The past few months have been very eventful - it almost feels like I spent more time away from my desk than at it! This started towards the end of April, when I left for Italy for the first GraWIToN school.

The first in a series of schools in the GraWIToN program, the initial lecture week fulfilled multiple purposes: we were to gather a foundation of knowledge in the field of gravitational physics, which not only our research but also the more specialised later schools would build upon. We were also instructed on the topics of presentation and outreach, aiming to prepare us for explaining our research and field to the general public by means of written and spoken word. Last but not least, this was the first time the twelve of us ESRs got to meet each other!

Some of us had already met by virtue of working in the same academic and/or geographic location, but here we finally had the opportunity to get to know each other properly. It was great to have this ample opportunity for both personal and academic exchange, and be able to put into perspective our own work with all its highs, lows, challenges and outlooks. While the days were filled to the rim with lectures by some of the leading experts in all aspects of gravitational physics, workshops and practical work, the nights were ours to explore the wonderful city of Pisa together as spring inexorably brought Tuscany back to life from its brumal hibernation. I can't say that I wasn't exhausted as we returned to Germany after three weeks, but it was a time well spent, with many new perspectives gained and friendships begun.

Even so, the return to Hanover was only temporary. Only a few weeks later, a similar event took place organised by the Max Planck Institute instead - all first year PhD students were relocated to Scotland for a week for another set of lectures. As fate would have it, we were joined in this school by students and staff from the University of Glasgow, some of which were already familiar from time spent together in Italy! And so, among the scientific and cultural exchange, there was also a reunion of sorts. Having returned from Scotland recently, however, I am gladly looking forward to the German summer, and to finally spending some uninterrupted quality time with my own research project: the fiber ring resonator.

According to the title of my PhD thesis, I will be working on stabilising high power fiber lasers. Hence, it isn't unexpected that my first research project is fundamentally based on optical fiber. In optics, cavities are an essential tool. From the simple Fabry-Perot consisting of just two mirrors opposing each other, to ring or bowtie cavities consisting of three, five or even more mirrors - they are everywhere. Although they fundamentally all fulfill the same task in providing a resonant space for light to circulate in, their applications are varied, from providing frequency references in order to stabilise a system to storing power as is done in the arms of Virgo and LIGO. However, most cavities in use today are air based and consist of mirrors fixed in space. As we are moving toward designing completely fiber-based lasers, however, we are aiming to find fiber-based alternatives to the components found in today's lasers. Hence, I am investigating how fiber cavities might be useful for designing the next generation of lasers for gravitational wave astronomy.

A fiber cavity is also called a fiber ring resonator. The name betrays the design - it is, fundamentally, a fiber ring in which light resonates. The ring is formed by joining the two ends of a length of fiber together. Light can enter this ring by coupling into it from another fiber. Optical fibers consist of a small core guiding the light, surrounded by cladding of a different refractive index. Ordinarily, the light is confined to the core - however, when the cores of two fibers are brought close enough together, light can couple from one fiber into the other by means of evanescent waves. Using this method to couple light into our fiber ring resonator, we can also observe the light coupling back out of the resonator to investigate the properties of the cavity, for example by comparing the output of it to that of a reference cavity of very high quality.

However, numerous challenges await before this stage is reached! In producing the fiber ring, the ends are joined by a process called fusion splicing. The fiber ends are heated to very high temperatures so the glass softens and liquefies, then are pushed together so that they melt into one another and form a connection indistinguishable from the rest of the fiber. In order to produce a working fiber cavity, this splice connection needs to be of a very high quality, minimising the light that is lost passing through the joint. However, splicing fibers is an art of its own and involves many parameters to be adjusted, that differ depending on the fiber and equipment used. Another process that needs to be mastered is coupling light from a laser source into the fiber - the core is only a few thousandths of a millimeter in diameter, so focusing a spot of light just right to enter the fiber is no easy task either. These challenges are not new, and have been conquered by many scientists and engineers before us. Nonetheless, we have to find our own way to emerge victorious on our way to scientific insight.

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