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TUTORIAL: PM fiber termination

Polarization maintaining fibers such as PANDA, Bowtie, or Tiger types are not optically symmetrical and have strong internal birefringence caused by stress-applying members. The internal birefringence is significantly higher than normal levels of bend-induced birefringence. In this way it preserves the state of polarization when the laser beam is correctly aligned to either of the two axes.

Termination with or without locking the key
All PM fibers can be terminated with or without key alignment. In certain applications, the key is simply not supplied, which allows the user to freely rotate the connector ferrule. Key alignment is the most time-consuming part of the process. Therefore, it is advisable to order terminated PM fibers with key alignment only when necessary to reduce cost.

Critical specifications
The two most critical measures of a terminated PM fiber are extinction ratio and key alignment accuracy. Extinction ratio can be easily degraded by inappropriate adhesives or untested curing procedures. A PANDA type of PM fiber can maintain a 25-35 dB extinction ratio. After connectorization, however, the figure can be as low as 10-15 dB if not processed with great care.

Crosstalk and extinction ratio
Fiber orientation misalignment can cause crosstalk between TE and TM modes even when the extinction ratio is good. For example, a perfectly terminated PM fiber with 27 dB extinction ratio can generate 15 dB cross talk due to key misalignment. We stress both extinction ratio and key alignment accuracy.

Connector selection
FC/PC, FC/APC, SC/PC, and SC/APC are the most commonly used connector types for PM fiber termination. When ordering FC type connectors, be sure to specify narrow or wide key. Narrow keys are commonly used with standard singlemode fibers while wide keys provide tighter fit between the key and the slot on the FC bulkhead for better accuracy.

Loose buffer and bare fiber
Standard PM fibers such as the PANDA type often come with 250-um or 400-um acrylic jacketing. 900-um loose tubing is often used to provide protection. Bare fiber is recommended, however, for most applications especially when the fiber has 400 um jacketing.

Fiber length
Over specified the fiber length is not only costly but also a potential source of extinction ratio degradation. It is also very important to keep the entire fiber in a natural position free of twists, bends, stress, or temperature variation to ensure optimum performance.

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