Tunable Tm:fiber laser under 2μm

Ting Zhao

Supervisors: Deyuan Shen

Fiber lasers are efficient, powerful and versatile waveguide resonant devices that comprise glass optical fiber waveguides for optical gain and Fabry-Pérot resonators for optical feedback. Rapid developments in fabrication capabilities now allow fibers composed of ultralow-loss silicate glass with low scattering, impurity losses and material imperfections, thus providing enormous flexibility in the characteristics and quantity of light that can be generated from fiber lasers. Passively air-cooling an optical fiber is simple owing to its large surface-area-to volume ratio. Optical excitation using multimode semiconductor diode lasers is straightforward and efficient with cladding pumping-particularly when the axial symmetry of the fiber is broken and efficiently excites the single-mode core of the fiber to create near-diffraction-limited output and a brightness enhancement of at least three orders of magnitude. The wavelength range around 2μm is part of the so called “eye safe” wavelength region which begins at about 1.4μm. Laser systems that operate in this region offer exceptional advantages for free space applications compared to conventional systems that operate at shorter wavelengths. Interest in the Tm3+-doped fiber laser originated from its emission band in the range of 1400-2700nm lying between the bands of Er3+ and Nd3+ ions. Since the advent of double-cladding configuration of fiber and the recent technological development of high-power laser diodes, configuration of fiber and the recent technological development of high-power laser diodes, output power (performance) of Tm3+-doped fiber lasers has scaled exponentially. Up to date, the maximum output achieved in Tm3+-doped fiber lasers has been comparable with that from Yb3+-doped fiber lasers. Laser beam in the 2~3μm wavelength range has wide applications. First, it is a good candidate in laser microsurgery due to high absorption of water in this spectral region thus can provide high-quality laser tissue cutting and welding. In addition, this wavelength-range laser has potential applications in environment monitoring, LIDAR, optical-parametric-oscillation (OPO) pump source, and so on.

For obtaining laser emission in the mid-infrared wavelength region, the Tm3+-doped fiber is an excellent candidate due to several unique advantages it possesses. First, the Tm3+-doped fiber has a strong absorption spectrum that has good overlap with the emission band of commercially available AlGaAs laser diodes, which have been significantly developed and are being developed with an unprecedented speed. Second , the specific energy-level structure of Tm3+-ions provides the Tm3+-doped fiber laser a special advantageous energy transfer process-the 3H4+3H6-3F4+3F4 cross relaxation process. In this process, two excited-state ions can be obtained with depletion of just one absorbed pump photon. With an appropriately high doping level, the cross relaxation process can offer a quantum efficiency close to two, which greatly improves the efficiency of the Tm3+-doped fiber laser. Thirdly, the Tm3+-doped fiber has a very broad emission band, spanning over more than 400nm. This feature offers the Tm3+-doped fiber laser an especially high-degree wavelength tunablility, which is very useful in applications such as spectroscopy, atmospheric sensing and so on.

We report a widely tunable Tm:fiber laser double end pumped by a high power 792nm diode laser, with much improved output in terms of power level and linewidth using VBG as wavelength selective element. The laser was tunable from 1898 to 2000nm, with >14W output power over a tuning range of 102nm and a spectral linewidth of 0.4nm. Over 15W of diffraction limited (M2~1.5) CW output power was generated for 54.4W of launched pump power at 792nm, corresponding to a slope efficiency with respect to launched pump power of 35.5%.

In this report, we’ll have a brief introduction of the detail of the tunable Tm3+-doped fiber laser.

参考文献：

[1] P. Myslinski, X. Pan, C. Barnard, J. Chrostowski, B. T. Sullivan, and J. F. Bayon, “Q-switched thulium-doped fiber laser,” Opt. Eng. 32(9), 2025-2030 (1993).

[2] L. Esterowitz, “Diode-pumped holmium, thulium, and erbium lasers between 2 and 3 μm operating CW at room-temperature,” Opt. Eng. 29(6): 676-680 (1990).

[3] R. C. Stoneman and L. Esterowitz, “Efficient, broadly tunable, laser-pumped Tm-YAG and Tm-YSGG CW lasers,” Opt. Lett. 15(9): 486-488 (1990).

[4] S. W. Henderson, P. J. M. Suni, C. P. Hale, S. M. Hannon, J. R. Magee, D. L. Bruns, and E. H. Yuen, “Coherent laser-radar at 2 μm using solid-state lasers,” IEEE Trans. Geosci. Remote Sens. 31(1): 4-15 (1993).

[5] Stuart D. Jackson, “Towards high-power mid-infrared emission from a fiber laser,” Nature Photonics, 6: 421-431 (2012).

[6] P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-doped fiber lasers: fundamentals and power scaling,” IEEE J. of Selected Top. In Quant. Electr., 15(1): 85-92 (2009).

Optical Transmission Properties of Silicon Nanoparticle Arrays

Ziyi Wang

Supervisors: Rongjun Zhang

Interference-based antireflection (AR) coating is designed to provide high absorbance of solar cells near the solar spectrum. But recently, silicon based nanoparticle array structures, such as nanosphere, nanowire, nanopillar, and nanocone, have been proved as promising building blocks for high efficiency solar cells for their broadband low reflectivity of light. Theoretical understanding of optical absorption within silicon nanopillar and nanocone arrays through visible light to near-infrared region could be helpful in further solar cell development.

A transfer-matrix method (TMM) of transmittance and reflectance was carried out in our simulation. And the DDSCAT 7.2 code package was employed to calculate normalized scattering cross section (Qsca), normalized absorption cross section (Qabs) and the electric field distributions. All optical constants used in our simulations are taken from ref 13. To confirm the reliability of TMM and DDSCAT7.2, we compared our simulation data with various references and good agreements were observed.

The reflectance reaches to a minimal value when Qsca is maxima. With the structure’s gradual changing diameter, light with all wavelengths could find an optimum resonance condition in nanocone arrays. Thus leads to the reflectance lower than 2.5% in the whole spectrum. The resonance mode in the cone overlaps with the substrate and proposed a leaky channel for light coupling into the substrate. So that most light will be scattered forward and absorbed, which means the efficiency of solar cell will be increased.The impurities and surface defects raised the extinction coefficient and trapped the light in the particles, thus reduced the reflectance and enhanced the absorption in the infrared region.

In summary, all-solar-spectrum absorption nanocone arrays with varies height and base diameter were compared with simulations. The simulation indicated that broadband high absorbance is due to the Mie resonance.

参考文献

[1] Shah, A. et al. Thin‐film silicon solar cell technology. Progress in photovoltaics: Research and applications 12, 113-142 (2004).

[2] Spinelli, P., Verschuuren, M. & Polman, A. Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators. Nature communications 3, 692 (2012).

[3] Qiu, Y., Hao, H.-C., Zhou, J. & Lu, M. A close to unity and all-solar-spectrum absorption by ion-sputtering induced Si nanocone arrays. Optics Express 20, 22087-22094 (2012).

Structre and Optical properties of SrTiO3 thin films deposited by radio-frequency magnetron sputtering at various annealing temperatures

Xinyu Yi

Supervisors: Jing Li

As an important functional material, Strontium titanate (SrTiO3) thin film has a wide range of applications in many fields because of its high dielectric constant, low leakage current, good thermal stability and dielectric tunability. Among all the chapters, undertaken by the introduction and the deposition of SrTiO3 thin films, focused on investigating the influence of the annealing treatment on the films’ structure and physical properties, the work is finished. The dissertation mainly includes the following contents.                                                                                                                                                                                               

Strontium titanate thin films have been prepared on p-type Si( 100) substrates by radio-frequency (rf) magnetron sputtering. The films were deposited at room temperature and annealed at various temperatures(500-800℃).The structure and optical constants of SrTiO3 thin films annealed at different temperatures were analyzed by x-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and Spectroscopic ellipsometry. The results showed that the as-deposited film was amorphous with stoichiometric ratio of about 1:1:3. The film turned to be polycrystalline after annealed at 600oC. The crystallinity and surface roughness became larger with higher annealing temperature. The refractive index and extinction coefficient of SrTiO3 films annealed at different temperatures were analyzed, and the band gaps of each thin film samples were also estimated from Tauc’s law.

参考文献

[1] X. B.Yan, K. Li, et al. The Resistive Switching Mechanism of Ag/SrTiO₃/Pt Memory Cells [J]. Electrochemical and Solid-State Letters, 2010, 13(3): H87-H89.

[2] S. B.Lee, A. Kim, et al. Reduction in high reset currents in unipolar resistance switching Pt/SrTiO_x/Pt capacitors using acceptor doping [J]. Applied Physics Letters, 2010, 97(9): 093505.

[3]  S. W. Lee, J. H. Han, et al. Atomic Layer Deposition of SrTiO₃ Thin Films with Highly Enhanced Growth Rate for Ultrahigh Density Capacitors [J]. Chemistry of Materials, 2011, 23(8): 2227-2236.

[4] G.Shirane, Y. Yamada. Lattice-Dynamical Study of the 110°K Phase Transition in SrTiO₃ [J]. Physical Review, 1969, 177(2): 858-863.

[5] C. S.Kang, C. S. Hwang, et al. Preparation and electrical properties of SrTiO3 thin films deposited by liquid source metal-organic chemical vapor deposition (MOCVD) [J]. Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers, 1996, 35(9B): 4890-4895.

[6]  A.Kosola, M. Putkonen, et al. Effect of annealing in processing of strontium titanate thin films by ALD [J]. Applied Surface Science, 2003, 211(1-4): 102-112.

[7] Y.Du, M. S. Zhang, et al. Optical properties of SrTiO₃ thin films by pulsed laser deposition [J]. Applied Physics A: Materials Science & Processing, 2003, 76(7): 1105-1108.

[8] X. M. Ding, X. Yang, et al. Surface physics and surface analysis [M], Shanghai: Fudan University. 2007.

[9] L. Y. Chen, X. W Feng, et al. Design of a scanning ellipsometer bysynchronous rotation of the polarizer and analyzer [J]. ApplOpt, 1994, 33(7):1299-1305.