Classification based on Modes of propagation

1. Classification based on Modes of propagation Two types of modes exist in both, the step- and the graded-index fibers: • Single-mode: As the name implies, a single-mode fiber sustains only one mode of propagation. • Multimode: Multimode fibers contain many hundreds of modes. Prepared by Edmond Fernandes

2. Single Mode and Multimode Prepared by Edmond Fernandes

3. Single Mode Step-index • This fiber has a core of constant refractive index but its diameter is only 8-12 µm. • This narrow core limits the propagation of waveguide modes to a single one as the angle required for the higher modes would not be achievable in this configuration. • The step in refractive index is small (about 1%) but this is sufficient to produce the total internal reflection required to confine the wave to the core of the fiber. Prepared by Edmond Fernandes

4. Cladding Core Core Cladding Prepared by Edmond Fernandes

5. Multimode Step-index • The fiber has a core of constant refractive index surrounded by a cladding of lower refractive index • Step-index fiber is so called because the refractive index of the fiber “steps” up as we move from the cladding to the core of the fiber. • The diameter of the core is about 50-100 µm and this is wide enough to allow several different waveguide modes to propagate down the fiber. Prepared by Edmond Fernandes

6. Multimode Step-index Fiber Prepared by Edmond Fernandes

7. Multimode Graded-index • The core is similar in size to that of multimode step-index fiber • However, in this case the refraction index is higher at the centre and decreases outwards in a specified non-linear fashion toward the lower refractive index of the cladding. Prepared by Edmond Fernandes

8. Multimode Graded-index • This graded refractive index profile causes the modes of higher orders which travel at higher velocities to be reflected in the outer portion of the core and thus travel a longer distance than the lower order modes which are slower but are confined to the inner portion of the core. Prepared by Edmond Fernandes

9. Graded-Index Fiber Prepared by Edmond Fernandes

10. Multimode Graded-index Fiber Prepared by Edmond Fernandes

11. Graded-index Fiber Prepared by Edmond Fernandes

12. Overview of the different fibers Prepared by Edmond Fernandes

13. Number of Modes in a Fiber? • Modes are sometimes characterized by numbers. • Single mode fibers carry only the lowest-order mode, assigned the number ‘0’. Prepared by Edmond Fernandes

14. Number of Modes in a Multimode Fiber • The number of modes that can propagate in a multimode fiber depends on: • The fiber’s numerical aperture ‘NA’(or acceptance angle θa) • Its core diameter ‘a’ • The wavelength of the light ‘λ’ Prepared by Edmond Fernandes