Biomedical subjects

Chang-Qing Xu

Publications and source records attributed to Chang-Qing Xu.

7 recordsLinked to original sources

PubMed2006

Long-period grating refractive index sensor with a modified cladding structure for large operational range and high sensitivity.

What is believed to be a novel long-period grating (LPG) refractive index sensor with a modified cladding structure is proposed and studied. In the proposed structure, the cladding of the fiber has a two-layer structure, i.e., a cladding layer of low refractive index with a reduced radius and an overlay of high refractive index. The sensitivity of the structure-modified LPG sensor to the ambient refractive index change as a function of the cladding layer and overlay parameters is investigated by way of modeling. It is found that an increase of the ambient refractive index causes a field redistribution of the cladding mode into the overlay when the parameters of the overlay are properly selected. It is shown that by reducing the radius of the cladding layer, the operational range of the LPG refractive index sensor can be as large as 0.195 (from 1.244 to 1.440) with a minimum sensitivity of 660 nm/refractive index, which represents a 31% increase of operational range in comparison with the operational range obtained from the reported structure. The design guidelines for achieving this large operation range and high sensitivity are explained by investigating the dependence of the cladding modes on the radius of the cladding layer.

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PubMed2006

Picosecond-pulse wavelength conversion based on cascaded second-harmonic generation-difference frequency generation in a periodically poled lithium niobate waveguide.

The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.

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PubMed2006

Modeling and optimization of Q-switched double-clad fiber lasers.

The numerical modeling of actively Q-switched fiber lasers is systematically presented. On the basis of typical Q-switched ytterbium-doped double-clad fiber lasers under forward and backward pump, the dynamic characteristics of pulse energy, pulse width, population inversion, and stored energy at tens-of-kilohertz repetition rates are studied by using the traveling-wave method. The laser performance is further investigated for different fiber core diameters, doping rates, cavity lengths, fiber losses, signal and pump wavelengths, reflectivities of output coupler, and switching speed of an acousto-optic modulator; the laser optimization is also quantitatively discussed. Some simulation results are also compared with previous experimental results.

Journal Article↗

PubMed2006

Changes of nitric oxide, oxide free radicals, and systolic arterial blood pressure in rats with experimental lymphatostatic encephalopathy.

The model of lymphatostatic encephalopathy was established by occluding and removing profound cervical nodes in rats, and the kinetic alteration of nitric oxide (NO), maleic dialdehyde (MDA), free radical scavenger (CuZn-SOD) and arterial systolic blood pressure were determined on different days after the blockage. The results showed that the level of NO significantly decreased at 1 day (P<0.05) and further decreased at 3, 5 and 7 day (P<0.01). The levels of MDA at 1, 3, 5 and 7 day significantly increased, but the contents of CuZn-SOD significantly decreased compared with the control (P<0.01). There was negative correlation between the levels of MDA and CuZn-SOD, but there was no relationship between MDA an NO. Arterial systolic blood pressure decreased progressively after cervical lymphatic blockage. The results showed that NO, oxide free radicals and the disturbances of the cardiovascular regulation may play important roles in lymphatostatic encephalopathy.

Aldehydes↗

PubMed2005

[Effect of peroxisome proliferation activated receptor-gamma on neuronal cell death induced by hypoxia and ischemia in rats in vitro and in vivo].

OBJECTIVE: To observe the effects of peroxisome proliferation activated receptor (PPAR)-gamma on neuronal cell death induced by hypoxia/reoxygenation and ischemia/reperfusion. METHODS: Cortical neural cells of fetal SD rats were cultured for 12 days and exposed to hypoxia/reoxygenation so as to establish a hypoxia/reoxygenation model. Another primary fetal rat cortical neuronal cells were pre-treated with different concentrations of GW9662, antagonist of PPAR-gamma, then underwent hypoxia for 3 hours, re-oxygenated for 21 hours. MTT was added one hour after to measure the cell viability. Eleven male SD rats underwent right middle cerebral artery occlusion (MCAO) using suture and reperfusion. Eleven rats underwent sham operation. after the rats were killed and their brains were taken out. Nucleoprotein was extracted from the cultured primary cortical cells and the cerebral cortexes of the rats and co-cultured with [gamma-(32)P]-labeled PPAR-gamma probe, EMSA to detect the PPAR-gamma binding activity. RESULTS: The PPAR-gamma activity of the cultured fetal rat cortical neurons that underwent hypoxia/reoxygenation significantly increased: It began to increase 1 hour after hypoxia and peaked in the 3rd hour of hypoxia, when the neurons underwent hypoxia for 3 hours and were reoxygenated for 2 hours, the binding activity still remained at a high level, and basically returned to the level of the untreated group 8 hours after reoxygenation. Data were quantified with control group as 100, 3 h of hypoxia was 160.3, and 2, 4, 8 h after reoxygenation were 157.5, 136.6, 103.3 separately. One hour after reperfusion the PPAR-gamma binding activity of the cortical cells at the ischemic side of the rats began to increase and peaked at the 4th hour, significantly higher than those of the cortical cells at the opposite side and of the sham operation group (both P < 0.01) then remained at a high level for the following 24 hours. The survival rate of the cultured neurons that underwent hypoxia for 3 hours and reoxygenation for 21 hours was significantly lower than that of the untreated neurons. Data were quantified with sham surgery group as 100, the side of MCAO and the contra side in surgery group were 144.8 and 102.6 separately. The survival rate of the neurons that were pretreated with GW9662 and then underwent hypoxia/reoxygenation was significantly higher than that of those without pretreatment (P < 0.01) with the peak protection effect of GW9662 at the concentration of 2.5 - 10 micromol/L. Data were quantified with control group as 100, hypoxia/reoxygenation group was 184, GW9662 group was 105. The PPAR-gamma binding activity of the primary cortical neurons pretreated with 5 micromol/L GW9662 for 30 minutes and than exposed to hypoxia for 3 hours and reoxygenation for 2 hours was significantly lower than that of the only hypoxia/reoxygenation group (P < 0.01). CONCLUSION: PPAR-gamma is involved in the pathogenesis of neuron death induced by hypoxia/ischemia and may become a new target of treatment of ischemic stroke.

Anilides↗

PubMed2004

Understanding multipeak phenomena in actively Q-switched fiber lasers.

Dynamics of split pulses in actively Q-switched fiber lasers are numerically investigated, and the mechanism that produces multipeak phenomena (MPP) is precisely illustrated. It is shown that the first transient pulse in a cavity is initiated by the quick switching of a Q-switching element, which actually injects part of the amplified spontaneous emission into the laser cavity in the form of a pulse. Then the multipeak structures are related to the evolution of the perturbation caused by the switching and are determined by the rise time of the switching and by conditions of photon density and population inversion.

Journal Article↗

PubMed2004

Cascaded wavelength conversions based on sum-frequency generation and difference-frequency generation.

A novel wavelength conversion scheme based on cascaded sum-frequency generation (SFG) and difference-frequency generation (DFG) is proposed and demonstrated in a MgO-doped LiNbO3 quasi-phase-matched waveguide. In this scheme, two pump wavelengths are set outside the communication band. It is shown that the same conversion efficiency can be achieved by use of two pump sources with lower output power (P1, P2) in this scheme compared with the conventional cascaded wavelength conversion technique based on second-harmonic generation and DFG with a single higher-power pump beam (P = P1 + P2) that is due to the use of a larger SFG nonlinear coefficient. The results significantly influence the selection of a suitable nonlinear interaction scheme for practical applications.

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