全光纤型圆偏振起偏器的工作原理及理论分析

Principle and theoretical analysis of all-fiber circular polarizers

提出了以高扭转率旋转应力型双折射光纤来实现手征光纤光栅,并进而实现易于在线制成的全光纤圆起偏器.采用耦合模理论分析表明,以小于1mm的扭转周期旋转领结光纤或熊猫光纤可以实现圆起偏器.与已有的基于特殊光纤的手征光纤光栅不同,旋转光纤基于商用应力高双折射光纤,容易与普通光纤连接.耦合模分析还揭示了这类由旋转光纤形成的手征光纤光栅中圆偏振模相耦合的偏振选择机理和实现起偏器的必要条件.数值分析表明,对数值孔径为0.1865、拍长为1.25mm的熊猫光纤以右手螺旋方向进行旋转,当旋转周期和光纤长度分别为0.375mm和33.2mm时,该起偏器在谐振波长1550nm处输出左旋圆偏振光.

A new suggestion of making chiral fiber gratings and all-fiber circular polarizers by spinning strain-induced high birefringence fibers was presented.By means of coupled-mode theory,it is shown that a circular polarizer might be achieved by using a spun Bow-Tie fiber or Panda fiber with short pitches of less than one millimeter.Unlike the special chiral fibers previously mentioned,these strain-induced high birefringence fibers are commercially available and are easy to be spliced with common fibers.The coupledmode analysis also reveals the scheme of polarization-selectivity between coupled modes and the necessary conditions for achieving polarizers in these kinds of chiral fiber gratings.A numerical simulation was done on a spun Panda fiber,and it is shown that in a right-handed rotating birefringent fiber with a numerical aperture of 0.186 5,a beat length of 1.25millimeter,a rotation pitch of 0.375millimeter and a fiber length of 33.2millimeter,a left-circular polarization light would output at the wavelength of 1.55micron.