Oxygen 760nm spectroscopy - quick 'n' dirty

[JGarry]

'Morning all,

I wish to use a 760nm laser to measure the concentration of molecular oxygen in a gas. I've a laser (multi-mode, not TDLAS:QLD-760-10S), a photodiode (Si), and I've built an oven to hold the laser diode at a constant temperature to avoid mode-hopping.

Is it really as 'simple' as modulating the diode current at some frequency, amplifying the PD output, and then deconvolving the sampled output with the modulating frequency?

At RTP, with 20% O2, I've a gut feeling that 1m path length ought to give something like 0.1% absorption of the total laser power.

Any comments about technique?

 

[Charles Link]

Just a quick comment or two=I have done a fair amount of experimental spectroscopy and radiometry, but haven't ever attempted something quite like this. Would it be possible to find a wavelength where oxygen has a resonant absorption, or at least a stronger absorption? I think it's going to be difficult to detect and quantify absorptions that are less than 1.0%. It would be much easier to work with numbers like 25.0 % absorption or 30.0 % absorption.

 

[JGarry]

Regrettably, the thicket of (narrow and faint) lines around 760nm appears to be the canonically 'best' choice for molecular O2 detection. Correlation methods (Luo et al., Oxygen measurement by multimode diode lasers employing gas correlation spectroscopy, 2009) appear to work in a non-realtime fashion for path lengths as short as 3cm - but they use a ramped laser current to always tune for optimal absorption.
I think.

Thanks for the interest though, and 'yeah' - oh that O2 had deeper bands!

 

[Charles Link]

Regrettably, the thicket of (narrow and faint) lines around 760nm appears to be the canonically 'best' choice for molecular O2 detection. Correlation methods (Luo et al., Oxygen measurement by multimode diode lasers employing gas correlation spectroscopy, 2009) appear to work in a non-realtime fashion for path lengths as short as 3cm - but they use a ramped laser current to always tune for optimal absorption.
I think.

Thanks for the interest though, and 'yeah' - oh that O2 had deeper bands!

Just an idea: I don't know if it's practical or not: Is there a wavelength that you could pick up through scattering? Detecting a weakly scattered component, e.g. $90^o$ to the main beam, especially with a lock-in amplifier and/or boxcar integrator, might be a whole lot easier than resolving differences from 100% that are quite minute. You might be able to accurately quantify even a very small scattered component. $\\$ Editing: An additional idea would be to bring the main beam to a focus inside the gas sample and collect the scattered light from that focused point with a lens.

 

[JGarry]

:/ Any scattering (say, Rayleigh) will be pretty much identical for O2 as it is for N2 (we're looking at O2 in a N2 buffer) - given the similar sizes of the molecules.

So basically, (I think) it comes down to either having a temperature-held laser diode (and a calibrated photodiode sensing a fraction of the beam) to establish beam power, or building a reference path (beamsplitter, mirrors, etc.) that passes through either pure O2 or pure N2.

<tosses coin>

Again, thanks for the commentary - glad that this tickles some folks' neurons too!

 

[Charles Link]

One other alternative that might prove easier would be to take a small sample and mix with 100% hydrogen (It might require a slight heating, and it will give a slight "pop"), and see how much water you get, which could be readily measured even with IR spectroscopic techniques=assuming there is no water in the original sample.

 

[JGarry]

:D
Lasers *and* flames! What can go wrong?