60
- Unscrew slightly each counter screws by two turns,
- The next steps consists in maximizing the signal’s power by turning the adjustement screws one by
one:
o Start with one adjustment screw and steer it slowly until the optical signal reaches a
relative maximum. Continue steering this screw a little more so that the power decreases by
a few percents.
o Repeat the operation with the next adjustment screws, proceeding using the same
maneuver. After having the three screws adjusted, start a second round and re-adjust the
first screw, and so on. Make sure to always adjust the screws in the same sequence (for
example, (1) -> (2) -> (3) -> (1) -> (2) -> (3)). The maximum power level is typically achieved
after three rounds of adjustment.
- Secure the optimum position (at maximal power) by tightening the counter screws,
- Your transmission ratio is now maximized
5-4 Issues with back reflection
Some of the laser sources composing the combiner can sustain damage from optical feedback.
This damage occurs on semiconductor laser diodes when their output is collimated and subsequently retro-
reflected. In these conditions, the retro-reflected beam focuses back onto the laser diode facet and can
create a power density high enough to melt this facet. This results in an irreversible degradation of the laser.
Once this process begins it is not possible to repair the laser, moreover this failure mechanism is not covered
under Oxxius’ standard warranty.
For this reason, Oxxius recommends the following precautions be taken when working with the beam
released by a combiner:
1) All reflective surfaces in the optical path should be angled slightly so that surface
reflections will not be retro-reflected,
2) Use angle-polished fibers when coupling the beam into optical fibers,
3) Avoid using mirrors placed at normal incidence into the light path. If mirrors must be placed
at normal incidence, then an isolator is required.
4) Avoid focusing the beam the onto a reflective surface without installing an isolator into the
light path,
5) Avoid sweeping the beam back and forth across the laser during alignment operations (for
example utilizing a retro-reflected beam to align through a pinhole in a confocal microscope). If such
a possibility is unavoidable, use an isolator at the output of the laser diode during the alignment
process to prevent permanently damaging the laser
Even if back-reflection does not harm all laser sources, it will likely disturb power monitoring and thus induce
instabilities of the output. In order to assert this cause of instability it is recommended to start the emission
while the output shutter(s) are closed (using a beam stop is also possible), then to note if the laser is more
stable in these conditions.
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