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Optical properties of erbium-doped silica fibers obtained by sol–gel method
Spectrochimica Acta Part A 55 (1999) 369 – 373
Optical properties of erbium-doped silica fibers obtained by sol–gel method E.M. Pawlik a,*, W. Stre˛k b, P.J. Dere´n b, J. Wo´jcik c, G.E. Malaskevich d, V.E. Gaishun e Institute of Telecommunications and Acoustics, Wrocl*aw Uni6ersity of Technology, Wrocl*aw, Poland Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wrocl*aw, Poland c Department of Optical Fibers Technology, Maria Curie-Skl*odowska Uni6ersity, Lublin, Poland d Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, Minsk, Belarus e Gomel State Uni6ersity, Gomel, Belarus
a b
Received 15 November 1997; received in revised form 25 May 1998; accepted 26 May 1998
Abstract In this paper, the authors discuss optical properties of silica glass and fibers, both doped and non-doped, obtained by the sol–gel method. Transmission and luminescence characteristics of glasses thus obtained were measured. It was found that the 1.5 mm emission was invoked in the fiber exposed to the 0.97 mm excitation. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Sol –gel glass; Optical fiber; Erbium-doped
1. Introduction Erbium-doped optical quartz fibers are the subject of extensive investigation in recent years, due to their potential applicability in optical amplifiers. Of interest are both the preparation of the fibers and the fibers’ transmission and luminescence qualities. Traditional erbium-doped fibers, obtained by various methods [1,2] are single-mode ones with the core diameter of a few micrometers [3,4]. * Corresponding author. Tel.: +48-71-3202119; fax: + 4871-3203189.
Below are presented transmission and luminescence properties of a fiber obtained by the sol– gel method and doped with erbium. Preliminary reports on spectroscopic properties of erbium doped fibers have been presented by us in [5–8]. Since the fibers exhibit relatively high attenuation, 200 mm core-dia., step-index-type specimens were employed to simplify the investigation. High concentration of OH − groups is the main technological factor which accounts for the increased attenuation and lower efficiency of luminescent emission in these fibers. A significant reduction in hydroxyl ion concentration can be
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 1 9 5 - 4
370
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 1. Attenuation of erbium (III)-doped fiber versus wavelength.
achieved at the production stage by thermal treatment of xerogel in the atmosphere of freon. In spite of some serious problems which are encountered in obtaining fibers with low OH − ion content, production and investigation of the fibers obtained by the sol – gel method and doped with erbium is highly prospective. This method offers the advantage of a relatively simple production procedure of the vitreous material, which is moreover carried out at low temperatures.
2. Materials and preparation Starting materials substrates for the production of sol – gel glass were Si (OC2H2)4, hydrochloric acid as catalyst, powdered silicon dioxide SiO2 and distilled water. In order to obtain the sol, hydrolysis of tetraethyloorthosilicate has been carried out in the acid water solution (H2O:Si(OC2H2)4:HCl molar ratios were as 1:16:0.01). Colloidal suspension has been prepared by adding the powdered silicon dioxide and the activator to the sol obtained. Then neutralization of the activated sol-colloidal suspen-
sion system has been done by the addition of ammonia solution. The resulting sol was dried and vitrified in controlled time and temperature conditions and then left to cool down by itself. Vitrification conditions were diversified by running the process in air, in vacuum, and in the atmospheres of F2 and Cl2. In the case of F2 and Cl2 atmospheres, OH − ions were removed, respectively, converting the sol into a high-transparency glass. Some of the monolithic xerogel samples were impregnated with water solution of ErCl3 · 6H2O. Sol–gel glasses usually contain a considerable amount of OH − groups, which reduce the efficiency of luminescent emission. A significant reduction in hydroxyl ion concentration is possible by thermal treatment of xerogel in the freon atmosphere. For some samples, xerogel was annealed in C2Cl3F3 freon atmosphere at 1000°C for 2 h. The samples were then vitrified in air during 2 h. Then rods were formed from small samples of the doped and non-doped glass, which were subsequently used in the drawing of PCS-type fibers with the core diameter of 200 mm.
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 2. Attenuation of undoped-fiber versus wavelength.
3. Experiments and results The attenuation of erbium (III)-doped and undoped fibers was measured in the optical range 700–1700 nm (Figs. 1 and 2). The measurements employed the cut-off technique [3]. Spectrum of the doped fiber exhibits characteristic absorption
371
bands of the Er3 + due to the transitions from the 4 I15/2 ground level to the 4I13/2, 4I11/2, 4I9/2 and 4 H11/2 levels. The maximum at 977 nm of the 4I11/2 absorption band matches well the wavelength of the pump diode. Both the doped and undoped fiber exhibits two peaks due to the concentration of the OH2 − ions. The second overtone of the OH − vibration makes the fiber opaque from 1360 to 1520 nm (Fig. 1), whereas the third overtone appears at 960 nm. The doped, as well as undoped, fibers reveal high attenuation in the IR area, which may prevent the amplification of signals. However, an attempt was undertaken to obtain luminescent excitation, both in the raw glass samples, and in the fiber obtained from them. The emission spectra were measured using a THR 1000 monochromator by Jobin Yvon, an argon laser (Carl Zeiss Yena) was used as an excitation source. The luminescence spectrum of Er3 + ions in a bulk glass sample, after excitation by an argon laser (l=488 nm), is presented in Fig. 3. The characteristic peak has been observed at 1534 nm, which is an evidence of the presence of erbium ions. This peak was assigned to the 4I3/2 4I15/2 transition (see diagram in Fig. 5).
Fig. 3. The 300 K luminescence spectrum of Er3 + ions in the bulk glass sample, after excitation by an argon laser.
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E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 4. Spectrum of IR diode laser used for excitation of the fiber.
In order to obtain the highest luminescent emission efficiency possible for the erbium-doped fibers, a semiconductor laser have been selected with a determined emission band. The spectrum of the 100 mW laser which was used in the investigation to induce luminescent emission, is shown in Fig. 4. The laser light was introduced into a 0.5 m long investigated fiber by means of a specially constructed collimator and a resulting
Fig. 6. The room temperature luminescence of the fiber excited by IR diode laser.
luminescent emission was observed. Fig. 6 presents the emission obtained from the fiber sample. The luminescent radiation was observed also at 1534 nm. This intensity was low and constituted 1/100 of the pumping diode’s basic intensity. The obtained signal was thus weak due to the strong attenuation at 1500 nm (Fig. 1).
4. Conclusions According to our knowledge this is the first report of an emission of erbium doped fibers obtained by the sol–gel technique. The results would improve in absence of the OH groups in the fiber. Our future effort will be directed towards better preparation of the sol–gel glass.
References
Fig. 5. Diagram of Er3 + -ion energy levels in a glass sample.
[1] B.J. Ainslie, J. Lightwave Technol. 9 (2) (1991) 220 –227. [2] W.J. Miniscalco, J. Lightwave Technol. 9 (2) (1991) 234– 250. [3] International Standard EC 793-1, Optical fibres, Part 1, Generic specification, 1989. [4] M.J.F. Digonnet (Ed.), Rare Earth Doped Fiber Lasers and Amplifiers, Marcel Dekker, New York, 1993. [5] P.J. Deren, K. Maruszewski, W. Strek, G.E. Malashkevich, E.N. Poddenezhny, Proc. SPE 2780 (1996) 332 – 335. [6] E.M. Pawlik, W. Strek, G.E. Malashkevich, I.M. Melnichenko, E.N. Poddenezhny, Proc. SPIE 2780 (1995) 364 – 366.
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373 [7] E.M. Pawlik, W. Strek, J. Wojcik, C. Wojciechowski, G.E. Malashkevich, J.M. Melnichenko, E.N. Poddenezny, Proc. SPIE 3186 (1996) 56–59.
.
373
[8] E.M. Pawlik, W. Strek, P.J. Deren, G.E. Malashkevich, E.N. Poddenezhny, I.M. Melnichenko, V.I. Gaishun, Proc. SPIE 3176 (1997) 299 – 302.
Optical properties of erbium-doped silica fibers obtained by sol–gel method E.M. Pawlik a,*, W. Stre˛k b, P.J. Dere´n b, J. Wo´jcik c, G.E. Malaskevich d, V.E. Gaishun e Institute of Telecommunications and Acoustics, Wrocl*aw Uni6ersity of Technology, Wrocl*aw, Poland Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wrocl*aw, Poland c Department of Optical Fibers Technology, Maria Curie-Skl*odowska Uni6ersity, Lublin, Poland d Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, Minsk, Belarus e Gomel State Uni6ersity, Gomel, Belarus
a b
Received 15 November 1997; received in revised form 25 May 1998; accepted 26 May 1998
Abstract In this paper, the authors discuss optical properties of silica glass and fibers, both doped and non-doped, obtained by the sol–gel method. Transmission and luminescence characteristics of glasses thus obtained were measured. It was found that the 1.5 mm emission was invoked in the fiber exposed to the 0.97 mm excitation. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Sol –gel glass; Optical fiber; Erbium-doped
1. Introduction Erbium-doped optical quartz fibers are the subject of extensive investigation in recent years, due to their potential applicability in optical amplifiers. Of interest are both the preparation of the fibers and the fibers’ transmission and luminescence qualities. Traditional erbium-doped fibers, obtained by various methods [1,2] are single-mode ones with the core diameter of a few micrometers [3,4]. * Corresponding author. Tel.: +48-71-3202119; fax: + 4871-3203189.
Below are presented transmission and luminescence properties of a fiber obtained by the sol– gel method and doped with erbium. Preliminary reports on spectroscopic properties of erbium doped fibers have been presented by us in [5–8]. Since the fibers exhibit relatively high attenuation, 200 mm core-dia., step-index-type specimens were employed to simplify the investigation. High concentration of OH − groups is the main technological factor which accounts for the increased attenuation and lower efficiency of luminescent emission in these fibers. A significant reduction in hydroxyl ion concentration can be
1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 1 9 5 - 4
370
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 1. Attenuation of erbium (III)-doped fiber versus wavelength.
achieved at the production stage by thermal treatment of xerogel in the atmosphere of freon. In spite of some serious problems which are encountered in obtaining fibers with low OH − ion content, production and investigation of the fibers obtained by the sol – gel method and doped with erbium is highly prospective. This method offers the advantage of a relatively simple production procedure of the vitreous material, which is moreover carried out at low temperatures.
2. Materials and preparation Starting materials substrates for the production of sol – gel glass were Si (OC2H2)4, hydrochloric acid as catalyst, powdered silicon dioxide SiO2 and distilled water. In order to obtain the sol, hydrolysis of tetraethyloorthosilicate has been carried out in the acid water solution (H2O:Si(OC2H2)4:HCl molar ratios were as 1:16:0.01). Colloidal suspension has been prepared by adding the powdered silicon dioxide and the activator to the sol obtained. Then neutralization of the activated sol-colloidal suspen-
sion system has been done by the addition of ammonia solution. The resulting sol was dried and vitrified in controlled time and temperature conditions and then left to cool down by itself. Vitrification conditions were diversified by running the process in air, in vacuum, and in the atmospheres of F2 and Cl2. In the case of F2 and Cl2 atmospheres, OH − ions were removed, respectively, converting the sol into a high-transparency glass. Some of the monolithic xerogel samples were impregnated with water solution of ErCl3 · 6H2O. Sol–gel glasses usually contain a considerable amount of OH − groups, which reduce the efficiency of luminescent emission. A significant reduction in hydroxyl ion concentration is possible by thermal treatment of xerogel in the freon atmosphere. For some samples, xerogel was annealed in C2Cl3F3 freon atmosphere at 1000°C for 2 h. The samples were then vitrified in air during 2 h. Then rods were formed from small samples of the doped and non-doped glass, which were subsequently used in the drawing of PCS-type fibers with the core diameter of 200 mm.
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 2. Attenuation of undoped-fiber versus wavelength.
3. Experiments and results The attenuation of erbium (III)-doped and undoped fibers was measured in the optical range 700–1700 nm (Figs. 1 and 2). The measurements employed the cut-off technique [3]. Spectrum of the doped fiber exhibits characteristic absorption
371
bands of the Er3 + due to the transitions from the 4 I15/2 ground level to the 4I13/2, 4I11/2, 4I9/2 and 4 H11/2 levels. The maximum at 977 nm of the 4I11/2 absorption band matches well the wavelength of the pump diode. Both the doped and undoped fiber exhibits two peaks due to the concentration of the OH2 − ions. The second overtone of the OH − vibration makes the fiber opaque from 1360 to 1520 nm (Fig. 1), whereas the third overtone appears at 960 nm. The doped, as well as undoped, fibers reveal high attenuation in the IR area, which may prevent the amplification of signals. However, an attempt was undertaken to obtain luminescent excitation, both in the raw glass samples, and in the fiber obtained from them. The emission spectra were measured using a THR 1000 monochromator by Jobin Yvon, an argon laser (Carl Zeiss Yena) was used as an excitation source. The luminescence spectrum of Er3 + ions in a bulk glass sample, after excitation by an argon laser (l=488 nm), is presented in Fig. 3. The characteristic peak has been observed at 1534 nm, which is an evidence of the presence of erbium ions. This peak was assigned to the 4I3/2 4I15/2 transition (see diagram in Fig. 5).
Fig. 3. The 300 K luminescence spectrum of Er3 + ions in the bulk glass sample, after excitation by an argon laser.
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E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373
Fig. 4. Spectrum of IR diode laser used for excitation of the fiber.
In order to obtain the highest luminescent emission efficiency possible for the erbium-doped fibers, a semiconductor laser have been selected with a determined emission band. The spectrum of the 100 mW laser which was used in the investigation to induce luminescent emission, is shown in Fig. 4. The laser light was introduced into a 0.5 m long investigated fiber by means of a specially constructed collimator and a resulting
Fig. 6. The room temperature luminescence of the fiber excited by IR diode laser.
luminescent emission was observed. Fig. 6 presents the emission obtained from the fiber sample. The luminescent radiation was observed also at 1534 nm. This intensity was low and constituted 1/100 of the pumping diode’s basic intensity. The obtained signal was thus weak due to the strong attenuation at 1500 nm (Fig. 1).
4. Conclusions According to our knowledge this is the first report of an emission of erbium doped fibers obtained by the sol–gel technique. The results would improve in absence of the OH groups in the fiber. Our future effort will be directed towards better preparation of the sol–gel glass.
References
Fig. 5. Diagram of Er3 + -ion energy levels in a glass sample.
[1] B.J. Ainslie, J. Lightwave Technol. 9 (2) (1991) 220 –227. [2] W.J. Miniscalco, J. Lightwave Technol. 9 (2) (1991) 234– 250. [3] International Standard EC 793-1, Optical fibres, Part 1, Generic specification, 1989. [4] M.J.F. Digonnet (Ed.), Rare Earth Doped Fiber Lasers and Amplifiers, Marcel Dekker, New York, 1993. [5] P.J. Deren, K. Maruszewski, W. Strek, G.E. Malashkevich, E.N. Poddenezhny, Proc. SPE 2780 (1996) 332 – 335. [6] E.M. Pawlik, W. Strek, G.E. Malashkevich, I.M. Melnichenko, E.N. Poddenezhny, Proc. SPIE 2780 (1995) 364 – 366.
E.M. Pawlik et al. / Spectrochimica Acta Part A 55 (1999) 369–373 [7] E.M. Pawlik, W. Strek, J. Wojcik, C. Wojciechowski, G.E. Malashkevich, J.M. Melnichenko, E.N. Poddenezny, Proc. SPIE 3186 (1996) 56–59.
.
373
[8] E.M. Pawlik, W. Strek, P.J. Deren, G.E. Malashkevich, E.N. Poddenezhny, I.M. Melnichenko, V.I. Gaishun, Proc. SPIE 3176 (1997) 299 – 302.