## Collimating beam with optical axis

Hi, I need help or any sudgestions with the next scenario -

I'm designing a sensor for a wind wave tunnel - can be found in the google search using the key words 'wind wave flume'
The sensor should give me the speed and frequency of the cappilary gravity waves created in the tunnel by a wind in it.

The sensor is of two main units; the bottom unit under the tunnel and the head unit above the tunnel.

On the bottom unit placed a laser propagating a beam toward inclined spinning mirror (1800 rpm) so that the laser beam creates a holow cone shape. Then the beam passes a plano-convex lens in order to enter perpendicularly to the bottom of the tunnel. This way no diffraction appears in the air-water interface.

When the water are still (no wind) I'll get a circle shape on the surface of the water, BUT when wind exists in the tunnel, diffraction will occure in the water-air interface. If I'll know the diffraction ammount, I'll be able to calculate the speed and frequency of the waves.

In order to measure the diffraction I decided to use a PSD (Position Sensetive Detector) placed on the head unit which entered into the tunnel above the water surface, with as less as possible interference to the wind in.

Since I want to keep a reasonable distance from DC wave height (~40mm) the delta's are quite big, somewhere aroung 12mm in every direction (X and Y), so it makes me to use a large area PSD something like 45x45mm, but this PSD is very expensive!

I'd like to use a small PSD something like 12x12mm and in order to use such a PSD I need some optic system of a lens or two, to decrese the difractions and collimate them with optical axis before they reach the PSD surface. The problem is that wave's slopes are changing rapidly (different diffraction gradient) every single moment, so I don't know what optical system will work for me.

Can someone help me with this one?

Thank you,
Gary Diamant.
 Recognitions: Science Advisor This seems like an overly complicated method, especially for someone with what appears to be very little optics experience. Plus, your 'magic step': "If I'll know the diffraction ammount, I'll be able to calculate the speed and frequency of the waves" seems doomed to failure, frankly. Are you really just trying to measure the surface height, or surface slope? You may be better off trying to track light that is *reflected* off the surface. Why not just image the air-water interface? What are the relevant length and timescales?

 Quote by Andy Resnick Plus, your 'magic step': "If I'll know the diffraction ammount, I'll be able to calculate the speed and frequency of the waves" seems doomed to failure, frankly.
Hi,

we did a preliminary experiment to see if this will succes, we took a laser pointer, pointed through the bottom of the tunnel upwards, on top of the tunel laid an A4 sheet, the motion of the laser beam was captured with a video camera 30fps which was set above the A4 sheet.
After some hours with MATLAB we received quite good expected results.

We are not trying to measure surface height or surface slope as it is, currently we are interested in waves frequencies, their 2D representation and yes, by the way, we will get the speed and slopes if we will know the refraction deltas in each direction.

 Quote by Andy Resnick track light that is *reflected* off the surface
What do you mean?

 Quote by Andy Resnick Why not just image the air-water interface?
Do you mean, why not use a hight speed camera? Well money I'll guess will be the best answer.

 Quote by Andy Resnick What are the relevant length and timescales?
of what?...

This project is a part of doctorate that my supervisor working on, he investigates the cappliary gravity waves (look for 'Gravity–capillary waves' in google) and an unknown doppler effect in the Dispersion relation of this Gravity-capillary waves (Or something like this...)