Photodiode Current Calibration - Newport LDC-3706 Series User Manual

Photodiode Current Calibration

1. Setup an optocoupler with a forward emitter current of 40 mA or greater and a maximum
collector output current of 10 mA or less in a current divider circuit. The emitter must be
in series with R2 (72Ω, 5W resistor). R2 and the emitter must be in parallel with R3
({3.3Ω, 25W for the LDC-3726} or {1.25Ω, 100W for the LDC-3736 & LDC-37620}).
2. Connect this circuit across the laser output terminals of the LDC-3706 instrument (pins
{4, 5} and {8, 9} for the LDC-3726 or LDC-3736) on the 9-pin connector or (pins {A1, A2,
3} and {A3, A4, 5} for the LDC-37620) on the 9W4 connector.
3. Measure and record the exact resistance of a 1Ω metal film resistor. A 4-point probe
resistance measurement is recommended.
4. Connect the 1Ω metal film resistor (R1) to the emitter of the optocoupler detector and
connect the photodiode cathode and anode input terminals of the LDC-3706 instrument
(pins 6 and 7 of the 9-pin connector) to the free end of the 1Ω metal film resistor and to
the collector of the optocoupler detector, respectively. Connect a calibrated DMM in
parallel across the 1Ω metal film resistor to measure the voltage across this resistor.
5. Select constant-power mode operation. Set the Power Limit to its maximum.
6. Navigate to the "MDI" selection in the Laser Calibration submenu of laser parameter
menu tree.
7. Select the MDI calibration mode by pressing the SET button while the "MDI" selection is
displayed. Then, press the LAS button to start the calibration procedure. Wait for the
photodiode current to ramp up and settle, then calculate the actual photodiode current
output by using Ohm's Law:
Where V is the accurately measured voltage across the resistor with a precision
multimeter and R is the accurately measured load resistance.
8. Turn the ADJUST knob until the display shows the value of the photodiode current
measurement as calculated from step 6. Then press the LAS button.
December 2019
Figure 5.1 – I Calibration Circuit
PD
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104 LDC-3706 Series Laser Controller