27
Input Optical
Pulse Energy,
E (
λ
)
Detector's Voltage
Pulse
Integrate Voltage
to find
E (
λ
)
E (
λ
)
V (t)
E (
λ
) =
V (t) dt
R (
λ
)
1
∫
Figure 8 – Measuring Laser Pulse Energy via a Thermopile in INTG Mode
A recommended procedure is:
i.
Plug in a Newport High-Power detector via its associated calibration
module and then turn the meter on. Set MODE to DC CONT, set AUTO on,
set the measurement wavelength to the desired value and then execute
AUTO CAL per Section 2.4.4. Now set ZERO on and then MODE to INTG.
ii. Press the R/S key to begin the integration. Before the optical pulse arrives,
the display may reflect the integral of detector noise due to ambient
temperature fluctuations.
iii. Trigger the laser pulse and watch the display increase at a much faster
rate due to the detector signal from the heat flow.
iv. When the display increase begins to slow down and again reflect integra-
tion of the detectors noise component, press the R/S key a second time to
stop the integration. The final display value represents the pulse energy.
NOTE
If the integrated result of the pulse signal is much larger than the integral of
the detector’s noise component, the display will show a steady reading after
the pulse. If the integrated result of the pulse is not much larger, then error
in the measurement will arise due to the uncertainty generated by integra-
tion of the noise component terms.
NOTE
The time constant of a thermopile detector determines the amount of time
that one should expect to wait when making an integrated energy measure-
ment of an optical pulse. Typically, an accurate value will be arrived at 5
time constants after the arrival of the optical pulse. Newport High-Power
detector time constants are listed in the Newport Catalog and in each
detector’s manual.
2.8.6
Using the Model 2835-C as an Exposure Controller
This application uses the INTG measurement mode along with triggering
features to configure the Model 2835-C as an exposure meter-controller. Use
the following setup and procedure:
i. Connect a BNC cable between the Model 2835-C’s trigger input and trigger
output BNC connectors. Using a T-connector, connect the trigger output
BNC to your shutter’s external control BNC.
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