In a single-mode fiber, the output of the source laser is transmitted to each other at right angles through two linear polarization modes. Imagine this is an ideal single-mode waveguide:
- The core and cladding are perfectly round and concentric;
- No bending, no loss (no absorption, no scattering);
- The core material is completely uniform (no impurities, bubbles, voids or other defects);
- The laser wavelength is greater than the cut-off wavelength, and all laser energy is limited to the core (no higher-order modes);
- The temperature of the optical fiber and the light source remains constant;
- No lateral stress (no external stress from cables, placement, supports, etc., even assuming no gravity or air pressure).
In this hypothetical situation, the two polarization modes will reach the far end of the fiber with the same phase and the same power. Along the length of the fiber, power from one mode to another does not couple. If the laser output has a modulated signal, the two polarization modes will carry a signal with no dispersion and no crosstalk.
Of course, this imaginary situation is impossible. The glass materials and waveguides made are not perfect. There are sub-micron asymmetry and non-uniformity. In addition, single-mode optical fibers are subject to lateral stress in cables and placed in air or underground networks. In closures, hand holes, cabinets, and other structures, cables may bend and even coils may be loose. These phenomena can cause polarization modes to propagate at different group velocities. Therefore, the modulated signal at the fiber receiving end will be affected by dispersion. In the worst case, digital “1 and 0” or analog waveforms cannot be distinguished.
If left uncorrected, this polarization mode dispersion can limit the distance or bandwidth of the fiber optic communication system. Therefore, optical fiber cable, and system designers have developed techniques to reduce or compensate for this dispersion. The fiber manufacturer optimized the pre-forming and stretching process to minimize asymmetry, non-concentricity, and lateral stress. In addition, the drawing tower is also equipped with equipment that rotates the fiber when it is drawn. This helps to control the polarization characteristics of the fiber. The cable manufacturer then squeezes the tube around the fibers to isolate them from external stress on the cable. In telecommunication systems, digital electronic devices include dispersion compensation features, such as the use of chips with forwarding error correction algorithms in the receiver.
Therefore, the polarization can be effectively managed in the communication fiber. However, in many non-telecommunications applications, two polarization modes are required to propagate in a controlled manner. For example, in some interference sensors, the goal is to keep the two modes separate and then recombine them to analyze their phase interference mode. This provides a way to accurately measure motion, vibration, or other phenomena that affect fibers. The goal of these applications is to minimize the power coupling from one polarization state to another or to keep the two polarization modes propagating in two separate paths-therefore, “polarization maintaining” fiber.
PM fiber solves some of the same problems as single-mode communication fiber-minimize the influence of external stress and bending on the polarization mode of the fiber. For example, in gyroscopes and some sensors, PM fibers are wound in compact coils, but there is still a need to prevent power coupling from one polarization mode to another. Therefore, PM fibers have built-in geometric features or stress-applying “parts” (SAPs) to maintain the separation of the two polarization modes and minimize the effects of external stress. There are several ways to construct asymmetric geometric features and saps in the fibers, resulting in several types of PM fibers.
1. PM fiber types can be traced back more than 40 years ago
The research on PM fiber can be traced back to the 1970s, so the development time of this technology is about the same as that of telecommunications single-mode fiber. In the 1970s and 1980s, many well-known governments, military, corporate, and university laboratories reported the development of PM fiber. Examples include articles and patent applications by researchers from AT&T Bell Labs, Corning, Fujikura, Hitachi, NTT, and the University of Southampton. For example, Fujikura’s website states that the company “first produced PANDA (Polarization Maintaining and Absorption Reduction) PM fibers in the 1970s.”
Another company, Fibercore, was separated from the Optical Fiber Group of the University of Southampton in 1982. In 1982, the company produced low birefringence fiber, and in 1983 it produced bow tie type high birefringence PM fiber. SAP designs, coatings, dopants, and fiber cross-sections have evolved since the early 1980s. (The “bow-tie” pattern seen on the cross-section of the fibre roughly describes the SAP pattern.) and wavelength characteristics. Abundant PM fiber arrays to meet the different needs of sensing, telecommunications components, and fiber lasers. At present, there are more than 20 manufacturers and distributors worldwide that provide PM fibers, and many of them provide multiple types of PM fibers.
2. Key features
The attenuation and tensile strength of PM fibers are critical, just as they are for other special optical fibers. PM fibers also have two specifications-beat lengths and retention (H) parameters-to characterize their birefringence characteristics. These are complex measurements, but they are very important for characterizing the ability of the fiber to maintain two polarization modes.
The two axes in PM fiber are sometimes referred to as the “slow axis” and the “fast axis” because they have different refractive indices. This means that the light waves in the two polarization modes will have different phase velocities. The beat length is a measure of the difference in phase velocity between two polarization modes. A shorter beat length means higher birefringence and greater separation between the two modes.
The measurement range of the beat length of PM fiber ranges from less than one millimeter to several centimeters. A beat length of 1 mm is considered very short. The 2mm racket length is usually available and is often used for tops. The tact length of standard single-mode fiber used in telecommunications is measured in meters. The beat length is wavelength-dependent, so a specific wavelength is used to analyze and report measurement results.The h parameter is the polarization extinction ratio per unit length. It is used to describe how the polarization of the fiber on one axis is maintained over the length of the fiber. The h parameter is measured using standard techniques for measuring polarization crosstalk. The measured value is expressed as the change in the optical power transmitted on one axis per unit length of the optical fiber, which is also at a specific wavelength.
3. Preview concept-how to achieve birefringence
Birefringence is caused by a special shape or “built-in” SAP. Like other fibers, SAP is a silicon-based glass, but they are doped with different coefficients of thermal expansion (CTEs). When the fiber is stretched and cooled, the SAP cools and shrinks at different speeds, causing permanent stress in the glass. The results show that the optical fiber has asymmetric stress zones with different refractive indices.
Three commercial PM fiber types include sap: 1. 2) bow tie, 3) elliptical stress layer fiber. The fourth type, elliptical core fiber, is described as using form birefringence instead of SAP. There are other ways to achieve birefringence. An example is the use of longitudinal air holes or voids in photonic crystal fibers. Panda type and bow tie type are the most widely used and are favored by many manufacturers of gyroscopes, other sensors, and telecommunication components.
There are many trade-offs between these three SAP types, including the following:
- The level of birefringence that can be achieved partly depends on the proximity of the sap to the core, and other factors;
- The size of the stress area and the degree of asymmetry required to achieve high birefringence may affect the complexity and strength of manufacturing;
- The ability to uniformly create stress areas throughout the fiber;
- Preform size and fiber length-bow tie and elliptical stress layer PM fiber depends on the MCVD process, but PANDA can use an external deposition process, such as OVD or VAD;
- Mechanical properties such as fiber strength and crack resistance;
- Deal with the complexity of the prefabs, especially if the SAP is close to the core, and retain the SAP shape during the drawing process.
PANDA fiber has two longitudinal boron-doped glass cylinders located at the core on opposite sides of the cladding. Compared with core glass and cladding glass, borosilicate glass has a lower refractive index and higher CTE, which results in a stress zone after stretching and cooling. The manufacturing process relies on drilling two holes longitudinally in the preform, inserting boron-doped rods, and drawing the preform to achieve parallel and evenly spaced borosilicate cylinders on both sides of the core. This sounds like a small step, but it is an oversimplification. In practice, there are many complicated procedures, including:
- The position and size of the hole are completely correct, without cracks, defects, or impurities,
- Obtain or manufacture precisely doped and uniform borosilicate rods to make them completely suitable for holes,
- Treatment of pre-formed ends to manage induced stress;
- Keep the sap round in the drawing process, and many other detailed steps.
Bow tie fibers also use boron dopants in the two longitudinal apps. Like PANDA fiber, the sap is made in the cladding on both sides of the core. One of the main differences with PANDA fiber is that the bow tie type sap is wedge-shaped or trapezoidal. Another difference is that they are prefabricated on the MCVD lathe. Instead of drilling and inserting the rod cladding, fabricated the bow tie to weaken the deposited layer of diamond glass near the core, and then selectively etch a part of the layer in the two opposite core regions, and then deposit it with other dopants Fill in these areas with glass.
In this way, SAP can be deposited closer to the core, thereby achieving higher birefringence under less stress. But just like the PANDA process, there are many high-precision steps and “techniques” to make SAP. Basically, the complexity of drilling and inserting boron rods are replaced by the complexity of etching and doping, including:
- Control the flow of etchant (usually a fluorine compound) to match the size and position of the etching burner;
- Etch two areas, precise volume, and positioning and evenly along the length of the preform;
- Manage the position of the deposition burner and the flow of dopants through multiple steps to deposit the correct core and cladding glass in the etched area;
- Control the collapse of MCVD preforms to achieve the correct bow tie SAP shape and position;
- Correctly handle the end of the preform to avoid problems in the stress area;
The bow tie process allows for changes in the size, location, and shape of the stress area. This allows PM fiber manufacturers to balance optical and mechanical properties while customizing the level of birefringence for different applications. Since the size of the preform is limited by the diameter of the MCVD deposition tube, the bow-tie process cannot provide flexibility in terms of the number of fibers that can be extracted from a preform.
The elliptical stress layer fiber preform is the same as the bow tie preform, which is made on the MCVD lathe. Several thin boron-doped glass rings have also been attached to the cladding of the elliptical stress layer PM fiber. However, the elliptical stress layer fiber relies on a processing process to remove part of the cladding, rather than chemical etching in the bow tie process. In this step, the circular or circular symmetrical preform is processed into two planes relative to the core.
Then, by carefully controlling the temperature and stretching tension, this plane is performed (approximately rectangular) and drawn out to obtain a round fiber. That is, when the blank is heated and stretched, the plane disappears or becomes a circle. At the same time, the borosilicate layer (the original ring shape) becomes elliptical, forming an asymmetrical stress area in the cladding. If the fiber is carefully drawn, the fiber core can remain round. The complexity of this approach includes:
- Careful pre-form deposition and processing to avoid stress mismatch;
- Along the length of the preform, uniform and precise machining of the plane side surface;
- Prepare processed preforms to minimize surface defects;
- Control the stretching temperature to obtain the correct viscosity and fiber shape;
Elliptical core fiber also uses MCVD preforms, but there is no stress zone doped with boron. In contrast, the core glass is deposited asymmetrically, so the fibers are round when stretched, but the core is elliptical. To store the iron core in this way, the turning of the lathe must be stopped. One side of the tube is coated with core glass. When the preform collapses under the internal vacuum, the outer edge of the cladding remains circular, but the core glass becomes asymmetrical. The resulting PM fiber has lower birefringence than SAP fiber, but also has less internal mechanical stress.
Other types of PM fibers include polarized fibers that only propagate one mode, circularly polarized fibers (the polarization mode rotates down the fiber), rare-earth-doped PM fibers, and PM fibers with special coatings. These types of coatings have their own manufacturing complexity, such as spinning during the stretching process, dealing with stress problems caused by cte and different dopants, and dealing with coating materials with different mechanical properties.
4. Application and business needs
Many special fibers are used for very short lengths-such as meters or tens of meters-for sensing, gain media, Bragg gratings, etc. On the other hand, PM fibers are usually used in applications requiring more than one kilometer, usually wound in tight coils of gyroscopes, accelerometers, and hydrophones. Shorter length PM fibers are also used in telecommunications pigtails, optical coherence tomography systems, hydrophones, fiber lasers, and other sensor applications.
PM fibers used in gyroscopes and other interference sensors are usually “bare”, that is, the fiber is covered, but not in the cable with strength members and outer sheath. In many cases, the coated fibers are rolled into units, which can be less than 20 mm in diameter. In addition, many PM fibers are used in defense and high-reliability applications, with key strength and life specifications. For example, some interference sensors using PM fiber are used in deep water (high pressure) subsea applications.
Some gyro coils may require hundreds of meters or kilometers of PM fiber. In gyro applications, the production of optical fibers must ensure that these lengths of optical fibers have no joints. For telecommunications components and other braided applications, some PM fibers have connection ends, using some marking or keying of axial position. Other PM fiber applications can also use splicing, and several companies provide specialized fusion splicing equipment.
5. From the preform to the stretch is correct
Since the 1980s, with the increase in production, the price of PM fiber has dropped by at least an order of magnitude. Nevertheless, PM fiber is still much more expensive than communication fiber due to the complexity of production, the limitation of preform size, order quantity, and output factors. For example, a complex problem in production is that before measuring key parameters such as beat length and h parameters, the preform must be completed and drawn.
This means that you must carefully follow every step of the recipe, from burner and gas flow setting deposition, to drawing, coating, and winding. In addition, the detailed formula of PM fiber varies from one MCVD machine to another and from one type of PM fiber to another. The calibration of the flow controller and other machine settings are crucial to setting up and following these recipes.
