Interferometers

Guide

Points for Assembly

It is easy to assemble an interferometer once you master the knack of it. The following are some points that require attention during assembly.

First of all, to align the optical axis of all components, adjust the height of holders using posts and post holders. To make the laser beam horizontal to the installation stand, adjust the angle of a He-Ne laser. Roughly arrange each component while keeping some space required for adjustment of the holders. To enter light correctly into optics, adjust components in sequence starting from the laser light source.

When installing components, firmly fasten various joints or clamps so that holders do not move.
Such parts include the coarse/fine switching clamps for elevation and azimuth of a mirror holder, or clamp of a post holder, the on/off of the lever of a magnet base.
If joints or clamps are not fastened, vibrations tend to occur, making stable observation of interference fringes difficult.

Points for Assembly
1. Spatial Filter Holders

A spatial filter holder is a device consisting of an objective lens and a pinhole, and makes a diverging ray from a laser beam while at the same time eliminating distortion in beam wavefronts or diffraction rings caused by particles to convert it to a clean Gaussian distribution.
Adjust the position of the spatial filter component so that the laser beam can be perpendicular to the center point of the objective lens.Move the pinhole away from the objective lens using the micrometer of the objective lens stage, and find a weak light that passed the pinhole (A).Then move the vertical and horizontal axes of the pinhole, and find the position where the passed light becomes maximum (B).When gradually moving the objective lens closer to the pinhole, the light that passed the pinhole becomes bright, but if the objective lens keeps approaching toward the pinhole, the light becomes darker.When it gets darker, adjust the pinhole and find the position where the light becomes bright.Repeat this operation until the brightness reaches maximum and there are no diffraction rings (C).

Spatial Filter Holders
Spatial Filter Holders
2. Collimated Light

The lens component receives light diverging from the spatial filter component, and converts it to thick collimated light.
For collimation adjustment, place a screen at a distance, and adjust the lens position toward the direction of the optical axis so that the beam diameter projected to the screen is the same as the beam diameter immediately after the lens.

Collimated Light
3. Splitting of a Beam and Recombination

A collimated light is split and re-combined by using a half mirror.
A half mirror is positioned so that a beam is flexed to a right angle, but a plate type half mirror causes shade on part of the beam because of its thickness and holder frame, limiting observation of interference fringes to a small area at the center of the half mirror. (It is especially noticeable in a Michelson interferometer.) To solve this problem, attach the half mirror in the reverse orientation (the coated surface faces the retaining ring side) and adjust the half mirror component so that the clear aperture of the transmitted beam is maximized.
* A beamsplitter holder (BHAN) that will not cause shade on a beam even when the coated surface is facing the front is also available.

 

Install the mirror component so that the collimated beam fits in the clear aperture of the mirror, and adjust positioning of each holder so that the two beams are projected on the screen to be over lap at the same size.
When a wedged plate type half mirror is used, a transmitted beam is slightly refracted due to the refractive index of the half mirror.
The two optical paths divided by the half mirror, therefore, are not at precisely 90 degrees, but it will not affect measurement or observation of interference fringes.

Splitting of a Beam and Recombination
4. Angle Adjustment of the Beam

Even if the two beams are perfectly superimposed by the adjustment in ③, interference fringes cannot be observed in most cases. To observe interference fringes, the parallelism of the two beams needs to be set to one minute or less.
There are various adjustment methods, but here we introduce one method that uses an adjustment lens.
Prepare an adjustment lens that has a long focal length (adjustment lens component IFC2-AL), insert it between the half mirror and the screen, and adjust the lens position so that the focal spots of the beams are on the screen. Picture (A) To completely superimpose the two spots, make fine adjustment by moving only the mirror of one optical path. Picture (B) (To make the adjustment easier, make the spots as small as possible and somewhat diminish the brightness of the spots.)
You can observe fine interference fringes when you remove the adjustment lens. Picture (C) (Repeat the aforementioned steps until fringes are visible.)
Next, while keeping the interference fringes visible, gradually increase the spacing between fringes by making fine adjustment on one side of the mirror holder. (At this time, if you adjust both mirror holders at the same time, the adjustment may become irreversible.)
Adjustment of the azimuth (θy) of the mirror holder increases or decreases the horizontal spacing between fringes, and adjustment of the elevation (θx) of the mirror holder increases or decreases the vertical spacing between fringes.
The interference pattern most suitable for observation is when the number of vertical or horizontal interference fringes is three or four. Picture (D)

Angle Adjustment of the Beam

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