Alignment

We aim to investigate spatial correlations of single photon pairs and for this a clear alignment procedure is needed. Without such a procedure the quality of the beams and the correlations are easily compromised for higher count rates. In this section we provide a protocol that we use to align our set-up.

We define two requirements that a successfully aligned set-up has to meet.

• The lensL1 has to focus the pump beam exactly in the centre of the PPKTP crystal. This ensures that the intensity of the pump beam is maximum inside the crystal, which guarantees a maximal efficiency of the PDC-process.

34

5.2 Alignment 35

• In our research we are interested in the correlations between photon- pairs that are created from single pump photons. To establish that we always collect both photons in a pair, we have to ensure that both zoom fiber collimators collect light from the exact same spot in the crystal, even when we translate the collimators in the x-yplane. To accomplish this, the collimators should be placed in the exact far field of L2 and the crystal in the exact focus of L2.

In principle the distance of the far field behind a lens is equal to its focal distance (f = 250 mm in our case). However, in our set-up the PDC-light passes a relatively large 2” beam-splitter cube (n = 1.53), which acts as a thick concave lens. Taking this into account, we find a distance to the L2 lens for the exact far field of 267.6 mm.

We use the following strategy to align our set up and fulfil the require- ments stated above.

1. We place apertures on the beam path of the blue pump beam and use a periscope to point the pump beam through these apertures.

2. We place the beam-splitter perpendicular to the pump beam in the beam path. We make use of the pump beam’s reflection on the beam splitter to optimise the angle of the beam-splitter with respect to the pump beam.

3. Using two single mode fibers (=5µm) we feed light from a Helium- Neon laser (λ =632.2 nm) back through the collimators. We secure the computer-controlled translation stages on the optical table so that the red beam hits the beam-splitter exactly in the middle and passes exactly through the apertures placed earlier. In this way we make sure that the collection modes of both collimators are parallel to each other. If necessary the collimators can be tilted to adjust the angle of the laser.

4. We place a Spiricon beam profiler at the exact location of the crys- tal and lens L2 at its approximate position perpendicular to the red laser beam. The two red laser spots should now be visible on top of each other. To find out the precise location of the L2 lens in which the crystal is in its exact focus we translate the collimators in thex- y plane. If the focus is at the exact crystal’s position both the spots should not move with respect to each other. By iteratively adjusting the position of lens L2 while monitoring the change of the two laser

36 In search of the four-photon peak

spots the exact right position of lens L2 is found. The collection part of the set-up should now be aligned. We constructed a black enclo- sure for this part of the set-up to reduce noise due to background light.

5. We put the beam profiler at the position of the PPKTP crystal in the beam path of the pump beam and place the lens L1 mounted on a x-y-z translation stage at its approximate position. We monitor the exact size of the beam spot while translating the lens parallel to the beam path. Once the beam spot has reached it smallest diameter, the pump beam is optimally focussed.

6. Finally, we place a pinhole at the exact position of the crystal. To let the pump beam pass the crystal, lens L1 is translated perpendicular to the beam path. The blue pump laser should now pass the pinhole from one side, while the two red lasers should pass it from the other side. The set-up is now aligned. The PPKTP crystal and filters can be placed on their intended positions and the collimators can be con- nected to the single photon detectors via multi-mode fibers.

After following the prescribed procedure the set-up should be completely aligned. However, in practise still some fine-tuning is necessary in order to collect a clear signal. To achieve this we follow some additional steps.

7. We tune the crystal temperature to the phase-matching temperature (φ=0) and position the collimators so that they collect photons from the centre of the closed PDC disk. We slightly adjust the position of the L1 lens to optimise the count rates of both detectors.

8. We set the temperature of the crystal back to achieve a negative value of φ and scan one detector in the x-y plane, while monitoring the photon-counts on each position. A ring should now be visible. It might occur that some parts of the ring light up more intense than others. In that case we modify the position of the lens L1 in the x-y plane, so that the intensity distribution is symmetrical for the entire ring (see figure 5.5a). The same ring should be visible when the other detector is scanned.

9. It is convenient to have the home-position of the computer-controlled translation stages at the exact centre of the PDC-ring. To achieve this, a cross-section is made of the ring in both x- and y-direction. The 36

5.2 Alignment 37

data is fitted by two Gaussian distributions (see figure 5.5b). By tak- ing the mean of the centre positions, the centre position in horizontal- and vertical direction can be determined. The home-values of the translation-stage is now reset to these values. By repeating this pro- cedure a couple of times for each detector, the exact centre of the ring is found after a few iterations.

Figure 5.5: a.Single photon counts of the SPDC-ring for a 5 mm PPKTP crystal at 54 degrees displayed for both single-photon-detectors 1 and 2. The colour-scale is the same in both figures. Detector 2 seems to have slightly less counts than detector 1. A smaller detection efficiency of detector 2 can be an explanation for this.

b. Horizontal and vertical cross-section of the count rates of detector 1 extracted from the left image of a. By fitting two Gaussians to the data-points the approxi- mate centre of the ring is determined. This can be used as an alignment-strategy to set the home-values of the detectors at exactly the centre of the ring.

38 In search of the four-photon peak