Polarization maintaining fiber (PMF, Polarization Maintaining Optical fiber) has been more and more widely used in the optical fiber communication and optical fiber sensing system as it has a strong polarization of linearly polarized light retention ability, and has good compatibility with the ordinary single-mode fiber. There has already set off a PMF research and application climax on abroad in 1980s. Polarization maintaining fiber has a great variety. The most widely used so far mainly is high birefringence PMF. It makes use of stress-induced or elliptical core method for high birefringence in the fiber, which makes the phase of the two orthogonal polarization mode fiber propagation constant varied greatly and reduce the mode coupling caused by the interference of bending, micro-bending, twisting and so on. The main structure is as panda fiber, bow-tie fiber, elliptical cladding type fiber, elliptical core optical fiber etc, please find the picture as below:
Picture1 Type of PMF
Polarization maintaining fiber coupler is an optical coupler with the application of polarization maintaining fiber. It is a key component to achieve polarization maintaining coupling, splitting and multiplexing. Its biggest feature is stable transmission of two orthogonal, linearly polarized lights , and can maintain their polarization state constant. Therefore, it has been becoming the key components of the various military interfereometric sensors and coherent communication, and also the basic elements of fiber optic gyroscope and hydrophone which are with high-precision and high-performance. Polarization maintaining optical fiber coupler enables the one linearly polarized light transmitted in a polarization maintaining fiber to divide into two lights in the coupling region of a special structure and maintain the state of polarization unchanged. Fused tapered polarization maintaining fiber couplerí»s principle   is shown as Picture 2. When the polarized light is inputted along the PM axis of the PM coupler (PM splitter/ PM PLC splitter)í»s input port, then divide to 2 lights Pout1 and Pout2 and transmit along the PM axis of the fiber.
The main parameters of the PM fiber coupler are: coupling coefficient, additional loss, polarization crosstalk, extinction ratio and so on. The additional loss of the fiber coupler has reflected the indicator of the device manufacturing process quality and the inherent loss of the device fabrication process . Extinction ratio and polarization crosstalk are the specific parameters of PM fiber coupler, both of them have reflected its PM performance .
II. Application and Development Status at Home and Abroad
In recent 20 years, the developed countries have invested a lot of manpower and material resources on the development of PM coupler (PM splitter/ PM PLC splitter) and already achieved rapid development. Currently, PM coupler has reached the practical stage and been on behalf of the current international standards. Its additional loss is between 0.3dB-0.5dB, extinction ratio is between 20-25dB, coupling ratio is typically 50/50 í└ 5%. However, its price is still expensive, such as the PM coupler (PM splitter/ PM PLC splitter)í»s quotation of U.S. 3M Company is $ 2,000 / pcs, which indicates that the coupler is still in the primary stage of practical use  . The units who are dedicated to coupler study are not so many. Beijing University of Aeronautics and Astronautics, North Glass University, Chengdu University of Electronic Science and Technology, Zhejiang University and the Ministry of Electronics Industry 23 Section, National Defense Science and Technology University and other units have promoted coupler study and also had some products available already. Recently, our extinction ratio is actually up to 22-25db and best up to 30db, reached the international level; coupling ratio is typical 50/50 í└ 5%, similar to the aboard; additional loss is about 0.5db, a bit lower than aboard. Domestic has attached great importance to PM coupler (PM splitter/ PM PLC splitter) so far, especially with the great demand of PM coupler in fiber-optic gyro inertial system , the state has increased investment on the study, which has accelerated the practical process of PM coupler (PM splitter/ PM PLC splitter).
óˇ. Various Manufacturing Methods and Their Features
There are two manufacturing methods of PM fiber coupler. One is cold method, the traditional optical cold process: rub off some of the fiber along the fiber axis of the PM fiber and glue the rubbed PM fiber together. This method is complicated, difficult and demanding, moreover, the finished coupler is large volume, poor temperature stability and poor practical value, so it is rarely used at present. The other is fused biconical taper method, currently most of the PM fiber couplersí» production is used this method by manual operation : strip the coating of 2 fibers in the middle of them about 20mm, clean up, direct axis under the microscope and fix them by UV glue, and then place them in a dedicated micro-torch to fuse their sides together and form a bipyramid to achieve the lightí»s transverse coupling and then package to form coupler by manual operation. But the price of the device is expensive, because manual operation has high operation requirements for the operator, and its low speed, low output, low yield, poor performance consistency. However, Military and Civilian fields have a growing demand for PM fiber couplers with high-performance and low-cost. Therefore, we need to develop a simple economic automatic PM fiber coupler production system, including multi-station automation axis, taper, packaging, testing and other engineering process.
IV . The Critical Technology
Ordinary single-mode fiber coupler has got mature control technology now, but PM fiber coupler still has many technical difficulties in the actual manufacturing due to its special structure, so there is no practical automatic manufacturing device so far. The main aspects of its critical manufacturing technology are as below:
1. Axis system
Manufactured the ordinary fiber coupler, it may be melted and stretched after the twisting of the two optical fibers, the two optical fibers are easily coupled together. Absolutely, manufactured the PM fiber coupler, it can not be twisted the two optical fibers, or it will damage the two optical fibersí» parallel. Therefore, it must use the axis system to make sure the PM axes of the two optical fibers to remain parallel. The structure of common CCD vision axis technology is as below:
Picture 3 Structure of CCD Vision Axis Technology
The light source irradiates the optical fibers from below, amplified by the microscope, the light through the PM fiber will image on the CCD. Due to the different refractive index of fiber core and pigtail, the light will be presented different dark or bright stripes. Using the computer to process the image according to the stripes, we can verdict the PM axes of the two optical fibers. The computer can drive the multi-axis motion platform to make the two optical fibers parallel according to the images.
ó§. Market and application prospects
The development of PM fiber coupler has provided critical optical devices for the development of coherent optical communication and fiber optic gyroscope. Take FOG as an example, with its unparalleled advantages, it has replaced the cumbersome operation and a lot of the day-to-day maintenance of the mechanical rotating mass gyroscope, but also overcome the locking phenomenon of the ring laser gyroscope and eliminated the need for anti-mechanical jitter measures to maintain the sensitivity of the small speed . Compared with traditional gyroscope, FOG has a prosperous prospect on its application for its easy-to-use and more accuracy. In addition, FOG can also be applied to the aircraft navigation systems and navigation and motion detection of car, train and ship, such as the control and navigation of antenna, camera stabilizer, robots, forklifts, agricultural machinery; pipeline mapping of gas, electricity, communications, electric cable and fiber optic cable and so on. Therefore, with the rapid development of coherent optical communication and fiber optic gyroscope, the demand of PM coupler (PM splitter/ PM PLC splitter) will be also increasing.
Coherent optical communication systems and fiber optic gyroscope and fiber optic sensor are all need PM fiber coupler with high extinction ratio and low additional loss. PM fiber coupler has a prosperous prospect on its application in both in military and civilian area. In order to attract more users, the products should still be paid more effort on reducing additional loss, narrowing shape length, reducing the cost to make the FOG much compacter and cheaper.