Alignment Table of Contents

• Alignment procedure home page

• Steering/Levelling of the pump laser

• Down Conversion: alignment of A/B detectors (you are here)

• Polarizing Beamsplitter: alignment of B'

• Interferometer Alignment

• Hardy's Test

• Hong-Ou-Mandel Interferometer Alignment

Alignment of two Spontaneous Parametric Down Conversion beams

This is very difficult, as these beams are non-definite and (worst of all) invisible. This guide will be your sherpa.

Initial Alignment Data Here's some data from the very first attempts at setting up this experiment. This data is not up to par with the following tables.

Step 1: BBO/HWP Placement

First take the BBO crystal and place it in the beam path, about 10-15 inches/holes from the second mirror. Make sure the white lines on the BBO crystal mount are vertical(i.e. perpendicular to the plane of the table.) Although it doesn't matter which BBO face the beam is incident upon, make sure the knobs are pointed away from the down-converting beams (see picture  of overall setup). Adjust the height such that the beam hits the direct center of the crystal. To do this properly, take a white note card, and place it in front of the crystal. Look to see where the blue dot is. Put on a pair of the orange goggles to reduce dispersion effects. Also, make sure that the white line on the BBO crystal mount is aligned as vertically as can be.

With the BBO at the proper height, retro-reflect the crystal. The following describes retro-reflecting, and you may skip ahead to the next paragraph if so desired. Turn the three adjustment knobs such that the crystal sits roughly perpendicular to the table. Take another note card (or the same one,), bend it at the bottom to create a base, and put a small hole in it (the smaller, the better). Place this note card directly in front of the blue laser mouth such that the beam goes through this hole. Now you should see a second blue dot on the note card (see above picture). This is the dot produced from the beam reflecting off the BBO face. If you don't see it, try placing the note card in front of next closest optical piece (iris 1 - I1). Adjust the three knobs such that the small reflected dot hits the center of the main beam. This assures you that the beam is hitting the BBO face precisely orthogonal.

With the BBO retro-reflected, place the 405nm half-wave plate (HWP) in between I1 and the BBO. It does not matter which face the beam is incident upon, just make sure you can easily see the numbers. Make sure the beam is hitting the exact center, and retro-reflect it. The polarization angle(θ) will greatly affect count rates. For now, turn the HWP to 0.

Figure 1: Definition of Angles: 

Figure 1 depicts an optical component and  is meant to help define the terms "tilt" and "turn" as they are used in this experiment. "Tilt" refers to the rotation about the vertical axis of the optical component and is denoted by the angle φ. If it helps, when the component is attached to a rod, rotating the rod will change the component's tilt. The Rotational Stage of the QWP also adjusts φ. "Turn" refers to the rotation of the component about the axis of the beam (so long as the beam is orthogonal to the optical component, which is the case after you have retro reflected the component). The angle of this turn is denoted by θ. All of the wave plates are secured in mounts that allow you to adjust θ

Step 2: Detector Rail

Take the ruler, two dogs, and a post. Fasten the ruler perpendicular to the beam using the the two, inverted dogs. Screw the post directly into the optical table, and butt one end of the ruler up onto it to make sure the ruler is exactly perpendicular. See the pictures below.

Place the A and B detectors so that their bases touch the ruler. Make sure that the bases of the detectors are sitting at around 40 inches/holes from the face of the BBO. You can now linearly slide the two detectors with ease. Place detector A on stage-right. and B on stage-left.

Step 3: Initial alignment of detector A collimating lens

1. Use trig to find the distance along the ruler at which each detector should sit. Rule of thumb is that for 40 holes, the detector should sit roughly 2 holes outward. Place detector A at this location and dog it down. This is so the detectors are nearly aligned with the 3-degree cone of down converted light coming from the BBO.

2. Place the beam height iris in front of detector A, and use your eye to match the detector height to the iris height (doesn't have to be exact).

3. Now we want to setup the HeNe laser to roughly align Detector A. Make the HeNe laser parallel to one of the dotted lines on the table, and also level it horizontally using two irises (similar to leveling the pump laser beam). Also, make the HeNe beam shoots in the same direction as the pump laser(refer to Figure 2 below). Make sure to leave enough room to place Detector D between the irises and the HeNe laser.

4. In this step, you will transmit the HeNe laser light through Detector A. This will allow you to ensure Detector A is pointed towards the BBO. Place the spare detector (Detector D) in front of the HeNe laser beam. Connect detector D to detector A (use FCB1 adapter for extension) . Carefully adjust the height and tilt of Detector D(note: using a ruler makes adjusting the tilt easier) until it's orthogonal to the beam and the red collimated light shines into Detector D and out of Detector A. Adjust Detector D until the beam coming from Detector A reaches its maximum intensity.  Refer to the below figure for what the beam should look like. Now adjust the tilt of Detector A until the beam is centered on the BBO crystal. Reconnect collimator A to the fiber coupler and turn off the HeNe. Detector A is now (roughly) aligned with the BBO.

5. Place the 810nm bandpass filter directly in front of detector A's lens. Height align it and make sure that the face will be roughly perpendicular to the down converted beam. This does not have to be perfect; a small deviation in angle has very little effect. Don't dog down the filter as you will want to adjust it as you align A.

6. Switch to the safe green lights and turn off the fluorescent lights. Open the LabView program and turn on the SPCMs. For alignment, set the gate window to 0.1-0.5s.

7. Confirm that the background counts are between about 200-400 Hz. Now turn on the blue pump laser. You should hopefully see photon counts that are above the background level. This is down-converted light! If you don't see an increase, try adjusting the HWP by 20 degrees or until you reach a maximum.

8. Tweak the H/V tilt adjustment knobs of detector A until a maximum count rate is reached. This can be done fairly easily by determining which direction the counts increases in. Then perform a sweep by turning the knob at a constant rate. Allow the counts to peak out and go a little past. Then use the graph to make sure you are at the maximum counts as you turn it back. If you haven't adjusted the intensity of the pump beam, you should be getting a count rate at or above 50000(with the bandpass filter in place).

9. Carefully adjust the horizontal position of the collimating assembly. If done carefully rough adjustment can be done by hand without the base dogged down. For finer adjustment dog down the detector base with only slight pressure. Then use something to gently tap THE BASE to move it back and forth. As you move it watch the A count rate with the LabView program. Turning the ACER LED monitor so that you can see it; this should not greatly affect the count rate and will make adjustments much easier. When you reach a maximum with the horizontal movement, readjust the horizontal tilt. You should then check to make sure the position is still a maximum. Iteratively adjust the position and tilt until you are certain you have found a maximum. When moving the detector you may find it necessary to adjust the 810nm bandpass filter. If you make a mistake and no longer get counts, it is probably because the collimator is no longer pointing at the BBO. If this happens, it may be easiest to shine the !HeNe out of the collimator again and aim it at the BBO(If you do need to do this, you will need to remove the 810nm bandpass filter from in front of Detector A, otherwise it will block the HeNe beam. Since it is on a flip mount, you can just flip it down.)

10. Very carefully adjust the vertical height of the detector until a maximum is reached. This is important, so here is a breakdown of this process:

    • Note the count rate, use a notebook or something...

    • Use the small vertical adjustment knob on top of the detector mount(use a one the the screw drivers to do this) Loosen the mount, raise it by only a millimeter, tighten the mount.

    • Tweak the H/V adjustment knobs until maximum is reached. Note final count rate. Compare.

    • If the count rate is roughly the same, continue to lift 1mm at a time. If the count rate drops, proceed to lower 1mm at a time, tweaking the knobs as you go, and noting the count rates. Mark this spot, if you desire.

    • When the count rate drops noticeably below the max rate, begin to raise the detector in 1 mm increments until the max count rate is reached. mark the low spot, if desired.

    • This will not be the last time you adjust the vertical height, so don't pour your heart into this step, but, you know, don't simply read this sentence first, and not iterate through the above. That would be most...unwise.

• 1. Dog down the detector.

  2. Turn the 405nm HWP until a maximum count rate is seen. Note the angle at which this occurs.

11. Tweak θ for the BBO until the detector A count rate is at a maximum. This should occur near the point where the white lines on the BBO crystal mount are vertical.

Step 4: Initial Alignment of Detector B

Before moving on, it is important to clarify something. When we align Detector B, we want to maximize alpha, the anti-correlation parameter. This can be done by maximizing AB coincidences or minimizing the AB accidental coincidence. As you know, the accidental coincidence rate is greatly affected by the laser intensity. For now, when you are maximizing alpha, don't adjust the laser intensity.

1. Repeat steps 1 through 10 of Step 3 for Detector B. However, instead of maximizing the B count rate, you will be maximizing the AB coincidence count, and thus the anti-correlation parameter. Maximize the coincidences by iteratively adjusting the horizontal and vertical positions.

2. Do not proceed until you are very certain on the location of Detector B. If you are certain of its position, follow steps 11 through 13. Remember to maximize AB coincidences, not B counts.

Step 5: Final location of Detector A

1. Iterate a few more times through steps 7 through 11 of Step 3. Be very careful, you won't need to move the detector much. This time around you're looking to maximize AB coincidences.

2. Properly fasten down both detector A.

3. Realign the 810nm bandpass filter. Select an angle that does not slightly reduce counts.

4. Fasten the filter properly.

Step 6: Final location of detector B

1. Follow steps 1 through 3 of Step 5.

2. Compare your results with this page: Down Conversion Experiment. (By maximizing coincidence counts and reducing laser intensity, it was found the maximum alpha is found when the count rates of A and B are twice that of the background noise.)

3. Once you have finished comparing your data, you should......

Step 7: Grab a beer

You've earned it.

Move on to Polarizing Beamsplitter: alignment of B'