Walking A Beam Through Two Irises
Goal
The goal is to make a laser beam pass through the centers of two same-height irises after two steering mirrors. The first iris defines the near point on the desired optical axis. The second iris defines the far point. When the beam is centered on both irises, the beam position and angle match the line selected by those two apertures.
This is a geometric alignment method. It does not require the downstream optic to be installed yet, which makes it useful when setting beam height, preparing a beam expander path, or establishing a clean reference line across an optical table.
This simulation shows a Z-fold made from two kinematic mirror mounts and two same-height irises on a 25.4 mm table-hole row. The wide view shows the two mirrors walking the beam onto the iris row. The close-up view shows the near and far iris readouts during each correction. The top-down panel shows why the method controls both lateral beam position and propagation angle.
Download the walking-beam alignment movie
The top-down view is useful when checking whether the beam path remains continuous from mirror to mirror and then through both irises.
Why This Works
A straight beam is fully specified by two points. Two irises therefore define the desired path: the beam must pass through the first aperture and then through the second aperture without clipping.
The two mirrors provide two coupled controls. A first-mirror adjustment changes where the beam lands at the near iris and also changes the downstream angle. A second-mirror adjustment changes the downstream angle and therefore moves the far iris spot strongly. Because the controls are coupled, a single pass is rarely perfect. Alternating between the near iris and the far iris converges on the line through both aperture centers.
Use small iris openings only after rough alignment is close. Starting with a larger opening prevents accidental clipping and makes it easier to see which mirror adjustment is moving the beam in the intended direction.
Setup Geometry
Use a low-power visible alignment beam, wear appropriate laser safety eyewear, and place beam blocks wherever the rough beam or mirror reflections could leave the intended table path.
Place Iris 1 as close as reasonably possible to the second steering mirror. This near iris gives a sensitive readout of the beam position just after the mirror pair. Place Iris 2 farther downstream on the same desired path. A longer distance between the two irises makes angular error easier to see, so the final alignment is more accurate when the far iris is placed farther away.
A table-hole row is a convenient reference for a straight horizontal path. When using the holes as a geometric reference, place each mirror so that the mirror surface is centered on the intended turning point of the hole pattern. The outside of the mirror mount is not the optical reference; the reflective surface is.
Before beginning, set the mirror actuators near the middle of their travel. This leaves adjustment range in both directions. Also center the beam on both mirror surfaces during rough placement. If the beam starts near an edge of a mirror, later corrections can clip the beam or force the actuator to the end of its range.
Right-angle reflections are convenient because they make the geometry easy to read on a rectangular optical table, but they are not required. Two steering mirrors are sufficient to laterally translate a beam onto a new parallel path.
Step-By-Step Protocol
Set both irises to the same height and place them on the desired beam path. Put Iris 1 close to Mirror 2 and Iris 2 farther downstream on the same reference line.
Open both irises enough that the rough beam can be found safely without clipping.
Place two steering mirrors before the first iris. Start with the mirror actuators near mid-travel, the beam centered on both mirror faces, and the reflective surfaces located on the intended table-hole turning points.
Roughly steer the beam so that it reaches the first iris and continues toward the second iris.
Adjust the first mirror to center the near iris. Watch the beam spot at Iris 1 and use small, deliberate mirror motions.
Adjust the second mirror to center the far iris. Watch the beam spot at Iris 2 and bring it to the same-height aperture center.
Return to the first mirror and re-center Iris 1. The second-mirror change usually perturbs the near iris slightly.
Return to the second mirror and re-center Iris 2.
Iterate until both iris spots stay centered after each correction.
Reduce the iris openings and repeat the final corrections. Stop when the beam passes cleanly through both apertures at the working aperture size.
Troubleshooting
If Iris 1 centers but Iris 2 moves far away, the mirror corrections are too large or the initial Z-fold geometry is too coarse. Open the far iris, make a rough second-mirror correction, and then restart with smaller movements.
If Iris 2 centers but Iris 1 no longer transmits, return to the first mirror. This is the normal coupling between the two controls, not a sign that the method failed.
If the beam clips both irises at different heights, recheck the mechanical height of the iris centers before continuing. Walking the beam cannot fix a reference line defined by apertures at different heights.
If the beam shape changes or becomes crescent-shaped at an iris, the beam is clipping. Open the iris, recover a full round spot, and only then close the aperture again.
If a mirror actuator runs out of range, do not keep forcing the correction with the knob. Recenter the actuator, reposition the mirror mount so the beam is again near the center of the mirror, and restart the rough alignment.
If the beam is centered on both irises but walks across a mirror surface, the mechanical layout is probably not centered on the intended turning points. Move the mount so the reflective surface, not the mount body, coincides with the reference geometry, then repeat the alignment.