Multiple polarizers experimental apparatus

Apparatus description

The setup for the construction of the multiple polarizers twin experiment is composed of three main components: (i) the supporting 3D printed plastic parts whose schematics are available here, (ii) a raspberry Pi running the control software over the internet and performing the video streaming and (iii) the low-level slave controller electronics comprising the sensing and the experiment motorisation.

The following drawings document the construction of the plastic parts:

World Pendulum standard manufacturing drawings

descrição dos desenhos explodidos

Pendulum holder showing the pivot support structure for the 50mmx9mm triangular pyramid.

Pendulum string support to avoid elongation errors. The cable is fixed by soldering it into an M4 brass screw 40mm long or an intravenous needle.

Pendulum string fulcrum detail showing the screw to hold the wire. Remanium is very ductile and easily wound around the screw.

Fulcrum support structure to be held on top of the pyramidal pendulum support. Strong M6 or M8 screws are used.

Optical path

incluir esquema da optica

Optical path alignment

The main body of the apparatus have the light traveling in parallel rays through the cascade of polarizers later being focused in the sensor (photo-diode). It is crucial for a good signal-to-noise reading to have the system perfectly aligned. For that end, the linear position of the emitting LED and the photo-diode receiver can be adjusted according to the following procedure:

1. Assemble only the first lens in the system and the light source;
2. Energize the LED and follow the emerging image from the exit, eg. projecting it in a wall a couple of meters apart;
3. Move the LED position in order to have an output image the closer to the size of the exit circle (~30mm);
4. Put in place the second collimating lens in order to focus the light in the photo-diode;
5. Using a voltmeter for reading the collected light intensity to the photo-diode terminals, move back and forward the photo-diode position in order to maximize the signal;
6. Firmly glue the light source and photo-diode positions.

Optical path calibration

Once the support structure is in place, we start to calibrate the absolute position of each polarizer; effectively all the polarizers will have a small offset systematic error and is important to note their value.

The first fixed polarizer shall be positioned with a couple of degrees in order to avoid starting the experiment from a maximum, allowing for a easily observation of such maxima. Consider to have it around ~15º and well secured, eventually with glue. Then start the calibration procedure by inserting the second polarizer and rotating it till the first maximums are detected (usually two). Take note of their value and leave the second polarizer at rest in that position. Now insert the third polarizer and repeat the procedure for the maximums and do this for the rest of them.

You will end up with a table of maximum transmission angles, leading to the reference value of maximum intensity in the cascade of polarizers.

Later, when performing the experiments this values of offsets must be consider in order to eliminate the systematic error of the system.

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Links

• Portuguese Version (Versão em português)
• Spanish Version (Versión en español)