Numerical Study On Multicore-fiber Saturable Absorber

Ultrafast pulse lasers have obvious advantages of high peak power,narrow pulse width,broad spectrum etc.They have been widely applied in such fields as huge volume communication,probing the micro-world,controlled thermonuclear fusion,biomedicine and so on.At present,passively mode-locked fiber laser is considered as the excellent light source outputting ultrafast pulses.Saturable absorber is the key components to passive mode-locked laser to achieve mode-locked.Its function is to realize the saturated absorption of optical pulses.Common clamping device include:semiconductor saturable absorption mirror,carbon nanotubes,graphene,Kerr lens and nonlinear deflection clamping.The main work of this thesis is to study a new kind of saturable absorber:multi-core fiber saturable absorber.The influences of the number of multi-core,coupling coefficient and nonlinear coefficient on the saturated absorption characteristics are analyzed detailed.The main contents are as follows:Firstly,for the structures of two-,three-,five-and seven-core fibers the coupling coefficients and the nonlinear coefficients of each structure are numerically obtained.Then the saturable absorption characteristics can be numerically simulated by solving the coupled-mode equations for different fiber structure.The results show that multi-core optical fibers can exbit the absorption for low-intensity parts and the transparence for high-intensity parts in transmitted pulses.Furthermore,increasing the numbers of cores can't improve the saturable absorption.Consequently,the twin-core fiber structure is discussed as follows:In the case of twin-core fiber,the coupling coefficient,which affect the characteristics of saturable absorption,are dependent on fiber structure and operating wavelength.The results show that,a decrease of coupling coefficient in fibers can effectively reduce the saturation power of saturable absorber.And a good filtering characteristics can be obtained when the length of fiber is appropriate.For example,when the length of fiber is 81 times of half beat length,the twin-core fiber can act as a band-pass filter in the wavelength range of 1530,1570nm.For the influence of the nonlinearity of fibers on saturable absorption,we numerically analyzed the case of filling highly nonlinear liquid in the fiber cores.The results show that the improvement of nonlinearity in fibers can also decrease the saturation power of saturable absorber effectively.For asymmetric twin-core fiber,the influence of the differences of core size and refractive index on the saturable absorption were analyzed.The results showed that the asymmetry can both decrease the coupling efficiency and increase the saturable power.Finally,we numerically simulated the mode-locked fiber lasers with the two-core fiber as saturable absorber.It proved that this class of saturable absorber can be used to generate ultra-short pulses by analyzing results in time and spectral domains for the fiber lasers.