We have a pump laser beam that is coming out of a single mode fiber and butt coupled into a PPLN waveguide crystal. We are capturing the beam at the output of the crystal with the focusing lens in an attempt to collimate it into a beam. The emerging beam does not appear to be Gaussian however and also does not seem to be behaving well through the optics. Namely, it will not collimate. More specifically, we see the following “window blind” diffraction pattern far from the crystal, image attached to this question. Can anyone shed any light into

  1. why the emerging beam is not Gaussian
  2. why the beam cannot be collimated? Our beam diameter coming out of the single-mode fiber (25 um) is larger than the waveguide itself (5 um), perhaps this is a factor?

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