As can be seen from the data graphs, the interferograms were of the expected form. The interferogram of the HeNe laser was sinusoidal with some noise. One possible explanation for this is due to speckle in the laser. This explanation was not realized until after the experiment had concluded. It would not be very difficult to determine if speckle was the cause and to eliminate it given a little more time. The FFT of the interferogram yielded one major peak that greatly dominated a couple of symmetric peaks that were close to this main peak, see graph. These peaks are probably the result of a slightly less than perfect sine curve or result from noise in the system, mechanical or speckle.
Interferogram for the HeNe laser:
FFT for the HeNe Laser:
The interferogram for the Na lamp matched the general form of what was expected, a beat pattern. There were difficulties in keeping the baseline of the graph constant. Most of this inconsistency was removed by using a PMT with a pinhole entrance. However, some randomness remained. With the inclusion of the PMT and associated equipment is also the inclusion of more variables that could introduce some error, possibly through thermal effects. The rest of the presentable data was modified in excel to correct the non-constant baseline. I took an FFT in Origin of the best data that I could get. The output raises some questions, some of which deal with how Origin displays FFT’s. Origin puts the main peak at the origin and then plots the rest of the graph relative to this peak, I think. Two other symmetric peaks appeared equidistant on either side of the main peak. This could result either from noise symmetric in frequency from the main peak, or it could be a mirrored reflection of the doublet peak. Getting a calibration of the x-axis in terms of energy would determine which is correct.
Interferograms for Na lamp:
Close up of above graph, note the non-constant baseline, but the fringe pattern is still noticeable:
FFT for the Na lamp:
There were difficulties calibrating the speed of the motor and therefore the energy separation in the FFT for the Na lamp. The motor could have been calibrated using the information obtained in the HeNe scan, but the speed of the motor needed to be changed in order to greatly reduce the amount of time spent taking data. Data was taken at the greatest possible speed that would still resolve the individual fringes. For future reference, Excel can only graph 32,000 data points in one 2-d graph.
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