The Stildies Of Novel Passively Mode Locked Lasers And Phenomenon Of Self Mode Locking In Raman Lasers

Laser technology had experienced a great development since the first successful laser was reported. The parameters of laser have perfected constantly, such as average output power, beam quality, pulse duration, and cost. Compared to other laser systems, all-solid-state lasers have the advantages of high efficiency, high stability, compact structure, and long lifetime. Due to these advantages, these kinds of lasers have been widely used in the fields of communication, material processing, medical, light display and so on.Generally, pulses with pulse durations of picoseconds and femtoseconds can be called as ultrashort pulses. The ultrashort pulses have the advantages of ultrashotr pulse duartion, high peak power, and broad spectrum and so on. Mode-locking technology is an efficient way to generate ultrashort pulses. Mode-locking technology can be divided into active mode-locking and passive mode-locking and the latter one can genetate much shorter pulse using saturable absorbers. In the recent20years, semiconductor lasers for optical pumping and optical saturable absorbers have dramatically improved these mode-locking lasers which have extremely short pulse durations (shorter than10fs), extremely high peak power (with amplifer, PW) and extremely high pulse repetition rates (greater than100GHz). Today, saturable absorbers used in passively mode-locked laser include Semiconductor saturable absorber mirror (SESAM), single-walled carbon nanometer tube (SWCNT), and Graphene. Compared to SESAM, SWCNT and Graphene have the advantages of broadband absorption, short recovery time, and easy fabrication.Double tungastates (KRE(WO4)2,RE=Gd, Lu and Y) crystals have been proposed to be ideal laser crystals with excellent properties of physics and chemistry. The high efficient laser operations have been obtained using rare earth ions doped KRE(WO4)2crystals. At the same time, KRE(WO4)2crystals belong to monoclinic system, space group C2/c. Due to their low symmetry, this kind of crystals have broader emission spectra, which are beneficial for mode-lcoking. Disordered crystals attract more and more attention as hosts for rare-earth ions in mode-locked lasers, because inhomogeneous broadening provides broader spectral emission than common crystals while compared to the even more broadband laser glasses, they exhibit higher thermal conductivity. Calcium-lithium-niobium-gallium garnet (CLNGG) is one of the typical disordered crystals.In this thesis, the work is focusd on novel passively mode-locked lasers. During experiments, we achieved some passively mode-locked lasers operating at different wavnlengths, using SESAM and SWCNT as saturable absorbers. Around1μm, Nd:K.Lu(WO4)2crystal and Yb:CLNGG crystal were used as the laser gaim medium; around1.5μm, a Cr:YAG crystal was used as the laser gain medium; around2μm, a Tm,Ho:(WO4)2crystal was used as the laser gain medium. Different SWCNTs and SESAMs with different paramaters were used to obtain mode-locking, and analyze the influence of the parameters of the saturable absorbers on mode-locking performance. We studied the characteristics of the passively mode-locked lasers and explained theses phenomena. We also studied the self mode-locking phenomenon in passively Q-swtiched Raman lasers.The main contents of this thesis are as follows:1. The characteristics of diode-pumped CW and passively mode-locked Nd:KLu(WO4)2lasers were studied.3.3W of1070.2nm CW laser was obtained with an absorbed pump power of4.4W. A Z-type cavity was used to archieve the passively mode-locked laser using SESAM as the saturable absorber. The maximum output power was1.4W, corresponding to a single pulse energy of6.8nJ. The pulse duration was11.5ps, corresponding to a peak power of0.6kW. During experiment, we obtained the phenomenon of pulse splitting at high pump power.2. The characteristics of a diode-pumped passively mode-locked Yb:CLNGG laser were studied. When the SWCNT was used as the saturable absorber, the minmum pulse duration of90.2fs was obtained, using an output coupler with a transmission of0.4%. When the transmission of the output coupler was3.0%, a maximum output power of90.5mW was obtained with a pulse duration of137.3fs. Appearance of CW component in the spectrum was the limit to obtain higher output power. When a SESAM was used as the saturable absorber, the shortest pulse duration of55.6fs was obtained, using an output coupler with a transmission of1.0%. When the transmission of the output coupler was3.0%, the maximum output power of150.5mW was obtained with a pulse duration of115.6fs. 3. The characteristics of a passively mode-locked Cr:YAG, using SWCNT as the saturable absorber, were studied. A pulse duration as short as49.6fs was obtained at1509.4nm. We studied the different performances of passively mode-locked lasers at different pump powers and found the optimum pump power for mode-lokcing. We used Frequency-Resolved Optical Gating (FROG) technology to measure the dispersion in the cavity.4. The characteristics of a Ti:sapphire laser pumped passively mode-locked Tm,Ho:KLu(WO4)2were studied. When SWCNT was used as the saturable absorber, a pulse duration as short as2.4ps was obtained. The spectrum centred at2058.9nm with a width of1.7nm, corresponding to a time-bandwidth product of0.3. When the transmission of the output couple was3.0%, the maximum output power of110.7mW was obtained. A pulse duration of7.2ps was obtained using SESAM as the satuarble absorber. During the experiment, we tested different saturable absorbers with different parameters and changed the dispersion in order to get fs pulses, unfortunately, we cound not get fs pulses.5. The self mode-locking phenomenon in all-solid-state Raman lasers was demonstrated. A intracavity passively Q-switched Nd:YAG/SrWO4/Cr:YAG Raman laser was used to study the phenomenon of self mode-locking. The self mode-locking phenomena with different cavity lengths, different small signal transmissions and different pump powers were studied. When the cavity length was longer than50cm, stable self mode-locking of Raman laser can be obtained. Self mode-locking phenomenon with100%modulation depth can be obtained with different pump powers and different small signal transmissions.The main innovations of this thesis are as follows:1. The characteristics of diode-pumped CW and passively mode-locked Nd:KLu(WO4)2lasers were studied. A pulse duration as short as11.4ps was obtained with a SESAM as the saturable absorber. This is the first report about passively mode-locked Nd:KLu(WO4)2laser. Our experiment demonstrated this kind of crystal is a candidate to get high power mode-locked laser with ps pulse duration.2. Passively mode-locked Yb:CLNGG laser, using SWCNT as the saturable absorber, was demonstrated for the first time. A pulse duration as short as90.2fs was obtained with a0.4%output coupler. This is the shortest pulses to our knowledge for a diode-pumped1-μm laser using transmissive SWCNT.3. A passively mode-locked Yb:CLNGG laser, using SESAM as the saturable absorber, was demonstrated for the first time. A pulse duration as short as55.6fs was obtaind with output power of60mW. This is the shortest pulses to our knowledge for Yb doped isotropic crystals.4. The characteristics of a Thsapphire laser pumped passively mode-lcocked Tm,Ho:KLu(WO4)2laser were demonstrated for the first time. Pulse durations as short as2.4ps and7.2ps were obtained, using SWCNT and SESAM as saturable absorbers respectively.5. The mode-locking phenomenon in passively Q-swtitched Raman laser was studied for the first time. Our experiment demonstrated that there was no obvious influence of small signal transmission and pump power on self mode-locking phenomenon.