Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Communication through cables and optical fibres.

  1. Oct 28, 2009 #1
    I'm in the first year of my engineering course and I've only completed the first four months of the course. As you can imagine,I don't have a very good grasp over my subjects yet,but I'm trying really hard!
    In our physics syllabus,we have a few things about communication through coaxial cables,optical fibres etc. about which whenever I look up in library books I find things which are beyond my level.Please give me a basic explanation of certain things I really must have a good idea about,I don't need all the mathematical descriptions-just the intuitive idea.
    Please bear with me while I cover a very short portion of my course and tolerate my dumb questions!

    Firstly,I have trouble understanding how pictures,movies and sounds can be represented as electrical signals.Then,in fibre optics,they are represented as light waves and certain systems like 'decoding' and 'encoding' systems are responsible this interconversion--what is basically happening in these systems?

    Also,what does 'digitization' of information mean? How can light waves carry information in the form of 'on-off pulses' called bits and bytes? (Aren't bits and bytes used in computers??)

    Finally, antennae are responsible for generating waves-- after transferring the waves into the cable,does the electrical wave propgate through the oscillation of electrons of the metal or the oscillations of the atoms in the metal?

    Please help me out! As you can see, I lack a basic undersanding of electrical communication--I'm in deep waters!
  2. jcsd
  3. Oct 28, 2009 #2


    User Avatar
    Science Advisor

    I won't answer all your questions, but I can maybe start you off on your way. At your level (even as an Electrical Engineer) you don't need to understand really well, or at anything other than a "is used in" level many of the concepts you touch upon. And the breadth of what you're asking is probably why you're having troubles finding anything; you'll find articles, books, and How It Works on some of the individual elements, but not on everything all together.

    In any case, let's start with sound waves. What we perceive to be sound are pressure waves that travel through the air. When they hit your eardrum, the sound causes it to vibrate in a way which makes some mechanical components move about, and which eventually get converted into electrical signals (nerve impulses) which go to your brain. Please don't ask me to elaborate upon this point, as this is a whole other discussion (the finer points of which I really don't know--look up the Wikipedia article on hearing probably as a starting point).

    So, if you have a vibrating membrane, you get sound. Now, imagine you have an electromagnet that's pulling a piece of paper (let's call it a membrane) with a chunk of iron on it. If you put a high voltage on the magnet, it'll pull the piece of paper back pretty far. If you put a low voltage on the magnet, it won't pull it back as far. Now if turn it on and off very rapidly, it causes the membrane to vibrate, which starts displacing air, and, in doing so, makes sound. Congratulations, you have a speaker.

    I mention that if you have high voltage, the electromagnet pulls the membrane back far, while if you have low voltage, it doesn't pull it back as far. The amount of voltage you can apply to your speaker is analog; it can accept any voltage value within its limits. If you apply a digital signal, it still works. Instead of applying, say, 10.354V, you can only apply 1 to 12V in 1 V increments. The fact that you apply discrete set voltages (instead of having a continuous range of voltages which you can apply) means that you're applying a digital signal.

    Now where do you get an electrical signal (digital or analog) from in the first place? Your ear does it pretty well, converting pressure waves into electrical signals for your brain. You can't really harvest ears, but you can use a speaker in reverse. Sound hitting the membrane will cause it to vibrate, will cause the iron piece at the back to move inside the coil, will cause an electrical signal to form. That's a (type of) microphone.

    Your quick and dirty, very (very) simplified response to what is sound, and how do we humans capture and make it electrically.
  4. Oct 28, 2009 #3
    Suppose we can adequately represent the color of a single pixel in a flat panel display as various combinations of a red, green, and blue LED. Suppose also that the brightness of each LED can be on an intensity scale of 0 to 1024 (called 10-bit because 210 = 1024). So by communicating three 10-bit words containg the three color intensities is sufficient to send this information (plus parity bits, start bits, stop bits).
    The conversion of the analog color values to digital values is done by an ADC (analog to digital converter) like in digital cameras.
    Bob S
  5. Oct 29, 2009 #4
    Thanks MATLABdude and Bob S.
    I do realise that my questions cover a very broad area of study,which makes answering them quite tiresome.I have read through the links you gave me and they have certainly helped a lot,but even after that I can't understand how light waves can be used to transmit sounds,since we can't have electromagnets or piezoelectric devices to respond to light waves.

    Also,in regard to propagation through antennae, I found a basic explanation of it in http://wiki.answers.com/Q/How_does_antenna_work ,but I have not had my doubts cleared.

    In regard to propagation of electromagnetic waves through coaxial cables,or optical fibres, I have a big problem in understanding modes,which infact we have in detail in our syllabus. Referring to http://www.tpub.com/neets/tm/106-10.htm , it says that there are 4 modes in which electromagnetic waves can be transmitted-TEM,TE,TM and hybrid--how can waves be transmitted when the waves in the transverse modes run along the diameter of the cable? How are these modes qualitatively different?

    Lastly, in optical fibres, light travels by total internal reflection--microwaves are also electromagnetic waves--can we say that even microwaves travel by total internal reflection?

    Please don't get frustrated by my endless questions,please just give me basic explanations wherever possible,I'll carry on trying to find more information on the net in the meantime.
  6. Oct 29, 2009 #5
    The current in the circuit is totally driven by the potential difference caused when the electromagnet in the diaphragm accelerates through the coil, so it is alternating, and contains all the information about the sound. If these electrons are oscillated up and down an antenna, they will radiate electromagnetic waves at the frequency of their oscillation, so the waves will in some way represent the pressure field at the diaphragm. I think this is correct...
  7. Oct 30, 2009 #6
    So I may conclude that it's the electrons in the antenna that oscillate to produce waves and not the atoms of the metal (which would cause the vibration of an air column around them,producing sound waves).

    Also, this variation in the electromagnetic field caused by the electromagnet cannot be caused by light waves,so my question remains as to what allows light waves to porpagate audio-visual signals.

    Please could someone also elaborate on modes of waves,I've already read a few pages on them (one page,I've mentioned about in my previous post),but I still can't get things clear.
  8. Oct 30, 2009 #7
    Yes, you can definitely conclude that- the atoms do vibrate due to thermal excitations (in fact you can call these sound waves in a solid), but that is not what carries the alternating current- it is the free electrons of the conductors that flow back and forward radiating EM waves in the antenna.

    Why not?

    audio -------(microphone electromagnet)---> current ---(antenna)---> EM waves ----(areal)---> current -----(loudspeaker electromagnet)----> audio waves
  9. Oct 30, 2009 #8
    Thanks for confirming that I've finally got it right, MikeyW.

    Also,I think I have finally understood how light waves can propagate audio-visual signals--the vibrating membrane (vibration caused due to pressure of sound waves) produce electromagnetic waves (at a particular wavelength,the electromagnetic waves may be light waves) and these light waves cause vibrations of the electrons in the recieving antenna,which again cause a sound-producing membrane to vibrate--finally reproducing the sound. Is that right?

    If that's okay,I can perhaps bring a few more points into consideration....most importantly,I want to repeat my question as to the basic difference between the method of propagation of light waves and microwaves....don't they both travel by total internal reflection?

    Also,what is the basic difference between wave guides and other cables,why are optical fibres smaller in dimensions than a coaxial cable, since the dimensions of a wave-carrier are fixed by the waves travelling within them—since wave guides and optical fibres the same kind of waves,shouln't their dimensions be alike?

    Finally,what enables optical fibres carry more information at a time than waveguides or coaxial cables--is it that we can have many light waves travelling through an optical fibre at a time,and why do the losses become higher at higher frequency of the wave?
    Last edited: Oct 30, 2009
  10. Oct 31, 2009 #9
    Can someone please come back to me,here!! I desperately need some help,else I'll have to cram everything in my book before the exams!!
  11. Nov 1, 2009 #10


    User Avatar
    Science Advisor

    Unfortunately, I think you're chasing down the individual trees, and not seeing the forest as a whole. Perhaps my experience is outmoded, or perhaps you have a program which is radically different from the way my introductory courses were scheduled, but the intro courses were there to develop the intuition that you took later on, and eventually said, "Ah ha!" to in an Epiphanic moment where everything suddenly makes sense once all the background pieces fit into place.

    That said, it sounds as if you perhaps skipped some lectures or missed some notes / discussion and now no longer know what it is that you should know, nor what to focus on in your suddenly-very-large textbook. If this is indeed the case, I would suggest doing both of the following:
    1) Consult the course syllabus
    2) Send a polite, non-panicky well-phrased e-mail to your professor / TA asking where in the syllabus you are now at (EDIT: and where the exam cut-off point might be), and if you might be able to ask them to clarify one or two things (and actually make it one or two things; don't swamp them with everything in the course)

    ...Unless this is one of those survey everything (but nothing in depth) type courses. In which case, I quote a History teacher of mine from high school:
    "First, I took summary notes on what I'd read, and what was given in class. Then, every week, I'd take summaries of the notes. Then, at the end, I'd summarize the summaries, and study them and the first-order summaries!"

    Regarding one or two of your most recent set of questions; an optical fiber is a type of waveguide: one for light. Typically, when you have audio that travels over light, it's possible to modulate very high frequency noise with the low-frequency audio (my friends made one for their final year electronics project), but it's typically digitized first. It's a whole lot easier to send a 1 (burst of light) and 0 (no light) versus analog light levels.

    Consider visible light (electromagnetic waves of optical frequency). Their wavelength is a few hundred nm (let's assume 600 nm--orangish red). Using the formula speed of light = wavelength * frequency, this works out to 5 x 10^14 Hz, 500 THz! In contrast, most coax (of the electrical variety) is usually rated to carry only a few gigahertz (10^9 Hz) The difference is enormous!
  12. Nov 2, 2009 #11
    Well, I know I have a lot to learn,and things will get clearer as I learn more,but some things,like those I asked in post no. 8 seem to me to be very basic,which I could understand to some extent even at this stage.

    Also,reading the last part of your last post, MATLABdude,it suddenly occurred to me that when the pressure oscillations in the air due to audio waves make an electromagnet move,producing electromagnetic waves,the frequency of the generated electromagnetic waves should match the audio waves.......but then we should get different frequencies of electromagnetic waves being produced every moment (to match with the audio waves)!!

    Also,pleeeeeeease briefly answer my most recent questions,I just cannot stop pondering on them,I'll try to understand as much as I can.
  13. Nov 3, 2009 #12
    P.S My problem is intensified by the fact that my teacher is very vague and doesn't seem to be too sure of certain concepts himself.
  14. Nov 3, 2009 #13


    User Avatar
    Science Advisor

    To which of your questions do you refer? You asked a whole lot of them...

    Also, we're an internet forum. We cannot cover in a few paragraphs what you've covered in weeks and dozens of textbook pages (although we can supplement it).
  15. Nov 4, 2009 #14
    Well,you,know,I'm asking these questions from only one chapter in our entire 5-chapter syllabus in physics and our teacher spent perhaps a maximum of 5 lectures on the chapter I'm referring to,which means he spent 2 lectures on the portion including wave-guides upto optical fibres. Again,our physics book is a really quite a thin book,and the chapter in concern was only for 30 pages.

    Besides, my entire paragraph contains only 3 questions,really-I'll just enumerate them....

    1. What is the basic difference between the method of propagation of light waves and microwaves....don't they both travel by total internal reflection?

    2.What is the basic difference between wave guides and other cables,

    (meaning why are optical fibres smaller in dimensions than a coaxial cable, since the dimensions of a wave-carrier are fixed by the waves travelling within them—since wave guides and optical fibres carry the same kind of waves,shouln't their dimensions be alike?)

    3.What enables optical fibres to carry more information at a time than waveguides or coaxial cables
    (my guess is that it is because we can have many light waves travelling through an optical fibre at a time.)
    Last edited: Nov 4, 2009
  16. Nov 4, 2009 #15


    User Avatar
    Science Advisor

    1. Context-free, both microwaves and optical light are electromagnetic radiation, which will just propagate outwards from a (non-directional) source, like ripples in a pond when you throw in a pebble (except that there'd be 3D ripples). Even when you have a laser, and really good lenses, over a great enough distance, the light will still diverge and spread out laterally (goes from being a narrow point to a very big spot).

    In order to get very long distance transmission from point-to-point (without huge amounts of broadcasting power), you need to confine the energy. Hence, the need for things like optical fibers (for light), or coax cable (for microwave frequency stuff). In a piece of fiber optics, it is total internal reflection (light bounces from side to side without losing energy), whereas for coax, it's control of the radiation.

    If you had a regular piece of wire (like, say, what your electricity comes in on), any AC you put into it will radiate outwards to some extent (i.e. it'll act as an antenna, which is why you can pick up 50 or 60 Hz nearly everywhere). With low frequencies, most of the power from the source still gets to the destination, and very little of it gets lost by radiation (way more of it gets lost through Ohm's law). When you send very high frequency AC through that same line, almost all of it will be radiated away before it gets to its destination. In coax, you still get radiation from the centre conductor, however the radio waves that come off of it are confined to the space between the centre conductor and the outer conductor. At the frequencies its designed to work with, the signal mostly stays put and goes down the centre conductor, and so I may be misrecollecting here, but coax is not designed to be a wave guide.

    2. An optical fiber *is* a wave guide! And no, coax is not a wave guide, IIRC (see above). As a hand-wavy argument about why there's a size difference between the conductors, the wavelength of light is a few hundred nm. The wavelength of something with a frequency of, say 1 GHz, is 30 cm.

    3. Optical fiber *is* a wave guide! Frequency of example coax: 1 GHz. Frequency of light, a few hundred THz (see my first posting). You can modulate (make small changes to the basic plain-jane sinusoid, which is the only way you can convey any information) one much faster than the other.

    Clearly, this is a very high-level, not so in-depth introductory survey course. Wikipedia cannot form the basis of your knowledge, but it can give you an intro, and I suggest reading a few of the articles, which should help to reinforce what you've already learned in class:
  17. Nov 5, 2009 #16
    I will try to provide a general summary of what you appear to be asking, I will try and put it in basic language because this topic can get complex quite quickly.

    Optic fibre - Think of it as a tube that carries light, at one end is a light source (laser or LED) and at the other and is an electronic device that is sensitive to light, ie the light from the lazer cause it to output a voltage or something similar that can be interpreted by a processor.

    A Coax cable works just like a normal electrical cable, the shielding allows it to operate at the frequencies generally used.

    A waveguide works in a similar method to an optic fibre, however the EM radiation is reflected by conductive walls, not dielectric total internal reflection. Remember light is EM radiation just a much shorter wavelength.

    Digitization is taking analog data such as a voltage and turning it into a binary representation a computer can understand. If you use 8 bits, which equals a byte incidently, you can represent that voltage as one of 256 (2^8 or 00000000 to 11111111 in binary) values.

    You asked about modes of transmission, I'll assume you just mean what TE, TM, TEM and hybrid actually mean in english. Look up transverse modes in wikipedia, you'll see the basic definitions. You'll also see that the maths can get complex if you go much further.

    Putting it all together, to transmit information about a sound or picture etc, the information must be sampled. ie the sound level at point t, equals x. This value of x is digitized to a binary value. Lets say it's 8bits of data, say 100. This means the value is 100/256 in terms of a proportion of the maximum possible value. In binary this is 01100100. This is basically encoding. putting it back together in a usable form at the other end is decoding.

    If this were being sent by an optic fibre somewhere, it would turn the laser on and off in the sequence of the binary number.

    If it were just a digital copper type transmission, a voltage would be applied or removed according to the sequence.

    It gets a little more complicated when it comes to antenna, coax and waveguides because the signal sent via these mediums is usually modulated by the digital data. It will transmit an audio signal using EM radiation at 100's to 1000's MHz, which is far beyond what we hear, but variations to the base signal can be detected by decoding hardware at the other end. So you can alter aspect of the base signal such as amplitude, frequence of phase in line with you digital data.

    I hope this clears some things up, but I think you need a better grasp of the underlying concepts in your questions if you want to do well.
  18. Nov 5, 2009 #17
    Thanks for the replies.

    Please forgive me,but its very late at night here,I'll look into your posts tomorrow first thing after college.
  19. Nov 6, 2009 #18
    Is it that no other electromagnetic wave (except light waves) can undergo total internal reflection,or is it that waveguides aren't suitable for total internal refelction?

    Uptill what I know allready, an electromagnetic wave consists of an electric and magnetic field,both at right angles to eachother and to the direction of their propagation....after reading about TE,TM,TEM,I can't picturise how an electric field can propagate along the fibre while the magnetic field of the same wave can propagate in a different direction.....morover,what is the benefit of such a strange way of light propagation?

    Does this mean that the amplitude of the propagating wave is measured by plotting it as an analog signal on a graph,and accordingly thevalue of the amplitude in terms of bits (like 100) is taken for subsequent analysis?

    Well,in regard to this,if we have a sound of varying pitch or intensity,to interpret this sound into an electromagnetic wave,we should get different pitches and intensities every moment and this would require different electromagnetic waves everymoment!
  20. Nov 6, 2009 #19
    Is this radiation due to the fact that electromagnetic waves travel shorter distances while moving from one side of the optical fiber to the other when the frequency is high,resulting in lower angle of incidence at the side of the fibre,and hence more penetrating power?

    Why do we need the diameter of the optical fibre to be close to the wavelength of the propagating light--afterall,its total internal reflection that's going on,which can occur even if the dimensions of the fiber and the wavelength are not similar.
  21. Nov 12, 2009 #20
    I've waited patiently for a long time. Snoogans,could you please look into the references I drew to your last post.
  22. Nov 13, 2009 #21
    waveguide aren't suitable for TIR, they operate on a dielectric-conductor boundary. Optic fibre operates on a dielectric-dielectric boundary with differing index of refraction.

    I don't know how to explain the other points any further, i suggest you find someone you can talk to face to face. Forums are difficult when the questions are as varied as yours.
  23. Nov 15, 2009 #22
    Well,thanks anyway. I do realise that my questions cover a large portion,which is too large to contain in a forum.

    The problem is that I'm in the very beginning of my mechanical engineering course,and I have been very intrigued by this topic,which is covered very biefly in my physics book. I just hope that I will get an opportunity to solve all my problems in future,if I get a chance to read the subject in greater depth.I hope I don't have to leave my questions unanswered even after I pass out as an engineer!

    Still,if anyone can contribute,please do.
    Last edited: Nov 15, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook