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Need help with my university project
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garydia1
4.99mW Green Laser Toy
Joined: 18 Nov 2007
Posts: 3
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 Posted: 11/18/07, 3:35 PM Post subject: Need help |
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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. |
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fnugget
Elite Laser
Joined: 21 Jul 2005
Posts: 131
Picture(s): 2
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| Posted: 11/24/07, 2:20 PM Post subject: ugh |
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I need to freshen up on stuff like this...
but for short, I think you'd get a large divergence if you try to condense it. |
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garydia1
4.99mW Green Laser Toy
Joined: 18 Nov 2007
Posts: 3
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| Posted: 11/24/07, 4:13 PM Post subject: Why is tha |
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Hi, thank for response.
Can you explain it scientifically, why is that behaves so? |
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mda
4.99mW Green Laser Toy
Joined: 30 Nov 2007
Posts: 2
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| Posted: 11/30/07, 4:05 PM Post subject: |
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Hi,
a few points...
first I would be surprised if power is an issue in this case, unless you have a lot of background illumination. Ideally you should put the whole tank in a dark room. If this isn't practical then consider using a wavelength filter and an optical-chopper/lockin-amplifier. Commercial units can be very expensive, but red cellophane will do nicely for the filter, and if you should be able to knock together a basic chopper system for a few tens of dollars.
In your case this is slightly complicated by the rotating mirror thing but I'm sure there's a way to make them work together.
The amount of power loss in the expander depends on whether it has a spatial pinhole filter or not. Cheap units, less than $1000 generally do not, so the power loss is minimal and practically zero if the optics are AR coated. Reversed pinhole expanders could possibly be very inefficient because the pinhole focusing is optimised for the "small side".
Finally, (see previous post) you are probably overlooking some fundamental aspects of beam focussing.
Look up "gaussian beam" on wikipedia. The HeNe beam size is that way for a reason... the beam will spread considerably over a few meters, because the Rayleigh range for that spot size is about 2m. By reducing the spot size 10 times, the Rayleigh range reduces 100 times to 2cm, making spreading much worse. If you can accurately focus that's ok, but I suspect with the water moving around this will be a problem.
Given the PSD's I've seen the regular beam size is good... you actually want the beam to cover approximately one quadrant to get a nice linear response. |
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garydia1
4.99mW Green Laser Toy
Joined: 18 Nov 2007
Posts: 3
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| Posted: 12/02/07, 3:17 PM Post subject: |
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Thank you,
that was very informatively, I'll talk with my instructor and we will decide what should we do next. |
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