Introduction to Power Meters
Advancing laser system applications have created the need for more beam precision, as well as more demanding and exact power and energy output requirements. These developments necessitate the need for highly accurate power/energy measurement devices.
Although the basic technology behind power/energy meters has changed little over the past 10 years, they have adapted to specialized applications for diverse purposes, including: measuring the pulsed light of IPL (Intense Pulsed Light) hair removal medical device lasers, highly sensitive meters for laser generated photolithography and special geometry meters adapted for tightly confined locations, etc. Still, the three basic measuring devices remain:
- Thermopile – Detectors which measure the amount of heat flowing through the detector by the heating effect on an array of thermocouples that measure the temperature drop across the thermopile; therefore, the reading is independent of ambient temperature. If the temperature of the cooled periphery goes up – the temperature of the inside of the thermopile goes up commensurately and the temperature drop provides the reading. As the mainstay of measurement devices, thermopile is virtually wavelength independent, durable and reliable – though it cannot measure repetitive pulses and is ill-suited for very low powers and energies.
- Photodiode – A photodiode detector based upon a semiconductor P-N junction that converts laser light impingement onto a photodiode into a current. When photons of light with an energy greater than the distinguishing band gap of the photodiode strike the detector – they generate an electron hole pair which is collected within the attendant circuit. These detectors are highly sensitive, possess a wide dynamic range with robust linearity at low powers. However, they are highly wavelength dependent, susceptible to saturation at low powers therefore exhibiting nonlinear effects – thus well-suited to low power detection.
- Pyroelectric – As laser light strikes the absorbing surface of the detector, heat generation polarizes the crystal and thereby creates an equal/opposite charge on the two surfaces of the detector. The surface of the detector is metalized thus the charge is collected onto the parallel capacitor – irrespective of where the laser beam hits the surface. Therefore, the charge of the capacitor is proportional to the pulse energy. With each subsequent pulse, the voltage on the capacitor is read, the capacitor is then electronically discharged – and ready for the next pulse. Pyroelectric detectors are highly sensitive and useful for repetitively pulsed applications and effective to kHz PRFs. Their heightened sensitivity may require an attenuator/diffuser positioned in front of the sensor crystal to minimize the energy resident onto the pyroelectric crystal.