Zhigang Jiang

Influence of energy pooling and ionization on physical features of a diode-pumped alkali laser

In recent years, a diode-pumped alkali laser (DPAL) has become one of the most hopeful candidates to achieve the high power performance. A series of models have been established to analyze the DPALs kinetic process and most of them were based on the algorithms in which only the ideal 3-level system was considered. In this paper, we developed a systematic model by taking into account the influence of excitation of neutral alkali atoms to even-higher levels and their ionization on the physical features of a static DPAL. The procedures of heat transfer and laser kinetics were combined together in our theoretical model. By using such a theme, the continuous temperature and number density distribution have been evaluated in the transverse section of a cesium vapor cell. The calculated results indicate that both energy pooling and ionization play important roles during the lasing process. The conclusions might deepen the understanding of the kinetic mechanism of a DPAL.

Analysis of characteristics of a diode-pumped rubidium vapour laser using a kinetic algorithm

We consider diode-pumped alkali vapour lasers (DPALs), which make it possible to achieve a high output power. Characteristics of such DPALs strongly depend on the physical properties of buffer gases and on structural parameters of a vapour cell. Special attention is paid to a diode-pumped rubidium vapour laser (DPRVL): We have investigated the effect of different conditions on its characteristics. The results show that the linewidth of the D2 line of a DPRVL and the fine-structure mixing rate between two excited energy levels, which are two crucial factors in implementing a high-power DPRVL, increase with the pressure of buffer gases and the temperature of the vapour cell. It is demonstrated that the population ratio of two excited energy levels is close to that corresponding to a thermal equilibrium as the pressure of buffer gases and the temperature of the vapour cell become higher. We have found that the optimal values of the methane pressure, the cell temperature and the cell length can be determined through a kinetic analysis. The conclusions can be valuable for designing configurations of an end-pumped DPAL.

Influence of deviation in central wavelengths of both a seed laser and a pump LD on the output features of a DPAL-MOPA system

A master oscillator power amplifier (MOPA) is thought to be a suitable equipment to realize the power scaling for a diode pumped alkali laser (DPAL). In fact, the characteristics of a DPAL-MOPA system strongly depend on the central wavelengths of both a seed laser and a pump laser due to the extremely narrow nature linewidth for atomic alkali. In this report, a theoretical model of an end-pumped DPAL-MOPA system is first developed to study the influence of deviations in central wavelengths on the output features. Then, the relationship between the environmental parameters and the output linewidth as well as the output power is analyzed. The results reveal that the deviation in central wavelengths of both a seed laser and a pump LD will lead to a dramatic decrease of the output power for a DPAL-MOPA system. The conclusions are thought to be helpful for design of an end-pumped DPAL with high powers.

Influence of excitation on physical features of a diode-pumped alkali laser

In recent years, a diode-pumped alkali laser (DPAL) has become one of the most hopeful candidates to achieve the high power performance. A series of models have been established to analyze the DPAL’s kinetic process and most of them were based on the algorithms in which only the ideal 3-level system was considered. In this paper, we developed a systematic model by taking into account the influence of excitation of neutral alkali atoms to higher levels on the physical features of a static DPAL. The procedures of heat transfer and laser kinetics were combined together in our theoretical model. By using such a theme, the continuous temperature distribution has been evaluated in the transverse section of a cesium vapor cell. The calculated results indicate that the excitation plays an important role during the lasing process, which might deepen the understanding of the kinetic mechanism of a DPAL.

Reviews of a Diode-Pumped Alkali Laser (DPAL): a potential high powered light source

Diode pumped alkali vapor lasers (DPALs) were first developed by in W. F. Krupke at the beginning of the 21th century. In the recent years, DPALs have been rapidly developed because of their high Stokes efficiency, good beam quality, compact size and near-infrared emission wavelengths. The Stokes efficiency of a DPAL can achieve a miraculous level as high as 95.3% for cesium (Cs), 98.1% for rubidium (Rb), and 99.6% for potassium (K), respectively. The thermal effect of a DPAL is theoretically smaller than that of a normal diode-pumped solid-state laser (DPSSL). Additionally, generated heat of a DPAL can be removed by circulating the gases inside a sealed system. Therefore, the thermal management would be relatively simple for realization of a high-powered DPAL. In the meantime, DPALs combine the advantages of both DPSSLs and normal gas lasers but evade the disadvantages of them. Generally, the collisionally broadened cross sections of both the D1 and the D2 lines for a DPAL are much larger than those for the most conventional solid-state, fiber and gas lasers. Thus, DPALs provide an outstanding potentiality for realization of high-powered laser systems. It has been shown that a DPAL is now becoming one of the most promising candidates for simultaneously achieving good beam quality and high output power. With a lot of marvelous merits, a DPAL becomes one of the most hopeful high-powered laser sources of next generation.

Composite thermally-induced focusing of a probe beam with Gaussian-distribution propagating through a cesium cell end-pumped by a LD

Diode-pumped alkali lasers (DPALs) have been developed in the recent years for their high Stocks efficiency, good beam quality, compact size and near-infrared emission wavelengths. Dividing a cylindrical vapor cell into many cylindrical annuli, we first obtain parameters such as the population density distributions, transition rates of pump photon absorption, and transition rates of laser photon emission of every annulus by use of a mathematical algorithm combining the procedures of heat transfer and laser kinetics together. After exploring the radial temperature distribution at the transverse section of a cesium vapor cell, we acquire the refractive index n and the thermal-optic coefficient dn/dT of each cylindrical annulus when the cesium vapor cell is end-pumped by a LD module. And then, we undertake a ray-trace theme to investigate the propagation of the incident beam with a perfect Gaussian distribution inside the vapor cell. By adopting the algorithm based on the complex self-consistency analyses in lens-like media, the intensity distributions of the probe beam outputted from an enclosed cesium vapor cell have been systematically evaluated for different pump powers. It has been seen that the intensity distributions of a Gaussian-distribution probe beam are different from those of a probe beam with flat-top-distribution. Finally, we get the effective focal length of a thermally-induced lens in each cylindrical annulus for different pumped-powers. It is obvious that the thermally-induced focusing phenomena for a Gaussian-distribution probe beam are much more serious than those for a flat-top distribution probe beam. These results will be helpful to improve the beam quality of a high-powered DPAL.

Phase distortion of a probe beam transmitting through a transparent medium bulk

A highly feasible full-field phase-measuring methodology has been developed to investigate the phase distortions of a probe beam transmitting through a LiB3O5 (LBO) crystal under different conditions. The results show that wavefront phase exhibits inhomogeneous distribution when the crystal is heated and the phase difference becomes small as time goes by when the heating temperature is kept unchanged. The technique provides an easy and feasible way to accurately measure the phase images of a probe beam transmitting through a crystal. The procedure provided in this report can be also used to study the rapid phase changes that take place in other types of optical materials.

Evaluation of population densities of a rubidium laser pumped with a narrow-linewidth Ti:Sapphire laser

In the recent years, alkali vapor lasers have become the most promising candidates for realization of a light source with both good beam quality and high output power because of their excellent performances. A rubidium laser is a typical kind of alkali vapor lasers. In this study, we developed a theoretical mode to evaluate the population densities of three levels as well as mode-matching efficiency of a rubidium laser pumped by a narrow-linewidth Ti:Sapphire laser under two different focal conditions. In the evaluation, at least two values of the output power of a rubidium laser must be known, which are usually obtained from the experiment. We performed a series of experiments by using two pump lenses with two focal lengths of 150 and 200 mm, and acquired two corresponding values of the laser output power. Then, we applied the experimental results in the theoretical calculation and then obtained the population densities of three levels and mode-matching efficiency of the rubidium laser. The study demonstrates that the outputted pump power can be used for evaluation of the population densities of three levels and mode-matching efficiency of an alkali laser under the different experimental conditions. The study might be valuable to better understand the physical features of an alkali vapor laser and to optimize the configuration of a high-powered alkali laser in the future.

Narrowing the linewidth to a desired value with a reflecting volume-Bragg-grating (RVBG) for high-powered LDs

In recent years, reflecting volume-Bragg-gratings (RVBGs) have been considered as the perfect elements to narrow the spectrum with highly adjustable parameters. In fact, the effects of narrowing the linewidth of high-powered LDs are determined by the parameters such as the grating thickness, refractive index modulation, and grating spatial frequency. By use of the theoretical regime, the relationships between the effective diffraction efficiency and these parameters of a RVBG have been systematically investigated to narrow the linewidth to a desired value. Although the results reveal that a higher effective diffraction efficiency may be achieved by adopting three parameters mentioned above with higher values, by considering the difficulties and confinement in production process of a RVBG, the characteristic parameters should be carefully selected as some appropriate values. From the evaluation, one also understands that the narrow linewidth of a RVBG-coupled LD and the high effective diffraction efficiency cannot be arbitrarily realized at the same time for a real case. The conclusions are thought to be valuable in construction of a practical narrow linewidth LD system.

Output features of two types of diode-pumped alkali vapor lasers in time domain

Diode-pumped alkali lasers (DPALs) have gained rapid development in the recent years due to their great potential for realization of high-power lasers. Relaxation oscillation (RO) is a common phenomenon related to the dynamic process in time domain. Generally, the generation of spikes in RO may affect the output stabilization of a DPAL. In this paper, we develop a kinetic model to investigate the output characteristics in time domain. Using such a mathematical model, a comparative study on the RO features of a diode-pumped rubidium vapor lasers (DPRVL) and a diode-pumped cesium vapor lasers (DPCVL) is theoretically carried out with different physical parameters including the cell temperature, buffer gas pressure, pumping power and reflectance of an output coupler. The analyses should be valuable for design of a steady high-powered DPAL.