCSE
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Grade E

Optical fibres are used extensively for transmission of data.  They consist of very fine strands of very pure glass.  They look like this:

Light bounces down the optical fibre like this:

Digital signals are sent as pulses of light down the optical fibre.  The advantages of digital over analogue are:

  • There is no interference;
  • There is no loss of signal quality or strength.

Therefore digital signals give good quality pictures and sound.  CDs are no longer just a music medium; they can carry a lot of data (700 MB for a CD).  They have now been overtaken by DVD (digital versatile disc) which can hold 10 GB of data.

Digital signals (pulses of ON and OFF) can be compared to analogue signals (a continuous wave).

                                                                                   

Grade C

Total Internal Reflection occurs when light in glass hits a boundary with air.  It's all to do with refraction.  You will have done refraction before.  Here is a picture to remind you of what it's about.

Note that the angle of incidence is measured from the normal line (a line at 90 o to the surface).

When a ray passes from glass into air, it bends away from the normal.  Look at what happens to the emergent ray in the picture above.  Now look at the picture below (ignore the strange symbols on the diagram):

Notice that as the angle of incidence gets bigger, the angle of refraction gets even bigger.  In picture C, the angle of refraction is 90o and the angle of incidence is the critical angle.  Above the critical angle we have total internal reflection  and the angle of incidence = angle of reflection.  For glass, the critical angle is about 42o.

Optical fibres are used in endoscopes, which doctors use to examine inside a patient.

In optical fibre cables, the signals travel at about 200 000 000 m/s.  The pulses are produced by an infra red laser, since glass is particularly transparent to infra red light.  Although the glass is very pure, the signals are reduced, so every 10 km or so there is a booster, to bring the signal back to its previous level.

The advantages of optical fibres are many:

  • Cables are much lighter;

  • Many thousands of signals can be sent at the same time;

  • The cables don't get hot and burn out;

  • They are cheaper, too.

Grade A

Digital signals are, of course, used a lot in computers.  Manufacturers have developed all sorts of other digital applications, such as the digital camera.  These were incredibly expensive at first, but have come down considerably in price.  The advantages are:

  • If you take a bad picture, the frame can be instantly deleted;

  • The memory card can hold hundreds of pictures;

  • It is easy to transfer them over to the computer;

  • There is a wide range of software to enhance pictures.

Data are stored on chips on bank cards (Chip 'n PIN).  Similarly biometric data such as iris patterns (as unique as fingerprints) can be stored in a digital passport.  The idea is to reduce the risk of false passports.  The idea is controversial.