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MTN003813_D02
PDB2xx Series
3 Operating Instructions
Note
Do not exceed a maximum power density of 4 W/cm
2
for maximum linearity performance when
measuring focused or small diameter beams. Always try to illuminate the whole detector active
area to prevent nonlinearities. Equal power densities on both detectors are important for max-
imum common mode noise suppression (CMRR).
The PDB2xx Series can be used in balanced mode (both inputs are illuminated) as well as in
single detector mode. In single detector mode, the RF OUTPUT swing depends on which
INPUT is used: it is positive for INPUT+ and negative for INPUT-.
In single detector mode, the optical input power should be below the specified CW saturation
power (see Technical Data ) to avoid saturation of the RF OUTPUT amplifier.
In balanced mode the power difference between the optical inputs should be less than the CW
Saturation Power. If necessary, use external neutral density filters or attenuators to reduce the
input light level.
Attention
The optical damage threshold is 20 mW. Exceeding this value will permanently damage the
photodiodes!
3.4 Electrical Output Ports
The Thorlabs PDB2xx Series has three BNC output connectors:
· MONITOR +
· MONITOR -
· RF OUTPUT
RF OUTPUT delivers an output voltage proportional to the difference between the photo cur-
rents of the two photodiodes. This voltage can by calculated to:
RF,OUT opt,1 opt,2
U = (P -P )× (λ)×GÂ
with:
Â
(
l
) - responsivity of the photodiode at given wavelength
P
opt,1
and P
opt,2
- optical input power
G - transimpedance gain of the RF output
The responsivity
Â
(
l
) for a given wavelength can be read from the individual curves in section
Performance Plots in the Appendix to estimate the RF OUTPUT voltage. Please note that
the given responsivity curves represent typical values - individual responsivity may deviate. The
maximum output voltage swing of the RF OUTPUT can be found in the Technical Data sec-
tion.
The optical input saturation power (see Technical Data ) of the balanced detector is the mini-
mum value, as it is given for the wavelength with the highest detector responsivity. At other
wavelengths, saturation will be reached at higher input power levels. The output signal should
not exceed the maximum output voltage to avoid saturation. Therefore the optical input power
(or the power difference between the optical input ports 1 and 2) should not exceed CW Satura-
tion Power listed in Technical Data .
MONITOR Output Ports
The signal monitor output ports (MONITOR+ and MONITOR-) allow to measure the input power
level of the two input ports separately. MONITOR+ and MONITOR- can therefore be used to
assess low frequency modulated signals at the individual detectors. The maximum output
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