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laser communication experiment

Discussion in 'Mathematics and Physics' started by PG1995, Dec 21, 2013.

  1. rumpfy

    rumpfy Active Member

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    This is another of those difficult questions.
    I'm not certain exactly what is a 'single ray'. I tend to believe this is another of those 'simplifications' that is often used. In optics there are polarisers and collimators and these things work on the light 'ray'.
    The 'Numerical aperture' is a measure of the light spreading from the end of a fibre for both the light entering, and leaving, the fibre. The NA therefore allows 'rays of light' within a certain solid angle, to be coupled into the fibre.
     
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  2. steveB

    steveB Well-Known Member Most Helpful Member

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    No, it doesn't mean that. One ray with particular angles and position of entry on the face of the fiber end would be associated with one mode only.

    However, the "ray picture" is not a very good way to understand what modes really are.

    Crudely, rays with different angles will have different arrival times at the other end of the fiber. This can be thought of as the rays having a different propagation time (or propagation constant) which means the arrival time is smeared into a wide range at the other end, with different arrival times for each ray.

    Rays can enter in a way that puts them crossing the central axis, or they could enter in a way that makes them bounce at skewed angles keeping them far from the central axis. So, modes not only have different propagation constants, but they have different spatial distributions.

    The problem with the ray picture is that it predicts a continuum of propagation constants and an infinite number of modes, but in reality there is a finite number of modes and the propagation constants are quantized to particular values.
     
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  3. PG1995

    PG1995 Active Member

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    Thank you, rumpfy, Steve.

    I think I was wrongly picturing the modes. Is there some simple way of explaining or thinking of modes? steveB : Sorry, I can't see any explanation of modes in your post and Google also didn't help me. EDIT: This is from the book I'm using.

    What I conclude is that as diameter of the core increases, a fiber become multimode from single-mode or monomode.

    Multimode fibers have got many problem associated with them. The book says that in spite of all the issues, a multimode fibers still find wide applications in intermediate and short-distance networks. I don't find this a good reason to to use multimode fibers when single-mode fibers can eliminate all those problems. I think it has got to do with high cost and sophistication required to handle single-mode fibers with a very tiny diameter.

    Thank you.

    Regards
    PG

    Helpful links:
    1: single mode fiber
    2: multimode fiber
    3: http://www.multicominc.com/active/manufacturer/multicom/Fiber Optics/singlemode-multimode.html
    4: http://www.fiberoptics4sale.com/Merchant2/multimode-fiber.php
     

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    Last edited: Jan 17, 2014
  4. dave

    Dave New Member

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  5. steveB

    steveB Well-Known Member Most Helpful Member

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    I'm sure there are simple ways to help you visualize it. But, it would be easier at a blackboard. I 'll keep thinking of a way. For now, I'll try the idea of thinking of the modes on a drum. You have studied modes on a vibrating string, I think. You have a fundamental tone and higher harmonics that are allowed by the wave equation and boundary condition. A drum head is like a two dimensional vibration, if we minimize the damping and allow a tone to be produced.

    Look at the animation is the following link. It shows a mode of a drum vibration. Fiber modes are a lot like this in that there is a two dimensional field distribution in the cross section of the fiber.

    http://en.wikipedia.org/wiki/Vibrations_of_a_circular_membrane

    To truly understand the modes, one must solve the electromagnetic wave equation that results from Maxwell's equations. The following reference shows this.

    http://www.ece.msstate.edu/~winton/classes/ece4853/fibers/WeaklyGuidedFibers.pdf

    Yes, certainly. Also, the refractive index difference between the core and cladding, as well as the wavelength of light determine the modes. Even a single mode fiber will have many modes for visible light, and is only single mode for 1300 nm and 1550 nm operation.

    That is exactly it. In particular, think about coupling light into such a small core. It is impossible to couple LEDs efficiently into a single mode fiber. Only a laser mode can be coupled well, and even then only with special lenses and very precise alignment and stabilization. It is interesting that when single mode fiber was initially considered, critics laughed at the idea saying the coupling light from sources and coupling from fiber to fiber would be impossible. This is a good lesson because such critics always underestimate human ingenuity to solve the next problem. What people do now far surpasses what anyone thought would be possible. Lasers can be couples to single mode fiber with only a few percent loss. Fibers can be fusion spiced together will essentially no loss at all (for all practical purposes). Fiber to fiber mechanical couplers/connectors now have losses less than 0.1 dB and are super stable.

    Hence, single mode fiber is very practical and effective, however cost will naturally be higher to deal with the tolerance and alignment issues. This is why multimode fibers and plastic fibers still find use, even though they are not as advanced as the single mode technology. Engineering and manufacturing always consider the cost/complexity needed relative to the application.
     
    Last edited: Jan 17, 2014
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  6. rumpfy

    rumpfy Active Member

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    PG,
    Theres a few things in my mind coming out of steve's reply at post 22.
    I think your original question was ;" if a single very small diameter ray of light entered an optical fibre, how would that ray of light become converted to a multi mode ray of light bouncing off the core/cladding interface"; at least that was how I saw it. Hence I gave the answers I did.
    I said;"I am not exactly sure what is a 'single ray'.
    Steve said;"The 'ray picture' is not a very good way to understand what modes really are".
    I thought we were both saying 'lets be careful here'.
    Your followup queries about single mode and how useful it is, didn't get to the other point that for some applications, a large diameter core can couple significant light from say a LED or other 'unrefined' light source and provide a useful technical solution. For example, there is an international spec for a 4 air data cable with data transmission rates up to 100 Meg bits per second. This stuff is pretty useless really for transmission lengths longer than 45 meter and there is an alternative based on plastic fibres which does the job. Its about horse for courses; whats the best/whats the most economical. These questions are asked relentlessly and its why engineers are always known for 'always changing their mind'.
    Has the questions in your mind been answered to your satisfaction?
     
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  7. PG1995

    PG1995 Active Member

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    You have phrased the question I had in mind better than I could. Thanks. I won't say that I understand it completely but I believe for the present I'm satisfied with my little understanding. I will give it some time and then might return to the question later. At the moment I'm tight on time and final exams are just around the corner. Thank you.

    Regards
    PG
     
    Last edited: Jan 18, 2014
  8. rumpfy

    rumpfy Active Member

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    Good luck in your exams.
    pr.
     
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  9. trash

    trash Member

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    Sorry for the late reply, I've just picked up on this thread.
    No. This one is pretty simple. Photons don't interact with each other. You can't bounce photons off other photons.
    The saying is; You can't "see" light.

    Radio interference can come from many sources. They can be directly on the same frequency like another signal jamming. Or it can be on another frequency and for a number of reasons signals can mix (intermodulate) to produce a new signal which interferes. And then there is sources of noise. Both internal to the equipment and external from the rest of the universe.

    Rumpfy is pretty close to the money with regards to fibre. Laser diodes are what is used for fibre communications. They are monochromatic lasers, there's plenty of information on places like wiki with regards to how they work. Specific wavelengths are used because of the attenuation windows in the fibre.
    The laser diode is not actually switched off. The reason for this is that it takes longer for the laser to turn on rather than transition between power levels. So technically it's AM (Amplitude Modulated).
    Though it's not uncommon for it to be described as 'pulsed'. Most of the laser diodes I see are 0.5mW power. Though these days SFP's come in a few different ratings.
    But to correct rumpfy, the first fibre comms in Australia did use 144MBps. I installed a lot of them in the 80's. The 565MBps systems didn't start being installed until about 1990.
    These were PDH systems. SDH started to be installed in the late 90's. The first systems were 2.5GBps rings. Then 10GBps. I stopped working specifically with fibre in 2000.
    Now I see 100MBps line systems and DWDM. Where several wavelengths are used on the same fibre to increase the capacity. I haven't even stopped to look at the long haul submarine cables.
    I heard somebody say 1TBps a couple of years ago.
    Now there are developments with Frequency modulating the lasers and even phase shifting them to modulate QAM. I makes my head spin these days.

    The reason as rumpfy mentioned is the fibre windows. Glass is a very good attenuator of UV light. It's pretty hard to get sunburn through a closed window :)
    In Wave Division Multiplexing the lasers are said to be different colours, but they are actually all so close together in frequency that they would be much the same colour if they visible to the eye.
    So you might say the first laser is red, the second, redder and the third reddist.

    I can shed a bit of light on the difference between single mode and multimode fibres. It's not correct to think of them in terms of laser beams shinning down the fibres.
    It's best that you forget that lasers are even used. Think of it in terms of ordinary light. Though just a single colour for simplicity.
    Single mode fibres are almost exclusively used these days. Their construction is now cheap and their losses acceptable.
    Multimode fibres were cheaper to manufacture and have (had) lower losses. There problem is that the wave front smears causing optical dispersion.
    This puts a limit on how fast you can modulate the light.
    Single mode fibre doesn't suffer from this problem, well at least not at the same rate that multimode fibre does, and not at lower data rates.
    As data rates and distances are increased they become more of an issue with single mode fibres.

    A really good description is to think of fibres as pipes and light waves like ping pong balls.
    If the pipe is large enough (multimode) the balls fall down the pipe with almost no friction. Some bounce off the walls on the way down and the balls arrive at slightly different times at the bottom.
    A single mode fibre the balls can only travel single file. They arrive exactly as they left.
    If the fibre mode is too small, then obviously the balls of a lower frequency are too big to fit down the pipe. If they are a high frequency (small balls) then the pipe will multimode.

    ---

    When it comes to atmospheric propagation, it's a whole new story.
    Generally we look at the properties of the atmosphere. It conducts orange light best and blue light worst.
    The sky is blue from reflection and orange sunsets give you a hint of it's path losses.

    So the people I know playing with atmospheric communications choose high power red LED's for their propagation. They also choose PIN diodes for their receivers because they are most
    sensitive to red light. I've been slowly working away at the other end of the spectrum with blue lasers and PMT tubes which are much more sensitive, but more sensitive to blue light.
    By comparison I have a lot more ground to cover than my solid state friends, and they're not at all threatened for distance records yet. I have much more trouble keeping noise out than
    getting light into the tube.
     
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