Alignment Table of Contents

• Alignment procedure home page

• Steering/Leveling of the pump laser

• Down Conversion: alignment of A/B detectors

• Polarizing Beamsplitter: alignment of B' (you are here)

• Interferometer Alignment

• Hardy's Test

Alignment of the Polarizing Beamsplitter and Detector B'

Important Note:

In this section, you will you use the visible HeNe beam to align Detector B'/C. Using the visible HeNe beam makes this task a lot easier. In order to do this, the HeNe beam must be aligned so that it mimics the path of the down-converted light in the B-arm. There are two methods you can use to accomplish this, which mostly differ by their placement of the irises. The method outlined on this page places the irises in the B-arm. The method outlined here(Hardy Alignment) places the irises in the HeNe beam path. It is recommended that you use this second method. (Note: this method refers to a Detector E, which is the name given to Detector D when it used to transmit the HeNe beam directly to Detector B.)

By setting irises in the B-arm, the path of the down-converted light will be defined, allowing the rest of the experiment to be aligned using a (visible) HeNe laser. This makes the process a bit easier.

Step 1: Setting B-arm irises

1. Insert two irises, one directly in front of detector B's lens, and one up-range(closer to the BBO, but still in the B-arm). Adjust their height and position until you can completely close them, and observe no change in counts (NOTE: in 2014, we weren't able to achieve it. This step is very difficult. When we closed the irises completely, the counts rate dropped dramatically). Fasten properly.

   • For the iris down-range, it must be placed so that, of course, counts are not impeded. Furthermore, it must be placed such that the blue laser passes as close as possible, but does not collide with the iris edge. This will be the max distance down-range that this iris could be placed.

2. The placement of the 810nm bandpass filter:

   • Make sure to use the flip mount.

   • Place before the 800nm HWP, and directly after the first B-arm iris.

Step 2: Aligning the HeNe

1. Begin by placing the HeNe laser in an orientation that mimics the blue 405nm laser, but is a few inches back

2. Two mirrors will now be used to mirror-walk the beam directly down the B-arm iris path.

   • The first mirror, which is not in the way of anything, will be a stationary, silvered mirror.

   • The second mirror is a similar mirror, but since it will sit in front of the BBO crystal, it must be on a pickoff mount.

3. Disconnect the B-detector from its SPCM, and walk the laser through the irises. If you have it right, it should still be able to shine through the collimating B-detector lens while both irises are in a nearly-closed position.

   • Note the the 10nm bandpass filter must be flipped down to allow for the HeNe to pass!

Step 3: Insert the Polarizing Beamsplitter (PBS)

1. Place the PBS such that the reflected light is directed out of the B-arm at an angle of ~90-degrees.

   • Orient the PBS such the the vertical and horizontal adjustment knobs adjust the reflected beam in the vertical and horizontal direction. Make the exit beam roughly parallel with the table.

   • Another thing: the laser light must be incident on the triangular prism (or: on the half) with the dot on it.

Step 4: Placing the B'/C-detector

• The B' Detector is called both the B' and C-detector. Know that they are the same.

1. Place the C detector (no optical fiber connected) in the path of the laser light which is reflected from the PBS.

2. Adjust the height and horizontal position such that the laser is seen projecting from the backside of the collimation lens. This means you're close to the actual position.

3. Now connect one end of an optical fiber to the back of the lens, and place the other end in front of a white piece of paper. A folded piece of note card works well. When the laser is correctly transmitting through the optical fiber, you should see a strong projection onto the note card.

   • This spot can be very tricky to get, as it is the same thing you did with the alignment of the A/B-detector down-conversion.

   • However, now you're doing it visually, so you'll know when you've got the 'sweet spot.' The C-detector is now roughly aligned.

   • The pattern projected on the note card or wall can tell you a lot of information about the alignment. If brightness is not a good indicator, the pattern can provide additional information. When on the edge of alignment, the pattern shows up as very course with a bunch of fine "fingers." As the alignment gets closer, the number of features goes down and the pattern morphs into a single bright spot. I recommend playing around with this and getting comfortable using it to align the detectors.

4. Follow a methodical procedure resembling the alignment of the A/B-detectors in order to maximize the signal through the fiber. Once roughly aligned, attenuate the signal using a combination of the three (OD3-5) neutral density filters, and then send the signal into a SPCM. Adjust the knobs to maximize counts.

Step 5: Placing the 800nm HWP

1. This is simple: just place it a few holes in front of the PBS. Make the laser hit dead center, and align the retro-reflection with the vertical axis of the first B-arm iris. Since there's no vertical adjustment of the HWP, just forget about it. It will work just fine.

2. You are now able to run the HeNe Laser Light (B/B' detector) anticorrelation experiment.

Move onto: Interferometer Alignment