Shown below are the plots of the amplitude and phase of the Mephisto laser light modulated with a chopper as a square wave at about 1 kHz.  The color bar for the phase should run from -pi to pi, and it does when I don't accidently comment out the color bar function.  Anyway, the phase is consistently pi/4 or pi/4 plus or minus pi.  Usually all three of these phases occur within the same image, as shown below.  Also, the amplitude is a factor of two or so higher than it should be where this phase jump occurs.  I think these problems are associated with the nature of the square wave.  However, there is a software bug that appears to be independent of the input data: there is a rounding error that causes the amplitude to jump to infinity at certain points.  This happened for only a dozen or so pixels so I deleted them from the amplitude plot shown below.  I am currently working on a more robust code that will use the Newton-Raphson method for nonlinear systems of equations.