POLARIZATION MAINTAINING FIBERS AND THEIR APPLICATIONS

Regular circular-core optical fibers have very low birefringence (refractive index dependence on polarization), and the guided light polarization state can change during propagation. Along the fiber length, some birefringence can be induced due to external perturbations (load, bend, etc.) or imperfections. This brings a random power coupling between the polarization modes (polarization crosstalk) and unpredictable output polarization states that change over time.

As part of fiber optic communication systems, the randomly polarized light generated polarization-mode dispersion (PMD). PMD is a phenomenon where polarization states of light travel at different speeds, leading to signal degradation and limiting communication distances. That was sufficient motivation to develop Polarization Maintaining (PM) fibers.

PM fibers have been under development since the 1970s. The original goal was to provide a solution that could preserve the polarization of light signals, ensuring reliable and high-quality transmission in fiber optic communication systems. Even though PM fibers are less used in telecommunications today, they are used in many other applications, as we will see.

There are different ways to design and build PM fibers. Coractive PM fibers are produced in what is called the PANDA configuration (Polarization Maintaining AND Absorption Reducing).

Key Characteristics of a PM fiber

PM fibers present some key characteristics that differentiate them from standard optical fibers:

Birefringence:

PM fibers exhibit high birefringence, meaning they have a significant difference in refractive index between orthogonal polarization axes.

Polarization State Maintaining:

In PM fibers, the polarization orientation of the transmitted light is preserved. In addition to being able to minimize unwanted polarization effects, such as polarization-mode dispersion (PMD), they maintain the integrity of the polarization state over long distances.

Slow and Fast Axes:

The PANDA design of PM fibers incorporates Stress-Applied Parts (SAP) to induce birefringence. These create two orthogonal axes of refractive index within the fiber—the fast axis (perpendicular to SAP) or the slow axis (parallel to SAP). These axes correspond to two orthogonal polarization states of light, effectively preserving the polarization orientation of the transmitted signal. When light is polarized along these axes, the fiber’s birefringence causes it to propagate at different speeds depending on the propagation axis selected. Light along the slow axis propagates slower than light propagating the fast axis.

Low Polarization Crosstalk:

PM fibers have low crosstalk between polarization axes, which means that polarization states of light travelling on one axis remain isolated from those travelling on the other.

Applications for PM fibers

PM fibers are required when polarization of light is necessary for the application. In telecommunication, they are used in some fiber optic networks where components, such as lithium niobate modulators, can be sensitive to the signal polarization state.

Other applications can be found in fiber optic sensing and interferometry. These fibers enable accurate and stable sensing of physical parameters such as pressure and rotation. For example, fiber optic gyroscopes require PM fiber.

Additionally, PM fibers are used in ultrafast fiber lasers, which produce very short pulses of light, typically on the order of femtoseconds (10-15 seconds) or picoseconds (10-12 seconds). Lasers in that range are used in micromachining, medical and other industrial and scientific applications.

PM fibers are also part of light frequency doubling fiber laser designs. For example, 1064 nm light can be “frequency doubled” towards a 532 nm green light output, which is needed for processing highly reflective materials at a more standard wavelength.

If you’re looking for PM fibers or any other specialty optical fibers, don’t hesitate to contact us. We’re celebrating 25 years in business, so you can be sure our team can help you find the best fiber for your specific needs.

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