Duan Xiaoming

A Graphene-Based Passively Q-Switched Ho:YAG Laser

A 2.09-μm in-band pumped passively Q-switched Ho:YAG laser is demonstrated. Single layer graphene deposited on a quartz substrate is used as the saturable absorber for the Q-switched operation. The minimum pulse width of 2.11 μs is obtained at an average output power of 100 mW, corresponding to a pulse repetition frequency of 57.1 kHz and the pulse energy of 1.75 μJ. The beam quality factors M2 of the Q-switched laser are 1.18 and 1.22 in the horizontal and longitudinal direction, respectively. The optical-to-optical conversion efficiency of the passively Q-switched laser is 4.3%, which is the highest conversion efficiency in the 2 μm wavelength, to the best of our knowledge. It shows clearly that the Ho:YAG crystal is a potential gain medium in the 2 μm range for the graphene application.

Progress of a novel amorphous and nanostructured Si-B-C-N ceramic and its matrix composites prepared by an inorganic processing route

The development of novel, high-temperature structural and multifunctional heat-resistant materials for service under high-temperature oxidation, severe thermal shock, ablation by combustion gas flow, and other harsh conditions has become one of the most pressing needs in the aerospace industry. Amorphous and nanocrystalline Si-B-C-N ceramic and its matrix composites have great potential applications in the fields of high-temperature structural and multifunctional heat-resistant ceramics because of their special structures and outstanding high-temperature properties. The preparation of dense Si-B-C-N ceramic bulks is greatly limited by the organic polymer precursor pyrolyzing method (organic processing route). Mechanical alloying and post-hot-pressing technologies (inorganic processing route) using inorganic powders as the raw materials were pioneered by researchers from the Institute of Advanced Ceramics at Harbin Institute of Technology about 10 years ago. The inorganic processing route, which is very simple and effective in the preparation of materials with uniform microstructures and excellent properties, has been used to obtain dense Si-B-C-N ceramic bulks and structural components resistant to high temperatures. The inorganic processing route has become an indispensable complement to the organic processing route and has resulted in improved and enriched experimental data and understanding of Si-B-C-N ceramics. Current research achievements in the field of SiBCN ceramics and their matrix composites obtained by the inorganic processing route (including microstructural characteristics, evolution laws, mechanical and thermophysical properties, the resistance to oxidation, thermal shock and ablation, and related mechanisms) are summarized and future trends are predicted.

Judd--Oflet analysis of spectrum and laser performance of Ho:YAP crystal end-pumped by 1.91-μm Tm:YLF laser

The Ho:YAP crystal is grown by the Czochralski technique. The room-temperature polarized absorption spectra of Ho:YAP crystal was measured on a c-cut sample with 1 at% holmium. According to the obtained Judd-Ofelt intensity parameters Omega(2) = 1.42 x 10(-20) cm(2), Omega(4) = 2.92 x 10(-20) cm(2), and Omega(6) = 1.71 x 10(-20) cm(2), this paper calculated the fluorescence lifetime to be 6 ms for I-5(7) -> I-5(8) transition, and the integrated emission cross section to be 2.24 x 10(-18) cm(2). It investigates the room-temperature Ho:YAP laser end-pumped by a 1.91-mu m Tm:YLF laser. The maximum output power was 4.1 W when the incident 1.91-mu m pump power was 14.4W. The slope efficiency is 40.8%, corresponding to an optical-to-optical conversion efficiency of 28.4%. The Ho:YAP output wavelength was centred at 2118 nm with full width at half maximum of about 0.8 nm.

High Efficient Continuous-Wave Ho:YAG Laser Pumped by a Diode-pumped Tm:YLF Laser at Room Temperature

We present a high efficient continuous wave Ho:YAG laser pumped by a diode-pumped Tm:YLF laser with a Fabry–Perot etalon tuning at 1.91 μm. The maximum output power reaches 7.2 W when the absorbed pump power is 10.8 W. The slope efficiency (relative to the absorbed power) is 74.1%, and the Tm:YLF to Ho:YAG optical conversion efficiency of 60%, then the diode-to-Holmium optical conversion efficiency achieved is 21.0%. The wavelength is 2090 nm when the transmission of output coupler is larger than 20%. The beam quality factor is M2 ~ 1.15 measured by the travelling knife-edge method.

Passively Q-Switched Tm,Ho:YVO4 Laser with Cr:ZnS Saturable Absorber at 2 μm

A stable diode-pumped passively Q-switched Tm,Ho:YVO4 laser with Cr:ZnS saturable absorber is reported. The shortest pulse duration of ~500 ns with the central wavelength of 2041 nm is obtained at the pump power of 7.4 W, corresponding to the pulse energy of 3.5 ?J at repetition rate of 65 kHz.

A Resonantly Pumped Q-Switched Er:Lu2SiO5 Laser

We describe the cw and Q-switched actions of a novel erbium-doped crystal Er:Lu2SiO5 pumped by a MgO:PPLN-OPO at 1536 nm. An efficient 1580.9 nm cw laser with the output power of 608 mW is obtained when the incident power is 4.5 W, corresponding to a slope efficiency of 11.1%. In the Q-switched operation, the pulse energy up to 1.2 mJ is obtained at a repetition rate of 200 Hz.

Judd--Ofelt analysis of spectra and experimental evaluation of laser performance of Tm 3+ doped Lu 2 SiO 5 crystal

This paper reports that the TM3+:Lu2SiO5 (Tm:LSO) crystal is grown by Czochralski technique. The room-temperature absorption spectra of Tm:LSO crystal are measured on a b-cut sample with 4 at.% thulium. According to the obtained Judd-Ofelt intensity parameters Omega(2)=9.3155 x 10(-20) cm(2), Omega(4)=8.4103 x 10(-20) cm(2), Omega(6)=1.5908 x 10(-20) cm(2), the fluorescence lifetime is calculated to be 2.03 ms for F-3(4) -> H-3(6) transition, and the integrated emission cross section is 5.81 x 10(-18) cm(2). Room-temperature laser action near 2 mu m under diode pumping is experimentally evaluated in Tm:LSO. An optical-optical conversion efficiency of 9.1% and a slope efficiency of 16.2% are obtained with continuous-wave maximum output power of 0.67 W. The emission wavelengths of Tm:LSO laser are centred around 2.06 mu m with spectral bandwidth of similar to 13.6 nm.

A Diode-Pumped 1617 nm Single Longitudinal Mode Er:YAG Laser with Intra-Cavity Etalons

We demonstrate the continuous wave p-polarized single longitudinal mode (SLM) operation of an Er:YAG laser at 1617.6nm pumped by a diode-laser with three inserted Fabry—Perot (FP) etalons at room temperature. The Brewster angle inserted FP is applied to obtain the p-polarized laser. For free running, the maximum output power is 570mW with a pump power of 12.5 W. An incident pump power of 12.5W is used to generate the maximum p-polarized single longitudinal mode output power of 78.5mW, corresponding to a slope efficiency of 1.6% and an optical-to-optical efficiency of 0.61%. The beam quality M2 is measured to be 1.15 at the highest SLM output power. This stable polarized SLM oscillation is encouraging due to its application for an injection-locked system used as a master laser.

Efficient Diode-Pumped Tm:YALO3 Laser with a Pump Recycling Scheme

By using a pump recycling configuration, the maximum power of 8.1 W in the wavelength range 1.935-1.938 mu m is generated by a 5-mm long Tm:YAlO3 (4 at. %) laser operating at 18 degrees C with a pump power of 24 W. The highest slope efficiency of 42% is attained, and the pump quantum efficiency is up to 100%. The Tm:YAlO3 laser is employed as a pumping source of singly-doped Ho(l%):GdVO4 laser operating at room temperature, in which continuous wave output power of greater than 0.2 W at 2.05 mu m is achieved with a slope efficiency of 9%.

Progress on the formation of ceramics and ceramic-based composites through geopolymer precursors

Geopolymers have the advantages of being low cost, environmentally friendly, fabricated at low temperature, and possessing sound heat resistant and tunable thermal properties. They have extensive potential applications in the aerospace, nuclear, and chemical industries. As geopolymers are prepared at room temperature, diverse kinds of reinforcement, such as ceramic fillers, carbon nanotubes, and short or continuous fibers, can be easily applied. Therefore, together with proper high-temperature heat treatment, the geopolymer technique provides a novel route for the preparation of high-performance ceramics and their composites. This would be coupled with the advantages of being environmentally friendly and low cost. Upon heating, the geopolymer underwent evaporation of water, dehydroxylation, particle rearrangement, viscous sintering, nucleation and crystallization, and a new kind of glass ceramic, leucite, with high-strength, high melting point, and adjustable coefficient of thermal expansion, was obtained. On increasing the cesium substitution in place of the potassium, the amount of stabilized leucite increased and with 20 at% cesium substitution, leucite was fully stabilized in the cubic phase and the thermal expansion coefficient decreased sharply. After proper heat treatment, the Cf/geopolymer composite could be directly converted into the carbon fiber-reinforced leucite ceramic matrix composites, and the mechanical properties were greatly enhanced. For the carbon-fiber-reinforced K-based geopolymer composite heat treated at 1100℃, flexural strength, work of fracture and Youngs modulus reach their highest values, increasing by 102.3%, 29.1%, and 84.7%, respectively, relative to their original state before heat treatment. The increased strength was attributed to the cracks blunding and release of residual strength based on the integrity of the carbon fiber. The impregnated composites also showed considerable isothermal oxidation resistance, and after being oxidized at 900℃ for 60 min, the composites still retained approximately 50% of the original value. Therefore, geopolymer technology together with high-temperature heat treatment provides a new method to fabricate composites with low cost and high specific strength, which might be attractive for applications at elevated temperature. We report the progress on the thermal evolution, crystallization kinetics, and evolution of the microstructure and properties of geopolymers and carbon-fiber- reinforced geopolymer composites. Future research directions are also presented.