THORLABS CLD101 Series, Руководство по эксплуатации

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6 Appendix
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6.4 Laser Control Tutorial
Structure of a laser diode
The "heart" of a butterfly laser is the laser
diode chip. It emits into two directions -
towards the collimator and into the reverse
direction. The power in both directions is
exactly the same. As for this reason, opposite
to the output collimator a photodiode is placed,
also known as monitor diode. The current of
this photodiode is proportional to the optical
output power of the laser and used for
observing and controlling the optical power.
The collimator focuses the laser beam and
feeds it into the optical fiber ("fiber pigtail").
Often, an isolator is placed between collimator
and fiber in order to suppress back reflections
from the fiber end back into the laser.
Important for a stable operation of the laser is a constant chip temperature, as this influences both
the emitted wavelength and optical output power. For temperature control, a TEC (Thermoelectric
Cooler, aka Peltier element) is used, it can cool or heat depending on the direction of the current
through the TEC. A thermistor placed close to the laser substrate is sensing the temperature.
Laser Operating Mode
A laser diode can be operated in constant current (CC) or constant power (CP) modes.
In CC mode, the injection current of the laser is maintained constant with a high accuracy. It can be
changed rapidly, so the CC mode is preferred for modulation of the laser output signal at
frequencies up to several hundreds of kHz.
In CP mode, the optical output power of the laser chip is maintained constant. This is achieved by
keeping constant the monitor current: The laser controller compares the actual monitor current with
it's set value and controls the injection current of the laser accordingly. This control loop is reacting
relatively slowly, that's why in CP mode the laser is used rarely for modulation.