Ultra-short pulsed Tm- and Er-doped all-fiber MOPA lasers and supercontinuum sources.
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- Institutt for fysikk 
The present work describes the results of the development of a family of novel diode-pumped ultra-short and short pulsed Er- and Tm-fiber laser systems in the “eye-safe” near- and mid-Infrared (mid-IR) wavelength range for various scientific and industrial applications, and the development of several sensitive and precise scientific tools and setups for spectroscopic diagnostics and optical measurements. The lasers are united by the common concept of master-oscillator – power amplifier (MOPA), are diode-pumped and produce short and ultra-short pulses (from nanoseconds to femtoseconds) in the wavelength range between 1.5 and 2.5 µm. The Tm-fiber lasers can be broadband and tunable and produce high-spectral-brightness radiation between 1.8 – 2.5 µm. The research work that I have done at the Laser Physics group is largely experimental and comprises both, development of own measurement and characterization devices as well as laser systems. This happened naturally, because the development of new femtosecond lasers is intimately connected with the necessity to measure and characterize components with high precision. Moreover, such measurement devices (e.g. dispersion and autocorrelation measurement instruments) are not commercially available in this wavelength range. For that reason, the work consisted of three major parts: • development of measurement and characterization methodology and setups; • development of various ultra-short-pulsed and broadband Tm-fiber laser systems; • development of high-peak power short-pulsed Er-fiber MOPAs for ranging applications. My research work at the Norwegian University of Science and Technology (NTNU) started with the study of optical and waveguide properties of optical fibers, and with the construction and development of devices for ultra-short pulse characterization. For the precise measurements of dispersion parameter in optical fibers in mid-IR wavelength region the dispersion setup was specially developed, tested and used for the investigation of the new fibers for our purposes. It operated in two wavelength ranges: from 1.7 to 2.0 and from 2.3 to 2.45 microns, with a resolution up to 20 nm. To the moment of the development, the setup allowed the broadest dispersion measurements in single-mode, short-length optical fibers in the wavelength range around 2 μm. All the dispersion results were used in the laser experiments. For ultra-short pulse characterization three autocorrelators operating in wavelength ranges of around 1.8, 1.9 and 2.3 μm were constructed. Autocorrelators are capable of measuring of pulses with picosecond and femtosecond durations, with down to 1% accuracy. Another part of the research has been devoted to the detailed study of energy transfer processes between active ions in the co-doped glasses. This is a very interesting and essentially important work for the development and fabrication of any new laser material. In this work, I performed the complete analysis of energy transfer (ET) processes between Yb3+ and Tm3+ ions in relation to the new compositions of core glass of Yb/Tm-doped Y-co-doped silica glass fibers based on nano-modified glass. The large part of the experimental activity was aimed at the development of short and ultra-short pulsed and broadband laser sources. This work resulted in the development of four fiber MOPA laser systems and two broadband supercontinuum (SC) sources: - The first laser system is the Er/Yb-doped two-stage MOPA laser with peak power of more than 4 kW and an average output power of 10 W. The initial duration of the seed pulse of ~100 ns was strongly shortened down to 3.5 ns as the result of the amplification, revealing a powerful technique for a short intensive pulse production for light detection and ranging (LIDAR) applications and micromachining laser systems. - The second laser is the highly stable all-fiber femtosecond SESAM mode-locked thulium-doped fiber (TDF) one-stage MOPA. The system was configured as either a high-power or a high pulse repetition rate Raman soliton source with the spectral tunability of 240 nm from 1.98 to 2.22 µm. In both configurations, pulses with the duration of few hundred femtoseconds have been obtained. Both Raman soliton lasers show a large potential for the development of compact, low cost, and high-power ultra-short-pulse sources. Such systems are promising for the use in experiments, where smoothly tunable pulses with relatively high energies are necessary, for example, for sensing, spectroscopy, material processing, as pump sources for optical parametric oscillator (OPO) or for the laser driven on-chip electron accelerators. In the high-power configuration, the Raman soliton of more than 0.5 W of optical average power could be produced in the active fiber of the Tm-doped amplifier. At the high-frequency Raman configuration the emission of ~5 nJ spectrally tunable pulses with the repetition rate of 83 MHz was demonstrated. It is important to note that the soliton spectral position was tunable in the range of 1.98 - 2.22 μm by changing only the amplifier pump diode current – the first practical realization of the electronically tunable femtosecond laser in this wavelength range. - The third system generates high-energy supercontinuum directly from MOPA. Supercontinuum has superior spectral flatness of better than 1 dB covering the wavelength range of 1.9-2.5 μm, with the total output energy as high as 0.284 μJ, and the average power of 2.1 W at 7.5 MHz repetition rate. The source shows a large potential for the development of compact, low-cost, and high-power ultra-short pulse lasers systems for sensing and spectroscopy. - Later, I have also demonstrated the first femtosecond semiconductor saturable absorber mirror (SESAM) mode-locked TDF laser based on the active silica fiber with highly Ge-doped core with the normal dispersion in the spectral region near 1.9 µm. The stable laser operation was obtained in all dispersion regimes (from anomalous to normal) at the wavelength of ~1.88 µm. The pulses from the master oscillator were amplified in the core-pumped TDF amplifier based on the same active fiber as the laser. The system had several rounds of development and after the last upgrade I was able to increase output pulse energy up to 31-nJ corresponding to 500 mW average power in normal dispersion regime. The system is still in progress of development and I believe that all the regimes can be improved. This is planned for future experiments. - As “external” broadband sources, the femtosecond pulse nonlinear broadening in a conventional step-index single-mode chalcogenide fiber and the synchronously pumped subharmonic OPO, based on the novel periodically patterned GaP crystal, were demonstrated. The compact Tm-fiber MOPA pumped OPO emitted at central wavelength of 4.18 µm and had the overall width of 1400 nm and spectral full width at half maximum (FWHM) of 1135 nm.