基于渐变折射率多模光纤的锁模光纤激光器

Mode⁃Locked Fiber Lasers Based on Graded Index Multimode Fiber

结合渐变折射率多模光纤的可饱和吸收效应和马赫-曾德尔干涉滤波效应,提出了一种渐变折射率多模光纤-单模光纤-渐变折射率多模光纤(GIMF-SMF-GIMF)结构,在环形掺铒光纤激光器中实现了单波长可调谐和双波长可切换的锁模脉冲运转。基于GIMF-SMF-GIMF的锁模结构,获得的波长可调谐范围为1550.9~1558.9 nm,最短脉冲宽度为0.6 ps。在泵浦功率增加至110 mW时,改变腔内激光的偏振态,实现了中心波长1531.6 nm/1556.2 nm处的单、双波长的可切换锁模激光输出。结果表明,基于GIMF-SMF-SIMF的全光纤结构在掺铒光纤激光器中具有波长可调谐和双波长锁模的潜力,对高速大容量光纤通信系统中的多通道光源的设计具有借鉴意义。

Objective Wavelengthtunable and multiwavelength modelocked fiber lasers are essential in fields such as communication technology,fiber sensing,and ultrafast photonics.While wavelength tuning is commonly achieved by incorporating additional tunable filters,most of these filters function solely as filters and require separate modelocking techniques.Recently,the saturable absorber based on graded index multimode fiber(GIMF)has gained attention due to its advantages of easy fabrication,high damage threshold,and strong antiinterference capabilities.It can serve both as a modelocked device and a filter.In this study,we propose a method featuring a simple structure,high loss threshold,and low insertion loss to achieve wavelengthtunable laser output and dualwavelength switchable modelocked pulse output of erbiumdoped fiber lasers.Methods We propose a graded index multimode fibersingle mode fibergraded index multimode fiber(GSG)structure,which functions both as a saturable absorber for modelocking and a filter for wavelength tuning.The GIMF length is fixed at 11.2 cm to align with the selfimaging period,and the GIMF 1 and GIMF 2 have identical parameters to create a symmetrical structure.We theoretically calculate the length range of the singlemode fiber that meets the requirements and conduct comparative experiments within this range.Experimental results show that when the length of the singlemode fiber is between 2.7 cm and 5 cm,the GSG structure functions effectively as a modelocked device and filter with wavelength tuning capability.We test the GSG structure with a fiber laser and use a digital oscilloscope,optical spectrum analyzer,autocorrelator,radio frequency analyzer,and power meter to measure output pulse characteristics including spectrum,pulse width,repetition rate,and output power.Results and Discussions Testing the GSG structure with a fiber laser reveals several key observations.At a pump power of 15 mW,we observe continuous wave output on the oscilloscope.As the pump power increases to 24 mW,we see Qswitched modelocking.At 46 mW,we achieve a stable modelocked state,with no pulse splitting or harmonics detected up to the maximum pump power of 289 mW.When the pump power is set to 87 mW,the center wavelength of the pulse is 1558.6 nm,with a pulse width of 0.6 ps and a 3 dB bandwidth of 5.95 nm(Fig.5).By gradually adjusting the polarization controller,we achieve modelocked pulse output with tunable wavelengths.The central wavelength of the modelocked pulse can be stably tuned across the range of 1550.9 nm to 1558.9 nm(Fig.6).At a pump power of 110 mW,we observe both modelocked pulse and continuous wave outputs.By further adjusting the polarization controller,we obtain dualwavelength modelocked pulse switching at central wavelengths of 1531.6 nm and 1556.2 nm(Fig.7).In the stable modelocked state,adjusting the polarization controller alters the transmission characteristics of the GSG structure.These changes affect the wavelengthdependent gain and loss equilibrium points within the cavity and the polarization hole burning effect.Additionally,variations in the distribution of gain peaks in the erbiumdoped fiber contribute to wavelength tuning and switching.Conclusions We present an allfiber structure based on GIMFSMFGIMF that combines the saturable absorption characteristics of GIMF with the MachZehnder(MZ)filtering effect of the GSG structure to achieve singlewavelength tunable and dualwavelength switchable modelocked pulse outputs in erbiumdoped fiber lasers.With a polarization controller adjustment,we obtain a modelocked pulse with a repetition rate of 8.5 MHz,a pulse width of 0.6 ps,a signaltonoise ratio of 64 dB at 87 mW pump power,and a wavelength tuning range of 1550.9‒1558.9 nm.At 110 mW pump power,we achieve modelocked pulse switching and dualwavelength modelocked laser output.The GIMFSMFGIMF allfiber structure,with its simplicity,high loss threshold,and low insertion loss,provides a valuable reference for designing compact,multifunctional fiber laser modelocked devices.