Three Types of Optical Fibre

Optical fibres are classified based on

(i) Material, (ii) Number of modes, (iii) Refractive index profile

Classification based on material

It can be classified into two types. a. Glass fibre  b. Plastic fibre

a) Glass fibre

If the optical fibres are made by the fusing mixtures of metal oxides and silica glass, then it is known as glass fibre.

Examples

The glass fibres can be made by any one of the following combinations of core and cladding.

GeO₂ −SiO₂ core ;SiO₂ cladding

P₂O₅ −SiO₂ core ;SiO₂ cladding 

b) Plastic fibres

If the optical fibres are typically made of plastics, then it is known as plastic fibre.

Examples

The plastic fibres can be made by any one of the following combinations of core and cladding.

Polystyrene core – Methylmethacrylate cladding.

Polymethylmethacrylate core – Co-polymer cladding.

The advantage of plastic fibres are more flexible, easy to install, less weight and less expensive than glass fibre. It is tough and durable than glass fibre.

The disadvantage is it exhibits considerably greater signal attenuation (energy loss) than glass fibres.

Classification based on number of modes

Depending on the number of modes of propagation the optical fibres are classified as
a. Single mode fibre  b. Multi mode fibre

a. Single mode fibre

If there is only one path for the light to propagate down the cable, it is called a single mode fibre.

• Single mode fibre has a very small core diameter so that it can allow only one mode of propagation.
• Ordinary light cannot be propagated through this single mode fibre.
• The optical loss is very less.
• The cladding diameter must be large compared to the core diameter.
• Generally single mode fibres are called step index fibres.
• It can be excited or used only by using semiconductor laser diaode.
• Example : cable TV transmission, telephone transmission.

Structure

Band width                     : More than 50MHz.

Advantages

1. It facilitates to transmit signal to longer distance.
2. It is free from intermodal dispersion.
3. It has higher bandwidth, thus used in long haul communication.

Disadvantages

1. It is difficult to launch the light through the fiber.
2. It is difficult to achieve end to end connection of similar fibers.

Application

Because of it its high band width, they are used in long haul communication systems.

b. Multi mode fibre

If there is multi path for the light to propagate down the cable, it is called a multimode fibre.

• Multimode fibre has a large core diameter so that it can allow many modes of propagation.
• Ordinary light can propagate through this multimode fibre.
• The optical loss is more.
• The cladding diameter is less compare to core.
• It can be excited or used by LED.
• Example: LAN (Local Area Network)

Structure

• Core diameter                : 5−10 μm.
• Cladding diameter        : Generally around 125 μm.
• Buffer jacket diameter : 250−1000 μm.
• Numerical aperture      : 0.08 to 0.15
• Band width                     : More than 50MHz.

1. It is easy to launch the light through the fiber.
2. It is easy to achieve end to end connection of similar fibers.

Disadvantages

1. It used to transmit signal to shot distance.
2. It suffer intermodal dispersion.

Application

Because of its less bandwidth it is very useful in short haul communication systems.

(1) Difference between Single mode fibre and Multi mode fibre

Classification based on the refractive index profile

The optical fibres are classified into two types,
a. Step index fibre    b. Graded index fibre

a) Step index fibre :

In this fibre, the variation in refractive indices of air, cladding and core varies by step by step. Thus, it is known as step index fibre.

• A step index fibre has central core with uniform refractive index.
• The cladding has less refractive index than core.
• It is used in under sea-level transmission.
• It involves both single mode and multimode fibres.

b) Graded index fibre :

In this fibre, the refractive index of the core is non-uniform. It is maximum at the centre and decreases gradually with distance towards the surface. Thus, it is known as graded index fibre.

• It is a multimode fibre with a large core diameter.
• The cladding has a lesser refractive index than that of core.
• At interface, the refractive index of core and cladding is equal.
• The graded index fibre will be of multimode system.

Fig 1.4 Multi step index

Based on refractive index and mode of propagation, optical fibres are further classified into three types. They are given below:

(c) Graded index multi mode fibre

(a) Step index single mode fibre

• Single mode fibers are those that accept only one light ray that first reaches the core.
• This enables the light pulse to make a cleaner signal at the receiving end.
• The bandwidth of light it carriers is also higher.
• The main drawback of a single mode fibre is the smaller fibre core diameter. It makes harder to couple light into the fiber.
• Hence, the manufacturing of most single mode fibre optic components, such as isolator, attenuator, etc., is expensive.
  

Fig 1.5 Step index single mode fibre

(b) Step index multi mode fibre

• It is the simplest type of optical fibre whose core diameter ranges from 100 to 970μm.
• Light signals can enter the core at different angles, hence following different paths (modes) within the core.
• The time interval between each output signal will, therefore, be different.
• Here a pulse of light that entered as a narrow beam gets dispersed. This spreading of the optical pulse is called modal dispersion.
• Information loss can occur when one pulse overlaps with an adjacent pulse.
• This type of fibres is basically used in data links that do not require very large bandwidths (frequency).

Fig 1.6 Step index multi mode fibre

(c) Graded index multi mode fibre

• Modal dispersion is reduced in a multi mode graded index fibre.
• The refractive index for each layer decreases away from the central axis. Hence, light travelling near outside of the core has a faster average velocity; light travelling near the center of the core has the slowest average velocity.
• As a result, all the rays arrive at the other end almost at the same time.

Fig 1.7 Graded index multi mode fibre