Re: Ref: Laser Metrology/ Encoders

--- In CAD_CAM_EDM_DRO@yahoogroups.com, stan <stan.distortion@...> wrote:

>
> So not much chance of an el-cheapo interferometer type thing, it's
>super accurate lab type stuff all the way.

Yes and no.

>I was thinking of waves as the surface of the water going up and down
>which seems like a bit of a misconception regarding light.....

Not really, water is a good example of a transverse wave. But your
ideas about polarisers were a "little" off.

> So all in all the 2 beams are necessary as the phasing of the beams
>is what makes the wavelength measurable.

Two beams from the same coherent source. A laser is an example, you
can think of it as outputing a single wave with a wavelength that
varies very little at least for a reasonable period of time.
Non-coherent sources act like lots of sources stuck together, all of
the waves are at different phases (and/or wavelengths) and so overall
no interference will be seen it will just average out.

> And a laser pointer isn't really a usable source as the wavelengths
>are messed up (not sure if its the lengths or the alignment or if I'm
>barking up the wrong tree altogether).

You can get a laser pointer to create interference and people do this
when making simple holograms but the length of time the laser keeps
outputting the exact same wavelength is probably short. This means
that when you split the beam to create two they will only interfere if
they travel about the same distance before being combined because if
the distances are different the light will conist of photons emmited
at different times and these may vary in wavelength. The varience
will probably vary over time so again you don't see interference. The
time the source remains coherent is the coherence time and the
distance that corresponds to is the coherence length. The former also
relates to how pure the source is spectrally. HeNe is good for
coherence length and you can pick them up cheap sometimes.

> Besides all that, something would be needed to count all the phases
> and there are around 4 million of them a meter if the sensor is
>beside the laser and the beam is being reflected back from whatever
>is being measured. Actually, that bit isn't too bad and 4000
>divisions per mm is a usefull number.

This is the essential problem, because there are so many waves per
meter the frequencies you can expect are high. If you can keep track
of the sinewaves you get then the vibration issues may not be an issue
as you can filter them out of the decoded signal. Assuming of course
you don't really need um precison after all.

>Could you give me a better idea on laser pointers or cd drive lasers
>as a light source?

Err not really, the coherence length will probably vary a lot between
models but I seem to rember that is can have something to do with the
physical size of the laser cavity, obviously small in diode laser. I
once made a simple interferometer with one and I think the coherence
length was less than 10mm.

>The concept doesn't seem to tricky but something to hold it all
> together on a vibrating machine looks difficult, it seems like it
>would need 3 or more side by side to tell which direction the thing
>is moving.

You could use two detectors to provide a quadrature signal. There may
be other ways.

>Oh, and 'if you need to ask, you can't afford it' prisms, but it
>would be nice even to see it working on a steady surface.

Have a look at edmund optics to get an idea of cost. Not so massive.

>Looking over that, I'm assuming the phases stay where they are if the
> distance stays the same and aren't going in and out at some speed

related to the speed of light. is that right?

Your signal relates to the relative phase of the two beams so if
nothing moves they stay in phase.

I'd leave your CD drive alone.

Graham