Monday, July 31, 2017

Aus Jena Jenapol Interphako Polarisationsmikroskope Shearing Interference Microscope


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.

            An optical waveguide viewed under an ordinary microscope reveals little information about itself.  Below is an image of a multimode 1x2 splitter. 
 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.

 Interference picture of a singlemode fiber. The fiber is surrounded by index matching fluid to reduce distortions created by the round fiber cladding. To completely eliminate the distortions, an index fluid perfectly matched to the index of the fiber clad must be used.
 Same fiber without interference.
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.

I restored a Jenapol for my personal lab, and developed a great appreciation for the fine engineering and workmanship that went into these microscopes.   This Jenapol had been used as a parts donor, and had a few missing and broken pieces.   The eyepiece assembly was gone, so I machined a monocular eyepiece tube that uses readily available WILD eyepieces.  Unlike many other microscopes, there are features that can only be used while viewing through the eyepiece, and are not visible through the camera port.  In addition, the optics required at the camera port are not the same as the eyepiece port. At the camera port, I am using a Kodak universal microscope adapter with a homemade 7mm to C mount adapter made from a Zeiss microscope part.  However while this assembly fits on the eyepiece port, the optics are completely wrong, and it does not work.   The WILD eyepiece works well, but the adapter tube length is critical. Otherwise the internal crosshair is out of focus.
Later I found a Nikon binocular assembly and discovered that Jena eyepieces fit it perfectly. I machined an adapter to attach the Nikon part to the microscope.  The modification performs as well as the original part. The monocular assembly is still useful for installing a camera, or using my Nikon eyepiece micrometer.

The next thing I did was eliminate the unreliable and expensive mercury arc light source.  Here I used an inexpensive automotive HID headlamp conversion kit.  At only 55 watts, it matches the brightness of the 250 watt mercury lamp.   I believe that the smaller arc length in the HID lamp allows for more efficient coupling of the light.  Here is a picture of the light projected onto a cardboard box. At this level of beam expansion, the intensity is brighter than noontime sunshine.

The lamp runs on 12 volts, and allowed me to replace the large mercury are power supply with this compact 12 volt, 20 amp supply, which I bolted to the lamp housing.
 The broadband HID lamp works well with my 551nm filter and polarizer.

My monocular eyepiece can be fitted with either WILD eyepieces or a WILD c-mount camera adapter.  This is very useful, for when the Jenapol was conceived, readily available electronic vision systems did not yet exist.  As a result, most Jenapol operations are done using the eyepieces, and the camera port is merely for taking photographs.   Unlike many microscopes, the camera port and eyepiece port take unique optical paths, and some features, like the polarizartion analyzer, are not viewable through the camera port.  Additionally, the camera port and eyepiece port have different focal lengths, and are not interchangeable.  Therefore, one cannot simply take the camera from the camera port and put it on the eyepiece port, even though it will fit.  It will be completely out of focus.

       I designed my monocular eyepiece to be in focus using either a WILD eyepiece or camera adapter.  Now I can put a camera on either port if I desire.  At right:  Eyepiece tube with modified WILD camera adapter.  Below: Exploded view of the camera adapter.

The modified Kodak Universal Adapter:   This works extremely well, with a highly uniform image.

Many parts of this microscope were seized up, for the grease Aus Jena used appears to turn solid after 30+ years.  The lamp adjustment shaft seized and snapped off, and I had to make a repair.
 I found this easily accomplished by drilling and tapping the shaft, and then using a 3mm screw to replace the broken portion.  This is as strong as the original part.

I have written an English language instruction manual for the Interphako, and I am working on lamp conversion instructions and a maintenance & repair manual.  
 I have also translated a very useful book published by Jena on interference microscopy.
If there is any interest in these topics, please contact me.

Jenapol  Instruction and Operation Manual translated to English.  My user manual is more than a simple translation.  It is enhanced by combining several sources of information, plus  knowledge gained from my 30 years of experience in using this microscope.    As a result, it is far superior and more informative then the original manual.  
Price for both books is $60.  

Interference Microscopy Principles and Applications translated into English.