The Aus Jena Jenapol shearing interference microscope is a rare instrument that was built in the 1980’s in what was then East Germany. At the time we purchased ours, we were told that there were only about 10 of these in the United States, and we had two of them.
Shearing interference microscopes work by splitting the image and then recombining the two images slightly out of phase. The two images can be both spatially offset, and offset in phase as desired. For evaluation of polymer optical waveguides, we typically offset the two images 50 microns in the horizontal direction. We then adjusted the optics to produce fringes with a period of approximately 25 microns. The spacing of the period is not critical, since the observed offset in the fringes changes proportionally. We therefore adjusted the fringe period to best observe the details we were most interested in. The fringe spacing can also be adjusted to infinity, and produce dark and light areas representing variations in refractive index.
The dimensions can be accurately measured, and any defects can be observed, but no information regarding the performance characteristics of the waveguide can be detected.
However, if a waveguide is analyzed using the Jenapol, a considerable amount of formerly invisible information is revealed. The delta index of the waveguide can be accurately measured by measuring the shift in the fringes from inside the waveguide . The shift is directly proportional to the index difference. From this, the numerical aperture of the waveguide can be derived
Equally useful, the variations of index inside the waveguide can be seen. This was very useful in our research to develop planar waveguides with a graded index.
Below are two waveguides that appeared the same under a conventional microscope: The left image has fringes with a round profile while the right has a flat top profile. These profiles provide an accurate indication of how the waveguide will operate without having to actually test the waveguide.
The waveguides were connected to an 850nm source. Both the near field and near fields were analyzed. Below are images of the near fields. These tests confirmed the predictive accuracy of the Jenapol.
When features are too small for analyzing with interference fringe patterns, the fringe spacing can be set to infinity, and the microscope is now sensitive to small variations in index.
These photos are of a 3 micron thick slice of a polymer film with two singlemode waveguides in it. In left side photo, the waveguides are nearly invisible, for the only difference between the waveguides and the surrounding material is an index difference of .001. However, when viewed in the image contrast mode, the waveguides are easily visible.
Another application for the Jenapol is identifying stresses inside a transparent material. The following photos are of a glass fiber inserted into a polymer. Under a conventional microscope, it looked perfect. However, the Jenapol revealed considerable distortion caused by the fiber adhesive diffusing into the polymer.
The shearing interference microscope can be a great asset to photonics R&D in ways that were probably not envisioned by the microscope’s designer over 30 years ago.