One of the standard detectors is used for the wavelengths between 200 to 1100 nm and the other for 780 to1800 nm. Newport uses two standard detectors that are sent to NIST with an exception of 818-IS and 918D-IS series detectors, which are sent to NRC (National Research Council Canada) for calibration on an annual basis. A deuterium lamp is used in the ultraviolet range up to 310 nm and a tungsten lamp is used thereafter in the visible and near infrared. Three gratings and two light sources are used by the monochromator to maximize the signal to noise performance over the 200 to 1800 nm wavelength ranges. The detector calibrations are done using a double monochromator in order to minimize stray optical noise, especially in the ultraviolet. The disk thermopile also has a much faster natural response time The primary difference is that the heat flows radially through the disk, which can handle more average power, especially with blown air or water-cooling. The thermocouples generate a voltage corresponding to this difference, just like in the wafer thermopile. The laser power heats the absorber in the center and creates a temperature difference between the center and the edge. One set of junctions is arrayed under the aperture while the alternate set is near the edge of the disk, which is attached to a massive heat sink. The disk is made of two sets of junctions laid out radially. That is when the disk thermopile shows its value. When a lot of average power is absorbed and has to flow through the small gap containing the thermocouples, the temperature becomes hot enough to damage the thermocouple junctions. One is the wafer type thermopile and the other is the disk. There are two kinds of thermopiles used in laser power measurement. Energy densities greater than 3 J/cm 2 and peak power densities above 100,000 MW/cm 2 can be handled this way depending on the wavelength. Unlike the broader band materials, which absorb the energy right on the surface, the energy is absorbed throughout the thickness of the material. How much is shown by the spectral absorptivity response curve for the material.įor applications that require an extremely high concentration of power and energy in a small area and a small time period for a single wavelength, a volume absorber would be necessary. A fraction is reflected that can vary from a few percent to 50 percent of the total optical power, depending on the material and intended application. This material absorbs most of the light energy from the laser and converts it to heat. That is because its properties define much of the performance of the detector, especially its resistance to pulse damage. The optically absorbing material is one of the most important parts of the detector. Please see Thermopile Sensor Physics for additional information, The monitor measures this voltage to provide the laser power reading in Watts. That voltage is proportional to the temperature difference, which is proportional to the laser power. This temperature difference causes the thermopile to generate a voltage. There is a temperature difference across the thermo electric device as the heat flows through it. The laser energy absorbed by that material is converted to heat. One side of the material is heated by the laser and the other side is a heat sink. That is, a temperature difference is used to create a voltage. However, thermopiles for laser power measurement are used in the opposite fashion. 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. The basic laser high-power (>1 Watt) detector is essentially a thermopile. Please see Photodiode Sensor Physics for additional information. Because of its low shunt resistance (50 kΩ typical), tens of picoamperes can be resolved at best. An exception to this rule is when the shunt resistance of the photodiode is small as with the Germanium photodiode ( 818-IR and 918D-IR). Typically, measurements can be made down to the sub-picoampere regime with good reproducibility, even at room temperatures. The photocurrent produced by the photodiode is measured directly by the power meter using an operational amplifier circuit known as a transimpedance amplifier. To put these effects in perspective, if a detector were biased as in the photoconductive mode, the dark current would be about three decades larger than the noise equivalent current of an unbiased detector. Thus, the shot noise contributed by the dark current is also eliminated. Consequently, this almost eliminates the dark current altogether. When a photodiode is used in the photovoltaic mode the voltage across the diode is kept at zero volts. Where q is the charge of an electron, I dark is the dark current, and I photo is the photocurrent.
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