The Studies Of Intracavity Frequency-doubled Raman Lasers And New Mode-locked Lasers

Stimulated Raman scattering (SRS) is an effective method for frequency conversion, which belongs to the third-order nonlinear optical process. By using the exiting laser sources and the SRS effect in Raman crystals, the solid-state Raman lasers can generate many new laser lines. The wavelengths of the generated scattered light are determined by the pump wavelengths and the Raman shifts of the Raman crystals. The laser spectrum with SRS can extend from the ultraviolet to the near infrared by using different pumping sources and Raman crystals. In recent years, some excellent Raman crystals have been produced with the improvement of crystal growth techniques. The all-solid-state Raman lasers also attract much interests for the advantages of compactness, high efficiency and high stability, and have wide applications in many fields such as information, communication, medical treatment, measurement, military affairs, and so on.The yellow-orange lasers with the spectral region from 550 nm to 650 nm have widely applications in metrology, remote sensing, medicine, and so on. But they are hardly obtained by frequency doubling Nd-doped lasers. With the Nd doped～1.06μm laser and the SRS in Raman crystal, the～1.18μm first Stokes wavelength can be generated. An efficient yellow-orange laser can be obtained by intracavity frequency doubling the first Stokes Raman laser.With the development of the laser technology, the ultra-short lasers also attract much attention. Ultra-short lasers usually have advantages of narrow pulse width, high peak power and broad laser spectrum and so on. They have been widely applied in such fields as defense, nuclear fusion, laser measurement, nonlinear frequency conversion, medical treatment, laser cutting, laser radar and so on. It is an important way to obtain picosecond and femtosecond laser pulse by using passively mode locking laser with saturable absorber.In this thesis, by using Nd:YAG as the gain medium, we studied the laser characteristics of the GdVO4, SrWO4, KLu(WO4)2 and BaW04 Raman lasers, respectively. By using the KTP intracavity frequency doubling of the LD pumped actively Q-switched Raman laser, the efficient yellow lasers were obtained. We also studied the laser characteristics of the LD pumped Nd:CYA, Nd:LSO and Nd:LYSO passively mode-locked lasers with SESAM as saturable absorber and the Er-doped fiber mode-locked laser with grapheme as the saturable absorber. The main contents of this thesis are as follows:1. An efficient LD end-pumped intracavity frequency-doubled Raman laser was demonstrated by using an Nd:YAG ceramic gain medium, an SrWO4 Raman medium and a KTP frequency doubling medium. With a coupled cavity, a 2.93 W 590 nm laser was obtained at an incident pump power of 16.2 W and a pulse repetition frequency of 20 kHz, the corresponding conversion efficiency was 18.1%.2. The characteristics of the LD end-pumped actively Q-switched Nd:YAG/BaWO4 Raman laser were studied. A 1.92 W 1180 nm laser was obtained with an optical-to-optical conversion efficiency of 17.4%. By using the KTP intracavity frequency doubling of the LD end-pumped Nd:YAG/BaWO4 Raman laser, a 2.83 W 590 nm laser was obtained. The corresponding optical conversion efficiency from diode laser to yellow laser was 17.9%. With a diode-side-pumped actively Q-switched intracavity frequency-doubled Nd:YAG/BaWO4/KTP Raman laser, an 8.3 W 590 nm laser was obtained at a 15 kHz pulse repetition frequency. The corresponding optical conversion efficiency from diode laser to yellow laser was 6.57%.3. The characteristics of the LD end-pumped actively Q-switched Nd:YAG/GdVO4 Raman laser were studied. At a repetition rate of 20 kHz and an incident pump power of 7.4 W, an average output power of 1.3 W was obtained, and the corresponding optical-to-optical conversion efficiency was 17.4%. Compared with Nd:GdVO4 self-Raman laser, the actively Q-switched Nd:YAG/GdVO4 Raman laser had higher average output power and conversion efficiency at the same experimental conditions.4. A diode-end-pumped actively Q-switched intracavity Nd:YAG/KLuW Raman laser was demonstrated. With a c-cut KLuW Raman medium, a 2.5 W 1178 nm Stokes laser was obtained at a 25 kHz pulse repetition frequency. The corresponding conversion efficiency from the pump laser to the Raman laser was 20.1%. By utilizing a b-cut KLuW crystal, 14 wavelengths including the first to ninth order Stokes wavelengths for the 88 cm-1 Raman shift (1074 nm,1085 nm,1095 nm,1106nm,1117nm,1128 nm,1139nm, 1151 nm and 1163 nm), the 1064 nm fundamental wavelength and other four wavelengths for the 757 cm-1 (1157 nm,1169 nm and 1182 nm) and 907 cm-1 (1178 nm) Raman shifts were obtained. This was the first time to obtain up to ninth order Stokes laser for the nanosecond pulse to our knowledge.5. A diode pumped passively mode locked Nd:CaYAlO4 (Nd:CYA) laser is demonstrated for the first time. Using a V-shaped cavity and a semiconductor saturable absorbing mirror, self-started mode locking of the laser was experimentally achieved. The mode-locked pulses were as short as 3.9 ps at a central wavelength of 1080.2 nm. The mode-locked laser produced a maximum average output power of 2.25 W with a slope efficiency of 23.2%.6. The passively mode locking operation of a diode-end-pumped Nd:LSO crystal was first experimentally investigated. Stable CW mode locking of the laser was achieved with a SESAM. The mode-locked pulses had a pulse width of 12.3 ps. When 14.5 W pump was absorbed,1.42 W average output power of the mode-locked pulses was obtained. The mode-locked laser had a slope efficiency of 16.7%.7. The diode-end-pumped dual-wavelength mode-locked laser based on Nd:LuYSiO5 crystal was demonstrated. With a SESAM, the 1075.8 nm and 1078.1 nm mode-locked pulses were achieved simultaneously with an 8.9 ps pulse duration. By frequency beating, ultrahigh repetition rate ultrafast pulse was generated with a 997 fs pulse width and a 0.59 THz repetition rate. A 1.7 W mode-locked laser was obtained with a 12.7 W pump power, the slope efficiency was 24.3%.8. The nanosecond square pulse erbium-doped fiber laser mode-locked with a graphene was demonstrated at a high repetition rate of 20.25 MHz. The generated pulsewidth was tunable from 1.7 ns to 7.2 ns depending on the pump power. The pulse energy was boosted to 1.09 nJ from the initial 0.02 nJ without distortion. The noise-like phenomenon was observed for the first time in an erbium-doped mode-locked fiber laser with a grapheme as the saturable absorber. The optical spectrum of the laser emission had a bandwidth as broad as 14 nm. The pulse width was 311 fs.The main innovations of this thesis are as follows:1. The efficient intracavity frequency-doubled actively Q-switched Raman laser was designed and studied. With a LD end-pumped actively Q-switched Nd:YAG/SrWO4/KTP laser, a 2.93 W 590 nm laser was obtained. The corresponding conversion efficiency was 18.1%, which was the highest conversion efficiency when the result was published. With a LD side-pumped actively Q-switched Nd:YAG/BaWO4/KTP laser, a 8.3 W 590 nm laser was obtained, which was the highest result in intracavity frequency-doubled actively Q-switched Raman laser to our knowledge.2. A LD pumped actively Q-switched KLuW Raman laser was demonstrated for the first time. With a c-cut KLuW Raman medium, a 2.5 W 1178 nm Stokes laser was obtained at a 25 kHz pulse repetition frequency. The corresponding conversion efficiency was 20.1%. By utilizing a b-cut KLuW crystal,14 wavelengths including the first to ninth order Stokes wavelengths for the 88 cm-1 Raman shift, the 1064nm fundamental wavelength and other four wavelengths for the 757 cm-1 and 907 cm-1 Raman shifts were obtained. This was the first time to obtain up to ninth order Stokes laser for the nanosecond pulse to our knowledge.3. The LD pumped Nd:CYA, Nd:LSO and Nd:LYSO passively mode-locked lasers were studied for the first time. With an Nd:CYA as gain medium, a 2.25 W CW mode-locked pulses were obtained with a 3.9 ps pulsewidth at a central wavelength of 1080.2 nm. With an Nd:LSO as gain medium, a 1.42 W CW mode-locked pulses were obtained with a 12.3 ps pulsewidth at a central wavelength of 1074.8 nm. With an Nd:LYSO as gain medium, the dual-wavelength mode locking phenomenon was observed. A 1.7 W mode-locked laser was obtained with an 8.9 ps pulse duration. By frequency beating, ultrahigh repetition rate ultrafast pulse was generated with a 997 fs pulse width and a 0.59 THz repetition rate.4. The nanosecond square pulse erbium-doped fiber laser mode-locked with a graphene was demonstrated at a high repetition rate of 20.25 MHz. The generated pulsewidth was tunable from 1.7 ns to 7.2 ns depending on the pump power. The pulse energy was boosted to 1.09 nJ from the initial 0.02 nJ without distortion. The noise-like phenomenon was observed for the first time in an erbium-doped mode-locked fiber laser with a grapheme as the saturable absorber. The optical spectrum of the laser emission had a bandwidth as broad as 14 nm. The pulse width was 311 fs.