飞秒激光直写高反射率中红外光纤布拉格光栅

High Reflectivity Mid-Infrared Fiber Bragg Grating by Femtosecond Laser Direct Inscription Method

采用飞秒激光逐线直写法,在氟化物光纤中制备出了窄带宽、高反射率的中红外光纤光栅,其中心波长为2964.34 nm, 3 dB带宽为1.24 nm,反射率高达99.27%。该工作有利于构建"全光纤化"中红外光纤激光器,对推动国内中红外光纤激光器核心器件的全自主化具有重要意义。

Objective Mid-infrared fiber lasers have attracted much attention due to high beam quality,high photoelectric conversion efficiency,excellent heat dissipation performance,and prospect of all-optical fibers.They have important applications in ranging,remote sensing,medical and health care,absorption spectroscopy,atmospheric communication,polymer materials processing,infrared countermeasures,and basic science.A fluoride fiber with a low phonon energy and a low transmission loss is usually chosen as the medium for producing and conducting a midinfrared laser.The structure of the space cavity mirror laser has such problems as low coupling efficiency,poor robustness,and easy deliquescence of fluoride fiber end faces.A fiber Bragg grating(FBG)based on fluoride fibers can be used to construct all-fiber lasers,which can effectively solve the above problems.However,the preparation of a mid-infrared fluoride fiber Bragg grating has not been reported in China.It is urgent to study the preparation technology of this key device.In this work,a mid-infrared FBG with a central wavelength of 2964.34 nm and a reflectivity of 99.27%is directly prepared in a fluoride fiber by femtosecond laser line-by-line direct writing method.This work is helpful to improve the operation stability,reliability and miniaturization of a mid-infrared fiber laser,and promote the application of corresponding mid-infrared fiber laser technologies.Methods In this paper,an FBG is prepared by femtosecond laser direct inscription method,and a 513 nm femtosecond laser is selected as the source.The fiber moves uniformly through a high precision three-dimensional displacement platform.The FBG testing device consists of a supercontinuum source,a coupled lens,and a spectrometer.The supercontinuum source(0.5-3.1μm)is coupled to a ZBLAN fiber with an FBG through a pair of coupling lenses(lens material of CaF_(2),focal length f=15 mm),and the end is directly connected to the spectrometer for transmission spectral testing.By comparing various preparation methods of FBGs,the femtosecond laser line-by-line inscription method has the advantages of high efficiency,low cost,flexibility,and good spectral characteristics.We choose the femtosecond laser line-by-line inscription method to achieve the preparation of a mid-infrared FBG.By studying the effects of laser energy and grating length on the preparation of an FBG,the appropriate writing parameters are selected.The preparation of FBGs with different central wavelengths is realized by changing the period interval of each FBG.Results and Discussions By comparing the grating fringe qualities under different pulse energies(Fig.2)and FBG lengths(Fig.4),a second-order FBG with a physical length of 6 mm is written under 150 nJ energy and 100μm/s inscription speed at 2964.32 nm central wavelength(corresponding to a grating pitch of 1.994μm).The resulting transmission spectrum of the FBG is shown in Fig.4.It reveals a strong and sharp Bragg resonance with a reflectivity of 99.27% at 2964.34 nm.The refractive index modulation depth is 7.66×10^(-4).We study the effect of grating length on the reflectivity of a grating.The reflectivities of gratings with different lengths of 2.991 mm,3.988 mm,4.985 mm,and 5.982 mm are 48.2%,63.47%,81% and 99.27%,respectively.The grating length is linearly related to its reflectivity.Compared with other works,this paper uses shorter length and faster writing speed to inscribe a high reflectivity FBG(Table 1).In addition,by modifying the grating period interval,we change the grating period to 2080 nm and adopt the same writing parameters.We inscribe an FBG with a reflectivity of99.12% at 3090 nm(Fig.5).In the transmission spectrum,there is a side lobe with a central wavelength of3091.2 nm and a depth of 6.62 dB to the right of the main response peak.When the FBG is applied to the laser,because the main peak is 13.92 dB higher than the side lobe,the gain of the main peak increases,and the parasitic oscillation generated at the side lobe can be effectively suppressed.Therefore,it is completely feasible to write an FBG in the direction of long wavelength by using an InF3 fiber with a long transmission wavelength as the writing sample and adjusting the grating pitch.The response peak of an FBG prepared in this paper is located in the region of the Dy^(3+) fluorescence emission spectrum,and it has more applications in Dy^(3+) doped fiber lasers.Conclusions In this study,we use the femtosecond laser line-by-line method and a 50× dry objective to study the influence of laser pulse energy on grating fringes.The pulse energy of 150 nJ,the inscribe speed of 100μm/s,the line length of 50μm,and the period pitch of 1.994μm are selected as writing parameters.A mid-infrared FBG with narrow bandwidth and high reflectivity is prepared based on a fluoride fiber.The central wavelength is 2964.34 nm and the reflectivity is up to 99.27%.The mid-infrared FBG with a central wavelength of 3090 nm and a reflectivity of 99.12%is prepared by adjusting the period pitch of this FBG.And it is expected to achieve a long band FBG writing by this method.This work is beneficial to the construction of an "all-fiber" mid-infrared fiber laser,which is of great significance to promote the autonomy of the core device of a mid-infrared fiber laser in China.