4.2 Controlling the PM100A
20
with different coatings. Black broad band coating as absorber does not need a
wavelength correction because the response is nearly flat from the UV to the mid-IR.
Special hard coatings can withstand high power levels and show also a good
spectral flatness. Further Thorlabs offers special heads for Excimer lasers with large
beam diameters and short pulse lasers with coatings for very high peak power levels.
The output voltage of a thermal sensor is linear to the incident laser power, as long
the thermal system is properly zeroed. The main application area for thermal sensors
is the measurement of high power levels from 100mW. Thorlabs offers also a special
thermally isolated thermal head with flat response and on power levels starting in the
µW range.
4.2.5.2.1 Readout acceleration
Thermal sensors show a fairly slow response. When laser power hits the active area
it takes, depending on the sensor type, from some 1 up to 20 seconds until the
system has settled and the power reading shows the right value.
Through special circuitry this time can be shortened to approximately 1-3 seconds by
„predicting‟ the final power value. The PM100A automatically uses the right
adjustment of the electronics to the time constant of the thermal sensor.
Nevertheless this circuitry has the disadvantage of inducing some noise to the
measurement value. Therefore the acceleration circuit can be disabled / enabled in
the system Menu: MENU \ Accelerator On/Off
4.2.5.2.2 Custom Thermal Elements
Custom thermal elements can be used for power measurements with the PM100A.
Therefore the PM100A needs to be set to the „Thermopile‟ default setting in the
system menu:
MENU \ Adapter Type \ Thermo
To perform measurements, a thermal element and an interlock must be connected to
the sensor input (see chapter 7.4 for the connector pin out). The operation is similar
to thermal power sensors with following exceptions:
- The response value in V/W is indicated instead of the operation wavelength.
- When pressing the λ key the response value (for one wavelength) can be set.
- In system menu on page 3, the time constant (
0 – 63%
) of the connected
thermal sensor can be set. This time constant value adjusts the acceleration
circuit for the measurement prediction.
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