Frequently Asked Questions

Input Light Intensity

What is current non-linearity?
What is the maximum light power I can irradiate on the detector before the output becomes non-linear? At what level does a typical photodiode saturate?
Can the photodiode be damaged by very high incident light power on the active area?
How can the light intensity received by the photodiode be reduced?

1. What is current non-linearity ?
A photodiode is non-linear when an increase or decrease in the amount of incident light power does not generate an equal amount of photocurrent in the same photodiode with a tolerance of +/- 1%. This is the range where responsivity remains constant within +/-1%. That is:

Above this range, also called dynamic range, the device may saturate completely at any incident light power level.

2. What is the maximum light power I can irradiate on the detector before the output becomes non-linear? At what level does a typical photodiode saturate ?
The maximum light power receivable by the detector before it saturates depends on the photodiode itself, the reverse voltage applied Vbias (Volts), and the resistance R (Ohms) of the circuit in which the detector is used and the wavelength used.

The maximum photocurrent created in the linear range is Imax = Vbias / R (A mperes) and the corresponding Optical power is given by: Pmax = Imax / Rl (Watts) where Rl is the responsivity of the photodiode and varies with the wavelength.

In the Photovoltaic (PV) mode, the previous definition is still correct but the voltage to consider is the silicon built-in value Built-in = 0.3 Volt. In the Photoconductive (PC) mode, the reverse voltage can reach up to 30 Volts (depending on the device). It is easy to understand that PC mode photodiodes have a wider dynamic range.

To give an idea, Max is between 1 and 100 milliwatts..

3. Can the photodiode be damaged by very high incident light power on the active area?
Once the Saturation point is reached, the behavior of the photodiode becomes more and more non-linear until the Damage threshold point is reached. At this point the photodiode is no longer able to turn excess in incident optical power into a photocurrent output and all the extra energy received by the device would be absorbed, typically as heat. This heat inside the device can easily damage it irreversibly as it endangers the fragile wire bonds on the surface of the photodiode.

4. How can the light intensity received by the photodiode be reduced?
To reduce the light intensity received by the photodiode, one could use optical instruments such as beam splitters, filters or polarizers placed on the optical path between the source and the detector.