LEAKY Mirror Data

We have taken data in our “Leaky Mirror” configuration that attenuates our high power laser down to detectable single photons. To understand this, you may have to study our apparatus first. In short, the “Leaky Mirror” study demonstrates that if there are regenerated photons to be time correlated to our pulsed laser, we would be sensitive to seeing them.

We do the leaky mirror calibration data as follows: In the laser box, we reflect most of the laser light by 90 deg into our power meter instead of going down the magnet. We expect less than 0.1% of the light to make it through this mirror. In our PMT box, we have another highly reflective mirror that reflects most – all but another 0.1% or so – of the residual light back through the magnet back into our laser box where it absorbed. Now, in the PMT box, what’s left of our laser spot has diverged from about 6mm to 10mm in size. On the front of our PMT, we have first a 10 micron pin hole that should capture about 10^-6 of the area. Next we have two absorptive optical filters that should have a combined reduction of about 10^-7. Any residual photons will hit our very sensitive photocathode on our PMT. The combined attenuation is nominally 10^-21 and since our laser puts out a few 10¹⁷ photons per pulse, we expect something like 1 photon to make it for every 100 pulses or so. Indeed that is what we observe ... with relatively large uncertainties on the exact attenuation.

In the figure below, you see the time difference (in ns) between our laser firing as measured by a photodiode in our laser box and the arrival of a photon in our PMT as measured by the PMT in our PMT box. The actual number of -248ns is understood perfectly in terms of a 158ns delay for the GPS time synch signal to get from QuarkNet board mounted on our PMT box to the QuarkNet board mounted on our laser box. Other relevant cable delays contribute about 42ns delay. The remaining 48ns is the speed of light time for the photon to travel from our laser, through vacuum and the attenuation, to our PMT. Occasionally, there is no photon detected and the closest PMT pulse is a noise pulse that occurs at a random time. In the top figure, you see the spike of our laser-generated photons and some noise pulses. In the bottom figure, you see a zoom in the region of the laser-generated photons that shows that 99% of those photons arrive within a 10ns window as expected for the width of our laser pulse. This 10ns window is our a priori search window. We would expect something like 0.25 noise pulses in this 10ns window which demonstrates the power of the time correlated method.

Time Difference (with offsets) between laser photons and PMT hits (ns).

The bottom figure shows a zoomed-in region of the top figure.

In addition, we use the “Leaky Mirror” data to check our sensitivity to polarization. We use a commercial polarizing filter (for telescope use) to analyze the polarization of the leaky mirror laser light. We do this without and with the ½-wave plate to rotate our polarization. The ½-wave plate is oriented 10 degrees away from its axis so that we do get “Leaky photons” in all cases, but it is reduced for the case when the polarization doesn’t match analyzer. The data behaves as expected.