9
Also, depending on the material used, you will be able to measure
either the crosstalk sum in real-time or only the maximum power.
Measuring the crosstalk helps, but an accurate alignment can
be done without it.
Step 1: Speed check the fundamental beam. Turn on the
source and power the fundamental mode. Note that the
channel labels are indicated on the device and the performance
report. Using a detector card, make sure that the device is well
powered. In a standard component, the waist radius is 300
µm, meaning that the spot is small. You should see a Gaussian
beam. If this is not the case, check the channel labels and the
proile of the other modes.
Step 2: Speed check the modes. Illuminate all modes one by
one. Verify that, at irst sight, the proile of the model agrees
with that of the performance report.
Step 3: Prepare the setup. In coniguration 1 or 2, L1 should be
positioned in a 1-x platform to align the focal point and/or the
telescope. Also, make sure that the light goes right through the
middle of the lens to avoid aberration. M1 should be positioned
on a kinematic mount with at least 2 degrees of freedom.
Step 4: Align the beam. Use the circulator on the fundamental
channel. The objective is to send light and to measure the
power relected by the mirror and demultiplexed in the same
mode. With the fundamental mode on, play on the orientation
of M1 and position of L1 to optimize the received power on the
fundamental mode. A irst rough alignment can be performed,
which rapidly becomes very sensitive.
At the best alignment position, insertion losses for the
fundamental mode should be close to the value on the
performance report x2.
If a multi-channel power meter is available, this enables the
crosstalk with the other channels to be measured. This makes
the alignment easier, as in the optimum position the sum of
all crosstalks from the fundamental mode to the higher order
modes should be at a minimum.
Step 5: Measurements. In this step and when you are happy
with the alignment, you can measure the system performance
for each channel: Insertion loss and crosstalk as indicated in
the annex.
Back-to-back measurement with a MUX and DEMUX
coniguration
Step 6: Measurement of the second MPLC. Repeat steps 1 to
5 for the second MPLC.
Step 7: Prepare the back-to-back setup. In the coniguration
of 2 MPLCs, it is recommended to have 2 telescopes, one after
the MUX to increase the size of the beam, and the other before
the DEMUX. Even if the 2 MPLCs are close to one another,
the beam will have a small divergence leading to a larger beam
size at the DEMUX input. Using 2 telescopes ensures that the
beam is the right size at the reception side and facilitates the
alignment process. The alignment procedure is then similar to
that of a single MPLC except that the circulator is no longer
needed.
MPLC 1
MUX
L1
M1
300 µm radius waist colimated beam at
the output of the multiplexer & input of
the demultiplexer
Two lenses are used as a
telescope to adapt the beam size.
L2
MPLC 2
DEMUX
L3
M2
L4
A small divergence will occur during
propagation. Adpating the 2nd telescope
ensures to have the correct beam size
Figure 3: Illustration of a back-to-back coniguration.
