Elsevier

Optics Communications

Volume 277, Issue 2, 15 September 2007, Pages 329-334
Optics Communications

An improved method of controlling rare earth incorporation in optical fiber

https://doi.org/10.1016/j.optcom.2007.05.053Get rights and content

Abstract

The process parameters involved at various stages of solution doping method for fabricating rare earth (RE) doped optical fiber have been systematically investigated to optimize the process conditions and achieve better control over RE incorporation. It is observed that the RE concentration, uniformity and the overall fiber quality are strongly influenced by the solution properties and dipping parameters. The soaking solution needs to be judiciously selected depending on the porous layer characteristics to improve the fiber quality and process efficiency. An interesting observation not hitherto reported is the influence of Al ion concentration in the solution on RE incorporation into the core. The investigation helps to obtain the optimum conditions necessary to produce fibers of given specification and achieve greater reproducibility.

Introduction

Rare earth (RE) doped optical fibers are now-a-days in great demand for their potential applications in optical amplifier, fiber lasers and sensor devices [1]. Among the various techniques [2], [3], [4], [5], [6], [7] developed for making the fibers, the MCVD process [8] coupled with solution doping method is commonly adopted as the process offers greater flexibility in doping rare earths in various concentrations and variation in RE/codopant proportion. However, control of RE concentration in a repeatable manner and the doping non-uniformity are still major problems in this process, which contribute towards high cost of the fibers. Investigations have been reported [9], [10], [11], [12], [13] on the influence of different process parameters on the ultimate fiber properties, but the observations are not complete in respect of correlating the parameters at different stages of the process so as to obtain the optimized conditions necessary to produce fibers of given specifications and achieve better reproducibility.

The solution doping process involves two major steps viz. deposition of porous core layer comprising refractive index raising materials like Ge- or P-oxide at lower temperature by MCVD process and soaking of the porous deposit with rare earth containing alcoholic/aqueous solution. Since the porous deposit acts as precursor for solution impregnation, its morphology and porosity influence to a great extent the final RE concentration and its distribution along the length of the preform and fiber. During the solution doping step, the composition of the soaking solution, the Al/RE ratio, the nature of solvent, dipping period are found to be the key factors to achieve the desired fiber properties.

In an earlier work [14] the authors reported the change in surface morphology and pore size distribution due to variation in soot composition. It was found that average pore size changes from 0.5 to 1.5 to 4.5 μm due to modification in the host composition from SiO2 to SiO2–GeO2 (12.5 mol% GeO2) to SiO2–P2O5 (12.8 mol% P2O5). The influence of deposition temperature on porous layer morphology for a fixed vapor phase composition was also evaluated indicating that lower deposition temperature results in pores of smaller size and uniform size distribution. As the temperature is increased, the pores are partially collapsed leading to both larger and smaller size pores with non-uniformity in their size distribution. Since the average pore size plays significant role during solution impregnation and any variation in pore size leads to non-uniformity in final RE distribution, judicious choice of vapor phase composition as well as deposition temperature are the key factors to fabricate fibers with good optical properties.

The present work constitutes the second phase of the investigation where a systematic study has been carried out on the influence of parameters involved during solution doping step in relation to the deposited soot characteristics to optimize the process conditions for enhanced repeatability and improved fiber performance.

Section snippets

Experimental

Following the MCVD process, porous layers of SiO2–GeO2 soot compositions were deposited inside silica tubes of 20 mm diameter and 1.5 mm thickness (Suprasil F-300 variety) at temperatures in the range of 1200–1300 °C. The proportion of SiCl4 (99.998% purity from Sigma–Aldrich, USA) and GeCl4 (99.999% purity from Alfa Aesar, Germany) was controlled to adjust the composition and refractive index difference. The total flow of the input vapor mixture was kept constant at 0.6 l/min in all the

Results and discussion

Experimental result shows that with increase in strength of the solution, the viscosity of the solution increases considerably. The variation in viscosity as shown in Fig. 1, corresponds to a change of viscosity values from 1.98 to 19.5 cP for a variation in the concentration of AlCl3 from 0.15 to 1.0 (M) in an ethanolic solution. The nature of solvent also plays a significant role on solution characteristics. The viscosity of a 0.3 (M) AlCl3 solution is found to increase from 1.3 cP to 6.7 cP by

Conclusion

The investigation reveals the interdependence of soot layer characteristics with the solution properties and the dipping parameters. The density, viscosity and pH of the solution are found to vary appreciably with change in solution strength and nature of solvent. About 10 fold increase in the viscosity is observed for a variation in the AlCl3 concentration from 0.15 to 1.0 (M) in ethanolic solution. The selection of solution of appropriate viscosity becomes an important criteria depending upon

Acknowledgements

The authors are thankful to the Director, CG&CRI for his constant encouragement and support in carrying out the investigation. They also like to thank the colleagues of Fiber Optics Laboratory for their valuable suggestions in the experimental work. Thanks are also due to SEM-ESCA laboratory, especially to Dr. Mrs. S. Sen, for their constant help in SEM/FESEM analyses of the porous deposit samples. One of the authors Anirban Dhar is thankful to CSIR, India for providing Senior Research

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