A thermopile is simply an array of thermocouples connected in series and close together. The most common application is when a voltage is applied to cool one side of the thermopile and whatever it is bonded to.
Thermopiles for laser power measurement, however, are used in the opposite fashion. That is, a temperature difference is used to create a voltage. On one side, the material heated by a laser (or any other radiation source), and on the other side is a heatsink. The laser's energy that is absorbed by the material is converted to heat.
With the hot absorber on one surface and the cold heatsink on the other, there is a temperature difference across the thermo-electric device as the heat flows through it. This temperature difference causes the thermopile to generate a voltage. That voltage is proportional to the temperature difference, which in turn is proportional to the laser power. At Gentec-EO, we carefully design and build the thermopiles for our laser power detectors to make sure that we offer the best performance for your application. Our products in the UP series are based on the thermopile technology.

Specification

Maximum average power (continuous) : 50 W
Maximum average power (1 minute) : 90 W
Spectral range : 0.19 - 20 μm
Typical rise time : 0.6 sec
Typical power sensitivity : 0.65 mV/W
Power calibration uncertainty : ±2.5 %
Repeatability : ±0.5 %