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3 Operating Instruction
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3 Operating Instruction
· Connect the optical source(s) to the optical input(s). The FC adapter will accommodate
multi-mode as well as single-mode fiber.
Attention
Please note that for PDB440X the INPUT- is on top and INPUT+ is on the bottom.
Attention
Be very careful about the force when connecting the fibers to the optical inputs! The photodi-
odes are mounted such that the gap between the protective diode glass and the fiber tip is as
small as possible. For this reason, be very careful when tightening the fiber connector, in order
to avoid damages to the glass window that protects the photodiode.
· The MONITOR output can be used to conveniently check the detected signal from each
input beam. The maximum output voltage swing of the MONITOR outputs is 10V for high
impedance loads (1.5 V into 50 W loads). Saturation of the MONITOR outputs will occur at
optical input power greater than 1 mW.
· The RF output signal must not exceed the maximum RF OUTPUT voltage swing (see
Technical Data ) to avoid saturation.
· For balanced operation illuminate both photodetectors simultaneously and use either the
RF OUTPUT or the MONITOR outputs to fine-tune the optical power balance by observing
voltage on a digital voltmeter or other low-frequency measurement device.
· After finishing measurements, turn the power supply off.
Note
To prevent saturation of the balanced amplifier make sure that the power difference between
the optical inputs remains less than the saturation power level (see "RF OUT CW Saturation
Power" in the Technical Data section ).
Attention
Do not exceed a maximum power density of 4 W/cm
2
for maximum linearity performance when
measuring focused beams, fiber outputs, or small diameter beams. The damage threshold of
the photo diodes is 20 mW! Exceeding this value will permanently destroy the detector!
Note
For free-space beam applications, the FC adapters can be removed in order to have direct ac-
cess to the photodetectors.
3.1 Operating Principle
Thorlabs product family of PDB440x and PDB450x detectors Balanced Amplified Photodetect-
ors consist of two well-matched photodiodes and an ultra-low noise, high-speed transimped-
ance amplifier that generates an output voltage (RF OUTPUT) proportional to the difference
between the photo currents of the two photodiodes, i.e. the difference of between the two op-
tical input signals.
Additionally, the unit has two monitor outputs (MONITOR+ and MONITOR-) to observe the op-
tical input power level on each photodiode separately. Due to their increased cut-off frequency,
these outputs can also be used to measure low frequency modulated signals on each detector
separately.
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