# How to measure the bandwidth of a photodetector?

• Durianpastry
In summary, the conversation discusses the concept of bandwidths in photodetectors, specifically the electrical bandwidth used to calculate shot noise. The speaker is unsure of how to measure the electrical bandwidth and asks for clarification. Suggestions are given to consult the datasheet or manufacturer, or to derive a test based on the application. The conversation also mentions using a network analyzer to measure the high frequency characteristics of parasitic elements in the photodetector.
Durianpastry
TL;DR Summary
how to measure the bandwidth of a photodetector and to calculate the shot noise?
I am new to photodetectors. I know there are usually two concepts of bandwidths of photodetectors, i.e the spectral and electrical.
What I do not understand is the electrical bandwidth.
The electrical bandwidth is used to calculate the shot noise of a photodetector. However, the papers usually don't show how to get the bandwidth.
I searched some references, and the bandwidth is related to capacitance and series resistance. So may I measure the EIS to get the capacitance and resistance to calculate the bandwidth?
Is the bandwidth used to calculate the shot noise the same with 3dB frequency?
But it seems that they are not the same?
I am so confused...
May someone tell me how to measure the electrical bandwidth of photodetectors?

Why not just look at the datasheet for your detector, or ask the manufacturer? They have already characterized them.

sophiecentaur and berkeman
Do you have an application that this is important - or is it just a general question? It would seem - given a specific application, you could derive a test based on that vs how the OEM characterized it.

Do you have an application that this is important - or is it just a general question? It would seem - given a specific application, you could derive a test based on that vs how the OEM characterized it.
Well, it is a general question about the bandwidth.
DaveE said:
Why not just look at the datasheet for your detector, or ask the manufacturer? They have already characterized them.
Because I fabricate one photodetector by myself and I do not know how to characterize it...

If you apply a DC reverse bias and block all radiation from hitting the junction, then the equivalent circuit is basically a capacitor (bias dependent, of course), a small series resistance, and a large shunt resistance. You can connect this to a network analyzer (or other instruments) with appropriate DC blocks to measure the high frequency characteristics (like S11, for example) of these parasitic elements.

DaveE said:
If you apply a DC reverse bias and block all radiation from hitting the junction, then the equivalent circuit is basically a capacitor (bias dependent, of course), a small series resistance, and a large shunt resistance. You can connect this to a network analyzer (or other instruments) with appropriate DC blocks to measure the high frequency characteristics (like S11, for example) of these parasitic elements.
Thank you!

berkeman

## 1. How do I measure the bandwidth of a photodetector using a light source?

To measure the bandwidth of a photodetector, you will need to use a light source with a known frequency and intensity. This can be a laser, LED, or other light source. The light source should be directed onto the photodetector, and the output signal should be measured using a oscilloscope or other signal analyzer. By varying the frequency of the light source and measuring the corresponding output signal, you can determine the bandwidth of the photodetector.

## 2. What is the significance of measuring the bandwidth of a photodetector?

The bandwidth of a photodetector is a measure of its ability to respond to changes in light intensity. It is an important parameter to consider when choosing a photodetector for a specific application, as it determines the speed and accuracy of the detector. A higher bandwidth allows for faster response times and better resolution.

## 3. How can I calculate the bandwidth of a photodetector from the measured data?

The bandwidth of a photodetector can be calculated using the rise time or fall time of the output signal. The rise time is the time it takes for the output signal to rise from 10% to 90% of its maximum value, while the fall time is the time it takes for the signal to fall from 90% to 10%. The bandwidth can be calculated using the formula: bandwidth = 0.35 / (rise time + fall time).

## 4. Can the bandwidth of a photodetector be improved?

Yes, the bandwidth of a photodetector can be improved by using a higher quality detector with faster response times, or by using amplifiers and filters to enhance the signal. Additionally, optimizing the setup and alignment of the light source and photodetector can also improve the bandwidth.

## 5. Are there any factors that can affect the accuracy of bandwidth measurements for a photodetector?

Yes, there are several factors that can affect the accuracy of bandwidth measurements for a photodetector. These include noise in the system, temperature fluctuations, and non-linearity of the detector. It is important to carefully control these factors and perform multiple measurements to ensure accurate results.

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