Probing the erbium ion distribution in silica optical fibers with fluorescence based measurements

https://doi.org/10.1016/j.jnoncrysol.2011.07.024Get rights and content

Abstract

Accurate determination of the rare earth dopant distribution in optical fibers enhances our understanding of the fiber manufacture process and enables further improvement in the design of fiber based products such as optical fiber lasers and amplifiers. Here a simple theoretical model consisting of an ensemble of rate equation systems, characteristic of the most likely electronic transitions that take place in the vicinity of erbium (Er3+) doped silica glasses, is developed and solved. Through this theoretical study it is established that information about the relative Er3+ ion distribution in fibers can be inferred by simply monitoring the backscattered fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. Following these theoretical studies a fluorescence intensity confocal optical microscopy (FICOM) scheme was employed to investigate the Er3+ ion distribution profiles in a range of silica optical fibers. The validity of the proposed theoretical model was confirmed through a comparison of the Er3+ ion distribution profiles acquired using the FICOM technique and those obtained from the application of a powerful analytical ion probe.

Highlights

► A method for investigating the Er ion distribution in optical fibers is presented. ► The link between fluorescence based data and Er dopant distribution is established. ► Information from the analysis of various fiber samples with this technique is shown. ► An ion-microprobe is also used to study the Er distribution within the same samples. ► The proposed theoretical model was confirmed through a subsequent data comparison.

Introduction

Photonic devices, such as optical fibers, fiber lasers, and fiber amplifiers play an important role in today's society. Their capabilities have found use in a vast range of scientific and industrial environments amongst which the communications field has been influenced the most. In particular, although optical communication technologies were initially introduced to improve traditional information exchange, they have recently become a key factor behind the tremendous growth in internet traffic, and optical technologies will be even more important in enabling and supporting the future expansion of internet traffic.

Optimizing the design and fabrication of such devices has become an ongoing challenge to optical scientists and engineers. For example, it has been demonstrated that the properties and performance of Er3+ doped fiber lasers and amplifiers (EDFA) are related to a number of parameters such as the fiber glass material, the waveguide characteristics, and the distribution profile of the Er3+ ions. Accurate knowledge of the latter has been found to be vital for the optimal design and operation of these devices [1], [2], [3], [4]. Various techniques have therefore been developed and investigated in order to acquire information related to the distribution of the dopants in the fibers.

To date, two different approaches have been employed to measure the distribution of Er3+ or other rare earth (RE) ions in optical fibers. In the first case, the concentration of dopants and their distribution, together with the refractive index profile (RIP) and other parameters are usually measured in the fiber preform, prior to the fiber drawing process. Some of the most important techniques employed to reveal the location of dopants in bulk samples such as fiber preforms, are based on the utilization of methods that are commonly used in the field of analytical chemistry and material science. These techniques include secondary ion mass spectroscopy [5], inductively coupled plasma atomic emission spectroscopy [6], and X-ray microprobe analysis [7]. The distribution of dopants in fiber preforms has also been examined using the electron probe microanalysis (EPMA) method and its counterpart systems [8], [9]. The resultant dopant concentration and distribution is then scaled down to match the drawn fiber dimensions assuming that there are no defects affecting the resulting fiber profile during the drawing process. This assumes that the profile remains unchanged during the drawing process. Huntington et al. [10] has shown that this assumption is not always valid for the determination of the RIP. This is believed to be the consequence of various effects such as the diffusion of elements that takes place because of the high temperature environment created during the drawing process. As a result it is not always valid to simply relate the dopant concentration profile extracted from preform measurements to fiber dimensions.

In the second approach, measurements are made directly on the drawn fiber. Once again a number of the techniques utilized for the analysis of the RE distribution in optical fibers have been borrowed from the analytical chemistry and materials science field. Such techniques include nano-secondary ion mass spectroscopy (Nano-SIMS) [11], time of flight secondary ion mass spectroscopy (ToF-SIMS) [12], transmission electron microscopy [13] and Raman microscopy [14]. Although these techniques can offer adequate spatial imaging resolution for the direct investigation of optical fibers and they can offer information about the distribution of other dopants within the fiber core region, they normally require complex and time-consuming sample preparation and the use of high cost instrumentation. In an attempt to overcome such limitations, a number of optical imaging schemes have been applied over the years in order to provide information about the RE ion distribution in the core of optical fibers directly from the investigation of their cleaved endface [15], [16], [17]. With the exception of the work of Petreski et al. [15], where a fluorescence lifetime confocal optical microscopy technique was developed for the investigation of praseodymium-doped optical fibers, the other optical based systems were capable of providing information about the Er3+ ion distribution in optical fibers with the application of a fluorescence intensity confocal optical microscopy scheme [16], [17].

In the latter cases, the intensity of the backscattered fluorescence originating from the de-excitation of the 2H11/2 and 4S3/2 upper energy levels (Fig. 1) was monitored and taken as an indication of the local dopant concentration of the Er3+ ions at a point. By scanning the excitation beam across the center of the fiber core, two-dimensional images or line-scans of the relative Er3+ ion distribution were acquired. However, the acquired intensity profiles were assumed to be directly related to the relative Er3+ ion distributions without establishing that the resulting backscattered fluorescence signal was indeed proportional to the erbium ion concentration. Care must be taken regarding the reliability of the intensity based measurements given that cooperative effects in RE doped fibers may yield incorrect values of dopant concentration [18]. In this work, the issue of the dynamics of different emission lines observed in Er3+ doped optical fibers and their relationship to the total Er3+ ion concentration is addressed by developing and solving a system of rate equations characteristic to the most likely electronic transitions observed in a Er3+ doped silica glass. This exploration validates the use of direct pumping of the 2H11/2 (at 514 nm, as shown in some of our previous work [17]) or the 4F7/2 (at 488 nm in the work of [16]) levels with the subsequent detection of the backscattered fluorescence signal (around 550 nm) from the de-excitation of the 4S3/2 level as a measure of the relative Er3+ ion distribution in optical fibers. Furthermore, we extend the relevance of this theoretical model as applied in our previous work with the use of a confocal optical microscope [17] to our more recent efforts in trying to improve spatial resolution using a Near-field Scanning Optical Microscope (NSOM).

Section snippets

Theoretical modeling

The main focus of this article is to identify the specific electronic transitions that take place in Er3+ doped silica glasses, which could be related directly to the concentration or distribution of the erbium ions within the core of optical fibers. The issue remains whether fluorescence intensity based measurements could possibly be related to the erbium dopant profile. As long as the process of de-excitation of an energy level is free from any possible inter-ionic cross-relaxation or

Experimental procedures

A total of five Er3+ doped silica optical fibers (Table 3) having estimated erbium ion concentrations that ranged between 100 ppm and 7600 ppm were studied. In all cases the fiber preform was manufactured using the MCVD technique in conjunction with solution doping, with the silica core matrix also incorporating aluminum, phosphorous and germanium as network modifiers. Based on the above theoretical model, information about the way the Er3+ ions are distributed in the core region of these fibers

Results

Normalized transverse profiles displaying the relative Er3+ ion distribution in the core region of the investigated fibers using the FICOM and NanoSIMS schemes are co-plotted in the series of graphs displayed in the following figures (Fig. 4, Fig. 5).

Discussion

A good agreement is evident between the extracted distribution profiles with the two different techniques. Any apparent discrepancies between the obtained data with the two different schemes are mainly a direct consequence of the two following reasons. First, measurements with the two techniques were acquired from different sections along the length of each fiber, suggesting that possible elemental redistributions induced during the preform collapse process or the fiber draw process may result

Conclusion

Advances in the use of rare-earth doped silica optical fiber lasers and amplifiers in telecommunications and a range of industrial applications depend critically on an accurate knowledge of the rare-earth dopant distribution within the core of the optical fiber or device. Here, a simple theoretical study of the spectroscopic characteristics of Er3+ doped silica optical fibers has established that information about the Er3+ ion distribution in optical fibers can be obtained by exciting the Er3+

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