Transcript Presentation

Alignment Using a Beam
Triangle
Opti 521
Phil Scott
2
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?
3
Defining an Optical Axis

Optical Axis is Defined by Surfaces

Plano Convex

Defined by Center of Curvature and surface normal
Center of Curvature

Spherical Surfaces

Defined by both Centers of Curvature
Center of
Curvature 2
Center of Curvature 1
4
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
5
Defining a Mechanical Axis

The Mechanical Axis is Defined by Mounts

Lens Barrel

Tube that contains all lenses in a single housing
Aitc-group.com

Cage System


Rods that connect and align mounts
Iris Pair

Define two point in free space
Newport.com
Thorlabs.com
6
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 2
Lens Center
Center of Curvature 1
7
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
8
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
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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
10
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.
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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
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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
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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??*
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*Insert applause here