Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271PULSED EYE-SAFE LASER SYSTEMS BASED ON SRS AND OPOV.A. Orlovich, V.I. Dashkevich, A.P. ShkadarevichB.I. Stepanov Institute of Physics, National Academy of Sciences of BelarusNezalezhnasti Ave. 68, 220072 Minsk, BelarusE-mail: v.orlovich@dragon.bas-net.byAbstract. The design and performance characteristics of several types of eye-safe laser sourcesconver<s<strong>trong</strong>>tin</s<strong>trong</strong>>g the emission of pulsed neodymium lasers with the 4 F 3/2 – 4 I 13/2 and 4 F 3/2 – 4 I 13/2 workingtransitions by stimulated Raman scattering (SRS) and optical parametric oscillator (OPO) aredescribedKeywords: Eye-safe radiation, Stimulated Raman scattering (SRS), Optical parametric oscillator(OPO), Ring extracavity Raman laser, Ring OPO, Intracavity 3 rd Stokes SRS laser, Self-SRS laser,SRS-active Nd:KGW crystal, KTP crystal, Nd:KGW pump laser.I. INTRODUCTIONPulsed laser sources genera<s<strong>trong</strong>>tin</s<strong>trong</strong>>g in the eye-safe spectral region (1.4 – 1.8 μm) find a wideapplication in environmental protection (atmos<strong>ph</strong>eric data measurements, pollution detection),range finding, active imagining and in several other areas related to the free-space propagation oflaser radiation. At present, there are two main approaches to develop eye-safe lasers. The firstone consists in direct lasing in this spectral range. Erbium doped glass is a prime candidate forthe generation of 1.54-μm eye-safe radiation, especially when codoped with Yb 3+ ions. Howeverthe thermal conductivity of Er-Yb glasses is very low which is disadvantage to create lasersystems with high repetition rates. For example, the thermal conductivity of glasses, that wasespecially designed for the high power lasers, does not exceed 0.9 W/(m K). A great deal ofeffort has been devoted to the synthesis and research of Er-doped crystals having the much largerthermal conductivity. Nevertheless, practical realization of high average-power laser action inEr-Yb crystals meets a number of obstacles. Among these are the multi<strong>ph</strong>onon 1.5-μmluminescence quenching and the low quantum yield leading to a high lasing threshold.In view of an imperfection of Er-laser media the second approach includes the nonlinearopticalconversion of the radiation of neodymium lasers by means of stimulated Ramanscattering (SRS) and optical parametric oscillator (OPO) is widely used to design eye-safe lasers.This paper reviews several types of eye-safe laser sources developed with the use of the secondapproach [1–4]. The results of investigation of their characteristics are reported.II. EXPERIMENTSOptical arrangements of the developed eye-safe sources are shown in fig. 1. In case of theSRS conversion, the Raman laser and the pump laser both use cylindrical AR-coatedNd(3%):KGd(WO 4 ) 2 (Nd:KGW) rods cut along the crystallogra<strong>ph</strong>ic [010] direction. Since theNd:KGW crystal is a Raman-active medium it is possible to realize so called a self-stimulatedRaman laser in which one rod is used for genera<s<strong>trong</strong>>tin</s<strong>trong</strong>>g and conver<s<strong>trong</strong>>tin</s<strong>trong</strong>>g the laser radiation. A schemeof such the laser is very simple. It is a two-mirror standing wave cavity including the 50-mmlong Nd:KGW rod and a passive (V 3+ :YAG) or active (LiNbO 3 ) Q-switch (fig. 1a). Since the34
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271oscillation of the electrical vector E in the linear polarized laser radiation of the Nd:KGW passesalong the optical indicatrix axis N m , SRS-shif<s<strong>trong</strong>>tin</s<strong>trong</strong>>g at the self-frequency Raman conversion modeis excited in the 901.5 cm -1 line. To obtain the eye-safe radiation, the mirrors are coated to makethe very high Q-factor of cavity at 1.351 μm ( 4 F 3/2 – 4 I 13/2 laser channel of Nd:KGW) and toprovide an output coupler M2 at 1.538 μm (1 st Stokes).Figure 1b shows the optical layout of the eye-safe source based on the extacavity Raman laserdriven by a separate multimode Nd:KGW pump laser. Both of lasers use ∅4x50 mm 3 Nd:KGWcrystals. The actively Q-switched (LiNbO 3 ) pump laser with the 4 F 3/2 – 4 I 13/2 working transitiongenerates the pulses of 22-25 ns in duration and up to 36 mJ in energy. The Raman laser mirrorsare coated to make the output coupler (M4) at the 1 st Stokes and to double passing the pumpbeam. The optical isolator prevents optical feedback from the linear Raman cavity, in otherwords, marking the high Q cavity at 1.351 μm by end dead mirrors M1 and M4, which leads tothe intracavity Raman conversion mode. The source can use either the 767.5- (E // N g ) or the901.5-cm -1 (E // N m ) Raman lines of the Nd:KGW crystal mounted in the SRS-laser and convertsfundamental laser emission into 1.507- or 1.538-μm eye-safe radiation, respectively.(a)(c)(b)Fig. 1. Optical arrangements of the developed eye-safe sources with (a) self-frequency Ramanconversion, (b) extracavity Raman laser, (c) ring nonlinear converter and (c) intracavity Ramanlaser of the 3 rd Stokes.Figure 1c shows the optical arrangement of the eye-safe source that consists of a ring-cavityRaman laser and the mention above pump laser. The ring cavity is advantageous to designcompact laser systems since there is no s<strong>trong</strong> optical feedback from the Raman converter(optical isolator is not needed) and output beams having different wavelengths of radiation canbe spaced apart. The rods 1 – 3 used in the SRS-laser have the diameter of 2.8 mm and the lengthl c =22 mm. They are oriented for N m -polarezed pumping. The ring cavity consists of three planemirrors. The optical axial contour of the ring cavity is an equilateral triangle, the perimeter ofwhich does not exceed 93 mm. The input mirror M1 is transparent (T > 96%) for the pump. Theoutput coupler M2 is partial-transparent at 1538 nm. To reduce the pump beam size in theRaman laser, a 2.5 X telescope is used.For Nd-doped laser media, the cross section of the stimulated 4 F 3/2 – 4 I 13/2 transition issignificantly lower than that of the traditional 4 F 3/2 – 4 I 11/2 transition at λ=1.06 μm. The 4 F 3/2 –4 I 11/2 channel provides a substantially higher pulsed energy output. However, SRS wavelengt<strong>hc</strong>onversion of a given transition into an eye-safe spectral range requires the generation of higherStokes components than in the case of the 4 F 3/2 – 4 I 13/2 channel, since the Raman frequencies ofthe SRS crystals commonly available for practical use lie in the range of 700–1100 cm -1. Figure(d)35
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