Ultrashort and short pulsed fiber laser development for transparent material processing, imaging and spectroscopy applications

Abstract

Since the invention of the laser in the 60s, the main advances in laser technology were done in two directions; shorter pulses and higher powers. In order to achieve this purpose, many laser types are developed and always replaced with simpler, smaller, cheaper alternatives that can deliver the same or better parameters. In the past 20 years, ber lasers have become an important alternative that can match and even enhance the performance of currently used lasers while reducing the complexity, costs and instability. Optical bers, which are the main components of ber lasers, were rst de- veloped just as a substitute for conventional cables since they offer much less attenuation in carrying signals over long ranges. So, most of the studies were focused on making the bers better for communication channels. After realizing that ber lasers offer better beam qualities, which is also a vital parameter for many laser applications, researches started nding ways to use bers for lasers and they achieved this in 80s by the rst ever utilization of low-attenuation ac- tive bers. After the invention of double-clad bers, utilization of diode lasers for pumping and development of efficient rare-earth doped bers, ber lasers became more than just a research topic in the laboratory and began to nd use in many applications. The utilization of ber lasers for short (nanoseconds) and ultrashort (picosec- onds, femtoseconds) pulse generation was a difficult task for researchers. The biggest challenge to overcome was nonlinear effects caused by the con nement of the beam into small volumes. By using smart designs like chirped pulse ampli - cation and highly doped lasers, pulse energies and peak powers close to solid-state ultrafast lasers can be achieved. These nonlinear effects were not just problems in the power scalability of ber lasers, on the other hand, they were an opportunity for new possible applications. For example, using these nonlinear effects inside bers, supercontinuum generation was demonstrated and found usage in many areas like spectroscopy, imaging and metrology. Today, more than 50 worldwide companies sell short-pulse ber lasers for ap- plications as diverse as ophthalmology, micromachining, medical imaging and precision metrology. Especially, ber-laser-based micromachining is routinely im- plemented in the fabrication processes for widely used consumer products. New applications of ber lasers are being continuously developed. Consequently, in this Ph.D. thesis study, new application areas of ber lasers are investigated. Ultrashort and short pulsed ber lasers are developed and uti- lized for biological and transparent material processing, spectroscopy and imag- ing. In the rst part of thesis study, we have demonstrated the use of a custom- built ber laser-based microscope system for nanosurgery and tissue ablation experiments. Through the use of custom FPGA electronics acting through ber- coupled AOMs, we are able to generate custom pulse sequence. Using this system, we have made photodisruption experiments in tissue level, cellular level and sub- cellular level. In the second part of this thesis study, in collaboration with Bogazici Univer- sity, we have developed a nanosecond ber laser system that is able to generate wavelength components of 600 nm to 1300 nm, developed speci cally for pho- toacoustic excitation. Using this system, we have made photoacoustic signal excitation in a ceramic sample and prepared the system for further experiments to generate photoacoustic images from biological specimens. In the third part of thesis study, in collaboration with ODTU, the development of a THz-TDS system driven by a novel Yb-doped ber laser whose repetition rate can be tuned, speci cally for fast scan THz measurements, is realized. Char- acterization of the built laser system is done considering the necessities for the OSCAT technique as an alternative method for fast scan THz measurements. Stability of the oscillator is examined in terms of power, spectrum and pulse duration with the changing repetition rate of the laser. Using this system THz waveforms are generated at different wavelengths and the system is prepared for further research in spectroscopy. In the last part of the thesis study, a high-pulse energy femtosecond laser system is developed and utilized for transparent material processing. The laser output is coupled to a fast galvo-scanner system, and a synchronized translation stage such that very wide areas (10 cm x 10 cm) are able to be processed with very high speed (2 m/s). Using this system, glass samples are cut, engraved and photodarkened.