Reseach On The Properties Of Gain Tailored Random Fiber Laser

The random fiber lasers(RFL)developed in recent years are novel fiber lasers.Quite different from conventional lasers,random lasers do not need definite optical resonator.Random fiber lasers have unique advantages,such as simplicity,relatively stable output and ultra-long-distance cavity.Due to these advantages,they have important research significance.In recent years,the use of their unique power characteristics,spectral characteristics,gain control and other properties has lead series of novel researches in optical sensing and communications.The initial research on random fiber lasers mainly focus on the random Raman fiber laser based on the conventional single-mode fibers.The unique properties and the advantages on the wavelength tunability,narrow band generation multiwavelength generation and high efficiency,high power generation by using the Raman gain based random fiber laser have been intensively studied.In fact,the realization of random fiber laser can base on the different types of gain mechanisms and their combination and regulation,such as the introduction of Brillouin gain,active fiber gain and the special designed fiber for tailoring the gain.In this thesis,we investigate the function of hybrid gain on the multiwavelength random lasing generation and the function of gradient Raman gain fiber on the high power random fiber lasing generation.First,the Brillouin gain based multiwavelength random fiber laser is investigated.By using the high power,narrow linewidth Brillouin pump and the bidirectional pumping half-open cavity design,up to six-order Brillouin random fiber lasing has been achieved with several hundreds of milliwatts output power.In order to reduce the threshold and increase the cascaded orders,the multiwavelength Brillouin-Erbium-Raman random fiber laser has been studied.The 1455 nm pump can provided Erbium-gain in the Erbium-doped fiber,and the residual 1455 nm pump can further provide Raman gain in the dispersion compensation fiber(DCF).With 0 dBm Brillouin pump power,Brillouin random lasing is stimulated with only tens of milliwatt 1455 nm pump power.A 23.2 nm 3-dB wide flat-amplitude multiwavelength Erbium-Brillouin-Raman RFL covering 290 Stokes lines with 0.08 nm channel spacing is achieved by optimizing 1455 nm pump.The results indicate the utilization of hybrid gain can improve the output properties of multiwavelength Brillouin fiber laser.Then,regard to the random Raman fiber laser,aiming to further increase the output power,we theoretically investigate the output properties of random fiber laser based on the gradient Raman gain.By utilizing the tapered fiber(the core diameter increases linearly along the fiber length),it can not only increase the pump power injected into the fiber,but also increase the threshold of 2nd-order random Raman fiber lasing,thus increasing the maximum output power of 1st-order random Raman fiber lasing.The influence of pump injection direction,fiber length and core diameter ratio on the output properties of tapered fiber based random fiber laser is numerically investigated.By comparison of tapered fiber based random fiber laser and conventional single-mode fiber based random fiber laser,the results indicate the output power can be significantly increased with tapered fiber.The study provides the guideline for designing tapered fiber based high power random fiber lasers.