Beat Analysis

The following data shows beat patterns for four different currents.  Looking at the beat pattern, we were able to determine a DD, as shown below, as this was the spacing between antinodes.  Since we knew the distance between two fringes was l/2 from our calibration, we were able to calculate the number of fringes in our beat by dividing DD by l/2.  Once we have the number of fringes, we can calculate the difference between the wavelengths by dividing the reading of the wavemeter by the number of fringes.  This difference can then be used to calculate the cavity length of the laser, as discussed in the theory section of our paper.  Also, we were able to compare the Dl with our spectra by measuring the spacing between peaks in our spectra.  Optimally, we would have wanted to compare the same currents, but limitations with the wavemeter made it impossible to measure the beam for the lower currents where we noted multiple nodes.  However, looking at our different spectra the Dl appeared to be consistent for different currents. 

All data taken at 20.5 degrees C

Average  Dl= 0.61897nm

Spectral Analysis

    The spectra analysis shows that below the threshold currency there were multiple modes, as predicted in our theory. 

Dl ~ 0.75 nm

The laser was observed to operate bi-modal near the threshold currency of 45 mA, and single mode above threshold.  However, there was not a clear transition between bimodal and single mode operation right at the threshold currency, and the two appeared to alternate between 45 and 50 mA.  The Dl measured between the modes for the multimode and bimodal operation  was consistent with the Dl calculated from our beat patterns. 

Cavity Length

      The following table shows two different cavity lengths calculated from the Dl obtained from the spectral data and the beat patterns as explained above.  The equation for the wavelength of standing waves in a cavity of length L is l=2nL/k.  Differentiating this with respect to k gives Dl= -2nL/k² or L= -l²/2nDl.  

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