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I am designing a transimpedance amplifier and was trying to simulate the circuit to study its characteristics. I came across this online design tool provided by Analog Devices that builds the transimpedance amplifier based on our input values. In the website the formula to calculate the photodiode current was given as,

Current(A) = Peak Light Intensity(w/m2) x Sensitivity of the photodiode(A/W) x Active Area of the photodiode(m2)

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I have the values of Sensitivity and Active area that I acquired from the datasheet of the photodiode. I also have the power of the laser source which is 5mW. How can I calculate the light intensity value using the power of the laser source? What other values should I consider for calculating the intensity of the laser source.

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It depends.

Are you able to focus (nearly) the entire laser beam onto the photodiode active area?

Then you can just use the beam power (5 mW) and ignore the active area factor.

Is the photodiode area smaller than the beam size?

Then you need to know the beam profile (often Gaussian, but it could be otherwise depending on the laser's output optics), and figure out the beam intensity in the part of the beam you are going to place the photodiode. For simulation it might be good enough to just make a rough estimate of what fraction of the beam you're going to capture (is it 50%, 5%, 0.5% ...?) --- or to use your simulation to work out what portion of the beam you need to capture to achieve the response you want and then design your optics from that requirement.

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