page_title: Thermopile laser sensor
section_title: Working principle and structure
hierarchical_section_title: Thermopile laser sensor / Working principle and structure
caption_attribution_description: English: Working Principle
context_page_description: Thermopile laser sensors are used for measuring laser power from a few µW to several W. The incoming radiation of the laser is converted into heat energy at the surface. This heat input produces a temperature gradient across the sensor. Making use of the thermoelectric effect a voltage is generated by this temperature gradient. Since the voltage is directly proportional to the incoming radiation, it can be directly related to the irradiation power.
  Unlike photodiodes, thermopile sensors can be used for a broad spectrum of wavelengths ranging from UV to MIR. Further, photodiodes are reverse biased and saturate for optical powers above a certain value, making thermopile sensors suitable for high power measurements.
  Pyroelectric sensor and calorimeter are commonly used for measuring the energy of laser pulses. Pyroelectric sensor can measure low to medium energies and are prone to microphonic effects. Calorimeters are capable of measuring high energies but have large response times.
context_section_description: As shown in Fig 2, a thermopile laser sensor consists of several thermocouples connected in series with one junction type (hot junction at temperature T₁) being exposed to an absorption area and the other junction type (cold junction at temperature T₂) being exposed to a heat sink. 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 then induces a temperature gradient across the sensor given as
   [K/m],
  where t is the thickness of the sensor.
  Due to the thermoelectric effect, the temperature difference causes an electrical voltage to build up within each thermocouple. This output voltage is directly proportional to the power of the incoming radiation. Since a large number of thermopiles are typically connected in series, voltages of several µV to V are reached.
  In general, a thermopile sensor consists of three elements: an absorber, the sensor element and a cooling body to dissipate the incoming heat.
caption: Figure 2:[8] Working principle of a thermal laser sensor (Adapted from figure 3 with permission)
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