# Aligning optical axis

1. Jun 9, 2009

### 0207

i have two optical lens assemblies performing separate functions individually. Now I want to allign their optical axis .. how do i achieve the alignment?

(actually i want to allign them back to back and ensure that their optical axes are concordant, if not overlaping ,then atleast parallel )

2. Jun 9, 2009

### Andy Resnick

Optical alignment techniques are not taught as much as they should be. First, each element has at least 4 degrees of freedom: if the propagation direction is 'z', each element can translate in 'x' and 'y', and in addition pitch and yaw rotations. If the assembly is nonaxisymmetric (off-axis parabola, for example) then there is also roll rotation. 'z' translation is not usually considered a degree of freedom, but it can be in some cases.

So the first step is to mount each assembly in a mount that allows independent control of the different degrees of freedom: 2 translation stages and 2 rotation stages. Putting the mount on a rail allows control over the 'z' axis as well. I cannot emphasize enough that taking time and effort to assemble a good mounting rig, with micrometer screws and minimal backlash, will make your life infinitely easier.

Once that is done, alignment is fairly straightforward. If the source is a laser, the easiest is to simply look at the backreflected spot- when the elements are aligned, the backreflection lies centered on the output window of the laser.

In general, the idea is to first establish an optical axis - the source is shooting stright down the mechanical axis of a rail, for example, and any folding mirrors maintain the proper propagation axis. Typically this is done with a pair of pinholes. Then insert each element, starting with the element closest to the source, and align it to maintain the optical axis, again using the pinholes.

Hope this helps.