What is polarization maintaining (PM) fiber?

Theoretically speaking, the fiber with a circular core should not produce birefringence, and the polarization state of the fiber will not change during propagation. However, in practice, the conventional optical fiber will be subjected to external forces and other reasons during the production process, so that the thickness of the optical fiber is uneven or bent, which will cause birefringence. When the fiber is affected by any external disturbance, such as wavelength, bend, temperature, etc., the polarization state of light can become chaotic as it travels through conventional fibers.

The application of polarization-maintaining fiber can solve this problem of polarization state change, but it does not eliminate the birefringence phenomenon in the fiber, but generates stronger birefringence through the design of the fiber geometry to eliminate the birefringence phenomenon. The effect of stress on the polarization state of incident light.

How does 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, resulting in a decrease in the polarization extinction ratio of the outgoing polarized light signal, thus affects the birefringence effect. 

How to make polarization maintaining fiber?

A common method of introducing high birefringence can be achieved by introducing geometrically symmetric inhomogeneous stress into the fiber core of the preform by adding two stress rods of modified glass composition 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. Another option is to use an elliptical core, the elliptical shape inherently producing some degree of shape birefringence, even in the absence of mechanical stress.

Structure of PM Fiber

Elliptic cladding type, bow tie type and panda type are three types of polarization-maintaining fibers that are widely used, all of which belong to stress-type polarization-maintaining fiber

What is fast axis and slow axis?

Polarization-maintaining fibers work by inducing a difference in the speed of light between the two perpendicular polarizations passing through the fiber. This birefringence creates two main transmission axes within the fiber, called the fast and slow axes of the fiber. The fast axis is an optical axis with a small refractive index and a faster light transmission speed, which vertically passes through the midpoint of the line connecting the centers of the two stress regions; the slow axis is an optical axis passing through the end points of the two stress regions, which is In the direction with a large refractive index, the transmission speed is slow.

What is a shot length?

If the polarization direction of the incident 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 period of power exchange between the two polarization components is called the beat length. The beat length is a very objective parameter of the degree of birefringence of the fiber. It has nothing to do with the length of the fiber, the polarization and alignment of the input light, etc. It can well reflect the periodic change of the polarization state of the fiber. Periodicity refers to the process of polarization state along the length of the fiber from linear deflection → elliptical deflection → circular deflection → elliptical deflection → linear deflection → elliptical deflection → circular deflection → elliptical deflection → linear deflection, and returns to the original linear deflection after an integer number of beat lengths. state.

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

What is extinction ratio?

How does the polarization-maintaining fiber realize the polarization-maintaining principle? Align the polarization direction of the polarized light with one axis, and the polarization component divided to 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 to which the polarization state of the fiber is maintained. 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, and the ratio of the polarization component along the original direction to the polarization component in the vertical direction is the extinction ratio. The extinction ratio is an important parameter to measure the quality of the polarizer. The larger the extinction ratio, the higher the quality of the polarizer.

ER ＝ 10 log（Pmax／Pmin）

How Polarization Maintaining (PM) patch cord Work

Polarization Maintaining Optical Connector realizes coupling alignment of polarization modes through precise connector keying. Compared with the traditional fiber jumper, the polarization-maintaining jumper has the advantages of polarization-maintaining fiber transmission of polarized light signals, which can ensure that the direction of linear polarization remains unchanged, improve the coherent signal-to-noise ratio, and achieve high-precision measurement of physical quantities.

The polarization-maintaining connector is an important component for the coupling of two polarization-maintaining fibers, ensuring that the polarization mode of the two polarization-maintaining fibers during coupling maintains the original polarization state of the polarized light and maintains a high extinction ratio for transmission. This requires precise docking of the slow axis or fast axis of the two fibers to minimize the θ angle error. By aligning the polarization direction of the polarized light with one of the axes, the polarization component divided into the other axis is small, thus maintaining the polarization state of the transmitted light. The polarization-maintaining angle and extinction ratio reflect the pros and cons 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 conducive to the polarization propagation of light in only one direction, thereby maintaining the polarization maintaining work. Panda-type stress bars are cylindrical, while bowtie-type stress bars are trapezoidal prisms.

Generally speaking, how well the polarization-maintaining fiber maintains the polarization state depends on the incident state of the polarized light, and the polarization state of the polarization-maintaining fiber is required to be aligned with the coupling in the fast and slow axis directions of the polarization-maintaining fiber. To ensure that the two fibers transmit with a high extinction ratio, it must be ensured that when the polarization-maintaining fiber is terminated through the fiber optic connector, the stress rod should be aligned with the connector key. Whether the Key key is aligned directly determines the link transmission function of the polarization-maintaining connector. This indicator is very critical.

The stress rod should be aligned with the key of the connector. The conventional production method on the market is to fill the ferrule with 353ND curing glue. When the glue is not fixed, align the stress rod (cat's eye) under the magnifying glass by rotating the core. Re-curing after preparation. 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 very easy to rotate, and the curing glue has a certain viscosity, which causes the core to rotate after alignment and before curing. Offset, the alignment angle of the "cat's eye" can only reach within ±5 degrees, and the extinction ratio of the entire jumper can only reach more than 20dB. Moreover, once the offset angle is too large, the connector end must be cut off and reattached, which wastes man-hours and makes it difficult to guarantee the quality.

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". By rotating the cured ferrule to replace the rotating optical fiber core, when the "cat's eye" is aligned, the ferrule tail is fixed to achieve the purpose of precise positioning. After verification, HYC series polarization-maintaining connectors, the alignment angle deviation can be controlled within ±1 degree, and the extinction ratio can reach more than 25dB.

Where does it application?

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 sensors such as fiber-optic gyroscopes, fiber-optic hydrophones, and fiber-optic communication systems such as DWDM and EDFA.

　SHENZHEN PUSMAI Technology Co.,Ltd taking the development of polarization-maintaining devices as one of the means to serve the society, actively promoting the development of polarization-maintaining devices, making their products increasingly mature and quite impressive, becoming the manufacturer with the most complete polarization-maintaining products in the world, and occupying a high position in the competition of many products Get ahead. The popularization and promotion of polarization-maintaining devices requires the sincere cooperation of more colleagues in the industry and more attention and research. It is believed that polarization-maintaining devices can make their due contributions to the development of human science and technology. 

PUSMAI product mainly focuses on polarization-maintaining optical fiber components and high-power optical fiber components in the 405nm~2050mm band, PUSMAI product PM/High power optic circulator , PM/High power optic isolator , PM/High power optic patch cord  , PM/High power optic optic swith and also provides customers with TP-PD, LD laser products. Solutions and customized services.