1. IntroductionThe optical fiber with pure quartz core and fluorinedoped glass cladding is a new kind of large core optical fiber that appeared in recent years. Some corporations have made it by POD technique (plasma outside deposition) [1], which decreased the cost of making large core fiber greatly in comparison with that from conventional MCVD technique. The diameter of the preform is greatly increased, and now it has replaced the conventional large core fiber and is widely used in many fields. Through many experiments, at last we made the fiber with the same structure as that from POD by the technique of “overcladding F-doped tube onto the quartz rod in high temperature” [2], i.e. “rod in tube” technique, in which the F-doped tube was fabricated by PCVD (plasma chemical vapor deposition), and the optical fiber preform was made by MCVD (modified chemical vapor deposition). The technique is characterized by simplicity, low cost, high end product efficiency, large ratio of “core diameter/fiber diameter” and the low loss of the optical fiber, etc.
The structure of this kind of fiber is quite different from that made by conventional technique. There are two main different points: one is that the fiber core material is pure quartz, another is that the cladding thickness is greatly reduced. The influence of the first different point on the fiber loss is obvious, and what influence will come about the loss of fibre brought by another different point? We want to be quite clear on this point, make it clear, we conducted a series of researches to confirm that, which include: 1) We cover the fiber with high and low refractive index polymers separately, and compare the changes between their loss spectra; 2) The influence of the glass cladding thickness with low-refractive-index coat on fiber loss spectra; 3) The influence of the concentricity error between core and cladding on the fiber loss spectra. We found the great changes on their loss spectra in the experiments, which shows that the influence of the above factors is obvious.
4. Discussion4.1. Influence of the Inner Polymer Coat of the FiberIn Figure 4, because the inner polymer coat of the 1# fiber has the low refractive index, the transmission light reaches and goes through the interface of the core and the F-doped glass cladding is reflected respectively back to the fiber core again by the glass cladding and interface of the inner polymer coat with low refractive index, and it continues to transmit in the fiber [4]. As a result, the transmission loss of 1# fiber can be relatively low. But the situation shown in Figure 5 is quite different, in which the fiber 2# is covered with the high refractive index polymer as inner coat, so the light energy is absorbed by the polymer coat material as it gets out of the core and reaches the interface between the F-doped glass cladding and polymer coat [5], thus resulting in a relatively high background loss of 2# fiber [6]. It’s the reason that leads to the different background loss between the above two fibers.
4.2. Influence of F-Doped Glass Cladding Wall ThicknessIn the Figure 5, the loss of 2# fiber is rapidly increased when the wavelength exceeds operation wavelength λ0 (0.808 µm). However, as wavelength exceeds critical wavelength λ2 (1.07 µm), the loss spectra presents a vibration character and keeps essentially a constant value. It means that the loss has nothing to do with wavelength.
As to multimode optical fiber, we know the light with operation wavelength λ0 is effectively limited and transmitted in the fiber waveguide with low loss when the cladding wall thickness with low refractive index is over 3λ0. When the wavelength of the light that transfers in the optical fiber increases, according to relationship of 3λ, in order to keep the light transmitting with low loss in the fiber, it demands the thicker glass cladding wall.
Table 1. Materials and the parameters and the structure of the preform and its optical fiber.
Through analysis, we think the loss spectra of optical fiber 2# in Figure 5 is the typical one of the fiber with pure quartz core and F-doped glass cladding. Though the cladding wall thickness of the fiber we designed is (110 µm − 100 µm)/2 = 5 µm, which has low refractive index,
but because of the process error of mechanical grinding and fluctuation of fiber diameter, the practical result is that some cladding wall thickness of fiber 2# is less than 5µm. We can see the result from the loss spectra in Figure 5, when the wavelength of light exceeded λ2 = 1.07 µm, the loss of the fiber would reach the highest value and presents a vibration character. It means the light transmitted in the fiber can not be controlled by the glass cladding. Currently, 3λ2 = 3 × 1.07 = 3.21 µm, so we know that the practical cladding wall thickness of the fiber 2# is 3.21 µm. In the opinion above, all the light has same significance when operation wavelength is over 1.07 µm. It will all leak out of the fiber as the leaking modules and absorbed and exhausted by the coating polymer with high refractive index, so the fiber loss opposite to long wavelength (over 1.07 µm) remains essentially a constant value in Figure 5. The loss has nothing to do with wavelength.
4.3. Infiuence of the Concentricity Error of the Fiber Core and CladdingThe loss spectra of 3# optical fiber in Figure 6 is obviously different from that in Figure 5 though they were all covered with high refractive index polymer as inner coat. Besides the larger background loss of the fiber 3#, the fiber loss increases gradually with the increasing of the wavelength. The loss spectra will present vibration when wavelength is over λ3 (1.5 µm).
Through analysis, we think the unusual loss spectra of the 3# fiber has something to do with the tolerance of the F-doped glass cladding wall thickness (concentricity error), which resulted from the mechanical grinding process, as is shown in Figure 7. The minimum wall thickness is δ1, and the maximum wall thickness is δ2 = 1.5 µm (critical wavelength) × 3 = 4.5 µm. Because the loss of 3# fiber is even larger than that of fiber 2# at operation wavelength 0.808 µm, it means that the error of cladding wall thickness leads to the wall thickness around δ1 thinner than 3λ0 = 3 × 0.808 = 2.424 µm, demonstrating that the light has leaked out of the fiber at the wavelength, whereas the cladding wall thickness of the other part of
the fiber is over 3λ0 = 2.424 µm, from Figure 7 we can see that the farther from δ1, the thicker of the cladding wall thickness will be on the fiber section.
As to meridian or precession light, the fiber with large concentricity error results in the result that the light transmitted in the fiber reaches to the aero nearby δ1 leaks out of the fiber and gets into the polymer coat with high refractive index, and it is absorbed and exhausted. This happens already in the range of short wavelength, so the background loss is raised up in the spectra of 3# fiber in Figure 6. Because of the large concentricity error offset structure of the fiber between its core and cladding, with increasing of the wavelength, there are more and more light leaks out of the fiber as leaking modules, which means the glass cladding controlling ability on the transmiting light is weaker and weaker. The geometric aero where the light is leaked out of the fiber became larger and larger, which is around δ1, the leaking light of high grade modules is absorbed and exhausted by polymer coat of high refractive index, and this finally leads to the fiber loss increasing larger and larger with the wavelength of light changing longer and longer. When wavelength λ is over λ3 and 3λ > 3λ3 = δ2, the glass cladding with low refractive index losed any controlling ability about the light which wavelength is over the critical point λ3. All transmitting light in the fiber changes into leaking light and absorbed and exhausted by the inner coating polymer, so the loss is not increased with the increasing of the wavelength. From Figure 6 we can see that the loss spectra of fiber 3# appeared vibration character which is similar with that of 2# fiber in Figure 5, when the wavelength increased longer enough.
5. ConclusionsWe know the light propagation constant 
, which transmits in the fiber, where n1 is refractive index of the fiber core, λ is the working wavelength, θ is the angle between vector k and the axis of the fiber, and each β is corresponding to one transmitting light module. With increase of the light wavelength λ that transmits in the fiber, β corresponding to each λ will become smaller and smaller. In terms of optics waveguide theory, when the propagation constant β of one transmitting light module reduces and exceeds one critical value β1 (i.e. optical wavelength λ increases and exceeds some value λ1), the transmitting module of light changes into leaking module, if optical wavelength further increases when the propagation constant β of one transmitting light module reduces and exceeds another critical value β2 (i.e. optical wavelength λ increases and exceeds some value λ2), the transmitting light will change its leaking module into radiation module, and lose all transmitting optical energy. This is the relationship between various transmitting optical wavelengths in the fiber and the energy carried by the corresponding module.
Through our experiment results, we can see it is still existed that relationship between the transmitting light wavelength λ and the energy carried by the corresponding module in this kind of fiber, and that because various inner polymer coat and the different glass cladding thickness and the concentricity error between the fiber core and cladding, the relationship is changed more complicated. But from another point, this maybe give us a chance, we can effectively influence the character of the fiber through adjusting some design on it. This provides us with some chances about the potential application of the special fiber.
In addition, from the experiments and discussion above, we have known that the loss of the fiber with pure quartz core and F-doped glass cladding is influenced by many factors, some of which come from the special structure of the fiber, so when someone plans to fabricate this kind of fiber by our technique of “rod in tube”, he should propose the corresponding requests on materials and fiber structure and processing geometry accuracy so that he can ensure the quality of this kind of optical fiber.