| Compared with the conventional mode-locked lasers, the passively mode-locked fiber laser offers a number of advantages, including high efficiency, good stability, compact structure, small size, long lifetime and so on. It is widely applied in fields of optical communication, fast ignition laser fusion, laser plasma interaction, strong field physics, X-ray lasers and plasma diagnostics. It has been extensively studies in recent years. Yb (one of rare earth elements) can be doped into quartz fiber as gain medium of laser, or as energy transfer center when it is doped into quartz fiber together with other elements. Much interest has been paid to the investigation of Yb3+ doped fiber lasers, because Yb3+ has relatively simple level structure,wide absorption band and emission cross section. In the nonlinear polarization rotation(NPR) technique mechanism, electric fields of different intensity experience different nonlinear phase shifts because of self phase modulaton(SPM) and cross-phase modulation(XPM) effects, then combined with the principle of polarization filter, producing the intensity dependent loss of optical signal, to achieve the self-amplitude modulation (APM) ,realize mode-lock output at last. The passive mode-locked fiber laser based on nonlinear polarization rotation has significant progress recently, however ,the output power still needs to be improved. Futhermore, the lower pulse repetition rate also limits its applications, and there still exists lots of unsolved theoretical problems. Our research in this field is far away from foreign countries, so we must make our own mark in this field via scientific proof and innovation.According to deeply analyze the nonlinear polarization rotation mechanism, we construct the Yb3+-doped passively mode-locked fiber laser experimental system, in this thesis we study the continuous and pulse laser output state experimentally, analyze and explain the experimental phenomena, By optimizing the technique of laser system, the performance of laser improve significantly. The thesis consist of the four parts as follows:The first part,describe the application of mode-locked fiber laser, introduce the development prospects and its research background at home and abroad.The second part, the mode-locking principle of the passive mode-locked fiber laser based on nonlinear polarization rotation is stated systematicly, analyse the different characteristics while the laser system operating in the linear region and nonlinear region, and discussed the relationship of resonator parameters induced from the mode-locking time-domain equation, all of these help us to optimize the design of laser and understand the experimental phenomena.The third part, we study the single-wavelength tunable features of the laser system, realize the output of tunable wavelength by adjusting the azimuth of quarter wave plate and half wave plate, at last get the tunable range from 1030 nm to 1054 nm, tuning width up to 24 nm. In addition, through the method of joint a fiber solenoid into the cavity fiber greatly extended the laser tuning range, wavelength range can be achieved from 1021 nm to 1078 nm, width up to 57 nm. Experimentally measured laser threshold of the traditional structure is 49 mW, after joint fiber solenoid the threshold up to 75 mW. After adding fiber solenoid the laser's slope efficiency decreased from 35% to 28%, the main reason is that the bending of fiber solenoid bring additional oscillation optical transmission loss, but that loss can be reduced by change diameter of the fiber solenoid. Measured the output power instability is 0.11% at 81.8 mW with no fiber solenoid when the laser wavelength is 1030 nm, while use the fiber solenoid has no effect for the output stability of this laser. During the experiment we found that change the pump power can affect the output of single wavelength regularity and obviously, case study of 1032.77 nm and 1047.74 nm, we analyzed the evolution of wavelength process when increase pump power, found that it is have a smallest step of 0.345 nm when wavelength is moving, deviation of not more than 0.005 nm, then calculated the coefficient of linear birefringence equal to for this laser system. At the same time we studied the relationship between wave plate rotation angle and laser wavelength, found some experimental phenomena different from the previous' theoretical simulation, and propose amendments to this problem. Next, experimental study of the dual-wavelength tunable laser system output characteristics, and found two dual-wavelength output state consist of equal amplitude dual-wavelength and unequal amplitude dual-wavelength output, When the 980 nm pump lasers output power kept at 200 mW, by adjusting the quarter wave plate and half wave plate's azimuth, achieved Yb4 .39×10?43+-doped fiber ring laser tunable dual-wavelength output, obtained a continuous tuning width 13 nm equal amplitude dual-wavelength and continuous tuning width 5 nm unequal amplitude dual-wavelength output, the two wavelength interval of 19 nm, deviation of less than 2 nm. At the pump power 300 mW, we measured the dual-wavelength output power, maximum output power is 87.8 mW, the conversion efficiency of 29%, the minimum output power is 10.6 mW, the conversion efficiency of 3.5%. Shows that the conversion efficiency of different output wavelengths have a huge difference, which reflects the difference gain between different wavelength. we take 1029 nm-1048 nm dual wavelength output as an example test what will happened to its output characteristics when the pump power changed. When the pump power is 200 mW, by adjusting the wave plate in front of the polarization beam splitter(PBS) make the amplitude of two wavelength equal, and then increase or decreased the pump power to observed the variation of the laser wavelength, and give a explanation. Finally, illustrated the principle of multi-wavelength output in the passive mode-locked fiber laser based on the nonlinear polarization rotation, and realized from the dual-wavelength to seven wavelengths output, measured the range of its pumping power and maximum output power, summing up the main technical points which achieved multi-wavelength output;The fourth part, achieved a fundamental frequency mode-locked laser output, and measured the threshold of mode-locked is 142 mW, repetition rate is 29.93 MHz, single pulse energy is 3.4 nJ, at last give a reasonable explanation to the hysteresis phenomena of mode-locking. Experiments show that while the fiber without interference, the output pulse width,amplitude and its frequency spectrum will changed by adjust the wave plates, the reason mainly due to different state of polarization controls the coefficients of self-amplitude modulation for the nonlinear polarization rotation saturable absorption body. At higher pump power state, observed the passive harmonic mode-locked, achieved second,third and fifth harmonic mode-locked output, and made a detailed study of the second harmonic output. Experiments show that, the polarization state made a direct influence for the time interval and the relative amplitude between the primary and secondary pulse, it is have a great difference for the splitting threshold of the primary pulse at different state of polarization control, we measured the maximum splitting threshold is 314 mW and the minimum splitting threshold is about 227 mW.Through this thesis, a deeper understanding of the passively mode-locked fiber laser based on nonlinear polarization rotation is made, and found some experimental phenomena worthy of more intensive study, in addition proposed some technical program to improve the output characteristic of the laser system, overall we made our own contributions for this field of research. |
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