PhD defence by Andrea Arduin

Abstract

The invention of the laser in the 1960s revolutionized science and technology, ushering in a new era of light-based research and innovation. A laser is a device that converts energy, such as electrical current, into a focused beam of high-quality light. The first laser, created by Maiman, used a ruby crystal to emit red light. However, only a few materials have the necessary properties to create lasers, limiting the range of colors they can produce. Fortunately, special techniques called nonlinear processes can change the color (or more accurately, the wavelength) of a laser beam, expanding the range of colors available.

After the first crystal based lasers, scientists developed fiber lasers, which use optical fibers—thin glass strands about as thick as a human hair—to confine light. By adding tiny amounts of special materials called rare earth dopants to the glass, these fibers can also produce laser light. But, again, only a few wavelengths are possible, so the ability to convert them remains crucial. Another limitation of fiber lasers is their power output. Because optical fibers are so thin, they can only handle moderate amounts of light. Intuitively, making the fiber larger would increase its power, but this also causes the light inside to spread into different patterns, known as modes, which can reduce the laser’s effectiveness.

Recently, researchers have taken a new approach: instead of avoiding these complex light patterns, they’re learning to control and use them. This allows to use larger fibers, increasing the energy output of converted fiber lasers by a factor of 100. This thesis explores using a process called fiber optic Cherenkov radiation in these large optical fibers to convert powerful infrared fiber lasers into visible light, achieving higher power levels than possible before.

Supervisors

• Principal supervisor: Associate professor Jesper Lægsgaard, DTU Electro, DenmarkCo-supervisor: Associate professor Lars S. Rishøj, DTU Electro, Denmark

Evaluation Board

• Group leader/PhD Xiaomin Liu, Max Planck Institute, Germany
• Lead Development Engineer Poul Kristensen, OFS Fitel, Denmark

Chairman

• Associate Professor Edmund Kelleher, DTU Electro, Denmark

Master of the Ceremony

• Professor Peter Uhd Jepsen, DTU Electro, Danmark