Teachspin Metal Film Beam Splitter

Setup 1: From Teachspin's Modern Interferometry Using a Metal-Film BeamSplitter

Based on Teachsping's Modern Interferometry attached as a file below.

A schematic of the setup is shown below.

First the laser should be set to the approximate height such that it passes through the middle of the metal-film beamsplitter. In the figure below, the metal-film beamsplitter has been placed on a translation stage so that the measuring arm mirror could be placed on a translation stage.

Before securing the beamsplitter in place, the beam was aligned with two mirrors to "walk the beam" so that the beam was aligned parallel to the table and to the holes in the breadboard. In order to achieve this, two irises mounted on identical bases were set to be the same height as the laser and were then each set against two screws on the breadboard, one near the second mirror, and one on the opposite end of the table.

Using the two screws for each mirror ensured that the beam was parallel to the holes in the breadboard. See figure below for example of the iris and screw setup.

Next the metal-film beamsplitter attached to the translation stage was placed in the beam path with the side labeled front, shown below, on the side of the incoming laser.

In order to ensure that the beamsplitter is was at a 45 degree angle from the incident beam, irises and screws were used to align the exiting beam at a 90 degree angle from the incident beam using the same method as before. In order to make fine angle and height adjustments one screw securing the base of the beamsplitter to the translation stage was loosened and the adjustment screw was used to adjust the height. Note that this is a tricky process because tightening the screw attaching the beamsplitter to the translation stage will also affect the height. Very small adjustments can be made by tightening the screw all the way, then loosening the screw very slightly, adjusting the height adjustment screw slightly, and then tightening the screw fully and repeating this process until the beam is aligned properly.

Next a mirror was secured to the table along the perpendicular beam to be used as the reference arm. The mirror was aligned properly by aligning the reflected beam passing back to the laser with the laser output hole. A mirror attached to a translation stage was secured to the optical breadboard along the non-deflected beam at approximately the same distance as the reference arm. It is important to have the two mirrors as close to the same distance as possible because the distance different noticeably affect that amplitude of the signal. To align this mirror with the other, a third mirror was placed along the third arm and aimed roughly against a far wall. The mirror attached to the translation stage was adjusted so that it would interfere with the other beam. Interference occurs when any straight line interference has gone away and only circular fringes remain. Note that this is very difficult and requires very fine adjustments. Another good way to see if the beams are interfering is to adjust the translation stage that the mirror is attached to very slowly and see if the fringes are moving.

The second 50/50 beamsplitter was then placed in front of the first beamsplitter, detectors(What kind?) were placed with irises in front of them as shown in the figure below. The irises are used so that the detectors are not overloaded with the intensity of the beam.

As shown in the figure above, a half-wave plate was also placed in front of the laser. The half-wave plate must be adjusted while the table is being tapped or the translation stage is being moved to see when a maximum signal is visible.

The output of the detectors when viewed in X-Y mode on an oscilloscope is on the order of several mV, so two SR-560 pre-amplifiers were used to better view the signal and reduce noise using filtering. The settings are shown in the image below. Note that a 50 Ohm resistor was attached to the input signal.

When slowly turning the adjustment knob on the translation stage, the oscilloscope should show an elliptical shape like in the figure below.