Optical Spectrum Analyzers
Chapter 5: Using the Free-Space Input
Page 17
STN053070-D02
To begin the procedure of aligning an external free-space light source, turn on the alignment beam and proceed
with the alignment, following the instructions in Section 5.3.1. Clicking the “Help” button in the “Free-space
Alignment” window in Figure 12 will open a PDF file with a tutorial with further details and examples.
5.3.
Free-Space Beam Alignment Example
The following section describes the procedure for aligning the beam from an (external) source to be colinear
with the OSA's beam path. The alignment beam emitted by the OSA is accessible to the user and is provided
as a reference for aligning the user's free-space beam path to be colinear with the OSA's beam path. After
following the instructions in Section 5.3.1 for a rough initial visual alignment, the software will guide the final
alignment using instrument measurements as described in Sections 5.3.2 and 5.3.3.
5.3.1. Initial Visual Alignment
Figure 13 below shows an example alignment setup, the most important part of which is to include two mirrors
in kinematic mounts. When two mirrors rather than one can be tipped and tilted, the conditions on the position
of the light source are much less stringent. Use the mirror closest to the test source to steer the input source
into the OSA, using a viewing card (such as Thorlabs Item # VRC2) and/or an alignment plate (such as Thorlabs
Item # CPA2) to aid in the alignment. Then, use the mirror closest to the OSA to point the alignment beam
emitted by the OSA at the center of the input source. Repeat these steps until the test source beam and
alignment beam overlap over the entire beam path.
When activating the alignment beam on the Redstone OSA, the DC signal of the detectors will be available, the
optimization of which will most often lead to good interferometric signals. Since the DC signals don’t require any
motor motion, the update rate is several Hz and the manual alignment process can thus be completed in a
shorter amount of time.
For the OSA207C, please note that due to the optical coatings used to obtain broad wavelength operation,
multiple red alignment beams may be visible. The center beam is the one to which the input IR source should
be aligned.
A portion of the red alignment beam light is coupled out from the Calibration output port on the Redstone OSA
front panel, which can aid in the alignment if the light source is fiber-coupled and emits outside of the visible
range. The test source fiber can then be replaced by a fiber connected to the Calibration output, and the beam
path of the test source is visualized.
If the input test source is sensitive to feedback from the alignment beam, position a suitable dichroic mirror as
close as possible to the test source.
