Archive for September, 2011
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: More >
Q: Why does a liquid remain hot or cold for a long time inside a thermos flask?
Answer: Do you know that thermos flask has double glass wall? The air is trapped in between this double wall of glass. When a liquid hot or cold is put into a thermos then this air does not allow the heat to loose through it. Do you know why? Yes, you are right, because the air is an insulator or poor conductor of heat.
This is the only simple reason that trapped air in double glass will not allow the heat to loose and this is the reason that a liquid remain hot or cold for a long time inside a thermos.
Q: Why is cooking quicker in a pressure cooker? More >
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: More >
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. More >
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.
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.
If we understand the basic theory of the semiconductor, then we know that semiconductors has holes and electrons. But question here arises what are basically holes? Let us discuss:
Most of us know that electrons move and the vacancy electrons left is called hole. But the question again arises here, if hole is just the vacancy then why it has positive charge. Let us discuss this now:
Suppose there are 3 electrons and a vacant space
1. e e e _
Now the third electron moves to the right, that is
2. e e _ e
So now the hole is at third position and we can say that the it moves to the left.
Now the second electron moves to the right, that is
3. e _ e e
so the hole moves to the left
and now first electron moves to the right
4. _ e e e
So the hole again moves to the left.
I know that you all know this. But here the basically only electrons are moving. Holes are just fictitious entity and a mathematical trick. Holes are just introduce to make things easier.
Therefore, it is only a convention that we call hole has a positive charge. In other words, one more reason can be that when an atom loses an electron, we say that it has now positive charge. Similarly, one electron leaves a place, we say that by convention, that vacancy or hole has positive charge.
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.
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
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: More >