Construction of a Tabletop Michelson Interferometer

The pupose of this page is to describe the work being done in the construction of a tabletop Michelson Interferometer. The purpose of this device in the scope of the project is two fold. First, the interferometer can be used to analyze the vibrational characteristics of a diamond waffer suspended from a wire frame. Second, the interferometer can be used to study the surface profle of a diamond waffer, allowing us to see the efects that different cutting and mounting techniques have on the final product. The main work does this semister has been on the vibrational aspects of the diamond mounting.

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.

Images of Set-up
The following are images of the complete Michelson set-up

Estimating Camera Sensitivity
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.


 * Determining the angle of the diffraction minimum for a circular aperture

Camera Calibration
Analysis of videos taken of the vibraton of the diamond waffer can lead to information about both the frequency and the apmlitude of vibration. To perform simultaneous measurments of both parameters, a precise camera calibration was necessary.

Calibration Set-Up
The following is an image of the device sed or the camera caibration:

IMAGE OF CALIBRATION DEVICE

This calibraton device was used to determine the change in pixel location of the center of the lens flairs based on agular displacment. To do this, the camera was mounted horizontally on a 1.40m long bar. The camera end was free to rotate about a pivot located at te opposite end of the rod, above which a mirror was mounted. A linear mcroadjustment translation stage was placed below the camea end.

Image Analysis
To find the location of the center of the flair on each image, several lines of code were imlimented:

MATLAB CODE

The first line imports the image, in this ase IMAG0001.jpg, convert it to a matrix, and separated the green channel. Next, this matrix is imaged in false color in terms of intensity from blue(low) to red(hgh). INSERT FALSE COLOR IMAGE ON RIGHT OF THIS PARAGRAPH The last line is the actual search function. The search functions by finding the location with the smallest difference between the intensity(a(m,n)) and the fit function we apply to image. We use a gaussian distribution with user defined parameters as our fit. The code for this function is: function out=gauss2(x,y,p)

[xx,yy]=meshgrid(x,y);

out=p(1)*exp(-0.5*((xx-p(2))/p(3)).^2-0.5*((yy-p(4))/p(5)).^2) + p(6);

The parameters p(1) to p(6) are, in numerical order, (1) the amplitude of the function, (2) x location, (3)sigma x, (4) y location, (5) sigma y, and (6) an offset applied to eliminte background noise. The function takes user inputs as an initial guess, then loops the search process to find more accurate values for the parameters. Arbitrary values are chosen for p(1), p(3), p(5) and p(6) to begin, and are then readjusted based on the first output parameters. Typicall, the closer these fit parameters are to the true paramters, the better the functin is at defining p(2) an p(4). By knowing the change in pixel locaton betwen photos, as well as th angular displacment, a value for pixel change per mrad in the xdiection can be determined. This proess is then repeaed for the y direction. Once these tweo parameters are known, the video analyss can be used to fin both frequency nd amplitude.

Video analysis
The same process used for the calibration images can be used to analyze the motion of the center of the lens flair over im. Becaue of the number of images involved in a video at 1200 fps, a batching process was developed to automate the proces. First, a built in Linux program, ffmpeg, is used to separate the images from the video. Next, the automtion script is applied to the folder contaning the video stills. This script uses the same code as the image analysis, but loops the search pocess through a large group of images.


 * Interferograms


 * Analysis of heat equation for diamond mounting