Gain-Switched Tm: YAG Ceramic Laser Resonantly Pumped at 1617 nm

Jianing Zhang

Supervisor: Deyuan Shen

High peak power 2 μm nanosecond pulsed lasers are widely used in scientific and technical areas due to the low attenuation in the atmosphere and ‘eye-safe’ aspects in this spectral region. They are required for remote sensing, laser ranging, industrial machining, as well as laser surgery and therapy. In addition, they serve as seed sources for wavelength conversion in optical parametric oscillators and mid-infrared supercontinuum (SC) generation. High energy 2 μm pulses on the nanosecond scale are usually obtained from a Tm-doped or Ho-doped Q-switching or gain-switching bulk crystal laser. A number of actively Q-switched Tm-doped crystal lasers with several mill joule pulse energy have been reported. Commonly, Tm: YLF, Tm: YAG, or Tm: YAP crystal was employed as the gain media. The long life time of the laser emission energy level and good thermal conductivity in these crystals are beneficial for pulsed operation with high energy. Recently, transparent laser ceramics have attracted much interest as an ideal substitute for single crystals due to the higher doping concentration and economical fabrication. Moreover, UV light coming from the up-conversion process will be scattered by the grain boundaries in the ceramic, which results in high laser efficiency. Actively Q-switched Tm: YAG ceramic lasers have been investigated and the laser performance appeared to be better than that of the single crystal laser. However, neither gain-switched Tm: YAG single crystal or ceramic laser has been reported. Although the peak power and pulse energy of gain-switched lasers are usually low compared to that of Q-switched lasers, they have their own distinct advantages including the simplicity in operation and extreme flexibility in pulse repetition rate tuning. In principle, there is no lower limit in operation repetition rate of a gain-switched laser since the active medium is pumped with short intensive pulses and hence does not suffer from amplified spontaneous emission (ASE) as those of continuously pumped Q-switched systems. On the other hand, the upper limit of the repetition rate of a gain-switched laser is its relaxation oscillation frequency which usually in multi-megahertz range.

We have built a numerical model to simulate the output from a gain-switched Tm: YAG ceramic laser pumped with an acoustic-optically modulated continuous wave (CW) Er: YAG ceramic laser at 1617 nm. 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 Tm: YAG ceramic laser, offering extreme flexibility in pulse repetition rate tuning.

参考文献：

[1]   Y. L. Tang, L. Xu, Y. Yang, J.Q. Xu, “High-power gain-switched Tm3+-doped fiber laser”, Opt. Express, vol. 18, no. 22 pp. 22964-22972, Oct. 2010.

[2]   J. Kwiatkowski, J. K. Jabczynski, W. Zendzian, L. Gorajek, M. Kaskow, “High repetition rate, Q-switched Ho:YAG laser resonantly pumped by a 20 W linearly polarized Tm: fiber laser”, Appl. Phys. B, June 2013.

[3]   Y. L. Tang, L. Xu, M. J. Wang, Y. Yang, X. D. Xu, and J.Q. Xu, “High-power gain-switched Ho:LuAG rod laser”, Laser Phys. Lett., vol. 8, no. 2, pp.120-124, 2011.

[4]   S. Zhang, X. Wang, W. Kong, Q. Yang, J. Xu, B. Jiang, Y. Pan, “Efficient Q-switched Tm:YAG ceramic slab laser pumped by a 792 nm fiber laser”, Opt. Commun., vol. 286, pp. 288-290, 2013.

[5]   S. Zhang, M. Wang, L. Xu, Y. Wang, Y. Tang, X. Cheng, W. Chen, J. Xu, B. Jiang, Y. Pan, “Efficient Q-switched Tm:YAG ceramic slab laser”, Opt. Express, vol. 19, no. 2, pp. 727-732, Jan. 2011.

Dispersion measurement in Whispering gallery mode(WGM) microcavity

Zhenmin Chen

Supervisor: Lei Xu

It is well know that dispersion is always one of the key parameters of the optical system. We know that the four wave mixing (FWM) in high speed communication system is related to the dispersion parameters of the system. WGM microcavities are widely studied across many fields in photonics for research and technology. They are used for filters and switches in optical communications, nonlinear optics. Therefore, the dispersion of the microcavity still plays an important role in these areas.

In microcavities, dispersion contains both geometrical part which relate to the shape of the cavity and material part as other optical devices. According to the theory of microcavity ，we can know microcavity dispersion has a certain relation with its FSR. Therefore, through the precise measurement of FSR in the microcavity ,we can obtain the dispersion parameter of microcavity.

Methods for measuring dispersion have been reported with use of optical frequency comb and sideband measurement. Here we will give a new way to measure the dispersion parameter of microcavity based on a Michelson interferometer.

参考文献：

[1] P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth & T. J. Kippenberg,Nature, Vol 450|20/27 December 2007

[2] P. Del’Haye, O. Arcizet, M. L. Gorodetsky, R. Holzwarth, T. J. Kippenberg, Frequency Comb Assisted Diode Laser Spectroscopy for Measurement of Microcavity Dispersion , Nature Photonics  3, 529–533 (2009). 

[3] Johann Riemensberger, Klaus Hartinger, Tobias Herr,Victor Brasch, Ronald Holzwarth, and Tobias J. Kippenberg, Dispersion engineering of thick high-Q

silicon nitride ring-resonators via atomic layer deposition, Optical Express ,Vol. 20, No. 25

[4] Li, Jiang; Lee, Hansuek; Yang, Ki Youl; Vahala, Kerry J, Sideband spectroscopy and dispersion measurement in microcavities, Optics Express, Vol. 20 Issue 24, pp.26337-26344 (2012)

A comparative study of the enhancement of molecular emission in a spatially confined plume

Feiling Huang

Supervisor: Jian Sun

Based on pulsed laser ablation of a sample and optical emission spectroscopy (OES) measurement of the ablation plume, laser induced breakdown spectroscopy (LIBS) has been developed as a popular and useful method for chemical analysis. The ability to obtain rapid, multi-elemental, in-situ analysis represents a unique advantage of LIBS over other analytical techniques. LIBS has shown a strong potential for performing direct elemental analysis of various materials including solids, liquids, and gases. It is also possible to obtain information about molecular species in the plume and the interaction of the plume with the ambient air through the detection of molecular emission bands with LIBS.

Although LIBS has so many advantages and has been widely used, the relatively low detection sensitivity compared with other spectroscopic analytical techniques is one of the main shortcomings of LIBS for trace analysis. Many efforts have been devoted to developing new instrumental and analytical approaches including double-pulse LIBS, femtosecond LIBS, combination of LIBS and laser induced fluorescence, resonance-enhanced LIBS, and LIBS with spatially confining plume, aiming at enhancing the LIBS signals, and consequently improving the sensitivity and lowering the detection limits of LIBS. Among these approaches, LIBS with spatial confinement is the one that needs the least modification of the LIBS system and is the easiest to be performed.

In this report, we focused on the enhancement of the optical emission from molecular species in an ablation plume by spatial confinement. A pulsed laser beam ablated a graphite target in the atmospheric air and induced a carbon plume expanding in air, generating a shock wave which propagated supersonically away from the target. The propagating shock wave was reflected by a metal disk which situated in the way of the propagation of the shock wave. The reflected shock wave propagated backwards and encountered the expanding plume, confining the plume expansion. Time-integrated and time-resolved OES measurements were used to study the enhancement of the plume emission. The temporal behaviors of the shockwave were also examined by probe beam deflection (PBD) technique to complement and compare with the results obtained by OES measurements. The mechanisms responsible for the enhancement of the plume emission as well as the processes occurring in the plume were also discussed.

参考文献：

[1]Jagdish P.Singh, Surya N. Thakur. Laser-induced Breakdown Spectroscopy[M]. Oxford: Elsevier B.V., 2007: 10.

[2]R. Hedwig, “Confinement effect in enhancing shock wave plasma generation at low pressure by TEA CO2 laser bombardment on quartz sample,” Spectrochim. Acta, B At. Spectrosc. 58(3),531-542(2003).

[3]X. Zeng, S. S. Mao, C. Liu, X. Mao, R. Greif, and R. E. Russo, “Plasma diagnostics during laser ablation in a cavity,” Spectrochim. Acta, B At. Spectrosc. 58(5), 867-877(2003).

[4]L. B. Guo, C. M. Li, W. Hu, Y. S. Zhou, B. Y. Zhang, Z. X. Cai, X. Y. Zeng, and Y. F. Lu, “Plasma confinement by hemispherical cavity in laser-induced breakdown spectroscopy,” Appl. Phys. Lett. 98(13), 131501 (2011).

[5]A. M. Popov, F. Colao, and R. Fantoni, “Enhancement of LIBS signal by spatially confining the laser-induced plasma,” J. Anal. At. Spectrom. 24(5), 602-604(2009).

[6]A. M. Popov, F. Colao, and R. Fantoni, “Spatial confinement of laser-induced plasma to enhance LIBS sensitivity for trace elements determination in soils,” J. Anal. At. Spectrom. 25(6), 837-848 (2010)

[7]P. Yeates and E. T. Kennedy, “ Spectroscopic, imaging, and probe diagnostics of laser plasma plumes expanding between confining surfaces,” J. Appl. Phys. 108(9)093306-093312 (2010)

[8]X. K. Shen, J. Sun, H. Ling, and Y. F. Lu, “Spectroscopic study of laser-induced Al plasmas with cylindrical confinement,” J. Appl. Phys. 102(9)093301-093305 (2007)

[9] Z. Wang, Z. Hou, S. L. Liu, D. Jiang, J. Liu, and Z.Li, “Utilization of moderate cylindrical confinement for precision improvement of laser-induced breakdown spectroscopy signal,” Opt. Express 20(S6), A1011-A1018(2012).

[10] Z. Hou, Z. Wang, J. Liu, W. Ni, and Z. Li, “Signal quality improvement using cylindrical confinement for laser induced breakdown spectroscopy,” Opt. Express 21(13), 15974-15979(2013)

Fabrication of Al doped ZnO film by electron cyclotron resonance plasma assisted pulsed laser deposition

Qinghu You

superviser: Jiada Wu

In the last decade, transparent conducting oxides (TCO) materials have been actively studied^[1]. Among these TCO materials, tin-doped indium oxide (ITO) film is the most commonly selected TCO electrode in flat panel displays including organic light-emitting diodes, liquid crystal displays and windows in solar cells due to its combined properties^[2]. However, since indium is a rare and an expensive element, the cost of ITO films is very expensive. Recently, doped zinc oxide films (for example, ZnO films doped with impurities, such as B, Al, Ga and In) have been studied as alternate materials to replace ITO because they are non-toxic, inexpensive and abundant materials^[3]. Among them, aluminium-doped ZnO film (AZO) is a wide band gap semiconductor, which shows good optical transmission in the visible wavelength regions (400–700 nm). Furthermore, AZO films have a lower electrical resistivity, which is similar to that of ITO film^[4,5].

There were various methods to produce ZnO film, such as the metal-organic chemical vapor deposition (MOCVD)^[1], the sol–gel method^[6], spray hydrolysis ^[7], sputtering ^[8] pulsed laser deposition (PLD)^[9], and some latest novel methods, such as atomic layer deposition (ALD)^[10]. Compared with the techniques mentioned above, we developed a versatile method for AZO synthesis based on plasma assisted pulsed laser deposition by coablation of two targets with two pulsed laser beams. In the reactive oxygen environment and with the assistance of oxygen plasma generated from electron cyclotron resonance microwave discharge, the ablation of a metal Zn target resulted in the deposition of a ZnO host film, whereas the ablation of a metallic Al target provided the host with Al atoms for doping in the growing ZnO film. The dopant concentration could be independently controlled to vary in a wide range by changing the pulse repetition ratio of the two laser beams or the laser fluence on the target for dopant supply.

参考文献：

[1] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, Bristol and Philadelphia, 1995.

[2] C.G. Granqvist, Thin Solid Films 730 (1990) 193.

[3] H. Kim, J.S. Horwitz, S.B. Qadri, D.B. Chrisey, Thin Solid Films 420 (2002) 107.

[4] H. Kim, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. Lett. 74 (1999) 3444.

[5] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451.

[6] T. Minami, H. Sonohara, J. Appl. Phys. 33 (1994) 1693.

[7] Y. Natsnme, H. Sakata, Thin Solid Films 372 (2000) 30.

[8] S. Bose, S. Ray, A.K. Barua, J. Appl. Phys. 29 (1996) 1873.

[9] B.J. Lochande, M.D. Uplane, Appl. Surf. Sci. 167 (2000) 243

[10]Geng Y, Guo L, Xu S S, et al. Influence of Al doping on the properties of ZnO thin films grown by atomic layer deposition[J]. The Journal of Physical Chemistry C, 2011, 115(25): 12317-12321.