Shunyan Wang

Diode-pumped passively mode-locked Nd:GSAG laser at 942 nm

Stable mode-locking laser pulse at 942nm by diode-pumped Nd:GSAG laser has been demonstrated for the first time. We obtained 8.7 ps pulses at repetition rate of 95.6 MHz and average output power of 220 mW.

Optimization of physical conditions for a diode-pumped cesium vapor laser

A diode-pumped alkali laser (DPAL) is thought to provide the significant promise for construction of high-powered lasers in the future. To examine the kinetic processes of the gas-state media (cesium vapor in this study), a mathematical model is developed while the processes including normal 3-enegry-level transition, energy pooling, and ionization are taken into account in this report. The procedures of heat transfer and laser kinetics are combined together in creating the model. We systemically investigate the influences of the temperature, cell length, and cell radius on the output features of a diode-pumped cesium vapor laser. By optimizing these key factors, the optical-to-optical conversion efficiency of a DPAL can be obviously improved. Additionally, the decrease of the output power due to energy pooling and ionization is also shrunk from 1.63% to 0.37% with the pump power of 200 W after optimization. It suggests that the effects of energy pooling and ionization should be decreased apparently under the optimal conditions. Basically, the conclusions we obtained in this study can be extended to other kinds of end-pumped laser configurations.

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.

Theoretical evaluation of a continues-wave Ho3+:BaY2F8 laser with mid-infrared emission

In this paper, we build a theoretical model to study a continues-wave (CW) Ho3+:BaY2F8 laser by considering both energy transfer up-conversion (ETU) and cross relaxation (CR) processes. The influences of the pump power, reflectance of an output coupler (OC), and crystal length on the output features are systematically analyzed for an end-pumped configuration, respectively. We also investigate how the processes of ETU and CR in the energy-level system affect the output of a Ho3+:BaY2F8 laser by use of the kinetic evaluation. The simulation results show that the optical-to-optical efficiency can be promoted by adjusting the parameters such as the reflectance of an output coupler, crystal length, and pump power. It has been theoretically demonstrated that the threshold of a Ho3+:BaY2F8 laser is very high for the lasing operation in a CW mode.

Theoretical analyses of a 1.617-um laser with a MOPA configuration

In the recent years, lasers around 1.6 μm are attracted much attention since their wavelengths fit the atmospheric transmission window and can be used for applications in a range of fields including laser radar, gas sensing, and free-space communications. As one of the lasing wavelengths of an Er:YAG medium is just located in the 1.6 μm region, such a laser has been gaining more and more extensive applications in the near infrared. Until now, rare literatures have been found in the MOPA (Master Oscillator Power Amplifier) study of a 1.617 μm Er:YAG laser because the effect of upconversion will become greater while a higher doping concentration is adopted. In this study, we theoretically analyze the amplification features of a 1.617 μm Er:YAG seed laser by using a multiple MOPA configuration. In the simulation, a kinetic model is established to investigate how the doping concentration, crystal length, and pump power affect the amplification efficiency of a seed laser. The results would be helpful to construct a feasible 1.617 μm laser system.

Analyses of absorption distribution of a rubidium cell side-pumped by a Laser-Diode-Array (LDA)

A diode-pumped alkali laser (DPAL) has been regarded as one of the most potential candidates to achieve high power performances of next generation. In this paper, we investigate the physical properties of a rubidium cell side-pumped by a Laser-Diode-Array (LDA) in this study. As the saturated concentration of a gain medium inside a vapor cell is extremely sensitive to the temperature, the populations of every energy-level of the atomic alkali are strongly relying on the vapor temperature. Thus, the absorption characteristics of a DPAL are mainly dominated by the temperature distribution. In this paper, the temperature, absorption, and lasing distributions in the cross-section of a rubidium cell side-pumped by a LDA are obtained by means of a complicated mathematic procedure. Based on the original end-pumped mode we constructed before, a novel one-direction side-pumped theoretical mode has been established to explore the distribution properties in the transverse section of a rubidium vapor cell by combining the procedures of heat transfer and laser kinetics together. It has been thought the results might be helpful for design of a side-pumped configuration in a high-powered DPAL.

Mode filling factor of a laser with end-pumped configuration

The mode filling factor is an important parameter especially in designing an end-pumped laser system. Although the end-pumped structure has been used for several decades, an overlap coefficient is often confused by many laser technicians. So far, there have usually been several conventional expressions using different formulae. A researcher often puzzles by some problems such as “which equation is a suitable one” and “how accurate the equation is.” Thus, it is necessary to construct a precise theory to systematically analyze the mode filling factor and investigate the difference among several traditional repressions. In this paper, an analytical model is first developed to study the mode filling factor for a pump beam and a laser beam with the modes in any order numbers. Then, several cases are discussed in which a pump beam and a laser beam are with a fundamental mode and a high-order mode as well as a hybrid mode, respectively. It has been demonstrated that the nonignorable deviation in the mode filling...

Study on energy transfer induced by collisions between rubidium and cesium

As two main atomic alkalis, rubidium (Rb) and cesium (Cs) have the similar energy-level structures. The energy transfer caused by collisions between rubidium and cesium atoms is a crucial factor for a vapor system. When such a vapor is irradiated with one component of rubidium resonance doublet, energy transfer will be induced by inelastic collisions between the excited rubidium atoms and the unexcited rubidium atoms, and between the excited rubidium atoms and the unexcited cesium atoms as well. It is noteworthy that the energy transfer between atomic rubidium and cesium is performed as cross relaxation. In this study, we theoretically investigate the effects of cross relaxations between the upper-state levels of atomic rubidium and cesium on the population distribution of the gas media. It has been demonstrated that the intensity of cross relaxations in this system is too weak to greatly affect the population distributions of atomic rubidium and cesium under the different temperatures. The conclusion might be helpful to better understand the physical features of alkalis.

Analyses of mode filling factor of a laser end-pumped by a LD with high-order transverse modes

Although the concept of the mode filling factor (also named as “mode-matching efficiency”) has been well discussed decades before, the concept of so-called overlap coefficient is often confused by the laser technicians because there are several different formulae for various engineering purposes. Furthermore, the LD-pumped configurations have become the mainstream of solid-state lasers since their compact size, high optical-to-optical efficiency, low heat generation, etc. As the beam quality of LDs are usually very unsatisfactory, it is necessary to investigate how the mode filling factor of a laser system is affected by a high-powered LD pump source. In this paper, theoretical analyses of an end-pumped laser are carried out based on the normalized overlap coefficient formalism. The study provides a convenient tool to describe the intrinsically complex issue of mode interaction corresponding to a laser and an end-pumped source. The mode filling factor has been studied for many cases in which the pump mode and the laser mode have been considered together in the calculation based on analyses of the rate equations. The results should be applied for analyses of any other types of lasers with the similar optical geometry.

Diode-pumped alkali lasers with a gradient temperature configuration

A diode-pumped alkali laser (DPAL) is one of the most promising candidates of the next-generation high-powered laser sources. Until now, a single-heater structure has been widely adopted to control the temperature of an alkali vapor cell in plenty of the DPAL studies. However, for an end-pumped DPAL using a single heater, most pump power can be absorbed by the gain media near the entrance window of a cell due to the large absorption cross section of atomic alkali. As a result, the temperature in the pumping area around the inputted window will be much higher than those in the other positions of the vapor cell. Such a large temperature gradient would bring about some negative influences on the output performance of a DPAL. Additionally, in the worst case, the inputted cell window may even be damaged, especially when the pump intensity becomes very high. To solve the problem, we put forward a new scheme by using a gradient heating process in which several heaters are simultaneously utilized to anneal an alkali vapor cell. In this technique, the temperature at the entrance window is set to be lower than that of the other side. Using this novel method, one can not only achieve a homogeneous absorption of pump energy along the cell axis, but also decrease the possibility of the window damage in the DPAL configuration. The theoretical simulation of the laser output features by use of multiple heaters has been carried out, and the optimum condition in temperature gradient is also discussed in this paper.