# Converting oscilloscope voltage to laser power

• kelly0303
In summary, the author has a photodiode connected to an oscilloscope and is trying to measure the power output from the device. The author has found that the power output is proportional to the laser frequency and the resistance of the diode. The author has also found that if the diode is reverse biased, the power output will be increased.
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!

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:

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berkeman said:
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

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)?

Can you post a link to the datasheet?

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'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.

berkeman said:
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.

DaveE said:
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)?

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.

kelly0303 said:
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.

DaveE
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​

## 1. How do I convert the voltage reading on my oscilloscope to laser power?

The conversion from voltage to laser power is dependent on the specific characteristics of your laser system. You will need to refer to the manufacturer's specifications or consult with a laser expert to determine the appropriate conversion factor.

## 2. Can I use a standard oscilloscope to measure laser power?

Yes, an oscilloscope can be used to measure laser power, but it is not the most accurate method. It is recommended to use a dedicated laser power meter for more precise measurements.

## 3. What is the relationship between voltage and laser power?

The relationship between voltage and laser power is not a direct one. It is affected by various factors such as the laser's efficiency, beam profile, and wavelength. Therefore, a conversion factor is needed to accurately convert voltage to laser power.

## 4. How can I determine the accuracy of my voltage to laser power conversion?

The accuracy of your conversion can be determined by comparing the results with those obtained from a calibrated laser power meter. It is recommended to perform this comparison periodically to ensure the accuracy of your conversion factor.

## 5. Are there any other methods for measuring laser power besides using an oscilloscope?

Yes, there are other methods for measuring laser power, such as using a pyroelectric sensor, thermopile sensor, or photodiode sensor. These methods are more accurate and reliable than using an oscilloscope. However, they may also be more expensive and require specialized equipment.

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