Setup 2: Dobosz Usuda and Kurosawa

Sources

This setup is based on a design published by:

M. Doboszy, T. Usuda and T. Kurosawa, "Methods for the calibration of vibration pick-ups by laser interferometry: I. Theoretical analysis," Meas. Sci. Technol. 9 (1998) 232–239.

Theory

Click here for the theory.

Setup

This setup uses a quarter wave plate, a polarizer, a non-polarizing beamsplitter, and a polarizing beamsplitter. As in the first setup, the beam was first aligned to be parallel to the table and the holes in the breadboard using two mirrors. A schematic of the setup is shown below where QR is the quarter wave plate, P2 is the polarizer, NBS is the non-polarizing beamsplitter, WP is a polarizing beamsplitter, PD1 and PD2 are photo detectors, M1 is a mirror and is the reference arm, and CCR and M2 is the location of the measuring arm where setup 2 placed a mirror on a translation stage.

A quarter wave plate was placed in between the laser and the first mirror as shown in the figure below. The quarter wave plate acts to cause the beam to be circularly polarized and can be placed anywhere in the path of the laser before the laser hits the first beamsplitter.

Next a non-polarizing beamsplitter was placed and aligned so that the split beam was perpendicular to the incoming beam using the iris method described in the previous setup section with the difference that this beamsplitter is much simpler to align. The two mirrors for the reference arm and measurement arm were placed in the same way as the first setup, but this time the two reflected beams were aligned to be off axis from the incoming beam. This was done because it makes the data much better, but it is not currently clear why this is the case. A polarizer was also placed between the non-polarizing beamsplitter and the mirror of the reference arm. This polarizer serves to make the light coming back linear polarized instead of circularly polarized. A figure of the two arms is shown below.

Next a polarizing beamsplitter was placed in the path of the recombined reference and measuring arms. An iris and detector were placed in the path of both output beams from the polarizing beamsplitter as shown below.

The photo-detector signals were sent to individual pre-amplifiers as before. To begin the settings were kept the same, but the amplification may have to be lowered if the amplifiers are overloading. The oscilloscope output while moving the translation stage is shown below.

Application of a Retroreflector

A retroreflector is used to reflect an incident beam parallel to that beam independent of the angle of the incident beam. A retroreflector is useful in situations where the object that is being measured moves in such a way that a mirror would quickly go out of alignment. In these situations a retroreflector and a mirror can be used to keep alignment by having the mirror aligned to reflect the beam back into the retroreflector and then interfere in the interferometer setup chosen. As long as the retroreflector does not move out of alignment to the point that the retroreflected beam is no longer hitting the mirror, the interferometer will still function normally with no phase change due to rotation of the retroreflector. A sample setup image is shown below.