NOTE: Each optical table component needs to be bolted into optics breadboard (board with the tapped holes). Please do not tighten these as hard as you can; a snug turn is good enough. If something feels really hard to turn, or that it requires a lot of your strength, then please stop. Something isn’t right.

5.1 Initial Laser Alignment

1. Flip the switch on the back of the HeNe laser if it’s not already on.  After a few seconds to warm up, a red beam should be emitted from the front.
2. The laser and two mirrors will already be on the table, mostly aligned. You’ll have to adjust the angle of those mirrors so that the beam is going in a straight line relative to the optics table. Use a ruler to make sure the beam is at the same position vertically and horizontally for as long a distance as you can.

5.2 Beam Expander

1. Insert the first lens in the beam expander (it’s the smaller of the two lenses). Make sure the beam is going through the lens as close to the center as possible, and that the lens is as perpendicular to the incoming beam as possible.
2. Use a white piece of paper to look at the beam. You should see it shrink at the focal point of the lens, then rapidly expand. Use a pen and mark the approximate width of the beam at two different distances after the focal point. From these approximate width measurements, determine the divergence of the beam, and thereby determine its minimal beam width.
3. Insert the second lens of the beam expander. Vary the distance of the second lens so that the output beam is well collimated.  It is VERY important that the output from the beam expander is well columnated.
4. Align the photodiode so that your measured voltage output is at a maximum. Note that the max output of the sensor is 5 V, so use the beam attenuators if you are measuring over 4.5 V.
5. The end result should be a well-collimated beam with the attenuator wheel set so that the voltage from the photodiode is less than 4.5 V.

5.3 Beam Profile

1. Use the mounted bubblewrap or whatever other component is handy to make the beam profile worse. We want a messy beam that we can fix later.
2. Align the profile-measuring edge so that it’s just about to block the beam. Make sure the translation stage has enough length to fully block the beam. Note that one full turn of the knob equates to 0.5 mm of translation.
3. Start taking photodiode measurements as you move the edge across the beam profile. You can take larger step-sizes at first (0.5-1.0 mm), but when the signal starts to change, you will want to take steps of about 0.1-0.2 mm. Take measurements until the signal drops to about 5% of the initial value. Then move in 1-mm steps for 2 or 3 more measurements.

5.4 Pinhole Diffraction

1. Add the pinhole at the focal point of the beam expander. This is very difficult to do. The pinhole is only 15 m in diameter, and must be placed exactly at the focal point in order to work properly. When the pinhole is at the exact coordinates, the output will be an Airy pattern, as discussed in Section 3.1.
2. Use the iris to block the Airy rings to allow only the central region of the Airy pattern through. This should be a purely Gaussian beam profile.
3. Use the profile-measuring edge and the photodetector to measure the profile of the beam.