How is laser power output measured?

One specification that is always in the description of lasers is its power level, but have you ever wondered how these laser manufacturers measure laser power?

There are a lot of different ways to measure this parameter. Three technologies are often found in industrial power detectors: photodetectors, thermopiles and water calorimeters.

Let me present how each technology works as well as their advantages and disadvantages.

Laser power measurement with photodetectors

Photodetectors are mainly used for power in the picowatt to milliwatt range. They are made from semiconductors with a p-n junction that converts photons coming from a light source into an electric current. The more photons available, higher is the current.

The wavelength range of a photodetector depends on the semiconductor used. For example, Germanium photodetectors are often used with the 1550 nm wavelength that is used in telecommunication since it is compatible with this wavelength.

Photodetectors have a short response time which makes them the preferred choice in applications where power variations need to be quickly monitored. Even if photodetectors are made for lower powers, it is possible to add attenuation, a pick-off or an integrating sphere in order to use photodetectors at higher power.

One of the downsides of these detectors is their absorption that depends on the wavelength. It is important to select the correct wavelength; otherwise, the measurement will be way off. A good example of photodetectors is our PH series.

Thermopile laser power detectors

Thermopiles are a type of power detectors that uses temperature sensors called thermocouples. These sensors are made from two different metals. When the temperature of the junction between them changes, a difference in potential is created. The number of volts created is related to the temperature of the junction. When used for laser power measurements, multiple thermocouples in series are used to get a more accurate measurement.

In front of the thermocouples, there is what is called an absorber. This absorber needs to be able to resist to high power density and to absorb most of the laser power in order to avoid back reflections. It is even better if the absorption is almost the same at every wavelength.

Then, the laser power is absorbed and the detector heats-up. When the temperature of the detector becomes stable, the voltage created by the thermopile is directly proportional to the power level and can be calibrated. Thermopiles are usually used for power levels from the milliwatt range to a few kilowatts. In order for the temperature to remain stable, various cooling options are used. The simplest one is to use a heatsink in order to improve convection. If the power is above 100 W, it is required to use active cooling like a fan or a water circuit in order to extract the heat and keep the detector temperature stable. Good examples of thermopiles can be found in our UP series.

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Water calorimeters for high power

Water calorimeters are used for lasers in the kilowatts range. These detectors have a water circuit connected to a water chiller that keeps the temperature and flow constant when the water enters the detector. As water flows in the circuit, the temperature of the water rises.

This parameter is directly related to the heat created by the laser. It is possible to relate the flow rate and temperature difference of the water when it exits with the laser power.  Because water needs to pass through the entire detector before reaching the temperature sensor and flow meter, these detectors are quite slow. However, they are really effective for high power lasers.

The absorber of these water calorimeters also needs to be made from a material that absorbs most of the laser power and transforms it into heat. Since most high power laser applications require the beam to be focused, kilowatt level lasers tend to have small beam sizes and, therefore, high power densities.

They need to also minimize the back reflections since 5% of a kilowatt level laser is still a lot of power and can damage the equipment around the detector. This is why many high power detectors have a tube in front of the absorber to block back reflections. Our HP series is an example of water calorimeters.

These are the three main ways to measure laser power, but there are other technologies that are less common such as pyroelectric detectors or integrating sphere power detectors, but can also be used in certain applications. They can be harder to calibrate or only work with a certain type of lasers. If you are interested in any of these technologies, you can contact your local Gentec-EO representative, we’ll be happy to help.

Myriam Blanchet
Myriam Blanchet is Gentec-EO’s Inside Sales Representative for US and Canada. She has a Bachelor in Engineering Physics with a concentration in Signals and Communications. She grew up in Quebec’s Photonics Cluster which sparked her interest in everything light related.
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