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Old 06-01-2010, 01:27 PM #1
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Arrow Reflecting a Beam to the Same Point Regardless!?

We're working with a Ti:Sapphire Laser pulsed at 80MHz. Our setup in a nutshell is having the main beam separated into a main beam that will excite the sample of XYZ head on, and a probing beam that will hit the sample at some angle, and be reflected off and into a prism that will separate the linear and nonlinear parts into 2 PMT's respectively. Our plan is to vary the wavelength of this from 700-1000nm.

My Problem

I've been told that as we vary the wavelength from 700-1000nm the linear and nonlinear beams will walk around some, so they will not line up with the PMT.
Where will the angles change due to wavelength? and how much travel can I expect?

Also, if the above is true, how can I rig my setup so that i don't have to move the PMT's everytime I change the wavelength? A concave mirror, rotating stage?


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Old 06-01-2010, 01:54 PM #2
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Default Re: Reflecting a Beam to the Same Point Regardless!?

I didn't follow a lot of that, but I have a feeling it's something to do with Snell's law.

Basically each material as a refractive index, and the angle or refraction/reflection vary's depending on the refractive index of the material and the wavelength of the light.

When light passes into another transparent medium (air, glass, perspex) it slows down. The more optically dense the material, the more the light slows.

Sorry it's been a while since I last went over it so a few things in there may be slightly incorrect. It may be something worth looking into though.
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Old 06-01-2010, 02:07 PM #3
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Default Re: Reflecting a Beam to the Same Point Regardless!?

Yes, I know Snell's Law, but I'm not sure where wavelength comes into the equation? or, again, where this is even a factor?
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Old 06-02-2010, 01:02 PM #4
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Default Re: Reflecting a Beam to the Same Point Regardless!?

If the material your prism is made with has dispersive properties (and they all do!), different wavelengths will refract at different angles according to Snell's Law. I don't think there's an automated way to get your beams to line up for the whole range of wavelengths (short of something that can compute and adjust). You might be able to get away with it if you don't pass the light through a prism, but use only mirrors, but I'm not sure to be honest.
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Old 06-02-2010, 04:42 PM #5
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Default Re: Reflecting a Beam to the Same Point Regardless!?

Dispersion. The index of refraction is dependent on wavelength, and therefore changes with wavelength. This changes the relationship between angles of incidence/transmission in Snell's law, because the n in the lens/medium is changing, thereby changing the angle. It also manifests as a change in the focal length, and therefore the NA, of lenses by the same mechanism.

As far as lenses, achromatic lenses can sometimes be used to correct for this, ie their focal length remains the same independent of wavelength, but may not be usable for you, and may not help anyway depending on what you're doing with them. As far as mirrors, be careful about your mirror design and plan for the dispersion. With just a metal film front-side mirror, the angles shouldn't really change, but I don't know your optical set-up.

-----------------------

ETA: Oh, I think I just figured what you're doing, and I literally just finished doing a lab for a class like this, I think. You're trying to detect the difference in power in the 2 beams, right? The lab we did involved using a monochromator coupled in a fiber. This fiber then split into 2 fibers: the first going directly into a photodiode and the second going to an optical assembly that bounced the light off of a sample, using a couple of optical elements to focus the light on the sample and then focus it onto another photodiode. The outputs of both photodiodes, as well as the signal from an optical chopper in the monochromator, were then all fed into a lock-in amplifier, which gave a precise ratio of the 2 powers.

Does this idea in any way apply to what you're doing?

ETA Again: Reading again, not really that much as far as what you're asking. Nevermind.

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