Diffusion of laser light for measuring laser power

[jodyflorian]

Hi,

I've decided to make my own meter for measuring laser power to around 10% accuracy if possible. Optical power meters are way too expensive for me though.

One building block I've decided to use is a phototransistor, because I've found one which has very good linear response - when measuring mW/m^2. (Its datasheet has a graph showing collector current for the said power per area). The data sheet also has a response graph for various light frequencies which will be easy to use since lasers have a very specific color e.g. 532nm for green).

However, the hurdle I'm having trouble getting over is the diffuser. In principle, (please correct me if I'm wrong), if I know the following properties for a diffuser, it should be easy to calculate the laser power, given:

1) Distance of phototransistor from the diffuser
2) Placement position of phototransistor with respect to the diffuser
3) Light scattering pattern of the diffuser for various angles (inc. reflection)
4) Absorbtion factor of the diffuser.
5) (values of 3 and 4 for the given laser frequency)

However, I'm having trouble finding anything I can use as a diffuser where 3, 4 and 5 are known. I don't have many, if any, tools to do this myself...
a) Multimeter
b) Oscilloscope
c) Lux meter
c) Three lasers of unknown power (in the 1 to 20 milliwatt range, I expect)

Please could anyone suggest a way of going about this, perhaps...
i) Buying cheap LEDs of known frequency and output power for calibrating
ii) Buying a diffuser with the above known properties (3, 4, 5)

Thanks :)

 

[jodyflorian]

Forgot to mention...

Already discussed trying to do this with the Lux meter at:

https://www.physicsforums.com/showthread.php?p=1941217

But the accuracy due to unknowns was becoming a problem - a Lux meter just doesn't seem useable.

 

[Andy Resnick]

I'm confused- why do you want to use a diffuser? There are good reasons (polarization, stability, etc), but when measuring optical power, the diffuser is something called an 'integrating sphere'. Perhaps you could construct an inexpensive one that meets your needs- I bet spraying the inside of a ball with that popcorn paint stuff (seen on 1970's era ceilings) would work fine.

 

[jodyflorian]

Lol @ popcorn paint. Brilliant - now I know the term! "Integrating Sphere".

I'm just trying to find the cheapest way of measuring laser output so I can judge lasers that I buy off ebay, and optical power meters seem to go for no less than £150 so about $250.

The main hurdle I'm coming up against is that anything I build myself seems to need calibration with lasers of known power output - which I dont' have!

I was thinking that if I bought a diffuser with known properties, and use a phototransistor with known properties, I wouldn't need to calibrate it.

The only alternative apparently involves measuring the cross sectional area of the beam, which my gut feel says would be really quite inaccurate and a fair amount of work for each laser that I test... (This would use the Lux meter and I'm not comfortable with the calculations involved!)

I really like the idea of using an Integrating Sphere, but I assume if I build my own it would still need calibration! I've just done a search on google and found a few suppliers but no prices and no specs... should I just look harder, or am I facing a brick wall here? I'd need to know the proportion of light falling on the sphere's surface vs the amount being absorbed, for given frequencies, I assume? Is this a typical value that should really be readily available from the manufacturers or am I missing the point? Just a thought as well... I have a horrible feeling that a ready made integrating sphere could cost more than £150!? DOH!

Thanks!