A europium-doped (Eu-doped) silica optical fiber is fabricated using modified chemical vapor deposition (MCVD) technology. Europium fluoride (EuF3) material is introduced into the fiber core with a high temperature vaporizing technique. Its concentration is approximately 0.11 at %. The outer and core diameters of doped fiber are approximately 122 and 9 μm, respectively. Refractive index difference (RID) between core and cladding is approximately 2%. A magneto-optical effect measurement system, which is based on the Stokes polarization parameters method, is set up to analyze its magneto-optical properties. The Verdet constant of the Eu-doped optical fiber is −4.563 rad T−1m−1, which is approximately double than that of single mode fiber (SMF) at 660 nm.
© 2016 Optical Society of America
Optical fibers and bulk glasses as magneto-optical devices have become promising candidates for the optical modulators, optical isolators, magnetic field sensors, optical current transducers, etc [1–3]. Although they have certain advantages, such as small size, high sensitivity, immunity to electromagnetic interference, the bulk glasses are less operation, misalignment and low intergration compared to the optical fibers, and the main disadvantage of optical fibers is that they are low magneto-optic properties in terms of the Verdet constant (SMF:~2.4 rad T−1m−1 at 660 nm and ~0.7 rad T−1m−1 at 1310 nm). In recent years, a magneto-optical fiber with high Verdet constant has become an important research topic. Rare-earth ions such as Eu3+, Ce3+, Tb3+, Pr3+ exhibit large Faraday effect in the visible to ultraviolet range. The transition of rare-earth ions leads to the Faraday rotation. L. Sun et al. reported the Verdet constant of the terbium (Tb)-doped silicate fiber, whose concentration is 56 wt.%, is −24.5 ± 1.0 rad T−1m−1 at 1053 nm [4,5]. Y. Shiyu et al. reported the Verdet constant of the tellurite glass fiber is 28 ± 0.5 rad T−1m−1 at 633 nm . P. Zu et al. proposed a novel magnetic field fiber sensor based on magnetic fluid-filled photonic crystal fiber (PCF)  and P. R. Watekar demonstrated a CdSe quantum dots doped optical fiber with high Faraday rotation . However, these researches focused on high doped multi-component oxide and fluoride bulk glasses and glass fibers, whose transmission loss is relatively high, and the polarization is easily degraded, which results in its poor stability. It is difficult for these magneto-optical fibers to be used widely. Different from glass fibers, silica optical fibers contain GeO2 and SiO2, and its substrate material is pure silica, which is of higher softening temperature, smaller thermal expansion coefficient and lower transmission loss.
We propose that doping rare-earth ions in silica optical fiber contributes to increasing the Verdet constant. Among the 14 ions in the lanthanide series, Eu3+ exhibits relatively large Faraday effect, which is because of the lower 4f-5d transition energy. Besides, the paramagnetism of rare-earth ions is related to unpaired free electrons of 4f electron shell. The number of unpaired free electrons of ion Eu3+ is 6, which is relatively more among the lanthanide series. Therefore, the trivalent Eu ion with large Faraday rotation is good candidates for magnetic-optical application such as magnetic film. What’s more, the chemical property of the europium fluoride materials is relatively stable. In this paper, the europium fluoride materials is introduced into the silica fiber core with high temperature vaporizing technique, and an Eu-doped silica optical fiber is fabricated using MCVD. At the same time, a magneto-optical measurement system is designed to analyze its magneto-optical properties in the different wavelengths. The Eu-doped silica optical fiber fabricated is utilized as magneto-optical fiber, for the first time, which can be widely applied in optical modulators, optical isolators, magneto-optical switches, and sensors of current and magnetic field, etc.
2. Fabrication of Eu-doped silica optical fiber
An Eu-doped silica optical fiber is fabricated using MCVD process in combination with high temperature vaporizing technique. The fabrication procedure is briefly described as follows . Firstly, a pure silica soot is deposited on the inner surface of substrate tube, whose outer diameter and wall thickness is 18 and 2 mm, respectively. Secondly, EuF3 powder (Strem Chemicals, Inc 99.99%-Eu) is vaporized at the entrance of silica substrate tube with above ~1500 °C. The homogeneous gas phase of EuF3 and the pure silica soot are deposited onto the inner surface of silica substrate tube, layer by layer. Then, the deposited materials are sintered by an external oxyhydrogen flame. The amounts of dopants are controlled by a number of deposition layers. During the doping process of EuF3 material, nitrogen gas flow is used as a gas carrier to prevent the vaporized EuF3 from being oxidized. Then the tube is collapsed around the temperature 2000 °C to form a preform. The preform is drawn into fiber and coated protective layer on fiber in the fiber drawing tower.
The element compositions of the doped materials in core layer of the fiber are analyzed by Scanning Electron Microscope (JSM-6700F cold field emission SEM, Japan) combining with Energy Dispersive Spectrometer (EDS) (SEM-EDS, MX80-EDS, OXFORD, England). Spraying metal on cross-section of the fiber is carried out, as shown in the top right corner of Fig. 1. The concentrations of Eu and F ions are approximately 0.11 and 0.11 at%, respectively, as shown in Fig. 1. In addition, Binding Energy of EuF3 is larger than that of Eu2O3. Its chemical properties are more stable, and not easy to be oxidized.
The optical fiber preform is drawn into optical fiber, and its outer and core diameters of doped fiber are about 122 and 9 μm, respectively, as shown in Fig. 2. Refractive index difference between core and cladding of the fiber sample is analyzed by optical fiber index analyzer (S14, Photon Kinetics Inc., USA). Its RID is approximatively 2% which is larger than that of SMF. It is caused by the role of Eu doping in the fiber core.
3. Magneto-optic measurement system
Actually, birefringence due to temperature, anisotropy of the fiber core materials and the existence of internal residual stress, can impact on the parameter ε in Eq. (1). Therefore, the detection of Faraday rotation cannot simply depend on the polarizer-analyzer method . In order to analyze the complex Faraday rotation of the polarized light plane, we present an explicit and effective method, which is combined with the Stokes vector and the Poincare sphere. The Stokes parameters can be obtained by Eq. (2) :Eqs. (3)-(6).
Through the measurement of the Stokes parameters, the azimuth and ellipticity are determined . A magneto-optic experimental measurement system has been built, as shown in Fig. 3. The input light source is the fiber-coupled Fabry-Perot laser diode (S1FC660, S1FC808, S1FC980, S1FC1310 and S1FC1550, Thorlabs). The light through a collimating lens (F220FC-B, Thorlabs) and a polarizer (LPVIS050-MP2, Thorlabs) is coupled into the fiber, which is clamped on the bare fiber launch system (MAX350D, Thorlabs). The solenoid, whose length is 30.4 cm, is composed of copper-wire coil. The fiber is placed in the solenoid axis. The magnetic field is produced by using a solenoid with a direct-current (DC). The direction magnetic field is parallel to the axial direction of the fiber. The data of the Stokes parameters, the azimuth and ellipticity can be collected from the polarimeter (PAX5710, Thorlabs). The states of polarization (SOP) can be displayed on the Poincare sphere model, which is constructed by the built-in polarimeter software. Faraday rotation of the magnetic-optical fiber is determined under external varied magnetic field strength on the coil axis from 0 to 140mT.
4. Experimental results and discussion
A measurement experimental setup is used to determine the linear and elliptical states of polarization, and then to measure its azimuth. Three kinds of silica optical fiber samples are Eu-doped silica optical fiber (self-fabrication), commercial Erbium-doped fiber (Nufern EDFC-980-HP C-Band EDF) and commercial SMF (Coring SMF-28e). Faraday rotation of three kinds of fiber samples are measured when the input light wavelength is 660 nm under different magnetic field densities, respectively. Each kind of samples, twelve groups of measured data are collected. The measured values are plotted against the applied field from 10.92 mT to 131.07 mT, as shown in Fig. 4. The experiment results show, for three fiber samples, Faraday rotation () is increased with the increasing of the magnetic field (), and the rotation shows a linear relationship with, which is basically consistent with theory, as shown in Eq. (7) .
Figure 4 shows that the rotation increased linearly proportional to the applied magnetic field, and the Faraday rotation of Eu-doped optical fiber is obviously lager than that of the other samples. What is more, Faraday rotation of Eu-doped optical fiber is about twice times than that of SMF at = 131.07mT and 660nm. The slope of vs curve determines the magnitude of the Verdet constant. The slope of Eu-doped optical fiber is maximum, which means its Verdet constant is larger than the others samples.
For the rare-earth ions, 4f electron shell is not filled and magnetic moment produced by unpaired free electrons of 4f electron shell, which is the main cause of strong magnetic . Moreover, because of the exterior 5s and 5p electron shell shielding effect, the effect of compound ligand field on the inner 4f electron shell is very small. Under the effect of external magnetic field, the Faraday rotation is easily caused by the transition of rare-earth ions . Therefore, unpaired free electrons in (n = 1,2…7) electron shell govern the magneto-optical properties of optical fibers, when the ions, such as Eu3+, Er3+, are doped into the silica optical fiber. And the number of unpaired free electrons of ion and Er3+ ion is 6 and 3, respectively, which may contribute to Faraday effect. Therefore, magneto-optical property of Eu-doped optical fiber is more stronger than that of EDF.
From a microscopic point of view, Faraday rotation is caused by the electronic transition of . With the applied magnetic field, Zeeman splitting of atomic energy levels result in magneto-optical rotation, which there exist the difference between left-handed photons and right-handed photons [17,18]. For the Eu-doped silica fiber, there are many interactions of fluoride containing Eu3+ ions, which have not only spin orbit interaction, but also the exchange interaction between the electrons. These effects can be equivalent to an effective field, such as L-S coupling, and an external magnetic field, such as H, as shown in Fig. 5(a), which can cause the ground state energy level splitting [19,20]. What is more, the electronic transitions of ground state energy level splitting have influence on the magneto-optical effect.
For the Eu3+ ions, it is the electronic transition of . Its orbital angular momenta (L) and spin angular momenta (S) are both 3, and exists L-S coupling, which results in 2J + 1 non-degenerate energy levels arising from effective field splitting (7FJ (J = 0, 1…6) of the 2S + 1LJ multiplet states of Eu3+ (4f6) . The main interaction of ions with the external magnetic field is the Zeeman splitting interaction, which splits into terms with values of the total S and L-momenta, can take the matrix form L + 2S. The Verdet constant is determined by the matrix elements of the operator L + 2S, and the L-operator is the reason for the difference of the values, which means the magnetic circular dichroism is based on the Zeeman effect . In the presence of the Zeeman effect, there are many splitted energy levels with different values of mL. The ineuqlity on the transition of the ∆mL = ± 1, correspond to with left and right-handed photons, makes contributions to the Faraday effect, as shown in Fig. 5(a).
According to literatures [22–24], there exist seven spin-orbit states belonging to the ground multiplet 7FJ (J = 0, 1, 2,…, 6) and three lowest excited states of 5DJ (J = 0, 1, 2). With the external magnetic field, electrons of 4f orbit in Eu3+ ion are excited, which exhibit a multiplicity both emission and absorption. Its emission spectrum results from transition between the ground multiple to the three lowest excited states, such as, , . However, emission spectra transitions from the 5D0 state are much stronger in intensity than other spectrum lines, which originates from the 5D1,2. And then the is strongest emission spectrum at 611 nm corresponding to an electric dipole transition [25–27]. The is emission spectrum at 658 nm, which is shown in Fig. 5(b).
In our experiment, the input light wavelength 660 nm is used to measure the Verdet constants of the three fiber samples (Eu-doped fiber, EDF, and SMF). The Verdet constant of Eu-doped fiber is larger than that of other two kinds of fibers. And then the other different input light wavelengths are also measured, as shown in Table 1.
Table 1 shows that Verdet constant of the Eu-doped silica optical fiber is largest in three samples at the different wavelengths. Especially, the Verdet constant of the Eu-doped optical fiber is −0.978 rad T−1m−1 at 1550 nm, which is approximately double than that of SMF.
Splitting effect of the 4f of Eu3+ is stronger than that of Er3+, that is, the possibility of dipole transition from 4f to 5d of Eu3+ becomes larger than that of Er3+, which result in an unequal between the right and left circularly polarized light. That may be the main reason that the Verdet constant of the Eu3+ is larger than that of Er3+.
Besides, the concentration of the rare-earth ion will determine the Verdet constant directly. The concentration dependence of the Verdet constant is investigated by obtaining accurate density data for each fiber sample and converting the mole fraction concentration into a dopant ion per unit volume concentration scale. The concentration of Er ions in EDF is approximately 0.50 at%. The concentration of Eu ions in the Eu-doped fiber is 0.11 at%, as shown in Fig. 1. According to our experiment results, the magneto optical properties of rare-earth Eu3+ ion obviously strong than that of Er3+ ion in the visible spectral band.
The Verdet constant of the three fiber samples under different light wavelength are plotted, and then polynomial fitted to the wavelength dependent data, as shown in Fig. 6. The wavelength dependence of the measured Verdet constants is described well within experimental error. The wavelength dependence effect of the Verdet constant, which was originally proposed by Van Vleck and Hebb , is described by Eq. (8)29,30]. On the whole, the fitting curve is approximately inversely proportional to the square of the wavelength, which is consistent with Eq. (8).
EuF3 materials are successfully introduced into the optical fiber core region with high temperature vaporizing technique. An Eu-doped silica optical fiber is fabricated using MCVD technique. With EDS and RIF analysis methods, we confirmed the concentration of Eu ions in the Eu-doped silica optical fiber is approximately 0.11 at%. Its RID is about 2%, which is larger than that of SMF. It results from the role of EuF3 doping in the fiber core. A reliable magneto-optical measurement system is built. The Verdet constants of Eu-doped fiber, EDF and SMF with different wavelengths are measured, which are −4.563, −3.379 and 2.413 rad T−1m−1 at 660 nm respectively. In particular, the Verdet constants of Eu-doped fiber is much larger than that of other two samples at 1550 nm, which is widely applicated. Furthermore, Faraday effect of Eu-doped silica optical fiber is obviously enhanced. The large Verdet constant can be generated by the rare-earth ion Eu3+. EuF3 doped in the fiber core is an effective method to increase the Verdet constant. The magneto-optical fiber with high Verdet constant can be used for high sensitive magnetic field sensors, magneto-optical switches, and magneto-optical modulators, etc.
National Natural Science Foundation of China (NSFC) (61275051, 61227012, 61275090, 61475095, 61475096, 61520106014); Science and Technology Commission of Shanghai Municipality (STCSM) (14511105602, 14DZ1201403, 15220721500); Open Project of State Key Laboratory (SKLD11KZ03, SKLSFO2015-01), and Laboratory for Microstructures of Shanghai University, China.
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