Z‐focus approach

Autofocus 🔎

The best optical system in the world is not going to do well if the sample is not positioned at the focal plane of the lens.

For now, the Medjed system is planned to use a knife-edge (or offset-beam) approach to manage autofocus. For this we use a standard diode laser. The diode laser used (a Thorlabs L637P5) is a 5mW 637nm laser. 637 nm as to not expose the resist, and 5mW to be just on the Class 3B laser threshold - rendering it quite safe provided you dont position your eyeball in the focal plane of the objective.

Caution

Please evaluate any claims w.r.t. safety yourself. Run the numbers yourself, ask your laser safety officer, or wear appropriate safety goggles.

The laser diode is a standard planar semiconductor design, so we get a strongly elliptical beam profile (8° vs 34° for parallel and perpendicular axis).

Collimation 🔴

We would like to collimate the light emitted from our diode. For that we can use a single lens (if we are willing to accept an elliptical beam). Choosing the focal length of the lens is going to come down to the desired beam diameter. Let's say we aim for a 11mm diameter. (that should still fit nicely within a 45° rotated 1-inch mirror). The focal length of our lens should then be (using a purely geometrical argument, as in this Thorlabs document)

f = (11mm/2) / tan(30°/2) = 18.0mm

So we might select something like the Thorlabs ACL2018U-A (f=18, ⌀=20) as the collimating element; mounting it 18mm from the diode. Note that the 11m diameter is only for the perpendicular axis.