Construction of a Tabletop Michelson Interferometer

Abstract
All atoms are made up of protons and neutrons, which are in turn comprised of quarks. The particles “gluing” the quarks together are known as gluons. The interaction of quarks and gluons is still not entirely understood, specifically how they are confined in the nucleus such that lone quarks are never seen. The nuclear physics group at the University of Connecticut is part of the GlueX Experiment, which hopes to probe directly the gluon bond and understand its mechanical properties. The probe used by GlueX is a high energy (small wavelength), polarized photon (particle of light) generated by a technique known as coherent bremsstrahlung. This technique involves radiation of photons as a high energy electron beam passes through a carefully oriented diamond wafer. Because of the potential for contamination, it is necessary to suspend the diamond wafer rather than mount on a ridged mount. The diamond is currently suspended on thin tungsten wires. Proper orientation ensures a high degree of polarization, which requires stable mounting of the diamond. The goal of my research is to investigate the mechanical properties of the mount and eliminate the possibility of vibration of the crystal.

The pupose of this page is to describe the work being done on the development of a system to analyze these vibrations. Two different techniques will be used to determine the amplitude and frequency of these vibrations. First, a direct imaging technique will be used. A Michelson interferometer will also be constructed, and will be used in not only the vibrational research, but also in the analysis of the diamond surface structure.

Parts List
The following is a lis of the parts used in the construction of the inerferometer, along with a brief descripton of the parts.

Estimating Camera Sensitivity
Originally the parts list called for a 1mW laser. There was a concern, however, that this laser would not be powerful enough to produce interferograms. It is critical to understand the sensitivity of the camera to light from the interferometer, given the high intended image acquisition speed. The camera purchased for this setup, Casio EX-F1, has a movie frame rate capability of 1200 Hz. The following information allows an order of magnitude estimate of the sensitivity. (The camera uses a CMOS sensor. Note that lx=lm&middot;m2)


 * a sample CMOS chip, Micron's MT9P401, has sensitivity of 1.4 V/lx&middot;s and supply voltage of 2.8 V yielding 2 lx&middot;s of light energy to saturation.
 * Light intensity conversion - 320 lm/W given:
 * a 100 W incandescent light bulb is measured to have the perceived intensity of about 1600 lm
 * a rough figure of efficiency for a 100 W is 5%

Using these conversions, the sensor pixels saturate at 6.3&times;10-3 W&middot;m2&middot;s. At 1200 Hz acquisition rate, assuming 100% duty cycle, the saturation figure is 5.2&times;10-6 W&middot;m.

Now, let us assume that only about 5% of 1 mW laser light reaches the sensor due to cleanup in the beam expander and the light transmitted through the diamond. If the light is expanded to a 2 cm diameter beam, the beam at the sensor is rated at 1.6&times;10-8 W&middot;m2

The two order of magnitude shortfall means that very little of the dynamic range of the sensor will be used, leading to a low signal to noise ratio. For this reason, a 5mW laser was chosen over a 1mW.

Choosing a pinhole
It is crucial that any spherical aberration are removed by way of pinhole. The laser spot is passed through a pinhole, which acts to expand the beam. This expansion creates a patter known as an Airy pattern. To correctly remove the spherical aberrations, the beam must be expanded such that the only the first ring is sent through the second lens, and the rest is blocked off. To determine the proper pinhole size for the design, the following analysis was performed.

Design of a mounting device for diamond surface analysis
During the last test as CHESS it was determined that the wire mount was not suitable for diamond analysis because the rocking curve was skewed by the high vibrational frequency of the diamond-wire system. In order to perform the surface analysis, a new mounting device is in development. This device is composed of two stretched mylar sheets with each sheet attached to one half of the mount. The diamond will be placed between the sheets. This advantage of this type of mount is that it will create very little stress on the diamond sample, preventing the sample from warping. This type of mount cannot be used in the final beam path at JLab because the high temperatures in the vacuum chamber (~500oC) will deform the mylar sheets.

In order to use this mount in the diamond analysis, the center area of the mylar must be sufficiently flat. To ensure this flatness, a large sheet of mylar (100 $$ \mu m$$, most likely 24" x 24") will be placed over and attached to a drum with an 18" diameter. The large diameter will ensure that any small wrinkles at the edges will not propagate all the way to the center region.  The mount will then be glued to the mylar and while the glue is still tacky, the mylar will be stretched. The mylar will be stretched by placing a weighted container (approximately 100 pounds) on top of it. To verify that the center region is flat, the mount will be placed in the interferometer, and the interferogram will be analyzed using the process found here.  This will give us an image of the surface of the mylar.