Ultrafast Dynamics Characterization Of Fiber Laser And Its Sensing Applications

In recent years,fiber lasers and optical fiber sensors have become a research hotspot in the fields of optical communication and optical sensing.With the advantages of high beam quality,compact structure,and good heat dissipation,fiber lasers have been widely used in optical fiber sensing,precision spectroscopy,and other fields.For the advantages of anti-electromagnetic interference,corrosion resistance,high sensitivity,as well as high temperature and pressure resistance,optical fiber sensors have been widely used in many fields such as petrochemical,aerospace,and transportation.In the oil and gas extraction industry,to ensure the oil extraction safety and efficiency,real-time monitoring of the parameters such as temperature,stress,and torque is required.However,with the continuous increase of the scale of the oil exploitation,the geological structure and environment of the medium-deep oil well exploitation site have become extremely complex.The application of traditional electrical sensors has encountered great challenges due to the factors such as high temperature,high pressure,and strong magnetism.Therefore,the research of sensing technology with high stability,high signalto-noise ratio,high sensitivity,and strong networking capability has become an urgent need.Optical fiber sensors are passive components that use optical fiber as the sensing medium and realize the measurements based on optical principles.They can work stably under the conditions of the high temperature and pressure in the wells,which will bring new opportunities for the development of mid-deep resources exploration and dynamic monitoring technology.Aiming at the application requirements of optical fiber sensors in the oil and gas extraction industry,the thesis conducts research from three aspects: the research of pulse laser source,stability analyses,and sensor design.The main research contents include the design of a pulsed fiber laser based on a thulium-doped fiber(TDF)saturable absorber,the ultrafast dynamic characterization of fiber laser breathing effect and vector rotation effect,and the realization of fast response sensors for temperature,strain,and torsion.The specific research work of this thesis is as follows:(1)The saturable absorption effect of TDF near 1.6 μm wavelength was achieved and an L-band compact Q-modulated fiber laser based on a TDF saturable absorber was designed.The Q-switched fiber laser uses 1.4 m long erbium-doped fiber(EDF)as the gain medium and 1 cm long TDF as the saturable absorber.The maximum repetition frequency,maximum peak power,and pulse width of the output pulse were 11.4 k Hz,11.7 m W,and 19.5 μs,respectively.The experimental results show that the fiber laser has good stability.(2)The soliton spectral breathing effect in mode-locked fiber lasers was investigated.By using the time-domain stretched dispersion Fourier transform(TS-DFT)technique,the experiments found that there is a stable spectral breathing soliton in all-polarizationmaintaining mode-locked fiber lasers operating in anomalous dispersion region,and confirmed that the spectral breathing effect is a transition state in the process of soliton building up and splitting.When the spectral breathing effect occurs,the Kelly sidebands,energy exchange sidebands,and spectral envelopes in the spectrum of the laser output pulse will all undergo periodic wavelength and intensity modulation.Moreover,this effect exists before the soliton splitting in both the all-polarization-maintaining EDF laser with strong birefringence and the single-mode EDF laser with weak birefringence.(3)The soliton vector rotation effect in the mode-locked fiber lasers was investigated.Double-period polarization rotation-locked vector solitons(PRLVS)with single,double,triple,quadruple,and quintuple soliton states were obtained from a mode-locked laser made of the single-mode fiber(SMF).The dynamic evolution of these PRLVS was experimentally observed using a polarization-resolved real-time spectroscopic measurement device based on the TS-DFT technology.It is found that the spectral center wavelengths of the two orthogonal components of PRLVS are different,and the Kelly sideband wavelengths generated on the same polarization axis are also different.When period-doubling polarization rotation occurs,the spectrum of PRLVS will have a periodic wavelength modulation.In addition,by adjusting the birefringence parameters in the cavity,the laser can output PRLVS with both period-doubling vector rotation effect and spectral breathing effect.(4)A fast demodulation method based on the TS-DFT is proposed for temperature and strain fiber sensing.This method maps the FBG transmission spectrum into the time domain through TS-DFT technology and then uses high-speed digital sampling technology to quickly detect the wavelength offset,so as to realize the rapid demodulation of the sensing signal.A tandem FBG array sensing experimental system was built based on a homemade all-polarization-maintaining mode-locked fiber laser,and different demodulation strategies were used to achieve fast demodulation of temperature and strain information in the time domain.The demodulation rate can reach 42.2 MHz,temperature sensitivity of 12.92 ps/°C,and strain sensitivity of 0.64 ps/με were obtained.(5)A fast optical fiber torsion sensor based on the vector soliton projection technique was designed.Experimental and numerical results show that the spectral depression wavelength of the soliton molecule produced by the soliton projection linearly varies with the twist angle of the fiber.This linear relationship can help realize the detection of the twist angle and twist direction of the optical fiber.The measured twist sensitivity of the sensor is 0.46 nm/rad/m.Since the light source of the twist sensor is a vector soliton,the twist angle can be rapidly demodulated in the time domain by TS-DFT technology.Finally,a detection speed of 14.65 MHz and a temporal distortion sensitivity of 0.28 ns/rad/m were experimentally obtained.