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 - 4271reflection at the interfaces of layers with different refractive indices. From the reflection a<s<strong>trong</strong>>tin</s<strong>trong</strong>>terfaces of the structured materials, we can find the reflected light has a significant intensity ifoptical path difference equals k λ03( k is an integer), then the λ 03 is called resonant wavelengthin the cavity.GlassPolymerAir-GapFig. 2. SEM-Image of the air-gap between glass and polymer layers.Figure 3 shows experimentally observed green emission spectra from Er-ions doped in theoptical fibers, when the pumping power of 70 mW at wavelength of 976nm for both cases: (a)Narrow emission mode at 537 nm obtained from random cavity created by silica glass fiber – airgap – polymer coated layer and (b) Broad-band emission spectrum with two peaks at 522 nm and547 nm obtained when the random cavity was destroyed.Intensity (arbitrary units)1400012000100008000600040002000537.1 nm522.9 nmPumped power of 70 mW at 976 nm(a)547.2 nm0520 525 530 535 540 545 550 555 560 565 570Wavelengh (nm)Fig. 3. Experimentally observed emission spectrum from Erbium ions doped into optical fiber:(a) Lasing spectra at 537nm by the coupled <strong>ph</strong>oton-atom mode in the random cavity, and (b) broad-bandupconversion fluorescence emission spectrum with two peaks at 522nm and 547nmLight emitted by Er-ions from center of the fiber can be confined to random cavity under theappropriate resonance conditions by reflections of the light at the interface of glass – air gap andair gap – polymer. In our case, the cavity resonant wavelength of 537 nm was not equal to522nm and/or to 547 nm, that responded to radiative transitions 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2in Er-ions, respectively. The measured line-width of laser spectra was of 0.2-0.25 nm that meansthe Q-factor of random cavity was of 2100-2800. Figure 4 demonstrates the intensity of 537 nm(b)132
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271narrow line-width emission as a function of the pump intensity at 976 nm. Upon increasing thepumped intensity, the 537 nm green light intensity stays in the linear lasing regime. It should benoted that the lasing threshold was very low (at pumped power of 2-3 mW for 6m-long Er-dopedfiber and the laser output remains linear with respect to the optical pump power even at sometens times above threshold.Lasing wavelength at 537nm from side-band of fiber)Intense of laser emission at 537 nm (a.u)140001200010000800060004000200000 10 20 30 40 50 60 70 80 90 100 110 120Optical Pumped Power (mW)Fig. 4. The 537 nm–lasing emission intensity versus the 976nm-laser pump power. Inset: Spectrum at537nm versus pumped power.For tes<s<strong>trong</strong>>tin</s<strong>trong</strong>>g formation of a random cavity which has structure of glass-air gap-polymer cover,we destroyed the air-gap by three ways: (i) removing the coated polymer cover, (ii) destroyingair-gap by depressing polymer cover and (iii) covering polymer layer by water and/or alcohol.For the first two cases (without air-gap in the structure of the fiber) the emission at 537 nm wasdisappeared, that mean the cavity of laser was absolutely destroyed (see Fig.5). In the last case,the 537nm-emission intensity was not disappeared. Figure 6 shows the spectra of 537 nmemissionsfrom the Er-doped fiber with polymer layer covered by water drops. The intensity ofthe emitted light at 537 nm was decreased by scattering on the fiber surface.Without random cavity (removing polymer layer)With random cavity created by glass-air gap-polymer layerOptical Intensity (arbitrary units)120010008006004002000510 520 530 540 550 560 570Wavelength (nm)Fig. 5. Spectrum of light emission from Er-ions in fiber. (1) Without air-gap in the fiber, the emissionwas disppeared; (2) With air-gap thickness of about 950 nm, the light at 537nm with narrow line-widthwas emitted.(2)(1)133
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Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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