Laser Power Measurement Principle
Comparison of Detection Technologies
The three main characteristics and differentiators of laser power detectors are 1) wavelength, 2) rise time and 3) power threshold.
The most common detectors for low powers are photodiodes due to their good power resolution and fast response time. Light is absorbed in the material and converted into an electrical current proportional to the radiation power. Depending on the material composition, each wavelength results in a different signal and only a narrow bandwidth of wavelengths lead to an electric signal at all, as can be seen in the chart on the right. Besides the widely available Si, Ge and InGaAs are also used as photodetectors. The absorption characteristics of those two materials extend into the NIR spectrum, up to 1800 nm. When electrically biased appropriately, InGaAs can even absorb at higher wavelengths. All available photodiodes together cover a total range of UV, VIS, and NIR.
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The working principle of thermopile sensors is fundamentally different from that of photodiodes (to learn more about this, click here). In an absorption layer on the sensor surface, the incident radiation is transformed into heat energy. This heat energy is measured by thermopile sensors. Thermopile sensors are therefore sensitive to radiation of all wavelengths, as long as the absorptive coating is efficient. Broad band absorbers are typically used and hence the spectrum from DUV to MIR lies within the detection range.
Due to their working principles, photodiodes and thermopile detectors have different signal response times.
Photodiodes are very fast and react from 1 ns to incoming radiation. When read out with commercially available power meters, the speed is lowered due to the electronics and results in a rise time of 0.1 – 0.2 s. Therefore, photodetectors are suitable for fast measurements and a good choice, if the wavelength range and the power range (see below) are appropriate.
Thermopile sensors are slower in their signal response. Since the sensors are based on thermal transport, the rise time is related to the thermal mass that needs to be heated. Therefore, larger or thicker thermopiles have longer response times than thinner or smaller ones. Peltier elements respond to incoming radiation with a minimum rise time of 1.8s due to their large thermal mass. Conventional disk-type sensors have minimum response times of 1s. greenTEG’s Thermal Laser Power Detectors are considerably thinner and are able to achieve response times of 0.1 s.
Both detector types (photodiodes and thermopile sensors) cover a large dynamic range of powers, (see graph on the right) and are therefore applied in different settings.