This thesis focuses on research I have done on ytterbium-doped femtosecond fiber lasers. These lasers operate in the near infrared region, lasing at 1030 nm. This wavelength is particularly important in biomedical applications, which includes but is not limited to confocal microscopy and ablation for surgical incisions. Furthermore, fiber lasers are advantageous compared to solid state lasers in terms of their cost, form factor, and ease of use. Solid state lasers still dominate the market due to their comparatively high energy pulses. High energy pulse generation in fiber lasers is hindered by either optical wave breaking or by multipulsing. One of the main challenges for fiber lasers is to overcome these limitations to achieve high energy pulses. The motivation for the work done in this thesis is increasing the output pulse peak power and energy. The main idea of the work is that decreasing the nonlinearity that acts on the pulse inside the cavity will prevent optical wave breaking, and thus will generate higher energy pulses. By increasing the output energy, ytterbium-doped femtosecond fiber lasers can be competitive with solid state lasers which are used commonly in research. Although fiber lasers tend to lack the wavelength tuning ability of solid state lasers, many biomedical applications take advantage of the 1030 µm central wavelength of ytterbium-doped fiber lasers, so the major limiting factor of fiber lasers in this field is simply the output power. By increasing the output energy without resorting to external amplification, the cavity is optimized and cost can remain low and economical. During verification of the main idea, the cavity was examined for possible back-reflections and for components with narrow spectral bandwidths which may have contributed to the presence of multipulsing. Distinct cases of multipulsing, bound pulse and harmonic mode-locking, were observed and recorded as they may be of more interest in the future. The third-order dispersion contribution from the diffraction gratings inside the laser cavity was studied, as it was also considered to be an energy-limiting factor. No significant effect was found as a result of third-order dispersion; however, a region of operation was observed where two different pulse regimes were found at the same values of net cavity group velocity dispersion. Results verify the main idea and indicate that a long length of low-doped gain fiber is preferable to a shorter, more highly doped one. The low-doped fiber in an otherwise equivalent cavity allows the nonlinear phase shift to grow at a slower rate, which results in the pulse achieving a higher peak power before reaching the nonlinear phase shift threshold at which optical wave breaking occurs. For a range of net cavity group velocity dispersion values, the final result is that the low doped fiber generates pulses of approximately twice the value of energy of the highly-doped gain fiber. Two techniques of mode-locking cavities were investigated to achieve this result. The first cavity used NPE mode-locking which masked the results, and the second used a SESAM for mode-locking which gave clear results supporting the hypothesis.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 1977 / Vita. / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Physics

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21788号 / 工博第4605号 / 新制||工||1717(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 近藤 輝幸, 教授 辻 康之, 教授 大江 浩一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(理学) / 甲第24123号 / 理博第4851号 / 新制||理||1694(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 高橋 義朗, 教授 石田 憲二, 教授 田中 耕一郎 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Master Oscillator Power Amplifiers (MOPA) are laser systems that utilize a seed and pump amplification system to boost the output power of high-quality lower power seeding signals. MOPAs can generate high gain while avoiding many of the nonlinearities that negatively affect resonance-based lasers that are known to feature higher internal intensities. Additionally, MOPAs provide an easy alternative to the construction of novel laser technologies for higher output power as they can be easily combined with existing laser sources to amplify their output power. This thesis outlines the design of an ytterbium-doped fiber laser (YDFL), featuring a MOPA architecture. The YDFL is constructed to amplify a continuous wave single mode signal, at 1064nm, from 366mW to 16.4W while maintaining high spectral purity and beam quality. This laser is being developed with the intention to seed a subsequent MOPA YDFL for amplification to 1.5kW, for use in following thermal blooming experiments. As a result, the laser being developed in this work must have high spectral purity, centered near 1064nm, and a narrow linewidth, less than 0.25nm. Methods for limiting instabilities within the MOPA amplification stages are developed and the final seed laser emission quality is demonstrated in this work.

Ces dernières années, le développement des lasers femtosecondes s’est massivement orienté vers des sources à fortepuissance moyenne pour des applications autant scientifiques - par exemple la génération d’impulsions XUV - qu’industrielles. Cettethèse a consisté à développer une source laser permettant d’amplifier des impulsions ultra-brèves à très haute cadence par un laserà fibre avec d’un côté le développement d’un laser de pompe femtoseconde fibré (<600fs) à haute cadence (100kHz), et de fortepuissance (50W) et de l’autre des amplificateurs paramétriques optiques ultrabrefs (<10fs) pompés par cette même source. Pour cela,au sein d’une architecture à dérive de fréquence de fort étirement (2ns), nous avons exploité les propriétés des fibres photoniques à trèsgros coeur dopé à l’Ytterbium qui, pour approcher des énergies proches de 1mJ, nécessitent d’effectuer des études d’endommagement etde préparation des fibres. Nous avons alors pu démontrer une puissance maximale de 90W mais d’excellents résultats ont été établis dansun régime stable et robuste pour lequel ce laser délivre une puissance de 60W avec des impulsions de durées inférieures à 400fs. Aprèsdoublage en fréquence, ce laser a permis alors d’amplifier en deux étages dans des cristaux de BBO des bandes spectrales supérieuresà 300nm centrées autour de 800 nm avec une énergie par impulsion de 19[mu] J (1.9W). Avec un système d’étirement et de compressionbasé sur la combinaison de lames de silice et de miroirs à dérive de fréquence, ces impulsions ont pu atteindre une durée finale de 9.7fs.Ainsi, ces deux sources permettent d’ouvrir la voie à de vastes champs d’investigation en physique moléculaire et atomique. / In recent years, the development of femtosecond lasers has been heavily oriented towards high average power sources forboth scientific experiments - such as XUV pulses generation - as well as for industrial applications. This work has been devoted to developa laser source able to amplify ultra-short pulses at a very high repetition rate. In one hand, we develop a high average power (50W)pump laser based on a Fiber Chirped Pulse Amplification (FCPA) technology delivering 400 fs pulses at a high repetition rate (100kHz).In the other hand, a multistage ultrafast optical parametric amplifiers (<10fs) pumped by this source has been then implemented. Toachieve this, we took the benefits of the Ytterbium-doped large-core photonics fibre’s properties in order to approach energies closeto 1mJ. Even in a highly stretched chirped pulse architecture (2ns), using this kind of technology, required to perform studies overdamage and preparation processes of fibers. Thereby, we demonstrated a maximum output power of 90W, but excellent results havebeen obtained in a stable and robust regime in which this laser delivers 60W with pulse durations shorter than 400fs. After frequencydoubling, this laser was sent as a pump into a two-stages - non collinear parametric amplifier made with BBO crystals and a spectrumdelivered a by a CEP-Stable-6fs Ti :sa oscillator has been amplified around 800nm over a spectral bandwidth larger than 300nm witha pulse energy of 19[mu] J (1.9W). By using a stretching and compression scheme based on the combination of silica wedges and chirpedmirrors, the final pulses have been then recompressed down to 9.7fs. These laser systems can be now used to pave the way for vast fieldsof investigation in molecular and atomic physics.

Atomic precision measurements provide a strong testing ground for new theoretical ideas and fundamental laws of physics. Measurement of the Lamb shift in the hydrogen atom is one of the best examples towards this -it resulted in the birth of QED in 1949 by Dyson, Feynman, Schwinger and Tomonaga. The precision measurements of the hyperfine structure in hydrogen and deuterium by Nafe, Nelson and Rabi indicated that the g-factor for the electron was not exactly 2 as predicted by Dirac, but slightly greater, due to QED effects. Thus the precision measurements are indispensable not only for developing new theory but also for the verification and fine-tuning of theoretical parameters. Precision measurement of hyperfine structure provide valuable information about the nucleus structure, which is helpful in fine tuning of atomic wave-functions used in theoretical calculations. The aim of the work reported in this thesis is the measurement of hyperfine frequency and the observation of hyperfine structure constant in alkali atoms and in Yb atom. This thesis is organized as follows. In Chapter 1, an introduction to the importance of Alkali atoms and Yb atom in the field of precision measurement will be discussed. The scope of this thesis is also discussed in this chapter. In Chapter 2, an introduction to hyperfine structure starting from the beginning of the atomic physics will be discussed. We have discussed about the LS-coupling, jj-coupling, and the influence of the atomic nucleus on atomic spectra. We have also discussed the Zeeman effect and Doppler broadening. In chapter 3, the detail of experimental technique used in this thesis as copropagating satabs, hyperfine frequency measurement using AOM scan, AOM lock and ring cavity has been discussed. Experimental technique to observe the EIT signal in two electron Yb system has been discussed, which can be improved the precision in frequency measurement because of the narrow line-width. In chapter 4, we describe the co-propagating saturated-absorption spectroscopy and its application in frequency measurement. Saturated-absorption spectroscopy (satabs) in a vapor cell is a standard technique used to stabilise the laser frequency. In normal satabs we are getting some extra peaks known as a crossover peaks because laser interact with different velocity group in a vapor cell. In satabs the crossover peaks are stronger and often swamp the true peaks. So we have developed a technique of co-propagating satabs to remove the spurious peak, which has several advantages over conventional satabs. The co-propagating satabs signal appears on a flat background (Doppler-free) with good signal-to-noise ratio and does not have the problem of crossover resonances in between hyperfine transitions. We have adapted this technique to make measurements of hyperfine intervals by using one laser along with an acousto-optic modulator (to produce the scanning pump beam). In chapter 5, we describe the measurement of the hyperfine interval in the 2P1/2 state of 7Li using the SAS technique in hot Li vapor. This technique produces spurious ground crossover resonances that are more prominent that the real peaks. So we have used this ground crossover to measure the hyperfine interval using AOM locking technique. We have developed a technique to measure the absolute frequencies of optical transitions by using an evacuated Rb-stabilized ring-cavity resonator as a transfer cavity. In chapter 6, we study the wavelength-dependent errors due to dispersion at the cavity mirrors by measuring the frequency of the same transition in the Cs D 2 line (at 852 nm) at three cavity lengths. The spread in the values shows that dispersion errors are below 30 kHz, corresponding to a relative precision of 10−10 . We give an explanation for reduced dispersion errors in the ring-cavity geometry by calculating errors due to the lateral shift and the phase shift at the mirrors, and show that they are roughly equal but occur with opposite signs. In chapter 7, we describe precision measurement of hyperfine structure in the 3P2 state of 171,173Yb, and see an unambiguous signature of the magnetic octupole coefficient C in 173Yb. The frequencies of the 3P23S1 transition at 770 nm → are measured using a Rb-stabilized ring-cavity resonator with an accuracy of 200 kHz. In 173Yb we obtain the hyperfine coefficients as A = − 742.11(2) MHz and B = 1339.2(2) MHz, which represent a two orders-of-magnitude improvement in precision, and C = 0.54(2) MHz. Using atomic-structure calculations for two-electron atoms, we extract the nuclear moments quadrupole Q =2.46(12)b and octupole Ω = 34.4(21)b × µN . The observation of nuclear octupole moment in two-electron atoms, to the best of our knowledge, was never reported before. In 171Yb we obtain the hyperfine coefficient A = 2678.49(8) MHz. Using this measurement as well as the previous measurement of A coefficient from our lab, we have compared the hyperfine anomalies for 1P1, 3P1 and 3P2 states. In chapter 8, we describe the EIT in two electron system of 174Yb from 1S0(Fg = 0) 3P1(Fe = 1). We have observed the EIT in degenerate two level system and → after lifting the degeneracy by applying the magnetic field we are getting five peaks. We have also observed the EIT in 173Yb. In 173Yb there are three degenerate two level system Fg =5/2 Fe =3/2, Fg =5/2 Fe =5/2, Fg =5/2 Fe =7/2. →→→ We have observed the same type of EIT signal for all the three transitions Fg = FFe = F, ±F + 1. → In Chapter 9, we give a broad conclusion to the work reported in this thesis and suggest future avenues of research to continue the work started here.

L’auto-doublage de fréquence dans des cristaux optiques non linéaires dopés avec des ions ytterbium est actuellement le meilleur moyen d’obtenir une émission efficace dans le vert, tout en garantissant une bonne compacité, un faisceau faiblement divergent et une bonne monochromaticité. Deux nouvelles matrices ont été étudiées en ce sens : BaCaBO3F un oxyfluorure à fusion congruente, et LaxGd1-xSc3(BO3)4 un membre de la famille des huntites à fusion non congruente mais pouvant être obtenu par la méthode Czochralski. Après l’étude des conditions de synthèse et de croissance, les propriétés optiques de l’ion ytterbium dans ces matrices ont été caractérisées, et l’effet laser en régime continu autour de 1µm a été démontré pour la première fois. Pour améliorer les performances d’auto-doublage de fréquence dans des cristaux de YCa4O(BO3)3:Yb, deux pistes ont été envisagées : stabiliser l’émission laser infra-rouge en travaillant sur la cavité résonnante (traitement de surface des optiques), et améliorer le rendement de conversion de Génération de Seconde Harmonique de type I en effectuant le premier suivi expérimental de la courbe d’accord de phase hors des plans principaux à 1064nm. / Self-frequency-doubling in nonlinear optical crystals doped with ytterbium ions is actually the best way to combine both efficient emission in the green region, low divergent beams and good monochomaticity. Two new compounds have therefore been studied: BaCaBO3F a congruent melting oxyfluoride and LaxGd1-xSc3(BO3)3 a huntite-type crystal with incongruent melting that can however be obtained by the Czochralski method. We first studied the synthesis and growth conditions, and then the optical properties of the ytterbium doped crystals. Laser effect in continuous wave operation mode was demonstrated for the first time. In order to improve the self-frequency-doubling performances of the Yb:YCa4O(BO3)3 crystals, we focused on both stabilizing the 1µm laser emission (special anti-reflection coatings) and on maximizing the type I Second Harmonic Generation process at 1064nm (first direct measurements of both the conversion efficiency and the phase-matching directions out of the principal planes).