Transmission Characteristics Of The Single/multi-cavity Structure Based On Asymmetric Fiber Bragg Gratings

Highly compact single-wavelength fiber lasers with ultranarrow bandwidth and high power,as well as multi-wavelength fiber lasers with or without switchable wavelength have wide applications in many areas such as in optical communication,high-precision spectrum and wavelength division multiplexing(WDM).Since the resonant cavity is the core of the laser,designing cavities with different structures for different targets is a frequently used method.In those structures,the Fabry-Peort(F-P)cavity is the most common one.Since fiber Bragg grating(FBG)is a fundamental device in fiber optics,correspondingly the FBG-based F-P structure is the most common cavity structure for the fiber laser.In this paper,we investigate the realization of single-wavelength,dual-wavelength and even multi-wavelength fiber lasers by using the asymmetric FBG-based F-P cavities.The investigations are carried out both theoretically and numerically.According to the coupled mode theory and the transfer matrix method,we mainly focus on the discussions of the transmission characteristics of the single/multi-cavity structure based on multiple asymmetric FBGs.The work is specified as follows:(1)An all-fiber F-P cavity structure based on two asymmetric FBGs is proposed,which can achieve ultranarrow single-longitudinal-mode transmission at the designed wavelength.The periods(or the Bragg wavelengths)of the two FBGs are different,which results that the two FBG stop bands are partially overlapped.The overlapped region can be controlled by tuning the FBG periods and only one wavelength is allowed to transmit through the cavity structure.Hence,the single-longitudinal-mode transmission is realized.If we use a long cavity in this case,the single-longitudinal-mode transmission will have an ultranarrow bandwidth.Such an all-fiber structure has applications in ultranarrow single-longitudinal mode fiber laser.(2)The dual-transmission properties of a two-cavity structure which is composed of three asymmetric FBGs are investigated.In this work,we firstly obtain the general expression of the transmission of a multi-cavity structure.When the expression is applied to the two-cavity structure,we can achieve the dual-wavelength transmission by properly choosing the parameters of the FBGs.The experimental feasibility of the structure is also discussed.Especially,the inaccuracy of the cavity length,the temperature and strain variations on the dual-wavelength transmission are analyzed.The all-fiber structure proposed here can be extended for multiple transmission with equal or even unequal spacings.The structure has potential applications in dual-or multi-wavelength fiber lasers.