The majority of Kerr frequency comb experiments is done in fiber optics. And because typically single mode fiber is used the polarization has to be adjusted at some points. This is typically done with a Fiber Polarization Controller (FPCs). And of course this has to be included also in the experimental layouts in the publications. Here is a brief review of different shapes that FPCs can take in this context.
Of course we start with the root-paper of Kerr frequency combs:
This paper was not just the seed of the field but this incarnation of an FPC appears also to be the grandfather of FPC images in the Kippenberg group. The fiber is imaged as a sophisticated layered structure with three different parts. Please note the floating nature of the three rings which seem to be without apparent contact to the fiber underneath. Contrasting the perfectly round nature of the rings are the framing boxes. A well balanced piece of vector graphic art.
Other early publications in the field decided to go for a less fancy fiber structure.
A simple black line. The three rings are well grounded and connected. Not just to one common fiber, no, they are directly connect to each other. A very light yet stable structure which might resist any attempt to adjust the polarization.
And then there were the first Kerr frequency combs in chip-based microresonators from the Weiner group. Potentially inspired by military applications (after all DARPA is funding this research quite well) was a new form of FPC: the rock solid version, which was maybe designed to survive a direct hit by a missile without changing the adjusted polarization.
Although not quite a fully fledged Kerr frequency comb publication, this version of an FPC is certainly worth noting.
Maybe it is the biased view from people who work on “real” Kerr frequency combs, but does it not look like a bio hazard sign? Or is it just a tribute to the 2012 Summer Olympics in London? Anyways, one important paper of the field of Kerr frequency combs in 2012 came from the Kippenberg group. It should define their paper’s FPCs for years to come.
One can clearly see how the heritage of the sophisticated three colored fiber structure of the grandfather paper lives on in this version. But the three rings seem to have lost their floating nature. The overall design is clearly influenced by the fact that this was also the first work of the Kippenberg group that included on-chip ring SiN resonators. I would describe the design as three evanescently coupled ring resonators with windows etch into the top cladding. While tiny ring resonators have been made in that group afterwards, the etched windows might hint for future developments to come.
But of course other group were first with SiN and ring resonators. Notable the collaboration of Gaeta and Lipson. So here is a FPC that comes from their time at Cornell.
Again a rather simple design but not quite as simple as the second version above. Again, three simple rings tightly interconnected. But each of them features a white, fuzzy spot right in the center. Maybe this is the PhD student longing for the light at the end of the PhD tunnel? Or maybe it is just the focus on the details.
Silicon (oxy-)nitride did not remain the only material for integrated structures. Other groups with other skills came along and brought there own interpretation of the theme “FPC” to the literature.
A simple variation of the three rings scheme. Not as sophisticated as the three-colored scheme but not as simple as the black-line version either. A well-chosen middle ground. The color choice shows a clear patriotic inspiration from the Stars and Stripes. The fundamental ideals of independence and freedom are, similarly to other designs, reflected by the three rings which are only very loosely coupled.
The freedom of movement is also at the basis of the next version.
This version is clearly inspired by the three-colored fiber designs shown above. This is not a surprise as the first author is the same as in the grandfather paper. However, as he had moved on to another research group, he adapted his original design, using some of the ideas that other publications had put forward in the mean time. Now the three rings are firmly interconnected. No floating of any sort. But also no independence of the rings anymore. Also the visual contrast of the rectangles is gone. Two steps forward, one back, I would say.
A similar heritage is also clearly visible here.
Again, clearly visible is the signature three colored, blue fiber design. However, this design has a certain twist to it. The middle ring seems to have been affects by too much Cherenkov radiation to stand up straight, it seems.
Back to a more solid design. Straight, black-grey-white.
Again, three (too) well connected rings. Filled with gray matter. It gives the whole structure substantially more weight. This seems to have been an important part of the experiment.
Also from the LA area but apparently more inspired by the Californian way of life of lightness and Flower Power is the following design.
It comes from the group of Vahala at Caltech. The shape is clearly inspired by the Olympic/bio hazard version above but it is not as entangled anymore. The green color shows that some parts of the world start to think about the required changes for a green future of our planet. Of course enabled by Kerr frequency combs. Or maybe it is just another example greenwashing. Only the future can tell.
The trend of going-green quickly spread.
Just a year later this interpretation of a FPC was published. Again, a hint towards a sustainable future but with three solid black disks. Gone is the lightness. Maybe someone still has to get rid of the dark coal heritage before going entirely green.
But green and blue are not the only colors available. With the increasing amount of publications on Kerr frequency combs, color shifts to the violet were maybe seen as a proper way to differentiate.
Here also the rectangles made a come-back. Fresh color, contrast of shapes, solid disks and proper connection to the linear fiber. Certainly a note-worthy version of an FPC.
Despite all the breakthrough FPCs published in Nature, Science, Nature Photonics as well as Optica, Nature Communications and countless other journals, some solid constructions just remain.
As the three-shades-of-blue design had been moved to NIST, also this design seem to have adapted by NIST from another group. Interesting.
Taking about the three-shades-of-blue design. The version of 2012 survived without major changes the last couple of years.
A bit broader stroke for the boxes but the main flaws remain. Barely connected rings that resemble rather ring resonators on a waveguide than one continuous fiber in an FPC.
With this we are nearly at the end of this gallery.
This is a new interpretation from NIST. Three disks, overlapping and well connected to the fiber. Wait, are these …. micro-toroids? Oh, what a reference to the original work. Nice.
And just to make this point clear: not everything that has three rings is a FPC:
So, I hope everyone has now plenty of inspirations for the next manuscript. Have fun writing and don’t forget to pay attention to the small details such as an original depiction of an FPC.
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