Quadrant photodiode selection advice

[Doc]

Hi all,

I have a post related to this question over in the 'general engineering' forum located here:
https://www.physicsforums.com/threads/beam-centering-using-a-photodiode.926814/#post-5854037

However, my question is a little more specific hence the new thread.

I need to select an appropriate quadrant photodiode for a laser beam centering application. I am a mechanical engineer with almost no electronics experience so I would very much appreciate some guidance/feedback.

I would like to make a list of different photodiodes and then select the most appropriate from that list. The laser will operate at two separate power levels: 20 W operationally, and 1 mW for alignment purposes. This is what I think I need to do:

1. Determine the amount of power likely to be incident on the photodiode.
2. Calculate the saturation power of different photodiodes. If the incident power > saturation power I will need to place an ND filter before the photodiode. The saturation power is basically the upper limit of what the photodiode can tolerate before its output becomes non-linear.
3. Calculate the minimum detectable power for different photodiodes. This is the noise floor?
4. Calculate the signal to noise ratio (SNR) for different photodiodes.

I have been using some resources to try to get this done including:
https://www.hamamatsu.com/resources/pdf/ssd/e02_handbook_si_photodiode.pdf
http://www.hamamatsu.com/jp/en/community/optical_sensors/articles/wits/index.html

But I'm getting a bit confused with a few different things.

Number 1. is fine, I have done that.

Number 2. I am having trouble with. I have an equation from one of my resources:
P_sat = (V_B + V_R) / (R_S + R_L)σ

The datasheet I am looking at is at the link below:
(https://www.hamamatsu.com/resources/pdf/ssd/s5980_etc_kpin1012e.pdf)

Where:
V_B is the bias voltage or contact voltage. Apparently this is 0.2 to 0.3 Volts though the guide doesn't specify where this number comes from.
V_R is the reverse voltage, found on the datasheet. Although, the datasheet can specify the maximum reverse voltage, but also lists other values for reverse voltage when specifying other quantities i.e. dark current is 0.3 nA when the reverse voltage is 10 V. Which one do I use?
R_S is the series resistance. I don't know where this comes from, nor does it seem to be on any datasheet I have seen.
R_L is the load resistance. 50 Ohms.
σ is the photodetector photosensivity, I have this from the datasheet.

I have tried calculating this and I get something like 0.5 W (that seems really high?), when the guides seem to say that saturation power of a typical photodiode is around 10 mW.

Number 3. I am also having issues with. The equation I am using is:
P_min = NEP×√B

Where:
NEP is the noise equivalent power. I have this from the datasheet.
B is the bandwidth. This I am really confused about and have no idea what to use. I assume that a greater bandwidth implies a more sensitive detector?

I think number 4. is okay.

One other thing I am wondering about is the two different power levels. I could have a couple of different scenarios here:
a. The incident power of both power modes is < saturation power of the diode. That is fine, no ND filter is needed. Will I need to amplify the signal (current output from the photodiode)? How would I even know if I needed to amplify the signal?
b. The incident power of the high power mode is > saturation power of the diode, but the low power mode is < saturation power of the diode. I will need an ND filter to protect the photodiode in high power mode, but will this make the signal too weak in low power mode? I am assuming that any amplification is done in electronics and cannot be activated/deactivated in-situ? If there is an amplification electronics stage to amplify the low power signal, then the high power signal will also be amplified? Is that a problem?

Okay I think I'll leave it there. I know that I have asked a LOT in this post, but any help or guidance would be appreciated.

Regards,
Doc

 

[Baluncore]

I replied to your other thread.
We need an accurate plot of intensity across the beam, with the aperture shown.