The Studies On Self-modulations And Instabilities Of Lasers

In1960, Theodore Maiman invented the ruby laser considered to be the first successful optical or light laser. Lasers differ from other sources of light because they emit light coherently, including spatial coherence and temporal coherence. Due to the excellent characteristics of laser devices, the researches on the laser technology and application have been the focus of the psychics in recent50years. Now, the wavelength generated by the laser devices can be extended from the ultraviolet to the far infrared by using different laser technologies and different crystals. With the progress of the technology, laser devices have been used in an enormous variety of applications, including optical communication, medical treatment, measurement, military affairs, and so on.Lasers are viewed as nonlinear systems capable of exhibiting a wide variety of nonlinear dynamics. Laser systems would become unstable (even chaos) from continuous wave emission and show a number of nonlinear dynamics when lasers are subjected to the outside disturbances and the system noises. As a result, such dynamics would induce departures of laser emission from stable state, and are called instabilities of lasers, including self-mode locking phenomenon in all-solid-state Raman lasers, the instabilities of Raman laser induced by the variations of Raman crystal’s temperature, dark pulse induced by optical feedback in semiconductor laser, modulation instabilities in fiber lasers and dark soliton formation in fiber lasers. Such different dynamics can be used as a testing ground for verifying the theory behind the nonlinear dynamical systems. So the researchers in academia considered it an important branch of laser physics. Nowadays, both applied and basic researchers are studying the laser dynamics, and the control of the chaos has become an important topics.In this thesis, a variety of nonlinear dynamics in laser systems (all-solid-state laser, semiconductor laser and fiber laser), all of which are essential for the development of the light wave technology and have been the most widely used lasers in the applications, have been systematically studied, including the self-mode locking phenomenon in all-solid-state Raman lasers, the instabilities of Raman laser induced by the variations of Raman crystal’s temperature, dark pulse induced by optical feedback in semiconductor laser, modulation instabilities in fiber lasers and dark soliton formation in fiber lasers. On the one hand, the physical mechanisms of the self-modulation in all-solid-state Raman lasers and dark pulse emission induced by optical feedback in semiconductor laser are studied, and the conditions of generating such phenomena are presented. On the other hand, new types of lasers inducing tunable Raman laser, fiber lasers with high repetition rate pulses and dark soliton emission fiber lasers are achieved by virtue of the instabilities of lasers.The main contents of this thesis are as follows:1. The instabilities in time domain of the all-solid-state Raman lasers are demonstrated. The self-mode locking phenomena of Nd:YAG/BaWO4Raman lasers with different cavity lengths are studied. It is found that the Raman lasers can show perfect Q-switched mode locking pulses when the Raman laser are designed with longer cavity length, and the pulse duration of the ultrashort pulses is about240ps. And the experimental results show that there is an close relationship between the modulation depth of the mode locking and the pump power level. We present the experimental conditions for the self-mode locking: pump power near the stimulated Raman scattering (SRS) threshold and a relatively long cavity length. Furthermore, typical Q-switched Raman lasers can be operated with short cavity length and high pump power.2. The characteristics of the self-mode locking phenomena of second-stokes in all-solid-state Raman lasers are studied. The first-stokes line at1103nm and second-stokes line at1145can be obtained in an LD side-pumped actively Q-switched Nd:YAG/BaWO4Raman laser. The laser line at1103nm is generated through oscillation and that at1145nm is generated in single pass. Under a diode pump power of115W, an average output power of9.4W Raman laser is obtained. It is found that the laser line at1145nm generated in single pass shows more obvious self-mode locking modulation compared with that at1103nm generated through oscillation.3. The instabilities of Raman laser induced by the variations of Raman crystal’s temperature are studied. By virtue of such instabilities, a tunable Nd:YAG/YVO4intracavity Raman laser is achieved. The center wavelength can be tuned over a0.49nm range from1175.76nm to1175.27nm when temperature of the Raman crystal is adjusted from5℃to150℃. Simultaneously, the output power of the Raman laser decreases from550mW to75mW and the pulse width changes from8.0ns to12.3ns. There is a sharp decrease of the output power when the temperature changed from 400K to410K. The tunable Raman laser operates with near linear relationship to the temperature and has good beam quality.4. The instabilities of quantum well semiconductor lasers induced by optical feedback are studied. The lasers can show different dynamics when the lasers are subject to different optical feedback strength. It is found that under appropriate operation conditions the laser can also emit a stable train of dark pulses. The repetition frequency of the dark pulse is determined by the external cavity length. Splitting of the dark pulse is also observed. We speculate that the observed dark pulse is a kind of temporal cavity soliton formed in the laser. Based on the well-known Lang-Kobayashi equation, we theoretically study the dynamics in the semiconductor lasers with delayed feedback. When the lasers operate without optical feedback or with high feedback strength, the semiconductor lasers can operate at cw state without instabilities.5. The modulation instabilities of fiber laser operated in abnormal dispersion regime and normal dispersion regime are demonstrated, respectively. By virtue of such instabilities, fiber lasers can emit high repetition rate pulse trains, and the repetition rates are30GHz and26GHz, respectively. The corresponding pulse durations of such lasers are710fs and5.36ps. In the experiment, the mode locking fiber lasers are self-started by the nonlinear polarization rotation technology. And the measured spectra also confirm the phenomena of modulation instability.6. As a new type of instabilities in fiber laser, dark soliton is studied. According to the nonlinear Schrodinger equation (NLSE), dark soliton can be formed in normal dispersion fibers. Based on NLSE, the formation of dark solitons in all normal dispersion fiber lasers is demonstrated. The observed oscilloscope trace of dark soliton with pulse duration25ps evidences the existence of dark soliton in all normal dispersion fiber lasers. And, it is found that the dark pulse observed in fiber laser is generated due to the mutual interaction in the bunch of dark solitons.7. The dark soliton formation in a dispersion managed fiber laser is studied. Despite of the existence of a piece of abnormal dispersion fiber which does not support dark solitons, our experimental results show that as far as the average cavity dispersion is in the normal regime, dark solitons could still be formed in the fiber laser. Numerical simulations have well confirmed the experimental observations.The main innovations of this thesis are as follows: 1. Clear physical explanation of the mechanism of the Raman self-mode locking is presented for the first time. We conclude that the self-modulation originates from the self-modulation in fundamental lasers, and there is a temporal cleanup effect in the progress of SRS. As a result, the self-mode locking in Raman pulses would be more obvious.2. A high power diode-side-pumped Nd:AYG/BaWO4Q-switched Raman laser at1103nm is studied. Through carefully design the intracavity laser, the first-order Raman laser radiation at1103nm was obtained by the relatively weak Raman shift line332cm-1. When the pulse repetition rate was set at15kHz, the average output power obtained was up to9.4W under a diode pump power115W. This is the first time to report the high power Raman laser by a relatively weak Raman shift line.3. A tunable crystalline Raman laser by varying the temperature of Raman crystal is demonstrated for the first time. Nd:YAG and α-cut YVO4crystals are selected as the laser and Raman gain media, respectively. The center wavelength of this NdiYAG/YVO4Raman laser can be tuned over a0.49nm range from1175.76nm to1175.27nm when temperature of the Raman crystal is adjusted from5℃to150℃.4. The dark pulse with high repetition rate in quantum well semiconductor lasers is studied. It is found that under appropriate operation conditions the laser can also emit a stable train of dark pulses. The repetition frequency of the dark pulse is determined by the external cavity length. Splitting of the dark pulse is also observed. This is the first report of such instability in quantum well semiconductor laser. It is concluded that the train of dark pulses is not the unique phenomena of quantum dot semiconductor laser, but is a universal phenomena induced by optical feedback.5. Fiber lasers operated in abnormal dispersion regime and normal dispersion regime can emit high repetition rate pulse trains based on the modulation instabilities. The pulse repetition rates are30GHz and26GHz, and the pulse durations are710fs and5.36ps, respectively. It is found that modulation instabilities in fiber lasers are ubiquitous.6. The oscilloscope trace of an real dark soliton is presented for the first time, which evidences the existence of the dark soliton in normal dispersion fiber lasers. The formation of dark soliton in dispersion managed fiber lasers is demonstrated. Despite of the existence of a piece of abnormal dispersion fiber which does not support dark solitons, our experimental results show that as far as the average cavity dispersion is in the normal regime, dark solitons could still be formed in the fiber laser. Numerical simulations are in consistent with the experimental observations. This is also the first report of the dark soliton in dispersion managed fiber lasers.