Optical Setup; Initial Visual Alignment; Final Alignment Using The Osa - THORLABS Redstone OSA305 Quick Start Manual

Redstone™ OSA305
3.3.1.

Optical Setup

To simplify the alignment, two mirrors in kinematic mounts should be used between
the test source and the OSA, where one mirror is close to the source and the other
is close to the OSA free-space port. Please see Figure 4 and Figure 5 for
suggestions of mirror placement. If the beam height of your experimental setup is
not consistent with the free-space input, the two mirrors and the distance between
them make it possible to adjust the beam height to that of the OSA's input. Note
that there is no need to keep the optical path parallel to the optical table until after
the mirror closest to the OSA.
3.3.2.

Initial Visual Alignment

Use the mirror closest to the test source to center the input beam on the OSA input.
A viewing card, such as Thorlabs Item #s VRC2 or VRC6S, and/or an alignment
plate, such as Thorlabs Item # CPA2, can be used to aid in the alignment. Then,
open the "Free-space Alignment" window from the Instrument menu, activate the
alignment beam, and use the mirror closest to the OSA to point the alignment beam
at the center of the input source. Repeat adjusting the two mirrors in turn until both
conditions are fulfilled; the two beams then overlap over the entire beam path. To
further optimize the alignment, the DC signal value of the detectors are shown
whenever the alignment beam is active. Maximizing these values will most often
lead to good interferometric signal values.
3.3.3.

Final Alignment using the OSA

Once the system is roughly aligned, deactivate the alignment beam in order to see
a spectrum in the main software window. Change the settings to Low Resolution
and Low Sensitivity and start a continuous measurement. Use the "Free-Space
Alignment" window to adjust the gain levels. First, adjust the gain levels until the
signal values lie between about 10% to 80%. Note that if the source is weak, the
sensitivity may need to be increased, as this enables higher gain levels for some
detectors. However, this will also slow down the measurements. If the source is
strong, the attenuation filters can be activated through the "Free-Space Alignment"
window.
To optimize the alignment, there are several software analysis tools that can be
used. In addition to the signal values in the "Free-Space Alignment" window, it is
recommended to survey the detected optical power by using the Power tool (under
the Analysis menu) and to observe either the interferogram or spectrum. A time
series measurement of the signal and/or power can be started via the "Long-Term"
functionality (see Section "Long-Term Analysis" in the manual) to get a reading of
how these values change as you adjust the two mirrors.
Once the desired software tools are selected, repeat the alignment procedure
described above but utilize the OSA readings instead of the viewing card to
optimize the alignment. Remember to decrease the gain level and/or activate the
attenuation filter if any detector gets close to saturation.
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Chapter 3: Using the OSA
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