Please Explain Why Again?

Dutchelm05 · Post by **Dutchelm05** » Tue Jan 08, 2008 10:55 pm

Can someone explain why in a single beam set up why you can't use an external laser source to help increase light to an object? Let's say I just do my normal set up (single beam) now add a laser pen, expanded it and shined it on the object (sync'ed with the exposure). Since the object shouldn't care about much about Mode or maybe even polarization since it is scattering anyway, wouldn't it simply cause more interference with the reference beam?

Anyway, just wondering and back in the beginners corner

Thanks for humoring me
Tony

Colin Kaminski · Post by **Colin Kaminski** » Tue Jan 08, 2008 11:59 pm

The laser needs to be precicely the same frequency. This is to a degree that is difficult, if not imposible to achieve with two lasers. Now, this starts to sound like one of those things everyone says but no one has tried. So, if you get two lasers that are very close if frequency, say tied to an atomic emission standard, then give it a try!

Dutchelm05 · Post by **Dutchelm05** » Wed Jan 09, 2008 2:54 am

Thanks Colin,
I just happen to have an Acme atomic emission meter in my pocket

So why? (as my 4 year old would ask endlessly). If light hits an object, should it not scatter and interfere with the reference beam? Would the result be nothing, noise, blur or something else?
I know 532nm lasers pretty well I guess but not sure what variability it has from one laser to another.
Thanks for the reply,
Tony

Colin Kaminski · Post by **Colin Kaminski** » Wed Jan 09, 2008 5:23 am

You know I was thinking about this, We used to have a poster that had the best hairbrained ideas. The problem was that they all had a basis in science. I could never decide if he truely had enough experience to talk the way he did, so anyhow, I thought to myself, what would holodisk do? He would bounce one laser off a microscope slide and take that few percent bounce and feed it back into a second matching laser and turn it on. This would seed the cavity with precisely the right frequency of light to make two lasers coherent with each other. The problem is this would be like doing an experiment in quantum mechanics. The set up would be very finicky at first until you had experience. In theory you should be able to make it work. It might be fun to play with, with a couple of laser pointers. Not your argon ion lasers!

JohnFP · Post by **JohnFP** » Wed Jan 09, 2008 1:47 pm

If light hits an object, should it not scatter and interfere with the reference beam?

Yes, as Colin stated, if the frequency is off just a little then you would not get a standing wave. The closer the frequencies the slower the stand wave would creap but they would have to be exact to get the standing wave to stand still.

I started to play with this and here is what I was going to do but never finished it. I could never get that Siemens LGK 7626 laser withe the piezo mirror from a fringe locker to lase. I still have it set up though. Take two lasers of relative same frequency, like two 632.8nm HeNe's. Remove the rear miror of one of the lasers and put the piezo mirror of a finge locker in. Then get them both to lase. Steal a little of each beam and create a Michelson with the beams. Expand the fringes from the Michelson and place the detector head of the fringe locker at the fringes. Then hopefully, the fringes can get locked because the piezo mirror would lengthen or shorten the one laser cavity as needed to keep the frequencies the same.

RaulBSP · Post by **RaulBSP** » Wed Jan 09, 2008 3:42 pm

Hello

I'm very dilettant in this optical stuff, but I read somewhere, and have the following
rough mechanical model for explaining this kind of phenomena myself:
"Continous" laser light departs from laser in packs. All photons inside one pack
have the same phase, so light is coherent only inside this pack. Every new pack
departing from laser has different phase, and it changes chaotically from pack to pack.
This explain coherence length, that is the length along which all photons belong to the same pack. Only photons with the same phase difference make the same interference pattern and when photons from differents packs coincide on the emulsion as object an reference beams, they form different patterns, that destroy each other.
In the case of different lasers, even if they have the same frequency, the phase of the packs
(as result of a chaotical proccess) will be different all the time and that is more important, that difference will be "different" with every pack , and there will be a sequence of chaotically changing interference patterns that will kill each other.

Saludos,

Raúl.

Dutchelm05 · Post by **Dutchelm05** » Wed Jan 09, 2008 4:31 pm

Colin Kaminski wrote:hairbrained ideas

Gee thanks Colin

Thanks Raul,
Yes that all makes sense, I thought though once the photon hit the object and scattered it would no longer matter regarding phase. I also thought in the case of KTP and YAG the variablity in frequency wouldn't be that big since it bound to the materail's properties.