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The refractive index that light in a fiber experiences will be slightly different depending on the polarization orientation of the guided mode. In a HiBi fiber this is due to deliberately induced birefringence, though there will always be some small waveguide asymmetry in a singlemode fiber. This means that parts of the light at various polarization orientations will propagate with different phase velocities, and therefore separate as they propagate along the fiber – this is a dispersion effect, hence polarization mode dispersion.

The PMD can be measured via interferogram, where it is related to the differential group delay, DGD, between fast and slow axis. Strictly speaking the PMD is affected by both the delay between the axes, GVD, and any polarization dependant loss, PDL, though it is often assumed to be valid that there is no PDL in a fiber under test. This may not be the case if the deployment of the fiber is such that guidance is favored on one axis, for example if the fiber is tightly coiled or there is microbending.

Related Products: PM Coupler Fiber, PM Erbium Doped Fiber, PM Gyro Fiber, Polyimide Coated PM Fiber, Standard PM Fiber, Telecoms PM Fiber

Related Terms: Beat Length, Birefringence, Birefringent Fiber, Bow-Tie Fiber, H-Parameter, Highly Birefringent (HiBi), Polarization Dependent Loss (PDL), Polarization Extinction Ratio (PER), Polarization Maintaining (PM) Fiber, Polarization Mode, Polarization Mode Dispersion (PMD), Polarizing Fiber