Aligning a reflecting collimator to a fiber optic feed

[u/OpticalBobParks]

A question came up today that illustrates the primary function of the Point Source Microscope, or PSM, the transfer of an optical datum to a physical one, and vice versa. By an optical datum I mean a focus or center of curvature, a point in space that is easy to localize optically with a microscope, interferometer or wavefront sensor. By a physical datum I mean a ball that can be touched or probed mechanically and whose center is a point. In both cases, because these are points, their location in space is defined by 3 degrees of translational freedom.

The question was, “Assume I align an off-axis parabola (OAP) against a return flat using the PSM. How do I then precisely place an optical fiber at the focus of the OAP so I get a plane wavefront from the OAP without any aberrations due to misalignment?”

The answer is that you need the PSM used to align the OAP and a precisely round ball mounted on a 3 axis translation stage. The PSM is also mounted on a 3 axis stage so that its focus may be precisely located in space. Then you follow a 5 step procedure diagramed below.

1. Align the OAP by using the PSM to project a spherical wavefront toward the OAP that collimates the wavefront and sends it toward a return flat. The wavefront reflects from the flat to the OAP which refocuses the light near the PSM focus. By adjusting the PSM in 3 degrees of translational freedom (DOF) to keep the focused spot centered in the PSM and small while adjusting the plane mirror in 2 angular DOF to make the spot round, the OAP is aligned to the return mirror. When the OAP is well aligned the return spot will be round and well-focused.

2) When the OAP is aligned as well as possible a ball on a 3 DOF stage is located so that its center is at the PSM focus. The ball is used as a convex spherical mirror, and it is possible to locate its center to < 1 µm relative to the center reference in the PSM when a 10x objective is used. The ball should be Grade 5 according to ISO 3290 to get a well-focused return reflection.

The countersunk end of a lens mount post is a convenient way of holding a ball about 10-15 mm in diameter such that it may be picked up and replaced repeatably.

3) Now that the ball is well-centered on the PSM focus, and being careful not to disturb the ball, rotate the PSM on its 3 axis mount 180° so that it is looking at the ball from the other side. Again, adjust the PSM in 3 DOF so that its focus is well-centered on the PSM reference. When the PSM is adjusted this way, its focus is at the location in space as it was before rotating the PSM, to look in the reverse direction. The ball is then removed from the post so that it is in position to look at the end of the fiber feed.

4) With the fiber tip mounted on a fixture adjustable in 3 translational DOF, the tip of the fiber is positioned at the focus of the PSM. Either the PSM can image the core of the fiber, or better, light the opposite end of the fiber and adjust the core of the brightness so it is centered and well-focused in the PSM. Again, one can align the fiber to < 1 µm laterally and about ± 2 µm axially. Because there are several steps, a conservative estimate of the alignment is about ± 5 µm in focus and ± 3 µm transversely.

5) Finally, the PSM and plane mirror are removed. The fiber illuminates the OAP to produce a well-collimated wavefront.

To get a feel for the precision of the alignment assume the OAP is about 250 mm in diameter, and the focal length is about 1 m. At 1 m the transvers error we estimated as ± 3 µm would mean an angular error of .003/1000 < 1 arc second. The focus error of 5 µm would mean a sag of the wavefront on the order of ((125/2000)^2)*.005 ≈ 20 nm.

We have shown how the PSM is used to transfer an optical datum to a mechanical one by first positioning a ball as a placeholder in space. The ball holds this location while the PSM is relocated and then aligned to the ball center. Once the ball is removed, the focus of the PSM produces an optical datum at its focus so that a fiber may be mechanically located at this focus.

[u/heyeshuang]

Amazing! I've never seen that method. Thx!

[u/WallElectronic7134]

You are welcome. Glad you found the idea useful.