Investigation And Optimization Of Distributed-feedback Fiber Laser

After decades of development, fiber lasers and fiber amplifiers have been widely used in many areas, including communications, sensing, medical treatment, and industry and agriculture. In recent years, with the development of doped fiber production and fiber grating fabrication, the technology that fabricating gratings directly in doped fibers to constitute a linear cavity fiber laser has become increasingly more sophisticated. The linear cavity fiber laser is widely used in sensor field due to its small size, light weight, low noise, narrow line-width, and easily multiplexing capability, etc. Distributed feedback fiber laser (DFB) is formed by a π-phase shifted fiber Bragg grating written on a continuous length erbium doped fiber. The kind of laser which cavity length is usually of the order of centimeters is used to provide the coherent light output with narrow line-width and low noise.DFB fiber laser is widely used in the sensor field because of its high sensitivity, anti-electromagnetic interference, dynamic monitoring range, easy multiplexing capability. As the research started later in domestic, the detailed analysis and investigation of its characteristics is very important for its practical application. In this dissertation, through the theoretical and experimental study on the characteristics of DFB fiber lasers, the main research results of this dissertation are carried out as follows:1. The working principle of DFB fiber laser is described around the cavity structural design, and also the design of asymmetrical DFB fiber laser:the phase shift position type and the refractive index modulation depth type. Both the two types can obtain stable single laser output, and its working principle is also introduced.2. The fabrication system based on the UV argon ion laser and dither mask method is introduced. The system is able to produce any structure of the complex refractive index fiber, can accurately control the phase shift size and position, and fabricate multiple fiber lasers based on grating characteristics and different doped fiber. Finally, a brief description of the DFB fiber laser fabricated by our research team is given, as well as different package designs.3. The spectral characteristics of DFB fiber laser is investigated by the transfer matrix theory. The effect of grating parameters, such as the phase shift size, phase shift position, grating length, refractive index modulation depth on the transmission spectrum is analyzed. A novel method based on a distributed feedback laser diode (DFB-LD) continuous wavelength-scanning spectrum for acquiring the precise spectra of phase-shift fiber gratings is presented by experiments. Its main advantage is that it can get nearly continuous transmission spectrum of the phase-shift fiber grating. We experimentally demonstrate measurements of phase-shift fiber grating spectra with a resolution of23.5femtometer. A Signal-to-Noise Ratio (SNR) advantage can also be provided owing to a much higher spectral density of DFB-LD than ASE source.4. The laser threshold and slope efficiency of DFB fiber laser is investigated by theoretical simulation and experimental analysis. The two different types of DFB fiber lasers, symmetric and asymmetric DFB fiber laser thresholds and slope efficiencies are compared. Experimental results show that the output powers at both ends of symmetric DFB fiber laser are substantially equal, and the symmetric DFB fiber laser can obtain a stable one-way output. Through the theoretical and experimental verification, we obtain that the maximum laser output ratio of this asymmetric DFB fiber laser structure can reach to100:1.5. The relative intensity noise (RIN) characteristics of DFB fiber laser is investigated in detail. The theoretical model is deduced using rate equation theory. The contributions of several parameters, such as Er3+doped concentration, pump power and coupling coefficient on the RIN characteristics are analyzed theoretically. In experiments, the influence of external environmental noise is measured; the RIN of DFB fiber laser increases3.6dB/(Hz)1/2when the environmental noise increases7dB. The pump power fluctuation influence is also investigated. Compared to the theoretical analysis, the results obtained from the experiments testify the important role of coupling coefficient in the RIN characteristics of DFB fiber laser.6. The intensity response of DFB fiber laser to external acoustic excitation is investigated theoretically and experimentally. The theoretical model is deduced using rate equation theory. The transfer function for external acoustic excitation modulation has been obtained, and the intensity response characteristics of DFB-FL to external acoustic excitation are described by simulation. In experiments, an intensity modulated sensing system based on DFB fiber laser is constructed. The intensity response property of DFB fiber laser to external acoustic excitation with different pump power has been analyzed. Through experiments we observe a signal of16.5kHz with25dB in the RIN spectrum as the external acoustic pressure is10.25Pa. Both the theoretical and experimental results confirm that DFB fiber laser has potential to be used as intensity-type acoustic sensor. Acoustic pressure sensitivity with an average value between-165dB re μW/μPa and-185dB re μW/μPa at frequencies ranging from800Hz to9kHz of the intensity-type sensor has been obtained by experiments for the first time to the authors’ knowledge.7. Several important technical issues in a sensor array are analyzed, such as the pump absorption and external laser feedback, splice, fiber end reflection and Rayleigh scattering. A very desirable feature of asymmetric DFB fiber laser is undirectionality, and this obvious advantage has important applications to design sensor array. Larger output powers are obtained from shorter ends of all the four asymmetric DFB fiber lasers. The output flatness of the sensor array system presents a good performance with the applications of asymmetric DFB fiber lasers, especially, the back three have mostly the same amplitudes and the power equilibration among them is less than0.5dB.8. The RIN characteristics of the four element sensor array are investigated in detail. Relaxation oscillation frequency plays an important role in the investigation of sensor array. Because the relationship between the relaxation oscillation frequency and the actual absorbed pump power is not affected by external laser injection. By calibrating the relaxation oscillation frequencies, the corresponding pump powers are obtained, and the actural RIN can be obtained. The actual RIN and the measured RIN with different external laser injections of DFB-FL1and DFB-FL4are obtained and compared. The eaperimental results confirm that the RIN induced by the front external laser injections can be effectively reduced by using asymmetric DFB fiber lasers.