Aligning the laser optics

This tutorial assumes the laser optics are almost completely disassembled.

Section I: Laser collimators

Assemble the Thorlabs parts as shown below. The green electrical tape is needed because the diameters don't quite match between the collimator and the slip ring. There are a few key considerations that can lead to errors:

Considerations

  • First, the metal posts must sit at the very bottom of the post holders, else there will be a slight vertical tilt to the collimator.
  • Second, The slip rings must be tightened firmly to the metal posts, else they will rotate and the collimator will have the freedom to move side to side after mounting. It is recommended that you place a long Alan key in the small hole in the side of the metal posts, just to have something to hold on to, then try tightening the slip ring onto the metal post as firmly as you can.
  • Third, the thumbscrew that tightens the metal post into the post holder must be tightened quite firmly, and for this it is necessary to use a hex wrench or Alan key into the center of the thumbscrew. If the thumbscrew happens to be near the small hole on the metal post, tightening the thumb screw may cause the post to rotate slightly and the thumbscrew will seat itself (undesiredly) into the hole. In these cases it is useful to detach the post holder from the rail carrier (there is a screw underneath the rail carrier that can be loosened), and then rotate the post holder until the thumbscrew will no longer be near the hole in the side of the metal post.

collimatorholder.png

Section II: Lens arrays

The lens arrays have a single cylindrical plano-convex lens (AR coated to 1070 nm) which is oriented so that the only focusing is in the vertical direction. These lenses are mounted inside a 1" lens tube using a retaining ring, which can be unscrewed using the red Thorlabs spanner wrench (SPW602). The laser light should be incident on the cylidrical side and the flat side should be facing the chamber. It is imperative to minimize the gaps where light can escape between the laser and the chamber, so the lens tube should be flush with the collimator on one side and flush with the window block on the other side.

The spot size at the sample position is determined exclusively by the distance between the lens and the sample (and the focal length of the chosen lens). Spreadsheets detailing calculations of the spot size are given in the spreadsheets in the Google Drive, found here.

Section III: In-Plane Viewing Optics Arrays

The in-plane viewing optics arrays are located on windows (lasers) 1 and 4 of the chamber.

[Work in progress]

Assembly guide can be found here

  • Place round laser line mirror in holder, tighten nylon tip set screws carefully to not crack mirror.
  • Do the same with rectangular dichroic mirror.
  • Use three thumbscrews to attach the two plates to each other. Be mindful of the mirror sitting between them, be careufl not to smack it into the metal.

The circular clamp is first placed on the window as shown below, and tightened using set screws on the shoulder. These should be tightened evenly, because if one is sticking out more than the other, then it will prevent the next clamp from sitting flush around the circular outer diameter.

The next clamp is a square piece with a ciruclar cut-out in the middle, which mounts onto the circular clamp which should already be installed on the window. This clamp is tightened using two shoulder screws. It is imperative that these are secure, because if these fall off at any point while the lasers are on, that could be a dangerous situation.

Next, there is a lens tube that can screw onto the top face of the assembly of the two plates. The distance of the lens from the bottom of the tube controls the size of the spot at the sample position. Make sure that the lens and the tube are mounted at an angle where the lens focuses the light exclusively in the vertical direction. This can take a few tries to get right: it is recommended to install the lens tube onto the plate assembly first (without the lens inside). Then, with the lens tube secure and unable to rotate freely, install the lens in the correct orientation and tighten the retaining rings.

Next, there is a 45-degree kinematic mount from Thorlabs that holds a round, broad-band mirror which can be rotated along two axes in the mount. This is then followed by a custom aluminum piece that uses Thorlabs slip rings to hold onto the IPG Photonics collimating optics at the end of the laser fiber.

After assembly, metal shards from the threads of various parts can fall onto the lenses and mirrors. Carefully blow away (using "canned air") any particulates off the surface of these optics to avoid local heating and melting of these parts. Ensure that this is done before the beam is first turned on.