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# Alignment

### Introduction

When we transfer an optical setup from the drawing board into the optical table, we need to align optical elements, such as sources, lenses, mirrors, filters, etc. in a very accurate way. Light travel in stright lines between points of impact in the optical elements. To obtain optical system with high accuracy, there are several techniques that help us in the alignment process. In this course, we will learn and experience in some of these techniques.

### Beam orientation

We will start with the method to align two beams into the same point. To align a beam, we need a way to change its orientation. One option to do so is by  changing the orientation of the source. However, in many cases, the source is bulky, therefore, it is preferable to control the direction of the beam by adding a mirror on an optomechanical stage at the beam line. The optomechanincal element allows to change the orientation of the mirror with at least two degrees of freedoms that are controlled by micrometers knobs. By rotating this micrometers knobs, the beam can be oriented with high accuracy. This is demonstrated in Fig. 1, where we use two mirrors in optomechanical elements for orienting two beams parallel to the table toward the same point although the two sources are in different hights and orientated at an angle to the table.

Fig. 1. Example of two mirrors orienting two beams into the same point

Exercise 1

• The student will direct two beams to the same spot as demonstrated in Fig. 1.
• Load the file “alignment_ex1.opt” (press the button below to download the file). In the file, we see two beams of light hitting mirrors and a detector.
•

• Your task is to bring the beams to the central point on the detector. This can be done by rotating the micrometers knobs on the mirror optomecanical stages.

While optical systems are invariant to rotation and translation, it is easier when the setup is oriented according to the optical table. In this case, all the elements are at the same height and the beams are aligned along the holes of the table.

• Orient the beams parallel to the table. For this task, load the file “alignment_ex1c.opt” with the two detectors on each beam.
• Your task is to orient both of the beams so they will travel parallel to the table. Make sure that the beam hits the two detectors at the same height. For maximal sensitivity and accuracy, it is suggested to have one detector close to the mirror and the other as far as possible

Exercise 2

In order to orient the beam in three dimensions, one should use two mirrors. One directs the beam perpendicular to the table and the other directs it parallel to the table at the correct direction. In this lesson, the student will learn how to direct beams along the table and how to change from the plane’s orientation with two mirrors. This is demonstrated in Fig. 2.

Fig. 2. Use of two mirrors to change the plane of beams of laser

• Load the file “alignment_ex2.opt” (press the button below to download the file). In the file, there is a beam traveling at a height of 100mm above the table and we orient it to travel at 120mm at 90 degrees.

• Change the orientation of the lower mirror until the beam travel parallel to the table in a different hight from the table.

When orienting a beam to a specific direction, it is helpful to use pinholes, so when the light travels through the pinholes, we are confident in the beam orientation. Also, it is important to use pinholes when we alternate the same beam toward different directions, thus when we want to return to a desired orientation, the pinholes help us align the beam.

• Load the file “alignment_ex2c.opt” (press the button below to download the file). Here you will use two mirrors for orienting a beam through two pinholes.

Start with the mirror that is far from the pinhole and orient the beam so it will pass through the first pinhole. Here we use detectors instead of pinhole. Therefore, you need to direct the beam to the center of both detectors.

• Next, orient the second mirror so the beam will travel through the second mirror.

Exercise 3

When orienting a lens, we are aiming to hit the center of the lens to minimize aberrations. In this case, we place the lens on a translation stage in addition to the rotating stage. For orienting the lens, we use both the reflection and the transmission from the lens. Here, we will address plano-convex lens, which is easier to handle, yetthe same technique is suitable for any lens.

• The student will learn how to identify the position of a lens and how to fix it.
• Load “alignment_ex3.opt”, and run the simulation. As evident, the lens is not oriented correctly. Place and orient the lens correctly by resorting to the reflection from the lens and without looking at the source position and orientation.
• First, we will use the reflection from the plano surface of the lens, so it will direct back toward the source.
• Next, we will use the reflection from the curved surface of the lens to orient the position of the lens. We need to make sure that the reflected light, from the curves surface is focused at the center of the input beam. Then, we know that our lens is oriented correctly.

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