Cooling of Laser Power Meters and Laser Beam Stops
Laser output power is a fundamental parameter which requires monitoring regardless if it is a scientific or industrial application. When the laser average power exceeds a few hundred watts, water cooling is recommended to prevent damage to the detector. Aspen Systems visited IPG Photonics in Marlborough with a laser power measurement device from Gentec-EO and a custom Aspen Systems liquid chiller.
How Laser Power Detectors Work
The basic laser power detector is essentially a thermopile. The more familiar application for thermopiles, in fact where the common name “thermo electric cooler” comes from, is when a voltage is applied to cool one side of the thermopile and whatever it is bonded to. Thermopiles for laser power measurement however are used in the opposite fashion. That is, a temperature difference is used to create a voltage. On one side is material heated by the laser and on the other is a heat sink.
The laser energy absorbed by that material is converted to heat. With the hot absorber on one surface and the cold heat sink on the other, there is a temperature difference across the thermo electric device as the heat flows through it. This temperature difference causes the thermopile to generate a voltage. That voltage is proportional to the temperature difference which in turn is proportional to the laser power. The monitor measures this voltage to provide the laser power reading in watts. The figure shows the fundamentals of the thermopile-based power detectors.
What’s the problem?
Thermopiles used in laser power detectors have a limited heat load before they reach their damage threshold. Focused laser beams of only a few tens of watts can create localized damage on the detector. It is always recommended to defocus the laser beam to spread the power density over a greater area of the detector. Even with a defocused laser beam, when the power exceeds a few hundred watts, air or water cooling is required.
Aspen to the Rescue – how did we save the day?
Aspen Systems has developed a range of liquid chillers well suited to cool laser power meters. The new Rack Mount Chiller (RMC) series includes the RMC-800 and RMC-1100. These turn-key systems offer impressive cooling power in a 2u (3.5 inches tall) package.