Introduction
A beam of light can be thought of as being compod of two orthogonal electrical vector field components that vary in amplitude and frequency. Polarized light occurs when the two components differ in pha or amplitude. Polarization in optical fiber has been extensively studied and a variety of methods are available to either minimize or exploit the phenomenon. In this tutorial, basic principles and technical background are introduced to help explain how the polarization in fiber optics works. For more general accounts on polarization, plea e Polarization.舒逸
Polarization Manifestation in Optical Fibers
Birefringence in Optical Fibers
Birefringence is a term ud to describe a phenomenon that occurs in certain types of materials, in which light is split into two different paths. This phenomenon occurs becau the materials have different indices of refraction, depending on the polarization direction
of light.
胞组词Birefringence is also obrved in an optical fiber, due to the slight asymmetry in the fiber core cross-ction along the length and due to external stress applied on the fiber such as bending. In general, the stress-induced birefringence dominates the geometry-induced one. MYTEST TEXT
Polarization Maintaining (PM) Fiber
A specialty fiber called the Polarization Maintaining (PM) Fiber intentionally creates consistent birefringence pattern along its length, prohibiting coupling between the two orthogonal polarization directions. In any design, the geometry of the fiber and the materials ud create a large amount of stress in one direction, and thus create high birefringence compared to that generated by the random birefringence. There are a number of designs available commercially, using various stress inducing architectures, such as Panda and Bow Tie PM Fibers available with various cut-off wavelengths.
Poincare Sphere
The Poincare sphere is one of the conventional ways of describing the polarization and changes in polarization of a propagating electromagnetic wave. It provides a convenient way of predicting how any given retarder will change the polarization form. Any given polarization state corresponds to a unique point on the sphere. The two poles of the sphere reprent left and right-hand circularly polarized light. Points on the equator indicate linear polarizations. All other points on the sphere reprent elliptical polarization states. An arbitrarily chon point H on the equator designates horizontal linear polarization, and the diametrically opposite point V designates vertical linear polarization.
苦的反义词是什么
Figure 1. Poincare sphere reprentation of polarization states.
Measurable Polarization Properties
DOP
Degree of Polarization (DOP) is defined as
DOP = Ipol / (I欧式鞋柜pol + Iunp)
Where Ipol and Iunp are the intensity of polarized light and unpolarized light, respectively.
When DOP = 0, light is said to be unpolarized, and when DOP = 1, it is totally polarized. Intermediate cas correspond to partially polarized light.
PER
Polarization Extinction Ratio (PER) is the ratio of the minimum polarized power and the maximum polarized power, expresd in dB. Any polarization component will specify this value as a specification.
PDL
Polarization Dependent Loss (PDL) is the maximum (peak to peak) variation in inrtion loss as the input polarization varies over all its states, expresd in dB.
PMD
白癣皮Polarization Mode Dispersion (PMD) is actually another form of material dispersion. Singl
e-mode fiber supports a mode, which in fact consists of two orthogonal polarization modes. Ideally, the core of an optical fiber is perfectly circular. However, in reality, the core is not perfectly circular, and mechanical stress such as bending, introduce birefringence in the fiber. This caus one of the orthogonal polarization modes to travel faster than the other, hence causing dispersion of the optical pul.
The maximum difference in the mode propagation times due to this dispersion is called Differential Group Delay (DGD), who unit is typically given in picoconds. Becau of its dynamic properties, PMD does not have a single, fixed value for a given ction of fiber, but has a distribution of DGD values over time. The probability of a DGD with a certain value at any particular time follows the Maxwellian distribution shown in Figure 2. As an approximation, the maximum instantaneous DGD is about 3.2 times the average DGD of a fiber.
了解的解怎么写
Figure 2. Maxwellian distribution of DGD.
Controlling Polarization in Optical Fiber
绳艺故事Methods of Controlling Polarization
Controlling the polarization state in optical fiber is similar to the free space control using waveplates via pha changes in the two orthogonal states of polarization. In general, three configurations are commonly ud.
In the first configuration, a Half-Wave Plate (HWP) is sandwiched between two Quarter-Wave Plates (QWP) and the retardation plates are free to rotate around the optical beam with respect to each other. The first QWP converts any arbitrary input polarization into a linear polarization. The HWP then rotates the linear polarization to a desired angle so that the cond QWP can translate the linear polarization to any desired polarization state. 玻尿酸的使用方法
An all-fiber controller bad on this mechanism can be constructed, with veral desirable properties such as the low inrtion loss and cost, as shown in Figure 3. In this device, three fiber coils replace the three free-space retardation plates. Coiling the fiber induces stress, producing birefringence inverly proportional to the square of the coils’ diameters. Adjusting the diameters and number of turns can create any desired fiber wave plate. Becau bending the fiber generally induces inrtion loss, the fiber coils mu
st remain relatively large.
Figure 3. Polarization control using multiple coiled fiber
The cond approach is bad on the Babinet-Soleil Compensator. An all-fiber polarization controller bad on this technique is shown in Figure 4. The device compris a fiber squeezer that rotates around the optical fiber. Applying a pressure to the fiber produces a linear birefringence, effectively creating a fiber wave plate who retardation varies with the pressure. Simple squeeze-and-turn operations can generate any desired polarization state from any arbitrary input polarization.
Figure 4. Polarization control using Babinet-Soleil compensator principle.
Polarization controllers also can be made with multiple free-space wave plates oriented 45° from each other. An all-fiber device bad on the same operation principle would reduce the inrtion loss and cost. The retardation of each wave plate components varies with the pressure of each fiber squeezer. The challenge is making the device reliable, co
mpact and cost-effective.
Piezoelectric actuators drive the fiber squeezers for high speed. Becau it is an all-fiber device, it has no back reflection and has extremely low inrtion loss and polarization-dependent loss. All new 25xxP Series Polarization Control instruments employ the fiber squeeze technique.
