# Estimate minimum detectable light pulse in a photodiode (5 ms, 44 kHz modulated)?

I'll use an IR LED modulated at 44 kHz to illuminate particles in air or water, and will use a silicon photodiode to detect weak pulses of scattered light as they pass through the LED's collimated beam.

Photodiode is shielded from direct light, so it will exhibit only dark current except for say roughly 5 ms pulses, when light scattered by the particle illuminates the detector.

An example of a detector's specs are shown below, it needs to be a large area detector for geometrical reasons.

I'm trying to make a ballpark estimate of how weak of a pulse I can reliably detect without getting a lot of false positives from noise.

I'll amplify the signal then use a simple diode + bandpass filter (say 100 Hz to 1 kHz) to remove the 44 kHz but pass the envelope of the 5 ms pulses.

With a bandwidth of ~1 kHz, detectors dark current of 200 pA means every 5 milliseconds there will be about 6 million electrons, with an RMS (shot noise) of about 2500 e-. So if my 5 ms light pulses produce say 10,000 or 20,000 e-h pairs and I set a threshold there, I'm in the right ballpark.

The 0.55 A/W at 850nm means each photon has about an 80% chance of making an e-h pair.

However, I haven't used the capacitance of the photodiode at all, nor the NEP (1.5E-14 W/Hz^1/2 @ 1550 nm) which curiously is specified at a wavelength for which silicon is blind. In other words, I haven't designed an amplifier and threshold discriminator yet.

Is disaster waiting for me around the bend, or is this likely to work with a reasonable low nose amplifier and bandpass filter?

Some spec data for an example 10x10mm PIN photodiode: https://www.thorlabs.com/thorproduct.cfm?partnumber=FDS10X10

• If you're doing anything at this low a light level, you might want to look into a photomultiplier instead of a photodiode. – Hearth Jan 20 at 16:31
• A phototransistor may be a cheaper option, too, but they are slower to operate than photomultipliers or photodiodes. This may be acceptable to you; 5ms is probably long enough to activate a phototransistor. – Hearth Jan 20 at 16:41
• My version of this datasheet says the NEP was measured at 960 nm (ie the peak sensitivity) – D Duck Jan 20 at 20:16
• Use a lens to increase the solid angle of light scattered from the particles. You can then use a smaller, less expensive photodiode. – D Duck Jan 20 at 20:21
• Simply put, unless you specify the particle density in the suspension, there is no possible way to answer the question. Once you do that, you then need to specify both the particle reflectance and the optical setup you've got. And, just out of curiosity, are attempting to detect single particles? – WhatRoughBeast Jan 22 at 0:42