Research On The Polarization Properties Of 2×2 Single-mode Fiber Coupler

2×2 single-mode fiber coupler is a kind of important fiber passive component, widely used in optical fiber communication, coherent light measurement and optical sensors. With the development of fiber technique in many fields, 2×2 single-mode fiber coupler is required to be of better performance. Polarization property is one of the most important properties, which influences the performance of the whole system.In this paper, the fabrication methods of single-mode fiber coupler, polarization-maintaining fiber coupler and fiber polarization beamsplitter, other kinds of the couplers, and the devices consists of coupler are introduced. The coupled mode theory is reviewed and the coupled coefficient is derived. The effect of the machining deviation during the fabrication process on the coupling ratio of fiber coupler is analyzed. Several brief models including a dumbbell model, a parabola model and an exponential model are given. Then the influences of the fiber core's parameters and the coupler's linear birefringence on the performance of a coupler are also theoretically analyzed.The investigations of the phase properties at the output ends of couplers include: (1) Under the condition of considering the coupler's loss but not the fiber's birefringence, the phase difference between the two output ends is studied. The results show that it shifts from the phase difference without considering the loss, and the shift is less than 3°. (2) Under the condition of taking account of the fiber's birefringence, the phase retardance between the two orthogonal components at the end of the through-out arm, the mathematical expression is given and the influences of the working wavelength, the environment refractive index and the effective coupled length on the retardance are investigated, theoretically and experimentally. The results indicate that with a linear polarized light input, the output light will be an elliptical polarized light. In this paper employing the principles of polarization optics, the splitting ratio of a coupler is analyzed; the effects of fiber's optical parameters upon the ration are also investigated. Then the influence of environment temperature on the ratio is researched theoretically and experimentally.The coupled mode equations concerning the polarization properties of couplers are derived; The matrices describing the polarization state variation are given. Exploiting the concept of DOP (Degree of Polarization), the polarization properties of 2×2 single mode fiber couplers are studied. The expression of the DOP at the output end of the throughout arm is given. The relations between the DOP and the parameters including effective coupled length, working wavelength, and the environment refractive index are simulated. Finally, the influences of environment temperature upon the DOP are investigated theoretically and experimentally.The work reported here might be helpful to the researchers working in fiber couplers research and fabrication techniques area.