LASER VÀ ỨNG DỤNG (TS. Nguyễn Thanh Phương) - CHƯƠNG 4

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Các loại laser và ứng dụng Nhắc lại: những yếu tố cấu thành laser • • • • • tương tác giữa ánh sáng và vật chất đảo mật độ tích lũy môi trường khuếch đại thích hợp buồng cộng hưởng quang học tương tác giữa một buồng cộng hưởng quang học và khuếch đại bên trog BCH: - phương trình tốc độ của laser - ngưỡng phát laser - bão hòa khuếch đại - so sánh mode và lọc lựa mode - we now want to get an overview about the different types of lasers, which are practically relevant....

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Nội dung Text: LASER VÀ ỨNG DỤNG (TS. Nguyễn Thanh Phương) - CHƯƠNG 4

LASER VÀ ỨNG DỤNG TS. Nguyễn Thanh Phương Bộ môn Quang học và Quang điện tử
Chương IV: Các loại laser và ứng dụng
Chương IV: Các loại laser và ứng dụng Nhắc lại: những yếu tố cấu thành laser • tương tác giữa ánh sáng và vật chất • đảo mật độ tích lũy • môi trường khuếch đại thích hợp • buồng cộng hưởng quang học • tương tác giữa một buồng cộng hưởng quang học và khuếch đại bên trog BCH: - phương trình tốc độ của laser - ngưỡng phát laser - bão hòa khuếch đại - so sánh mode và lọc lựa mode - we now want to get an overview about the different types of lasers, which are practically relevant. 17/04/2011 3
Chương IV: Các loại laser và ứng dụng IV.1. Laser rắn
IV.1. Laser rắn … Ruby (Rubin) continued optical and laser properties of ruby at room temperature 17/04/2011 9
6.3.2. Neodymium Lasers Laser rắn IV.1. Neodymium Lasers 17/04/2011 10
IV.1. Laser rắn • first realized with glass1961, with YAG 1964. • crystal - Nd:YAG is the most important material used for solid state laser systems. YAG stands for Yttrium-Aluminum-Garnet, Y3Al2O12, a colourless, isotropic crystal. For a Nd:YAG laser rod ~1% of the Y3+ ions is replaces by Nd3+ ions. The YAG-structure is very stable from lowest to highest temperature, its mechanical stability and workability (growing, grinding, polishing) as well as the achievable optical quality are good. absorption spectrum of Nd:YAG 17/04/2011 11
IV.1. Laser rắn … Nd:Lasers continued - Nd:YAG is a four-level laser, it is - level scheme of Nd:YAG homogeneously broadened - lasing is mainly supported by the R2 fast, non- sub-level of the 4F3/2 level. At room radiative decay temperature ~40% of 4F3/2 atoms τ ~240µs are in R2 ( Boltzmann). - "strongest" laser transition at 1064.1 nm - lower laser level is 4I11/2 with various sub-levels, which all give slightly different emission wavelength. - lower laser levels are thermally not populated, so inversion can easily be achieved, even for cw-operation. fast, non- radiative decay 17/04/2011 12
IV.1. Laser rắn • applications - material processing (cw and pulsed lasers) welding (Schweißen), marking, writing, drilling (Bohren) (sizes of few µm possible), cutting - illumination and ranging (military) - medical, especially ophthalmology (Augenheilkunde) - pumping of other lasers (e.g. frequency doubled Nd:YAG for pumping of Ti:Sa lasers) and non-linear optics (e.g. frequency doubling [532 nm], tripling [355 nm], quadrupling [256 nm], parametric conversion). - Nd:glass lasers and corresponding amplifiers are also used for laser fusion experiments 17/04/2011 13
IV.1. Laser rắn • discharge pumping - cw-lasers are pumped by diode lasers at ~ 810nm or various types of discharge lamps or filament lamps, pulsed lasers by flash lamps. - energy corresponding to non-radiative decays limits quantum efficiency to ~ 76%. Excess power (~24%) is converted into heat, which has to be dissipated. Light not absorbed by the pump bands is also partially converted into heat - examples for pumping cw-Nd:YAG lasers 17/04/2011 14
IV.1. Laser rắn … discharge pumping continued arc lamps - discharge lamps (cw) - typical electrical power: ~ 1…10 kW, ~ (100V, 50 A) - arc length ~ 50mm - lifetimes few 10h … ~ 1000 h - discharge/filament tube is mounted inside a flow tube which carries the coolant (liquid). filament lamps - typical electrical power: ~ 1kW, - filament length ~ 50mm - lifetimes ~100h 17/04/2011 15
IV.1. Laser rắn … discharge pumping continued - depending on requested power different "pump cavity" designs are used for discharge pumped lasers. Elliptical cross sections are the basis for many of these geometries, where the discharge tube is located at one focus and the laser rod at the other. 17/04/2011 17
IV.1. Laser rắn … discharge pumping continued - thermal loading pulsed Nd:YAG lasers as well as other solid-state systems can provide very high peak powers (many GW) and large pulse energies (many joules). Especially if lamps (~ 10 kW electric power each) are used for pumping, thermal loading of the crystal is a serious, power-limiting issue. Absorption of pump plight outside the pump band, and heating due to non- unity quantum efficiency • will induce thermal lensing through temperature dependence of the index of refraction. This modifies the resonator geometry dynamically! • thermal stress causes birefringence and can even lead to damage of the crystal. Reduction of problems arising from thermal loading requires • uniform pumping • good heat removal 17/04/2011 18
IV.1. Laser rắn … discharge pumping continued - slab geometry a slab geometry provides a number of advantages over rod-designs: • pumping is more homogeneous • larger surface per volume (better heat removal) • temperature gradients only in y-direction. ⇒ cartesian symmetry helps to avoid thermal stress induced depolarization problems (laser emissions is already polarized in the y-z plane due to Brewster cut of crystal) 17/04/2011 20
IV.1. Laser rắn … discharge pumping continued - different slab geometries exist single (dual) flash lamp design multiple flash lamp design 17/04/2011 21
IV.1. Laser rắn … discharge pumping continued - disk geometry beneficial for ultra-high power pulsed solid-state laser systems like those used for laser fusion (at Lawrence Livermore National Laboratory): - better cooling, - better beam quality - larger aperture (∅ >70cm ) - better gain uniformity 17/04/2011 22
IV.1. Laser rắn • diode pumping - high pumping efficiency, because diode lasers at 810nm match Nd:YAG absorption bands very well ⇒ reduction of thermal load problems (thermal lensing, thermal birefringence) ⇒ improved total electrical-to-optical efficiency - better pump beam quality: pump laser light can be focused into the gain volume (especially for end-pumped systems) - longer MTBF (mean time between failure): typically 10.000 h for diode lasers vs. a few hundred h to about 1000 h for discharge lamps. - operation simplified: reduced cooling requirements, no high voltage "spikes", no UV-light which degrades crystal, optics and coolant. - a single diode laser can provide a few W cw-power (typically not fundamental mode). Single transverse mode laser diodes with ~0.1 W up to 1 W output power exist. Sometimes broad stripe diode lasers, 1D-arrays ("bars") or 2D-arrays can be used. 17/04/2011 23
IV.1. Laser rắn … diode pumping continued - there are several geometries for optical pumping with laser diodes • end pumped systems (single and double) - pump light can be matched to mode volume 17/04/2011 26
IV.1. Laser rắn … diode pumping continued • side pumping of a rod - direct coupling (diodes close to amplifier) - coupling with optics - fiber coupling (!) • achievable: optical cw-pumping at ~10kW, cw-output typical 100W, up to ~1kW 17/04/2011 27
IV.1. Laser rắn … Nd:Lasers continued - physical, optical, thermal properties of Nd:YAG A MISER oscillator (Monolithic Isolated Single-mode End-pumped Ring), or alternatively, an NPRO (Non-Planar Ring-Oscillator): the crystal itself constitutes the amplifier, optical resonator, and optical diode to enforce uni- directional oscillation. T. J. Kane and R. L. Byer, Opt. Lett. 10 (2), 65 (1985) ; I. Freitag et al., Opt. Commun. 115, 511 (1995) 17/04/2011 29