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Increasing the signal
Once an optical signal is detected, the power meter’s sensitivity shall be changed accordingly. The next step
consists in maximizing the signal’s power by turning the adjustement screws one by one.
Start with one adjustment screw and tighten it slowly until the optical signal reaches a relative maximum.
Then fasten this first screw a little more so that the signal’s power decreases by a few percents.
Repeat the operation with the next adjustment screws, proceeding in the same way. After having adjusted
the three screws, 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.
Be aware that repetitive squeaks indicate that a screw is tightened too strongly, a situation that should be
avoided.
The coupling losses can be defined as being the difference between the power level monitored by the laser
source and the power actually measured at the far end of your patchcord. Before proceeding to the next
step, check that these coupling losses do not exceed 100 mW.
Fine adjustment and locking
Take note of the power delivered at the output fiber, and compute the transmission ratio (i.e. the measured
output power divided by the raw output power obtained at the same set point).
The fiber’s coupler adjustment is now near its optimum and you can proceed with the final phase: achieving
the maximum coupling efficiency by adjusting the three counter screws (the small pin screws, see figure 4-4)
Using the same procedure as before, start from a state where all three counter screws are equally loosened.
Then tighten slowly the first counter screw until the optical signal reaches a relative maximum, and fasten it
a little more so that the signal’s power decreases slightly.
Repeat the operation with the other counter screws, proceeding likewise. After adjusting the three screws,
start a second round and re-adjust the first counter screw, and so on.
At the end of the last round, tighten the last counter screw so as to maximize the output power level.
You can now gradually increase the output power up to its nominal set point. As a final check, make sure
that the transmission ratio has not decreased.
Checking the alignment
A possible way to check the alignment’s robustness is by pressing gently on the connector in all directions
(see Figure 4-6): when thus pressing, the signal’s power should decrease but it should then recover its initial
value once the pressure is released.
If this is not the case, the coupler’s alignment is not optimized. The test gives a hint about the direction
where more pressure should be applied to the fiber (i.e. which counter screw sould be tightened.) Use this
information to re-optimize the coupling.
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