Alignment

General Information

Beam height above table surface: 160 mm

Abbreviations

• SL - Scan lens
• Galvo - Galvanometer mirror
• VAT - Vertical alignment tool

Set Up

1. Ensure that the alignment laser is level with the table by checking its height at two separate, far apart locations on the bench.
2. Ensure that the alignment laser is collimated. Below is a table of wavefront radii of curvature at different distances from the laser as measured with a Shack-Hartmann wavefront sensor.
3. Set the alignment laser beam height to ~160 mm above the table surface.
4. Ensure that the galvanometer mirror is at that correct height above the table by carefully putting the alignment laser next to it and visually verifying that it would intercept the entire cross section of the beam.

Distance, cm	RoC, m
50 cm	-30 +/- 1
75 cm	-35 +/- 2
100 cm	-40 +/- 3

Excitation Path

Phase 1

Align Galvo and Scan Lenses 1 and 2

1. Set the galvanometer voltage to 0V
2. Put an adjustable ND filter in front of the laser beam.
3. Place two irises on a row of bolt holes on the table. Align the laser to them.
4. Insert the galvanometer mirror and place it so that the beam is roughly in the center of the mirror.
5. Use C-clamps to move the two irises after the mirror on a column of bolt holes. Position the galvanometer so that the beam passes through the irises at these new positions.
6. Close down both irises. Insert a mirror after the second iris and align the back reflected beam back through both irises.
7. Insert a coverslip after the ND filter to pick off the reflected beam.
8. Insert two new irises to define the path of the pick off beam. Their absolute position doesn't matter.
9. Insert scan lens one (SL1) and place it such that the distance between the mirror and the lens housing is approximately 170 mm. Use the speckle size from scattering from a white piece of paper to verify that the mirror is near or at focus.
10. Adjust positioning of SL1 until the pick off beam is recentered through the irises and the forward the beam goes through both irises. Forward and backward beams may not agree completely; this is OK. Align them as close as possible.
11. Insert SL2 and place it approximately 170 mm after the galvo. Align its transverse position so that the forward and backward beams pass through all irises.
12. Insert the Shack-Hartmann wavefront sensor after SL2. Adjust SL2's axial position until the output beam is collimated (absolute value of the radius of curvature is greater than about 30 m, the value of the beam's RoC directly out of the alignment laser).
13. Remove the wavefront sensor and iterate over the last two steps until transverse and axial alignments are good.

Align Objective and Tube Lens

1. Insert two mirrors after SL2 in a figure 4 configuration. Use two irises to align the beam into the 90 degree upward folding mirror. (One iris is on the mirror mount in the photos.)
2. Insert the vertical alignment tool (VAT) into the objective barrel. Use the tip/tilt adjustment screws on the folding mirror and adjust its position on the table until the beam is traveling approximately vertical and is centered along the axis where the objective will be placed. 2x tip/tilt adjusters and x/y positioning provide the full four degrees of freedom for beam positioning. You will not be able to exactly align the beam until the next step. Just make it approximate.
3. Remove the alignment disk from the lower cage plate on the VAT. Use the adjustment screws on the folding mirror to align the beam to the tool's iris at the far end of the tool. Insert the alignment disk. Use the adjustment screws on the mirror in the figure 4 assembly that is closest to the assembly to align the beam into the hole in the alignment disk. Iterate until good alignment through both irises is achieved.
4. Remove the VAT and insert the objective. Verify that the beam is traveling approximately vertically after the objective.
5. Almost certainly the beam will not being traveling perfectly vertical. This is OK for now. If it at least is close, use the tip/tilt adjusters on the folding mirror to redirect the beam vertically.
6. When debugging, you can also verify visually that the beam is entering the center of the objective's rear aperture.
7. Set the laser to the lowest possible power such that you can just see the beam leaving the objective. This is the most dangerous part of the alignment. Insert the tube lens. Adjust its height by raising and lowering the post in the post holder until the correct height is found for maintaining a vertical beam path. Lock this position in place with a C-clamp. You may also need to roughly adjust the lens's transverse and axial positions to find the beam and place the focus of tube lens in the objective's back focal to start this step.
8. Setting the vertical position first will confine beam displacements to a vertical plane so that you can stand outside of this plane for eye safety during alignment.
9. Next, find the axial position of the lens such that the focal planes of the tube lens and objective are coplanar. This can be done by eye by inserting an index card into the beam after the objective to observe its size while simultaneously moving the lens's axial position until its size is a minimum. There will be a large, maybe ~1 centimeter range of lens axial positions in which it will be difficult to identify when the spot size has been minimized. This means the precision of finding the coplanar point is relatively low.
10. Alternatively, use the beam view on the wavefront sensor as shown in the image below. Note that the beam's radius of curvature will not change much with lens axial position. Instead, use the x and y diameter readouts from the wavefront sensor and mark the range of lens axial position that have been tried. Use a method of bisections to find the tube lens axial position that minimizes the beam size.
11. Fix the tube lens to the table. Attach a C-clamp to the post so that the lens can be removed and replaced in the same position.
12. Remove the objective and tube lens. Reinsert the VAT, align to it as before, and mark where the beam hits the top of the table's enclosure.
13. Reinsert the tube lens and stop down the beam using one or more irises from previous steps. Verify that the beam hits the mark. If it doesn't, slightly adjust its transverse alignment, taking care to ensure that the axial alignment is minimally affected.
14. Reposition the iris right before the tube lens, after the figure 4 folding mirrors. (It will be slightly off because the last mirror in the figure 4 was used to align the beam to the VAT.) Stop it down. Insert an alignment disk and change the lens's tip/tilt alignment until the back reflections are centered on the axis.