Alignment Using a Beam Triangle Opti 521 Phil Scott

Presentation Overview Defining an Optical Axis Optical Axis Woes Defining a Mechanical Axis Mechanical Axis Woes Degrees of Freedom for an Iris Setting up a Beam Triangle Degrees of Freedom for a Mirror Aligning the Axes Inserting Elements Why a Triangle?? Why Does this Work?

Defining an Optical Axis Optical Axis is Defined by Surfaces Plano Convex Defined by Center of Curvature and surface normal Spherical Surfaces Defined by both Centers of Curvature Center of Curvature Center of Curvature 1 Center of Curvature 2

Optical Axis Woes We often design around an optical axis Elements have imperfections Optics can’t float Mounts have non-zero tolerances What Happens when a Lens is Decentered? s Δθ f Δθ=s/f

Defining a Mechanical Axis The Mechanical Axis is Defined by Mounts Lens Barrel Tube that contains all lenses in a single housing Cage System Rods that connect and align mounts Iris Pair Define two point in free space Aitc-group.com Thorlabs.com Newport.com

Mechanical Axis Woes Mechanical Errors Lead to Misalignment and Stresses Metal Bends Lens edges may not be well controlled Lens decenter due to tolerances Center of Curvature 1 Center of Curvature 2 Lens Center

Degrees of Freedom for an Iris All objects have 6 degrees of freedom X, Y, Z, roll, pitch, yaw Relevant degrees for an iris X, Y, sort of Z Small rotations do very little Iris Pair 2 X, 2 Y (4 Total Degrees) ΔZ should be large enough so that the small angle approximation applies Diracdelta.co.uk

Setting up a Beam Triangle Use two irises to define a mechanical Axis Use two mirrors to match the optical axis to the mechanical axis Start by aligning the light on the first iris

Degrees of Freedom for a Mirror All objects have 6 degrees of freedom X, Y, Z, roll, pitch, yaw Relevant degrees for a fold mirror Tip, Tilt, sort of Z Clocking and translation are irrelivant Mirror Pair 2 Tip, 2 Tilt (4 Total Degrees) These 4 degrees can accommodate the 4 degrees from the iris pair Diracdelta.co.uk

Aligning the Axes Rotating the first mirror will change the position where the light strikes the second mirror Rotating the second mirror changes the direction of the light entering the system Use mirror 1 to align the light on the front iris and mirror 2 to align the light onto the second iris. Iterate until the system is aligned* *Since the mirrors are finite in size, you may need to translate them if the beam walks off the edge of a mirror.

Inserting Elements Now that there is a mechanical axis with a source that is aligned to it, we can insert optics into the system. Recall: Decentered lenses lead to angular deviations (slide 4) Insert the lens between the irises and adjust until the light is once more centered on the second iris. This shows that the optical axis of the lens is aligned to the mechanical axis of the system to some geometric tolerance Wash, rinse, repeat for each optic

Why a Triangle?? Simple: It doesn’t have to be!! Any two mirror arrangement can work The beam triangle keeps the angles of reflection small so a larger beam footprint will fit on the mirror

Why Does This Work? The mechanical axis has four degrees of freedom Using the four degrees of freedom from the mirrors we can accommodate any reasonable geometry. Another way to think about it: There are two ways to define a line (in other words: an axis) Use both to define the same line so the axes overlap

Questions??* *Insert applause here