Fabrication Of Fiber Grating Using Femtosecond Laser

Fiber grating is a crucial passive fiber device,which has widely been applied in fiber sensors,fiber communications and fiber lasers.The most common method to inscribe fiber grating is based on ultraviolet exposure,and the photosensitivity of fiber is necessary during grating inscription.Thus,hydrogen-loading and thermal annealing cannot be neglected.The photosensitivity of fiber is not that important in femtosecond laser inscription method,which will be studied in detail in this thesis,so that hydrogen-loading and thermal annealing can be neglected.Moreover,femtosecond laser inscription is more flexible and phase mask is not necessary in some situation,which is potential for inscribing fiber gratings with different characteristics to satisfy with different applications.Although femtosecond laser inscription has been developed for more than 20 years,most of them were applied in fiber sensors.Only a few of them were applied in high power fiber lasers.In this thesis,the fabrication technique of fiber grating using femtosecond laser and its application in high power fiber lasers are studied.1.The analysis of coupled mode theory and the simulation of coupling coefficient were carried out.Firstly,the modal distribution was simulated using finite element method,and then,the model of coupling coefficient was established based on coupled-mode equation.The coupling coefficients of type Ⅰ fiber Bragg grating(FBG),type Ⅱ FBG and long period fiber grating(LPFG)were calculated using the aforementioned simulation model.The theoretical results illustrated that the cross refractive index profile of fiber grating showed large influence on the coupling coefficient.These research works provided theoretical guidance for the fabrication of fiber gratings using femtosecond laser.2.Different categories of LPFGs were fabricated by femtosecond laser direct writing technique,and the basic characteristics of these LPFGs was investigated in detail.The inscription setup based on long distance objective lens was established.A preliminarily study on ultra-small period LPFGs with period less than 10μm was carried out.This kind of LPFG possessed the characteristics of LPFG and FBG in the same time.Ultra-small period LPFGs showed potential in refractive index sensing and polarization dependent devices.The spectral characteristics of square wave modulated type Ⅱ long period fiber gratings(LPFGs)inscribed by femtosecond laser was studied theoretically and experimentally.Research results indicated that higher order harmonics refractive index(RI)modulation commonly exist together with the fundamental harmonic RI modulation in this kind of LPFGs.The existence of higher order harmonics resonance increased the insertion loss.A novel and simple direct inscription method for long period fiber gratings(LPFGs)of arbitrary grating period by sinusoidal intensity modulated femtosecond laser was proposed and verified.This method induced a sine refractive index(RI)modulation,with which higher order harmonics resonance that exists in the normal square-wave modulation method was reduced.It was beneficial for reducing the insertion loss of LPFGs.These research works revealed the influence of the refractive index modulation alone the fiber axis,and the insertion loss of type Ⅱ LPFG was reduced by suppressing the higher order resonance.3.The fabrication of FBG using femtosecond laser line by line inscription and its application was carried out.The line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs had been studied.Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling.By aligning the lines position across the core region,the apodized line-by-line FBG was achieved.The basic characteristics of TFBGs inscribed line-by-line was studied.Experimental result showed that the location of grating plane,the tilted angle and the harmonic order of TFBGs could affect the transmission spectrum.Besides,torsion(or polarization)could also influence the spectrum of TFBG.Erbium doped fiber Bragg gratings(EDFBGs)using femtosecond laser line-by-line inscription was realized,and an integrated fiber oscillator was realized based on this technique.The laser property of the integrated oscillator was also experimentally tested.These research works realized the fabrication of highly localized FBG,which were meaningful in integrated fiber optics.Besides,the insertion loss of FBG inscribed line-by-line was extremely low,which was potential for fiber laser system.4.The fabrication of FBG in large mode area double cladding fiber was realized by femtosecond laser and phase mask.A fabrication system using visible femtosecond laser and phase mask was established.A pair of FBGs near 1080 nm were fabricated and utilized to construct a high power fiber oscillator.The maximum output laser power is more than 3.2 kW with a slope efficiency of～77.9%,and the beam quality factor M~2 is about 1.28.This research work confirmed the reliability of visible femtosecond laser in fabrication of FBGs intended to be applied in high power fiber oscillators.By optimizing fabrication process,a pair of FBGs near 1070 nm were fabricated.A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs,and a record output power of 5 kW was achieved with a slope efficiency of～82.1%,and the beam quality factor M~2 is measured to be～1.6 at the maximum power.This research work realized the maximum output power using femtosecond laser inscribed FBG.5.A pair of FBG were inscribed in a large-mode-area double-cladding ytterbium-doped fiber using femtosecond laser phase mask scanning technique.In this way,an integrated ytterbium-doped fiber active cavity was realized,and no splicing point occurred in this cavity.Then,an all-fiber oscillator was fabricated using this active cavity.The maximum output power was 520 W.Due to the existence of nonradiative transition process,the temperature of FBG went up dramatically with the pump power.Thus,cooling was needed.This research work provided a scheme for diminishing the number of splicing points in high-power fiber lasers.