Optical Fiber Communication: Modes, Types, and Applications, Cheat Sheet of Electronics

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Optical Fiber Communication

EC- 4313

Lecture 03

Subject Taught By:

Dr. Surjeet Raikwar

Assistant Professor

Mode:

In optical fiber communication, a "mode" refers to a specific path or pattern that light can follow as it travels through the fiber. Based on the Number of Modes

1. Single mode fiber
2. Multimode fiber Based on Refractive Index
3. Step index optical fiber
4. Graded Index optical fiber Based on the Material Used
5. Plastic Optical Fibres
6. Glass Fibres

• Figure(a) shows a multimode step index fiber with a core diameter of around 50 μm or greater, which is large enough to allow the propagation of many modes within the fiber core.
• Figure (b) shows a single-mode or mono mode step index fiber which allows the propagation of only one transverse electromagnetic mode (typically HE 11 ), and hence the core diameter must be of the order of 2 to 10 μm.
• The single-mode step index fiber has the distinct advantage of low intermodal dispersion (broadening of transmitted light pulses), as only one mode is transmitted, whereas with multimode step index fiber considerable dispersion may occur due to the differing group velocities of the propagating modes. This in turn restricts the maximum bandwidth attainable with multimode step index fibers, especially when compared with single-mode fibers.

• However, for lower bandwidth applications multimode fibers have several advantages over single-mode fibers. These are: (a) The use of spatially incoherent optical sources (e.g. most light-emitting diodes) which cannot be efficiently coupled to single-mode fibers; (b) Larger numerical apertures, as well as core diameters, facilitating easier coupling to optical sources; (c) Lower tolerance requirements on fiber connectors. The normalized frequency is a dimensionless parameter and hence is also sometimes simply called the V number or value of the fiber. It combines in a very useful manner the information about three important design variables for the fiber: namely, the core radius a , the relative refractive index difference Δ and the operating wavelength λ.

Advantages of Step Index Fiber

• The following are the major advantages.
• It involves simple and easy design and manufacturing procedures.
• It has a constant and uniform refractive index within the core.
• It allows efficient light signal propagation due to its sharp variation in refractive index at the core- cladding interface.
• Highly cost-effective in terms of manufacturing and application.
• Robust against mechanical stresses and movements.
• Highly versatile; hence it is suitable for a wide range of applications. Disadvantages of Step Index Fiber
• It is not suitable for applications where high bandwidth and high information-carrying capacity is required.
• It suffers from modal dispersion that reduces the bandwidth and transmission distance of the fiber.
• It suffers from mode coupling and modal noise that affect the signal quality.

Example 1 - A multimode step index fiber with a core diameter of 80 μm and a relative index difference of 1. 5 % is operating at a wavelength of 0. 85 μm. If the core refractive index is 1. 48 , estimate: (a) the normalized frequency for the fiber; (b) the number of guided modes. Ans: Normalized Frequency= 75.8 & guided mode= 2878 Example 2 - A graded index fiber has a core with a parabolic refractive index profile which has a diameter of 50 m. The fiber has a numerical aperture of 0.2. Estimate the total number of guided modes propagating in the fiber when it is operating at a wavelength of 1 m. Ans: Normalized Frequency= 31.4 & guided mode volume for parabolic profile= 247 Example3 - Estimate the maximum core diameter for an optical fiber with the same relative refractive index difference (1.5%) and core refractive index (1.48) as the fiber given in Example 1 in order that it may be suitable for single-mode operation. It may be assumed that the fiber is operating at the same wavelength (0.85 m). Further, estimate the new maximum core diameter for single-mode operation when the relative refractive index difference is reduced by a factor of 10. Ans:a= core diameter 2.6  m and 8  m for refractive index difference is reduced by a factor of 10

Let us have a look at the curve shown below that represents the variation in the profile of the refractive index with various values of α:

Graded-Index Multimode fiber The diameter of core in graded-index multimode fiber is somewhat between 50 to 100 micrometer. The large diameter of the core allows multiple rays to propagate through the fiber. The light wave that travels inside the fiber changes its behavior with time while travelling inside it. As we have already discussed that the refractive index of the core at the axis is comparatively larger than at the other part inside it. Thus when light is allowed is propagate inside the fiber, then it travels from less dense medium to more dense medium. However, we are aware of the fact that for TIR to take place the light must travel from denser to rarer medium. So, the light ray despite being reflected gets refracted inside the core. Hence, the light on travelling gets continuously refracted and bends. Thus in case of graded-index multimode fiber, the light rays do not propagate by following a straight line, rather they follow parabolic path due to non-uniformity in the refractive index of the core.

Advantages of Graded-Index Fiber

• It can transmit a large amount of information.
• The distortion is comparatively small than step index fiber. Disadvantages of Graded-Index fiber
• These fibers possess low light coupling efficiency.
• It is somewhat expensive than step index fiber. Thus from the above discussion, we can say that in the case of graded index fiber the transmitted information signal can be propagated efficiently and the chances of dispersion are also less in this case.