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Practical Uses for Lasers

  1. Jan 21, 2008 #1
    Has anyone had any experience in using a laser system for measuring distance or location?

    I am looking for the laser emitter as well as the receiver as kind of like a matched pair. I haven't really been able to find anything other than laser emitters on their own, and wanted to know if anyone has used a laser "system"

    any links anyone has would be great...

    thanks
     
  2. jcsd
  3. Jan 22, 2008 #2

    berkeman

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  4. Jan 22, 2008 #3

    mgb_phys

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    For time-of-flight laser ranging you typically use APDs for the detector because you need high speed and sensitivity, you also need a timing chip capable of resolving a few 10pS.
    Or buy something like this http://www.mdl.co.uk/laser_modules/laserace-im/index.html
     
  5. Jan 22, 2008 #4
    yep, Photodiode, that's what I'm looking for.

    Perfect, thanks.

    pud
     
  6. Jan 22, 2008 #5

    berkeman

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    Interesting -- Avalanche Photodiode (APD) -- I need to read more about how to use them. This is an interesting link I came across while googling laser APD:

    http://www.analogmodules.com/Data_sht/7500.pdf
     
  7. Jan 22, 2008 #6

    stewartcs

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    Not sure if this is what you are looking for but I have one of these for measuring distance.

    http://www.stanleytools.com/default...atMax™+Tru-Laser™+Distance+Measurer

    CS
     
  8. Jan 22, 2008 #7

    Astronuc

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  9. Jan 23, 2008 #8

    mgb_phys

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    There are two main ways of doing laser range finders.

    Time of flight = fire a short (ns) infrared laser pulse and start a clock, when the return pulse is detected by a photodiode you stop the clock. Divide c=3E8 m/s by the time = distance. This is useful for long range (upto several km) but lower accuracy (a few cm) and is fast (100,000 points/s) it's used for imaging scanners and laser-radar.

    Phase = transmit a continual visible beam varying in brightness sinusoidally at a few kHz. Measure the brightness of the returned beam and compare the phase difference.
    From the phase you can tell what fraction of the 'wavelength' the distance represents.
    This is used for short distances (upto 10m) and high accuracy (mm) but is slow (5-10 seconds/reading) - it is used in the hand held DIY Disto measurers.
     
  10. Jan 31, 2008 #9
    I didn't think of varying the beams brightness....that's interesting....

    Say I had a continuous beam shooting from d=0 to d=1m. During the continuous beam, I want to swipe something through the beam (break the beam) at a location of 10cm. Then do the same thing at 13cm, then 25cm, then 80cm, etc. etc. Would there be a way to record the location of the breaking of the beam? Would this be possible? Accurate?
     
  11. Jan 31, 2008 #10
  12. Feb 1, 2008 #11

    mgb_phys

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    Remember you aren't measuring the brightness of the returned beam to calculate the distance from the loss of signal.
    You are comparing the phase of the change in brightness - eg, if you receive a maximum at the same time your are transmitting a maximum, the object must be a whole number of 'wavelegnths' away - where this is the wavelength of the sine wave not light.
    It has a problem that you need to integrate lots of waves to get a good answer so it is slow, and there is an ambiguity - the object could be 0.1, 10.1, 20.1 m away if the wavelength of the change is 10m. To solve this real instruments autaomatically make the measurements at a few different frequencies.

    Easily with a 'time of flight' system. These can make 10,000s of measurements/second and should give an accuracy of 1cm over that short range.
    A typical system is used for counting traffic on a freeway - a single laser across 6 lanes can count vehicles breaking the beam and measure which lane they were in. Obviously you miss object which were completely hidden behind another object - unless you have another beam measuring from the other side.
     
  13. Feb 1, 2008 #12
    Right, comparing wavelengths, you aren't going to be able to differentiate between one amplitude vs. another, one crest vs the other, etc.

    The traffic counting system is interesting. I'll have to Google that and see what comes up.

    Thanks
     
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