The Investigation Of Multimode-interference Filter Used In All Fiber Laser

The structure of SMS fiber filter is a kind of multimode fiber to excite high ordermode interference model of fiber devices based on the self-imaging effect. It can beproduced such as strain sensor, temperature sensor, optical lens, edge filter andband-pass filter. a variety of new optical devices using the fiber structure responsecharacteristic of spectral response and the environment, because the benefit from theeasy package advantage and connected optical fiber system, It can provide solutionsfor optical fiber laser. There is a wide application prospect in fiber communicationand sensors.In this paper, in order to achieve narrow linewidth all-fiber laser, using SMSstructure filter, from the theory to solve the single-mode fiber to some couplingparameter of multimode fiber, self-imaging theory to explain SMS, and study therelationship between the transmission spectra and the outer coating liquid refractiveindex multimode fiber.The multimode interference filter structure is simulated by beam propagationmethod based on the finite difference method, use RSOFT software to change themultimode fiber diameter, length and wavelength of incident light, observation ofmultimode fiber optical field distribution in response to internal. To verify thetheoretical feasibility of SMS as a multimode interference filter. In the case of fixedmultimode fiber, the incident light of different wavelengths in the SMS structure ofthe transmittance is different, only one wavelength of light can be able to just grin atthe output position, maximum energy output.Ring cavity erbium-doped fiber laser was built, discusses the role of isolator inthe ring cavity and the loss of its introduction. Choose a different length ofmultimode fiber to obtain different output wavelength, narrow linewidth filterfunction of SMS structure can achieve good verification. In order to realize thetuning of the laser, increase the sensitivity of multimode fiber, multimodeinterference filter in the tapering the spectral shift was found in fiber optic taper ofexternal coating liquid, in the low refractive index change of wavelength is about2nm. The new phenomenon of wavelength conversion was found in the experiment, repeated several times, can stable switch back and forth between the twowavelengths, the conversion from1540nm to1553nm, the fast wavelength canachieve the maximum13nm conversion, the fast wavelength conversion have broadapplication prospects in military and civil fields.