Thermopile Sensor Working Principle

Laser measurements based on heat

Thermopile sensors measure the radiation power of a laser beam. When a laser beam hits the surface of a thermopile sensor, the incident radiation is absorbed within the coating layer and transformed into heat. This heat is then flowing through the sensor from the top surface to the bottom, causing a temperature difference across the sensor. Due to the thermoelectric effect, the temperature difference causes an electrical voltage to build up within the sensor element. This output voltage is directly proportional to the power of the incoming radiation.

The absorber coating determines which spectral range of the radiation is detected. When using a broadband absorber with a flat spectrum, thermopile sensors are therefore sensitive to radiation of all wavelengths.

In many applications, photodiodes are used to measure laser power. The working principle of thermopile sensors is fundamentally different from that of photodiodes. The difference is explained in detail here.

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Crucial characteristics of thermopile sensors

The usability of thermopile sensors strongly depends on these 2 characteristics:

  1. Absorption Coating
    The absorption coating determines how well radiation from different wavelengths is converted into heat. Optimal coatings have a flat spectral dependence, which cause the same voltage response of the sensor at different wavelengths. All of greenTEG’s Laser Power Detectors have a flat spectral curve from 190 nm to 15 µm. Further information about the thermopile sensor elements is available at our .
  1. Arrangement of Thermocouples
    The temperature difference caused by the optical power, is transformed into an electrical voltage by so called thermocouples. For a good sensor performance, both the material but also the design need to be considered carefully. Three different arrangement types of thermopile detectors are available today.
  • Disk modules have a circular absorption area in the center. The thermopiles are arranged in a ray-like manner, pointing outward towards an aluminum ring acting as a heat sink. greenTEG does not manufacture these types of detectors. Ophir, one of the manufacturers of these detector types, has a good introduction here: thermopile disc detector introduction (external link).
  • Peltier elements with axially arranged thermocouples are sometimes used. These have a more compact design, as there is no passive area on the sensor. Wikipedia offers a great introduction about Peltier elements (external link).
  • Axially arranged thermocouples developed at ETH Zurich (see image on the right). This technology allows for thermocouples with considerably smaller diameter (>300 um diameter). The sensor design allows for smaller size, increased mechanical robustness and increased sensitivity.
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