Xinlu Zhang

Multiple-beam, pulse-burst, passively Q-switched ceramic Nd:YAG laser under micro-lens array pumping

A novel four-beam (named laserI, laser II, laser III and laser IV, respectively), passively Q-switched, pulse-burst ceramic Nd:YAG laser under 2 × 2 micro-lens array pumping was demonstrated for the purpose of laser-induced plasma ignition (LIPI). Multiple-beam output together with pulse-burst mode in which both high repetition rate and high pulse energy can be realized simultaneously were obtained to greatly improve the performance of LIPI. The pulse-burst contained a maximum of 5 pulses, 3 pulses, 2 pulses and 3 pulses for laserI, laser II, laser III and laser IV, respectively, and the corresponding repetition rate of laser pulses in pulse-burst was 10.8 kHz, 7.2 kHz, 6.8 kHz and 5.2 kHz, respectively. The output energy for single laser pulse in pulse-burst was in the range of 0.12 mJ to 0.22 mJ.

Diode-pumped continuous wave and passively Q-switched Tm,Ho:LLF laser at 2 µm

A compact diode-pumped continuous wave and passively Q-switched Tm,Ho:LuLiF(4) laser is demonstrated. The maximal output power of 381 mW at 2069 nm in continuous wave regime is obtained at an absorbed pump power of 1.5 W. By using a Cr(2+):ZnS saturable absorber, the maximum Q-switched average output power of 74 mW is obtained at 2055 nm. The pulse width and pulse energy are almost independent of the absorbed pump power, with the maximal values of 1.2 μs and of 13 μJ respectively. The pulse repetition frequency can be tuned almost linearly from 1 to 5.8 kHz by changing the absorbed pump power. Furthermore, a comparative analysis of laser performances with different output couplers is first carried out.

Theoretical and experimental investigation of actively Q-switched Tm,Ho:YLF lasers

The quasi-three-level rate equation of continuous wave end-pumped actively Q-switched Tm,Ho:YLF laser is given under considering energy transfer up-conversion and ground state re-absorption. The analytical formula of the pulse energy is derived. The theoretical results show that the overall laser performance is affected by up-conversion effect, and the energy transfer up-conversion effect reduces the pulse energy and the upper level effective lifetime. The practical example of the diode end-pumped actively Q-switched Tm,Ho:YLF is used to verify the present model.

Diode-pumped actively Q-switched Tm, Ho:GdVO 4 /BaWO 4 intracavity Raman laser at 2533 nm

A compact diode-end-pumped actively Q-switched intracavity Raman laser with Tm,Ho:GdVO(4) laser gain medium and BaWO(4) Raman gain crystal is demonstrated for the first time. The Raman threshold is as low as 2.0 W of diode power at 802 nm. The highest average output power of 186 mW at the first Stokes wavelength of 2533 nm is obtained at a pulse repetition rate of 1 kHz, and a pump power of 2.8 W, corresponding to a diode-to-Stokes optical conversion efficiency of 6.6%. The pulsewidth and pulse energy are 7.8 ns and 0.19 mJ, respectively.

Diode-end-pumped continuously tunable single frequency Tm, Ho:LLF laser at 2.06 μm

We report on a laser diode-end-pumped continuously tunable single frequency Tm, Ho:LLF laser near room temperature. For transmission of 5%, the maximum single frequency output power of 221 mW at 2064.4 nm was obtained by using two uncoated etalons. The single frequency Tm, Ho:LLF laser operated on the fundamental transverse mode with an M2 factor of 1.13, and the output frequency could be tuned continuously near 1.5 GHz by angle tuning only of the 1 mm thick etalon. Furthermore, the influence of output coupler transmission on the laser performance was also investigated. The single frequency laser can be used as a seed laser for coherent Doppler lidar and differential absorption lidar systems.

The effects of energy transfer upconversion on end-pumped Q-switched Tm, Ho : YLF lasers

Based on the model of continuous wave end-pumped quasi-three-level system, the influence of energy transfer upconversion (ETU) on diode-end-pumped actively Q-switched Tm, Ho : YLF lasers is investigated by the rate equation analysis. The analytical expressions of the fractional thermal loading and the pulse energy are obtained. The influences of ETU on the fractional thermal loading and the output pulse energy are analysed. The theoretical results show that the ETU reduces not only the pulse energy but also the effective upper level lifetime of the actively Q-switched Tm, Ho : YLF laser. The practical example of the diode-end-pumped Q-switched Tm, Ho : YLF laser has been used to verify the present model.

Energy transfer enhanced laser cooling in Ho 3+ and Tm 3+ -codoped lithium yttrium fluoride

We report a theoretical scheme for laser cooling of solids based on energy transfer usually found in rare-earth codoped materials. The cooling scheme enables a large enhancement in the cooling efficiency with regard to the standard anti-Stokes fluorescence cooling. A Ho3+ and Tm3+-codoped low-phonon crystal (LiYF4) sample is investigated to find that the cooling efficiency increases, and then decreases with the increasing of the resonant absorption. The optimal cooling efficiency is predicted to exceed 5%. The maximum cooling power density could be promoted greatly by applying the codoped cooling scheme. The cooling scheme is also valid for other rare-earth (for example, Tm3+ and Er3+, or Er3+ and Yb3+) codoped materials.

End-pumped optical bistability Tm,Ho:YLF laser

We experimentally demonstrate strong optical bistability in a 2 μm continuous wave Tm,Ho:YLF laser with a bulk crystal. The laser is end-pumped by a 792 nm fiber-coupled laser diode. The bistable region is as wide as 1.6 W and the jump power at the turning point is as high as 82.5 mW when the temperature of the laser crystal is kept at 253 K. The influences of the cavity length and the transmission of the output coupler on the characteristics of optical bistability are obtained. The theoretical analysis shows that the bistable output in the Tm,Ho:YLF lasers comes from the combined effects of the nonlinear saturation of ground state reabsorption, the energy transfer upconversion, and the excited state absorption.

Formation mechanism of optical bistability in end-pumped quasi-three-level Tm, Ho:YLF lasers

We report on optical bistability and its formation mechanism in an end-pumped quasi-three-level 2 μm Tm,Ho:YLF laser with only a bulk laser crystal. We have experimentally observed that the characteristic output versus input curve is not linear and shows bistability when the laser crystal is singly end pumped by a 792 nm laser diode. The width of the bistable region is 1.6 W and the jump power at the turning point is 82.5 mW. However, the laser shows linear output when the laser crystal is dual end pumped. The formation mechanism of optical bistability is explained by solving coupled rate equations in which energy transfer upconversion and ground-state reabsorption are considered.

Diode-pumped actively Q-switched Tm:YAP/BaWO 4 intracavity Raman laser

We report an intracavity Raman laser based on BaWO(4) Raman conversion in a diode-pumped actively Q-switched Tm:YAP laser for the first time. With an incident diode power of 17.5 W and a pulse repetition rate of 1 kHz, the maximum average output powers of 880 mW and 306 mW for the fundamental laser at 1.94 μm and the first Stokes laser at 2.36 μm were obtained, respectively. The pulse width and pulse energy of the first Stokes laser were 14.1 ns and 0.31 mJ, respectively. The Raman gain coefficient of the BaWO(4) crystal was estimated to be 1.1 cm/GW at 1.94 μm.