Investigation On High Power And Low Noise Tunable Single-frequency Fiber Laser For All Optics Communications

The works in this thesis are on the basis of two projects: New Single Longitudinal-mode Laser Array Source in WDM All Optical Communications funded by Science and Technology Development of Jilin Province, China. And Investigation on New Wavelength Tunable Single-frequency Fiber Laser funded by Special Funding of Optoelectronics sponsored by Changchun Science and Technology Development, Jilin Province, China.Narrow linewidth single-frequency fiber laser adopts erbium-doped fiber (EDF) as gain medium, it is of high coupling efficiency, high power, low noise, single longitudinal-mode, no chirp and it's tunable. This kind of fiber laser is one of the most attractile light source in optical communications, optical sensor and spectroscopy. Therefore, research in the tunable fiber laser of high power, low noise and single-frequency has practical significance in the development of technology of high-speed all optical distant communications.This works mainly focuses on the desiderated improvements of 1.55μm ring fiber laser in its single longitudinal-mode, wavelength tuning and high-speed modulation in order to meet the requirements of modern telecommunication. In view of the 1.55μm ring fiber laser, we develop the single-frequency fiber laser according to communications demand as the aim. In this paper, the gain, mode, tunable property, high-speed modulation and distance transmission of EDFs have been studied experimentally and theoretically, which include: 1. The gain characteristics of the Erbium-Doped Fiber is analyzed theoretically and investigated in experiment. A high flattening C+L band super-fluorescent fiber source with 70nm bandwidth and high output power is achieved.2. The single longitudinal-mode operation is realized by introducing a fiber Bragg grating (FBG) as narrow wavelength-selective element and a section of EDF pumped by 980nm laser diode (LD) as saturable absorber to produce standing-wave saturation effects in the cavity. The three energy level stabilization characteristic of the fiber laser is deduced and the cavity configuration of the fiber is optimized.3. A fiber-coupling scanning Fabry–Perot (F-P) interferometer is designed to measures fiber laser longitudinal-mode at 1.55μm. Overall length of the laser cavity is 18m and the laser longitudinal-mode spacing is 11.1MHz. The single longitudinal-modem was observed by the interferometer with free spectrum range (FSR) of 7.5GHz. The longitudinal-mode fabric is analyzed by the interferometer with FSR of 1.5GHz. The linewidth of longitudinal-mode was measured as 20MHz. Changing the cavity length to induce longitudinal-mode spacing to 3.7MHz to compare the structure of the longitudinal-mode under the various cavity lengths, the experiment approves the single longitudinal-mode of fiber laser precisely.4. Interference effects and saturable absorption effects arising from the laser oscillation in the saturable absorber are experimented. The influence of different lengths, concentration of saturable absorber on characteristics of laser output is analyzed. The relationship of saturation absorption with the signal optical intensity is educed and the bistable output characteristic is discussed in depth. It relates the absorber greatly, and resulted from saturable absorption effect.5. The relationship between the laser modes and the components in the ring cavity is investigated experimentally. Two orthogonal polarization modes are found. Then, the single longitudinal-mode is observed with a polarimeter in the cavity and the laser mode is optimized. 6. A framework for tuning wavelength is designed and applied in the single-frequency fiber laser. A tunable FBG is used as wavelength-selective element and an unpumped 2m-long EDF is used as saturable absorber. A single line polarization laser with tuning range of 42nm is achieved. The linewidth is measured as 25-kHz.7. Three high-speed typical rates in the optical communications, i.e. 1.25Gbps, 2.488Gbps and 9.953Gbps, are used in the modulation experiment of single-frequency fiber laser. The signal is transmitted in 84km long-distance experiments. The eye diagrams of optical transmission and reception are measured by an optical sampling broadband oscilloscope, the performance of long-haul transmission with high-speed modulation is perfect.With the experimental and theoretical analysis, there are some novel findings in the key technology and its application to fiber lasers in this paper as follows:1. It is the first time to investigate the mode structure of under changing the longitudinal-modes spacing with the ring cavity length. The modes of different cavity lengths are compared. The single longitudinal-mode is distinguished.2. It is the first time to study the saturable absorbtion characteristics in the erbium-doped fiber ring laser comprehensively and systematically. The relationship of the laser output with the absorber is obtained. The optical bistable phenomena the laser output curves showed is studied in depth experimentally. The imperfections of the other researchs are complemented with the experimental analysis.3. It is the first time to experimentally study the mode structures. 42nm output can be tuned for a single line polarization light, and output power is up to 20mW with low-noise and stable mode. This fiber laser can be applied to high-speed optical communication systems as an excellent carrier for long distance transmission source.4. For the first time the fiber laser is experimented in the high-speed electro-optical externally modulated transmission tests, and the results according with optical communications requirements are achieved through 84km long-distance transmission of signal. The signal eye diagrams of the multi-rate transmission and reception are measured, and the eyelids are thin. The results show that the coding jam and signal aberrance were not observed and signal-to-noise ratio for better. The high-speed transmission error rate is 10-12, and the system has run error free continuously for 24 hours.The single-frequency fiber laser in this paper can emit single-mode laser, with high power, low noise, wideband tunable continuous, no chirp with high-speed modulation in long-distance transmission, and the laser couples with optical system efficiently, small size, low cost. This product will be the most potential carrier sources in the new generation all optical communication systems.