# Determining the maximum input power of a photosensor for a given laser power input

I need to measure the average power of a 1550nm pulsed laser output using a photodiode. The datasheet of the Thorlabs FDG03 photodiode can be found here (or if these links fail, the product page with relevant links is here). I'm also planning to use the recommended circuit with Rload = 50 Ohm load and with 2.5V or 3V bias:

In my case, the pulsed laser's repetition rate is 1kHz, pulse width is 1ns and I can set the laser average power. At this page under "Pulse Calculations" tab, there is a formula which calculates the peak power of a pulsed laser as:

I'm trying to figure out what should be the maximum average power of the laser which does not damage the photodiode. My confusion is should I just look at the average power or the peak power.

The photodiode datasheet does not give detail about maximum optical input power. But it gives the maximum current as 5mA:

The responsivity is given as 0.85 A/W so I calculate the maximum power as:

Pin_max = 1000mW / 850mA * 5mA = 11mW

So if I'm not wrong the maximum optical input power of the photodiode is 11mW.

So in my case not to exceed this, I need to set the laser peak power to 11mW? If I do that and follow the equations below:

Pavg = 11mW * 1000Hz * 0.000000001 = 11nW

Does that mean I need to set the average max power of the pulsed laser to 11nW not to damage the photodiode? It is extremely small and I'm not sure if my way of doing is correct.

• Excuse me, but if I have interpreted the formula correctly, the peak power is: 11mW * 1ms/1ns= 11kW. Since I= Res*Pavg, I get : I=9.35mA. Commented Dec 21, 2023 at 8:38
• The coaxial cable "external" wire is not "grounded"? Commented Dec 21, 2023 at 8:41
• What is the beam angle of the laser and, at what distance is it relative to the photodiode? Commented Dec 21, 2023 at 10:42
• Just from experience and without looking at the datasheet, you will have to be very careful with that laser. At 1 KHz/1ns it will be very easy to evaporate the metallization right off the detector face regardless of the maximum photocurrent current rating. I recommend attenuating the laser as low as you can get away with. Commented Dec 21, 2023 at 14:57
• @Andyaka That is the question actually. How much attenuation I need(by means of distance or sphere attenuator ect.) not to damage the laser. For example, if I set the laser rep rate to 1kHz and average power to 1mW; after this stage how much optical power attenuation I need not to damage the laser? I just want to be sure how much attenuation I need before the photodiode. Commented Dec 21, 2023 at 17:55

My confusion is should I just look at the average power or the peak power.

The Example Calculation part of the Pulse Calculations tab of the page shows a calculation where:

• The energy per pulse is 11.8 pJ, which is described as seems low.
• The peak pulse power is calculated as 1.18 kW
• Which results in the conclusion:

It is not safe to use the detector to measure this pulsed laser emission, since the peak power of the pulses is >5 orders of magnitude higher than the detector's maximum peak optical input power.

Which suggests the peak power needs to be used.

The photodiode datasheet does not give detail about maximum optical input power. But it gives the maximum current as 5mA:

While the text in the question says the maximum current is 5 mA, the table has 10 mA. I assume the value of 5 mA is a typo, working through the calculations with a maximum current of 10 mA end up with the same maximum peak power of 11.7 mW for the photo detector.

Does that mean I need to set the average max power of the pulsed laser to 11nW not to damage the photodiode?

Is it possible to attenuate how much of the laser power reaches the detector?

Since photodiodes typically have relatively maximum peak optical input power limits, the laser pulses should be attenuated as necessary to protect the sensor.

Edit:

After getting a downvote realised that this answer doesn't necessarily address the following:

The photodiode datasheet does not give detail about maximum optical input power.

Perhaps there is a flaw in my thinking behind calculating the peak pulse power and you should ask Thorlabs about the peak pulse power allowed.

@Andy aka made a good point in a comment on the question that the beamwidth of the laser, and how far away the 7.1 mm2 active area of the FDG03 photodiode is from the laser, will affect how much laser power is received by the FDG03.