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'[PICLIST] "Lasers again" distance measurement'
2001\02\02@061809 by Bureriu Cristian

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Idea: To comapare the phase of the sent beam with the received. The
diference should give the distance if < light wavelenght (or multiple of it)
Please vote this idea. :-)

Sid

{Original Message removed}

2001\02\02@072329 by Andy Faulkner

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The basic method of operation is time-of-flight mode.
ie. measure the time it takes light to travel from the
transmitter to the object, and back again.

Example.
Pulse emitted from the device and detected by the
receiver 0.000001333 seconds later. Speed equals
distance over time. So distance = speed x time. The
speed of light is 300,000,000m/s and the time is
0.00000133s so the distance is 300,000,000 x
0.000001333 = 399metres. That's the distance the light
travelled from the laser to the target and back again,
so the distance to the object is half that, or in this
case 199.5metres

"I think"
Vote for this one

How you detect the returned signal, urm now thats
another issue.


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2001\02\02@105251 by Bob Ammerman

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Wavelength of light is _very_ small. This technique is indeed used to
measure small distances (actually more usually small changes in distance).
Not useful for any macroscopic dimensions.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: Bureriu Cristian <.....Cristian.BureriuKILLspamspam@spam@AT.SIEMENS.RO>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Friday, February 02, 2001 5:59 AM
Subject: Re: "Lasers again" distance measurement


> Idea: To comapare the phase of the sent beam with the received. The
> diference should give the distance if < light wavelenght (or multiple of
it)
> Please vote this idea. :-)
>
> Sid
>
> {Original Message removed}

2001\02\02@111157 by Robert Francisco

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Another way of measuring distance is to shoot a laser to a mirror( on a
taget)  and have the reflected beam 'interfere' with a ref beam. You can get
very high resolutions. Basically counting the destructive/constructive waves
of the laser.

Robertf

{Original Message removed}

2001\02\03@064745 by mike

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On Fri, 2 Feb 2001 10:09:25 -0600, you wrote:

>Another way of measuring distance is to shoot a laser to a mirror( on a
>taget)  and have the reflected beam 'interfere' with a ref beam. You can get
>very high resolutions. Basically counting the destructive/constructive waves
>of the laser.
Probably not much use for tractor-sized distances, but it does remind
me of a really fun demo...
Many He-Ne lasers have a rear mirror that lets some light pass (i.e.
not quite 100% reflective). Stick a photodiode to the back of this
mirror, and connect it to an audio amplifier and speaker.
Fire the laser at a mirror on the  wall, and line it up so the beam
returns right back to the laser (i.e. beam at exactly 90 degrees to
the mirror on the wall)

You can now 'hear' any tiny movement of the wall via the speaker, e.g.
leaning on it, as it moves by a few hundred light wavelegths per
second.
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2001\02\03@224717 by Jeethu Rao

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Well Sid,
   Can you please tell me how can the phase of a beam be determined ? I
know the phase of radio signals can be detected. But I don't know of a
method to find out the phase of a light beam.

Did you mean the phase of a Signal modulated by light ? If so, then I
believe its possible.

Jeethu Rao
http://www.jeethurao.com

{Original Message removed}

2001\02\03@225757 by David VanHorn

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At 08:51 AM 2/4/01 +0530, Jeethu Rao wrote:
>Well Sid,
>     Can you please tell me how can the phase of a beam be determined ? I
>know the phase of radio signals can be detected. But I don't know of a
>method to find out the phase of a light beam.

You simply mix the received light with the transmitted light.
Dark means out of phase, bright means in-phase.

It's used for sensitive distance measurement.
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2001\02\04@152719 by Peter L. Peres

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Isn't the modulated beam, phase measurement method a variation of Fizeau's
experiment ? (the one with the spinning slotted discs with slots 180
degrees out of phase - 18th century ?).

Peter

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2001\02\04@155708 by Bob Blick

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>Isn't the modulated beam, phase measurement method a variation of Fizeau's
>experiment ? (the one with the spinning slotted discs with slots 180
>degrees out of phase - 18th century ?).

We all stand on the shoulders of giants..

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'[PICLIST] "Lasers again" distance measurement + e'
2001\02\05@020358 by Bureriu Cristian

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This is what Like Dave VanHorn said:

       "You simply mix the received light with the transmitted light.
       Dark means out of phase, bright means in-phase."

I must agree with him but I agree with you too. You can easily modulate a
laser light by controlling the current. This allows you to send pulses by
connecting the laser to a square wave generator and then it's possible to
compare the phase between the incoming and transmitted pulse.

Another extreme idea:
Considering a non-modulated laser pointed to a window across the street.

We already know about:
- we can measure small distances (<wavelength) using the mixing of
transmitted and received beam using the dark/light zone method.
- we can detect small changes in distances greater than the wavelength using
the same method.

Actually this can be a spying method. The sound (e.g. speaking) produces
small movement of the windows but it's possible to measure it. Here, the
windows of a building can act like a vibrating, mobile beam reflector. The
rest could be simple by converting the measurement to electrical signal and
then to sound.

Sid Pryce


{Original Message removed}

2001\02\05@044313 by spam

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Dear laser enthusiasts.

I seem to remember that diode lasers has a very short coherence
length = not possible to make interfere with itself over more than a
few mm. (i.e. not even out of the housing).

You have to use gas lasers for that.
- but times may have changed...

Kent

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2001\02\05@111328 by Robert Francisco

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Sid,

I saw the same thing ( laser listener) published in Popular Electronics( I
think) back in 1987 maybe '86.


----- Original Message -----
From: Bureriu Cristian <@spam@Cristian.BureriuKILLspamspamAT.SIEMENS.RO>
To: <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Monday, February 05, 2001 1:03 AM
Subject: Re: "Lasers again" distance measurement + extreme idea


{Quote hidden}

using
> the same method.
>
> Actually this can be a spying method. The sound (e.g. speaking) produces
> small movement of the windows but it's possible to measure it. Here, the
> windows of a building can act like a vibrating, mobile beam reflector. The
> rest could be simple by converting the measurement to electrical signal
and
> then to sound.
>
> Sid Pryce
>
>
> {Original Message removed}

2001\02\06@203631 by Peter L. Peres

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The coherence length of lasers taken from 'standard' CD player pickups
(near IR) is 10cm or so. This is enough to do some experiments in 'open
air'. You will need a camera and a TV to watch with. A $70 B/W camera
module works perfectly for this.

Peter

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'[OT]: "Lasers again" distance measurement + extre'
2001\02\07@122235 by jamesnewton

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Audio Tone Generator -> Modulates output of RF VCO -> Linear AMP ->
Directional Antenna pointed at offending car -> Car Radio -> Car Speakers ->
Air in the Car -> Car Windows -> Laser Inferiometer mounted with Antenna ->
Audio Amp -> PLL -> Logic Inverter -> Ramp Wave Integrator -> VCO Frequency
Input.

If the Jerk is listening to his radio, he hears a loud "WELP" then a very
loud tone. Then you flip a switch that holds the VCO Frequency Input and
switches from the Tone Generator to a MIC and you say... "THIS IS THE VOICE
OF GOD, DON'T EVER DO THAT AGAIN"

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{Original Message removed}

2001\02\07@141823 by David VanHorn

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>
>This is what Like Dave VanHorn said:
>
>         "You simply mix the received light with the transmitted light.
>         Dark means out of phase, bright means in-phase."
>
>I must agree with him but I agree with you too. You can easily modulate a
>laser light by controlling the current. This allows you to send pulses by
>connecting the laser to a square wave generator and then it's possible to
>compare the phase between the incoming and transmitted pulse.

You can do both I suppose, modulate the beam with one carrier, and phase
detect that to get distance, then use the interference rate to get speed,
during the interval that the beam is on.


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2001\02\08@040313 by Andy Faulkner

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What is all this phase stuff you keep talking about.

Are you meaning that if you send a square wave
modulation down a lasers beam buy modulation, the
further you get away you get a change in the frequency
of the modulation.




--- David VanHorn <spamBeGonedvanhornspamBeGonespamCEDAR.NET> wrote:
{Quote hidden}

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Fax +44 870 132 3275.....................UK Dial 0870 132 3275
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2001\02\08@054150 by Roman Black

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Andy Faulkner wrote:
>
> What is all this phase stuff you keep talking about.
>
> Are you meaning that if you send a square wave
> modulation down a lasers beam buy modulation, the
> further you get away you get a change in the frequency
> of the modulation.

Hi Andy, switch the laser on/off at 1MHz. Sense
the reflected signal with a high gain amp with no phase
shift (someone suggested FM radio chip).
The reflected signal travels twice the distance.
Speed of light delay (time of flight) causes a phase shift
between the transmitted squarewave and received squarewave.

You can simply AND or XOR (etc) the two squarewaves to give
a pwm signal that depends on distance.
Changing the modulated frequency affects the possible
max range of the system.

People have suggested problems with isolating the receiver
amp from the transimtter RF noise. Also sensing phase
using the "hump" of the received signal, not the zero
crossing point. I have a design mostly finished for this
technique for hobby robotics and have learned a bit
from the discussion here so far. Thanks everyone! :o)
-Roman

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