Optimizing power to the detectors

To avoid dark-current errors and bit-noise errors in your data, all four PSD detectors need a high SUM reading. However the SUMs should not be allowed to saturate at 32767 (max integer in 16-bit A/D converters) because then the actual light value is unknown and certain variables like the trap positions (A_dist-Y, B_dist-Y) lose their calibration. A good rule of thumb is that when the laser powers are set at 100 milliWatts, all 4 SUM readings should read between 20,000 and 30,000.

The ND absorption filter in the Prism Box assembly reduces the laser beam power into the force-detector PSDs so that they do not saturate. An ND reflection filter may be flipped in front of the position PSDs to reduce their sensitivity so they do not saturate.

Laser power into the traps should be maximized by adjusting polarization of the light emitted from the optical fibers.

There is a problem getting enough SUM signal from the lightlever when the power is optimized to the traps. Bending the fiber leading to the wiggler changes polarization of light coming from the fiber end. The fiber is usually bent to optimize power at the traps and minimize light that passes through the polarizing beamsplitters (pbs) and hits the Beam Blocks

Bend fiber here to change polarization

giving maximum reflection

Beam block Beam block

and minimum wasted light

However bending the fiber to optimize trap power usually makes the lightLever power too weak. The pellicle actually directs light out of the plane of the drawing (although depicted flat in schematic below) and the pellicle tilts at 45 degrees to the beam which is near Brewster’s angle. Therefore the pellicle will reflect zero light to the position PSD if the beam from the fiber tip is optimized (linearly polarized) to reflect efficiently from the pbs cube.

One solution is to introduce an extra waveplate between the pellicle and the polarizing beamsplitter. That way light from the fiber can be elliptically polarized, and thus reflect partially at the pellicle. Then the new waveplate changes the elliptical light to linear in order to give maximum reflection efficiency from the pbs cubes.

extra waveplates

The extra waveplates must be very thin to fit into the space between the collimating lenses and the pbs cube. I use polymer “Retarder Film U27342” from Anchor optics.The film is cut into a ½” disc using a punch. Fast axis is marked by flat on disc.

Five pieces go into the threaded hole above the pbs cube, namely: (1) retarder-film disc (2) retainer ring (3) 8mm-Lens-Adapter (4) Collimating lens inside adapter (5) retainer ring. Two tools are used to place these parts: (6) spanner wrench for retainer rings Thorlabs SPW603 (7) depth gauge for setting position of collimating lens

Remove masking both sides12

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Mark the flat with a pen to indicate the fast axis of the polymer

Place the disc in the hole directly on top of the pbs cube. The fast-axis mark should go approx. 45 degrees to the sides of the box +/- 10 degrees.

Unfortunately the disc will fall out if you ever remove the pbs cube by opening the cover on the other side of the box.

Hold the retarder disc firmly against the pbs cube with a ½” retainer ring. Mark the position of the fast axis on the outside of the box.

Drop the collimating lens (already pressed into adapter ring) into the hole. Flat side of lens face up toward fiber end. Avoid getting dust/dirt into hole.

Screw retainer ring on to lens adapter to hold lens in place. Mark focal length of lens on outside of box.

Gauge sets depth of lens inside 8-mm adapter ring. The flat surface of the lens should sit 0.120 inches below the surface of the Prism Box (that depth is 0.070” for older style box).

Push down hard on the gauge and the lens will slide deeper inside the plastic lens adapter until it reaches the proper depth.

Depth Gauge

• Material: brass

Drill hole 0.295

0.350

Depth = 0.120

Note: fits Prism Box Frame ver.2 For older Prism Box frame, use depth = 0.070”

The end