THORLABS PDB440A, Руководство по эксплуатации, Страница: 13 / 29

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3 Operating Instruction
11
3.7 Recommendations
Thorlabs PDB440x and PDB450x detectors Balanced Amplified Photodetectors can eliminate
noise sources to allow precise measurements. The PDB440x and PDB450x detectors is de-
signed to be used in a dual beam setup: one optical path for measurement and one invariant
reference path. If set up properly, the PDB440x and PDB450x detectors can reduce common
mode noise for more than 35 dB over the specified frequency range. Below are given some re-
commendations 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 power densities on both detectors are important as well to obtain maximum possible
common mode rejection. Always try to illuminate the whole active area of the detectors to
prevent nonlinearities. Focused high power beams may lead to frequency response degrada-
tion, resulting in dramatically reduced common mode rejection.
· 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 on next page shows the maximum allowed path length difference in air to obtain a
desired CMRR. For fiber based application the maximum path length difference must divided
by 1.5.
· 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 PDB440x and PDB450x detectors Balanced Detector. Different
common ground points can also be tested.