# Temporal overlap of light beams

1. Aug 23, 2007

### scarecrow

Let's assume I can generate two individual light beams, one centered at a wavelength of 800nm (visible) and the other at 3450 nm (infrared).

Obviously the visible light has a higher frequency than the infrared light. Since all EM waves travel at the speed of light, how can one achieve temporal overlap of two different light frequencies?

I'm thinking that one must delay one of the light beams, but I'm uncertain of the correct equations to use to show this.

2. Aug 23, 2007

### mgb_phys

Not sure what you mean by temporal overlap?
At realistic intensities light doesn't interact with light (it can in very high power lasers).

3. Aug 23, 2007

### scarecrow

That's exactly what I'm using: high powered lasers. Sorry, let me clarify my setup:

I have two laser beams that need to be spatially and temporally overlapped, i.e. they need to be overlapped in space and time.

In other words, both laser beams need to hit the sample in the same spot and at the same exact time.

4. Aug 23, 2007

### mgb_phys

Sorry was overcomplicating things because you posted in the quantum forum!

If these are pulsed lasers and you just need the pulse to arrive at the target at the same time then you only have to worry about the dispersion of the medium. That is the refractive index of the material will be slightly different at the two wavelengths and so the speed will be different.
This site will calculate it http://www.luxpop.com/

For air at the two wavelengths you said, for a short lab bench distance the differenc is going to be very small, you will probabaly have more difficulty triggering the lasers that accurately.

The high power laser thing applies to VERY high power (giga-terra Watt) pulses where quantum effects begin to matter, here laser beams passing through each other can interact.

5. Aug 23, 2007

### scarecrow

I understand now.

thanks