Performing Basic Measurements 69 69
Figure 60 Measuring Laser Pulse Energy via a Thermopile in CW Integrate Mode
A recommended procedure is:
With an 818P High Power Detector connected to the meter, turn the meter on.
Set the Mode to CW Continuous. Set the Range to Auto and press the
Lambda (λ) key to set the measurement wavelength to the desired value. Set
Zero on and then change Mode to CW Integrate.
Before the optical pulse arrives, the display may reflect the integral of
detector noise due to ambient temperature fluctuations.
Trigger the laser pulse. The display will display a new integrated energy
measurement every second. The readings will start decreasing rapidly after
the first couple of measurements due to the decline in heat flow from the
relatively slow thermopile detector.
When the displayed reading drops again to the point of essentially displaying
the detector’s noise component, the individual stored readings may be
retrieved via the USB interface. Summing these readings will yield the pulse
energy.
NOTE
This method works best if the integrated result of the pulse signal is much larger
than the integral of the detector’s noise component. If the integrated result of the
pulse is not much larger, then error in the measurement will arise due to the
uncertainty generated by integration 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 measurement of an optical
pulse. Typically, an accurate value will be arrived at 5 time constants after the arrival
of the optical pulse. Refer to the detector’s manual for the proper time constant.
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