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Short note: Linearly polarized modes.

• When the difference between core and cladding refractive index becomes very small (∆ ≪1), more light energy spreads into cladding. Thus light guiding capacity of the fiber becomes weaker making the structure weakly guided in nature.

• For such weakly guided fibers it is observed that certain field components or modes are having similar field distribution and propagation constants. Such modes are grouped together and are known as degenerate modes or linearly polarized modes.

• By using weakly guided approximation it is possible to solve the boundary conditions for field components at core-cladding interface.

• Hence under weakly guided approximation natural modes degenerate into linearly polarized modes. The relationship between the natural HE, EH, TE and TM mode designations and the $LP_{lm}$mode designations is shown in Table. The mode subscripts l and m are related to the electric field intensity profile for a particular LP mode.

• $LP_{01}$ mode which corresponds to $HE_{11}$ mode never has cutoff condition and is considered as fundamental mode of optical fiber.

• V=2.405 defines cutoff condition for $LP_{11}$ and only one mode is supported by fiber $LP_{01}$. So, for single mode fiber V=2.405.

• In almost all practical fibers TE,TM or HE modes are not present light propagates in the form of Linearly Polarized modes briefly called as LP modes inside the fiber.

• The electric field intensity profiles for the lowest three LP modes, together with the Electric field distribution of their constituent exact modes are given below.

• Thus it is observed that there are only two possible orientations in the intensity pattern of LP modes either vertical or horizontal.