Tutorial - THORLABS MLDEVAL User Manual

MLDEVAL

6.5 Tutorial

Laser diode operating modes
Constant Current (CC) Mode
In this mode, the laser current is set to a fixed value. If the laser diode package contains a mon-
itor (photo) diode, it will not be used. As the laser output power generally depends not only on
the laser injection current, but also on the laser chip temperature, the output power may show a
temperature dependent drift.
The advantage of the CC mode is the capability to modulate the laser by an external signal.
Thorlabs offers two miniature laser drivers operating in CC mode:
MLD203CHB Constant current driver with high modulation bandwidth (DC to up to 100 kHz)
MLD203CLN Constant current driver with low output current noise. The low noise feature is
accompanied by a lower modulation bandwidth (DC to up to 1 kHz)
As in CC mode the monitor diode is not used, only the laser diode is connected and thus, the
CC miniature drivers support all laser diode pin codes.
Constant Power (CP) Mode
In CP mode, the laser output power is maintained constant by keeping constant the monitor
(photo) diode current. In other words, the monitor current is being observed and compared to
the set value. The comparator circuit controls the laser current in such way, that the monitor
current remains constant.
In fact, a CP driver can be modulated, either. However, the monitor current feedback loop is
comparably slow so that only low frequency modulation is possible.
Laser diodes come with different connection schemes of laser and monitor diode, particularly
with respect to the common pin. For this reason, Thorlabs offers two types of CP miniature
laser drivers, depending on the pin code of the laser (see
MLD203P1 Constant power driver for pin codes A, B and F
MLD203P2 Constant power driver for pin codes C and D
Note
The pin codes E, G and H are not supported as these laser diodes have no monitor diode.
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