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).
