High Repetition Rate Gain-switched Thulium Fiber Laser with an Acousto-optic Modulator

Jianing Zhang

supervisor：Deyuan Shen

Short pulse, high repetition rate lasers operating in the eye-safe 2 μm spectral region are widely used in areas of optical countermeasure, lidar, laser marking of plastics and medical surgery, etc. In addition, they serve as seed sources for wavelength conversion in optical parametric oscillators and mid-IR supercontinuum (SC) generation. Commonly, pulsed 2 μm laser output can be generated through Q-switching, mode-locking or gain-switching of a Tm-doped or Tm, Ho co-doped fiber laser. Gain-switched TDFL has attracted significant attention for its simplicity in operation and flexibility in repetition rate tuning.

Previously, gain-switched Tm-doped fiber laser (TDFL) was pumped with pulsed laser at 790 nm or 1.064 μm. The output power from a gain-switched TDFL pumped at 1.064 μm was limited by the inherent pump excited state absorption (ESA) loss. A 790 nm pulsed laser was therefore employed as pump source for efficient gain-switching of TDFL. However, the output from such a TDFL is chaotic with a series of spikes due to the delayed relaxation process from the upper pump energy level to the laser emission energy level. These spikes can be regulated through resonant pump scheme. Since the population of the emission energy level accumulates via thermal equilibrium instantly after the pump absorption, the cavity gain is switched on and off simultaneously with the pump pulse thus relaxation oscillation can be eliminated by adjusting the pump pulse duration. We report here a stable gain-switched TDFL pumped with an acousto-optically modulated continuous wave (CW) Er, Yb co-doped fiber laser at 1.56 μm. For a certain pump power-level, stable gain-switched operation can be realized by modulating the pump pulse duration to let only the first relaxation oscillation pulse in each pump period lase. In this pump scheme, the upper limit of pulse repetition rate is determined by the relaxation oscillation frequency of the TDFL, offering extreme flexibility in pulse repetition rate tuning.

The laser produced 1850 nm pulses with 2.5 μJ pulse energy and a pulse width of 75 ns at 500 kHz, resulting in a maximum average output power of 1.32 W and a slope efficiency of 54.7% with respect to absorbed pump power. Owing to the broad repetition rate tuning range, stable gain-switched pulses with a repetition rate up to 1.33 MHz were also obtained from this laser configuration. Such a gain-switched Tm-doped fiber laser will find applications for its broad repetition rate tuning range.

参考文献:

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[8]   Y. Tang, F. Li, J. Xu, “Narrow-pulse-width gain-switched thulium fiber laser,” Laser Phys. Lett., vol. 10, no. 3, Mar. 2013.

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Transport properties of random media composed of core-shell spheres calculated by suing the Energy-density Coherent Potential Approximation method

Yuchen Xu

Supervisor: Hao Zhang

In our daily life, light is more likely to propagate in random media than in uniform media. The term “random” refers to some kind of strong heterogeneity in space of the transport medium studied, and here the heterogeneity is induced by the refractive index fluctuation for light. Light undergoes multiple scattering in random media, which may leads to some interesting phenomena such as enhanced coherent backscattering or even Anderson localization. So far, several methods have been developed to study the transport properties of light in random media, such as T-matrix method, coherent potential approximation (CPA) method and energy-density coherent potential approximation (ECPA) method. The simulation results from ECPA method coincide the best with experiments. On the other hand, core-shell nanoparticle has been a hot topic recently. The chemical and physical properties of systems composed of core-shell nanoparticles can be easily tuned by modulating the morphology and constituent materials of the particles.

Motivated by these facts, we set about calculating the light transport properties of random media composed of core-shell spheres using the ECPA method. We first developed the ECPA algorithm for multi-layered spheres, then we carried out a series of calculations concerning the contribution from the morphology and dispersion of random media composed of core-shell spheres on the transport properties of random media in terms of scattering-cross-section efficiency factor, mean free path, velocity of electromagnetic energy, and diffusion coefficient. It is found that the core layer introduces more complicated resonant modes which lead to diverse possibilities to sharply decrease the transport of light within random media.

To study the mechanism of the new resonant modes, we developed another algorithm to simulate the field distribution within the coated spheres from the ECPA model. This work is still in progress.

参考文献：

[1] P. Sheng, Introduction to Wave Scattering, Localization, and Mesoscopic Phenomena, 2nd ed. (Springer, Heidelberger, 2006)

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[4] H. Zhang, H. Zhu, M. Xu, Transport properties of light in a disordered medium composed of two-layered dispersive spheres. Opt. Express 19(4), 2928–2940 (2011)

Controlled Growth of Crystalline g-C3N4 Nanocone Arrays by  Plasma Sputtering Reaction Deposition

 Leilei Guan

Supervisor: Ning Xu

Vertically aligned carbon nitride nanocone arrays have super high hardness, high aspect-ratio, good field emissivity and high light absorptivity making them highly attractive materials for a variety of potential applications. However, fabricating such well-ordered nanostructures through most of current methods is very difficult due to their extremely stringent growth conditions. Here, vertically aligned crystalline g-C₃N₄ nanocone (g-CNNC) arrays were firstly synthesized on Ni-covered Si (100) substrates with CH₄/(N₂+H₂) mixture feeding gases by abnormal glow discharge plasma sputtering reaction deposition. The experimental results show that the morphologies, structures, composition and photoluminescence of the as-grown nanostructures strongly depend on the CH₄/(N₂+H₂) ratio (0—1:10). The diamond-structured g-CNNC arrays were grown only at a CH4/(N₂+H₂) ratio of around 1:150 as a result of the sputtering of H⁺ to the graphite support to generate a lot of C/C⁺, the etching of H⁺ on the growing graphite structures, and the reaction between the active N/N⁺ and the sputtered C/C⁺ and CHn- radicals. At the higher or lower CH₄/(N₂+H₂) ratios, the silicon or diamond nanocone arrays were grown respectively due to the H⁺/N⁺-sputtering to the silicon substrate and the inhibition of CHn- (n<3) radicals on the H⁺-sputtering to the graphite support or the overwhelming sputtered C/C+ over the active N/N⁺. The growth mechanism of the g-CNNCs are different from the CNx nanocones reported earlier.

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