A Polarization Maintaining Fiber is a single-mode fiber that preserves and transmits the polarization state of the light entering into it. Usually, polarization-maintaining fibers are single mode, but in very rare cases, there are a few other modes too. This is because of the difficulty in producing strong and uniform birefringence in fiber glass on a large core area where many modes can be guided.
Figure 1: Polarization maintaining fiber
As a result of the imperfect alignment of the input polarization direction and due to the residual degree of mode mixing, the polarization extinction ratio of light output may be lower than that at the fiber input. Figure 1 shows a polarization-maintaining fiber. The mode mixing effect is strongly increased when mechanical stress is applied to these fibers. A high-quality polarizer (or polaroid filter) is used after the fiber for applications that require high polarization extinction ratio.
Polarization-maintaining fibers are also called birefringent fibers. Polarization maintaining fibers maintain uniform birefringence throughout the fiber length. Larger the birefringence better the prohibition of power coupling.
Principle of operation in polarization maintaining fiber
Two orthogonally polarized light modes are forced to travel through a fiber at different velocities or different propagation constants. This difference is made by creating an anisotropy inside the core of the fiber by making the geometry elliptic or by applying controlled uniaxial stress. The strong birefringence is the reason for the difference in propagation constants of the two polarization modes. The relative phase of these modes traveling in the same direction drifts away. This makes it difficult to cross-couple the optical energy and as a result, the polarization state of transmitted light is preserved.
Types of Polarization maintaining fiber
Based on the birefringence, the polarization-maintaining fiber is classified into two: Low birefringent fiber and High birefringent fiber.
Generally, the minimum value required to maintain polarization in a fiber is 10-4. For high birefringent fibers, the birefringence value is greater than 10-5 and in the case of low birefringent fibers, it is lesser than 10-5.
High birefringent fibers (HBFs) are produced by introducing a large structural asymmetry in the fiber during the manufacturing process. This is typically done by applying a high level of stress to the preform or by using materials with a large difference in refractive indices in different directions. One common method is to use a preform that has a high refractive index in the core and a low refractive index in the cladding. The preform is then heated and drawn into a fiber, creating a large structural asymmetry that causes the fiber to have high birefringence. High birefringent fibers are commonly used in applications that require precise control over the polarization of light, such as in fiber-optic gyroscopes and interferometry. They are also used in fiber-optic sensors for measuring strain, temperature, and other physical parameters.
Low birefringent fibers (LBFs) can be produced using the chemical vapor deposition (CVD) technique. In this process, a precursor gas is introduced into a reactor, where it is heated to a high temperature and deposited onto a substrate, usually in the form of a preform. The deposited material forms the core and cladding of the fiber. By carefully controlling the deposition process and the composition of the precursor gas, it is possible to create a fiber with a small structural asymmetry, resulting in a low birefringence. Low birefringent fibers are used in applications that require minimal polarization-dependent loss, such as in long-haul telecommunications systems and high-speed data transmission. They are also used in fiber-optic amplifiers, where the low birefringence allows for the amplification of multiple channels of data simultaneously.
High birefringent fibers are of two types: Single polarization fibers and Two polarization fibers.
The crosstalk in single-polarization fibers is constant and independent of fiber length. However, the crosstalk in two polarisation fibers becomes worse as the fiber length increases. Bow-tie fibers and Panda fibers are examples of single polarization fibers. These are stress-induced birefringent fibers with low optical losses. Panda fibers have a unique structure that consists of a central core surrounded by two orthogonal claddings. The light propagates in the core and is polarized in the direction of the cladding. Bow-tie fibers have a similar structure to panda fibers, but the core is made up of two smaller cores separated by a thin bridge. The two cores have orthogonal birefringence, which means that they have different refractive indices for light polarized in different directions. The schematic of Panda fiber and Bow-tie fiber are shown in figure 2.
Figure 2: Schematic of Panda fiber and Bow-tie fiber
There are also polarization-maintaining fibers with twofold rotational symmetry in the refractive index profile and are called form birefringent fibers. This symmetry is made by making the shape of the core elliptical. The attenuation is very high for these fibers.
Disadvantages of polarization-maintaining fibers
Applications
Polarization maintaining fiber is commonly used in telecommunications to connect a source laser and modulator since the modulator requires polarized light as input. They are also used in the aerospace industry for fiber-optic gyroscopes applications. Some special applications include fiber interferometry, fiber optic sensing, and slab dielectric waveguides.
PM fibers are also used in coherent optical transmission systems or long-distance bidirectional optical transmission systems. For transmission applications where the polarization plane of the optical signal is important, such as transmission lines for optical sensors and coupling for optical electrical integrated circuits, PM fibers are used.
They are also used in Raman amplifiers, lithium niobate modulators, and other polarization-sensitive systems to maintain the polarization of the incoming light and keep cross-coupling between polarization modes at a minimum. They are rarely used for long-distance applications since they are very expensive and have higher attenuation than single-mode fibers.
What are PM Fibers?
Polarization-maintaining (PM) fibers are designed to overcome standard optical fibers' limitations by preserving light polarization over long distances. They achieve this by incorporating a built-in mechanism that constrains the light to maintain its original polarization throughout its journey. This is particularly important in fields like fiber-optic communications, sensing, and certain types of scientific research, where the consistency of light polarization is necessary for accuracy and efficiency.
How Do PM Fibers Work?
PM fibers work by using a special design that includes stress-applying parts (SAPs) within the fiber. These SAPs create a birefringent effect, meaning they induce a difference in the refractive indices for light polarized in two perpendicular directions (commonly referred to as the fast and slow axes). By carefully aligning the input light along one of these axes, the fiber maintains the polarization of the light, preventing it from spreading into other polarization modes.
Applications of PM Fibers
Advantages of PM Fibers
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