Types of laser

Before we learn about types of LASER first let we understand what is full form of LASER. Here this article gives information about different types of laser mode to know more details about LASER in optical communication devices.

Single-Mode Lasers :


Single-mode lasers are preferred for high-speed long-distance communications. The single mode lasers that contain only a ( single longitudinal mode and a single transverse mode). Therefore, the spectral width of the optical emission is very narrow for this mode.

To restrict laser only for one longitudinal mode, its length of the lasing cavity is reduced such that only a single longitudinal mode falls within the gain-bandwidth of a device. Because of this device becomes hard to handle and output power is limited to a few mill watts only.

Modulation of Laser Diodes :


Imposing information on light stream is called as modulation of a laser diode.

Imposing information on light stream is called as (modulation of laser diode The / produce a varying optical power or by using an external modulator to modify optical power.

The limitation of the direct modulating rate of laser diodes depends on spontaneous (radiative) and stimulated carrier lifetimes and the photon lifetime

Spontaneous lifetime :


It is a function semiconductor band type structure and the concentration.

Stimulated carrier lifetime :


It is a function of optical density in the lasing cavity and it is typically 10 ps.

Photon lifetime :


In this laser average time that the photon resides in the lasing cavity before being lost by absorption.

Light Source Materials

In light source materials the spontaneous emission is due to the carrier recombination called the carrier recombination of electroluminescence.  

 To encourage electroluminescence is necessary to select an appropriate semiconductor type of material. The semiconductors depending on energy band-gap can be categorized into the following way :
  • Direct band-gap semiconductors 
  • Indirect band-gap semiconductors. 
  • Indirect band-gap semiconductors, electrons, and also holes on either side of band gap have the same value of crystal momentum. Hence, direct recombination is ban possible
  • In the indirect gap semiconductors, the maximum and minimum energies occur at different values of crystal momentum. The recombination in these semiconductors is quite slow.
  • In this laser, the active layer semiconductor material must have a direct band gap. Indirect bandgap semiconductor, electrons, and also holes recombine directly without need of the third particle to conserve momentum.
  • In these materials, the optical radiation is sufficiently too muchhigh. 
  • The peak output power is obtained at (810 nm ) The width of emission at half power (0.5) is referred to as (full-width half maximum (FWHM spectral width. For the given LED FWHM is 36 nm.
The fundamental quantum mechanical relationship between the gap energy E and frequency v is given as :

 E = hv

 E = hc / ɣ

 ɣ = hc / E


Where energy = joules, and wavelength = meter.  So expressing the gap energy in electron volts and wavelength in micrometers for this application.

What is light emitting diodes

A light-emitting diode (LED) is a component that converts the electrical signal into a corresponding light that is injected into the fiber. Basically, the light emitter is a key element in any fiber optic system. Essentially LED is a PN junction diode. 
p-n Junctions 

A basically conventional p-n junction is called as homojunction as the same semiconductor material.

The electron-hole recombination occurs in a relatively wide layer = 10 micrometers. As the carriers are not confined to the immediate vicinity of a junction, hence high current densities cannot be realized.

The carrier confinement problem can be resolved by sandwiching a thin layer of 0.1 micrometers between the p-type and n-type layers. 

 When the carrier confinement occurs due to bandgap discontinuity of the junction. Such a junction is called heterojunction and its device is called a double heterostructure. 

In an optical communication system when the requirement is an LED is 

1) In LED bit rate of 100-200 Mb/sec.
2) In LED optical power in tens of microwatts. 

LED Structures


Heterojunctions


A (heterojunction) is an interface between two adjoining single crystal semiconductors device with different band gap.

Heterojunctions are of two types, Antisotype ( p-n ) or,  Isotype (n-n or p-p)

Double Hetero-junctions (DH) 


In order to achieve efficient confinement of emitted radiation double hetero-junctions are used in LED structures.

In double hetero-junction, the crosshatched regions represent the energy levels of free charge recombination occurs only inactive InGaAsP layer.

A hetero-junction is a junction formed by dissimilar semiconductors. Double heterojunction ( DH) is formed by two different semiconductors on each side of the active region. The figure shows double heterojunction (DH) light emitter.


Double heterojunction emitter ( DH )
The two materials have different refractive indices and different band gap energies. The changes in band gap energies create the potential barrier for both holes and electrons. The free charges can recombine only in narrow, well defined active layer side.

A double hetero-junction (DH) structure will confine both electrons and holes a narrow active layer. Under forward bias, it will be a large number of carriers injected into the active region where they are efficiently confined. 

Main LED materials or Type :


  • Gallium Arsenide ( GaAs ) - infra-red
  • Gallium Phosphide ( GaP ) - Red, yellow and green
  • Aluminum Gallium Phosphide ( AlGaP ) - Green
  • Gallium Indium Nitride ( GaInN ) - near ultraviolet, bluish-green and blue
  • Aluminium Gallium Arsenide Phosphide ( AlGaAsP ) - High brightness red, orange-red, orange and yellow
  • Silicon Carbide ( SiC ) - Blue as a substrate
  • Gallium Nitride ( GaN ) - green, emerald green
  • Zinc Selenide ( ZnSe ) - Blue
  • Aluminium Gallium Nitride ( AlGaN ) - Ultraviolet

Advantages of LED structure :

  • Carrier recombination occurs in a small region 
  • light emission occurs in an optical waveguide, which serves to narrow the output beam
  • Smaller Size
  • Light in weight
  • Longer lifetime
  • Operate very fast
For detailed information

Application of LED diode :

  • Sensor application
  • Mobile application
  • Sign application
  • Indicator
  • Remote  control
  • Optoisolator
  • LED signal
  • Illumination
  • Automotive uses
For detailed information
Read more >> Application of LED

Characteristics of Light Source for Communication

A light source needs the following characteristics is given below :

Characteristics of Light source for communication:

  • It must be possible to operate the device continuously at a variety of temperatures for many years.
  • It must be possible to modulate the light output over a wide range of modulating frequencies.
  • For fiber links, the wavelength of the output should coincide with one of the transmission windows for the fiber type used.
  • To couple a large amounts of power into an optical fiber device the emitting area should be very small.
  • To reduce material dispersion in an optical fiber link, the output spectrum should be too narrow.
  • The optical output power must be directly modulated by varying the input current to the device.
  • High coupling efficiency.
  • Low weight and low cost.
  • Better linearity to prevent harmonics and inter-modulation distortion.
  • High optical output power.
  • High reliability.
  • The power requirement for its operation must be low.
  • The light source must be compatible with the modern solid-state device.
Two types of light sources used in fiber optics are full form of LEDs and laser
diode (LD). 

Advantages and disadvantages of optical fiber

Fiber optic system currently used extensively as the transmission line between terrestrial hardwired systems. The fiber optics are used for transmission of data from point to point location, so its very accurate result in fiber optic system. Here are the main advantages and disadvantages of optical fiber to learn or know more details about optical fiber.

Advantages of optical fiber :

  • Small size
  • Higher bandwidth
  • Less signal attenuation
  • Light weight
  • Immune to cross-talk
  • Fiber cable are more strength
  • Optical fiber have long life more than 100 or above years
  • Grater immune to tapping
  • Resistance to corrosive material
  • Long distance transmission is possible
  • Immunity to electromagnetic interference 
  • Low power loss so less signal degradation 
  • Much thinner and lighter than metal wires
  • Difficult to tap so they do not radiate electromagnetic energy and thus emission cannot be intercepted 
  • Long lifespan about 100 years

Disadvantages of optical fiber :

  • Unidirectional propagation
  • High initial cost
  • Optical fiber more tensile stress than copper cables
  • Installation and maintenance
  • Fiber joining process is very costly and require skilled menpower
  • Difficult to splice
  • Expensive to install 
  • Highly susceptible
  • It can only be used on ground mostly not used in mobile communication so limited applications

Element of optical fiber transmission link

In this block diagram that are a different block can be used in optical fiber communication. General block diagram of optical communication system consists of following important blocks :
  •     Transmitter
  • .    Information channel
  • .    Receiver
Block diagram of OFC systems

Message origin :

  • Generally, message origin is from a transducer that converts a nonelectrical message into an electrical signal.
  • Common examples microphones for the room a transducer that converts a non-electrical include microphones for converting sound waves into currents and video (TV) cameras for convert images into the current.
  • In the data transfer between computers, the message already in electrical form.

Modulator :


The modulator has two main functions :

  • Modulator converts the electrical message into the proper format
  • Modulator impresses this signal onto the wave generated by the carrier source
  • Two distinct categories of modulation are used i.e. analog and digital modulation system

Carrier source :

  • Carrier source generates the wave on which the information is transmitted. This wave is called the carrier.
  • In fiber optic system, a laser diode ( LD ) or a light emitting diode ( LED ) is used for the carrier.

Channel coupler :

  • Coupler feeds the power into the information channel. In channel coupler the  atmospheric optical fiber system is a lens used for collimating the light emitted by the transmitter side and  also have directing this light towards the receiver.
  • This channel coupler must be efficiently transfer the modulated light beam from the source to the optic fiber device.  
  • One of the advantages of this design is that it possibility of high losses.

Information channel :

  • The information channel is the path between the transmitter and receiver. In fiber-optic communications, a glass or plastic fiber is the channel.
  • Amplifiers are needed in very long links to provide sufficient power to the receiver. Repeaters can be used only for digital systems.

Optical detector :

  • The information being transmitted is detected by a detector. In the fiber optic system, the optic wave is converted into an electric current by a photodetector.
  • The current developed by the detector. this current is proportional to the power in the incident optic wave. Detector output current contains the transmitted information.
  • This detector output is then filtered to remove the constant bias and then amplified.

Signal processing :

  • Signal processing includes filtering, amplification. Proper filtering maximizes the ratio of signal to unwanted power.
  • For a digital system, decision circuit is an additional block. The bit error rate (BER) should be very small for quality communications.

Message output :

  • The electrical form of the message emerging, from the signal processor, are transformed into a sound wave or visual image.
  • Sometimes these signals are directly usable when computers or other types of  machines are connected through a fiber system.

Need of fiber optic communication

  • Fiber optics deals with the learning of propagation of light through the transparent dielectric wave-guide.
  • Fiber optic system currently used extensively as the transmission line between terrestrial hardwired systems.
  • The fiber optics are used for transmission of data from point to point location, So its very accurate result in fiber optic system.
The fiber Optic Communication system has emerged most important fiber optic communication system. This article is very helpful for daily need of fiber optic communication compared to the traditional system because of following requirements :
  • In the long-haul transmission system, it is need of low loss transmission medium
  • It is need of compact and least weight transmitters and receivers
  • There is need for increased span or area of transmission
  • There is need for increased bit rate-distance product
A fiber optic communication system fulfills these requirements, hence most widely accepted. So most of the telephone company are currently using optical fiber communication for long distance communication and many more advantages of optical fiber communication.

Evolution of fiber optic system

  • Fiber optics deals with the learning of propagation of light through the transparent dielectric waveguide. Fiber optic system currently used extensively as the transmission line between terrestrial hardwired systems.
  • The fiber optics are used for transmission of data from point to point location, so its very accurate result in fiber optic system, Now a day there are many advance technology upgraded in fiber optic system.
  • Optical fiber are very helpful for daily need of fiber optic communication compared to the traditional system because of the long-haul transmission system, as well as low loss transmission medium
  • This article is very helpful for how a new technology upgraded in the year of 1975 to 2015. So now let us talk about the evolution of fiber optic system.
1. First generation :

 The first generation of lightwave systems uses HaAs semiconductor laser and the operating region was near 0.8 micrometers. Other specifications of this generation are as under :
  • Bitrate  : 45 Mb/s
  • Repeater spacing: 10 km
2. Second generation :
  • Bit rate : 100 Mb/s to 1.7 Gb/s
  • Repeater spacing : 50 km
  • Operating wavelength : 1.3 micro meter
  • Semiconductor : In GaAsP
3. Third generation :
  • Bit rate : 10 Gb/s
  • Repeater spacing : 100 km
  • Operating wavelength :  1.55 micro meter
4. Fourth generation :

 The fourth generation uses WDM techniques.
  • Bit rate : 100 Tb/s
  • Repeater spacing : >10000 km
  • Operating wavelength: 1.45 to 2.62 micrometer
5. Fifth generation :

 The fifth generation uses raman amplification techniques and optical solitiors.
  • Bitrate : 40-60 Gb/s
  • Repeater spacing: 24000 km – 35000 km
  • Operating wavelength: 1.53 to 1.57 micrometer

LED structures

Light emitting diode (LED) is a component that converts the electrical signal into a corresponding light that be injected into the fiber. Basically, the light emitter is a key element in any fiber optic system. Essentially LED is a PN junction diode.

Heterojunctions


A (heterojunction) is an interface between two adjoining single-crystal semiconductors devices with a different bandgap.

Heterojunctions are of two types, Antisotype ( p-n ) or,  Isotype (n-n or p-p)

Double Hetero-junctions (DH) 


In order to achieve efficient confinement of emitted radiation double hetero-junctions are used in LED structures. In double hetero-junction, the crosshatched regions represent the energy levels of free charge recombination occurs only inactive InGaAsP layer.

A hetero-junction is a junction formed by dissimilar semiconductors. Double heterojunction ( DH) is formed by two different semiconductors on each side of the active region. Figure shows double heterojunction (DH) light emitter.

Double heterojunction emitter ( DH )
The two materials have different refractive indices and different bandgap energies. This structure can change in bandgap energies create the potential barrier for both holes and electrons and also some free charges can recombine only in narrowband, well defined active layer side.

A double hetero-junction structure will confine both holes and electrons are narrow active layer. So in DH junction under forwarding bias, it will be a large number of carriers injected into active regions where they are efficiently confined. One of the most advantages of LED serves to narrow the output beam.

Advantages of LED structure :

  • Carrier recombination occurs in a small region 
  • light emission occurs in an optical waveguide, which serves to narrow the output beam.

Laser Diodes

  • The laser is a basic device which amplifies the light, hence the LASER full form is an acronym for light amplification by stimulated emission of radiation.
  • The operation of the device may be described by the formation of an electromagnetic standing wave within a usage a cavity (optical resonate) which provides an output of monochromatic highly coherent radiation.
  • There are different types of LASER used like single-Mode Lasers and Modulation of Laser Diodes, depends on which types of application is used.
  • The output of laser diodes characteristics is depends on the drive current passing through it. In laser diode operating at low drive current, it operates as an inefficient LED full form when driving current crosses threshold value and lasing action begins.
  • An injection laser is also known as a laser diode or diode laser. It is a semiconductor device, Nowadays it can be used many application or devices like optical fiber, compact disk, remote control device etc.
  • In LASER diode the power current characteristics, the threshold current only spontaneous emission is emitted hence there is a small increase in optic power with drive current.

Principle :

  • The material absorbs light rather than emitting. Three fundamental processes occur between the two energy states of an atom. 
1. Absorption 
2. Spontaneous emission 
3. Stimulated Emission. 
  • Laser Action is the result of three process absorption of energy packets (photons) spontaneous emission, and stimulated emission. (These processes are represented by the simple two-energy-level diagrams) 
Where,
  • E1 is the lower state energy level.
  • E2 is the higher state energy level. 
  • Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states.
  • If E1, is lower state energy level and E2, is higher state energy level.
E = E1 - E2 = h.f.
You may also know advantages and disadvantages of laser diode know more details about laser diode.

Advantages of Laser Diode

  • Simple economic design.
  • Better modulation capability.
  • It gives high optical power.
  • Laser diode is smaller size as compared to other types of laser.
  • Production of light can be precisely controlled.
  • Ability to transmit optical output powers between 5 and 10 mW
  • Low spectral width (3.5 nm).
  • Laser diode has high coupling efficiency.
  • It can be used at high temperatures.
  • Laser diode is the cheaper device to produce laser output.
  • Ability to maintain the intrinsic layer characteristics over long periods.

 Disadvantages of Laser Diode 

  • A speckle pattern appears as two coherent light beams add or subtract their electric field depending upon their relative phases at the end of the fiber.
  • In laser diode use of large drive current produces unfavorable thermal characteristics and necessitates the use of cooling and power stabilization.
  • It produces more divergent laser beam
  • Expensive
  • Poorly absorb in hard tissue and hydroxyapatite

Advantages and disadvantages of LASER diode

The terms stand for LASER full form Light Amplification by Stimulated Emission of Radiation. It can produce coherent radiation in the visible or in an infrared spectrum when current passes through it. It can be used in optical fiber systems, compact disk players and also some remote control devices. Here this post gives information about the advantages and disadvantages of LASER diode to know more about it.

Advantages of LASER diode :

  • Simple economic design
  • It has low power as compared to other types of laser diodes
  • Better modulation capability
  • The laser diode has a high coupling efficiency
  • It can be used at high temperatures
  • It gives high optical power
  • In this diode cheaper device to produce laser output  
  • It has a smaller size as compared to other types of laser diodes
  • Production of light can be precisely controlled
  • Laser diode to transmit optical output powers between the range of about 5 and 10 mW
  • Low spectral width (3.5 nm)
  • Compact
  • It is easily manufactured in arrays
  • The delivered system not as expensive as hard tissue laser diode
  • Ability to maintain the intrinsic layer characteristics over long time periods

Disadvantages of  LASER diode :

  • It produces a more divergent laser beam
  • They require big and costly optics for a large source of size
  • It has a critical heating problem
  • Hight drive current to drive the large laser for pellets
  • Expensive
  • A speckle pattern appears as two coherent types of light beams to added or subtracts their electric field depending upon their relative phases at the end of the fiber optic device
  • Poorly absorb in hard tissue and hydroxyapatite
  • Current produces unfavorable thermal characteristics 
  • Necessitates the use of cooling and power stabilization

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Features of LED

Nowadays the light is collected from the edge of the full form of LED, in order to reduce the losses caused by absorption in the active layer and to make the beam more directional. Such a device is known as edge-emitting LED or LED. This article informs to know about more features include in ELDE and how to use LED in daily life.

Features of LED : 
  • A linear relationship between optical output and current. 
  • While we are using LED, Spectral width is around 25 to 40 nm or lambda is equal to 0.8 - 0.9 µm.
  • The modulation bandwidth is much large. 
  • Not affected by catastrophic gradation mechanisms hence LED is more reliable. 
  • Better coupling efficiency than the surface emitter.
  • Less temperature is sensitive. 
Usage :
  • It is mostly used for short-range narrow and also used medium bandwidth links. 
  • Long-distance analog links.
  • Suitable for digital systems up to 140 Mb/sec.

Advantages and disadvantages of LED

The term LED full form stands for a light-emitting diode is used in suited for short-range narrow and medium bandwidth links. LED can be suitable for the digital system technology up to 140 Mb / sec. Here this post gives information about the advantages and disadvantages to know more about LED.

Advantages of LED :

  • LEDs produce more light per watt
  • LED is more efficient 
  • Lower initial cost
  • Simple design
  • LED doesn't change their light tint as the current passing through them is lower 
  • Ease of manufacture
  • Simpler fabrication 
  • Simple drive circuitry
  • Simple system integration
  • Linear light output against current characteristics.
  • High reliability
  • Less temperature dependent
  • LED will achieve full brightness in few microseconds
  • LED does not contain mercury, unlike compact fluorescent lamps
  • Excellent CRI ( Color rendering index )
  • Environmental friendly
  • Quick turn ON and OFF
  • Good adaptability to coherent laser operation
  • No warm-up period 
  • Not affected by cold temperature 

Disadvantages of LED :

  • Refraction of light at semiconductor/air interface
  • The average lifetime of radiative recombination is only a few nanoseconds, therefore, modulation BW is limited to only a few hundred megahertz
  • Low coupling efficiency-usually low optical power coupled into the fiber (W)
  • Large chromatic dispersion
  • Driving LED hard in ambient temperature may result in overheating of the LED package, eventually leading to device failure
  • Incoherent light source
  • Spontaneous emission and non-linear output characteristics
  • Little effective in a wide area

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Number of channel in TDMA system

The number of channel slots that can be provided in a TDMA system is found by multiplying the number of TDMA slots per channel by the number of channels available.

N = m (Bt -2 Bguard) / Bc

m = Maximum number of TDMA users supported on each radio channel 
Bc =  Channel bandwidth
Bt = Total specturm allocation
Bguard = Guard band allocated at the edge of allocated spectrum
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