# Need help with my university project

1. Dec 2, 2007

### garydia1

Hi all, I'm a student at a mechanical engineering faculty, and I'm doing my final project now.

The purpose of the project is to built a sensor for measuring gravity-capillary waves in a wind-wave flume. The idea is to pass a laser beam from beneath of the aquarium through the aquarium bottom, through the wavy water, up to the air over the water and at the end the laser beam should point on a PSD (Position Sensetive Detector) which measures the X and Y deflections of the beam from the calibrated point over still water. Then I can use Snell law and various DSP to find the wave coeficient 'k' or waves wave lengh 'lamda', phase velocity 'C' etc...

As part of a project, I need to draw a circle with the laser beam (using a rotating mirror) with a diameter of 1.5mm over the water surface.
The smallest laser beam diameter I could find is 0.48mm and it's too big!
I'm going to use a NeHe 632.8nm 5mW or 10mW CW laser, and I thought to use a beam expander reversibly as a beam condenser, and here is the question:
Is it possible to pass a 0.48mm diameter laser beam through a beam condenser X10 to receive a 0.048 diameter laser beam without loosing the needed energy to pass through the perspex of the aquarium and the water, so the PSD will detect the beam?

Thank you for you attention,
Gary.

2. Dec 2, 2007

### Claude Bile

It should be possible to reduce the beam diameter down to 48 microns with the He Ne laser, but I'm not so certain about the CO2 laser due to its long wavelength.

Claude.

3. Dec 3, 2007

### garydia1

Ryleigh lenght

Thanks,

now, if I condense a beam by 10 times it will reduse the ryleigh lenght and depth of focus by 100 times, what should I do to prevent it?

4. Dec 3, 2007

### Claude Bile

Unfortunately there is not a whole lot you can do without introducing additional optics. Delivering light to the target via optic fibre can eliminate problems with depth of field, however it has the obvious disadvantage of no longer being a free-space delivery. Special objectives and collimators can also work to reduce this problem, but you then have the disadvantage of making your system less robust due to the degree of beam aligning that needs to be done.

Claude.