Increasing Popularity of polarization-maintaining Fiber

Optical networks have become popular due to increasing demands for bandwidth. It becomes important to use existing fiber-optic networks very effectively because installation of new fiber-optic networks is very expensive. Dispersion and attenuation are the main parameters that limit the optical networks. The uses of two polarization axes are improved by the efficiency of optical networks which are very much similar to the technology used in radio technologies. The use of two polarization planes is allowed because of the use of polarization maintaining splitter.

Theoretically, there is no birefringence in an optical fiber with a circular core, and in such an optical fiber the polarization state does not change during propagation. However, in reality, due to external perturbations or manufacturing imperfection in an optical fiber, a small amount of birefringence is always present. Such birefringence is inherently random, and in an optical fiber between two polarization modes, it results in random power coupling. The output polarization state then becomes unpredictable and also differs with time.

PM fibers need to meet critical optical and mechanical specifications, such as attenuation and tensile strength. In characterizing their birefringence properties polarization maintaining isolator have two specifications – beat length and holding (H) parameter. These are complex measurements, but characterizing how well the fibers maintain the two polarization modes they are necessary.

By intentionally inducing uniform birefringence along the entire fiber length a polarization-maintaining Fiber (PM Fiber, PMF) maintains two polarization modes, hence they prohibit random power coupling between two polarization modes.

In a single-mode fiber, the transmission of a source laser’s output is done with two linear polarization modes that propagate at right angles to each other. For a moment imagine that this fiber is an ideal single-mode waveguide:

• the fiber and source laser temperatures always remain constant;
• Polarization maintaining circulator does not have bends and no loss;
• there is perfect uniformity in the core material;
• Compared to the cut off wavelength the laser wavelength is greater, and all the laser energy is concentrated in the core;
• Lateral stress is zero (no external stress from cabling, placement, supports, etc., or even, hypothetically, no gravity or air pressure).

Detailed Explanation of Polarization-Maintaining Fibers

With two linear polarization maintaining modes propagating at right angles to each other, a source laser’s output is transmitted in the case of a single-mode fiber.

Compared to cutoff wavelength the laser wavelength is greater, and all the laser energy is confined in the core

It does not have any bends and losses;

There is uniformity in core material;

The cladding and core are perfectly concentric and round;

There is consistency in the fiber and source laser temperatures;

Lateral stress is zero.

There would have been no coupling of power from one mode to the other and it is not at all possible along the fiber’s length. If a modulated signal is carried by a laser output then these two polarization maintaining splitter modes will carry the signal without any dispersion and no crosstalk.

There is no perfection in the manufactured glass materials and waveguides. There is the presence of sub-micron asymmetries and non-uniformities. If single-mode fibers are being cabled and placed in aerial or underground networks then they may experience lateral stress. In handholes, cabinets, closures, and other structures the cable can experience bends or even have coils of slack.

If it is not corrected then this polarization-mode dispersion can have limitations with the distance or the bandwidth of a fiber optic communication system. Thus, to reduce or compensate for this dispersion, fiber, cable, and system designers have developed many techniques. Preform have been optimized by fiber manufacturers and to minimize asymmetry, non-concentricity, and lateral stresses they have drawn processes.

So, in telecom fibers polarization can be effectively managed. Through this, you can find a way to make accurate measurements of motion, vibration, or other phenomena that affects the fiber.

Similar issues are addressed by both the single-mode communications fibers and PM fibers. Few of them are minimizing the effect of external stresses and bends on the polarization maintaining circulator in the fiber. For building asymmetric geometric features and SAPs into fiber you will get many ways that will ultimately give rise to several types of PM fibers. To reduce or compensate for this dispersion, fiber, cable, and system designers have developed many techniques.

All About Polarization-maintaining Fibers

Optical fibers even if have a circularly symmetric design always exhibit some degree of birefringence because in practice you will always find some amount of mechanical stress or other effects which break the symmetry. The polarization maintaining of light changes in an uncontrolled way gradually.

Principle of Polarization-maintaining Fibers

By using a polarization-maintaining fiber the above problem can be fixed and it is not a fiber without birefringence but on the contrary a specialty fiber with a strong built-in birefringence (high-birefringence fiber or HIBI fiber, PM fiber). The polarization maintaining splitter of light which is launched into the fiber is aligned with one of the birefringent axes and even if the fiber is bent this polarization state will be preserved. The physical principle present behind this can only be understood in terms of coherent mode coupling. Due to the strong birefringence, the propagation constants of the two polarization modes are significantly different and because of it the relative phase of such co-propagating modes rapidly drifts away. During heating, the fibers are slowly stretched and tapered.

Therefore, both modes get effectively coupled by any disturbance along with the fiber and it takes place only if it has a significant spatial Fourier component with a wavenumber matching the propagation constants difference in two polarization modes. The usual disturbances in the fiber are too slowly varying to do effective mode coupling only if the difference is huge. In quantitative terms, compared to the typical length scale on which the parasitic birefringence varies the polarization beat length needs to be significantly shorter.

Few Ways of Realizing Polarization-maintaining Fibers

On the opposite sides performance on of the core, for introducing strong birefringence a commonly used method is including two stress rods of the modified glass composition. One can make bow-tie fiber with different techniques, where the stress elements have a different shape and reach closer to the fiber core so that you can get a stronger polarization maintaining circulator. Due to the strong birefringence, the propagation constants of the two polarization modes are significantly different and because of it the relative phase of such co-propagating modes rapidly drifts away.