We are currently doing an experiment in which we shine laser light through a sugar solution, and move a photo-diode down the tank of solution collecting data of light intensity as it moves down. However we are having a lot of trouble calibrating it, as vibrations and misalignment of the laser are emphasised as we move down the tank.

We want to be able to spread the beam in front of the photodiode so vibrations and misalignment don't ruin our results.

What are our thoughts so far?
-Tracing paper do diffuse light but is in solution so doesnt work as well.
-spreading the light at the laser end of the tank, the laser intensity is too small and the intensity decays significantly anyway as we move down the tank so not ideal.

Any Ideas would be very gratefully appreciated

Rob

 

Have you thought about using a pinhole or slit to cause beam spreading?

John

 

Not too sure about that as we want to keep the laser beam thin as it enters the tank, to stop it scattering and losing intensity too quickly, and if we put it in the solution, the issue is that it is too difficult to calibrate the equipment such that the laser beam stays on a chosen point as the photodiode runs down the tank. But thankyou all the same

 

You need PHYSICAL isolation of your equipment from earth.

without isolation from the SOURCE of the vibrations, there can be no solution to the vibration problem. Electrical 'work arounds' are not going to solve the basic problem of physical coupling of the equipment to a vibrating base stand.

solve the vibration issue mechanically and you won't have any need for our help here.

(Edit) I once used foam rubber pads and a styrafoam box to isolate a phonograph turntable from the bass tones that caused feedback. Something similar could be used here as well. Foam pads to uncouple the base from earth and a cover to isolate the equipment from air currents.

 

There is always this stuff - http://www.sorbothane.com/

Although a fixture that holds the laser source and the beaker would be a superior solution.

 

What wavelength of laser are you using? Can you paint or apply a fluorescent strip (e.g., fluorescein, but there are many possibilities) to the detector side of the tank. That will spread the light without diminishing its integrated intensity much, if you use the right dye.

John

 

There seems to be an assumption you have to be outside the beaker. It may be valid, but couldn't you simply make a water proof probe that is dipped into the liquid?

 

You need PHYSICAL isolation of your equipment from earth.

That's a good comment because optics experiments are often done on a vibration isolated optics table. OP - These tables weight about a half ton or more and are floated on special legs with compressed air to isolate from the ground vibrations. The steel top has tapped holes in a grid pattern to mount the components to.

Note that low frequency ground vibrations from trucks can travel very long distances, so OP don't think a basement cement floor is good enough, unless you are miles from any traffic.

A poor man's version of an optics table is a plywood box filled with sand resting on inflated inner tubes. The sand is also good for holding components and mounts in place.