Recommendations - THORLABS PDB48*C-AC series Operation Manual

3.6 Recommendations

Thorlabs PDB48xC-AC Balanced Amplified Photodetectors can eliminate noise sources to al-
low precise measurements. The PDB48xC-AC is designed to be used in a dual beam setup:
one optical path for measurement and one invariant reference path. If set up properly, the
PDB48xC-AC can reduce common mode noise for more than 25 dB over the specified fre-
quency range. Below are given some recommendations to achieve an optimal common mode
suppression:
· To obtain the maximum possible common mode rejection (common mode noise suppres-
sion), equal power levels on each photodetector are essential. Any power imbalance will be
amplified and hence decrease the possible noise reduction.
· Equal optical path lengths are very important for common mode noise suppression especially
at higher frequencies. Any path length difference will introduce a phase difference between
the two signals, which will decrease the noise reduction capability of the balanced detector.
The figure below shows the maximum allowed path length difference in fiber to obtain a de-
sired CMRR.
· Avoid etalon effects (interference fringes caused between two optical surfaces) in optical
paths. Using angle polished optical connectors will greatly reduce etalon effects in a fiber
based setup. Effects like residual frequency modulation, polarization noise, polarization
wiggle or spatial modulation can also degrade common mode noise suppression. For further
details contact Thorlabs. In general, reducing sources of differential losses in the optical
paths (other than the measurement itself) will improve the common mode noise reduction.
· Another critical point can be electrostatic coupling of electrical noise associated with ground
loops. In most cases an electrically isolated post (see Thorlabs parts TRE or TRE/M) will
suppress electrical noise coupling. Always try to identify the electrical noise sources and in-
crease the distance to the PDB48xC-AC Balanced Detector. Different common ground points
can also be tested.
© 2019 Thorlabs
3 Operating Instruction
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