The multi-pass ring cavity was constructed using only a pair of identical spherical mirrors, which is compact and can easily be aligned. The spatial hole burning effect and green problem can be eliminated in these ring cavities that can be applied to generate a single frequency laser. The characteristics of multi-pass non-planar and planar multi-pass ring cavities were analyzed, such as the reentrant conditions and cavity stability. The multi-pass ring lasers were successfully demonstrated by the reentrant condition simulations, the cavity length error between experimental result and simulation value were below 1.2%. Yb3+:YAG was used as the gain medium in this dissertation, it has many advantages compared with that of Nd3+:YAG. Such as high doping concentration, low quantum defect, long upper state lifetime, broad emission bandwidth and its wide absorption band. However, the quasi-three-level nature of Yb3+:YAG makes temperature control crucial for laser performance. A Ti:sapphire laser pumped Yb3+:YAG bulk crystal multi-pass continuous-wave ring laser was demonstrated with a slope efficiency of 50.3%, and a Yb3+:YAG crystal fiber ring laser was demonstrated with a slope efficiency of 54.7%. The thermal load in Yb3+:YAG was observed and compared with that of Nd3+:YAG. The passively Q-switched operation was obtained by a Cr4+:YAG saturable absorber. Due to the ring cavity configuration, the spontaneous noise from gain medium perturbs the population difference of the saturable absorber was reduced so that the timing jitter of the repetition period was restrained to around 11% while 33 ns pulses were obtained. A compact diode-pumped continuous-wave ring cavity with 25.0% slope efficiency was presented. Two main challenges are noticed in the high power laser diode end pumped configuration, mode-matching difficulty and huge heat load. The mode-matching problem can be solved by an appropriate cavity design, the laser-heated pedestal growth (LHPG) method was used to growth Yb3+:YAG crystal fiber with small surface to improve the heat dissipation. The fiber crystal laser was successfully generated and compared with that of bulk crystal. To our knowledge, this is the first demonstration of a Yb3+:YAG ring laser, and also the first demonstration of Yb3+:YAG crystal fiber ring laser.

Semiconductor has electronic bandgap because of the periodic potential barriers. Similarly, as shown in Yablonovitch and John¡¦s original idea in 1987, and the optical bandgap can be formed by arranging the dielectric material periodically, named photonic crystal. The innovation promotes vigorous development in the last twenty years. Many applications were discovered by using the idea of photonic crystal, such as waveguide, left-hand material, slow light, optical register, etc. Conventional fibers guide light in the core by the total internal reflection principle, but Russel and co-workers demonstrated fibers with a so-called photonic crystal cladding in 1996, and these fibers guide light by a new physical mechanism different from traditional fibers. Photonic crystal fibers can be simply divided into two groups, one is index guiding fiber and the other is photonic bandgap fiber. Both of them have 2D periodic structures with designed defect structure in the center. Hence light can be confined and guided by special defect modes. We have successfully demonstrated microstructured fibers which have 2D periodic structure by LHPG method. During the fabrication processes, capillaries may collapse due to the surface tension. We discuss the hole-collapse issue and our solution. Besides, the quality of fiber extremely depends on the stability of laser power of the LHPG system, so we design an efficient feedback control to improve it. We also discuss the fibers¡¦ SEM images and optical properties. Finally the future work refers to the drawing of 3D photonic crystal fiber and improving the sharp thermal gradient by using a sapphire tube.

The maximum capacity of an optical fiber transmission system more than doubled every year to match the fast-growing communication need. The technology break through in dry fiber fabrication opens the possibility for fiber bandwidth all the way from 1300nm to 1600nm. The fast increasing demand of communication capacity results in the emergence of wavelength division multiplexing (WDM) technology, which results in the need for ultra-wideband optical amplifier. Cr4+:YAG has a strong spontaneous emission that covers 1300nm to 1600nm. Besides, its absorption spectrum is between 900nm to 1200nm, which matches with the pumping source in current erbium doped optical amplifier. Such a fiber is, therefore, eminently suitable for optical amplifier applications. In this article, we will introduce the development of ultra-wideband optical amplifier using the double-clad Cr4+:YAG crystal fiber, which is grown by laser heated pedestal growth(LHPG) technique. Its material properties as well as optical gain will be characterized. By butt-coupling method, a low insertion loss of 4.2 dB was achieved in a SMF-CDF-SMF configuration, and it was measured to demonstrate a gross gain of 2.4 dB at 1 W bi-directional pump power. Moreover, theoretical models and numerical simulations have been developed to predict the experimental results. Numerical simulation indicates that the efficiency of mode overlapping between signal and pump is crucial to gain performance. The mode overlapping efficiency is about 25%~30% for our crystal fiber under current circumstances. In the future, we will make an attempt to reduce the index contrast between core and cladding for better mode overlapping efficiency. At the same time, we also try to grow crystal fiber of smaller core diameter to improve gain performance.

The rapid developments in optical and electronic technologies have accelerated developments of solid state laser technology. The diode-pumped solid state laser has the merits of the diode laser, such as compactness, low cost, and the merits of the solid state laser, such as high laser quality, high conversion efficiency, long lifetime, and simple structure. In addition, the diode-pumped solid state lasers have made it a feature star among lasers. One of the problems in solid state laser is the heat removal. The crystal fiber is used as the laser gain medium in this work to be able to reduce largely the volume of solid-state laser, and improve the heat disappearance. There are many different methods to grow crystal fibers, LHPG method one of the best because single crystal fibers can be grown with small diameters at very fast rate, and accurate control. We have gown high quality Nd:YAG crystal fiber with diameter of 23~285 mm. After cladding, grinding, polishing, and coating, we could ready to fabricate the Nd:YAG crystal fiber laser. We have successfully implemented diode-laser pumped Nd:YAG crystal fiber laser. The lasing threshold power is 143 mW, and the maximum output power is 38 mW. In the feature, we shall improve the cooling system, the cladding, and coating to further increase the conversation efficiency and output power.

Abstract The rapid developments in optical and electronic technologies have accelerated developments of solid state laser technology. The diode-pumped solid state laser has the merits of the diode laser, such as compactness, low cost, and the merits of the solid state laser, such as high laser quality, high conversion efficiency, long lifetime, and simple structure. So, it has been applied in electronics, communication and medicine widely. In this work, the crystal fiber was used as the laser gain medium, and coated with optical thin film at its end facets as the laser cavity to be able to reduce largely the volume of solid-state laser, and improve the heat dissipation. We used laser heated pedestal growth (LHPG) method to grow crystal fiber, which can grow with small diameters at very fast rate and accurate control. High quality Nd:YAG crystal fibers with diameter of 23~285 mm were grown. After cladding, grinding, polishing, and coating, we successfully fabricated the Nd:YAG crystal fiber laser. We discovered that the gradient concentration of Nd ions distributed over the cross section of crystal fiber is helpful in focusing lights. We have successfully implemented diode-laser pumped Nd:YAG crystal fiber laser with a slope efficiency of 28.9%. It is the best result up to this time as we know. The maximum output power is 80 mW. In the future, we shall improve the cooling system, the cladding, and coating to further increase the conversion efficiency and output power.

Abstract The maximum capacity of an optical fiber transmission system is more than doubled every year to cater the fast-growing communication need. The technology breakthrough in dry fiber fabrication opens the possibility for fiber bandwidth from 1.3 um to 1.6 um. The fast increasing demand of communication capacity results in the emergence of wavelength division multiplexing (WDM) technology, which results in the need for ultra-broadband optical amplifier. Cr4+:YAG has a strong spontaneous emission spectrum covers from 1.3 um to 1.6 um. In addition, its absorption spectrum is between 0.9 um to 1.2 um, which matches with the pumping source in current erbium doped optical amplifier. Such fiber is, therefore, eminently suitable for optical amplifier applications. In this thesis, we introduce the development of ultra-broadband optical amplifier using the double-clad Cr4+:YAG crystal fiber, which is grown by the laser heated pedestal growth (LHPG) technique. With the butt-coupling method, the insertion loss decreases to 2.0 dB ~ 2.9 dB in a SMF-Cr4+:YAG DCF-SMF configuration at signal wavelength from 1.26 to 1.64 um. A gross gain of 3.2 dB is demonstrated at 0.7 W bi-directional pump power at present. Moreover, theoretical models and numerical simulations have been developed to find out a better method for experiments. Numerical simulation indicates that the pump ESA will seriously impede the development of optical amplifier using the double-clad Cr4+:YAG crystal fiber. In the future, in order to reduce pump ESA we attempt to use cladding pump scheme instead of core pump scheme and to choose pump wavelength at 925 nm instead of 1064 nm,. At the same time, we will also try to grow crystal fiber of smaller core diameter and to extend its length to improve gain performance.

Cr4+:YAG is an attractive gain medium due to its broad 3-dB emission spectra all the way from 1253 nm to 1530 nm that just cover the low loss window of silica fiber. Such a broadband characteristic offers a potential to develop a broadband amplified spontaneous emission (ASE) light source, optical amplifier, and tunable laser. Growing the Cr4+:YAG bulk crystal into fiber form is necessary for generating larger gain by the better optical confinement of the waveguide structure. For the application of laser, it is superior to bulk crystal for reduced lasing threshold and better slope efficiency due to also the optical confinement effect and better heat dissipation. Laser heated pedestal growth (LHPG) method has been used to grow high purity crystal fibers due to its crucible free nature. A novel cladding technique, co-drawing LHPG (CDLHPG), was developed to solve core-reduction problem and obtained a double-clad fiber (DCF) structure. But the power fluctuation of heating laser caused large core variation of Cr4+:YAG DCF, and further impaired the optical performance. An innovating method for suppressing the fluctuation of heating power, sapphire tube assisted CDLHPG technique, was developed and combined with power feedback control program. By this technique, 10-£gm-core Cr4+:YAG DCFs which meet the adiabatic propagation criterion were fabricated. By comparing with ASE and optical amplifier experimental data, cross sections of pump absorption, emission, and excited-state absorptions (ESAs) of pump and signal were determined. Pump ESA loss limited the optical performance that could be solve by using cladding pump scheme. A record-low threshold Cr4+:YAG DCF laser with two slopes with respect to absorbed pump power was achieved at room temperature. The threshold pump powers were 2.5 mW and 96 mW in the low and high absorbed pump powers with the same output coupler transmittance of 3.8%, respectively. The slope efficiencies of the fiber laser were 0.4% and 6.9%, respectively. By numerical simulation, 56% slope efficiency can be achieved with a length of 7 cm and an output reflectance of 80%. Our group also firstly used the ASE as the light source of optical coherence tomography, an axial resolution of 3.5 £gm was achieved.

Recently, long-period fiber gratings (LPFGs) based on PCFs have been demonstrated by using heating or a mechanically pressure to induce periodic index variations along the fibers. However, LPFGs fabricated by these two methods suffer the structure damage. In this thesis we propose novel optically controllable LPFGs based on the photoresponsive photonic liquid crystal fibers (PLCFs) and no structure damage occurs during the fabrication process. The photoresponsive PLCF was filled with a LC mixture consisting of the nematic LC E7 and the photoresponsive 4MAB. The properties of the photoresponsive PLCF can be modulated by using laser irradiation. In addition, the transmission bands of the photoresponsive PLCF can also be tuned by controlling the 4MAB concentration or operation temperature. An optically controllable LPFG was fabricated based on the photoresponsive PLCF by using blue-laser irradiation through a mask with 700-£gm grating period. The measured resonant wavelength appeared at 1539 nm with the FWHM was 27 nm, and the maximum dip depth was about −15 dB with a 6.5-dB insertion loss. The LPFG was shown to be erasable by using a green laser. In addition, we have also investigated the effects of the number of grating period, 4MAB concentrations, operation temperatures, thermal recovery properties, and irradiation intensity on the LPFGs. Our proposed optically controllable LPFGs possess reversible property and are quite useful to be applied in tunable optical devices.

Because of the fast growing in communications, the quality of signal transmission in optical fiber becomes very important. Concurrently, photonic crystal fiber (PCF) consisting of a central defect region surrounded by multiple air holes is attracting much attention in recent years because of its unique properties, such as full photonic bandgaps, wideband, dispersion, endlessly single mode and birefringence, etc. This thesis is mainly focused on the development of the photonic band structures and propagation properties of PCF. And we propose a novel ideal about 3-D PCF, which can be fabricated using the laser heated pedestal growth (LHPG) method. In the thesis, we study the optical properties of 2-D and 3-D PCFs made by Pyrex using the software RSoft. From the result of simulation, the 2-D out-of-plane bandgaps for a hexagonal close packed structure appear between the air filling fraction range from 0.30 to 0.88 for the incident light of wavelength range from 0.7 to 1

Nesta tese estudamos as condições para o crescimento estável dos compostos óxidos de metais de transição Sr2MO4 e das soluções sólidas SrM1-xM\'xO3, (M,M\'=Ru,Ti,V) pela técnica de fusão a laser de pedestais. Os nutrientes e sementes policristalinos utilizados para produzir fibras monocristalinas crescidas por fusão a laser não foram, em geral, nem queimados nem sinterizados antes do seu uso. Dois caminhos diferentes de processamento foram seguidos para a preparação de amostras cristalinas. A melhor condição de crescimento cristalino encontrada foi a que envolve o uso de atmosfera gasosa isostática como ambiente de crescimento cristalino. Porém, enquanto que fibras monocristalinas altamente homogêneas de SrTiO3, Sr2RuO4 e SrVO3 foram obtidas desta maneira, foi muito difícil estabelecer condições aceitáveis de crescimento cristalino para as soluções sólidas. A forte influência da natureza química do ambiente de crescimento cristalino e dos reagentes químicos de partida na qualidade da fibra monocristalina foi investigada. Explicações para as principais dificuldades encontradas no crescimento tanto das composições extremas como das suas soluções sólidas foram inferidas usando argumentos termodinâmicos e cálculos semi-empíricos dos parâmetros importantes envolvidos no processo de crescimento cristalino. A caracterização das fibras monocristalinas incluiu algumas adaptações aos métodos tradicionais de caracterização da resistividade elétrica, microestrutura e composição em cristais volumétricos. / In this thesis we have studied the conditions for the stable growth of the transition metal oxide compounds Sr2MO4 and solid solutions SrM1-xM\'xO3, (M,M\'=Ru,Ti,V) by the laserheated pedestal growth (LHPG) technique. The polycrystalline seed and feed rods used to grow single crystal fibers have, in general, not been fired or sintered prior to use. Two different processing methods were followed to prepare single crystal samples. The best crystal growth condition found was one involving the use of isostatic gaseous atmosphere as the crystal growth ambient. However, while highly homogeneous SrTiO3, Sr2RuO4 and SrVO3 single crystal fibers were obtained in this way, it was very difficult to establish acceptable crystal growth conditions for the preparation of the solid solutions. The strong influence of the chemical nature of the crystal growth ambient and starting reagents on the single crystal fiber quality was investigated. Explanations for the main difficulties found to grow both the extreme compositions and their solid solutions have been inferred using thermodynamic arguments and semi-empirical calculations of important parameters involved in the crystal growth process. Single crystal fibers characterization included some adaptations to the traditional electrical resistivity, microstructural and compositional methods practised in bulk crystals.