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6 Measurement Considerations
17
6.6 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, install proper shielding of
the sensor from ambient and stray light.
6.7 Back Reflection
The surfaces of photodiodes, ND filters and even the black coatings of thermal sensors show a
small percentage of back reflection of the incoming light. If this back reflection hits for example
a laser diode or a HeNe laser, this may have an impact on the power stability of the laser. It is
therefore recommended to slightly tilt the power meter sensor with respect to the laser beam.
This way, the back reflection will not 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 is almost completely absorbed in the sensor.
6.8 Beam Diameter vs. Active Sensor Area
Most sensors are not completely uniform in their response over the active area. 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 values are given in the sensor specifications. The software displays the actual power
or energy density for the beam diameter inserted in the software. 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. This means that the beam size in the plane of the
sensors active area must not exceed the size of the active area.
6.9 Fiber Based Measurements
Laser light is emitted from an optical fiber tip in a conical shape with an angle twice the accept-
ance angle of the fiber. The acceptance angle is calculated as follows:
Θ =
n* arcsin (NA)
where NA - numerical aperture of the fiber
n - refraction index; in air n = 1
For typical single mode fibers the total angle 2*Θ of the emitted light is between 15° and 25°,
for an angled connector (APC), the cone is tilted by approximately 4° from the fiber axis.
This expansion of the beam has to be considered to avoid overfilling the detector which causes
wrong results. On the other hand, for measurements with high power fiber lasers a certain gap
between fiber tip and detector surface has to be kept to decrease the power density.
Thorlabs offers fiber adapters with the most common connectors that are verified with the
S12xC series optical sensors and with most thermal sensors.
For large divergence angles or fiber measurements that are critical to back-reflections it is re-
commended to use an integrating sphere sensor of the
S14xC series .
Another good choice for fiber based measurements are the fiber heads of the S15xC series .
They plug directly to the meter and have no cable between sensor and power meter. This min-
imizes measurement interference.