Many high-tech products are composed of a series of procedures, so each step is based on the accuracy and precision of the previous step. This is especially true for rare-earth-doped fibers, which require knowledge of preform manufacturing, fiber stretching, measurement, and a basic understanding of waveguide propagation and laser fundamentals. For example, the production of preforms requires knowledge and experience in several areas: handling chemicals, managing deposition processes, understanding glass materials and their characteristics, and glass processing skills such as grinding, shaping, stretching, straightening, and joining.
Some steps of making doped fiber preforms are carried out on machines under software control. Other parts of the process depend on the operator’s decision, and sometimes on their skills in using hot glass; these steps involve techniques and techniques. This article will lead you through the key steps, starting from the core pole. As mentioned in the previous article, we believe that the halide MCVD process is particularly suitable for achieving the best-doped fiber properties in many applications.
1. Core and Cladding Depend on different processes
An MCVD doped optical fiber preform has several layers of glass material. The erbium-doped fiber core is deposited in a high-purity quartz tube. The quartz of this starter tube becomes the first cladding glass. The subsequent steps use sleeves to increase the amount of cladding glass. In many applications, such preforms are “shaped” by grinding the flat surface of the surface. This hexagonal or octagonal shape is to maximize the mixing of the modes in the first cladding, thereby maximizing the absorption of the core.
The molded preform is then stretched, coated with a polymer, and subjected to verification tests. For some doped fibers called “double cladding”, the polymer coating is a low refractive index material used as a secondary cladding. From the beginning, the quartz material of the tube and sleeve is the primary cladding.
2. The size and shape of the preforms vary by application
In order to achieve the required fiber geometry, many preforms are made through tailoring. The amount of core material to be deposited, the amount of cladding glass to be added, and the final shape can all vary with the design of the fiber, which in turn is driven by the application of doped fiber. For example, the erbium-doped fiber used in telecom amplifiers and telecom transmission fiber has the same outer diameter and similar core-to-pack ratio.
For lasers and sensors, on the other hand, the shape, numerical apertures, and diameters of the core and cladding may vary significantly. These variables are driven by factors such as pump type, number of absorption and gain, temperature, modal quality, operating wavelength, and many other operating characteristics. For example, some fiber lasers use doped fibers with 20 μm core and 400 μm cladding. For fibers with an octagonal primary cladding, the 400-µm diameter measurement is the diagonal from one vertex to the other.
3. MCVD deposition and casing are building blocks
1) Deposit the doped core material.
Most fiber manufacturers source starting tubes from outside. Quartz rods, tubes, and other shapes in high purity are available for fiber manufacturers from several companies. The typical size of the deposition tube is 25 mm in outer diameter and 1 meter in length. Other sizes are available.
For custom sizes, the tube manufacturer’s process may mean that the customer must purchase a larger quantity. The typical specification for wall thickness is 3 mm, or 19 mm inner diameter. The fiber manufacturer can specify different sizes based on the number of core materials to be deposited and the planned geometric features. The doped core material is deposited on the inner wall of the tube at a thickness of 100 microns (or several hundred microns).
2) The collapse of the mandrel.
The result of the deposition is a hollow cylinder with doped material on the inner wall. This must be folded into a solid rod with a solid doped core. The goal is to get a straight rod with good core-cladding concentricity, geometric consistency, no contaminants-no moisture (low OH-). This step is done on a performing lathe while the rod is hot and turned. The deposition temperature is usually between 1800 and 1900°C. During the collapse, the temperature will rise several hundred degrees, and the burner will pass the rod at a slower speed.
The gas pressure in the pipe is carefully controlled together with the burner channel. This is done to maintain a good balance and surface tension to ensure that the tube collapses and has a uniform cylindrical shape. At this point, the collapsed rod is almost ready for the casing, which will build up the cladding material. After the collapse, the mandrel may not be straight. In this case, the mandrel must be straight so that it can be inserted into a straight sleeve. This is where the glass processing technology comes in. This process is done on the lathe, heated again, and the operator uses a hand tool to manually straighten the rod. Then, the last step before the casing operation is cleaning and fire polishing the rod.
3) Increase the cladding glass through the casing.
An integral part of all these pre-forming steps is the careful inspection, measurement, and testing of mechanical, geometric, and optical properties. For example, in order to plan the casing step to achieve the required fiber core diameter and core-to-pack ratio, the precise dimensions of the folded mandrel are required. Commonly used casing sizes are 32 mm in outer diameter, 20.5 mm in inner diameter, and 1 meter in length. There are different diameters and wall thicknesses to choose from, but the choice of the preform manufacturer may be limited by the burner size and other lathe parameters.
Depending on the size of the mandrel and sleeve, it may be necessary to stretch the preform before it will fit into the tube. It can be stretched on a lathe equipped with a movable tailstock, but this process is limited by the length of the lathe and the sag of gravity. Another option is to stretch the tower vertically, like a tower of attraction. This allows for greater length and better geometric control. In either case, heat and stretching exercises are carefully controlled. The process of pressing the sleeve onto the mandrel can also be performed on a horizontal lathe with a mobile burner, or on vertical equipment with a burner or a furnace, and also requires careful temperature and motion control. After crushing the casing, the result is a strong glass rod. Depending on the plan and the required size, it may be a prefab for drawing, or it may be a temporary unit for obtaining more casings.
4) Obtain multiple preforms from one mandrel.
In many cases, the geometric goal is achieved by stretching or “rattan” transition units, cutting them into shorter lengths, and pressing sleeves on each transition unit. In some cases, a rod with a sleeve may be stretched to a length of about 10 meters and cut into multiple parts. For such lengths, the drawing process is carried out on a vertical tower, and the processing speed used is much slower than the process used to draw the fiber.
There are different procedures to “cut” long sections. The choice depends on the core dopants, geometry, and other glass characteristics. These options include different temperatures and mechanical processes. Therefore, the casing can involve many complicated steps, producing four or more preforms from one MCVD deposited mandrel. Again, all of these require precise measurement, careful handling, and cleaning.
5) Drawing after final molding.
Some preforms can be drawn into circular cross-sections after the previous step. However, for some fiber laser applications, a pre-formed shape such as hexagonal or octagonal is more suitable to achieve better mixing of pump-laser output modes at the core. Why is this shaping advantageous? In a cylindrical fiber with a concentric core, the pump energy can propagate in the cladding, completely neglecting the doped core.
Over the years, different geometries have been developed to increase the power absorption of the pump. One strategy is to grind the outer edges of the preforms on one or more flat surfaces. This must be done carefully to minimize surface defects, which can cause problems in wire drawing and coating. Then, the stretching temperature must be optimized to avoid rounding off the desired shape.
As the temperature increases, the drawing speed and tension must be controlled to achieve the proper fiber diameter and strength. (Like making preforms, the drawing process of rare-earth-doped fiber also has many variables and complications. Topics related to drawing will be introduced in subsequent articles in this series.)
4. Conclusion
We have shown a series of steps-deposition, collapse, sleeve, rod, cutting, re-sleeve, and finally forming-from the beginning of the tube to the perform ready to be stretched. Each step includes careful measurement and testing. Each step has a yield factor. For example, at each stage, some glass at the end of the mandrel and tube will be lost.
The testing process may further reduce the total amount of deposited or casing glass. With these different output factors, the success of each step depends on the success of the previous step. Therefore, it is vital that this process starts with a good core rod. For many rare-earth dopants and required fiber characteristics, such as refractive index, NA, etc., the halide method can be used to obtain good core rods.
The number of fibers extracted from a preform depends on fiber diameter, drawing yield, and other variables. Generally speaking, one meter of rare-earth-doped optical fiber preform can produce one to five kilometers of optical fiber. Therefore, if one mandrel can produce four or more preforms, it can produce approximately 10 kilometers of fibers. One core rod can provide enough fibers for hundreds of lasers or amplifiers, depending on the number of fibers in the gain medium.
