THORLABS PM100USB, Руководство по эксплуатации, Страница: 18 / 48

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PM100USB
· For long term measurements in free space applications it is necessary to provide con-
stant ambient light conditions, or to best shield the light path from external light sources.
· The temperature should be stable over the time of the measurement.
6.3 Power Measurement of Pulsed Signals
The PM100USB will read the average value of a pulsed signal if the following conditions apply:
Photodiode Sensor
A photodiode sensor can follow short pulses in the nano second range. To achieve this, it is im-
portant that the pulse peak power is within the maximum power range of the sensor and that
the power range in the software is set such that the peak power will not exceed this range.
Otherwise, the reading will clip at the end of the range and lead to a wrong average value. To
find the appropriate power range, the Min-Max display function is very helpful. Depending on
pulse length and repetition rate, the bandwidth setting will influence the power reading.
Thermal Sensor
For a thermal sensor, pulse length, repetition rate and peak power are not critical as long as the
peak power is lower than the damage threshold of the sensor. Some thermal sensors react
very slowly and will integrate the power incident on the active area of the sensor. Therefore the
time resolution of the thermal sensor is limited.
6.4 Line Width of Light Sources
The line width of light sources can be neglected only when using a broadband thermal or pyro-
electric sensor.
Photodiode sensors show a strong dependency on the operating wavelength. For light sources
with a line width greater than 10 nm (e.g. LED) there may be an influence on the displayed
power. To achieve the best result for broadband light sources with a photodiode sensor it is ne-
cessary that the response curve is nearly linear over the line width. When entering the center
wavelength of the light source as operation wavelength, the PM100USB will show the optical
power for a symmetrical spectral response shape.
6.5 Temperature Effects on Thermal Sensors
Thermal sensors respond to any temperature differences that occur between thermal disc and
heat sink. The measurement result can be affected by airflow disturbances or by heating of the
heat sink, as for example during long-term exposure of the thermal disc to the laser beam.
To avoid disturbances it is recommended to shield the sensor from airflow and to zero it prop-
erly in the operating condition. That means for short term measurements zero the cold sensor,
for long term measurements zero the sensor when it is in a state of thermal stability (e.g. after
10 minutes light exposure).