Review of Femtosecond-Laser-Inscribed Fiber Bragg Gratings:Fabrication Technologies and Sensing Applications

Fiber Bragg grating(FBG)is the most widely used optical fiber sensor due to its compact size,high sensitivity,and easiness for multiplexing.Conventional FBGs fabricated by using an ultraviolet(UV)laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures.Recently,the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra.The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types,even fibers without any photosensitivity.Such femtosecond-laser-induced FBGs exhibit excellent thermal stability,which is suitable for sensing in harsh environment.In this review,we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs.Firstly,the mechanism of femtosecond-laser-induced material modification is introduced.And then,three different fabrication tech no logies,i.e.,femtosecond laser phase mask technology,femtosecond laser holographic interferometry,and femtosecond laser direct writing technology,are discussed.Finally,the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized.Such femtosecond-laser-inscribed FBGs are promising in many industrial areas,such as aerospace vehicles,nuclear plants,oil and gas explorations,and advanced robotics in harsh environments.

Single-mode helical Bragg grating waveguide created in a multimode coreless fiber by femtosecond laser direct writing

We demonstrate the fabrication of single-mode helical Bragg grating waveguides(HBGWs)in a multimode core-less fiber by using a femtosecond laser direct writing technique.This approach provides a single-step method for creating Bragg grating waveguides.Specifically,the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity.Effects of pulse energy,helical diameter,and helical pitch used for fabricating HBGWs were studied,and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters,including a diameter of 10μm and a helical pitch of 1.07μm.The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW.Moreover,five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch,and this technique could be used for achieving a wavelength-division-multiplexed HBGW array.Then,the temperature and strain responses of the fabricated single-mode HBGW were tested,exhibiting a temperature sensitivity of 11.65 pm/℃ and a strain sensitivity of 1.29 pm/με,respectively.In addition,the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700℃ for 15 h.The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process,and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of∼7 dB and a“blue”shift of 0.36 nm.Hence,such a femtosecond laser directly written single-mode HBGW could be used in many applications,such as sapphire fiber sensors,photonic integrated circuits,and monolithic waveguide lasers.

Cyclin A2/cyclin-dependent kinase 1-dependent phosphorylation of Top2a is required for S phase entry during retinal development in zebrafish

Cyclin-dependent kinase 1 (CDK1) plays an essential role in cell cycle regulation.However,as mouse Cdk1embryos die early,the role of CDK1 in regulating the cell cycle and embryo development remains unclear.Here,we showed that zebrafish cdk1^(-/-)embryos exhibit severe microphthalmia accompanied by multiple defects in S phase entry,M phase progression,and cell differentiation but not in interkinetic nuclear migration.We identified Top2a as a potential downstream target and cyclin A2 and cyclin B1 as partners of Cdk1 in cell cycle regulation via an in silico analysis.While depletion of either cyclin A2 or Top2a led to the decreased S phase entry in zebrafish retinal cells,the depletion of cyclin B1 led to M phase arrest.Moreover,phosphorylation of Top2a at serine 1213 (S1213) was nearly abolished in both cdk1 and ccna2mutants,but not in ccnb1 mutants.Furthermore,overexpression of TOP2A^(S1213D),the phosphomimetic form of human TOP2A,rescued S phase entry and alleviated the microphthalmia defects in both cdk1^(-/-)and ccna2^(-/-)embryos.Taken together,our data suggest that Cdk1 interacts with cyclin A2 to regulate S phase entry partially through Top2a phosphorylation and interacts with cyclin B1 to regulate M phase progression.

Ln-substituted polytungstates as efficient hetergeneous catalysts for Knoevenagel condensation reaction

Three isomorphic polytungstates,CsgK_(18)H_(10){[Sm_(2)(H_(2)O)_(4)W_(4)O_(10)(AsW_(9)0_(33))3]_(2)(N(CH_(2)PO_(3))_(2))}:46.5H_(2)O(1),CS_(10)K_(9)H_(18){[Eu_(2)(H_(2)O)_(4)W_(4)O_(10)(AsW_(9)0_(33))_(3)]_(2)(N(CH_(2)PO_(3))_(2))}.41.5H_(2)O(2),Cs_(10)K_(9)H_(18){[Gd_(2)(H_(2)O)_(4)W_(4)O_(10)(AsW_(9)O_(33))_(3)]_(2)(N(CH_(2)PO_(3))_(2))}:46H_(2)O(_(3)),have been successfully synthesized and characterized by routine methods,and demonstrated excellent catalytic activities in Knoevenagel condensation reaction as heterogeneous catalysts.Notably,catalyst 1 achieved higher reaction activity than catalysts 2 and 3,where a satisfactory reaction yield(95%)and high TON value(6380)could be obtained at moderate reaction condition.In addition,in the scale-up experiment,with the help of catalyst 1,7.8g benzaldehyde and 5.7g ethyl cyanoacetate could transform into corresponding condensation product with a satisfactory yield(83%)and impressive TON value(13.883).