Thermopile Laser Sensors
Thermopile laser sensors (Fig 1) are used for measuring laser power from a few µW to several W (see section 2.4). 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 (see section 2.1).
Unlike photodiodes, thermopile sensors can be used for a broad spectrum of wavelengths ranging from UV to MIR (depending on the characteristics of the absorption coating at different wavelengths). Further, photodiodes are reverse biased and saturate for optical powers above a certain value (typically in mW), 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 (mJ to J) and are prone to microphonic effects. Calorimeters are capable of measuring high energies (mJ to kJ) but have large response times.
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
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