Measuring laser pulse width & pulse repetition frequency (PRF) & Power.

[simonb]

Dear forum,

I am starting to gather the items needed to measure laser radiation in order to ascertain safety:-

1) Photodiode/amp to measure pulse width & PRF
2) Sensor to measure the optical laser power emitted from my blue 445nm laser module.

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1) Looking at the easier one first, does anyone have any suggestions on a suitable fast photodiode?

I have in the past experimented with the detection of infra-red (IR) laser diode pulses, typically around 900nm. I found that a relatively cheap BPW34 or SFH205/SFH221 IR photodiode was able to react quickly enough to the 200ns pulses at around 125Hz that certain er.. laser based automobile speed related measuring devices work using. (Solely for educational purposes)! Also very similar to some of the IR Electronic Support Measures (ESM) & Defensive Aids Suites that I have worked on professionally in military avionics.

Much of the difficulty was in the design of the amplifier & discriminating circuit that the photodiode feeds. Detecting such a tiny current against a background of ambient/naturally occurring IR was not easy.

I cannot seem to find any suitable photodiodes with a 1cm^2 detection area. Most are only 1 upto 5mm^2. Although presumably if you cannot avoid a smaller detector area, you can factor-up the measurement? E.g if it is 0.25cm^2 you would quadruple the reading? Or is it non-linear or more complicated than that?

The photodiode would also need to work with the visible blue (445nm) wavelength of my laser diode module. Whether you can use a broad-bandwidth device for a rangh of wavelengths, or have to use different specific photodiodes for each laser wavelength (445nm, 532nm, 632nm etc.)

Mention is made in various places to Hamamatsu, Centronic and UDT devices. Please does anyone know any part numbers? Are there any circuit/schematic diagrams for suitable photodiode amplifiers that work in conjunction with the photodiodes mentioned? Failing that I suppose the photodiodes' own datasheets would be a good place to start.


2) Measurement of Laser Power.

I understand there are various technologies to do this, including Silicon & thermopile. Complete meters with build in display & sensor, or separate sensor blocks that show their measurement on an external multimeter, such as a Fluke DVM. It also appears to be something that is not easily achieved on a DIY, semi-pro or home built basis. The implications of incorrect measurements could be gravely serious.

Is this the case, or are there possibilities for the skilled electronics engineer to make their own? Even if a few % is lost in accuracy.

I have used a professional meter in the past. It was basically a circular finned heatsink with a sensor in the centre, on a fairly heavy base. I think it was made by Coherent, but it was years ago.

As I no longer work for that professional laser display company any longer, I don't have access to that meter, or other useful things like an optical spectrum analyser. This was most useful, & could be used to display the wavelength(s)/spectral content of the laser emissions. It was useful to spot any stray 808nm Infra-Red in the beam.

I don't think I even want to know the cost of that analyser. Just glad I didn't drop it!

Thanks for reading, & I look forward to reading your suggestions.

Many thanks,

Simon B.

[mixedgas]

For a good Photodiode time, call Dr. Phil Hobbs

www.electrooptical.net

"Reaching the shot noise limit for 10$" has a good circuit overview for photodiodes.

You need a simple transimpedance amplifier, or as a alternative, just a good scope, a photodiode, and a 400 ohm resistor to load the PD.

Thorlabs.com will have larger cells for cheap. 1x1 cm cells are really just sold as solar cells these days, but they have lots of capacitance and are slow.

Steve