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 - 42711 Clean Cavity2 Water-coated Cavity18000537.1 nm Ppump = 100 mWIntensity (arbitrary units)1600014000120001000080006000400020000510 520 530 540 550 560 570Fig. 6. Spectrum of 537nm-emission for the case: (1) clean cavity created by glass-air gap-polymer, and(2) covering the polymer layer by water drops. The intensity of narrow line-width 537nm-light stillemitted, but it was decreased by scattering on the surface.We obtained series of green lasers emitted at 537 nm along the fiber. The lasing intensitydepended upon the pumped power at the position of random laser and the laser intensitymonotone decreased along the fiber from beginning pump position (see Fig. 7).12Wavelength (nm)Laser intensity at 537 nm (arbitrary units)18001600140012001000800600400200End-fiber pumped power of 2 mW at 976 nm)00 5 10 15 20Distance along fiber axis (cm)Fig. 7. Lasing intensity at 537 nm versus distance from pump position along fiberUsing the model of the coupled <strong>ph</strong>oton-atom modes in the cavity we proposed that the 537nmlasing emission from Er-ions in the fiber can be appeared by followings factors: A diode laseropera<s<strong>trong</strong>>tin</s<strong>trong</strong>>g at 976 nm pumps the Er-ions from their fundamental level 4 I 15/2 to 4 I 11/2 and a second<strong>ph</strong>oton transfers the excited ion to either 4 F 7/2 . This level decays very rapidly to the levels 2 H 11/2and 4 S 3/2 . The split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g of these levels is only some hundreds of cm -1 and the inversion can beachieved between the level 4 S 3/2 and the upper Stark levels of the ground state 4 I 15/2 [12]. In ourcase, the emission at 537 nm does not responded to radiative transition between excited state4 S 3/2 and fundamental state 4 I 15/2 , that mean the emitted <strong>ph</strong>oton is a result of the interactionbetween the resonant cavity <strong>ph</strong>oton and the excited ions on the upper levels 2 H 11/2 and 4 S 3/2 . TheEr-ion population on the state 4 S 3/2 may be more than on 2 H 11/2 , and the different value ofresonant <strong>ph</strong>oton energy and radiative energy of transition 4 S 3/2 - 4 I 15/2 is very small in experiment134
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271(it is equal to 42 meV). We can obtain the probability of interaction of <strong>ph</strong>oton-excited atom onstate 4 S 3/2 would be more than with atom on state 2 H 11/2 in the cavity.IV. CONCLUSIONWe have been experimentally observed the green lasing emission at 537 nm from Er-dopedoptical fiber pumped by laser diode of 976nm. The lasing wavelength at 537 nm did notresponded to any radiative transitions in the Erbium atoms. The experimental evidence showsthat, the <strong>ph</strong>oton-atom interaction appeared due to cavity randomly created between glass fiber-airgap-polymer layer cover, which has Q-factor of 2100-2800. From theory of interactions betweenthe cavity-resonant <strong>ph</strong>oton and the excited Er-ions on the upper levels we can explain the<strong>ph</strong>enomenon of the emission of <strong>ph</strong>oton energy differed from the radiative transition energy ofthe atoms. It is remarkable that we can obtain series of random cavity lasers along the Er-dopedsilica fiber. There is the first step toward a novel green laser with adjus<s<strong>trong</strong>>tin</s<strong>trong</strong>>g emissionwavelengths based on Er-doped fibers for optical sensors.V. ACKNOWLEDGMENTSThis work was partly supported by the Vietnam-Belarus Cooperation Project in Physics andby the Grant of MOET B2010-TN03-29.REFERENCES[1] Lawandy N M, Balachandran R M, Gomes A S L, Sauvin E, Laser action in s<strong>trong</strong>ly scatteringmedia, Nature, 1994, 368, 436-438.[2] Wiersma D S, Lagendijk A, Light diffusion with gain and random lasers, Phys. Rev. E, 1996, 54,4256-4265.[3] Wiersma D S, Cavalieri, A temperature-tunable random laser, Nature, 2001, 414, 708-709.[4] Vanneste C, Sebbah P, Selective excitation of localized modes in active random media, Phys. Rev.Lett., 2001, 87, 183903.[5] Takeda S, Obara M, Random microresonators enable localized-mode tuning, SPIE Newsroom. May2009. doi:10.1117/2.1200904.1605[6] Chaikina E I, Lizarraga N, Mendez E, Optical-fiber random lasers, SPIE Newsroom, 2009. doi:10.1117/2.1200906.1688[7] Burin A L, Ratner M A, Cao H, Chang R P H, Model for a Random Laser, Phys. Rev. Lett, 2001, 87,215503-1-4.[8] De Boer J F, van Albada M P, Lagendijk A, Transmission and intensity correlation in wavepropagation through random cavity, Phys.Rev. B, 1992, 45, p.658 -666.[9] Yablonovitch E, Inhibited spontaneous emission in solid-state Physics and Electronics, Phys. Rev.Lett., 1987, 58, 2059-2062.[10] Weisbuch C, Nishioka M, Ishikawa A, Arakawa Y, Observation of the coupled Exciton-<strong>ph</strong>otonmode split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g in a semiconductor quantum microcavity, Phys. Rev. Lett., 1992, 69, 3314-3317.[11] Bjork G, Yamamoto Y, Analysis of semiconductor microcavity lasers using rate equations, EEE J.Quantum Electron. 1991, 27, 2386-2396.[12] Von Klitzing W, Jahier E, Long R, Lissillour F, Lefevre-Seguin V, Hare J, Raimond J-M, HarocheS, Very low threshold green lasing in micros<strong>ph</strong>ere by up-conversion of IR <strong>ph</strong>otons, J.Opt. B:Quantum Semiclass. Opt. 2000, 2, 204-206.135
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Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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