Pyroelectric Sensor Physics

Typical operation of a pyroelectric energy sensor.
Figure 1: Typical operation of a pyroelectric energy sensor.

Pyroelectric Optical Energy Sensors

The configuration of a typical pyroelectric sensor and its operational output can be seen in Figure 1. A pyroelectric material, which is usually crystalline, possesses an electric polarization, even in the absence of an applied voltage. An incident laser pulse heats the crystal, which causes the material to expand and produce a change in the polarization. Charge builds up on opposite surfaces of the crystal which generates a current flow that charges a capacitor. This charged capacitor induces a voltage whose amplitude change is proportional to the original laser pulse energy. Since it is the change in temperature that produces the current, pyroelectric detectors respond only to pulsed or modulated radiation. They respond much more rapidly to variations in radiation than thermopiles and are unaffected by steady background radiation. The response of a pyroelectric detector depends on the thermal time constant (governed by the thermal mass and thermal connections from the element to its surroundings) and the electrical time constant (the effective resistance and capacitance of the detector circuit). Consequently, small detectors with small thermal mass can have extremely rapid response. Adding a black coating to give uniform absorption over a wide spectral range is often used in pyroelectric sensors. However, this coating increases the thermal mass of the detector, which lowers the frequency response. This results in a temporal response vs. sensitivity tradeoff.

Pyroelectric detectors are typically used to measure the energy of pulsed lasers where the pulses may vary in width from fs to ms and in energy from sub µJ to J. As discussed in Radiometric Measurement, the thermal and electrical time constants of energy detectors are chosen so that each pulse is integrated. The peak of the output voltage is a measure of the charge produced by the detector and therefore, of the pulse energy. The charge dissipates before the arrival of the next pulse. The integration time, or fall time, imposes limitations on the minimum interval between pulses or the maximum repetition rate which can be accurately measured. It is possible for pyroelectric sensors to be used to measure the powers of low-level CW or quasi-CW light sources. To accomplish this, the radiation must be modulated or chopped prior to being detected such that the detector produces an AC output signal. Since the frequency response of the detector is much lower than for measuring pulsed radiation, the increased sensitivity typically results in a higher dynamic range in this configuration.

Pulse energy equation- Qe
Equation 3. Energy Density or Fluence (F).

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