Laser Hut & Beam Path

To protect the lasers from radiation damage due to the electron beam and shield personnel from accidental exposure to hazardous laser light, all laser systems are located in a shielding house ("laser hut"). It is positioned in Hall A on the right-hand side of the target at a distance of approximately 5 meters as shown in Figures and . This placement limits the maximum angle for the hadron spectrometer to about 45$^\circ$, which is sufficient for the planned $^3$He experiments.

Figure: A topview of the laser hut shows the location of the interlocked door as well as the position of the laser table where all lasers and optics will be set up.

Inside the laser hut, an optical table with an anodized aluminum top supports all optics. The setup of the polarizing optics is shown in Figure .

Seven infrared diode lasers (four for longitudinal pumping and three for transverse pumping) and the related interlock control box are located on a 19" rack. Light is guided out of each laser and goes to an optical table via an optical fiber. Each beam is focused by a 2" diameter convex lens with a focal length of 88.3mm, then divided by a polarizing beam splitter cube into two linearly polarized rays. Because the laser light is initially unpolarized, both the direct and the split beam carry approximately half the power. To utilize the full laser power it is necessary to combine both beams and focus them onto the optical pumping cell. This is accomplished as follows:

The direct beam is reflected by a 3" diameter dielectric mirror which can be adjusted to steer it towards the pumping cell. The split beam passes through a $\lambda/4$ waveplate, is reflected by a 2" diameter dielectric mirror, and passes through the $\lambda/4$ waveplate again. The fast and slow axes of this $\lambda/4$ waveplate should be oriented at an angle of 45$^\circ$ to the horizontal or vertical direction. The linear polarization of this beam is thus rotated by 90$^\circ$, and is able to pass through the beamsplitter, essentially without reflection. The second passing through the splitter is necessary to achieve a very high degree of linear polarization for the split beam since the splitter only gives high polarization for direct beam ($T_P>95$%,$R_S>99.8$%).

Now both beams from each laser have identical linear polarizations. Each passes through a $\lambda$/4 waveplate that transforms its polarization from linear to circular. The orientation of each $\lambda$/4 waveplate is shown in Figure , note that all eight (or six for transverse pumping) $\lambda$/4 waveplates should have the same orientation.

The eight (or six for transverse pumping) resulting beams are carefully aligned to coincide on the pumping chamber of the cell. They enter the target chamber via transparent windows in the target enclosure and oven walls. There is one pair of windows for each polarization direction.

The total path length from the lasers to the pumping cell is about 5 meters. The beam path for transverse pumping is a straight line from the polarizing optics to the pumping cell (via the transparent windows). However, for any other pumping direction, e.g. longitudinal, the beam must be reflected twice such that it goes first down and then horizontally in the desired direction. To this end, a pair of adjustable 4" diameter mirrors is mounted on the outside of the target enclosure. For longitudinal pumping, these mirrors can be standard dielectric ones since they will be in a polarization-preserving compensating configuration (one mirror rotated by 90$^\circ$ with respect to the other).

Figure: Top view of the optics setup inside the laser hut. All items are on a laser table which is sitting on a mezzanine inside the laser hut

Under normal conditions, laser light will be completely absorbed by the rubidium in pumping cell. However, there is small amount of the laser light being reflected by the pumping cell wall or deflected by the pumping cell itself. There is a 4" diameter mirror on the left side (viewing towards downstream direction) to collect the light for the EPR measurement. The light is focused by two lenses (mounted on the oven) first, then guided by a pipe with mirror and additional focusing lenses in it, finally reaches the EPR photodiode which is located a few meters away from the target. At the meantime this light will also be viewed by a fiber and enters the spectral-analyzer, which is used to monitor the spectrum and intensity of the lasers.

The laser beam line is protected by beam pipes that extend from the laser hut to the target top cover. It is important to point out that the laser beam lines are fully enclosed from the rest of the hall.

The entire laser hut is a laser controlled area that requires special safety precautions (see Chapter 13 and Appendix A).