Hong-Ou-Mandel Interferometer Alignment

Alignment Table of Contents

• Alignment Home

• Pump Steering

• Down Conversion Alignment

• Polarized Red Laser Alignment

• Interferometer Alignment

• Hardy's Test

• Hong-Ou-Mandel Interferometer Alignment

Alignment Procedure

Step 1: Pump Steering

Place you pump laser and walk two mirrors so that the beam is parallel with the table. Refer to the pump steering page for more information on how to do this.

Step 2: Down Conversion Alignment

Follow the steps detailed by the down conversion alignment page.

Step 3: Insert Irises

Begin by height aligning 6 irises to the pump laser. Insert an iris into the path of the down converted photons that are going to detector A. Place the iris at least 13 inches away from the BBO and adjust it until the down converted photons pass through the center of the closed iris. This is best done by observing the count rate for detector A in the LabView program. Counts will decrease with the fully closed iris in the way, just maximize counts. Insert a second iris as close to detector A as possible and repeat count maximization.

Repeat this step for detector B.

Step 4: Insert the HeNe and First Beam Splitter

Figure 1: Placement of the HeNe and beam splitter are shown. The red line denotes the path of the HeNe beam.

In order to align the Hong-Ou-Mandel Interferometer, you need to be able to see the beam you are working with. However, the down converted photons are in the infrared and cannot be seen. So, you must align a HeNe laser so that it follows the path of the down converted photons. The way to do this is as follows:

1) Mount the HeNe to the table near the pump laser. With about 40 inches between the BBO and the detectors placed in Step 2, a space about 30 inches (on center) wide will be necessary to construct the interferometer. Try to keep the HeNe outside of this space when mounting. Unplug the detectors from the SPCMs before continuing. It is a very expensive mistake to make.

2) Use two mirrors to steer the beam into the beam splitter. Place the second mirror on a magnetic pick-off mount as close to the BBO as possible and screw the base on tight. Make sure to leave enough space to reach the knobs of the mirror. Note: The pick-off mirror should be attached by placing one side on the base and gently lowering the other side into place. This is to prevent the mount from snapping into place, which can misalign the mirror.

3) Place a non-polarizing beam splitter about 5 inches in front of the BBO crystal.

4) Adjust the positions of the splitter and mirrors until the beam is close to going through the set of irises for detector A. The beam need not actually pass through the irises for this step; just get the beam close enough that you can walk the mirrors in the next step.

5) Aligning the beam splitter so that the HeNe beam mimics the path of the down converted photons has a process that is analogous to walking mirrors. NOTE: I use "beam A" to refer to the beam that passes through the detector A irises and "beam B" to refer to the beam that passes through the detector B irises.

a) Walk the mirrors so that beam A passes through the detector A irises.

b) Adjust the knobs on the bottom of the splitter until beam B is level.

c) Turn the knob on the side of the splitter until beam B is parallel with the line going through the detector B irises.

d) Move the splitter until beam B goes through both detector B irises.

e) Repeat as necessary.

Once the beam splitter is aligned, you should open the irises.

Step 5: Insert Mirrors

Place one mirror in the path of beam A and one mirror in the path of beam B. The mirrors should be roughly in between the BBO and detectors A and B. Make sure that the mirrors are equidistant from the BBO and eachother; the optical path lengths of the two interferometer arms must be the same and you will want to pay attention to this detail now rather than later.

Unfortunately, this interferometer beams does not allow A and B to be aligned so that they are parallel to the grid of holes in the table. Rather, you must align the beams so that they make a 45 degree angle with this grid. An easy way to accomplish this is to take multiple screws and screw them into holes along a 45 degree diagonal. Place a ruler along the screws so that the ruler is aligned to a 45 degree angle. Now place two irises that were height aligned earlier along the ruler.

Adjust the height and tilt of the mirror until the beam goes through both irises. (This is a good opportunity to use SpectraSuite, and the OceanOptics spectrometer to maximize the beam intensity through the closed irises. In general, a neutral density filter is necessary to not saturate the spectrometer and allow the peak of intensity to be resolved at 632nm). Make note of what side of the screws the ruler is placed on. Repeat this alignment for the other mirror by placing a ruler along two screws so that it mirrors the first ruler with respect to the symmetry axis. This is depicted in Figure 2.

Figure 2: A depiction of the mirrors placed in Step 5 as well as the technique for aligning them.

Step 6: Translation Stage for Beam B

The Hong-Ou-Mandel Interferometer experiment requires that you be able to adjust the path length difference between beam A and beam B. So, you will need to add a translation stage with two mirrors mounted to it for beam B. One possible translation stage setup is shown in Figures 3 and 4 (Though, it is not recommended to use this set up, as previous attempts found it unstable. Perhaps a configuration in which a small optical bread board is mounted to the stage should be used. This would allow the mirrors to be screwed into it rather than placed on a rod that can be easily bumped or misaligned. Additionally, it would provide a set up less prone to vibration).

Place the translation stage in the path of beam B as shown in Figure 4. It is imperative that the stage is mounted at a 45 degree angle to the grid of holes. Otherwise, adjusting the translation stage during data taking could throw the interferometer out of alignment!

Screw two screws into the 45, and press the front of the stage against them (see Figure 3). Additionally, to ensure the stage is mounted correctly is to screw two table clamps together as shown in Figure 5. By doing this, these clamps become long enough that you can place one (or more) on each side of the stage and be able to screw them into holes that lie on a diagonal (see Figure 4).

Once the stage is in place, the mirrors need to be aligned so that beam B still follows a 45 degree angle with the grid after reflecting off of the mirrors on the stage. First, place two irises along rulers, similar to how you did it in Step 5, after the translation stage (i.e. place them on the right hand side of Figure 4 so that beam B arrives at the irises after reflecting off of the translation stage mirrors). This time, though, after placing the iris closest to the mirrors, clamp that iris down and move the ruler further away from the mirrors to place the second iris. You will want the irises to be further apart than 12 inches to get the best alignment possible. To align the mirrors, follow these steps:

1) Turn the knob on the stage so that the mirrors are as far forward as possible.

2) Using a spectrometer placed behind the far iris, walk the beam into the irises by adjusting the mirrors on the stage.

3) Turn the knob to move the mirrors all the way back.

4) Walk the beam into the irises by adjusting the mirrors on the stage again.

5) Repeat 1-4 as necessary.

6) Open both irises. When the mirrors are aligned, you should be able to turn the knob on the stage as much as you want and the intensity at 632nm will not change. If you can do this, you can move on. If not, go back to 1.

Figure 3: The translation stage with mirrors mounted to it. The two screws used for a guide are visible at the front edge of the stage, below the mirrors.

Figure 4: The translation stage placed in the path of beam B is shown. The red line denotes beam B and the arrows represent the direction of beam B.

Figure 5: Two table clamps screwed together.

Step 7: More Mirrors for Beam A

You now want to reverse the direction of beam A. Do this by adding two more mirrors into its path. It is best to use the pillars (shown in Figure 6) to mount these mirrors, as they are more impervious to vibration, and as you will see in the Troubleshooting section, they are easier to work with. However, you need to ensure that when you adjust the translation stage while taking data, the path length difference between beam A and beam B will be zero at some point (keeping in mind, the stage only has a 5 mm travel).

First, turn the knob on the stage so that the mirrors are not at their furthest forward or furthest back. Measure the distances between the mirror placed in Step 5 and mirrors on the translation stage. Now, place two mirrors in the path of beam A so that the distances between the mirrors along the path of beam A match those distances you just measured.

Using the same technique you have been using, walk these two mirrors so that beam A remains at a 45 degree angle, which is symmetric to the one for beam B's mirrors. The end result of this step is shown in Figure 6.

Figure 6: The mirrors added in Steps 5 and 7. The arrows on the red lines denote the direction of the HeNe beam. The pillar mounts for the stationary mirrors of beam A are clearly shown on the left.

Step 8: Second Beam Splitter

Find the point where beams A and B intersect. Secure a non-polarizing beam splitter onto an XY translating stage, and center it where the beams intersect (as seen in Figure 7). Dog the stage down well, you'll be adjusting knobs on it and do not want it to move at all.

Figure 7: Placement of the second beam splitter. Ignore the collimators placed at the outputs of the beam splitter for now.

Step 9: HeNe Interference

The first step to aligning this beam splitter is to adjust the splitter until the HeNe beam forms an interference pattern. When you first placed the beam splitter, there were probably four beams exiting the splitter: two from beam A and two from beam B.

One method of achieving interference is as follows:

Make slight adjustments to the position of the beam splitter by adjusting the stage, but only adjusting one direction/knob. Place a diverging lens where the beams exit the splitter to project them onto the wall (see Figure 8 below).

You will be able to see two interference patterns, in fact, one in each direction that the beam splitter sends the beams. Put a spectrometer in the path of one of these exiting beams. Use a neutral density filter to attenuate the HeNe and open SpectraSuite. Adjust the position of the spectrometer to maximize the intensity at 632nm.

Step 10: White-Light Interference

Observing interference fringes from the HeNe beam should have been pretty easy because of the HeNe's long coherence length. A white-light source contains a much greater range of frequencies and consequently has a much shorter coherence length. This step will be nowhere near as easy as the last, but it is necessary to ensure that the path lengths of beams A and B are the same.

First, place your white light source in front of the first mirror (of the ones placed in Step 4) as can be seen in Figure 9.

Figure 9. Placement of the white light source. The irises are present to help to collimate the white light source.

With SpectraSuite still open and the spectrometer in place, place a converging lens in front of the spectrometer as to focus the combined white light beam into the fiber optic cable. Begin by translating the stage to its forward position, and translating the stage by small increments backward. Because the interference is only visible in a range of tens of microns, translate the stage a couple ticks (on the course dial) back and one tick forward. If the alignment isn't precise, the interference will be subtle and you may blow over it.

Figure 10. From left to right: progressive transition from nominal spectrum to deep interference fringes as the stage is translated.

If white light interference isn't seen, repeat step 9. Alignment precise enough for white light interference can be achieved visually with the HeNe by maximizing the fringe width. When white light interference is present, it can be maximized by projecting the beam onto a note card, and adjusting the beam splitter knobs, similar to how the HeNe interference was maximized. Be weary, at this point the interferometer is PREPOSTERIOUSLY sensitive. If you adjust the beam splitter and lose interference, go again back to step 9. Maximum white light interference is achieved when a single fringe is observed, and the combined beams flicker similarly to the HeNe.

Step 11: Remove First Beam Splitter and Mirror

The Hong-Ou-Mandel Interferometer is aligned when you have both HeNe and white-light interference. If you are absolutely positive this is the case, then remove the beam splitter and mirror that obstruct the path of the down-converted photons. BE VERY CAREFUL FROM NOW ON! IF SOMETHING HAPPENS THAT REQUIRES THE INTERFEROMETER TO BE REALIGNED, YOU HAVE TO GO ALL THE WAY BACK TO STEP 4 OR EARLIER!

Step 12: Detectors

Place down two detectors: one in the path of each beam exiting the beam splitter. Make sure that they are equidistant from the beam splitter. Place an 810nm filter in front of both detectors. Move the translation stage all the way forward or back. Turn on the pump laser and follow a similar procedure to that found in Step 2 to align the detectors.

Once you have maximized the coincidence rate, you are done! Start taking data!

This interferometer design is taken from Armendáriz et al:

Armendáriz, G. et al. "Teaching quantum mechanics with the Hong-Ou-Mandel Interferometer." Proceedings of the SPIE, Volume 9289, id. 928908 7 pp. (2014).

Step 13: Troubleshooting

• • If white light interference still eludes you, measure the path lengths of each beam from where they leave the first beam splitter to where they hit the second. Compare the lengths of A and B. If the two lengths differ by more than a quarter inch, the A arm path must be changed. This can be done by sliding the second and third path A mirrors along a 45 in a similar motion as the transnational stage of path B. Loosen the clamps holding each mirror and place a transnational stage on the 45. To shorten path A, carefully push each mirror toward the beam splitter separately with the stage to shorten the path length by the desired amount. If path A is too long, construct an arm that wraps around the mirror post. Then move the stage away from the beam splitter by the desired amount. This process can be seen in Figure 11. This will slightly misalign these two mirrors and alignment should be continued from Step 7 on. The image below demonstrates this technique.

• If you must go back beyond step 4, for whatever reason (bumped a mirror, or the BBO, etc.) we recommend NOT removing the placed mirrors on their bases, but rather unscrewing the mirrors from the holder, which should allow a clear path from the BBO to the detectors placed in step 2. A handy trick, in case you must remove a component (such as a beam splitter) and wish to place it back in its "original" position, is to place small clamps butted against the base of the object, and dog them down. This provides a guide to place them back, but a forewarning comes, as it likely won't be in its exact original position.

For assistance, contact:

Mitchell Frand, Email: frand053@umn.edu

Jacob Christy, Email: chri3448@umn.edu