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Photosensitive Yb-doped double-clad fiber for fiber lasers A.S.Kurkov, O.I.Medvedkov, V.I.Karpov, S.A.Vasiliev, O.A.Lexin, E.M.Dianov Fiber Optics Research Center at the General Physics Institute of the Russian Academy of Sciences, 38 Vavilov Str., 11 7756,MOSCOW, Russia, fax: (095)135-81-39, E-mail:
[email protected]
A.N.Gur' yanov, A.A.Laptev, A.Umnikov, N.I.Vechkanov Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinin Str., 603600, Nizhny Novgorod, Russia.
Introduction High power lasers based on Yb-doped double-clad fibers have shown a potential for telecommunication, material processing, medical applications, etc. Development of high power diode arrays and progress in the field of fiber Bragg gratings allowed producing compact Yb-doped fiber lasers with output power up to 20 W [ 11 and radiation wavelength in the range between 1020 and 1150 nm [2]. As a rule Bragg gratings that compose a laser cavity are fabricated in a standard fiber and then spliced with an Yb-doped active fiber. Difference of parameters of standard and active fibers leads to additional losses of both pump and signal. Therefore it is evident that an improvement of laser efficiency can be achieved if Bragg gratings are written directly in an Yb-doped fiber which acts as a laser medium. In this paper we report fabrication of photosensitive double-clad Yb-doped silica fibers as well as properties of lasers based on this fiber type.
Fiber design To provide sensitivity of Yb-doped fiber to 244 nm light without hydrogen loading, a germanosilicate host glass matrix with relatively high germanium concentration (up to 12 mol.%) have been selected. Such glass composition decreases the core diameter and, therefore, absorption of the pump. Due to strong effect of background losses on laser output power in such fiber type a laser length should be limited by 20 t 50 m, and, therefore, Yb concentration has to be increased up to 180 0.5 + 1.5 wt.%. It was observed that heavily Yb-doped fibers based on SiO2GeO2 glass have background losses in the core as high as 200 + 250 dB/km. 8 o,6 Therefore an additional co-doping with A1203 has been used to improve the ytterbium solubility and to fabricate the oi Ov4 fibers with background losses less than os2 10 dB/km. In our experiments the inner cladding of rectangular and D-shape ' OIO;, geometry has been used. In both 10 20 30 40 50 cladding shapes a pump absorption Time, min efficiency was 0.9 + 1. Silicon ribbon Fig.1. Time dependence of reflection was used to fabricate the outer cladding coefficient of 4-mm-long Bragg grating. which resulted in fiber numerical aperture of 0.38 t 0.4. '
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Bragg grating formation To write Bragg gratings a frequency-doubled Ar-ion laser radiation in an interferometric writing scheme has been used. Typical power density was about 100 w/cm2. Grating transmission spectra were measured with the help of a Yb-luminescence source pumped at 980 nm and an optical spectrum analyser with a spectral resolution of 0.1 nm. Fig. 1 shows typical behaviour of reflection coefficient of 4-mm-long Bragg grating written in an Yb-doped fiber with 8 mol.% Ge02 without additional hydrogenation. It is clearly seen that the fiber tested exhibited the photosensitivity of type IIa, which is typically observed in fibers heavily (18 mol.% and more) doped with germanium [3]. Taking into consideration the Bragg wavelength :shift which took place during grating writing the total mean UV-induced refractive index change in the fiber core S of about 8 ~ 1 has 0 ~been estimated. It should be mentioned that type IIa photosensitivity is not observed in fibers with 8 mol.% GeO2 and this . phenomenon should be related with AI 0 and/or Yb codoping of the fiber core. To clarify this question further Pump power, W investigations have to be carried out. Fig.2. Dependence of laser output power on pump power for two pump wavelengths. Laser experiments Several lasers based on doubleclad Yb-doped fibers with Bragg gratings written directly in these fibers have been tested. To pump the fiber lasers two Opt0 Power semiconductor sources with wavelengths of 910 nm and 980 nm have been used. Laser cavities were formed by two Bragg gratings centered at the wavelength of about 1090 nm with reflection coefficients of 95% (high reflector) and about 40% (output grating). Cavity lengths varied in the range of 20 + 30 m. Fig.2 shows dependence of laser output power on pump power for the lasers based on the double-clad fiber with 8 mol.% of GeO2 for pump wavelengths used. Differential 0 efficiencies as high as 65% and rdatively low threshold of about 100 mW have been observed for both -10 pump wavelengths. Quantum 8 efficiencies of 78% and 72% have .-i .-20 been estimated for 910 nm and 980 E' 0 nm pump wavelengths, respectively. 5 5o Typical emission spectrum of laser based on a fiber with 1 wt.% of -40Yb203 and pumped at 910 nm is shown in fig.3. 1088 1090 1092 1094 1096 1098 1100 To improve laser efficiency WaElength, nm 1
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we fabricated the fiber with an increased Yb203 (1.5 wt.%) and GeO2 (10 mol.%) content in the fiber core. A laser based on this fiber had Bragg
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Fig.3. Emission spectrum of double-clad laser based on a fiber with I wt.% of Y b 2 0 3 in the fiber core.
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grating with reflection of 96% at 1084 nm as an input mirror, whereas the cleaved fiber end (R-4%) was used as an output mirror. Emission spectrum of this laser at a pump power of about 4W (h = 910 nm) is shown in fig.4. This spectrum consisted of several bands centered at 1036, 1084, 1134, 1190, 1259 and 1331 nm that can be due to stimulated Raman scattering. Indeed, frequency shifts between different bands are in the range of 410-430 cm-’ that corresponds to the main Raman band in germanosilicate glass.
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Wavelength, nm Fig.4. Emission spectrum of a laser based on the fiber doped by 1.5 wt.% of mol.% GeOz.
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It is significant that the intensity of the first anti-Stokes wave is even higher than that at the wavelength of Bragg grating. We suppose that this fact can be explained by efficient energy transfer from main emission band to anti-Stokes wave. This process can be accelerated by the phase matching of these waves which can be due to anomalous dispersion of Yb-doped glass near 980 nm band. In addition the wavelength of anti-Stokes wave corresponds to the maximum emission cross-section of Yb-ions. One can expect that the phenomenon of stimulated Raman scattering observed in our experiments can be used for intracavity cascaded Raman conversion to long-wavelength Stokes orders, if appropriate Bragg gratings are written in the fiber, as it was demonstrated for Ge- and P-doped fibers [4,5].
Conclusions We have fabricated photosensitive at 244 nm Yb-doped double-clad fibers that can be used for efficient fiber lasers. Bragg grating prepared in these fibers exhibited photosensitivity of type IIa, while Ge02 concentration is not sufficient for such grating type. Fiber lasers based on developed fibers have a differential efficiency as high as 65% for pumping at 980 and 910 nm. Strong changes of laser emission spectrum caused by stimulated Raman scattering inside a laser cavity have been observed for the fiber with increased Yb and Ge concentration. This effect can be used for the creation of Raman converters. References 1. D. Innis et al, Proc. of CLEO’97, Postdeadline paper. 2. H.M.Pask et al, ZEEE Journal of Selected Topics in Quantum Electronics, Vol.1, pp.2-13, 1995. 3. P.Niay et al, Optics Communications, Vol.113, pp.176-192, 1994. 4. S.G.Grubb et al, Proc. Optical Amplifiers and Their Applications, 1994, paper PD3. 5. E.M.Dianov et al, Electronics Letters, Vo1.33, pp. 1542-1544, 1997.
