What is Polarization Maintaining(PM) fiber connector?

Polarization maintaining(PM) optical fiber connectors are usually used for special applications, such as optical fiber sensing, interferometry, planar waveguides, coherent optical transmission, and long-distance bidirectional optical transmission systems. To understand polarization-maintaining connectors, we need to know what polarization-maintaining fibers are.

The PM fibers currently on the market basically have the following cross-sectional types:

![保偏光纤配图1.jpg](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/463940/12698cc7-788c-ea14-c4a9-7010ec884a8c.jpeg)


The fifth is the most common type, which also known as Panda fiber (Its cross-section looks like a panda face). What’s the difference between PM fiber and ordinary single-mode fiber?

Polarization-maintaining fiber is a special type of single-mode fiber. Ordinary single-mode fiber can transmit random polarized light, while PM fiber can only transmit polarized light in one direction. In polarization-maintaining fibers, the polarization of linearly polarized light waves propagating into the fiber is maintained during the propagation process, and there is little or no cross-coupling of optical power between the polarization planes. For some fiber components that require polarized light input (such as external modulators), the function of maintaining the polarization direction is very important. By generating stress in the fiber material itself, the polarization characteristics of light waves in a specific direction can be maintained. Therefore, the polarization maintaining fiber with the following structure appears.

![保偏光纤配图4.jpg](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/463940/32dac29c-1062-c505-652a-666256247146.jpeg)


The stress rod generates stress in the optical fiber, which only contributes to the polarization propagation of light in one direction. When the PM fiber is terminated through the fiber connector, the stress rod should be aligned with the connector key. Whether the keys are aligned directly determines the function of the PM connector link transmission. The index is very critical.

How to ensure that the stress rod and connector key are aligned? If we want to ensure accurate alignment, do we need to do some special design for the structure of the connector?

The stress rod should be aligned with the connector key. 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) with the magnifying glass by rotating the lens core, and then cure after alignment. The problem is that the fiber core is very easy to rotate during the baking process even after aligning the “cat’s eye”. And there is a certain viscosity of the cured glue, which causes the fiber core to rotate offset after alignment and before curing. 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. And once the offset angle is too large, the connector end must be cut off and re-headed, wasting man-hours and the quality is difficult to guarantee.
![保偏光纤配图3.jpg](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/463940/56e9e426-3a7d-904e-2bba-84d3751a46aa.jpeg)

In order to solve the above problems, HYC has independently designed a polarization maintaining fiber connector that can rotate the ferrule with 360 degree angle to align with the "cat's eye". The traditional rotating optical fiber core is replaced by rotating the solidified ferrule. When the "cat's eyes" are aligned, the ferrule tail is fixed to achieve the purpose of precise positioning. After verification, the alignment angle deviation of HYC series polarization-maintaining connectors can be controlled within ±1 degree, and the extinction ratio can reach more than 25dB.

About HYC Co., Ltd
Founded in 2000, HYC CO., LTD (HYC) is the world’s leading passive optical devices OEM/ODM manufacturer who engages in R&D, producing, marketing and providing service of fiber optical products.

Our main products are: Optical Fiber Connector, WDM, PLC splitter, MEMS optical switch and other optical passive basics devices. HYC products and solutions are widely used in 4G/5G, Telecom, Data Center,  Cloud Computing industry and other fields.
www.hyc-system.com