Ch5Sec4 - THE FIBER FORUM Fiber Optic Communications...

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THE FIBER FORUM Fiber Optic Communications JOSEPH C. PALAIS PRESENTED BY
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Joseph C. Palais 5.4 2 Section 5.4 MODES AND FIELDS IN STEP- INDEX FIBERS 5.4.1 Modes Recall the mode chart that we developed for the dielectric slab waveguide. It is repeated on the following slide.
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Joseph C. Palais 5.4 3 Modes Slab Waveguide Mode Chart n 2 n eff n 1 θ
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Joseph C. Palais 5.4 4 Modes Step-Index Fiber Mode Chart n 2 n eff n 1 0 1 2 3 4 5 6 Axial 2 2 2 1 2 n n a V - = λ π 90° θ c θ HE 11 TE 01 TM 01 HE 21 HE 12 EH 11 HE 31 HE 41 EH 21 TM 02 TE 02 HE 22 2.405 Critical Angle
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Joseph C. Palais 5.4 5 V is called the normalized frequency or V parameter . Notes: 1. The chart is universal, valid for any n 1 and n 2 . ) ( 1 , sin . 2 z t j o eff e E k n n β ϖ θ - = = Modes 3. For a fixed V, many modes can propagate.
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Joseph C. Palais 5.4 6 4. There are TE modes, TM modes, and hybrid modes (EH and HE). 1. For large V: Higher-ordered modes travel close to the critical angle. Lower-ordered modes travel nearly axially. 6. For a given mode, n eff varies with wavelength, producing waveguide dispersion. Modes
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Joseph C. Palais 5.4 7 7. Since n 1 = n 1 ( λ ), there is also material dispersion. 8. Since different modes have different ray angles there is modal distortion. For the slab, the number of modes is the integer part of λ 2 2 2 1 2 1 n n d N - + = Modes Multiply the answer by 2 if both TE and TM modes are included.
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5.4 8 For the SI fiber, if V > 10, the number of modes is approximately:
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Ch5Sec4 - THE FIBER FORUM Fiber Optic Communications...

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