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Old 11-11-2011, 06:11 PM #1
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Default Science fair experiment

I am 15 years old ( 10 grade) and just joined the forum. I am interested in doing a science fair project where I can test the intenisty of the laser beam after it travels through different types of lenses. I was thinking of using the IR thermometer I have seen used on this forum as a laser power meter to measure the effect on the beam strength (intensity/thermal output) ) after it passes through various lenses. I am hoping some may increase the strength and others lessen it. At least that is my initial hypothesis. I plan on purchasing some lens assortment from Meredith instruments for $25. I hope they have some of the following:Bi-Concave, Bi-Convex, Plano-Concave, Plano-Convex lesnes in the kit. My thought if I use a laser pointer or somehting inexpensive like that I would be simulaitng how I think a cutting laser works with an external lens concentratiing the beam strength to cut the metal.

Anyway I may be over my head here and am not making much sense. I would appreciate any help, ideas, suggestions or resources on how or where to get materials, etc. to do my experiment.

I went to Wicked laser site and saw expanded lens kit and youtube video. Are these external lenses like I think I want for my experiment or different collimating lenses found within the laser itself?

Thanks


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Old 11-11-2011, 07:05 PM #2
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Default Re: Science fair experiment

Hey,

Welcome to the forum. Here are my initial thoughts:

1) No lens will increase the power of a laser, ever. They will always decrease it. Conservation of energy, laws of thermodynamics, etc etc etc.

2) The lens type (concave, convex, etc) shouldn't have anything to do with how much energy is lost. It will only impact what the output shape is. As long as the output shape still fits on your power measurement sensor, the actual lens shape shouldn't really be a factor in the amount of power loss.

3) The characteristics of your lens that WILL impact power loss are:
- Thickness of the lens (how much glass the beam is passing through)
- Quality of the glass (how well the material itself transmits light)
- The surface coating (whether there is an anti-reflection treatment to prevent losses)
- Whether you're using a single lens, or a composite lens made up of multiple elements (which really just implies more glass to push the light through, and more potential for loss)
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Old 11-12-2011, 09:07 AM #3
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Default Re: Science fair experiment

Focussing a laser will increase the intensity (or irradiance as it's called in optics) as intensity is W/m^2. Measuring the intensity and the total output power is different. For the total power you just capture the beam entirely with an optical power meter.

For measuring the intensity you need to take the beam profile into account. One possibility is to sample the power to a known fixed surface that is smaller than the beams to be measured. A single mode fiber can be used for that, it will measure the power at a surface of a few micrometers in diameter. Because this surface is so small it will never average the power over a surface larger than the beam which would give false readings.
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Old 11-12-2011, 10:17 PM #4
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Default Re: Science fair experiment

Thank you for the feedback. If I am understanding you correctly, my experiment still has some merit in that the variety of lense types will impact the intensity of the laser beam. I think I should be able to measure that thermally if the beams are all positioned on the same size mass/area. I think an Infra red thermometer could be used to measure the temperature difference from one lense type versus another as they are targeted on a small black surface. If this makes sense please confirm or suggest any changes I need to make. Also do you suggest a certain laser that would help me run my experiments. I have a very minimal budget.

Thanks

I look forward to your suggestions or any others on the forum.
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Old 11-14-2011, 10:13 AM #5
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Default Re: Science fair experiment

The whole point of intensity/irradiance is power per unit area, summing over the whole beam area only leaves a power measurement and you'll only measure some losses of lenses you won't find anything about intensity.
A thermometer has a low dynamic measurement range, the intensity of a focusses/unfocussed laser has a huge dynamic range, up to 10^8 if done correctly. A thermometer also requires a high power laser which is dangerous. The laser itself may be affordable if your budget isn't too minimal, but don't forger on safety measures, definately if you're in public. Class 3b or class 4 lasers require a lot of safety measures, so don't expect that to be easy.
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Old 11-14-2011, 11:51 PM #6
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Default Re: Science fair experiment

Well, as this is a school experiment and not a design for a highly accurate laser power meter, i think there are options available.

You could set up a black target that is thermally conductive (perhaps a penny painted black) in a fixed environment. Shining a laser onto it will raise its temperature above ambient by some amount that increases with laser power.

As long as all the laser lights hits this penny, you can compare the losses in lenses etc.

I don't think this will be sensitive enough to detect the differences between simple glass lenses though, but if you throw in a ND2 filter etc you may find readable results.
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Old 11-15-2011, 03:44 AM #7
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Default Re: Science fair experiment

You'd probably be better off using some sort of light sensor and just working with comparative data. SInce you aren't really interested in the actual output power of the laser, just how much the lenses effect the power, using a light sensor (Maybe with an Arduino to make it easier) and comparing the readings would probably work better than a thermal measurement. Especially since you'd want to use a low power laser in public, which wouldn't work too well at heating anything up to a noticeable difference.

I'm not 100% sure of what you're trying to achieve here, the only thing that is going to make a difference to the laser power is the quality of the lens & coatings. The type and strength of the lens isn't gonna make much of a difference apart from thickness of the glass.
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Old 11-15-2011, 11:42 PM #8
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Default Re: Science fair experiment

An optical sensor would be more sensitive to small power differences indeed. You need a fairly large one to capture the light after its been through a set of lenses which pay prove to be a problem.

I doubt the thickness of the glass will be much of a issue tough: you incur most losses at the surfaces (partial reflection). Once light is inside a piece of good quality glass the losses are very low. I doubt you could measure a difference in loss between 1 mm of glass and 1 inch of the same glass, provided its optical grade.
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Old 11-16-2011, 02:26 AM #9
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Default Re: Science fair experiment

You can find lenses for less than that, look here> FS: AR coated lenses
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Old 11-16-2011, 09:43 AM #10
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Default Re: Science fair experiment

Quote:
Originally Posted by Benm View Post
An optical sensor would be more sensitive to small power differences indeed. You need a fairly large one to capture the light after its been through a set of lenses which pay prove to be a problem.
A large sensor will only measure total power, no peak intensity, so a focussed beam won't read higher than an unfocussed beam of equal power. Photodiode sensors are very sensitive but also have an angle dependent reflection that you need to account for or compensate if you want to do precision measurements. A good diffuser or integrating sphere would probably work, but this would still be a power measurement.

Just to repeat: intensity/irradiance is not equal to power, measuring lens losses is not equal to measuring peak intensities. The OP asks for intensity measurements, not power measurements. Everybody talks about measuring power and losses but the idea was to measure the intensity, so everybody misses the point of the experiment.

A simple setup would be a tiny pinhole, a photodiode behind it connected to a sensitive multimeter (at least 100nA resolution). Take a 1mW laser, plenty of power for this experiment and safe. Thermal measurements if intensity require very specialized equipment.
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