Photodetector for short laser pulse

Question

I'm looking at building a detection system for fast laser pulses (532nm wavelength for 250 ps duration). It seems to me that this will require a very fast photodiode with sub nano-secind rise time. Are such components manufactured and sold commercially, and if so, what is an estimate of the cost I can expect.

Answer 1

Photodiodes are not specified by rise time. They are specified by capacitance, and this capacitance combined with the gain and stability requirements of the transimpedance amplifier defines their rise time. Then there are tricks like bootstrapping and cascoding to reduce the effective photodiode capacitance so increase the speed for a given feedback resistor but the relationship is not straightforward.

What this means is no one can give you a price range because no oen can even narrow down what photodiode you might even need since you have not provided your level of illumination which means no one can narrow down what photodiodes you may be looking at. A huge part of photodiode cost is active area. Larger active area means more capacitance which means a slower photodiode, but also collects more light so better for low illumination. Since you are shining a laser at it you probably don't need a very sensitive photodiode.

However, you also did not specify detection or measurement. This matters because measurement requires a larger active area to capture the entire laser spot to deal with spatial variations in the laser spot and/or to not saturate the laser spot. The required active area could literally make the difference between needing a $1 photodiode and a $100 or even $500 photodiode.

A 10-90% rise time works out to approximately have the highest Fourier frequency component of the signal be:

$$BW = \frac{0.35}{t_{rise10-90}}$$

So a 1ns rise time would require a bandwidth of 350MHz.

You will need to read how a transimpedance amplifier works conceptually with a photodiode. This is a basic photodiode transimpedance amplifier calculations which you will also definitely need to read: https://www.ti.com/lit/ug/tidu535/tidu535.pdf?ts=1664111058976

It's a bit confusing so I will leave these bullet points:

1. Reverse biasing a photodiode reduces the capacitance which lets you operate it faster at the expense of some noise
2. A real transimpedance amplifier required a feedback capacitance to be stable
3. The feedback resistor in the amplifier determines the gain which means you need to know the photodiode response and illumination to know the gain you require
4. The feedback resistor and feedback capacitance together determine the bandwidth.
5. ...but wait...where does the photodiode capacitance come in to determine the bandwidth then? It doesn't really...at least not directly. Two things are at work: required gain and stability. Things that increase stability will put an upper limit on bandwidth but they don't determine the bandwidth.

Here is the convoluted relationship feedback resistance, feedback capacitance, photodiode capacitance, bandwidth, stability, gain, and opamp gain bandwidth product (GBWP):

• higher value feedback resistors means more gain but lower bandwidth
• higher value feedback resistors will increase the GBWP required for stability
• higher feedback capacitances mean lower bandwidth
• lower feedback capacitances will require higher GBWP to remain stable
• higher photodiode capacitance and higher op-amp input capacitances will require higher GBWP for stability

If your calculations tell you that to get your combination of gain and bandwidth require photodiode capacitances or feedback capacitances that approach <1pF, or even a few pF then your specifications are not feasible. In that case you need to start augmenting the basic transimpedance amp such as reverse biasing the photodiode (by far the simplest and almost free), bootstrapping it , and/or cascoding or other techniques.

If you don't understand any terms I used that doesn't matter. I didn't explain everything required anyways and what I did explain was in almost zero detail. You are going to need to read about these topics starting with how a transimpedance amp works and how to size components for them when using a photodiode (which is the link I posted). Augmenting the basic circuit is a whole other story: https://zeus.phys.uconn.edu/halld/hpd-3-2005/frontends.pdf

There are many levels of understanding between the two links.

For 350MHz you are going to need augmentation such as bootstrapping and/or cascoding.