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Pr3+-doped network of halides in silica: A new approach to 1.3μm light amplifying material
JOURIqA~ O F
ELSEVIER
Journal of Non-Crystalline Solids 215 (1997) 108-110
Letter to the Editor
pr3+-doped network of halides in silica: a new approach to 1.3 txm light amplifying material Sui Hua Yuan * Department of Physics, SW China Normal University, Chongqing 630715, People's Republic of China Received 17 July 1996; revised 26 February 1997
Abstract
A new approach is proposed to combine the advantages of halides (or chalcogenides) in offering good light amplifying environment for Pr 3÷ ions with the advantages of silica in offering good chemical shielding and mechanical support to the halides to make a new 1.3 i~m light amplifying material. The pr3+-doped halide glass network is compounded in the silica host. Preliminary samples consist of a network of Pr3+-doped chloride glass interwoven with a network of silica glass. The nodal pitch is 30 nm on average for both the chloride glass and silica glass skeletons. 1.3 Ixm fluorescence is observed on the pr3+-doped sample while 0.87 and 1.06 Ixm fluorescence are observed on the Nd3÷-doped sample with the intensity ratio ~ 2:1 close to that in Nd3+-doped fluorozirconate. © 1997 Elsevier Science B.V.
The demonstration of 1.3 Ixm amplification by a praseodymium-doped zirconium fluoride (ZBLAN) glass fiber amplifier (PDFA) was reported in 1991 [1]. Since then efforts have been made to develop a praseodymium-doped fluoride single-mode fiber with a large numeric aperture ( > 0.4) and the means of bi-directional pumping to ensure gain efficiency and a low noise figure (NF) to PDFA in which the 1.3 I~m quantum efficiency of praseodymium is small [2]. Recently a gain efficiency of 0.4 d B / m W and a large small signal gain of 42 dB and a quantum slope efficiency of 50% when operating as booster have been reported [3,4]. For practical use an even higher quantum efficiency is desired. Chalcogenide, mixed halide and indium fluoride glasses are being considered as new efficient hosts for praseodymium [3,4].
* Fax: +86-811 886 6796.
The reliability of the amplifier is also a considerable problem although some authors declare that the reliability problem is solved [5]. In addition any PDFA with non-silica as a host cannot be coupled to traditional silica fibers by welding. To solve the problems mentioned above we propose a material with a praseodymium-doped chloride glass network interwoven with a silica glass network. The nodal pitch is of the order of 10 nm for both the chloride and silica skeletons. A preliminary preparation has been made in our laboratory as a new 1.3 txm light amplifying material. Fluorine and chlorine ions are isoelectronic while chlorine ions have a larger mass and ionic radii. Therefore, chloride offers similar ligand fields to the praseodymium ions as fluorine does and phonons in a chloride have smaller energy than in fluoride. Assuming the line width of the lasing 1.3 Ixm light in the chloride composition is 30 nm as in the fluoride composition, the coherent length is A2/A A
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 1 4 7 - 6
S.H. Yuan / Journal of Non-Crystalline Solids 215 (1997) 108-110
~ 58000 nm. The lasing light from each of the praseodymium ions in chloride may be coherent. The Rayleigh scattering cross-section of 1.3 I~m light by a 10 nm diameter cluster is about 10 -21 cm 2 while the lasing cross-section of the rare-earths' ion is about 10 -20 cm 2 [5]. For a pr3+-doping concentration of 400 ppm in chloride [6], there are many praseodymium ions in each chloride cluster. Thus, the loss of 1.3 Izm light due to scattering would be minimal and dominated by the gain of light at 1.3 Ixm as long as the chloride network distributes in the silica host uniformly reducing density fluctuations. The praseodymium ions in the chloride would be shielded from the phonons in silica if the dimension of the network is appropriate. The phonons in silica with a higher energy can cause irradiative decay of electrons and are harmful to quantum efficiency [7]. The silica host also provides the praseodymium ion-doped chloride network with good chemical protection and mechanical support. Furthermore, if the concentration of the praseodymium ion-doped chloride network (or clusters) is reduced smoothly along one dimension to the edge of the composite material, the edge portion of the material may be weldable with silica fibers. Since lasing at many different wavelengths ranging from the visible region of the spectrum to almost 3 txm can be achieved by doping, many of the lanthanide series of elements in halide glasses [8] and especially flatter gain spectrum of 1.5 p~m light amplification can be achieved by doping erbium ions in fluoride glasses [9]; our approach may also provide diversity. The host porous silica glass made in our laboratory for preliminary experiment did not contain germanium as an index adjuster and was manufactured by phase separation and chlorine acid etching of a sodium-boron rich silicate glass. The sodium-boron rich silicate glass was prepared by melting at 1400°C the burden of composition 8.4Na20.0.9AI20 3 • 24.7B20 3 • 66SIO 2 (mol%). Phase separation of this glass was produced at a temperature of 560°C for 20 h. Then a sample was etched with HC1 (2 N) and NH4CI (34%) as buffer at ~ 93°C for 10 h to get porous silica glass. The porous silica glass was evacuated and then injected with the mixture solution of PrCI 3 and the burden chloride for the chlorine glass (BaC12-
109
PbCI2-CaCI2-NaC1) by sinking the evacuated porous glass in the solution for 20 h. The concentration of Pr 3+ ions was controlled in the range 50 to 1000 ppm (by the weight of Pr 3÷ ions in the chloride). The sample was then processed by drying and sintering for 2 h in vacuum at ~ 925°C, near to the melting point of the chloride glass and within the sag temperature range of the silica glass. Finally the sample was polished for measurement. Nd3+-doped samples were made as accompaniments to allow ourselves to become acquainted with the processes. The porous glass has a structure of an interwoven silica skeleton with an average dimension 0.5 Izm of the skeleton's width and of the pores in the skeleton, which was measured by a scanning electronic microscope (SEM, Hitachi S-2700). The porosities of the samples were measured as ~ 50% applying the Archimedean principle. The surfaces of the Pr 3÷doped and Nd3+-doped samples were observed by SEM. Fig. 1 shows the typical picture of the pr3+doped samples; the average dimension of the pr3+-doped chloride network (black portions in the figure) was about 30 nm according to the calibration in Fig. 1. The Nd3+-doped samples were similar to Fig. 1. The amorphous structure of the doped samples with doping concentrations less than 1000 ppm were verified by the absence of X-ray powder diffraction lines measured on a X-ray diffractometer (Rigaku D/max-3A). The fact that the praseodymium ion-
Fig. 1. Typical SEM micrograph of the samples of Pr3+-doped chloride networkcompoundedin silica, sinteredbetween900 and 950°C in vacuum.The black portionsare the pr3+-dopedchloride network.
I lO
S.H. Yuan/Journal of Non-CrystaUine Solids 215 (1997) 108-110
doped chloride network has an amorphous structure is important as it can ensure high quantum efficiency to the praseodymium ions by allowing for dipole transitions [2]. As the concentration was increased to 10000 ppm, typical lines of quartz crystal were observed on the background of the amorphous spectrum. The index, n, of the sintered porous silica glass was measured, n = 1.481. The indices of the Pr 3+doped silica glasses were in the range from 1.487 to i.496 depending on the doping concentrations. The fluorescence spectra of the pr3+-doped (100 ppm) and Nd3+-doped (100 ppm) samples were measured using pumping wavelengths at 1020 and 822 nm, respectively. Weak 1.3 ~ m fluorescence was observed on the pr3+-doped sample but the lifetime was not measured. Because of the small size of the sample it was not polished. The Nd 3+-doped sample was polished and the fluorescence spectrum was observed as shown in Fig. 2. Two peaks at 870 and 1060 nm are the electronic transitions of Nd 3+ ions, 4 4 4F3/2-419/2 and F3/2- Ill/2. The ratio of the intensities of the two transitions is about 2:1, similar to that observed in Z B L A N [10], which indicates that the environment provided by the chloride clusters for the fluorescence of the Nd 3+ ions is similar to that provided by Z B L A N and the phonons from the silica network are shielded effectively from intruding into the interior of the chloride clusters.
wa.velength (lz zn)
Fig. 2. Fluorescence spectrum of the sample of Nd3+-doped chloride network compoundedin silica. The two peaks at 870 and 1060 nm correspond, respectively, to the transitions 4F3/2-419/2 and 4F3/z-4Ill/2. The ratio of the intensities of the two transitions is about 2:1, similar to that observed in Nd3+-dopedZBLAN.
Although the approach of compounding a praseodymium doped chloride glass network in a silica host glass to make a new 1.3 txm light amplifying material is only in the preliminary stage, the results obtained indicate that this approach is likely promising. The problem found at present is mainly the preparation of homogeneous samples of appropriate size for processing and optical measurements. We have been doing experiments to produce larger samples adequate for processing and measurements. The gradient distribution of the pr3+-doped chloride clusters along one dimension to opposite surfaces of the material is more conveniently realizable using the tube samples than the block samples.
Acknowledgements The author is thankful to Dr M. Bass and Dr S.G. Grubb for measuring the fluorescence spectra of the samples. The author is thankful to Mrs Xiaowei Chen for her help in the preparation of the manuscript.
References [1] Y. Ohishi, T. Kanamori. T. Kitagawa, S. Takahashi, E. Snitzer, G,H. Sigel Jr., in: Tech. Digest of OFC'91, 1991, PDP 2. [2] T. Kanamori, Y. Terunuma, K. Fujiura, Y. Ohishi, S. Sudo, in: Proceedings of 9th Int. Symp. on Non-Oxide Glasses, 1994, p. 74. [3] M. Yamada, M. Shimizu, Y. Ohishi, J. Temmyo, M. Wada, T. Kanamori, S. Sudo, in: Tech. Digest of OAA'93, 1993, p. 240. [4] T.J. Whitley, J. LightwaveTech. 13 (5) (1995) 744. [5] Y. Ohishi, T. Kanamori, M. Shimizu, M. Yamada, K. Fujiura, S. Sudo, 10th Int. Conf. on IOOFC, 1995, ThA2-3. [6] A. Blixt, P.J. Nilsson, T. Carlnas, B. Jaskorzynska, IEEE Photon. Technol. Lett. 3 (11) (1991) 996. [7] T. Izumitani, H. Toratuni, J. Non-Cryst. Solids 47 (1982) 1. [8] M. Monerie, D. Ronarc'h, F. Auzel, Proc. 17th European Conf. on Optical Communication ECOC'91 (invited paper), 1991, p. 9. [9] D. Ronarch et al., OSA OAA' 93, Yohahama, Japan, 1993, paper PD 10. [10] M. Poulain, J. Lucas, Mater. Res. Bull. 10 (1975) 243.
ELSEVIER
Journal of Non-Crystalline Solids 215 (1997) 108-110
Letter to the Editor
pr3+-doped network of halides in silica: a new approach to 1.3 txm light amplifying material Sui Hua Yuan * Department of Physics, SW China Normal University, Chongqing 630715, People's Republic of China Received 17 July 1996; revised 26 February 1997
Abstract
A new approach is proposed to combine the advantages of halides (or chalcogenides) in offering good light amplifying environment for Pr 3÷ ions with the advantages of silica in offering good chemical shielding and mechanical support to the halides to make a new 1.3 i~m light amplifying material. The pr3+-doped halide glass network is compounded in the silica host. Preliminary samples consist of a network of Pr3+-doped chloride glass interwoven with a network of silica glass. The nodal pitch is 30 nm on average for both the chloride glass and silica glass skeletons. 1.3 Ixm fluorescence is observed on the pr3+-doped sample while 0.87 and 1.06 Ixm fluorescence are observed on the Nd3÷-doped sample with the intensity ratio ~ 2:1 close to that in Nd3+-doped fluorozirconate. © 1997 Elsevier Science B.V.
The demonstration of 1.3 Ixm amplification by a praseodymium-doped zirconium fluoride (ZBLAN) glass fiber amplifier (PDFA) was reported in 1991 [1]. Since then efforts have been made to develop a praseodymium-doped fluoride single-mode fiber with a large numeric aperture ( > 0.4) and the means of bi-directional pumping to ensure gain efficiency and a low noise figure (NF) to PDFA in which the 1.3 I~m quantum efficiency of praseodymium is small [2]. Recently a gain efficiency of 0.4 d B / m W and a large small signal gain of 42 dB and a quantum slope efficiency of 50% when operating as booster have been reported [3,4]. For practical use an even higher quantum efficiency is desired. Chalcogenide, mixed halide and indium fluoride glasses are being considered as new efficient hosts for praseodymium [3,4].
* Fax: +86-811 886 6796.
The reliability of the amplifier is also a considerable problem although some authors declare that the reliability problem is solved [5]. In addition any PDFA with non-silica as a host cannot be coupled to traditional silica fibers by welding. To solve the problems mentioned above we propose a material with a praseodymium-doped chloride glass network interwoven with a silica glass network. The nodal pitch is of the order of 10 nm for both the chloride and silica skeletons. A preliminary preparation has been made in our laboratory as a new 1.3 txm light amplifying material. Fluorine and chlorine ions are isoelectronic while chlorine ions have a larger mass and ionic radii. Therefore, chloride offers similar ligand fields to the praseodymium ions as fluorine does and phonons in a chloride have smaller energy than in fluoride. Assuming the line width of the lasing 1.3 Ixm light in the chloride composition is 30 nm as in the fluoride composition, the coherent length is A2/A A
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 1 4 7 - 6
S.H. Yuan / Journal of Non-Crystalline Solids 215 (1997) 108-110
~ 58000 nm. The lasing light from each of the praseodymium ions in chloride may be coherent. The Rayleigh scattering cross-section of 1.3 I~m light by a 10 nm diameter cluster is about 10 -21 cm 2 while the lasing cross-section of the rare-earths' ion is about 10 -20 cm 2 [5]. For a pr3+-doping concentration of 400 ppm in chloride [6], there are many praseodymium ions in each chloride cluster. Thus, the loss of 1.3 Izm light due to scattering would be minimal and dominated by the gain of light at 1.3 Ixm as long as the chloride network distributes in the silica host uniformly reducing density fluctuations. The praseodymium ions in the chloride would be shielded from the phonons in silica if the dimension of the network is appropriate. The phonons in silica with a higher energy can cause irradiative decay of electrons and are harmful to quantum efficiency [7]. The silica host also provides the praseodymium ion-doped chloride network with good chemical protection and mechanical support. Furthermore, if the concentration of the praseodymium ion-doped chloride network (or clusters) is reduced smoothly along one dimension to the edge of the composite material, the edge portion of the material may be weldable with silica fibers. Since lasing at many different wavelengths ranging from the visible region of the spectrum to almost 3 txm can be achieved by doping, many of the lanthanide series of elements in halide glasses [8] and especially flatter gain spectrum of 1.5 p~m light amplification can be achieved by doping erbium ions in fluoride glasses [9]; our approach may also provide diversity. The host porous silica glass made in our laboratory for preliminary experiment did not contain germanium as an index adjuster and was manufactured by phase separation and chlorine acid etching of a sodium-boron rich silicate glass. The sodium-boron rich silicate glass was prepared by melting at 1400°C the burden of composition 8.4Na20.0.9AI20 3 • 24.7B20 3 • 66SIO 2 (mol%). Phase separation of this glass was produced at a temperature of 560°C for 20 h. Then a sample was etched with HC1 (2 N) and NH4CI (34%) as buffer at ~ 93°C for 10 h to get porous silica glass. The porous silica glass was evacuated and then injected with the mixture solution of PrCI 3 and the burden chloride for the chlorine glass (BaC12-
109
PbCI2-CaCI2-NaC1) by sinking the evacuated porous glass in the solution for 20 h. The concentration of Pr 3+ ions was controlled in the range 50 to 1000 ppm (by the weight of Pr 3÷ ions in the chloride). The sample was then processed by drying and sintering for 2 h in vacuum at ~ 925°C, near to the melting point of the chloride glass and within the sag temperature range of the silica glass. Finally the sample was polished for measurement. Nd3+-doped samples were made as accompaniments to allow ourselves to become acquainted with the processes. The porous glass has a structure of an interwoven silica skeleton with an average dimension 0.5 Izm of the skeleton's width and of the pores in the skeleton, which was measured by a scanning electronic microscope (SEM, Hitachi S-2700). The porosities of the samples were measured as ~ 50% applying the Archimedean principle. The surfaces of the Pr 3÷doped and Nd3+-doped samples were observed by SEM. Fig. 1 shows the typical picture of the pr3+doped samples; the average dimension of the pr3+-doped chloride network (black portions in the figure) was about 30 nm according to the calibration in Fig. 1. The Nd3+-doped samples were similar to Fig. 1. The amorphous structure of the doped samples with doping concentrations less than 1000 ppm were verified by the absence of X-ray powder diffraction lines measured on a X-ray diffractometer (Rigaku D/max-3A). The fact that the praseodymium ion-
Fig. 1. Typical SEM micrograph of the samples of Pr3+-doped chloride networkcompoundedin silica, sinteredbetween900 and 950°C in vacuum.The black portionsare the pr3+-dopedchloride network.
I lO
S.H. Yuan/Journal of Non-CrystaUine Solids 215 (1997) 108-110
doped chloride network has an amorphous structure is important as it can ensure high quantum efficiency to the praseodymium ions by allowing for dipole transitions [2]. As the concentration was increased to 10000 ppm, typical lines of quartz crystal were observed on the background of the amorphous spectrum. The index, n, of the sintered porous silica glass was measured, n = 1.481. The indices of the Pr 3+doped silica glasses were in the range from 1.487 to i.496 depending on the doping concentrations. The fluorescence spectra of the pr3+-doped (100 ppm) and Nd3+-doped (100 ppm) samples were measured using pumping wavelengths at 1020 and 822 nm, respectively. Weak 1.3 ~ m fluorescence was observed on the pr3+-doped sample but the lifetime was not measured. Because of the small size of the sample it was not polished. The Nd 3+-doped sample was polished and the fluorescence spectrum was observed as shown in Fig. 2. Two peaks at 870 and 1060 nm are the electronic transitions of Nd 3+ ions, 4 4 4F3/2-419/2 and F3/2- Ill/2. The ratio of the intensities of the two transitions is about 2:1, similar to that observed in Z B L A N [10], which indicates that the environment provided by the chloride clusters for the fluorescence of the Nd 3+ ions is similar to that provided by Z B L A N and the phonons from the silica network are shielded effectively from intruding into the interior of the chloride clusters.
wa.velength (lz zn)
Fig. 2. Fluorescence spectrum of the sample of Nd3+-doped chloride network compoundedin silica. The two peaks at 870 and 1060 nm correspond, respectively, to the transitions 4F3/2-419/2 and 4F3/z-4Ill/2. The ratio of the intensities of the two transitions is about 2:1, similar to that observed in Nd3+-dopedZBLAN.
Although the approach of compounding a praseodymium doped chloride glass network in a silica host glass to make a new 1.3 txm light amplifying material is only in the preliminary stage, the results obtained indicate that this approach is likely promising. The problem found at present is mainly the preparation of homogeneous samples of appropriate size for processing and optical measurements. We have been doing experiments to produce larger samples adequate for processing and measurements. The gradient distribution of the pr3+-doped chloride clusters along one dimension to opposite surfaces of the material is more conveniently realizable using the tube samples than the block samples.
Acknowledgements The author is thankful to Dr M. Bass and Dr S.G. Grubb for measuring the fluorescence spectra of the samples. The author is thankful to Mrs Xiaowei Chen for her help in the preparation of the manuscript.
References [1] Y. Ohishi, T. Kanamori. T. Kitagawa, S. Takahashi, E. Snitzer, G,H. Sigel Jr., in: Tech. Digest of OFC'91, 1991, PDP 2. [2] T. Kanamori, Y. Terunuma, K. Fujiura, Y. Ohishi, S. Sudo, in: Proceedings of 9th Int. Symp. on Non-Oxide Glasses, 1994, p. 74. [3] M. Yamada, M. Shimizu, Y. Ohishi, J. Temmyo, M. Wada, T. Kanamori, S. Sudo, in: Tech. Digest of OAA'93, 1993, p. 240. [4] T.J. Whitley, J. LightwaveTech. 13 (5) (1995) 744. [5] Y. Ohishi, T. Kanamori, M. Shimizu, M. Yamada, K. Fujiura, S. Sudo, 10th Int. Conf. on IOOFC, 1995, ThA2-3. [6] A. Blixt, P.J. Nilsson, T. Carlnas, B. Jaskorzynska, IEEE Photon. Technol. Lett. 3 (11) (1991) 996. [7] T. Izumitani, H. Toratuni, J. Non-Cryst. Solids 47 (1982) 1. [8] M. Monerie, D. Ronarc'h, F. Auzel, Proc. 17th European Conf. on Optical Communication ECOC'91 (invited paper), 1991, p. 9. [9] D. Ronarch et al., OSA OAA' 93, Yohahama, Japan, 1993, paper PD 10. [10] M. Poulain, J. Lucas, Mater. Res. Bull. 10 (1975) 243.