Performance of Metal-Coated Mirrors with Different Protective Dielectric Layers. Part 1: Experiment




Jason R. Stockton, Benjamin (Adam) Catching, and Dr. Anna Petrova-Mayor CSU Chico, Department of Physics Introduction: Metal-coated mirrors are commonly used in optical instruments. When light is reflected off a mirror, its polarization may change depending on (1) the coating (material, number and thickness of layers), (2) the angle of incidence, and (3) the orientation of the mirror (azimuth angle) with respect to the incident polarization. Understanding of these changes is of particular importance for polarization sensitive instruments. We performed a series of measurements and characterized 4 sets of mirrors with different coatings (off-the shelf and custom designed). Here we present the experimental results and analysis and outline the theoretical background that we will use next to model the observed phenomena. Experiment: Vertically polarized beam at 1543.7 nm is incident on the laboratory beam scanner. The beam is steered in azimuthal and vertical (elevation) direction via the rotation of the first and the second mirror, respectively. The state of polarization (SOP) of the beam for any pointing direction is measured with a polarimeter. The experimental data are compared in figures 1-2 and summarized in table 1. Theory: Metal-coated mirrors are of low cost compared to multi-layer dielectric mirrors and are thus the choice for large optics (e.g for telescopes or scanners). The metallic coating is deposited on a glass substrate and covered with a thin dielectric layer (such as SiO2) that protects the metal against oxidation and scratches. The biggest disadvantage of metallic coatings is that they absorb light and therefore their reflectivity is limited to 96%–98%. The reflectivity can be enhanced by the use of several layers of dielectric coatings. The principle of operation of multi-layer coatings is explained in Fig. 3. The amplitude reflection coefficients and the accumulated phases upon reflection are different for the s- and p-polarization ()and(). Therefore, the state of polarization (SOP) of the reflected beam may differ from the SOP of the incident beam. We call this effect depolarization caused by the mirror. To calculate the transformation of the SOP of the incident beam, we plan to use the matrix method from Mayor et al. The transformation matrix, its elements, and the related coefficients are defined below. Further work is required for the calculations/measurements of the amplitude coefficients and the phases. Conclusions and Outlook: The obtained data show that the aluminum mirrors depolarize less than the gold mirrors and the enhanced coatings depolarize less than the protected coatings. The orientation of the ellipses for all mirrors consistently follows the azimuth angle. The next task is to develop and apply the theoretical model for the mirrors and compare the theoretical with the experimental results. Furthermore, our group had tested a new method for compensation of the depolarization effects caused by the mirrors. We will extend the theoretical model to this case in order to explore other options. Reference: Mayor, S. D., S. M. Spuler, B. M. Morley, E. Loew, 2007: Polarization lidar at 1.54-microns and observations of plumes from aerosol generators. Opt. Eng., 46, 096201. Thanks: We want to thank Scott Gimbal, Michael Li, and Robert Blanton for helping with the data collection.


My First Poster

So a good portion of my life this semester (Spring 2014) was consumed with the assembly of my first poster with my student co-author Adam Catching, and our VERY patient advisor, Dr. Anna Petrova-Mayor. WOW, that was a lot of work. Walking down the halls of the physics department you will find these posters all over the walls. It is all research done by students and their faculty advisors. They are simple looking, just a few equations, some diagrams or photos, and a couple of paragraphs. I used to wonder what the big deal was about them. It isn’t like they were writing a paper. HAHAHA, I was fooled. I am sure writing a good scientific paper is more difficult, BUT, putting together a poster is no walk in the park either. At least not the first time you try it.
Why? Well, you work for months on a piece of research. Months. It takes time to figure out if your question is a valid one and if so, you can only hope that it is answerable within your allotted time frame. Here you get to play with theory for a while. What should happen? Then you have to figure out how best to go about answering your question with your restrictions… mainly, that means time, equipment, human work power, and money. Mostly, money. Stupid money. Once you have a plan, you need to develop your experimental protocols and methodology. Then, start building your setup. Once your setup is good to go (not as easy as that) you get to take data. Realize your data sucks and you need to fix/ adjust/ completely redo/ something, and take it again. Eventually, you get good data and you can compare your (hopefully) good theory to your good data and see what the difference is. Finally, it’s time to publish! YEA!
Back to putting that poster together….
Now you get to talk about all that stuff you learned and figured out…. except you have only have just a few equations, some diagrams or photos, and a couple of paragraphs to work with. WHAT!?! How the heck am I supposed to organize and summarize all that STUFF in a way that actually gets the point across in that limited space? Fresnel equations, wave theory, you call that hard? HA! Summarizing all that work… THAT is hard!
Ok, so I am complaining a little. I really had fun with it. And (not supposed to use “And” as the first word in a sentence…. I don’t care… I’m a rebel!) AND, I didn’t actually have to do a lot of that. See, I came into this research project after it was already started. It wasn’t exactly easy to catch up though. I did have to learn the theory and figure out what the heck that thing was on the fancy looking table and how it all worked. That was quite a bit of work and I admit, I took it slowly. Still, I ended up the first author on this poster somehow.
I must extend my thanks to Dr. Anna Petrova-Mayor for allowing me to be active in this research and also to Scott Gimbal for answering all my questions and patiently pointing out that I was trying to figure out stuff he already did. They call that reinventing the wheel I think. THANK YOU!