Old Quadrature Setup

In Depth Setup for Setup 3

This page will detail the setup of setup 3.

Walking the Beam

After fixing the laser to the optical breadboard, the laser must be aligned using two mirrors. The first mirror should be set up to roughly reflect the laser at a right angle from the incident beam. The same should be done for the second mirror as well. In order to align the laser such that it is parallel to the table, two irises should be adjusted so that they are the same height. This can be done by setting the center of the iris to the height of the laser. One iris should be fixed to the table near the second alignment mirror in the path of the beam. The second iris should be placed at the far end of the optical breadboard along the path of the beam. Close both irises. Adjust the first mirror using the alignment screws such that the incident beam goes through the first iris. Open the first iris, and adjust the second mirror so that the beam goes through the second iris. Close the first iris and repeat this process until the laser beam goes through both irises when they are both closed.

Beamsplitters and Reference Arm

After removing the irises, a non-polarizing beamsplitter should be placed such that the split beam is approximately at a right angle from the incident beam. A mirror is then placed in the path of this beam and adjusted so that it is approximately reflecting the beam back on axis with the incident beam. Keep in mind that it is generally good to have the reference arm about the same length as the measuring arm. An easy way to do this is to adjust the mirror until only one dot is visible on the beamsplitter. This should be accurate enough, but to be more accurate the reflected beam should be visible on the front of the laser and can be adjusted until it is going directly back into the laser. A polarizing beamsplitter should then be placed on the opposite side of the non-polarizing beamsplitter from the reference arm. Again the polarizing beamsplitter should be set so the beams are split at approximately a right angle.

Measurement Arm

The mirror used in the measurement arm should be set up at approximately the same distance from the non-polarizing beamsplitter as the mirror in the reference arm is. The mirror should be aligned such that it is close to being on axis with the incident beam, but not on axis fully. A good way to do this is to align the reflected beam such that it is fully visible on the front of the laser(i.e. off axis enough that it does not enter the laser cavity). Next adjust the reference arm mirror so that it overlaps the measurement arm beam.

Quarter and Half Wave Plates

Next place a half wave plate in between the laser and the non-polarizing beamsplitter. Place a quarter wave plate between the non-polarizing beamsplitter and the reference arm mirror, and another quarter wave plate between the non-polarizing beamsplitter and the polarizing beamsplitter. Take a mounted mirror and temporarily place it between the quarter wave plate and the polarizing beamsplitter and adjust it so that the beam is visible on a wall. Adjust the reference arm mirror to fully overlap the measuring arm beam. Once this has been done properly, a slight touch on one of the adjustment knobs should produce visibly moving interference patterns. This is difficult to do and takes practice. If nothing is seen after attempting for a while, try adjusting the angle of one of the quarter or half wave plates. Once something is visible, remove the mirror and place photo detectors in front of the beams exiting the polarizing beamsplitter. The signals from these detectors must be amplified using a preamplifier with settings as shown in figure 1.

Figure 1: Preamplifer settings. Be sure to use a terminator on a T-Junction with the signal from the photodetector.

The output from the two preamplifiers can be observed in X-Y mode on a oscilloscope. Knocking the table should produce an elliptical pattern. The shape and amplitude of the ellipse is affected by the angle of the quarter and half wave plates. One at a time, rotate the wave plates until a maximum amplitude signal is visible. Going through each wave plate once should be enough.

Congratulations you have set up an interferometer, an image of the setup is shown below.

Page Written by Luke Molacek