Creating a Precisely-tunable Single-frequency Laser Source to Excite Whispering-Gallery Modes
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Category Archives: PHY 287
Report Fall 2011 Semester
Filed under PHY 287
Light Beat Frequency
Scope without input
Scope with control frequency
Scope with shifted frequency
Scope with beat frequency
Superposition of the two frequencies (Click to enlarge)
Beat Frequency (Click to enlarge)
Filed under PHY 287
Stepper Motor
Some of the links I read while hooking up the motor:
Motor Shield – Frequently Asked Questions
Stepper Motor – Connect and use a stepper motor with the Make Controller Kit
It is hooked up to M1 and the wire color order from the top is:
Red
Blue
White
Green
Yellow
Next up, using pwm to achieve continuos motion.
Filed under PHY 287, Side Projects
Drawing Plots
Octave and GNUplot are installed on my computer. I used MacPorts.
I made this picture using Octave & GNUplot.
Filed under PHY 287
Inner workings of an AOM
The crystal is subject to a compression waves, which are the acoustic wavelength. The incident light beam, when striking at the correct angle, forms a diffraction pattern corresponding to the acoustic wavelength.
The transmitted light is Doppler shifted based on:
Sources: The Acousto-Optic Modulator and Optical Heterodyning
Filed under PHY 287
Week 6
Today was spent setting up my laser, AOM and photodetectors. I read a lab manual on how to use an AOM, and I wanted to perform the demos it suggested. Unfortunately I could not find information on how to hook up the AOM. I need to ask Marty for help in hooking up the driver to it. The photodetectors are mounted to sliding tracks, so I can adjust them. The AOM has z axis freedom and can rotate in the x/y plane. One of the frequencies should emerge off center of the original transmission angle.
Filed under PHY 287
Week 6 Day 1 & 2
Recently I decided to stop writing daily updates. Writing daily reminded me that at times it is better to have a long term view. Holding a thought for about a week before moving forward and acting on it suits my pace of idea generation. This gives me time to thoroughly examine the idea and look for flaws in it. The past weeks thoughts have focused around whispering gallery mode resonators.
One of the publications has a chart showing pump transmission power vs frequency. I wanted to make an arrangement so I could take similar measurements. After speaking with Professor Metcalf he suggested using an AOM to change the frequency of light, and to use a quarter wave plate to rotate the polarization, so I could separate the two frequencies (the light travels through it twice, in the schematic). After the AOM the light should pass through a resonator, and it should peak when the frequency is one of the modes of the resonator. Ideally I would like the resonator to be a whispering gallery mode resonator, but at the time I do not have one. Alternatively I could use a Fabry-Perot resonator, which I think I will do for initial testing.
I was reading about the fabrication of WGMR, and I think it would be possible for me to construct some out of silica glass, using my lampwork glass shop. If I can learn how to measure the modes of the FP, then I could make several WGMR out of glass, at different sizes and slightly different shapes. Then I could use the WGMR instead of the FP.
A thought just occurred to me, I want to measure the frequency of the light and simply knowing the intensity is not enough. Could I calculate the frequency of the output light from the AOM? Should I interfere the two beams at some point to see what the frequency difference is? I can look up the process ‘hetrodyne’ for additional information.
Filed under PHY 287
Intensity vs angle of HWP through Vertical Polarizer
I have a 632nm half wave plate, and I needed to find the optical axis.
The axis HWP is horizontally polarizing the light at an angle of around 45 degrees. Now to record the intensity over 40-50 degrees and find the exact position for horizontally polarized light.
***UPDATE***
After sitting in the dark and rotating the HWP from 40 – 50 degrees the minimum intensity value is at 40 degrees. (measured with voltmeter). Now to find the exact maximum position. I’m going to look near the 0 point.
***UPDATE 2***
One maximum is at exactly 2 degrees. The second is at 88 degrees.
Filed under PHY 287
The quantum eraser & solving the half wave plate question
There were two things I wanted to try at the LTC. The first was to retry the quantum eraser experiment, using the steps as shown in the Scientific America article and my own thoughts about how to do it using a half wave plate. The second thing to do was confirm if the suspected half wave plate actually is a half wave plate.
Confirming the half wave plate
I had previously determined that this supposed half wave plate has an optical axis in the x and y direction which did not effect the transmitted light. At this point I was unsure if the plate was a half or quarter retarder. I know that a quarter wave plate would produce circularly polarized light if held at a 45 degree angle to in incoming polarization. To test for this I set up in this order:
Vertical Polarizer –> Horizontal –> No Dot
(Shows I have vertical and horizontal polarizer’s in place)
Vertical Polarizer –> Plate 0 degrees –> Horizontal –> No Dot
(Shows the Plate has no effect at 0 degrees)
Vertical Polarizer –> Plate 45 degrees –> Horizontal –> Dot
(Still not a confirmed HWP though, it could be a QWP)
Vertical Polarizer –> Plate 45 degrees –> Horizontal –> Linear Polarizer Rotated at random –> Light blocked at times
The fact that the light is blocked at times by the linear polarizer confirms the polarization is linear. Since the light is linearly polarized, but appears to have its axis rotated by 90 degrees, the plate is acting as a HWP.
The Quantum Eraser
The general idea is to split a beam, have one half be horizontally polarized, the other vertically polarized, recombine the two and see no interference. This part is working, using the method described in Scientific America. Then, when I place a linear polarizer at 45 degrees in the recombined beam I should see a return of the interference pattern. This is not happening, and I am not sure why.
Jones Matrix for what I expected to happen.
I tried creating the V and H polarization using two different methods. The first one was to simply place a vertical polarizer in one beam, and a horizontal polarizer in the other beam. This did remove the interference, but it is not returning when I rotate a linear polarizer in the recombined beam. (Although the dot never completely vanishes.)
IDEA: Try rotating before the magnifying glass. Or is it that the 45 degree polarizer has no knowledge that previously the light was from two different split beams? It could have been any horizontal or vertical light?
The second method for creating V and H polarization was to vertically polarize the entire beam, then place the HWP at 45 degrees in one of the beams, shifting the vertical polarization to horizontal. This did create an interesting effect. The interference fringes became about 4x finer. I should find a way to secure the HWP in this position so I could inspect these fine interference fringes in closer detail. The expected result was for the interference pattern to vanish. I should try rotating a linear polarizer in this interference pattern and see what happens.
Another random question not relating to rest of article:
Do two laser beams intersecting with each other cause any effects with each other? Is it possible to change polarization of light through magnetic fields?
Filed under PHY 287
Trying to make a quantum eraser at the LTC
Today’s visit to the LTC began with reading various web pages about Quantum Eraser experiments. To help understand the quantum eraser experiment I watched a few videos about the famous “Young’s Double Slit” experiment. The videos demonstrated how an observer influences the pattern created by photons (or electrons) sent through two slits. When the slit choice of the photons is unobserved they create a double slit interference pattern, but when observed the photons create two bars. The quantum eraser experiment relies on the same idea of observing which way photons in two interfering laser beams are traveling (by measuring the polarization of the two laser beams).
I decided to try out a simple experiment in a Scientific America article about a DIY quantum eraser experiment, and thought I would improve it by using a fine strand to create the diffraction pattern instead of the suggested piece of wire. This turned out to be unwise, because despite creating a very nice diffraction pattern the initial spacing was so fine I was unable to complete the second part of the experiment, which is to polarize the two halves of the “split beam” in orthogonal directions. I left this project alone, but I think I could get it to work next time by using a lens to increase the size of my laser dot, a wire to split the beam and creating a spliced polarizer, ½ horizontal and ½ vertical. If this is placed directly after the wire the interference pattern should disappear and then reappear if I place another polarizer into the beam after the initial polarization.
I continued to try out this idea using the large interferometer, which has an interference pattern. I placed a horizontal polarizer in one beam and a vertical polarizer in the other. The interference pattern disappeared, because the two beams where no longer interacting with each other, but a reduced intensity light still shone through. Rotating a polarizer in this light just reduced the overall intensity. I do not recall if it completely darkened the light at any point and I forgot to record it. I’ll have to check again.