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Investigation On All-fiber Raman Fiber Lasers

Posted on:2010-02-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z J QinFull Text:PDF
GTID:1118360308965887Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Distributed Raman fiber amplifiers (RFAs) hold a large number of advantages, including no restriction of gain bandwidth, low noise, and low nonlinear penalty. Now it is one of the hottest topics in broadband optics amplifiers. Pump sources with appropriate wavelength and high output power are one of the significant premises for RFAs being applied practically. Raman fiber lasers (RFLs) based on stimulated Raman scattering in optical fibers can emit any wavelength, which have been regarded as promising pump sources for RFAs. In contract to laser diodes, RFLs possess flexible designing for lasing wavelength, high output power, polarization insensitive, high beam quality, high conversion efficiency, and compatible with current fiber-optic communication system. Furthermore, RFAs with broadband and flat gain spectrum can be achieved by using multiple wavelength cascaded RFLs as pump sources.The main results of the dissertation are given as follows:(1)Considering the difference between the effective core area of Stokes we present an improved theoretical model applicable to any order cascaded RFLs. The analytical expressions of the output power, the threshold pump power, and the optimal Raman fiber length and output-coupler reflectivity which make pump-to-Stokes conversion efficiency maximum have been derived. An example of a fifth-order RFL is given to demonstrate the model. Comparing our analytical solutions with the published experimental results, we find an excellent agreement between them. Besides, the optimization of the fifth-order RFL is achieved. We find out that the optimization critier of large output power and low threshold pump power conflicts; the conversion efficiency is insensitive to the change of Raman fiber length near the optimal value while is sensitive to the output-coupler reflectivity, which brings great flexibility for choosing the fiber length; and the extra losses in cavities have a significant effect on RFLs characteristics.(2) We have proposed an efficient hybrid GA with excellent convergence to analyze and design multiwavelength cascaded RFLs, which combines both the advantages of GA and shooting method. An example of three-wavelength RFL has been used to demonstrate the method. Results show that the hybrid GA eliminates the disadvantages of conventional GA of converging slowly or oscillating near the optimal space and of shooting method of assigning initial shooting values properly.(3) The Stokes broadening of a first-order RFL operating at 1156nm is investigated experimentally. In the experiment, we find the spectrum of the intracavity Stokes takes on a "double-peak" shape with increasing the pump power, and some spectral sidebands positioned dissymmetrically to the center mode are also observed. The results verify the the interactions of intracavity four-wave mixing (FWM) of longitudinal eigenmodes are responsible for the nonlinear broadening. As a countermeasure against the broadening, one can use the effective reflectivitiy method to modify the theoretical model or employ special Raman fiber with high nonlinear coefficient to alleviate the Stokes broadening to some extent from the physical mechanism.(4) We present experimentally a multiwavelength RFL at 1550nm formed by Sagnac comb filter and broadband chirped fiber Bragg grating. The dispersion shifted fiber used in the cavity strengthens the FWM between/among the lasing channels which balance the channel output power automatically. We obtain a six-wavelength RFL with channel spacing of 0.54nm and 3dB-linewidth of 0.18nm. The maximal wavelength shift for each channel in half an hour is 0.06nm, and 0.8dB for power fluctuation.
Keywords/Search Tags:Fiber optics, Stimulated Raman scattering, Cascaded Raman fiber lasers, Hybrid genetic algorithm, Four wave mixing
PDF Full Text Request
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