Fiber Design and Fabrication
In this tutorial, we discuss the engineering aspects of optical fibers made using either silica glass or a suitable plastic material. Manufacturing of fiber cables, suitable for use in an actual lightwave system, involves sophisticated technology with attention to many practical details. we begin with silica fibers and then consider plastic fibers. Both types of materials have been used in recent years to make microstructured fibers too.
1. Silica Fibers
In the case of silica fibers, both the core and the cladding are made using silicon dioxide (SiO2) or silica as the base material. The difference in their refractive indices is realized by doping the core, or the cladding, or both with a suitable material. Dopants such as GeO2 and P2O5 increase the refractive index of silica and are suitable for the core. On the other hand, dopants such as B2O3 and fluorine decrease the refractive index of silica and are suitable for the cladding. The major design issues are related to the refractive-index profile, the amount of dopants, and the core and cladding dimensions. The diameter of the outermost cladding layer has the standard value of 125 μm for all communication-grade silica fibers.
The figure above shows typical index profiles that have been used for different kinds of fibers.
The top row corresponds to standard fibers which are designed to have minimum dispersion near 1.3 μm with a cutoff wavelength in the range 1.1-1.2 μm. The simplest design (a) consists of a pure-silica cladding and a core doped with GeO2 to obtain Δ ≈ 3 x 10-3. A commonly used variation (b) lowers the cladding index oer a region adjacent to the core by doping it with fluorine. It is also possible to have an undoped core by using a design shown in (c). The fibers of this kind are referred to as doubly clad or depressed-cladding fibers. They are also called W fibers, reflecting the shape of the index profile.
The bottom row shows three index profiles used for dispersion-shifted fibers for which the zero-dispersion wavelength is chosen in the range 1.45-1.60 μm. A triangular index profile with a depressed or raised cladding is often used for this purpose. The refractive indices and the thickness of different layers are optimized to design a fiber with desirable dispersion characteristics. Sometimes as many as four cladding layers are used for dispersion-flattened fibers.
Fabrication of telecommunication-grade silica fibers involves two stages. In the first stage a vapor-deposition method is used to make a cylindrical preform with the desired refractive-index profile. The preform is typically 1 m long and 2 cm in diameter and contains core and cladding layers with correct relative dimensions. In the second stage, the preform is drawn into a fiber by using a precision-feed mechanism that feeds the preform into a furnace at the proper speed.
Several methods can be used to make the preform. The three commonly used methods are modified chemical-vapor deposition (MCVD), outside-vapor deposition (OVD), and vapor-axial deposition (VAD). The following figure shows a schematic diagram of the MCVD process.
In this process, successive layers of SiO2 are deposited on the inside of a fused silica tube by mixing the vapors of SiCl4 and O2 at a temperature of about 1800°C. To ensure uniformity, a multiburner torch is moved back and forth across the tube length using an automatic translation stage. The refractive index of the cladding layers is controlled by adding fluorine to the tube. When a sufficient cladding thickness has been deposited, the core is formed by adding the vapors of GeCl4 or POCl3. These vapors react with oxygen to form the dopants GeO2 and P2O5:
GeCl4 + O2 → GeO2 + 2Cl2
4POCl3 + 3O2 → 2P2O5 + 6Cl2
The flow rate of GeCl4 or POCl3 determines the amount of dopant and the corresponding increase in the refractive index of the core. A triangular-index core can be fabricated simply by varying the flow rate from layer to layer. When all layers forming the core have been deposited, the torch temperature is raised to collapse the tube into a solid rod of perform.
The MVCD process is also known as the inner-vapor-deposition method, as the core and cladding layers are deposited inside a silica tube. In a related process, known as the plasma-activated chemical vapor deposition process, the chemical reaction is initiated by a microwave plasma. By contrast, in the OVD and VAD processes the core and cladding layers are deposited on the outside of a rotating mandrel by using the technique of flame hydrolysis. The mandrel is removed prior to sintering. The porous soot boule is then placed in a sintering furnace to from a glass boule. The central hole allow an effective way of reducing water vapors through dehydration in a controlled atmosphere of Cl2-He mixture, although it results in a central dip in the index profile. The dip can be minimized by closing the hole during sintering.
The fiber drawing step is essentially the same irrespective of the process used to make the preform. The following figure shows the drawing apparatus schematically.
The preform is fed into a furnace in a controlled manner where it is heated to a temperature of about 2000°C. The melted preform is drawn into a fiber by using a precision-feed mechanism. The fiber diameter is monitored optically by diffracting light emitted by a laser from the fiber. A change in the diameter changes the diffraction pattern, which in turn changes the photodiode current. This current change acts as a signal for a servocontrol mechanism that adjusts the winding rate of the fiber. The fiber diameter an be kept constant to within 0.1% by this technique. A polymer coating is applied to the fiber during the drawing step. It serves a dual purpose, as it provides mechanical protection and preserves the transmission properties of the fiber. The diameter of the coated fiber is typically 250 μm, although it can be as large as 900 μm when multiple coatings are used. The tensile strength of the fiber is monitored during its winding on the drum. The winding rate is typically 0.2-0.5 m/s. Several hours are required to convert a single preform into a fiber of about 5km length. This brief discussion is intended to give a general idea. The fabrication of optical fiber generally requires attention to a large number of engineering details discussed in several texts.
2. Plastic Optical Fibers
The interest in plastic optical fibers grew during the 1990s as the need for cheaper fibers capable of transmitting data over short distances (typically <1 km) became evident. Such fibers have a relatively large core (diameter as large as 1 mm), resulting in a high numerical aperture and high coupling efficiency, but they exhibit high losses (typically exceeding 20 dB/km). For this reason, they are used to transmit data at bit rates of up to 10 Gb/s over short distances (1 km or less). In a 1996 demonstration, a 10-Gb/s signal was transmitted over 0.5 km with a bit-error rate of less than 10-11. Graded-index plastic optical fiber provide an ideal solution for transferring data among computers and are becoming increasingly important for the Gigabit Ethernet and other internet-related applications requiring bit rates in excess of 1 Gb/s.
As the name implies, plastic optical fibers use plastics in the form of organic polymers for making both the core and the cladding. The commonly used polymers for this purpose are polymethyl methacrylate (PMMA), polystyrene, polycarbonate, and an amorphous fluorinated polymer poly(perfluoro-butenylvinyl ether), or PFBVE, known commercially as CYTOP. The PMMA plastic was used to make step-index fibers as early as 1968. By 1995, the technology had advanced enough that is was possible to make graded-index plastic fibers with a relatively large bandwidth. Since then, considerable progress has been made in making new types of plastic fibers with relatively low losses even in the wavelength region near 1.3 μm. The core diameter of plastic fibers can vary from 10 μm to 1 mm depending on the application. In the case of low-cost applications, the core size is typically 120 μm, while the cladding diameter approaches 200 μm.
Manufacturing of modern plastic fibers follows the same two-step process used for silica fibers in the sense that a preform is prepared first with the correct refractive-index profile and is the converted into the fiber form. An important technique used for making the preform for graded-index plastic fibers is known as the interfacial gel polymerization method. In this technique, one begins with a hollow cylinder made of the polymer (such as PMMA) destined to be used for the cladding. This hollow cylinder is filled with a mixture of the monomer from which the cladding polymer was made, a dopant with higher refractive index than that of the cladding polymer, a chemical compound that helps in initiating the polymerization process, and another chemical known as the chain-transfer agent. The filled cylinder is heated to a temperature close to 95°C and rotated on its axis for a period of up to 24 hours. The polymerization of the core begins near the inner wall of the cylinder because of so-called gel effect and then gradually moves toward the center of the tube. At the end of the polymerization process, one ends up with a graded-index preform in the form of a solid cylinder.
The fiber-drawing step is identical to that used for silica fibers. The drawing apparatus similar to the last figure is used for this purpose. The main difference is that the melting temperature of plastic is much lower than that of silica (about 200°C in place of 1800°C). The fiber diameter is continuously monitored using a suitable optical technique, and another plastic coating is applied to the fiber. This top plastic coating protects the fiber against microbending and facilitates its handling.
The following figure shows the loss spectra of several plastic fibers.
A PMMA fiber exhibits losses that typically exceed 100 dB/km. In contrast, losses of modern PFBVE fibers remain close to 20 dB/km over a wide wavelength range extending from 800 to 1300 nm and have the potential of being reduced to below 10 dB/km with further optimization. Similar to the case of silica fibers, material absorption can be divided into intrinsic and extrinsic categories. Intrinsic absorption losses in plastic fibers result from vibrational modes of various molecular bonds within the organic polymer used to make the fiber. Even though the vibrational frequencies of these modes lie in the wavelength region beyond 2 μm, their harmonics introduce considerable losses for all plastic fibers even in the near-infrared and visible region. Extrinsic absorption is related to the presence of impurities within the fiber core. Transition-metal impurities such as Fe, Co, Ni, Mn, and Cr absorb strongly in the wavelength range 0.6-1.6 μm. Even a trace amount as small as a few parts per billion can add losses in excess of 10 dB/km. Similar to the case of silica fibers, any residual water vapor results in a strong peak near 1,390 nm. This problem is less severe for PFBVE fibers because fluropolymers do not absorb water easily.
3. Cables and Connectors
Cabling of optical fibers is necessary to protect them from deterioration during transportation and installation. Cable design depends on the type of application. For some applications it may be enough to buffer the fiber by placing it inside a plastic jacket. For others the cable must be made mechanically strong by using strenghening elements such as steel rods.
A light-duty cable is made by surrounding the fiber by a buffer jacket of hard plastic. A tight jacket can be provided by applying a buffer plastic coating of 0.5-1 mm thickness on top of the primary coating applied during the drawing process. In an alternative approach the fiber lies loosely inside a plastic tube. Microbending losses are nearly eliminated in this loose-tube construction, since the fiber can adjust itself within the tube. This construction can also be used to make multifiber cables by using a slotted tube with a different slot for each fiber.
Heavy-duty cable, needed for submarine applications among other things, use steel or a strong polymer such as Kevlar to provide the mechanical strength. The following figure shows schematically three kinds of cable.
In the loose-tube construction, fiberglass rods embedded in polyurethane and a Kevlar jacket provide the necessary mechanical strength (left drawing). The same design can be extended to multifiber cables by placing several loose-tube fibers around a central steel core (middle drawing). When a large number of fibers need to be placed inside a single cable, a ribbon cable is used (right drawing). The ribbon is manufactured by packing typically 12 fibers between two polyester tapes. Several ribbons are then stacked into a rectangular array that is placed inside a polyethylene tube. The mechanical strength is provided by using steel rods in the two outermost polyethylene jackets. The outer diameter of such fiber cables is typically in the range of 1 to 1.5 cm.
Connectors are needed to use optical fibers in any actual lightwave system. They can be divided into two categories. A permanent joint between two fibers is known as a fiber splice, and a detachable connection between them is realized by using a fiber connector. Connectors are used to link fiber cable with the transmitter (or the receiver), while splices are used to join two fiber segments permanently. The main issue in the use of splice and connectors is related to the loss. Some power is always lost, as the fiber ends are never perfectly aligned in practice. Splice losses below 0.1 dB are routinely realized by using the technique of fusion splicing. Connector losses are generally larger. State-of-the-art connectors provide an average loss of about 0.3 dB. The technology behind the design of splices and connectors is quite sophisticated. You can refer to the book of "Optical Fiber Splices and Connectors" by C.M. Miller and S.C. Mettler for a very detailed description.