A fiber amplifier is constructed at 1014.8nm and then frequency doubled to produce 507.4nm. This could then be frequency doubled again to produce 253.7 radiation. The fiber amplifier consists of Ytterbium doped double-clad fiber cooled to low temperatures and incorperates a diode laser as the seed source. The amplifier is built in a two stage configuration with high power diode lasers at 980nm pumping each stage. The output of the fiber amplifer is then doubled in a PPLN crystal and redoubled in a BBO cavity. Measurements are taken throughout the system to determine ouput powers from the first stage and from the fiber amplifier as a whole.

Abstract 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. The fiber amplifier that is in common use can not fully cover the whole, which its range is from 1.3 to 1.6 £gm. Recently, the Cr4+-doped fiber has shown a broadband emission from 1.3 to 1.6 £gm. Therefore, it is eminently suitable for super-wideband optical source. In this study, we first propose and fabricate a Cr4+-doped fiber by employing a commercial drawing-tower method, which has good core diameter uniformity, the growth speed is up to 200 m/min, and the core diameter is less than 10 £gm. The central wavelength of the ASE spectrum is at 1310 nm, and a 3-dB bandwidth is 300 nm. The divergent angle of the Cr-doped fiber is 17 o ¡Ñ 15 o and it¡¦s also similar to a single mode fiber of 16 o ¡Ñ 16 o. Low-loss fusion splice can readily be done with the standard single mode fiber, and is beneficial when integrated with the broadband WDM couplers. Therefore, it is good for commercial production and application to light wave systems.

A fiber amplifier is constructed at 1014.8nm and then frequency doubled to produce 507.4nm. This could then be frequency doubled again to produce 253.7 radiation. The fiber amplifier consists of Ytterbium doped double-clad fiber cooled to low temperatures and incorperates a diode laser as the seed source. The amplifier is built in a two stage configuration with high power diode lasers at 980nm pumping each stage. The output of the fiber amplifer is then doubled in a PPLN crystal and redoubled in a BBO cavity. Measurements are taken throughout the system to determine ouput powers from the first stage and from the fiber amplifier as a whole.

The lasers operating in the wavelength range of 900 - 1000 nm have caused intense attention because they are in great demands for: 1. Highpower blue and deep UV laser generation 2. High power single-mode pump laser source 3. Light detection and Lidar , etc. And now, there are actually many different types of lasers can generate laser in this wavelength range. For example, Nd and Yb doped fiber laser, Nd and Yb doped glass and crystal lasers, OPO and SHG laser, etc. Among all this options, we decided to study the Nd-doped fiber laser for their outstanding advantages: 1. As fiber laser, it possess all the advantages of any fiber lasers have, such as: high power scalability, excellent beam quality, high spectral and intensity stability, super compactness, robustness and reliability. 2. Comparing to other rare-earth-ion, the Nd^3+ ions have a more broad emission wavelength range from 900-950 nm. My goals for doing this thesis research are:1.Experimentally and theoretically investigate Nd-doped fiber lasers and amplifiers at 9xx nm. 2. Develop 9xx nm single frequency fiber lasers and amplifiers. 3.Obtain directions for developing high power single-frequency Nd-doped fiber laser sources at 9xx nm. To achieve these goals, 1. Nd-doped fiber lasers at 934 nm were investigated. 2. Core-pumped and cladding-pumped Nd-doped fiber amplifiers are also investigated. 3. The simulation of the Nd-doped fiber amplifiers have been done.

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 fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A preform was assembled by using the grown Cr:YAG rod as core and the silica tube as cladding. The outer and inner diameters of the silica tube are 20 and 7 mm, respectively. The initial dimension of the Cr:YAG crystal rod had a length of 0.03 m and a diameter of 500 £gm. The Cr:YAG crystal was grown into a diameter of a 290 £gm with a length of 0.12 m by the LHPG method. The SMCDSFs had a 6 £gm core and a 125 £gm cladding. The transmission loss was 0.08 dB/cm at 1550 nm. The far-field pattern measurements indicated the single-mode characteristic when the propagation wavelength was longer than 1310 nm. In order to solve the interface of core and cladding, a novel rod-in-tube(RIT) perform was employed by inserting the Cr:YAG crystal rod of 0.03m length and 500 £gm diameter into the silica capillary tube, which had the same diameter with the drilled silica rod. The single-mode Cr-doped fibers had successfully been fabricated and the loss had been reduced to 0.03 dB/cm at 1550 nm with a 5 £gm core and a 125 £gm cladding. Furthermore, the SMCDSFs also had the single-mode characteristic when they operated in the optical communication window. The successful fabrication of SMCDSFs may be one step forward towards the achievement of utilizing the SMCDSFs as ultra-broadband fiber optical amplifiers to cover the bandwidths in the whole 1300 to 1600 nm range of low-loss and low-dispersion windows of silica fibers and a broadband source for enabling high resolution in optical coherence tomography (OCT).

A tunable erbium-doped fiber ring laser using an intra-cavity Fabry-Perot filter as the tuning element is investigated. Tuning is achieved by varying the applied voltage which controls the FP cavity length. The laser's wavelength is monitored using an optical spectrum analyzer to determine the laser's spectral characteristics under static conditions at different wavelengths over its tuning range of approximately 50nm. When the laser is tuned rapidly, the frequency versus time characteristic is determined using a fiber Fabry-Perot interferometer with a photodetector to convert the optical signal to an electrical signal. The core of the research is to determine the degree of spectral broadening of the laser as a function of the spectral tuning rate. The fringe contrast of fiber Fabry-Perot interferometer transmittance curves decreases with increase in the tuning frequency. The gain at a certain wavelength becomes a function of time putting an upper limit on the tuning frequency of the system. The carrier lifetime of erbium ions dictates the maximum achievable tuning speed.

Pulling bulk crystal into fiber is suitable for laser, amplified spontaneous emission (ASE), and optical amplifier applications in optical communications because of its structural similarity to silica fiber. Moreover, fiber configuration can confine pump light in a small cross-sectional area with a high energy density for a long distance. Among crystal fiber growth techniques, the laser-heated pedestal growth method (LHPG) was adopted. It is crucible free and can therefore produce high-purity, low-defect-density single crystals. However, interface loss of the crystal fiber is one of the main causes of optical loss. In order to reduce the optical loss, a proper method to clad the fiber is important for high device performance. For laser application, high-efficient Nd:YAG lasers were demonstrated using gradient-index crystal fibers. We used controlled profile of the active ion resulted in index difference of 0.0284 between the center and the edge of the fiber to confine the laser beam in the center region and thus reduced the interface loss. A laser output power of 80 mW was achieved with a slope efficiency of 28.9%, which, to our knowledge, is the highest ever achieved for diode-laser-pumped Nd:YAG fiber laser. For ASE and optical amplifier applications, Cr4+:YAG crystal fiber was studied due to its fluorescent spectrum just covering the low loss window of silica optical fiber. To reduce the fiber diameter and propagation loss, a novel cladding technique, codrawing LHPG (CDLHPG), was developed. Although fused-silica-clad fiber can be made with a 29-micron-diameter core and a propagation loss of less than 0.1 dB/cm, which is a factor of 7 smaller than that of an unclad fiber, it has almost no Cr4+ fluorescence in the core area due to the entering of SiO2 in YAG. With proper controlled growth parameters of the CDLHPG method, a double-clad fiber with a core diameter of 25 micron was successfully grown. Up to 2.36 mW of ASE with a bandwidth of 265 nm was demonstrated. After splicing the double-clad fiber with conventional single mode fiber, we successfully demonstrated the first transition metal-doped fiber amplifier in the optical fiber communication band. Up to 16-dB of gross gain at 1.47 micron was achieved.

"Hydroxyl ions are a common contaminant in optical fibers, and are responsible for strong absorption centered at 1380 nm that becomes significant over long optical path lengths. Recently, however, special fabrication methods have been developed that minimize the hydroxyl ion contamination, permitting use of the entire 1300-1700 nm spectral region for telecommunications. There is therefore interest in examining the Nd 4F3/2 to 4I13/2 transition for a potential optical amplifier at 1400 nm. In this thesis, the excited state absorption cross section and the overall gain/loss spectrum of neodymium in a silica glass fiber were determined for the 1200-1500 nm wavelength region using the pump-probe method. The ground state absorption cross section was also determined from transmission measurements, and the stimulated emission cross section was calculated using the fluorescence spectrum and the McCumber relation. Oscillator strengths for absorption and emission transitions were calculated in the 800-1600 nm wavelength range using the Judd-Ofelt method. The above procedures were followed for both the Nd-doped fiber, as well as an erbium-doped silica fiber. The shape of the Nd emission spectrum is also noteworthy, since the characteristic Nd peak at 1064 nm is not observed, although there is strong emission at 1092 nm. The pump-probe measurements revealed significant excited state absorption loss between 1200 and 1350 nm, due to excitation from the 4F3/2 to the higher 4G9/2 and 4G7/2 states. Between 1350 and 1475 nm, there was no net gain or loss that could be observed beyond the level of the noise. For the glass fibers studied, it appears that in the spectral region of interest for an optical amplifier, the stimulated emission and excited state absorption cancel one another out."

In this dissertation I report the development of high power pulsed fiber laser systems. These systems utilize phosphate glass fiber for active elements, instead of the industry-standard silica fiber. Because the phosphate glass allows for much higher doping of rare-earth ions than silica fibers, much shorter phosphate fibers can be used to achieve the same gain as longer silica fibers.This single-frequency laser technology was used to develop an all-fiber actively Q-switched fiber lasers. A short cavity is used to create large spacing between longitudinal modes. Using this method, we demonstrated the first all-fiber Q-switched fiber laser in the 1 micron region.In addition to creating high peak powers with Q-switched lasers, created even higher powers using fiber amplifier systems. High power fiber lasers typically produce spectral broadening through the nonlinear effects of stimulated Raman scattering, stimulated Brullion scattering, and self-phase modulation. The thresholds for these nonlinearities scale inversely with intensity and length. Thus, we used a short phosphate fiber gain stage to reduce the length, and a large core fiber final stage to reduce intensity. In this way we were able to generate high peak power pulses while avoiding visible nonlinearities, and keeping a narrow bandwidth.The immediate goal of developing these high power fiber laser systems was to generate narrowband terahertz radiation. Two different wavelengths were combined into the final amplifier stage at orthogonal polarizations. These were collimated and directed into a GaSe crystal, which has a very high figure of merit for THz generation. The two wavelengths combined in the crystal through the process of nonlinear difference frequency generation. This produced a narrowband beam of THz pulses, at higher powers than previous narrowband THz pulses produced by eyesafe fiber lasers.