The breakthrough technology in dry fiber fabrication has opened the possibility for using fiber bandwidths all the way from 1.3 to 1.6£gm. However, the fiber amplifier used in commercial product, such as erbium-doped fiber amplifier (EDFA), can not fully cover the whole fiber bandwidths from 1.3 to 1.6£gm with a single fiber amplifier. Recently, the Cr4+-doped fiber has shown a broadband emission from 1.3 to 1.6£gm. Therefore, it is interesting to develop a single fiber amplifier which can operate the wide bandwidth of the 1.3~1.6£gm emission. In this study, we have successfully fabricated and measured the Cr-doped fibers by using a commercial drawing-tower technique and a rod-in-tube method. The core diameters were 26 and 16£gm. The Cr4+ fluorescence spectrum showed a broadband emission from 1.2 to 1.6£gm. The radiation intensity was up to the order of nW. This indicates that the new Cr-doped fibers may be used as a broadband fiber amplifier. The advantages of using the drawing tower to fabricate the Cr-doped fibers are to have a better control of the core diameter, the fiber uniformity and circularity. Therefore, the Cr-doped fibers may have a potential for commercial production and application to lightwave communication systems.

Cr4+:YAG is an attractive gain medium due to its broad 3-dB emission spectrum ranging from 1253 to 1530 nm, which cover the low-loss window of the silica fiber. Such a broadband haracteristic 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 crystals for reducing lasing threshold and obtain the high slope efficiency due to the beter optical confinement and heat dissipation. A record-low threshold of 0.75 mW was achieved at room temperature. It is more than 500 times lower than any previously reported Cr4+:YAG lasers, and a slope efficiency of 6.9% was obtained. The ultralow-threshold lasing is made possible by the low propagation loss of 0.08 dB/cm and the high pump intensity of the core. Such a low-threshold operation makes the double-clad crystal fiber laser be compatible to present optical communication systems. Furthermore, we demonstrate that the proposed optimization algorithm can provide an efficient and direct approach to analyzing both fiber structure and bulk medium in terms of practical application requirements. The simulation results reveal that Cr4+:YAG DCF lasers offer advantages over bulk lasers in terms of low-threshold lasing and high efficiency, which are essential for their use in optical communication systems.

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 1.3 mm to 1.6 mm. The fast increasing demand of communication capacity results in the emergence of wavelength division multiplexing (WDM) technology, which results in the need for wideband optical amplifier. Cr4+:YAG has a strong spontaneous emission that covers 1.3 mm to 1.6 mm. Besides, its absorption spectrum is between 0.9 mm to 1.2 mm, which matches with the pumping source in current erbium doped optical amplifier. Such a fiber is, therefore, eminently suitable for optical amplifier applications. We have successfully fused the double cladding Cr4+:YAG crystal fiber with single mode fiber by fusion splicer. The crystal fibers are grown by the laser-heated pedestal growth technique. The splicing parameters are optimized to achieve an insertion loss of below 1 dB. Since, the core diameter tapering will increase the propagation loss and reduce the gross gain. Adiabatically tapered fiber is discussed. Simulations are performed to predict the loss, and compare with the experimental results, then find out the way to improve the gross gain. Numerical simulation indicates that the gross gain could reach 37.2 dB at 0.5 W pump, if the core diameter of the double cladding Cr4+:YAG crystal fiber is reduced to 5 mm. In the future, in order to increase gross gain we will improve the mode matching between the cores of single mode fiber and the double cladding Cr4+:YAG crystal fiber. Gradual change of the refractive index at the splicing region as well as high Cr4+ doping concentration can also improve the gross gain.

In this study, we have successfully demonstrated the use of laser scanning confocal microscopy in studying the fluorescence spectroscopy of Cr3+ and Cr4+ ions in Cr:YAG crystal fibers, double-clad crystal fibers, and glass fibers.

In the last years, intensive research on new tunable solid state laser materials has been carried out. For the spectral range from 1300 to 1600 nm the Cr4+ ion seems to be the most promising laser-ion. In order to meet the the demand of broad-band devices, we employed the Cr4+ doped YAG crystal fibers with high optical quality thin films design and coating (high index material TiO2, low index material SiO2) to achieve the development of high efficiency crystal fiber lasers. In this thesis, crystal fiber was used as the laser gain medium, and coated with optical thin films at both end faces (input face 900-1100 nm AR and 1300-1600 nm HR; output face 1300-1600 nm HR) as the laser cavity to reduce the loss and promote laser efficiency. During the experiment, we tried to optimize the coating conditions to acquire high density, stable index and low absorption coefficient thin films. With different thin film thickness and stacking designs, the electric field distribution was designed to be away from the laser interface and high index region to increase the laser-induced damage threshold and lifetime for high power pumping. In addition, CaO and Cr2O3 were deposited on Cr4+:YAG source rods before the laser-heated-pedestal growth to increase CaO and Cr2O3 doping concentration for higher fluorescence efficiency and signal saturation power.

Owing to the increasing demand on the telecommunication bandwidth and the advent of the OH--free dry fiber developed by Lucent Technology, a new transmission bandwidth for the fiber optical communication which is centered around 1400 nm has been created in recent years. However, the corresponding light sources, optical amplifiers and other devices needed in the new transmission window are still under development. This thesis is to dissert with respect to the Cr4+:YAG crystal fiber based ASE (amplified spontaneous emission) light sources which emit light with broadband wavelengths from around 1200 nm to 1600 nm. Our previous study showed that side-pumping configuration can achieve a maximum ASE power of 0.65 mW with a pump power of 11 W at 980 nm in wavelength, whereas the end-pumping configuration successfully generated an optimum ASE power of 2.45 mW with a pump power of 11 W using a 1064-nm Yb fiber laser. This thesis reviewed the Cr4+:YAG material and studied on the single and double-pumping configuration of ASE light source, numerical simulations of the L-I curve of both configurations, and of the emission and the gain spectrum of the material, plus the introductions of the experiments needed for the simulations will be reported.

Cr4+ doped Yttrium aluminum garnet (YAG) has a strong spontaneous emission that can generate near-infrared emission from 1.2 to 1.6 £gm. This broadband emission have aroused great interest in developing tunable wavelength lasers and amplified spontaneous emitter (ASE).In this dissertation, The valence states of Cr ions in Ca or Ca/Mg co-doped Cr:YAG single-crystal fibers are studied. The fibers were grown using the laser-heated pedestal growth (LHPG) method, followed by annealing treatments up to 1500 oC. The concentrations of the Cr3+ and Cr4+ ions in octahedral and tetrahedral sites in oxygen or nitrogen environments were characterized for the first time to our knowledge. Above 700 oC, migration of Cr4+ between octahedral and tetrahedral sites takes place, the ratio is about 4%; its relative stabilization energy was estimated. For Ca,Cr:YAG annealed in an oxygen or nitrogen environment, it was 0.25 and 0.3 eV, respectively. For Mg,Ca,Cr:YAG annealed in oxygen or nitrogen, it was 0.47 and 0.49 eV, respectively. For the Ca,Cr:YAG crystal fiber (Ca/Cr=113.1%) with oxygen annealing, about 35% and 2.5% of Ca ions took part in charge compensation for Cr4+ in the octahedral and tetrahedral sites, respectively. The density of oxygen vacancies depends on the concentration of Ca ions. The estimated ratios of the unreacted oxygen vacancies to total oxygen vacancies were about 63% and 88% for oxygen and nitrogen annealing, respectively. The main limitation on the concentration of Cr4+ in the tetrahedral site of YAG is the presence of unreacted oxygen vacancies. Furthermore, chromium ions tend to diffuse outward during the LHPG of YAG crystal fiber, in which the average Cr4+ ion concentration decreases significantly after each diameter-reduction step. The Cr4+ ions are replenished using an electron gun to deposit Cr2O3 and divalent-ion oxide (CaO or MgO) on the source rod circumference before growth. It was observed that Ca2+ has better efficiency to diffuse into the source rod more efficiently than Mg2+generating fewer defects and stacking faults. By CaO deposition and post growth annealing at 1350 oC under an oxygen environment, a 110% increase in Cr4+ concentration was obtained. The achieved Cr4+ concentration and the ratio of Cr4+ to total Cr were 1.76 10^18 cm^-3 and 5.5, respectively.

Rapid development of fiber-optic communications network requirements increasing in recent years, The WDM technology and invention of anhydrous optical fiber open the possibility for optical fiber transmission bands broaden form 1.3 £gm to 1.6 £gm. Chromium doped yttrium aluminum garnet crystal fiber has characteristic of 300 nm broadband. Therefore, it¡¦s strongly desirable to develop a broadband fiber amplifier, laser or other active components for extending the flexibility of system architecture design in optical fiber communication. A few-mode chromium doped yttrium aluminum garnet double-clad crystalline fibers has been demonstrated by employing a modified version of LHPG technique, means using silica-YAG crystal co-drawing and multiple core-tuning process by precisely controlled inter-diffusion between YAG core and silica tube. In this thesis, electron probe x-ray micro-analysis, energy dispersive spectroscopy and transmission electron microscopy were utilized to confirm this fiber structure and composition. This fiber has gross gain about 2.3 dB with dual pumped by few hundred mini Watt. Significantly reduce the pump power threshold. Compared with the last large core size, few-mode (small core size) chromium doped yttrium aluminum garnet double-clad crystalline fibers has lower heat effect and higher power efficiency. To enhance the optical properties towards few modes or even single mode, not only reduce the transmission loss, but improve the device efficiency. Key words: Laser heated pedestal growth, Cr4+:YAG, Double-clad Crystal Fiber, gain