Alignment Table of Contents
Interferometer Alignment (you are here)
Just as in the previous alignment of the B' detector, the interferometer is initially aligned with the red HeNe laser.
You will need three small irises (they come in various diameters) to perform the interferometer alignment.
Place them along the B-arm path just before the 800nm HWP, and set their height.
The best way to do this is to open the irises just enough such that the laser forms a ring (or 'moon') around the iris opening. When this moon is even on each side of the opening, your iris is centered.
Place an iris in the HeNe path just after the first mirror that it reflects off. Or: place it in between the first silvered mirror and the first pickoff (silvered) mirror. Adjust it's height so it is centered.
This iris will be used to retro-reflect the beams of the interferometer. It shall be called the 'HeNe retro-iris.'
The HeNe is currently following the B-arm iris path. It must be sent off to the interferometer. To break it off, place the longer-wavelength mirror (on a pickoff mount) into the B-arm path a few inches before the 800nm HWP.
Use two irises. Align them with the holes in the table. Use them to guide the laser light, thus making it level and parallel with the table. Make sure that the HeNe beam connects with the dead center of this pickoff mirror.
This can be tricky. The method that works best is to place the first iris about 3-4 holes (inches) in front of the mirror, and second mirror as far as possible away, while still being in-line with the optical holes.
In order to make sure they are truly parallel, take two square and identical plates (see picture), and bump them up onto two screws with are placed parallel to the desired path (again, see path). Press the iris against this plate and align its orthogonal axis with the orthogonal holes (again, see picture). This will standardize the iris placement.
Remember the orientation of the plate with respect to the beam direction. Make sure to keep the plates on the same side of the beam each time you place an iris.
Place the second, stand-alone mirror in the path of the sent beam a decent length away. Not quite at the edge of the table, but close to it. This ensures that you will have enough room for the interferometer.
Reflect this beam by 90-degrees in the direction opposite of the down-converted beam (i.e. send it towards the area where there's more room on the table). As always, make sure the beam is reflecting as close to the center as possible.
Use the same method as above to place the irises parallel to the holes in the table, and send the beam through these holes.
This beam is incident to the MZ interferometer, and by using above iris method, the beam will be roughly aligned.
Use a mirror-walking technique between the pickoff mirror (which broke away the HeNe beam), and the second (stand-alone) mirror to ensure that the beam passes through the center of each iris.
Now you have a beam which is very much level with the table, and parallel with the holes.
Place the Non-polarizing Beam Splitter (NBS) a reasonable distance from the stand-alone mirror (~7 inches).
The beam must be incident on the half with the dot on top, and have the beam follow the paths of the arrows on top of the NBS.
Using the two-iris method, align the reflected beam with holes orthogonal to the incident beam.
Here the placement of NBS1 greatly matters. First of all, make sure that (as always) the incident beam hits the dead center of the NBS face.
In order to make sure it's aligned horizontally, begin with aligning the moon of the beam with the far iris; you will probably be off on the near iris. Next, move NBS1 horizontally (very slowly) while very slowly rotating the NBS in order to keep it aligned with the far iris.
Now close the HeNe retro-iris almost all the way, and observe the retro-reflection. Tweak the three knobs until the retro-reflection becomes centered.
If it is placed correctly, the beam should be: a) entering/exiting the dead center of the NBS faces, b) centered on, and leveled with the two irises, and c) retro-reflected correctly on the HeNe retro-iris.
If not, then move the placement of the NBS1, rinse and repeat.
It does not matter if you place M1 (linear mount) or M2 (piezo mount) first, they are two identical arms.
Adjust the linear mount so that it is in the middle, or 'zero' position. This will be at 5mm.
Place the linear mount such that the mirror face sits exactly six holes from the location of NBS1.
Using the two iris method, reflect the beam by 90-degrees in the direction of the beam incident to the interferometer. Use a positioning technique similar to the one used to place NBS1. Make sure that the beam hits as close to the center of the mirror as possible.
Leave the far iris in place, at it will be used later.
The zero position of the piezo mount is harder to find than the linear mount. Place it where the top platform is even with the bulk of the mount.
The rest of the procedure follows identical to Step 4. Remember: six holes from NBS1, and leave the far iris in place.
NBS2 is placed six holes from both mirror-mounts, and it's orientation should be matched with NBS1 (i.e. the dot and arrows).
Follow the same alignment techniques as in Step 3. This means moving the near iris from in front of M2 and placing it near to NBS2.
It will be easier (maybe...) this time, as the two beams seen on each iris only have to be aligned with each other.
Retro-reflect the beam of NBS2 using the HeNe retro-iris.
As before, if placed correctly, the beam should be: a) entering/exiting the dead center of the NBS faces, b) centered on, and leveled with the three irises, and c) retro-reflected correctly on the HeNe retro-iris.
If not, move slightly the position of NBS2, rinse and repeat.
Note: Aside from the linear movement of the linear mounts, the only adjustment of the MZ interferometer should be made to the knobs of NBS2.
Now that the MZ interferometer is assembled, lets see some interference. Remove all irises, and place two optically-mounted note cards along the outputs of the two branches. Along the closer output, insert a bi-concave (diverging) lens. This expands the image.
Do you see interference? There are two types: 1) concentric interference seen in the two beam signals, and 2) linear interference fringes in the signal itself. If the MZ interferometer is aligned decently, the second fringe type should be easily observable.
Before we enhance these fringes, first set the beam orientation. Move the rough linear mount away from the zero position such that the two beams appear as separate on the note card. Now adjust the knob of NBS2 so that they are vertically identical. Then move the linear mount back into the zero position, or which ever position evenly aligns the two beams.
Now adjust the two knobs on NBS2 that control the vertical position of the beams. Watch for the linear fringes to become bigger/smaller. You want them to become bigger. When they peak, the fringes should flip in orientation. Adjust the two vertical knobs to that peak point.
Flip in orientation means: as you turn the knob in one direction, the fringe spacing will go from getting larger to getting smaller. At the peak point it will appear as though the fringes are vertical.
Adjust the horizontal knob of NBS2. This will have a dramatic effect on the fringes. Adjust until only a single fringe is observable; this happens when the fringes peak, or flip in orientation. At this point the note card image will appear to flicker with even the smallest of changes in path length.
This part is very cool. You can press on the table and observe the fringes move. They pulse when relaxing. You can blow on the laser beam - changing the index of refraction of the air (VERY slightly) causes a fringe shift.
Also note that when constructive maxima appear on one note card, destructive appear on the other; they oppose each other.
Now remove the closer note card, and place the D-detector lens in the path. Just as in the alignment of B' (Step 3), use the output signal of an optical fiber to find the 'sweet spot,' and thus roughly align the D-detector. This works best of you place the D detector as close as possible to NBS2, while still leaving space (1-2 inches/holes) in between.
Since the coherence-length of the HeNe laser is very long, it is quite easy to get HeNe interference. However, an incandescent source has a blackbody spectral profile, and will only interfere with itself when the path-lengths of the MZ interferometer are exactly identical. Therefore, we observe the spectral response of an incandescent light source, and find that when correctly set up, strange things happen to its spectrum. Getting this to work is by far the trickiest part of the set-up. You could have very nice HeNe interference, but still no spectral response.
Place an incandescent flashlight in between the pickoff mirror (which breaks the beam away from the B-arm) and the stand-alone mirror (which sends the beam into the interferometer). Place it as close to the stand-alone mirror as possible. The flashlight should be on a pickoff mount for easy removal.
Connect the output-end of the D-detector optical fiber to the Ocean Optics spectrometer and observe the spectral profile. It should appear in the form seen here.
Slightly adjust the 'rough alignment' linear mount, oscillating it until a maximum intensity is observed, then tweak the NBS2 knobs slightly, making sure to re-adjust the rough alignment linear mount back to maximum with each adjustment of the NBS2. The key thing to watch out for is the fringes in the spectrum. They will appear as small bumps on the top of spectrum at first, and once you see them, it will be quite easy to adjust until only a single fringe is observed. Note that the spectrum will wiggle A LOT as consecutive maxima and minima (in intensity) pass into the spectrometer. This will increase the difficulty of the single fringe, and so the final adjustment should be visual.
Once the fringes are observed, make small tweaks to the NBS2 knobs, and then use the linear mount to make 1-3 fringes viewable. Lock down the adjustment of the rough alignment linear mount.
Use the knob on the piezo mount to make sure the spectrum-fringes are minimized. Place an optically-mounted note card on the other output of the MZ interferometer. Do you see white-light fringes? If yes, follow the similar procedures for adjusting the HeNe signal interference until the white light begins to 'flicker' to the best of its capability. Be VERY careful. At this point the MZ is preposterously sensitive. Do not fear if you have to go back a few steps to re-align.
When you feel that the alignment is satisfactory, it's time to send single photons into the MZ interferometer.
Remove the flashlight from the beam path. Flip up the 810nm bandpass filter, and turn on the blue laser.
Maximize the D-detector counts. This is best done by placing something like a ruler on the table along which the D detector can be slid, making the adjustment easier. Perform this adjust similarly to the adjustment of the A/B detecotrs to the down-converted light.
Then, tweak slightly to maximize the AD coincidence counts, these will be important.
Note that if the MZ is set up perfectly this maximum should be hard to obtain due to the fluctuation of of max/minima (just as seen in the spectral response of the incandescent source). However, while aligned perfectly for the incandescent source, the MZ interferometer is mostly like not perfectly aligned to the single photon source.
Now, adjust the NBS2 until fluctuation in AD coincidence becomes large/readily apparent.
I left this intentionally vague, as all I can offer is guidance.
Be 'beyond sensitive' when adjusting NBS2. What I found effective was to apply as little pressure to a knob as possible, increasing until it lurches forward (say, one 'unit' of rotation).
Observe the response.
Rinse and repeat until the spectrum reaches a near-zero minimum, and a maximum D-count that is roughly 1/2 the max counts of detector A (only half the output intensity is reaching detector D; other half is sent out the second NBS2 path). Changes in path length should oscillate the signal between these two boundaries.
When satisfactory, insert the two 810nm HWPs between NBS2 and the two linear mounts. There's some room for error here. The beam doesn't have to hit the dead center, or be retro-reflected as far as I could tell. However, to be on the safe side, you could send the HeNe through and check. NEVER send the full-strength HeNe into a SPCM.
Finally, place the linear polarizer in between NBS2 and detector D. If all is well, the MZ interferometer is complete.
You've been working too hard.