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Since the aperture obstruction contributes to an energy leak from the core to the sidelobe of the far field spot, the Gaussian mode of the single-mode fiber (SMF) will receive less energy leading to the coupling efficiency (CE) degradation. A method based on diffraction optical element (DOE) and wavefront corrector is introduced to reshape the plane wave with hollow uniform intensity into the plane wave with uniform intensity to improve the core energy of spot. The SMF CE with and without DOE for different central obscuration ratio is studied by numerical simulation. The DOE is effective to improve the SMF CE. Without the DOE, the CE decreases from 81% to 54% when the central obscuration ratio increases from 0 to 0.4. Compared with 54% CE at the of 0.4, the CE can increase to 72%, 78% and 80% with 4 level, 8 level and 16 level quantized DOE, respectively. It is not necessary to use the DOE beam shaper when the central obscuration ratio is small. The 4 level, 8 level and 16 level quantized DOE can obtain benefits of the CE when the central obscuration ratio is above 21.6%, 11.5% and 7%, respectively.

This paper investigated the fire-retardant mechanism of the nano-LDHs in the intumescent system by the temperature programmed oxidation (TPO). Researches were also conducted to explore the function of the nano-LDHs in the composite fire-retardant agents in air and nitrogen atmosphere, respectively. The results indicated that the nano-LDHs species were responsible for the catalytic oxidation of the rich-carbon compound in oxygen atmosphere. In addition, the nano-LDHs species and their calcinated products at high temperature could increase the carbonaceous residue-shield of the carbon-rich materials, improve the quality and the graphitization degree of the formed char-layer, and accelerate the intumescence and expansion of the melting carbon-rich materials to a certain degree under the oxygen-free condition, leading to the carbonization and expansion of the intumescent layer.

The 946nm diode-pump microchip self-Q-switched laser of a chromium and neodymium codoped yttrium aluminum garnet crystal material (Cr4+Nd3+:YAG) is studied, especially about its physical mechanism of operation. The 4F3/2 → 4I9/2 transition of Nd3+ ion is beneficial to achieving laser oscillation in a quasi-three-level system based on coating the cavity mirrors of the microchip with films that suppress the 1064nm operation and enhance the 946nm laser. The Cr4+ ion is a saturable absorber. The initial loss Nt1 is high, which acts as the threshold for laser oscillation. The stable loss Nt2 is low because the Cr4+ ion is acceleratively bleached by the fast enhancement of the oscillating laser. The high Nt1, small Nt2 and fast progresses permit the oscillating laser of the Cr4+Nd3+:YAG to have a good self-Q-switched property whose full width at half maximum is about 4.2ns. Its highest laser power is about 5.7mW. Its peak power is about 150W. Its good fundamental transverse TEM00 mode results from the absorption bleaching established by both the pump and oscillating lasers, which suppress other transverse mode and allow the oscillation only in the fundamental transverse TEM00 mode.

High power fiber lasers and amplifiers are widely used in the scientific and industrial field. In order to meet the requirements for high output powers the effective area of fibers becomes larger and larger to reduce optical nonlinearities. With the increase of effective area, the number of high-order modes will increase. In the case of high output power, the spectral shift and broadening of the optical fiber will also affect the modal number and content. The number and content of fiber modes affect the pointing stablity and quality of the laser beam. The M2-parameter is commonly used to define the quality of the laser beam, but a small M2 number is not guaranteed for single mode operation. Therefore, the relationship between wavelength and transmission mode in fiber transmission is studied in this paper. We use the spatial and spectral Fourier transform (F2) method to establish a theoretical-experimental method of describing the relationship between wavelength and mode. This method can directly give out the modal content of optical fibers without any priori parameter such as the properties of fiber and requirement for setup accuracy. On the one hand, the theoretical modeling of wavelength affects modal content. In the simulation, the sources with the same wavelength bandwidth and different central wavelengths are used to test the fiber. The results show that the modal content and number of the fiber change with the wavelength bandwidth and center wavelength. The mode components of the corresponding optical fiber will change after changing the central wavelength. As the spectral width of the light source increases, the number of high-order modes increases. On the other hand, in order to further verify the relationship between wavelength and mode of fiber, the F2 method is used to measure the optical fiber modal content with different wavelengths. The final experimental results are in agreement with the theoretical results. The experimental and simulation results show that the mode field distribution of each mode varies with wavelength:the longer the wavelength, the larger the mode field is. The beam quality has little change with the wavelength except for those positions with frequency near the cutoff frequency, and the power ratio of each mode relates to the wavelength.

We investigate the relationship between the beam intensity and the four-wave mixing diffraction efficiency of the longitudinal-field multiple-quantum-well (LMQW) photorefractive device grown at low temperature. A optimum beam intensity is found. We also explain the different mechanisms of the beam intensity effect on the diffraction efficiency of the LMQW device and the transverse-field MQW device. Some advice on how to improve the diffraction efficiency is given.

A stable low-threshold self-Q-switched diode-pumped 946-nm Nd(3+), Cr(4+):YAG microchip laser operating in single frequency and fundamental transverse mode is reported. The output characteristics of the microchip laser were investigated under cw and pulsed pumping. By combination of the infrared microchip and an external frequency doubler, 473-nm pulses with a conversion efficiency of 18% were achieved.

A diode-laser pumped blue intracavity frequency-doubled self-Q-switched microchip laser of a chromium Cr4+ Neodymium Nd3+ codoped yttrium aluminum garnet crystal (Cr4+Nd3+:YAG) combined with a potassium niobate (KNbO3) crystal is developed. By coating the cavity mirrors with the films that suppress the 1064-nm operation and enhance the 946-nm laser, the 4F3/24I9/2 transition of the Nd3+ ion is facilitated to achieve the 946-nm laser oscillation. The 946-nm laser of the Cr4+Nd3+:YAG is self-Q-switched due to the saturable absorption of the Cr4+ ion. The full width at half maximum (FWHM) of the laser pulse at a 964-nm wavelength is about 5.07 ns. A self-Q-switched 473-nm laser pulse is sequentially obtained through intracavity frequency doubling of the 946-nm laser by the KNbO3 crystal, whose FWHM is about 4.30 ns. The 473-nm intracavity doubled laser has a good fundamental transverse TEM00 mode, because the self-Q-switched 946-nm laser has a good TEM00 mode that results from the absorption bleaching established by both the 808-nm pump laser and 946-nm oscillating laser. The constant FWHM results from both the response time of the self-Q-switching and the establishing time of the oscillating laser being much faster than the accumulated time of the pump laser energy. The constant peak power of the 946-nm self-Q-switched laser mainly depends on the modulation ability of the self-Q-switching.