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What is Beat Length?
Beat length is the distance over which the relative phase between two orthogonal polarization modes in an optical fiber changes by 2π. It is a key parameter for characterizing the polarization properties of polarization-maintaining (PM) and birefringent fibers.
A shorter beat length generally indicates stronger birefringence, meaning the fiber more strongly separates the propagation of its two principal polarization modes.
HOW Beat Length Works
Beat length results from the difference in propagation constants between the two orthogonal polarization modes of a birefringent fiber. Because the two modes travel at slightly different phase velocities, their relative phase changes as they propagate.
Two polarization modes
Light propagates along the two principal polarization axes of the fiber, which have slightly different effective refractive indices.
Phase difference
The difference in propagation constants causes the relative phase between the two modes to accumulate along the fiber.
Polarization evolution
After one beat length, the relative phase has changed by 2π and the polarization state completes one full cycle. Stronger birefringence therefore results in a shorter beat length
