as long as the index of refraction of SAPs is lower than or equal to the index of refraction of cladding, there will be no secondary oscillation mode through SAPs.
The most common shapes used for SAPs are the bow tie shape and the circle. These fibers are called bow-tie and panda fibers respectively. The cross sections of these two fibers are shown in the figure below. The modal birefringence used in these fibers represents geometric and stress-induced birefringence. The geometric birefringence is very small and can be ignored for the circular core fiber. It has been shown that the birefringence of these fiber cores can be improved when SAPs are placed close to the fiber core, but it must be placed very close to the fiber core so that there is no increase in fiber loss, especially if the material on the SAPs is not silicon dioxide. Panda fiber has been improved to achieve higher mode birefringence, very low loss and low crosstalk. Round PM fiber
The concept of circular birefringence can be introduced into the fiber, so the two right-angled polarization modes are circularly polarized clockwise and counterclockwise in the fiber -- the so-called circular PM fiber. The most common way to achieve ring birefringence in a circular (axially symmetric) optical fiber is to twist the fiber, which produces a difference in propagation constants between the oscillating main mode of circular polarization in clockwise and counterclockwise directions. Thus, the modes of these two circularly polarized waves are decoupled. It can also be considered that the external stress can change the azimuth Angle in the fiber length direction, which can generate ring birefringence on the fiber. If an optical fiber is twisted, a torsion stress is generated, resulting in optical properties associated with distortion.
The fiber core of the fiber can also be laid along the spiral path in the cladding, so that the ring birefringence can also be obtained. This causes the light to travel along a spiral path, forming an optical rotation. Birefringence can be achieved only because of the influence of geometry. Such a fiber can be used as a single mode fiber, and it will cause relatively high loss in high order mode.
The annular PM fiber with helical fiber core structure can be used in the field of sensing current according to Faraday Effect. Optical fibers can be made using bimetallic rods and preformed tubes, which spin the preformed tubes to form spirals during fiber drawing.
Linear PM fiber
There are two main types of LINEAR PM fiber, namely single polarization type and birefringence type. Compared with the two basic polarization modes, the main characteristic of single polarization mode is that it has a large transmission loss. For birefringence fiber types, the propagation constants between the two polarization modes in the main mode of oscillation are obviously different. Multiple optical fiber designs can be used to maintain linear polarization, which will be discussed later.
Edge slots and edge tunnels linear PM fiber
The edge-slot fiber integrates two slots with a refractive index lower than the cladding index. The slots are located on two sides of the central fiber core. This type of fiber has a W-shaped refractive index distribution along the X-axis and a step refractive index distribution along the Y-axis. The edge-tunnel fiber is a special example of edge-slot structure. In these linear PM fibers, geometric anisotropy is introduced into the fiber core to obtain birefringence fibers.
Linear PM fiber with stressed components
An effective method to introduce high birefringence into the fiber is to introduce the non-uniform stress with double geometric symmetry into the fiber core. As a result of the photo elastic effect, the stress changes the refractive index of the fiber core, which can be observed through the polarization pattern along the fiber spindle as well as the results of birefringence. The required stress can be obtained by using two equally and independently stressed components (SAPs) located in the cladding region opposite the fiber core. Therefore,
Tip: At present, the most popular PM fiber in the industry is the round Panda fiber. Panda fiber one of the many advantages over other PM fibers is fiber size and numerical aperture compared to conventional single-mode fibers. Minimum loss on the device is ensured when using both types of light.
Linear PM fiber with elliptic structure
The first proposed experimental study of practical low-loss single-polarization fibers on three types of optical structures has been carried out: elliptic core, elliptic cladding and elliptic sheath fiber. The early research of elliptic fiber core cable involves the calculation of polarization birefringence. In the first stage, the rectangular dielectric waveguide is used to estimate the birefringence of the elliptic core fiber. In the experiment of using PM fiber for the first time, a kind of fiber with dumbbell shape fiber core was manufactured. The polarization beat length can be reduced by increasing the refractive index difference of fiber core cladding. However, due to practical application limitations, it is not possible to increase the refractive index difference too much. Increasing the refractive index difference results in transmission losses, and splicing becomes more difficult because the core radius must be reduced. The typical birefringence value for elliptic fiber is higher than that for elliptic cladding fiber. But the loss of elliptic fiber core is higher than that of elliptic cladding fiber.
Linear PM fiber with refractive index modulation
For a single-polarized fiber that isolates the cutoff wavelength of two right-angle oscillations, a method to increase its frequency band width is to select a refractive index distribution that only allows one polarization state to be at the cutoff. The high birefringence can be achieved by introducing angular modulation to the inner cladding index of the three-layer elliptical cross-section fiber. In the study of three-layer elliptical cross-section optical fibers, a perturbation approach is adopted, in which the rectangular fiber core waveguide is assumed as the reference structure. In single polarization operation, birefringence tests on three layers of ellipsoidal fiber show that proper angular modulation of the inner cladding index can enhance birefringence and extend the wavelength range.
The refractive index distribution is called the butterfly profile. This is an asymmetric W contour, which is composed of a consistent fiber core and the cladding surrounding the fiber core. In the cladding, the contour has the maximum value of NCL, and changes upward in radius and Angle, and has the maximum descending condition along the X-axis. There are two properties of this shape to realize single mode single polarization operation. First, the shape is asymmetrical, which will make the propagation constants of the two main modes of oscillation at right angles different, and second, the attenuation within the castle ensures that each mode has a cutoff wavelength. Butterfly fibers have a weak conductivity, so the answer to the scalar wave equation can be used to determine the mode field and propagation constant. The answer relates to trigonometric functions and Mathieu functions, which are used to explain the correlation of transverse coordinates in the cladding of the fiber core. These functions are not orthogonal to each other, which require an infinite set of functions to account for the modal fields in different regions and to satisfy the boundary conditions. The resulting geometric birefringence graph, compared with the standard frequency V, shows that the degree to which the refractive index declines along the X-axis increases the asymmetry, thus increasing the maximum and V values of the birefringence. The peak value of birefringence is characteristic of non-circular fibers. Mode birefringence can be improved by introducing anisotropy into the fiber. For anisotropy, it can be achieved by assigning different refractive index distributions to the two polarizations of a mode. Geometric birefringence is less than anisotropic birefringence. However, the drop in the butterfly shape cladding can provide double polarization to the oscillating main mode cutoff wavelength, which is separated by a wavelength window in which it is possible to achieve single polarization single mode operation.
