Converting oscilloscope voltage to laser power

[kelly0303]

Hello! I have this photodiode which I connect to an oscilloscope using a normal BNC cable to an oscilloscope, terminated with a 50 Ohm resistor. I measure the output from an optical cavity (which is basically laser light), which in my case looks like a flat signal close to zero, and regular peaks (on the order of hundreds of mV amplitude) appearing from time to time (the details are not important, but I basically get a signal when the laser frequency, which I scan, matches the optical cavity length).

I would like to convert the maximum voltage in these peaks, call it $V_{max}$ to the laser power corresponding to that (basically the laser power output when my laser frequency is perfectly on resonance with my optical cavity), call it $P_{max}$. From the manual from that link I have the formula $R(\lambda) = \frac{I_{PD}}{P}$, where in my case, for $\lambda = 1064$ nm I get $I_{PD} = 0.45P$.

Also if I assume the resistance to be $R = 50$ Ohm (is this right?) and ignore the dark current, the measured voltage would be $V_{max}=RI_{max}=RI_{PD}=0.45RP$ so in my case I would get $P = \frac{V_{max}}{0.45R}=0.1/(50\times0.45)=4.5$ mW. Is this right or am I oversimplifying it (I don't need a super accurate result, but just a close estimate of the power)? Thank you!

 

[berkeman]

What bandwidth do you want to achieve? Optical diode detectors are not usually operated at zero bias; reverse bias improves bandwidth and gives you a better detector signal. Often done like this:

https://www.allaboutcircuits.com/te...hotoconductive-modes-of-photodiode-operation/

 

[kelly0303]

What bandwidth do you want to achieve? Optical diode detectors are not usually operated at zero bias; reverse bias improves bandwidth and gives you a better detector signal. Often done like this:

View attachment 325698

https://www.allaboutcircuits.com/te...hotoconductive-modes-of-photodiode-operation/

Ah I actually don't know much about this. What do you mean by bandwidth in this case? It is the frequency with which the peaks appear? Also how do I achieve reverse bias in practice (the diode has only one BNC output)?

 

[berkeman]

Can you post a link to the datasheet?

 

[kelly0303]

Can you post a link to the datasheet?

You mean the properties of the diode? It is in the link in the original post, this one: https://www.thorlabs.com/drawings/d...39C-F0B7-C7CF855725E31CB6/DET100A2-Manual.pdf

Thank you!

 

[DaveE]

I've done a lot of this in the past, and, sorry to say, there is no good way to do it with some sort of calibration standard. You have to measure it. That has to include the beam delivery stuff (lenses, apertures, mirrors, filters, diffusers, etc.) and the electronics (at least the 1st stage).

 

[berkeman]

I'm getting an error when I click on the link(s):

The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

 

[kelly0303]

I'm getting an error when I click on the link(s):

The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

Hmmm how about this one: https://www.thorlabs.com/search/thorsearch.cfm?search=DET100A2 This is the model and the documentation is attached to it on the website.

 

[kelly0303]

I've done a lot of this in the past, and, sorry to say, there is no good way to do it with some sort of calibration standard. You have to measure it. That has to include the beam delivery stuff (lenses, apertures, mirrors, filters, diffusers, etc.) and the electronics (at least the 1st stage).

I am not sure I understand. In my case I want to know the power after all the optical element (the light coming out of my setup goes directly into the diode). Shouldn't the conversion curve (Power to current) be enough to get that power (without caring about the optics before)?

 

[DaveE]

This one?
https://www.thorlabs.com/drawings/b...246-86BA-313A130BF3DE4C38/DET100A2-Manual.pdf

I didn't realize it was a detector assembly. You should ask Thor Labs about accuracy/calibration, they may have selected diodes, characterized, or calibrated them somehow. Si diode die responsivity isn't that well controlled, in my experience.

OTOH, I suppose we should have started by asking what sort of accuracy you need.

Shouldn't the conversion curve (Power to current) be enough to get that power

Yes, if you have it, it's accurate, and you realize exactly what light is illuminating the surface (i.e. how much of the beam is being sampled).

 

[hutchphd]

How accurate do you need to be? If you really want accuracy I would choose photoconductive mode rtaher than photovoltaic
What you have done should get you within a factor of 2 for sure. Be aware that the slope on the silicon photodiode spectral response falls off pretty fast above 900nm (you are at 45% at 1064 ) and that is Temp dependent.
If you are not rapidlly passing through the peak signal (>kHz) then the bandwidth should not matter.

 

[berkeman]

https://www.thorlabs.com/drawings/b...246-86BA-313A130BF3DE4C38/DET100A2-Manual.pdf

I didn't realize it was a detector assembly.

Yeah, that changes the question. Looks like that detector can be operated in either mode, and handles the reverse bias internally for photoconductive mode.

 

[TonyStewart]

This is a detector for 1064 nm. The DET100A2 sensitivity specs are ; [mA/mW]

1060​ 0.415​ 1080​ 0.295​	​
Vo = 0.391 mA*Rf/mW​	​
​	​