THORLABS PM100USB, Руководство по эксплуатации

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6 Measurement Considerations
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The selection of the best suitable pyroelectric energy sensor must be made with respect to the
following parameters:
Energy to Measure
The maximum allowed energy is limited by the absorption layer (excessive energy causes
mechanical damages on the layer). Here, the specified maximum temperature level must not
be exceeded. It is important to pay attention to the maximum energy density, to the average
power, and to the maximum power density during a pulse.
On the lower end, the detection is limited by the resolution and the minimum trigger level , as
well as by the noise level.
Beam Diameter
The active area of the detector should be selected in such that it has a slightly larger diameter
than the incident beam. The beam should not necessarily cover most of the sensor area, but
the maximum allowed energy density must not be exceeded.
Repetition Rate
The maximum pulse repetition rate depends on the combination of the internal capacitance of
the detector and the load resistor. The display of a power meter unit has an input impedance of
1 MΩ (similar to the typical input resistor of an oscilloscope).
Pulse Length
Energy sensors can detect and measure pulses with a duration from sub-nanosecond range to
approximately 2 ms. The maximum pulse duration depends on
· the electrical time constant of the sensor, given by detector capacity and load resistance
·
the thermal time constant of the sensor.
Usually the latter is the more significant.
Wavelength
The sensors are typically calibrated at one wavelength. A sensor specific correction curve is
stored in the memory of the DE-9 connector for other wavelengths. The wavelength dependent
response is nearly linear flat for black coating over a wavelength range from 185 nm to > 25
µm; the ceramic coating is also suitable for this wavelength range, but is not flat over the entire
wavelength range.
6.2 Reducing Noise for High Accuracy Measurements
Noise from grounding, cable capacitance, temperature effects, stray and ambient light, and de-
tector noise are interfering with the measurement. The impact of noise is high for measure-
ments of low optical power signal. Below are some recommendations on how to reduce inter-
ference to a minimum:
· The housing of power sensors is connected to the digital ground of the power meter and
should be linked to earth ground (e.g. via post mounting).
·
Energy sensors should be mounted as isolated devices, using the supplied isolation a-
dapters, because the housing is connected to the power meter analog ground.
·
The sensor cable conducts very small current or voltage signals. The cable capacitance
induces disturbances when the cable is moved. Whenever very small power or energy
levels are measured, the cable should be fixed in position.
· The bandwidth should be set to Low , when using photodiode sensors.
·
The acceleration circuit should be switched off when using thermal sensors.
·
Choose a suitable sensor: The detector noise is lowest with Si or InGaAs sensors.
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