Fiber Based Measurements; Energy Measurement Using Pyroelectric Sensors - THORLABS PM100USB Operation Manual

4 Measurement Considerations

4.9 Fiber Based Measurements

Laser light is emitted from an optical fiber tip in a conical shape, with an angle
twice the acceptance angle of the fiber. The acceptance angle is calculated by:
Θ = n* arcsin (NA)
where NA - numerical aperture of the fiber
n - refraction index; in air n = 1
Fore typical single mode fibers the total angle 2*Q 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
and getting 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-reflec-
tions it is recommended 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 console. This minimizes measurement interferences.

4.10 Energy Measurement using Pyroelectric Sensors

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 (too high energy
causes mechanical damages on the layer) and by the sensor material, that must
not exceed a certain temperature level. It is also important to pay attention to the
maximum energy density, to the average power and to the maximum power dens-
ity during a pulse.
To the lower end, the detection is limited by the resolution and the minimum set-
table trigger level of the display unit, as well as by the noise level. To minimize the
noise level and to avoid ground loops it is recommended to mount the detectors
using the supplied isolation adapters.
Beam Diameter
The active area of the detector should be selected in such way, that it has a
slightly larger diameter than the incident beam. Not necessarily the beam should
cover most of the sensor area, but the maximum allowed energy density must not
be exceeded.
Repetition Rate
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