What is optical amplifier

In advanced technology, an optical fiber amplifier is used in transmitting data in fiber optic communication systems. An amplifier is inserted at a specific place to boost optical signals in a system where the signal is a week. In a large network, a long series of the optical fiber amplifier is placed in a sequence along the entire network link. An optical amplifier is basically nothing but a laser without of feedback. It is a fiber optic device used to amplify optical signal directly without conversion into an electrical signal.  Optical amplification depends on the frequency of the incident signal and local beam intensity. It is a device that amplifies an optical signal without converting it into an electric signal. 

 Types of an optical amplifier called an EDFA and Raman amplifier also have their advantages and disadvantages. 

An optical amplifier is characterized by these characteristics given below  : 

• Gain - It is a ratio of output power to input power ( dB )
• Gain bandwidth of a system - Range of wavelength over which the amplifier is effective  Gain saturation of a system - Maximum output power, beyond which no amplification is effective 
• Gain efficiency of a system - Gain is a function of input power (dB/mW)
•  A noise of a system- Undesired signal due to physical processing in amplifier ''

Explore more information:

1.      Advantages and disadvantages of Raman amplifier 

GPRS system architecture

GPRS full form provides many devices without restricting data rates of transmission. The service precedence namely high, low, normal, class, reliability throughput and delay are determined by QoS profile.

GPRS uses GSM architecture for voice. In order to offer packet data services through GPRS, a new class of network node needs to be introduced as an upgrade to the existing GSM network.

The network nodes are called GPRS support nodes ( GNN ). The GPRS support nodes are responsible for the delivery and routing of data packet between the mobile stations and the external packet data network ( PDN ).

GPRS system architecture 

AUC full form - authentication center

BSC  full form - Base station controller

BTS full form - Base transceiver station

EIR full form - Equipment identity register 

HLR full form - Home location register

VLR full form - Visitor location register

GGSN - Gateway GPRS support node

ISDN -  Integrated system digital network

MSC full form -  Mobile switching center

PDN -  Packet data network

SMSC - Short message service center

PLMN  - Public land mobile network

SMS - GMSC - SMS gateway MSC

SMS - IWMSC - SMS internetworking MSC

SGSN - Serving GPRS support node

Efficiency of TDMA system

The efficiency of a full form of a TDMA  system is a measure of the percentage of the data that is transmitted. In this system, the transmitted data has information for providing overhead for the access scheme.

We can measure the frame efficiency of TDMA is a percentage of bits per frame that contain transmitted data.

So the number of overhead bits per frame is expressed as in this form : 

overhead  = Nr br + Nt bp + Nt bg + Nr bg 

Nr = Number of reference burst per frame

Nt = Number of  traffic burst per frame

br = Number of overhead bits per reference burst for system

bp = Number of overhead bits per preamble in each slot for system

bg = Number of an equivalent bit in each guard time interval for the system

So we have to calculate the total number of bits per frame (bT)  is :

bT  = Tf  R

Tf = Frame duration

R = Channel bit rate

The frame efficency is ( 1 - boverhead / bT ) * 100 %

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.

Explore more information:

1. What are the advantages and disadvantages of laser diode 

Operational efficiency of Laser diode

This article is very useful to know or learn about how to find different types of efficiency in the laser diode.

1. External  quantum efficiency :

The operational efficiency of the semiconductor laser is differential external quantum efficiency. It is the ratio of the increase in photon output rate for a given increase in the number of injected electrons.

                             ȠD = dpe \ dI( Eg)

Where, 

pe = Optical power emitted from a device

I = Current

Eg = Band gap energy expressed in terms of electron volts

2. Internal  quantum efficiency :

The internal  quantum efficiency for a semiconductor laser is defined as ɳ  given below :

ɳ = Number of photons generated in the laser cavity / Number of injected electron

The optimal value of internal quantum efficiency Ƞi is ranging between around 50 to 100 %.

3. Total efficiency :

The total efficiency is defined as :

ɳ_T = Total number of output electron / Total number of injected electrons

ɳ_T = Pe / I. Eg

 Pe / I  = ȠT . Eg

ɳ_T = ȠD ( 1 - Ith/I )

Where Ith - threshold current

I - injection current 

4. External power efficiency 

It is in converting electrical input to optical output is expressed as 

Ƞep = Pe / P  * 100

Ƞep = Pe / IV * 100 

Ƞep = ȠT ( Eg / V ) * 100 

 The Ƞep is also called as device efficiency.

Power current characteristics of laser diode

To shown in the figure the output optic power versus forwarding input current characteristics is plotted in the figure for a typical laser diode. 

To shown in the figure below the threshold current only spontaneous emission is emitted hence there is a small increase in optic power with drive current. 

At threshold when lasing conditions are satisfied. At that time when the optical power increases sharply after the lasing threshold because of stimulated emission.

The lasing threshold optical gain is related by threshold current density for stimulated emission by the expression  is given below :

 Gth =  β  Jth 

Where β is constant for device structure.

Power current characteristics of a laser diode

Optical characterstics of laser diode

The output of the laser 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, lasing action begins. 

Here to shown in figure graph comparing optical power of LED operation and LASER operation.

Optical characterstics of laser diode and LED

Applications of Injection laser

A LASER full form is one type of device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. 

An injection laser is also known as a laser diode or diode laser. It is a semiconductor device, Nowadays it can be used optical fiber, compact disk, remote control device etc

Application of injection laser: 

• Laser diodes are performed where too much high radiance is required. This high radiance is generated due to the amplifying effects of stimulated emission. So it can supply optical power in milliwatts.
• Where narrow linewidth of the order of 1 nm ( approximately 10 A°) or less is required narrow linewidth is useful in minimizing the effects of material dispersion.
• Laser diodes are preferred where modulation is needed in the high gigahertz range.
• Where temporal coherence is required.
• Where good spatial coherence is required, it allows the output to be focused by a lens into a spot that has a  too much greater intensity than the dispersed unfocused beam.

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

Read more >> Advantages and Disadvantages of LED 

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.