Properties of light

Light is electromagnetic radiation that has wave properties. Light waves can travel in a straight line. The light rays travel at the speed 3×10⁸ or 186000 miles/sec. in free space. Light is a basic form of energy produced by the luminous object. It can travel through a vacuum. A light source needs characteristics of light must be possible to operate the device continuously at a variety of temperatures for many years.

Let us now check it out the primary definitions which we will be using while discussing optical fiber.

1. Refractive Index :

Refraction is the bending of a light ray that occurs when a light ray passes from one medium to another.

So it usually happens when the light to them from one medium to the next. The measure of how much light refract in a medium call index of refraction.

Refractive index can be defined as, n of an optical parameter of a material or substance as the ratio of the speed of light(c) in the air to the speed of light(v) in another medium, such as glass, water etc.

Where, 

Refractive index  n = Speed of light in air (c) / speed of light in a material (glass-v)

n = Refractive index

c = Speed of light,  speed of light in free space = 3×10⁸ or 186000 miles/sec

v = speed of light in a material

Now will discuss the index of refraction in a different material

• Air - 1.003
• Vacuum - 1
• Water - 1.33
• Glass fiber - 1.5-1.9
• Diamond - 2.0 - 2.42
• Silicon - 3.4
• Gallium arsenide - 2.6

Due to the refraction of light, we have to observe the various effect in our daily life. The following some example of them.

• A stick partially dipped in water seems to be sent
• A clear pool of water always appears to be shallower than compare it actually is.

2. Reflection :

The law of reflection state that when the light ray incident upon a reflective surface at some of the incident angle i from imaginary perpendicular normal, the ray will be reflected from the surface at some angle r^r from normal which is equal to the angle of incident.

The angle of reflection =  r^r  = the angle at which the light is reflected from a surface is called the angle of reflection

                                                                  i =  r^r 

                           ^{The angle of incidence  is the same as = the angle of reflection}

The angle at which the light strikes a surface of the mirror with respect to the normal is called the "angle of incidence". The figure shows that the law of refraction and reflection.

 

3. Diffraction :

Diffraction is the slight bending of light as it passes around the edge of an object side. It refers to various phenomena that occur when a wave encounters an obstacle or silt. It is also defined as the bending of light around the corners of an obstacle or aperture into the region of the geometrical shadow of the obstacle. every cloud is a silver lining is the example of diffraction.

4. Dispersion :

The process on which light is separated into its colors due to the different degree of refraction, so the separation of visibility into its different colors is known as dispersion.

Working of photo diode

In the photodiode, convert the modulated light back into an electrical signal. As the intensity of an optical signal at the receiver is very low, the detector has to meet high-performance specifications.

The conversion efficiency must be very high at the operating wavelength. The speed of the response must be high enough to ensure that the signal distortion does not occur. It must be possible to operated continuously over a wide range of temperature for many years.

At present, these requirements are met by a reverse biased p-n photodiode. In these devices, the semiconductor material will absorbs a photon of light, which excites an electron from the valence band to the conduction band.

The photogenerated electron leaves behind it a hole, and so each photon generates two change carriers. This increases the material conductivity so-called photo conductivity resulting in an increase in the diode current. 

V-I characteristics of photodiode :

The characteristics of the photodiode are shown in clearly in the following figure. A photodiode is operated in a reverse bias mode. To show in figure photocurrent is nearly independent of reverse bias voltage which is applied. For zero luminance, the photocurrent is almost zero excluding for small dark current to shown in the figure.

V-I characteristics of a photodiode

Advantages and disadvantages of raman amplifier

Raman amplifier is also a relatively mature optical amplifier in the different amplifier system. In this  amplifier, the optical signal can be achieved by using stimulated Raman scattering, stimulated Brillouin scattering or stimulated for photon mixing giving parametric gain by injecting a high power laser beam into an optical fiber. Here this article gives the advantages and disadvantages of the Raman amplifier to know more details about the Raman amplifier. 

Advantages of Raman amplifier :

• Compatible with installed SM fiber
• Variable wavelength amplification possible
• A very broadband operation may be possible
• Can result in lower average power over a span, so it good for lower crosstalk
• Can be used extends  EDFAs
• Flexible gain wavelength
• Low noise
• Simple structure
• The non-linear effect can be reduced

Disadvantages of Raman amplifier :

• High pump power require
• Sophisticated gain control needed
• Noise is also an issue
• Low efficiency
• High cost

Advantages and disadvantages of EDFA

The most popular material for long-haul telecommunication application widely used in silica doped with erbium, which is known as an erbium-doped fiber amplifier. So we will check the advantages and disadvantages of EDFA to know more about EDFA. 

Advantages of EDFA :

• Gain is an excess of  up to 40 to 50 dB
• Low noise 3-5 dB, it is suitable for long-haul application
• High pump power utilization
• Good gain stability
• The high energy conversion efficiency
• High gain with little crosstalk
• This amplifier is fully compatible with the rest of the fiber optic transmission link
• Flatness can be improved gain flattening optical filter
• Amplify wide wavelength band in the range of  around 1530 nm  to 1560 nm region, with a relatively flat

Disadvantages of EDFA :

• Size of EDFA is large
• Fixed gain range
• Gain up flatness
• Optical surge problem
• High pump power consumption
• It cannot be integrated with other semiconductor devices

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Difference between LED and laser diode

LASER diode is also known as a laser diode and LED also known as a light emitting diode, both diodes have different characteristics in the way in which they emit light. When a LASER emits converged light, the output of an LED highly diverges. In a LASER diode, the operation of the device may be described by the formation of an electromagnetic standing wave within a cavity (called an optical resonator) which provides an output of monochromatic highly coherent radiation. This explanation on LASER  LED helps to compare the difference between LED and LASER diode. Here this article gives the information about the difference between LED and laser diode, to know more about LED and laser diode.

LED Diode :

• LED stands for the light emitting diode.
• LED is small size.
• LED are longer life, reliable, require little power.
• Coupling efficiency is very low.
• Here generation photon by spontaneous emission.
• Output power is linearly proportional to drive current.
• LED's produce a divergent and non-coherent light beam.
• Transmission distance is too much smaller and the response is so fast.
• Cost is low.
• The coupling efficiency of LED is very low.
• A wide range of wavelengths is available, Wavelength is 0.66 to 1.65 micrometer.
• Drive current for LED is to be maximum about 50 to 100 mA peak.
• LED has required no extra circuit just because of it is a simple circuit.
• The data rate is low.
• Their response is fast.
• LED's use with the multi-mode fibers.
• The bandwidth of LED is moderate.
• Light emitted of LED consist of various colors.
• The junction area is wider when use LED. 
• Two types of LED 1)Surface and 2)Edge emitter.
• LED generally consider an eye shape.
• Feedback is not required in LED.

LASER Diode :

• A LASER  diode stands for light amplification by stimulated emission of radiation.
• Laser's are bigger in size.
• Laser are longer life, less reliable and also require more power compare LED type of diode.
• Coupling efficiency is very high.
• Here generating photon stimulated emission.
• power is linearly proportional to the current above threshold.
• The laser produces the monochromatic and coherent light beam.
• Transmission distance is too much greater and also have the response is faster than LED.
• Cost is high.
• The coupling efficiency of the laser is very high compared to LED.
• Wavelength range is 0.78 to 1.65 micrometer.
• Require to drive current is Threshold current around - 5 to 40mA.
• In a laser diode, it has to require an extra circuit for isolation of temperature reaction.
• The data rate is high.
• Their response faster than LED.
• Laser use with single mode and multimode fiber.
• The bandwidth of the LASER is higher.
• While LASER consists of single colors.
• In case of LASER junction area externally small.
• Two types LASER one type of semiconductor LASER and other types of Gas LASER.
• LASER must be rendered eye shape especially for ϒ = < 1400 nm 
• Proper feedback is essential in a laser to treated as an optical source.

So in this above difference, we can conclude that LED and LASER both are generated light, but both diodes are different.

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