32
Semiconductor
(Newport Low-Power) detectors provide a current signal. The
2835-C is capable of 100fA resolution in order to provide the highest sensitiv-
ity performance with these detectors.
Thermopile
(Newport High-Power) detectors, provide a small voltage signal.
The 2835-C is capable of 125 nV resolution in order to reach the sensitivity
limits of thermopile detectors.
Pyroelectric
(Newport Energy) detectors, deliver a peak voltage signal, . The
2835-C is capable of capturing 2
µ
sec rise time voltage spikes so that it may
operated with the fastest of these detectors.
3.5
Thermopile Detector Signals
Thermopile detectors respond with a voltage signal that slowly to changes in
incident optical power. The time constant of most thermopile detectors is on
the order of 1 to 10 seconds.
Thermopile
Response
Input Optical Signal
Displayed Signal
1 sec
5 sec
Figure 11 – Thermopile Signals exhibit 1 to 10 second time constants.
3.6
Pulse Energy Detector Signals
A Newport Energy detector will respond to a single radiant energy pulse with
a voltage pulse at its BNC output. This pulse exhibits a sharp voltage rise to a
peak followed by a slower voltage decay which “undershoots” zero volts
before settling back to zero volts. When a detector is operated within its
proper limits, the voltage difference from immediately before the sharp rise to
the peak is linearly proportional to the radiant energy.
If a second pulse arrives before the “undershoot” rises back to zero volts, the
voltage rise from this pulse will start from an initial negative value. At sufficient
energy pulse repetition rates, a negative “baseline” voltage will develop from
which the voltage rise must now be measured to achieve accurate energy
readings. The 2835-C contains baseline capture circuitry which maintain its
accuracy specifications over rep-rates ranging from single pulse to 2 Khz.
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