Pre-Fabrication: Selection and Preparation of Host Material

The proper selection and preparation of the host material plays a crucial role in ensuring the high quality and optimal performance of the fabricated fibre. The host material used for the fabrication of optical fibres can be categorized into three broad groups – silica glasses, non-silica glasses and plastics or polymers. Typically, fused silica, which is also known as amorphous silicon dioxide, is the preferred choice for a host material as it has a number of highly advantageous characteristics:

 Fused silica has a wide wavelength range with good optical transparency, from 200 nm to 3000, with extremely low absorption and scattering losses at the order of 0.2 dB/km in the near-infrared spectral region of approximately 1550 nm. It can also be made transparent at the 1400 nm region by ensuring a low concentration of Hydroxyl (OH) groups³ in the fibre.

 Fused silica fibres can be drawn at temperatures of between 1600^oC and 2000^oC from silica based preforms, making them suitable for high temperature processing.

They are also easy to fusion splice, with low average losses of 0.1 dB or better, making them a highly preferred fibre for deployment in commercial optical networks.

 Fibres based on fused silica glass possess amazingly high mechanical strength against pulling and bending, highly stable chemically and also not hygroscopic.

Furthermore, the structure of the host material readily accepts various dopant materials, making it suitable for the fabrication of specialty optical fibres such as EDFs.

 Fused silica fibres also have a very high optical damage threshold, which is important in the development of fibre amplifiers and lasers. It also has a particularly low Kerr non-linearity factor, which is highly beneficial in preventing detrimental non-linear effects from occurring in the fibre, which will in turn affect the quality of the transmission.

3 Conversely, inducing a high concentration of OH groups will make the fused silica fibre transparent for wavelengths in the Ultraviolet (UV) region.

37 Vitreous or fused silica (SiO2) glass is made by cooling molten glass in such a manner that it does not crystallize, but rather remains in an amorphous state, with the viscosity of the molten glass increasing to a level where the glass molecules can no longer rearrange themselves in the form of a liquid due to fast cooling. SiO2 has a silicon-oxygen tetrahedron network structure, with a coordination number of 4. The tetrahedron structure of the SiO2 molecules links at all four corners, forming a continuous 3-dimensional network as shown in Figure 2.

Figure 2: Tetrahedron structure of SiO2, which has 4 oxygen ions connected to a single silicon ion. The solid lines represent the O^2- to O^2- bonds, while the dashed lines represent bonds between the Si⁴⁺ and O^2- ions.

In the SiO2 structure, the shortest Si-O link is approximately 0.162 nm, while the shortest O-O link is approximately 0.265 nm [44]. Each oxygen atom moves in two degrees of freedom, thus giving the SiO2 its various absorption bands. It is these characteristics that give fused silica glass its highly desirable characteristics. Vitreous germanium or GeO2, a non-silica glass also possesses a similar tetrahedral structure, with the same coordination number of 4. However, the ionic diameter of the germanium atom is larger than that of the silicon ion, thus making the Ge-O bond length slight greater at about 0.175 nm [45]. The structure of GeO2 glass is more compact than SiO2 glass, thereby making the interstitial volume of GeO2 slightly less

O

2-Si⁴⁺

O-O Bond

Si-O Bond

38 than that of SiO2.This manifests as structural defects inGeO2 glass, as a result of the formation of Ge-Ge bonds, thus reducing the overall popularity of GeO2 based glass.

The viscosity of GeO2 glass is near to that of SiO2 glass, as shown in the viscosity curves of Figure 3. The glass transition temperatures (Tg) and softening temperatures (Ts) of both glasses are also shown in Figure 3.

Figure 3: Viscosity of oxide glasses with glass transition (Tg) and softening (Ts) temperature

From the figure, it can be seen that GeO2 glass has a transition temperature of between 550 to 600^oC, while SiO2 glass has a higher transition temperature at about 1100^oC. Adding a minor amount of F (of about 3%) will lower the temperature of SiO2 glass to below 1000^oC, making the fabrication process slightly easier. The glass softening temperatures for GeO2 and SiO2 glass ranges at around 1000^oC and slightly higher than 1600^oC. B2O3 glasshas much lower transition and softening temperatures, at around 200 and 300^oC, making the fabrication process easier, but making the glass difficult to splice to conventional fibres. These characteristics become a very important factor in the selection of the host material.

Another important factor that must be considered in the fabrication of the EDZF is the refractive index of the fibre. In order to fabricate an optical fibre, two fused silica glass layers are needed, namely the inner layer (core) and the outer layer (cladding). It is here that the problem lies, as the core must have a higher refractive index than the cladding to ensure that Total Internal Reflection (TIR) is always preserved and thereby trapping light within the core layer [46]. At the same time, both

39 the core and cladding layers must be compatible in terms of their viscosity and thermal expansion coefficients in order to preserve the physical integrity of the fibre, thus ruling out the use of different host materials for the core and cladding sections of the fibre. To overcome this problem, the refractive indices of the host material for the core and cladding layers are instead controlled using Refractive Index (RI) modifying agents, which are typically added to the core layer during fabrication process to increase its RI and thus preserving TIR. The best RI modifying agents for fused silica fibres are found to be those oxides that are similar in dimension to SiO2 itself, such as GeO2, P2O5 or Al2O3. The RI can be changed by varying the percentages of the different dopants in the glass host, as shown in Figure 4. From the figure, it can be seen that increasing the concentration of dopants such as GeO2 and P2O5 in the silica host will increase the RI of the core layer, while increasing the concentration of B2O3

and F dopants in the silica host will have the opposite effect, reducing the RI of the glass [5].

Figure 4⁴: Effect of common dopants on refractive index of the silica host.

Thus, the choice is left to the manufacturer of the fibre, whereby the RI of the core can be increased, or the RI of the cladding decreased. The typical modifying agents used in the fabrication of commercial optical fibres are Al2O3,GeO2, P2O5, or B2O3, which forms aluminosilicate, germanosilicate, phosphosilicate or borosilicate glass. It is also prudent to note that F is typically the preferred agent for fabricating low-RI

4 Source: G. E. Keiser, Optical Fibre Communication, 2nd Ed., New York: McGraw Hill, 1991.

40 claddings as opposed to B2O3, as B2O3 possesses minor Infrared (IR) absorption characteristics which in turn will affect the transmission of light at the 1500 nm region. Furthermore, F ions are also used as glass formers, as they harden the fibre, allowing it easier drawing [47].

The same agents that are responsible for modifying the RIs of the core and cladding also play an important role in the fabrication of fibres doped with active ions such as Er2O3. These fibres, also known as rare-earth doped fibres, are crucial in the development of active fibre systems, such as fibre amplifiers and fibre lasers. In order to fabricate rare-earth doped fibres, a pure silica host cannot be used as it typically exhibits low solubility levels for rare earth ions. The reason for this is the structure of a pure silica glass host, which adheres closely to the ‘continuous random network’

model as proposed by Zacharison [48]. In this model, the structural tetrahedrons as illustrated in Figure 2 will share a bridging oxygen ion, thus forming a rigid network.

As trivalent rare-earth ions, such as Er³⁺ ions require the coordination of 6 to 8 oxygen ions, thus the introduction of Er³⁺ ions into the silica matrix will result in the silica tetrahedron being displaced to allow the dopant ion to bond to six oxygen ions.

This will allow a coordination balance to be achieved, with the 3+ charge of the erbium ion being balanced by three -1 charges from the oxygen ions, with two oxygen atoms behaving as a single charge⁵. As there are not enough ‘non-bridging’ oxygen ions in the structural matrix of silica, thus the Er³⁺ ions will tend to share oxygen ions, which in turn leads to a clustering of the rare earth ions. The close proximity of the Er³⁺ ions now allows energy to be transferred among these ions through Er-O-Er bonds, which reduces the ability of the Er³⁺ ions to impart energy to photons propagating through the doped fibre. This is the phenomenon known as concentration quenching, and the effects of concentration quenching and clustering limit the amount of Er³⁺ ions that can be incorporated homogenously into a pure silica fibre to only a about a hundred ppm/wt [49], [50]. As such, increasing the concentration of Er³⁺ ions in a silica host will require a change to the atomic structure of the silica host; typically by adding an additional ion into the tetrahedral structure. Both aluminum and phosphorous ions can be used as co-dopants in this manner, surrounding the Er³⁺ ion and shielding its charge. Trivalent Al³⁺ ions will act as a substitute for the Si⁴⁺ ions in the glass network, so that the three Aluminum ions can compensate for the charge

5 In this regard, the three -1 charges will thus be the contribution of six oxygen ions.

41 imbalance when in close proximity with the Er³⁺ ion. Phosphorous ions also play a similar role if incorporated into the glass structure, forming a P2O5 tetrahedron [49], [51]. Today, alumina-silicate glass has become the industry standard and preferred choice for the fabrication of rare-earth doped fibres for conventional applications.

In addition to the refractive index difference between the core and cladding layer as well as the ability to incorporate rare-earth ions, another important consideration in the development of the host material for the EDZF is the total dispersion of the material. The total dispersion, which comprises of material dispersion and waveguide dispersion, should be kept as close as possible to zero when selecting various dopants as well as their levels of concentration. The relationship between the Zero Dispersion Wavelength (ZDW) with respect to various dopant concentrations is shown in Figure 5.

Figure 5: The relationship between the dopant concentrations and the ZDW of the silica host

It can be seen clearly that increasing the concentration of dopants such as GeO2 and P2O5 in the silica host will result in the ZDW shifting towards the longer wavelength region, while higher concentrations of B2O3 dopants will in turn shift the ZDW of the silica host towards the shorter wavelength region. Therefore, the concentration of

Dopant Concentration (mol %)

5 10 15 20

1.25 1.30 1.35 1.40

B2O3

P2O5

GeO2

Zero Material Dispersion Wavelengths (m)

42 these dopants in the fabrication process is carefully controlled to allow the desired ZDW wavelength to be achieved.

Based on these criteria, the selected host material for the fabrication of the EDZF is a silica glass, which is procured in the form of a hollow silica substrate tube, with an outer diameter of about 20 mm and an inner diameter of about 16 mm. Once the selection of the host tube has been completed, the tube is cleaned thoroughly, to remove any contaminants, before being placed in the lathe of the MCVD rig. Upon mounting the tube securely, the next stage of the fabrication process begins, which is the deposition of the soot layer by the MCVD technique.