SPQI Equipment Cage Assembly

This page describes the cage assembly.

The cage assembly works like this:

Light coming from a Thorlabs optical fiber cable(the orange cable) is incident on a collimating lens. This lens focuses the beam, which passes through a Longpass filter. This filter greatly attenuates light with wavelengths under 780nm(See the graph below.) This light is then incident upon an another lens which focuses the beam into a pacer fiber cable(the black cable), which is connected to the SPCM. A rubber light seal is used to stop outside light from entering the assembly.

The second lens is mounted on an Adjustable Body mount. This allows for fine adjustment to make sure the focused light hits the the second lens in the sweet spot. NEVER adjust these knobs as that will bring the two lenses out of alignment and drastically reduce counts.

If you suspect a cage assembly (here referred to as the "bad" cage assembly) may be misaligned, you should do the following before adjusting the knobs on it: 1.) Double check that whichever detector you are using to capture photons is in the correct photon path and make sure nothing is in the path that shouldn't be there. Also, make sure the detector and the "bad" cage assembly are properly connected.

2.) If you already have a detector in a good alignment and getting a good amount of counts, connect this detector to the "bad" cage assembly by simply reconnecting the orange fiber optic cables. If you now get the same high amount of counts with this setup, then the "bad" cage assembly is working properly and isn't the problem. Chances are the detector you were originally trying to align is not in the right spot. IF you connect the good detector with the "bad" cage assembly and the count rate is not as high as it should be, then the "bad" cage assembly may need to be realigned.

3.)If you don't have a detector already in a good alignment, use a different cage assembly and see if there are any changes in count rate(make sure to turn on the proper SPCM when doing this.) Simply disconnect the orange fiber optic cable from the "bad" cage assembly and connect it to a different one. If you notice a drastic increase in counts, than the "bad" cage assembly needs to be realigned. If you don't notice a drastic change, then the issue is probably not with the cage assemblies and you should redo your alignment of your detector.

If, after completing the preceding steps, you determine that the "bad" cage assembly is not aligned properly, realign it using the adjustment knobs. Note that the proper alignment is very fine and it can be difficult the find the sweet spot. Iteratively adjust the three knobs, trying to sweep as large an area as possible using small adjuments at a time. Once you get near the sweet spot, you should notice a jump in counts. Keep adjusting until you find the maximum count rate, which hopefully should be consistent with the count rates you get using other cage assemblies.

Cage Assembly Cross section

Collimating Lense(In Mount)

Long Pass Filter(In Mount)

ThorLabs FGL780

The above graph gives the % Transmission of light as a function of Wavelength for the Long Pass Filter

-- Main.cardx013 - 01 Aug 2012