Tech Briefs Magazine - May 2022 - PIT-15

TECHNOLOGY LEADERS Topic
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Time (s)
Figure 2. 1 nm MIM of an XMS linear stage. Insert - MKS Instruments' XMS50-S Linear Motor Stage.
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Figure 3. Step and settle characteristics of an MKS linear motion stage 250 ms after being moved.
ler (as opposed to Resolution which
is a theoretical capability and not a
practical parameter) and can range
from 100 nm to 1 nm. Smaller MIM
comes at significant costs in terms of
alignment speed and beam power increments.
*
Repeatability is the ability to repeatably
position an object. It can be unidirectional
(always approaching the target
position from the same direction) or
bidirectional (approaching the target
position from either direction). This
parameter is important for quickly
finding the peak power location for
similar device designs. The XMS stage
Photonics & Imaging Technology, May 2022
shown in the insert in Figure 2 has 80
nm bi-directional repeatability.
* Position stability is the ability to maintain
a position within specified tolerances
over a specified time interval.
It is the sum of drift and vibrations,
which typically varies between 0.5 and
a few microns. Aligning fibers for assembly
steps such as bonding relies on
the positional stability of the motion
system. Figure 3 shows the positional
stability of an MKS Instruments linear
motion stage 250 ms after movement.
The stage exhibits less than 20
nm variation in position stability after
settling.
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* Other motion parameters include: axis
alignment, location of the gimbal
point, system stiffness, pitch/yaw, thermal
considerations, fixture design,
Abbe error, etc.
Representative Search Algorithms
Effective optical fiber alignment can
only be achieved using a positional
search algorithm appropriate to both
the application and the step in the alignment
procedure. Search algorithms can
be classified into two categories: 1) those
most effective for finding the first light;
2) faster and more precise algorithms
End of theoretical motion 246.8 ms
for peak power location.
* First Light Searches. There are two
primary approaches for first light
searches, raster scans and spiral scans.
Raster scans, the simplest search method,
scan a defined distance along one
axis, index the position by a defined
distance along another axis, then repeat
the cycle. Raster scans are one of
the quickest methods for finding the
first light of the beam. Spiral scans are
another approach used for first light
searches. This method searches the
general area of the beam by using a
spiral motion generated by synchronizing
controlled motion in the X and
Y axes.
* Peak Power Searches After first light
has been located, search algorithms
other than raster or spiral scans are
better for finding the peak power location.
The choice of the peak power
search algorithm depends on whether
the beam has a Gaussian distribution
or top hat profile having multiple
peaks. The following examples are representative;
a number of other methods
exist:
Hill climb is a simple 2D search for the
highest power. It is most effective for
beams that have a Gaussian profile and
when the optical power quickly increases.
The hill climb method, by itself, is not
effective in finding peak power with flat
beam profiles.
Centroid Search moves along one axis
and finds a peak, then moves along a
second axis to find the final peak. Centroid
searches are useful with top-hat or
multi-peak profiles.
Centroid Search moves along one axis
and finds a peak then moves along a second
axis to find the final peak. Centroid
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PIT Tech Leaders MSK 0522_1.indd 15
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ToC
4/18/22 1:08 PM
Following Error (µm)
Position (nm)

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Tech Briefs Magazine - May 2022

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