Applications of optical fibers

Last time I have discussed the various losses and their reasons in optical fibers. Toady we will discuss the various applications of fibers.

Optical Fibre Communication System

Optical fibers are mainly used in communication systems due to  rapidly increasing demands for telephone communications throughout the world, multiconductor copper cables have become not only very expensive but also an inefficient way to meet these information requirements. Continue reading “Applications of optical fibers”

Reasons for losses in optical fibers

When a signal passes through an optical fiber, then signal intensity losses may occur. This is called attenuation. There are many reasons for losses in optical fibers.

This loss is generally expressed in decibel (dB) and is defined as the ratio of injected optical power Pi into the fibre to the received optical power P0 from the fibre, i.e., attenuation  = 10/L Log (Pi) / (P0)  dB / km where L is the length of the fibre in km. Continue reading “Reasons for losses in optical fibers”

Dispersion in optical fibers

Pulse Dispersion: The broadening or spreading of the output pulse with the time is called pulse dispersion. This can happen due to the different reasons. Let us discuss them one by one:

1. Inter-modal dispersion: The term “Inter-modal” consists of two terms “inter” and “modal”. “inter” means “within different” and “modal” term comes from mode (mode means path followed by the light). Therefore, intermodal dispersion means the dispersion between the different modes of the fiber. Therefore this dispersion can not occur in mono-mode fibers. Thus it can only occur in multi-mode fibers.

Reason: Continue reading “Dispersion in optical fibers”

Couplers in optical fibers

Couplers:

The devices which are used to distribute light signal from one to many and from many to one optical fiber are called couplers. A few of the types of the couplers are:

1. Biconically Tapered  Directional Couplers: In this type, the multimode fibers are made bare by removing the sheath and then these fibers are placed side by side and twisted together. Then the twisted portion of the fiber is heated gently and stretched. If the signal is passing through the first fiber then it will now start passing through the second fiber. Thus the coupler now becomes one input having two outputs.

2. Beam Splitting Couplers: Continue reading “Couplers in optical fibers”

Connectors in fibers

Dear friends,

Last time I have discussed the permanent technique of joining the fibers that is called splicing. Today I will discuss another technique called connectors:

Connectors: Connectors are the temporary joints between two or more fibers. These can connected or disconnected as and when requires. A few different types of connectors are:

1. Ferrule connectors:

(Meaning of ferrule: A metal ring or cap)

In this method, the fiber ends are inserted into left and right end of the ferrule. Then these ferrules are slid into a tapered sleeve and a butt joint is formed between fiber ends. The joint can be fixed at its position by locking the ferrule arrangement. When in need, the fibers can be again separated by unlocking the ferrule. Continue reading “Connectors in fibers”

Splicers in optical fibers

Do you know how fibers are joined together to pass information? One of the methods is splicing. Let us discuss it:

Splicers: Splicers are the permanent joints between two or more fibers.

Splicing: Splicing is the technique to join the fibers.

These are used to extend the length of the fiber or repair the damaged fibers. The two common types of splicing are:

1. Fusion splicing: have you seen the welding done to join the iron? Fusion splicing is just the same. In this case, the fibers are made bare by removing the jacket/sheath. The fiber ends are then placed on adjustable vernier screws. The fiber ends are then aligned using these screws to high degree of accuracy. The fiber ends are then brought closer and using a micro electric arc lamp. The ends are then melt and then fused or joined by this method. The condition of the fibers is that they should have the same refractive index of the core.

2. Mechanical splicing: In this technique, splicing of fiber is done by mechanically.

a) V – groove splicing: In this method, the bared fibers are placed in a V- shape structure called V – groove. The two fibers are slide into the V- groove until they touch each other. The joint is made permanent by a sticky substance called epoxy resin. The epoxy resin should have same refractive index as the core of the two fibers.

b) Precision sleeve splicing: In this type, the bared fibers are placed into the glass sleeve whose inner diameter is slightly loose than diameter of the bared fibers. The fiber ends are then joined permanently by putting a sticky substance called epoxy resin through a hole in the middle of the fiber. The epoxy resin should have same refractive index as the core of the two fibers.

Solitons, optical sources and optical detectors in fibers

Question 1: Define solitons.

Answer: A soliton is a pulse or wave that travels along an optical fiber without changing the shape. It is experimentally found that due to fiber non-linearity, the refractive index of the fiber starts depending on the intensity of the light in addition to the wavelength of light. Therefore, the intensity of light itself can influence the velocity of the pulse in the fiber. Thus in solitons, the decrease in velocity due to the decrease in wavelength can be compensated by increasing the intensity of the low wavelength components in comparison to the high wavelength components of the wave. Therefore all the components of the wave travel with equal velocity in an optical fiber and pulse dispersion does not take place.

Question 2: Name the optical sources used for optical fibers.

Answer: Laser diodes and light emitting diodes are the most common sources. These devices require very less power for operation.

Question 3: What are optical detectors?

Answer: Optical detectors are the devices or instruments used at the output terminal of an optical fiber. These devices directly convert optical radiation into electrical signals and respond quickly to the changes in the optic power level.

Example: Photodiode, photomultiplier tube, PIN photodiode etc.

V- number or cut off frequency or normalized frequency in optical fibers

V- number or cut off frequency or normalized frequency in optical fibers is equal to

V = Пd(Numerical Aperture)/λ

Where d is the diameter of the core and λ is the wavelength of light passing through the core of the fiber.

As NA = √n12 – n22 or

NA = Sin θ or

NA = n1√2 Δ

Therefore, V number relation can be changed after putting the above relations in basic relation of V according to the need.

Significance of V number:

If V is less than 2.405 then the fiber is mono mode but if V is greater than 2.405 then fiber is multimode.

V number is also related with the number of modes is the fiber as:

N = V2/ 2 for step index fiber and

Number of modes for graded index fiber is N = V2/ 4.

Numerical aperture and acceptance angle

I have discussed about the principle of optical fiber in my earlier articles. Today let us discuss the terms related to fiber:

Numerical Aperture (NA): NA is the light gathering ability or capacity of an optical fiber. More the NA. the more efficient will be fiber. It is also known as figure of merit.

NA is related to refractive index of core (n1), cladding (n2) and outside medium (n0) as

NA = √n12 – n22/n0

If the medium is air then n0 =1, then

NA = √n12 – n22

Acceptance angle (θ): It is the maximum angle made by the light ray with the fiber axis, so that light can propagate through the fiber after total internal reflection.

Relation NA and acceptance angle:

NA = Sin θ

Acceptance cone: It is the cone in which the light incident at acceptance angle or less than the acceptance angle and then the light can propagate through the fiber after total internal reflection.

Fractional Refractive index change (Δ)= Δ = n1 – n2/n1

Relation NA and Δ:

NA = n1√2 Δ

Step index and graded index fiber

Mode of the fiber: As we have discussed in the earlier article about the light propagation through an optical fiber after total internal reflection. The path followed by the light in a fiber is called the mode of the fiber.

The light can pass through one path ( that is mono mode or single mode fiber) or more than one path (that is called multi mode fiber). On the basis of this, the fiber is divided into two types:

1. Step index fiber

2. Graded index fiber

Let us discuss the difference between the two: Continue reading “Step index and graded index fiber”