Reseach On Cascaded Operation And The Efficiency Property Of Random Fiber Laser

As a new branch of random lasers, random fiber laser has its unique advantages, such as simplicity, ―mode-less‖ spectra, relatively stable output, single-transverse-mode profile and ultra-long-distance cavity. Therefore, the research of random fiber laser is meaningful in both theoretical investigation and application area. Several study on the power performance and spectrum properties of this kind of laser and its applications in the area of optical communication and sensing have been reported in recent years.In the innitial works, the fiber length for the random fiber laser is relatively long(>50km) and the pump is depolarized. Due to the extremely small Rayleigh backscattering coefficient, the threshold for the random fiber laser is relatively high and the cascaded operation is difficult. The power and efficiency property of such kind of laser is lack of thorough investigations.In this thesis, the aim is to realize the low-threshold cascaded operation of random fiber laser and investigate its efficiency properties. Third-order random lasing has been achieved based on the Raman gain and the multiwavelength Brillouin-Raman random fiber laser has also been demonstrated. On the other hand, the efficiency properties have been thoroughly studied by using the short-length fiber or the polarized pumping scheme.First, third-order random lasing based on the Raman gain is experimentally demonstrated for the first time. The used scheme is the forward-pumped half-open cavity based on the fiber loop mirror(FLM). FLM provide the broad-band, strong point feedback for the first-, second- and third-order random lasing. The broad-band reflection of FLM can avoid the reduction of the effective reflectivity caused by the spectral broadening effect of the Stokes light, thus further reducing the threshold of the high-order laser and realizing the third-order random lasing with only 2.45 W pump power.Second, by introducing the hybrid Brillouin-Raman gain for the random lasing, multiwavelenght lasing can be achieved through the cascaded Brillouin effect. With the half-open cavity design and a novel mirror-less cavity by combining the ―active‖ dispersion compensating fiber(DCF) and the ―passive‖ single mode fiber(SMF), the discrepancies in power level and linewidth between neighboring channels that exist in the previous works with mirror-less open cavity can be solved. Utilizing the two schemes, 17 nm and 40 nm flat-amplitude bandwidth(3dB range) with uniform Stokes combs have been demonstrated, respectively.In term of the efficiency properties, we proposed a forward-pumped random Raman fiber laser based on the short fiber length. Through the numerical simulation, we thoroughly studied the influence of the fiber length and the mirror‘s reflectivity on the threshold, power distribution and optical conversion efficiency of the lasing. The results provide a guidline to design the high efficiency, high power random fiber laser.We experimentally verified the concept with 1km G.652 fiber, the maximum 7W of 1140 nm 1st-order random lasing is obtained with 70% of optical conversion efficiency.Furthermore, the polarization properties of random fiber laser operating via Raman gain are investigated for the first time by using the polarized pump. The results indicate that the output random lasing is partially polarized and the lasing properties can be significantly influenced by thestate of polarization(SOP) of the pump.With the polarized pumping, due to the different average gain coefficient for forward and backward Stokes waves, the slope efficiency of pump side output is significantly reduced comparing to the depolarized pumping.We use the modified power balance model to qualitatively study the impact of polarization evolution on lasing power properties.The proposed high-order random Raman fiber laser and the multiwavelength Brillouin-Raman random fiber laser provide novel ways for the applications such as the high-order Raman amplification and the Dense Wavelength Division Multiplexing(DWDM) in the area of optical communication and sensing. The study on the random fiber laser based on the short cavity and the polarized pumping pave a way to realize the high power operation or manipulation of the power distribution of the random fiber laser.