Temperature Effects On Thermal Sensors; Ambient And Stray Light; Back Reflection; Beam Diameter Vs. Active Sensor Area - THORLABS PM100USB Operation Manual

PM100USB

4.5 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 interfered by airflow
disturbances or by heating up the heat sink, e.g., during long-term exposure of
the thermal disc to the laser beam.
To avoid disturbances it is recommended to shield the sensor as good as pos-
sible from airflow and to zero it properly 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 ex-
posure).

4.6 Ambient and Stray Light

Ambient or stray light can strongly affect the measurement accuracy in free-space
applications. A permanent background light level can be subtracted by conducting
a zero adjustment. In case of varying ambient light like daylight or turning on/off
room light, the only solution is a proper shielding of the sensor from ambient and
stray light.

4.7 Back Reflection

he surfaces of photodiodes, ND filters and even the black coatings of thermal
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sensors show a certain kind of back reflection of the incoming light. If this back re-
flection hits for example a laser diode or a HeNe laser, this may have an impact
on the power stability of the laser, therefore it is recommended to slightly tilt the
power meter sensor with respect to the laser beam. This way, the back reflection
won't enter the laser.
If back reflections must be completely avoided it is advised to use a S14xC series
integrating sphere sensor where the incoming light gets nearly completely ab-
sorbed in the sensor.

4.8 Beam Diameter vs. Active Sensor Area

ost sensors are not completely uniform in their response over the active area;
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except integrating sphere sensors. To overcome uniformity issues the incident
beam should have a diameter larger than 10% of the active sensor area.
Another important point is the maximum allowed power and energy density of the
sensor. The maximum ratings are given in the sensor specifications. The
PM100USB can display the actual power or energy density for a known beam dia-
meter. For high power or high energy beams a good efficiency can be reached by
selecting a sensor with a detector size about 20% - 30% larger than the beam
diameter.
It is also important not to overfill the sensor. That means, that the beam size in
the plane of sensor's active area must not exceed the size of the active area.
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