Weihong Hua

Experimental research of a chain of diode pumped rubidium amplifiers.

In this paper, we have set up a diode pumped rubidium MOPA system with a chain of two amplifiers. The experimental results show an amplified laser power of 26W with amplification factor of 16.3 and power extraction efficiency of 53% for a single amplifier, and an amplified laser power of 11W with amplification factor of 7.9 and power extraction efficiency of 26% for a chain of two amplifiers. The reason for lower performance of cascade amplification is mainly due to the limited total pump power, which will be not sufficient for efficient pumping when assigned from a single amplifier into two amplifiers. The situation could be well improved by increasing the seed laser power as well as the pump power for each amplifier to realize high efficient saturated amplification. Such MOPA configuration has the potential for scaling high beam quality alkali laser into high powers.

Fluorescence enhancing mechanism of optical repumping in sodium atoms for brighter laser guide star.

With quantum Bloch equations and sodium cell based experiment, we systematically resolved optical repumping mechanism of sodium atoms, a path to brighter sodium laser guide star (SLGS) that would be accepted by worldwide observatories. Besides the former studies, we detailed the population distribution of sodium atoms with and without repumping, which makes the repumping mechanism easy to understand. Experimental results based on a buffer gas free sodium cell and a single frequency laser implies that the optimum repumping frequency offset is 1712 MHz, and the repumping power fraction should be 10-18%. The light intensity depended character of repumping is also carried out. We learned that future SLGS pumped by higher effective light intensity would benefit more from repumping. To the best of our knowledge, this is the first systematic investigation of repumping mechanism using a single mode sum-frequency laser, which gives thorough support for previous numerical work and shows that the lab bench experiment could be used as an intermediate link between theoretical modeling and on-sky test.

Nozzle design in cw hydrogen fluoride chemical laser

Different geometry nozzles used in CW hydrogen fluoride chemical laser are investigated. Four geometry parameters, including throat width; area ratio; axis length and base width are considered. The flow properties, laser outcoupling power and small signal gain (SSG) of a Fabry-Perot resonator are calculated. The results show that when throat width and area ratio increase, the power and SSG peak will decrease in varying degree; they also varied when axis length changed; larger base width is related to lower cavity pressure, and smaller base width is related higher cavity pressure.

Real-time measurement of temperature rise in a pulsed diode pumped rubidium vapor laser by potassium tracing atom based absorption spectroscopy

In this paper, we first propose and demonstrate a novel tracing atom based absorption spectroscopy method for the real-time measurement of the temperature rise inside the pump region of a pulsed diode pumped alkali laser (DPAL). By artificially adding potassium atoms into the gain medium of an operational rubidium laser, the information of the temperature rise can be obtained from the variation of the potassium absorption signal. Some important influencing factors are studied. Typical results show that, as the pump power (2 ms duration) increases from 22 W to 92 W, the temperature rise increases from 103 K to 227 K. As the pulse duration increases from 1ms to 5 ms, the temperature rise increases from 128 K to 314 K, and the heat relaxation time increases from 3.8 ms to 8.1 ms. The method is favored for its ability for real-time detection and high sensitivity, which provides a useful way for DPAL diagnostics.

Experimental measurement of ionization degree in diode-pumped rubidium laser gain medium

We use optogalvanic method to measure the ionization degree in the hydrocarbon-free rubidium DPAL gain medium. The results show that the ionization degree increases as pump intensity and helium pressure increase, and presents rollover as rubidium concentration increases. A maximal ionization degree of ∼6.45×10(-6) has been obtained with pump intensity of 0.82  kW/cm2, temperature of 150°C and helium pressure of 500 Torr. Theoretical estimation shows that energy pooling is the main process rather than photoexcitation for the subsequent ionization processes.

Method for remotely measuring the spin-damping time of mesospheric sodium.

By using the return flux enhancement factor due to repumping, the spin-damping time of mesospheric sodium can be estimated. Because the absolute return photon number does not need to be measured, this method is independent of sodium abundance. An example of how to find the spin-damping time using this method is given. As a result, it is shown that this method is sensitive and has the potential to improve the precision of the spin-damping time estimations of mesospheric sodium. Finally, the impact of the geomagnetic field on this method is analyzed.

Simulation of diffraction effects of anti-reflection microstructures used as intra-cavity optical elements

The anti-reflection microstructures (ARMs) have been widely used due to their many advantages as compared with traditional films, such as high transmittance at required wavelength, high damage threshold, and resistance to corrosive environments etc. Some recent applications use ARMs as intra-cavity optical elements in laser systems. Due to the presence of microstructures, ARMs may also add diffraction effects on the features of the transmitted laser beams or the resonators eigenmodes. In this paper, we simulate the diffraction effects of ARMs that used as intra-cavity optical elements, and propose some further considerations.

DPAL research in Changsha

Alkali laser has been one of the most promising paths to high energy laser during past dozen years. As the first group realized DPAL and XPAL lasing in China, we had done lots of theoretical and experimental works to further clarify the mechanism of alkali lasers, such as exploring scaling parameters design balance and MOPA configuration amplified spontaneous emission suppression in DPAL based on our self-developed fast converging algorithm, XPAL’s continuous wave operation threshold, performance degradation of VBG narrowed diode laser array and stacks due to conductive thermal flow, heat deposition induced gas dynamic parameters variation estimation, local atomic number density change measurement with single frequency tunable diode laser, ionization and other higher level nonlinear effects with opto-galvanometer method. Based on above research works, preliminary c onsiderations and conclusions for alkali laser scaling are given.

Calculation of transmittance of diode pumped alkali laser in atmosphere propagation

Diode pumped alkali vapor lasers (DPAL) is a rising high-energy laser. The wavelength of which is consistent with the response curve peak position of solar cell, and it has broad application prospects in laser directed energy transfer. The paper bases on the application of solar unmanned aerial vehicle (UAV) energy transfer in high altitude and longendurance conditions. For the first time by using the MODTRAN and FASCODE, we calculate the transmittance of Potassium, rubidium, cesium laser in the typical atmospheric conditions vertically and different angles of atmospheric slant path by the numbers, The result shows that DPAL has a very high atmospheric transmittance, and also a valuable reference in other applications with the atmospheric transmission.

Efficiency enhanced coherent combining of fiber lasers through mutual injection locking

Coherent combination of fiber lasers through mutual injection locking is demonstrated experimentally in this paper. By moving the mutual injection couplers from the output port to the high reflection feedback port of the lasers, a modified combining configuration is constructed with obviously enhanced slope efficiency as compared with the conventional one. The laser efficiency increases from 29.7% to 37.8% by this modification. The corresponding maximum output power enhancement of the combined laser is 26.6%. This modification increases not only the individual child laser powers but also the combining efficiency. The physical connotation of the modification on the improvement of the laser performance has been discussed.