With the sun already set for the year and polar night just beginning, a few CREATE students are in Eureka working on instruments which thrive on darkness! I work in the CRL lidar lab, using lasers and telescopes to understand the atmosphere.
With CRL Polar Sunrise campaigns largely taken up (and rightly so) by the collection of atmospheric measurements, this late fall campaign is an opportunity to do some of the calibrations on the lidar which take a while, or which benefit from dark skies.
One goal of my Ph.D. project is to characterize a linear depolarization ratio channel in the lidar so that it can be used to tell the difference between ice crystals and water droplets in the clouds that CRL observes all year long. To make these measurements, the lidar sends linearly polarized light into the sky. The green light waves are all travelling the same direction (straight upwards), and are all wiggling in the same plane. The light scatters off of anything it hits in the atmosphere directly above the lidar. Some of the scattered light is directed back downwards, where it is captured using a 1-m diameter telescope. By checking the polarization state of the backscattered light, we can determine whether it was scattered by droplets (which are spherical, so we’d measure the light all the same polarization as what we sent), or by ice particles (which are angular, and which scramble the polarization somewhat).
We line up our polarization detectors in the lab to correspond to the “parallel” and “perpendicular” directions of the light that’s returned. The tricky part is that some mirrors and lenses in our detection system affect the returned light before we can measure its polarization state. Figuring out whether the upstream optics effect the polarization (which would bias our results) is very important. The goal of this fall 2013 trip is to measure the contribution of these optics in several ways.
The best way to do this is to put completely scrambled light into the telescope from the top, and measure what we get in the detectors. If the light starts out completely unpolarized, we’d expect to see even numbers of photons in each of our polarization detectors. If the optics between our telescope and the detectors are introducing a bias, we’ll see that during this calibration measurement, and be able to account for this during data analysis.
That all sounds good, but what’s the best way to do this, in practice? Completely unpolarized light sources are kind of hard to come by, so we make our own by putting a depolarizer on our instrument. We try various sizes, in various places in the lidar. Because the telescope focuses the light, if we put the depolarizing material after the focusing is done, we only need a depolarizer that is 2 inches in diameter. There is a downside, however: we miss out on understanding the polarizing effects of the telescope itself. We’ve done this in the past, but we wanted to do it better now, and see the effects.
This trip, we’re going BIG on the calibration: As I write this, there is a 1-m diameter depolarizer on the entrance window to the lidar. We’re running the lidar as usual, but we’re scrambling all of the returned light, so that it is unpolarized as soon as it enters the telescope. You can see that we’ve carefully left the laser-exit-window uncovered so that the laser beam can get out, and we have anchored the depolarizer down so that it cannot blow away. We’ll find out in a day or two what the effects are!
So, where does one find a 1-m depolarizing optic which doesn’t suffer in cold conditions (below -30° C), or wet conditions (it might get snowed on or get frosty), and is not too breakable, and is light enough that I can carry it onto the roof of the lab? At the art supply store, of course!
We’ve tested various materials, and so far, the best (well, the best which costs less than $700 per square inch) seems to be archival-quality waxed paper called Glassine. In our test, we put polarized light through the glassine, and then rotated another polarizer in front of it. If the glassine is depolarizing, we’d expect to see no change in the amount of light coming through as we rotate the 2nd polarizer. With one sheet, we do a pretty good job. With two layered sheets, it’s perfect! We just need to balance the needs of a) getting a good depolarizer with b) not dimming our light by too much. Check it out:
We’re not the first people to use waxed paper for depolarizing purposes. My favourite reference is this one http://jeb.biologists.org/content/199/4/999.full.pdf, which talks about octopi (they have eyes which can use polarized light for extra information). We’ve also tested various kitchen brands of waxed paper ourselves, but have found the Glassine to work best.
With some weather luck, we’ll have one more piece of our calibration puzzle solved by the time we fly back South!
Ph.D. student at the University of Western Ontario