The important thing is not how you start but how you end!

…Or at least how it continues. Obviously, the work has just started, but after 5 months I can already have a look to my early periods retrospectively. The feeling of being lost at the beginning (typical when you start a PhD) has become motivation for the future, and now I know better what my role is in my institution and it really helps me in my daily work in the lab and office.

But GraWIToN is not only inside my lab, it consists of lot of people working either research or administration. Therefore, from time to time we have a meeting such the first GraWIToN School, that was carried out from the 20^th of April until the 8^th of May at the European Gravitational Observatory, in Pisa. It was really great to meet my colleagues from the other institutions and spend 3 weeks with them. In addition, the organization was really good (thanks to Erika, Elena and Michele) and we attended several lessons related to different areas of knowledge required in the research of gravitational waves: astronomy, optics, physics, digital signal processing, and many others imparted by helpful teachers.

Although not everything I learnt is directly applicable to my work at LZH, it was very interesting and opened my mind to a wider understanding of my topic inside the science and technology of gravitational waves detection. We also had outreach lessons, but I must say that the funniest one was the Arduino practical session :).

The days were a bit intensive (but I have to recognize that the great coffee breaks made it easier), however we also had time to visit the city and some other cities near Pisa. Can anyone imagine what the first place we visited was? ... Right! The well-known Leaning Tower of Pisa!

Back in Hannover I resumed my tasks. I had started a new set-up before going to Pisa and after this dose of motivation I couldn’t wait to continue mounting it. It is a fibre amplifier based on a Leakage Channel Fibre (LCF). Such fibre is sometimes a bit difficult to manipulate, at least for me because didn’t have previous experience with it, but after some time I had everything ready to start the experiments.

One of the main properties of this specific fibre is that it provides a flat-top beam profile with a Large Mode Area (LMA). In principle this kind of beams are not very useful for gravitational wave detector laser amplifiers, however it would raise the threshold of nonlinear effects and this can be a good feature for a high power amplifier. It happens because a Gaussian beam contains high intensity at its peak, that makes some nonlinear effects appear relatively soon when power is increased. In a LMA the intensity is reduced due to a larger area and besides this a flat-top beam would (Credit: www.optique-ingenieur.org) avoid high intensity peaks as the power is equally distributed over the available area.

Anyway, the Gaussian beam profile is still a must if you want to build a gravitational wave detector or upgrade the existing ones, so we must find a way to reshape the flat-top beam (or whichever) to a nice clean fundamental mode as, for example, by using Diffractive Optical Elements or DOEs. However, these DOEs work properly if the beam to be reshaped has a constant shape, and the overlapping of higher order modes would change the beam shape along the time due to the phase differences. That is why the next objective is to determine if the fibre can offer long-term beam shape stability and to determine the relation between the different modes that reach the output.

Omar de Varona Ortega

Laser Zentrum Hannover e.V.
Laser Development Department
Solid-State Lasers Group
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