Theoretically, a fiber with a circular core should not produce birefringence, and the polarization state of the fiber will not change during propagation. However, in practice, regular the optical fiber in the production process, will be subjected to external force and other reasons, so that the thickness of the optical fiber is not uniform or bending, etc., it will cause the phenomenon of birefringence. When the fiber is subjected to any external interference, such as wavelength, bending, temperature, etc., the polarization state of light will become chaotic when transmitted in the regular fiber.

What is polarization maintaining (PM) fiber?

Theoretically, a fiber with a circular core should not produce birefringence, and the polarization state of the fiber will not change during propagation. However, in practice, regular the optical fiber in the production process, will be subjected to external force and other reasons, so that the thickness of the optical fiber is not uniform or bending, etc., it will cause the phenomenon of birefringence. When the fiber is subjected to any external interference, such as wavelength, bending, temperature, etc., the polarization state of light will become chaotic when transmitted in the regular fiber.

The application of polarization maintaining fiber can solve the problem of polarization state change, but it is not to eliminate the birefringence phenomenon in the fiber, but to eliminate the influence of stress on the polarization state of incident light by designing the geometry of the fiber.

How does the polarization maintaining fiber affect the birefringence effect in the fiber? During the drawing process of the polarization maintaining fiber, when the linearly polarized light is transmitted along one characteristic axis of the fiber, part of the optical signal will be coupled into another characteristic axis perpendicular to it, which will eventually cause the decrease of the polarization extinction ratio of the outgoing polarized light signal, thus affecting the birefringence effect.

How to manufacture polarization maintaining fiber?

A common method of introducing high birefringence can be achieved by introducing a geometrically symmetric non-uniform stress in the fiber core of the preform by adding stress rods of two modified glass compositions on both sides of the core. The stress-type polarization-maintaining fiber mainly relies on the difference of the thermal expansion coefficient of the embedded stress rod and the fiber core to generate thermal stress, which leads to the change of the refractive index of the material under the action of thermal stress, resulting in the birefringence effect. An alternative is to use an elliptical core, the elliptical shape itself producing a degree of form birefringence, even in the absence of mechanical stress.

Structure of Polarization Maintaining Fiber

Elliptic cladding type, bow tie type and panda type are three kinds of polarization maintaining fibers which are widely used, and all belong to stress type polarization maintaining fibers.

What is the fast axis and the slow axis?

Polarization-maintaining fibers work by inducing a difference in the speed of light in two perpendicular polarizations passing through the fiber. This birefringence creates two major transmission axes within the fiber, referred to as the fast and slow axes of the fiber, respectively. The fast axis is the direction of the refractive index is small, the optical axis of the light transmission speed is fast, and it vertically passes through the midpoint of the center line of the two stress regions; the slow axis is an optical axis passing through the end of the two stress regions, which is the direction of the large refractive index, and the transmission speed is slow.

What is a long shot?

If the polarization direction of the incident linearly polarized light is consistent with the fast axis or the slow axis of the fiber, the polarization state of the light remains unchanged during transmission. If the polarization direction of the incident light forms an angle with the fast axis or the slow axis, two orthogonal polarization modes with different propagation constants will be excited at the same time, and the power between the two polarization components will be exchanged periodically, which is called the beat length. Beat length is a very objective parameter of the degree of birefringence of the fiber, which has nothing to do with the length of the fiber, the polarization and alignment of the input light, and can well reflect the periodic change of the polarization state of the fiber. Periodicity refers to the process of polarization state from linear polarization → ellipsometry → circular polarization → ellipsometry → linear polarization → ellipsometry → circular polarization → ellipsometry → linear polarization along the length of the fiber, returning to the original linear polarization state after an integer number of beat lengths.

The formula for the beat length is Lp = λ/B,λ wavelength, and B is birefringence. The smaller the beat length, the greater the light speed difference between the fast and slow axes, and the stronger the birefringence. Typical B value: ordinary fiber B = 10(-7), polarization maintaining fiber B = 10(-4)

What is the extinction ratio?

How does the polarization maintaining fiber realize the principle of polarization maintaining? If the polarization direction of polarized light is aligned with one axis, the polarization component divided into the other axis will be very small, thus maintaining the polarization state of the transmitted light. At this time, the extinction ratio (ER) parameter is introduced to reflect the degree of the fiber to maintain the polarization state. When the polarization direction of the polarized light is aligned with one of the fast and slow axes, two orthogonal polarization modes are generated by the components. The ratio of the polarization component along the original axial direction to the polarization component in the vertical direction is the extinction ratio. Extinction ratio is an important parameter to measure the quality of the polarizer, the greater the extinction ratio, the higher the quality of the polarizer.

ER = 10 log(Pmax/Pmin)

Working principle of polarization maintaining (PM) jumper

Polarization-maintaining jumpers (Polarization Maintaining Optical Connector) are used to achieve coupling alignment of polarization modes through precise connector key positions. Compared with the traditional fiber jumper, the polarization-maintaining jumper has the advantages of polarization-maintaining fiber transmitting polarized light signal, which can ensure the direction of linear polarization unchanged, improve the coherent signal-to-noise ratio, and realize the high-precision measurement of physical quantity.

The polarization-maintaining connector is an important component of the coupling of two polarization-maintaining fibers to ensure that the polarization modes of the two polarization-maintaining fibers during coupling maintain the original polarization state of the polarized light and maintain a high extinction ratio for transmission. This requires precise docking of the slow axis or fast axis of the two optical fibers to minimize the θ angle error. By aligning the polarization direction of the polarized light with one axis, the polarization component divided into the other axis will be small, thus maintaining the polarization state of the transmitted light. The polarization-preserving angle and extinction ratio reflect the degree of maintaining the polarization state.

How to align the "cat's eye"?

The stress rod is parallel to the fiber core, and the applied stress produces birefringence in the fiber core, which is beneficial to the polarization propagation of light in only one direction, thus maintaining the polarization maintenance work. The panda-type stress bar is cylindrical, while the bow-tie type uses a trapezoidal prism stress bar.

Generally speaking, the quality of maintaining the polarization state of the polarization maintaining fiber depends on the incident state of the polarized light, and the polarization state of the polarized light is required to be coupled and aligned with the fast and slow axis directions of the polarization maintaining fiber. To ensure that the two optical fibers are transmitted with a high extinction ratio, it is necessary to ensure that when the polarization maintaining optical fiber is terminated by the optical fiber connector, the stress rod should be aligned with the connector Key key. Whether the Key key is aligned directly determines the function of the link transmission of the polarization-maintaining connector. This indicator is very critical.

The stress rod should be aligned with the key of the connector. The regular production method on the market is to fill 353ND curing glue into the ferrule. When the glue is not fixed, the stress rod (cat's eye) should be aligned under a magnifying glass by rotating the fiber core, and then cured after alignment. The problem with this is that after aligning the "cat's eye", the connector needs to be moved for baking and curing, the core is prone to rotation, and the curing adhesive has a certain viscosity, resulting in a rotational offset of the core before curing after alignment, the alignment angle of the "cat's eye" can only reach within +/-5 degrees, and the extinction ratio of the whole jumper can only reach more than 20dB. And once the offset angle is too large, the connector end must be cut off and re-added, wasting man-hours, and the quality is difficult to guarantee.

In order to solve the above problems, Yiyuantong independently designed a polarization-maintaining connector that can rotate the ferrule 360 degrees to align with the "cat's eye. The solidified ferrule is rotated to replace the fiber core of the rotating optical cable. When the "cat's eye" is aligned, the ferrule tail is fixed to achieve the purpose of accurate positioning. After verification, the HYC series polarization-maintaining connectors can control the alignment angle deviation within ± 1 degree, and the extinction ratio can reach more than 25dB.

Applied to where?

Polarization-maintaining fibers are generally used in applications that are sensitive to polarization states, such as interferometers or lasers. It is also often used in fiber optic gyroscopes, fiber optic hydrophones and other sensors and optical fiber communication systems such as DWDM and EDFA.