This thread has got me really excited. So, I did a few simple tests with stuff I had around. First test was putting the beam of a m140 through a 30deg prism and shinning it on a target ~50 feet away. I measured the location of the spot a .5A then 1A then 1.5 amps. The the spot moved 3/16 of an inch from .5 to 1A then another 3/16 from 1 to 1.5A . This shows the wavelength is changing with current. However the diffraction amount is very small. So, I am not sure how, using a prism, we could get enough diffraction to get the beams to overlap in a small distance?
Next I tried a diffraction grating with too different m140's. The wavelength of these two were very different when measured at the same current. i didn't calibrate the grating with a HeNe so I don't know exactly what the WL were on the m140's. But it proves there is a significant deviation on the m140's.
Planters, is it possible the variation you see after the prisms is more due to the different angles the beams are entering the prism than due to different wavelengths? If your combiner is set to overlap the spots at a distance all the individual beams would be converging at some small angle before the prism.