A Complete Guide for Special Fiber

Optical fiber technology has revolutionized the communications industry. After decades of deployment, fiber-optic networks carry telephone, television, and Internet services, either partially to end users, or in many cases, all the way to homes. These services are common and used very well. For example, in 2018, American adults averaged 8.5 hours of “screen time” per day-using smartphones, TVs, tablets, or personal computers. With the Covid-19 pandemic, the average screen usage time of adults in 20201 has increased by more than 50%, reaching 13.5 hours per day.

The surge in users includes more video conferencing and other video services, which require more network capacity than web pages and audio communications. Fiber optic networks can usually handle higher loads with minimal interruption. In some cases, network operators have increased network capacity

“Light up” more fibers, by adding wavelength channels, or increasing the bit rate. This illustrates the huge capacity and flexibility of today’s fiber optic networks.

Since the 1980s, the performance of fiber optic systems in terms of bandwidth and distance has improved significantly. In the early days, the bandwidth of fiber optic systems was larger than that of copper systems, but it was far from today’s requirements. The distance performance is limited by the loss of the optical fiber, so every 100 kilometers or so, a relay station needs to receive the optical signal, clean it electronically (reshaping and retiming), and then resend it.

All this has changed with the development and use of a special optical fiber, which is doped with the rare earth element erbium (Er), which is used to make optical amplifiers. Erbium dopants provide optical gain in the band near the 1.55 μm low-loss “window” of the communication fiber. The Erbium-Doped Fiber Amplifier (EDFA) enhances the transmission signal optically, eliminating the need for electronic “remodeling-rescheduling-regeneration” repeaters. In addition, an EDFA can amplify multiple wavelength channels in the same fiber without crosstalk, thereby greatly increasing the bandwidth of the fiber system2. In this case, dedicated optical fiber supplements the use of communication optical fiber, thereby greatly improving bandwidth and distance performance.

1. Difference between special fiber and communication fiber

“Specialty optical fiber” can be defined as an optical fiber that does not comply with single-mode and multi-mode communication optical fiber standards. For single mode, the International Telecommunication Union (ITU) standard is widely adopted. For multimode, the Optical Fiber Technology Alliance (TIA FOTC) standard of the Telecommunications Industry Association is widely adopted. These MM fiber specifications are also standardized by the International Electrotechnical Commission (ISO-IEC) of the International Organization for Standardization.

In both cases, “widespread adoption” means that fiber optic specifications are used by fiber optic and cable manufacturers, as well as companies that produce transceiver components, connectors, and other products that connect to fiber optics. In addition, these optical fiber standards also refer to standards for local area networks, telecommunication systems, and other infrastructure. Optical fiber standards specify the geometric, physical, and optical characteristics of optical fibers.

Communication fibers are used to transmit modulated optical signals of specific wavelengths. The fiber design is optimized for low loss and modal characteristics, supporting distance and bandwidth requirements. Special optical fibers are used for applications other than signal transmission, such as amplifiers, sensors, lasers, filters, ring resonators, etc. To meet these different applications, there are hundreds of different specialty fiber types, with many variations in the glass composition, core and cladding structures, geometric characteristics, coatings, and specially tailored optical performance characteristics. Some major specialty fiber families have the following characteristics:

• Special doped glass, especially in the core;
• The structure that realizes the birefringence of the polarization-maintaining fiber;
• Multiple packages;
• Special refractive index profile, customizable emission characteristics, numerical aperture (NA), effective area, mode propagation, and other waveguide characteristics;
• Various combinations of these functions.

The following table shows examples of specialty fibers and some typical applications. In the table and the rest of this article, we discuss silicon-based fibers that can be deposited using chemical vapor deposition, at least for part of the performing process. (There are also special fibers based on chalcogenides, fluorides, and other glass materials. These non-silicon fibers use different manufacturing processes and are not discussed here.) As can be seen from the table, many special types of fibers are designed for sensing systems. Fiber-based sensors can measure a large number of chemical, physical, environmental, and biological parameters. The breadth of sensing applications is the key reason for the large variety of specialty fibers.

Communication fiber

Specialty fiber

(Both tables provide a partial listing of fiber types and applications, with some examples that illustrate the diverse range of applications and fibers available)

2. Measure fiber demand in kilometers or meters

As mentioned above, there are more products and types in the market in the field of specialty fibers. On the other hand, the output of communication fiber is much higher. For example, by 2020, the installed mileage of global communication optical fiber will exceed 450 million kilometers, of which more than 90% will be ITU G.652. D “standard single-mode” fiber. The rest include single-mode fibers that meet the standards for improved dispersion, effective area, and bending performance, and standard multimode fibers.

FOC estimates that the world’s total demand for special optical fiber is 1 million kilometers per year or less than 0.5% of the demand for communication optical fiber. The use of polarization-maintaining (PM) fiber in the gyroscope can be extended to hundreds of thousands of kilometers per year, but the demand for other types of fiber is much lower. In sensors, fiber lasers or other devices, many applications use only a few meters or even less than one meter.

3. Mass production vs. custom production

Most specialty fiber manufacturers provide multiple types, basically satisfying a large number of small-batch or customized orders. Therefore, there is no special optical fiber factory for mass production like the production of telecommunication optical fiber. Some quick comparisons:

Telecom Fiber

• The length of the precast parts can reach 3 meters, most of which are 1-3 meters. One company uses 6 meters precast parts.
• The diameter of preforms can reach 25 cm; most are 10 to 20 years old.
• The stretching speed can exceed 40 m/s (forced cooling), and the typical stretching speed is 10-30 m/s.
• If a 3-meter-long preform is used, the fiber length of a preform can exceed 5000 kilometers.
• The size of single-mode telecom fiber reel is usually between 10 and 50 kilometers, and the size of multi-mode is smaller.
• The price of standard single-mode (G.652.D) is less than USD 10.00 per kilometer or less than 1 cent per meter.

(Note: This is cheaper than kite line, fishing line, dental floss or pasta per kilometer.)

Specialty fiber

• The length of the general preform is one meter or less. Many specialty fiber manufacturers who use the MCVD process to make mandrels start with a one-meter-long deposition tube
• The mandrel from a 1-meter tube can be stretched and sleeved twice or more to produce multiple preforms, so a 1-meter deposition tube may produce 4 or 5 preforms.
• The diameter (outer diameter) of the deposition tube commonly used to make MCVD core rods is 2.5 cm.
• The diameter (outer diameter) of the commonly used casing is 3.2 cm.
• The preform prepared for the final drawing may have an outer diameter of a few centimeters.
• The stretching speed is usually less than 5 m/sec, and many are less than 1 m/sec, depending on fiber type, profile complexity, material (dopant), etc.
• The number of fibers extracted from a preform varies greatly because the outer diameter of special fibers varies greatly. (Almost all telecommunications fibers have a cladding diameter of 125µm.)
• The cladding diameter of the rare-earth-doped fiber is usually 400µm. For a preform of this size, after various production factors, a preform with an outer diameter of 3 cm can produce several kilometers of fiber.
• Special fiber preforms usually require multiple steps, such as socketing, socketing, cutting, re-slipping, etching, and adding stress bars or other elements. Therefore, the pre-forming processing time per gram of material or per kilometer of optical fiber is many times that of telecommunications optical fiber.
• The price of specialty fiber varies greatly, but it is many times more expensive than communication fiber. A rare-earth-doped fiber costs more than US$50 per meter and costs more than US$50,000 per kilometer—three orders of magnitude higher than telecom fiber.

In short, standard telecommunications fibers all use the same glass material and geometric parameters. The fiber demand of each factory is tens of millions of kilometers and thousands of tons of preformed materials are needed. In response to these requirements, manufacturers have invested in batch deposition, large preforms, and high-speed stretching systems. Some of these systems are highly automated and can run large batches with minimal throughput.

The demand for special fibers is much smaller and requires more processing. Manufacturers do not need mass production equipment. They are in great demand for highly skilled scientists, engineers and technicians with deep expertise in products and processing steps. Specialty fiber manufacturers usually have to research and prepare starting chemicals, customize fiber design and production processes according to application requirements, develop processing procedures, and troubleshoot production problems to make new or customized fibers.

4. Tailor materials and geometric properties for different applications

There are three key factors involved in designing and manufacturing fibers. The first is the glass component. The second is geometric features size and shape. The third, the refractive index profile, partially depends on the first two. In other words, the refractive index profile is determined by controlling the positions of different glass materials in the optical fiber.

The composition of the glass varies with the dopant. The goal is to control the absorption, scattering, dispersion, and other phenomena of the fiber at a specific wavelength. Control the glass composition of different “parts” in the fiber, and further adjust the performance of the fiber under many parameters. What we mean by “parts” mainly refers to the concentric layers from the middle of the fiber to the outside. (There are also some fibers that have non-concentric parts, such as stress areas, cylindrical holes, voids, etc.)

This is where size and geometric features come into play. The refractive index profile is the curve of the refractive index of different materials at the distance from the central axis. The figure shows the refractive index of the fiber cross-section. The refractive index can be modified with different dopants and different layer thicknesses to control loss, dispersion, bending performance, modal propagation, emission conditions, cutoff wavelength, and other optical properties.

In most optical fibers, the refractive index is symmetrical-the “feature” of the optical fiber is in a layer of concentric circles. However, polarization-maintaining fibers have asymmetric properties that achieve birefringence. Unlike the transmission of communication fiber, the goal of PM fiber is to transmit two orthogonal polarization modes separately. In many applications, these two modes are recombined to measure delay, loss, or other disturbances to sense various parameters. There are several ways to achieve asymmetric features-use different materials to apply stress in the glass or using ellipses or other core and cladding shapes. These characteristics require special manufacturing processes.

5. Complex shapes and features increase manufacturing complexity

By controlling the material, size, and refractive index profile, specialty fiber manufacturers can customize the transmission, gain, birefringence, sensing characteristics, and other characteristics of the fiber for different applications. However, these design solutions also bring about complex problems in preform manufacturing and fiber stretching.

Different glass materials and dopants have different coefficients of thermal expansion (CTE). In the MCVD process, the material is deposited at a high temperature and then cooled. When MCVD core rods or preforms are processed from materials with different CTE, it creates stress, which complicates the subsequent machining.

Careful procedures are required to avoid the risk of damaging the mandrel or preform, and to avoid introducing defects that can cause casing and stretching problems. These methods vary with the dopant and glass composition, as well as the index profile and size. Using different dopant concentrations and glass compositions can also cause problems for the MCVD process-controlling vapor pressure and deposition uniformity.

The incorporation of different dopants and the resulting stress factors also have an impact on wire drawing. As mentioned earlier, the outer diameter and coating of special fibers vary greatly, so the drawing process must address these complexities. In addition, the stretching process must maintain the expected distribution of dopants to avoid introducing defects, weaknesses, or other defects. This means that drawing temperature, drawing speed, preform feed rate, drawing force, coating temperature and other drawing tower variables must be carefully controlled.