External Modulation Operation - THORLABS EK2000 User Manual

OEM Laser Diode Driver Evaluation Kit
3.1.4.

External Modulation Operation

The laser output power can be controlled via an external modulation voltage
while operating in the Constant Power Mode. The laser output is inversely
proportional to the modulating voltage with 0 V being the laser fully on and 2.5 V
turning the laser fully off.
To use the external modulation, perform the following steps:
1. Set up the LD2000R for Constant Power.
2. Attach an external modulation source (e.g. function generator, D/A
converter, etc.) to the Analog Modulation Input (P10).
3. Apply power to the EK2000 and adjust the modulation input amplitude
and frequency for the desired output.
4. The laser output will now be controlled by the external modulation
voltage.
The graph above describes the characteristic of the modulation voltage. If the
LD2000R is set up to match a particular laser, the solid curve would represent
the output power of the laser as a function of the modulation voltage. A couple of
notes of interest:
1. If the PWR Limit control is set below the maximum output power, the
laser output will plateau (clip) at the PWR Limit level for modulations
below the PWR limit.
2. If the total feedback gain, RF, is not optimized for the operating laser,
the laser turn off point of the output will be different than 2.5V (usually
2468-D01
Clipping due to
Current Limit
being set too
low.
Chapter 3: Description
The turn-off
voltage is
dependent on
matching R to
fext
the laser. If the
feedback gain is
too high, the laser
will turn off at a
lower voltage
(V = R (I ),
off F Mon
where RF=R
F EXT
in parallel with
20 kΩ).
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