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.

The connection between the devices of optical fiber system is an important part of optical communication equipment. For reducing the power loss in single transfer process, we couple the light from one device to another by connecting with splicer and connector. In the optical fiber communication system, the fiber must be coupled with light source or detectors and optical amplifier. The way connect fiber by fusion splice is different from the mechanical connectors, which is small joint volume, higher mechanical strength and much stable after connecting. It is more suitable to apply on micro-package optical communication device. In the study, we confer with the temperature profile and thermal stress of fusion splice module during splicing Cr4+¡GYAG crystal fiber and single mode fiber by numerical simulations. Through adjusting the parameters, like fusion current, fusion place and the processes of splice to examine the trend of change of temperature and thermal stress.

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.