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Polarization-Maintaining Fibre Tutorial

polarization maintaining fibre


Polarization-Maintaining Fibre Tutorial

FS Official 2015-10-21

Introduction to Polarization

As light passes through a point in space, the direction and amplitude of the vibrating electric field traces out a path in time. A polarized lightwave signal is represented by electric and magnetic field vectors that lie at right angles to one another in a transverse plane (a plane perpendicular to the direction of travel). Polarization is defined in terms of the pattern traced out in the transverse plane by the electric field vector as a function of time.

Polarization can be classified as linear, elliptical or circular, in them the linear polarization is the simplest. Whichever polarization can be a problem in the fibre optic transmission.

FibreStore Polarization Coordinate System

More and more telecommunication and fibre optic measuring systems refer to devices that analyse the interference of two optical waves. The information given by the interferences cannot be used unless the combined amplitude is stable in time, which means, that the waves are in the same state of polarization. In those cases it is necessary to use fibres that transmit a stable state of polarization. And polarization-maintaining fibre was developed to this problem. (The polarization-maintaining fibre will be called PM fibre for short in the following contents.)

What Is PM Fibre?

The polarization of light propagating in the fibre gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fibre and on its temperature. Specialised fibres are required to achieve optical performances, which are affected by the polarization of the light travelling through the fibre. Many systems such as fibre interferometers and sensors, fibre laser and electro-optic modulators, also suffer from Polarization-Dependent Loss (PDL) that can affect system performance. This problem can be fixed by using a specialty fibre so called PM Fibre.

Principle of PM Fibre

Provided that the polarization of light launched into the fibre is aligned with one of the birefringent axes, this polarization state will be preserved even if the fibre is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fibre can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fibre are too slowly varying to do effective mode coupling. Therefore, the principle of PM fibre is to make the difference large enough.

In the most common optical fibre telecommunications applications, PM fibre is used to guide light in a linearly polarised state from one place to another. To achieve this result, several conditions must be met. Input light must be highly polarised to avoid launching both slow and fast axis modes, a condition in which the output polarization state is unpredictable.

The electric field of the input light must be accurately aligned with a principal axis (the slow axis by industry convention) of the fibre for the same reason. If the PM fibre path cable consists of segments of fibre joined by fibre optic connectors or splices, rotational alignment of the mating fibres is critical. In addition, connectors must have been installed on the PM fibres in such a way that internal stresses do not cause the electric field to be projected onto the unintended axis of the fibre.

Types of PM Fibres

Circular PM Fibres

It is possible to introduce circular-birefringence in a fibre so that the two orthogonally polarized modes of the fibre—the so called Circular PM fibre—are clockwise and counter-clockwise circularly polarized. The most common way to achieve circular-birefringence in a round (axially symmetrical) fibre is to twist it to produce a difference between the propagation constants of the clockwise and counterclockwise circularly polarized fundamental modes. Thus, these two circular polarization modes are decoupled. Also, it is possible to conceive externally applied stress whose direction varies azimuthally along the fibre length causing circular-birefringence in the fibre. If a fibre is twisted, a torsional stress is introduced and leads to optical-activity in proportion to the twist.

Circular-birefringence can also be obtained by making the core of a fibre follows a helical path inside the cladding. This makes the propagating light, constrained to move along a helical path, experience an optical rotation. The birefringence achieved is only due to geometrical effects. Such fibres can operate as a single mode, and suffer high losses at high order modes.

Circular PM fibre with Helical-core finds applications in sensing electric current through Faraday effect. The fibres have been fabricated from composite rod and tube preforms, where the helix is formed by spinning the preform during the fibre drawing process.

Linear PM Fibres

There are manily two types of linear PM fibres which are single-polarization type and birefringent fibre type. The single-polarization type is characterized by a large transmission loss difference between the two polarizations of the fundamental mode. And the birefringent fibre type is such that the propagation constants between the two polarizations of the fundamental mode are significantly different. Linear polarization may be maintained using various fibre designs which are reviewed next.

Linear PM Fibres With Side Pits and Side Tunnels:

Side-pit fibres incorporate two pits of refractive index less than the cladding index, on each side of the central core. This type of fibre has a W-type index profile along the x-axis and a step-index profile along the y-axis. A side-tunnel fibre is a special case of side-pit structure. In these linear PM fibres, a geometrical anisotropy is introduced in the core to obtain a birefringent fibres.

Linear PM Fibres With Stress Applied Parts:

An effective method of introducing high birefringence in optical fibres is through introducing an asymmetric stress with two-fold geometrical symmetry in the core of the fibre. The stress changes the refractive index of the core due to photoelastic effect, seen by the modes polarized along the principal axes of the fibre, and results in birefringence. The required stress is obtained by introducing two identical and isolated Stress Applied Parts (SAPs), positioned in the cladding region on opposite sides of the core. Therefore, no spurious mode is propagated through the SAPs, as long as the refractive index of the SAPs is less than or equal to that of the cladding.

The most common shapes used for the SAPs are: bow-tie shape and circular shape. These fibres are respectively referred to as Bow-tie Fibre and PANDA Fibre. The cross sections of these two types of fibres are shown in the figure below. The modal birefringence introduced by these fibres represents both geometrical and stress-induced birefringences. In the case of a circular-core fibre, the geometrical birefringence is negligibly small. It has been shown that placing the SAPs close to the core improves the birefringence of these fibres, but they must be placed sufficiently close to the core so that the fibre loss is not increased especially that SAPs are doped with materials other than silica. The PANDA fibre has been improved further to achieve high modal birefringence, very low-loss and low cross-talk.

PANDA Fibre and Bow-tie Fibre

PANDA Fibre (left) and Bow-tie Fibre (right). The built-in stress elements made from a different type of glass are shown with a darker grey tone.

Tips: At present the most popular PM fibre in the industry is the circular PANDA fibre. One advantage of PANDA fibre over most other PM fibres is that the fibre core size and numerical aperture is compatible with regular single mode fibre. This ensures minimum losses in devices using both types of fibres.

Linear PM Fibres With Elliptical Structures:

The first proposal on practical low-loss single-polarization fibre was experimentally studied for three fibre structures: elliptical core, elliptical clad, and elliptical jacket fibres. Early research on elliptical-core fibres dealt with the computation of the polarization birefringence. In the first stage, propagation characteristics of rectangular dielectric waveguides were used to estimate birefringence of elliptical-core fibres. In the first experiment with PM fibre, a fibre having a dumbbell-shaped core was fabricated. The beat length can be reduced by increasing the core-cladding refractive index difference. However, the index difference cannot be increased too much due to practical limitations. Increasing the index difference increases the transmission loss, and splicing would become difficult because the core radius must be reduced. Typical values of birefringence for the elliptical core fibre are higher than elliptical clad fibre. However, losses were higher in the elliptical core than losses in the elliptical clad fibres.

Linear PM Fibres With Refractive Index Modulation:

One way to increase the bandwidth of single-polarization fibre, which separates the cutoff wavelength of the two orthogonal fundamental modes, is by selecting a refractive-index profile which allows only one polarization state to be in cutoff. High birefringence was achieved by introducing an azimuthal modulation of the refractive index of the inner cladding in a three-layer elliptical fibre. A perturbation approach was employed to analyze the three-layer elliptical fibre, assuming a rectangular-core waveguide as the reference structure. Examination of birefringence in three-layer elliptical fibres demonstrated that a proper azimuthal modulation of the inner cladding index can increase the birefringence and extend the wavelength range for single-polarization operation.

A refractive index profile is called Butterfly profile. It is an asymmetric W profile, consisting of a uniform core, surrounded by a cladding in which the profile has a maximum value of ncl and varies both radially and azimuthally, with maximum depression along the x-axis. This profile has two attributes to realise a single-mode single-polarization operation. First, the profile is not symmetric, which makes the propagation constants of the two orthogonal fundamental modes dissimilar, and secondly, the depression within the cladding ensures that each mode has a cutoff wavelength. The butterfly fibre is weakly guiding, thus modal fields and propagation constants can be determined from solutions of the scalar wave equation. The solutions involve trigonometric and Mathieu functions describing the transverse coordinates dependence in the core and cladding of the fibre. These functions are not orthogonal to one another which requires an infinite set of each to describe the modal fields in the different regions and satisfy the boundary conditions. The geometrical birefringence plots generated vs. the normalized frequency V showed that increasing the asymmetry through the depth of the refractive index depression along the x-axis increases the maximum value of the birefringence and the value of V at which this occurs. The peak value of birefringence is a characteristic of noncircular fibres. The modal birefringence can be increased by introducing anisotropy in the fibre which can be described by attributing different refractive-index profiles to the two polarizations of a mode. The geometric birefringence is smaller than the anisptropic birefringence. However, the depression in the cladding of the butterfly profile gives the two polarizations of fundamental mode cutoff wavelengths, which are separated by a wavelength window in which single-polarization single-mode operation is possible.

Applications of PM Fibres

PM fibres are applied in devices where the polarization state cannot be allowed to drift, e.g. as a result of temperature changes. Examples are fibre interferometers and certain fibre lasers. A disadvantage of using such fibres is that usually an exact alignment of the polarization direction is required, which makes production more cumbersome. Also, propagation losses are higher than for standard fibre, and not all kinds of fibres are easily obtained in polarization-preserving form.

PM fibres are used in special applications, such as in fibre optic sensing, interferometry and quantum key distribution. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input. They are rarely used for long-distance transmission, because PM fibre is expensive and has higher attenuation than single mode fibre.

FS PM Fibre Solution: PM Fibre Patch Leads

FS PM fibre patch leads are based on a high precision butt-style connection technique. The PM axis orientation is maintained by using male connectors with a positioning key and a bulkhead female receptacle with a tightly toleranced keyway, ensuring good repeatability in extinction ratios and insertion losses.

Features:

 High extinction ratios of 20dB to 30dB  Low insertion losses, typically <0.2dB  FC, FC/APC, SC, SC/APC, ST, ST/APC, LC, MU, MTRJ, E2000 and other connectors available  Compatible with industry standard connectors  Wavelength of 360nm-1800nm available  Ø0.9mm, Ø2.0mm, Ø3.0mm protective outer jacket available  Fast /Slow axis alignment, Wide/Narrow key, panda type, bow-tie type, elliptical type available  Custom fibre sizes and cable lengths

Requirments for Using PM Fibres

Termination: When PM fibres are terminated with fibre connectors, it is very important that the stress rods line up with the connector, usually in line with the connector key.

Splicing: PM fibre also requires a great deal of care when it is spliced. Not only the X, Y and Z alignment have to be perfect when the fibre is melted together, the rotational alignment must also be perfect, so that the stress rods align exactly.

Another requirement is that the launch conditions at the optical fibre end face must be consistent with the direction of the transverse major axis of the fibre cross section.


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