Borate Glasses for Low Loss Optical Fibre

Borate glasses with composition 50B2O3 – (25-x) Bi2O3 – 25ZnO – xTiO2 where x = 0, 1, 2, 3, 4 (mol%) were succesfully fabricated using a convensional melt-quenching technique aimed at tailoring glass suitable for a low loss optical fiber fundament. For this purpose, glasses were characterized for their density, refractive index, reflectance, UV-Vis absorption and FTIR spectra. Density measurement was carried out by applying Archimedes principle. Refractive index was measured using Brewster’s angle method. UV-Vis spectrum was recorded within the range of 200-1100 nm and FTIR measurement was measured at IR range. Combining the spectra data recorded both from UV-Vis–NIR and FTIR, the theoretical minimum loss of the glass was obtained. In addition, the band gap energy of the present glasses was also calculated. From these data, it can be derived many other glass properties such as Oxygen Packing Density (OPD), ionic radius, and polaron radius.


Introduction
Borate glass (B2O3) has been extensively studied and applied in various applications [1]. Borate glass has optical properties such as high solubility of rare-earth ions, good thermal stability, and high chemical properties. Borate glass has very high phonon energy around 1300 cm -1 can cause nonradiation losses and reduce effective radiation emissions [2]. To form a glass, however, B2O3 cannot stand alone without the addition of alkali, alkaline earth or the addition of other glass formers [3]. Therefore, borate glass must be added to other glass formers to produce glass such as Bi2O3, ZnO, and TiO2.
The addition of alkali, alkaline earth or other glass-forming in this borate glass have been reportedly produced glass with very high transparent. Bi2O3 is a chemical oxide which has a high electron valence but has low field strength and high polarization [4]. The addition ZnO into the glass reduces the glass melting point and increases glass stability against crystallization, refractive index and ability to form glass [5]. Apart from the addition of Bismuth (Bi2O3) and Zinc Oxide (ZnO), the addition of other oxides is Titanium (TiO2). In borate glass the addition of TiO2 can increase the ability of glass formation, glass stabilization, chemical resistance, and tissue formation [6]. In this paper, borate glasses with compositions: 50B2O3 -(25-x) Bi2O3 -25ZnO -xTiO2 where x = 0, 1, 2, 3, 4 (mol%) will be fabricated and characterized for their densities, refractive indices, reflectance, absorption spectra both in UV-VIS-NIR wavelength range and IR range.

Experiment
Borate glasses with compositions: 50B2O3 -(25-x) Bi2O3 -25ZnO -xTiO2 where x = 0, 1, 2, 3, 4 (mol%) were fabricated by applying melt-quenching method. The starting materials were weighed using a digital scale a nitrogenized glove box and then mixed evenly using mortar for 10 minutes. Mixture was then transfer into an platinum crucible melted in electrical furnace at 900 o C for 1 hour. Casting was carried out by pouring the molten into a preheated mold and subsequently annealed at temperature of 300 o C for approximately 9 hours and cooled to room temperature at a cooling rate of 1 0 C/minute. For optical characterization purpose, glasses were polished to optical standard. Density and optical characterization, i.e., refractive index, absorption spectra (UV-VIS-NIR and FTIR), were carried out using methods as has been presented in our previous papers [7,8,9]. Figure 1-a shows how TiO2 concentration affects the average molecular weight, density and molar volume of the investigated glasss. Density of a glass is affected by average molecular weight and molar volume as expressed by (1) where ρ is the density value of glass, xi is the molar fraction and Mi is the molecular weight of the oxides that make up the glass. This competing factor as shown in Figure 1 shows a trend that according to equation (1) causes increasing value of density. The increase of glass density with increasing TiO2 concentration means that slow rearrangement during glass formation process occur as TiO2 was added into the mixture [10,11].

Oxygen Packaging Density
Oxygen Packaging Density (OPD) is a closeness measure of oxygen atoms making up the glass. Oxygen Packaging Density (OPD) can be used to measure variations in density and temperature of glass transition characterized by a glass matrix with spatial oxygen dispersion and can be related to molar volume [11,12]. Oxygen Packaging Density (OPD) can be calculated using the equation: where is the number of oxygen atoms in the glass and is the molar volume cm 3 /mol. As shown in Figure 2-a, OPD of the glass increases with increasing TiO2 ions into the glass. Polaron radius which increases with the increase of TiO2 as shown in Figure 2-b resulting in decreasing electrical conductivity of glass [13].

Absorption Spectra and refractive index
The bandgap of the amorphous glass system is very effective when investigated using the UV-Vis method [14]. The absorption spectrum of TiO2 doped borate glass was identified at a wavelength of 200-1100 nm and is shown in Figure 3. The absorption edge began to occur at a wavelength of 400 nm. As the TiO2 concentration increases, the absorption edge experiences a shift in wavelength. This is due to the increased concentration of non-bridging oxygen [15,16]. Using According to Davis and Mott optical absorption is a method that can be used to confirm optical transitions and optical band gaps [17]. Optical energy bandgap can be determined using the equation (3): (3) The decrease of bandgap energy as function of the change in TiO2 is shown in Figure 4a. The reduction in the energy band gap is caused by the addition of TiO2 ions to the B2O3 glass. The reduction occurred due to structural changes after the addition of TiO2 ions. According to Villegas &  Fernandez, changes induced by the TiO2 ion glass structure as a result of an increase in the amount of non-bridging oxygen [18]. Therefore, the addition of TiO2 in glass can reduce the optical bandgap.
As seen in Figure 4b, the refractive index of borate glass increases with increasing TiO2 ion concentration. The Ti ion in this glass is Ti 4+ [14]. The addition of Ti 4+ ions can increase the amount of Non-Bridging Oxygen (NBO). Since NBO is more polarizable than BO, this addition causes the increase in refractive index [19].

Reflectance
Reflectance is the ratio of light reflected from sample glass and aluminum mirror [20]. Aluminum mirror is used for comparison because of its greater light transfer. From the results of the study that the reflectance of B2O3 glass increases with the increase of TiO2 ions concentrations (Figure 5).  Figure 5. The reflectance spectra of TiO2 doped B2O3 glass.

FTIR
The FTIR absorption spectrum of borate glass with TiO2 doped has been analyzed, the absorption peaks formed occurred in the range 400-4000 cm -1 as shown in Figure 6 (a). Borate glass contains structural units such as BO3, BO4, triborate, and diborate. This structure shows the information of the boroxol ring in the glass system, this explains that the glass containing the structure BO3 and BO4 is connected randomly [21,22,23]. Figure 6-a is FTIR data for all the investigated samples. To see the exact location of the absorption peaks and to provide information about the bonds forming the peaks, all samples were deconvoluted. Nevertheless, only one sample is shown here, namely the BBiZTi sample with x = 3. Deconvolution was carried out within the range 400 -1500 cm -1 (Figure 6-b). As can be seen from Figure 6

Conclusions
The optical properties and physical properties of borate glass have been investigated, with the composition of 50B2O3 -(25-x)Bi2O3 -25ZnO -xTiO2 where x = 0, 1, 2, 3, 4 (mol%) using the meltquenching method. The data generated based on research states that the physical properties which include Density, Molar Volume, Oxygen Packing Density (OPD), and Ionic Radius with the addition of TiO2 ion concentration on B2O3 glass are increasing in the graph. while the optical properties which include absorption spectra, refractive index, bandgap, and reflectance of the resulting graphs are increasing. Whereas in deconvolution it produced 13 absorption bands. This is due to the influence of the addition of TiO2 ion concentration on B2O3 glass.