Energy-transfer Upconversion Effect Research Articles

High-performance diode-end-pumped Nd:YLF laser operating at 1314 nm.

A stable, efficient, and powerful 1314 nm Nd:YLF laser inband-pumped by a wavelength-locked narrowband 880 nm laser diode is demonstrated. The influence of mode-to-pump ratio on the performance of the diode-end-pumped Nd:YLF laser has been systematically investigated by taking into account the thermal effect and the energy transfer upconversion effect. For the optimum mode-to-pump ratio of 0.84, the maximum continuous wave output power of 21.9 W was extracted under the pump power of 70 W, which corresponded to the optical power efficiency of 31.3% and the beam quality of M2 ≈ 1.6. The resultant output power stability was determined to be 0.059% (RMS) within 1 h. In addition, by increasing the mode-to-pump ratio to 1.0, the near-diffraction-limited beam (M2 ≈ 1.3) was achieved with the output power of 17.0 W and the optical power efficiency of 24.3%.

High-power operation of double-pass pumped Nd:YVO4 thin disk laser

A simple, compact, double-pass pumped Nd:YVO4 thin disk laser is demonstrated. Its continuous-wave performance with different Nd doping concentrations and thicknesses is investigated experimentally. The maximum output power of 17.7 W is achieved by employing a 0.5 at.% doped sample, corresponding to an optical-to-optical efficiency of 46% with respect to the absorbed pump power. In addition, a numerical analysis and an experimental study of the temperature distribution, and thermal lens effect of the Nd:YVO4 thin disk, are presented considering the influence of the energy transfer upconversion effect and the temperature dependence of the thermal conductivity tensor. The simulated results are in good agreement with the experimental results.

Linear polarization output performance of Nd:YAG laser at 946 nm considering the energy-transfer upconversion**Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301401) and the Fund for Shanxi “331 Project” Key Subjects Construction, China (Grant No. 1331KS).

A theoretical model of quasi-three-level laser system is developed, in which both the thermally induced depolarization loss and the effect of energy-transfer upconversion are taken into account. Based on the theoretical investigation of the influences of output transmission and incident pump power on thermally induced depolarization loss, the output performance of 946 nm linearly polarized Nd:YAG laser is experimentally studied. By optimizing the transmission of output coupler, a 946 nm linearly polarized continuous-wave single-transverse-mode laser with an output power of 4.2 W and an optical–optical conversion efficiency of 16.8% is obtained, and the measured beam quality factors are and . The theoretical prediction is in good agreement with the experimental result.

The influence of energy transfer upconversion on Q-switched single resonator optical parametric oscillator

The influence of energy transfer upconversion on Q-switched single resonator intracavity optical parametric oscillator is investigated by a space-dependent rate equation model. The numerical analysis shows that the energy transfer upconversion effect causes the pump size to have an optimum for yielding the largest output power at a given pump power. To verify the presented model, the practical example of Q-switched Nd : YVO4—KTA single resonator intracavity optical parametric oscillator has been used. The optimum pump size, calculated by presented model is 320 μm which is reported 300 μm.

Modeling and optimization of actively Q-switched Nd:GdVO4 912 nm laser

The simulation and optimization of Q-switched Nd:GdVO4 quasi-three-level 912nm laser under Gaussian pump beam distribution is conducted with considering the spatial distribution of intracavity photon density. The steady state rate equation for the population accumulation involves the energy transfer upconversion effects while the rate equations with ground state absorption are used to describe the pulse generation process. The influence of the energy transfer upconversion and ground state absorption effects on the output characters of pulsed 912nm laser are estimated under different pump conditions. Based on the model, the optimized parameters of Q-switched Nd:GdVO4 quasi-three-level 912nm laser are obtained. This simulation method can be used to analyze and optimize the Q-switched quasi-three-level lasers under Gaussian beam pump.

Thermally induced diffraction losses for a Gaussian pump beam and optimization of the mode-to-pump ratio in an end-pumped Nd:GdVO4 laser

The analytical model of thermally induced diffraction losses for a Gaussian pump beam are derived as functions of the mode-to-pump ratio and pump power in end-pumped Nd-doped lasers considering the energy transfer upconversion effects. The mode-to-pump ratio is optimized based on it. The results show that the optimum mode-to-pump ratio with the thermally induced diffraction losses is less than 0.65, and it is less than the results in which the thermally induced diffraction losses are neglected. The theoretical model is applied to a diode-end-pumped Nd:GdVO4 laser operating at 1342 nm, and the theoretical calculations are in good agreement with the experimental results.

Design of a high gain single stage and single pass Nd:YVO_4 passive picosecond amplifier

A detailed comparison of the influence of pumping wavelength and crystal doping concentration on the performance of Nd:YVO4 amplifiers is developed through theoretical analysis. The effect of energy transfer upconversion and the strong temperature dependence of the emission cross section were taken into account. This study showed the interest of 808 nm pumping with low doping concentration crystals and the importance of the crystal temperature for the design of a high gain amplifier. Using these conclusions, we built a picosecond Nd:YVO4 master oscillator power amplifier reaching 10 W output power for only 50 mW of seed at 200 kHz in a single pass, single stage configuration. With a pulse duration of 22 ps, it corresponds to an output pulse energy of 50 μJ and to a peak power of 2.3 MW. With a same setup, a single pass small signal gain over 45 dB has been measured, and near 50% extraction efficiency was reached for a seed power of 3.5 W. The influence of the Nd:YVO4 amplifier temperature on the output power was also studied for different levels of gain saturation.

Quasi-three-level neodymium vanadate laser operation under polarized diode pumping: theoretical and experimental investigation

We propose a novel technique for pumping neodymium vanadate crystal in 4 F 3/2 → 4 I 9/2 transition with polarized diode light. With a theoretical model on quasi-three-level neodymium vanadate lasers including excited state absorption and energy transfer upconversion effects, the improvement on the laser performance of polarized pumping is evaluated. A maximum output power of 4.8 W in Nd:GdVO4 912 nm laser is achieved with the incident pump power of 21.8 W, the maximum output power increases about 85% and the slope efficiency is enhanced to 1.5 times towards the unpolarized pumping under the same condition. This technique is especially suitable for quasi-three-level systems end pumped by high-brightness fiber coupled diode sources associated with short neodymium vanadate crystals.

Effects of energy-transfer up-conversion and excited-state absorption in quasi-three-level Nd:GdVO4 lasers

We investigate in detail the effect of energy-transfer up-conversion (ETU) and excited-state absorption (ESA) in diode-end-pumping quasi-three-level Nd:GdVO4 lasers. The energy levels of Nd:GdVO4 crystals and the rate equation involving ETU and ESA effects are presented. The results of simulation show that the ETU effect is important in quasi-three-level lasers and can provide heating approximately two times higher than that induced by the ESA. Moreover, the relationship between the incident pump power and the 912 nm output power was simulated taking into account the ESA; a comparison with experimental data is also presented. As a result, we conclude that the ESA plays an important role in the high-power pumping field; the estimated equivalent cross-section σESA = (1.0 ± 0.5) ∙ 10−20 cm2.

Thermo-optic effects affecting the high pump power end pumped solid state lasers: Modeling and analysis

A complete solution of the thermal effect for continuous wave laser diode array fiber coupled high pump power end pumped actively solid state laser is presented. In this paper, the thermal loading resulted from energy transfer up-conversion effect for laser operated under different pulse repetition frequency has been taken into account in the overall thermal effects. The thermal loading transfer the absorbed pump power into non-uniform temperature distribution in the laser rod, and thus results in thermal strain, thermal stress, thermal dispersion of index, and thermally induced diffraction losses. For a practical laser cavity configuration containing the internal optical elements, we use spherical aberration theory to give a better explanation of experimental results via calculating diffraction losses quantitatively. According to analysis of thermal focal length experimental data, the results show that our calculated value of thermal loading under fixed Nd 3+ doped concentration laser crystal is not a constant but a function of pump power instead. The theoretical predicted curve of thermal focal length based on our model is in good agreement with experimental measurements. At low pump power, the thermal loading is a linear function varying with pump power, but it begins to saturate at pump powers exceeding 6 W. This phenomenon is consistent with our physical intuitions.

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.

Q-Switched Intracavity Frequency-Doubled Solid State Lasers: Influence of ETU Effect, Repetition Rate and ESA Effect

Our theoretical analysis showed that the overall intracavity frequency-doubled actively Q-switched solid state laser performance is affected by both pulse repetition rate and energy transfer up-conversion effect. The energy transfer up-conversion effect increased greatly and seriously degraded the laser performance especially when operating at high pumping power and low pulse repetition rate. The curves generated by numerical computation of the presented model can be used to predict the laser performance. We concluded that the pulse lengthening effect also appears with the influence of energy transfer up-conversion and is not related to the repetition rate. From the analysis, we found that the frequency doubling coefficient that Liu and Kim chose in their experiment was just the optimized value. Considering the effect of excited state absorption of saturable absorber into passively Q-switched intracavity frequency doubled solid state laser, the second threshold condition for this model is derived. A simple and very clear formula is obtained.

Modeling of active and passive Q-switched intracavity frequency-doubled solid state lasers

We present a model of high-power continuous-wave laser-diode pumped actively Q-switched intracavity frequency-doubled green laser, in which the energy transfer up-conversion effect was taken into account. Our theoretical analysis showed that the overall laser performance is affected by both pulse repetition rate and energy transfer up-conversion effect. The energy transfer up-conversion effect increase greatly and seriously degrade the laser performance especially when operating at high pumping power and low pulse repetition rate. The good agreement with the experimental measurements reported in previous paper demonstrates that energy transfer up-conversion effect have to be considered to predict the laser operational characteristics accurately. We also first derived the second threshold condition for passively Q-switched intracavity frequency doubled solid state laser by including the effect of excited state absorption of saturable absorber into this model. A simple, and very clear formula was obtained.

Influence of energy-transfer upconversion effect on the pulse in diode-pumped Cr :YAG passively Q-switched lasers

Influence of energy-transfer upconversion (ETU) effect on the output pulse in diode-pumped Cr:YAG passively Q-switched lasers is investigated. Using the ETU theory and Q-switching rate equations, the initial population inversion, the final population inversion and the threshold population inversion at the maximum photon density are achieved. The output pulse width and pulse energy depend on the relation between them. The predicted results are in good agreement with the experimental results. The method of reducing the ETU effect is also discussed and it is useful to optimize this kind of lasers.

Modeling of diode-end-pumped Q-switched solid-state lasers: influence of energy-transfer upconversion

The influence of energy-transfer upconversion (ETU) on diode-end-pumped actively Q-switched lasers is investigated by the space-dependent rate equation analysis. The numerical analysis reveals that as a consequence of the ETU effect there is an optimum pump size for yielding the largest pulse energy and a different optimum pump for yielding the highest peak power at a given pump power. The practical example of the diode-pumped actively Q-switched Nd:YAG laser has been used to verify the present model.

Repetition-rate dependence of thermal loading in diode-end-pumped Q-switched lasers: influence of energy-transfer upconversion

The fractional thermal loading in diode-end-pumped Q-switched lasers was estimated from the theory of cavity stability. The experimental results show that the thermal loading depends on the pulse-repetition rate. This dependence is attributed to the effect of energy-transfer upconversion (ETU). A theoretical model including the ETU effect was developed to analyze the fractional thermal loading in end-pumped Q-switched lasers. The theoretical predictions were found to be in good agreement with the experimental data.

Influence of energy-transfer upconversion on the performance of high-power diode-end-pumped CW lasers

The influence of energy-transfer upconversion (ETU) on the performance of diode-end-pumped lasers is studied by including ETU effects into the space-dependent rate equations. The dependence of ETU rates on the mode-to-pump size ratio and on the output mirror transmission is derived. Experiments for Nd:YAG and Nd:YVO/sub 4/ crystals are performed to validate the present model. It is found that the ETU process increases the sensitivity of the output power to the output mirror transmission but its influence on the output power is not significant for low output transmission.