Step index fiber

The step index fiber is a cylindrical waveguide core with inner core has a uniform refractive index of n₁ and the core is surrounded by an outer cladding with a uniform refractive index of n_2.

In this fiber, the cladding refractive index ( n₂ ) is less than the core refractive index ( n₁ ) but there is some abrupt change in the refractive index at the core-cladding interface for using simple step-index fiber. 

The propagation of light wave within the core of step index fiber takes the path of a meridional ray. Ray follows a Zig Zag path of straight line segment. 

Depending on the refractive index profile of fiber and modes of fiber there exist two types of step index fiber  called as : 

1. Single mode step index fiber type
2. Multimode step-index fiber type

1. Single mode step index fiber

Single mode step index fiber has a central core that is sufficiently so small so that there is essentially only one path for light ray through the cable. 

The light ray propagates in the fiber through reflections. Typical core size is 2 to 15 micrometer. so single mode fiber is also known as the fundamental or monomode fiber.

Single mode fiber will permit only one mode to propagate and also does not suffer from mode delay difference. 

These are primarily developed for the 1300 nm window but they can be also be used effectively with time multiplex and wavelength division multiplex system operating in 1550 nm wavelength region.

The core fiber of a single mode fiber is very narrow compared to the wavelength of light is to be used. Therefore, only one single path exists through the cable core through which light can travel. 

(a) Multimode step index fiber type,  (b) Step index fiber type

2. Multimode step index fiber 

Multimode step-index fiber is the most widely used in optical fiber communication. It is easy to manufacture and also its core diameter is 50 to 1000 micrometer. 

In this mode, the light ray is propagated using the principle of total internal reflection. Since the core index of refraction is too much higher than the cladding index of refraction, so the light enters at less than the critical angle is guided along the fiber.

Light rays passing through the fiber are continuously reflected off the glass cladding towards the center of the core at different angles and also have lengths, limiting overall bandwidth.

One of the disadvantages of multimode step-index fibers is that the different optical length caused by various angles at which light is propagated relative to the core devices, causes the transmission bandwidth to be fairly small. So multimode step index fiber is typically only used in an application requiring of less than 1 km.  

Some important point of step index fiber :

• The data rate is very slow.
• The ray path of light propagation is looked like Zig Zag manner.
• Step index fiber of two types of mono-mode fiber and multimode fiber whereas using step index coupling efficiency with fiber is too high.
• In step index fiber the diameter of the core is about 50 - 200 micrometer while in the case of multimode fiber and 10 micrometers in the case of single mode fiber type.
• Attenuation is more.
• The refractive index of the core is high.
• The bandwidth is low.
• It is mostly used in local network communication.

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1. Difference between step index fiber and graded index fiber 

Features of CDMA

The main features of the CDMA system are given below :

• The CDMA system users share the same frequency either  TDD or FDD can be used.
• CDMA has a soft capacity limit.
• In the CDMA system the number of users increases, the system performance decreases.
• While using in CDMA system near-far problem arises at the receiver end.
• Multipath fading can be reduced as the signal is spread over a large spectrum.
• CDMA is an interference-limited system.
• Higher frequency reuse. 
• CDMA uses the co-channel cell.  So the spatial diversity can be used to provide soft handoff.
• The channel data rates are very high. Hence, the symbol duration is short and less than the channel delay spread. A rake receiver can be used to improve the reception by collecting a time-delayed version of the required signal.
• CDMA is a protocol implemented in a wireless network device. It is the most famous protocol and implemented in all over the world because it offers several advantages.
  
• There is not a sheer cutoff in the number of users that it can support unlike TDMA full form or any other traditional cellular device. 
• TDMA and FDMA both systems are suffering from hard hand-off. CDMA system has to eliminate the hard handoff by using a technique logically labeled the soft hand-off. 
• CDMA has a too much higher user capacity than that of the TDMA disadvantages or FDMA disadvantages system.
• CDMA is a secures its signals by encoding the user transmission in a unique code.

Raman amplifier

Introduction to Raman amplifier :

Raman amplifier is an optical amplifier based on main to be Raman gain, which results from the effect of stimulated called as Raman scattering. In this medium, the active medium is generated or often at an optical fiber, although it can also be a bulk crystal, a waveguide in a photonic integrated circuit, or a cell with a liquid or gas medium device. An input signal can be amplified while a counterpropagating with a pump of a beam, the wavelength of which is typically a few tens of nanometers shorter. For silica fibers, the maximum gain is obtained for a frequency offset of around = 10-15 THz between pump and signal depending on the composition of the fiber core. Here this article gives information about the most important amplifier known as the Raman amplifier to better understanding this topic.

Features of Raman amplifier :

• It can be operated in the different wavelength region
• Provided that a suitable pump source is available
• In this amplifier the gain spectrum can be tailored by using different pump wavelength simultaneously
• This amplifier also requires high pump power, it also requires high pump brightness but it can also generate high output powers
• A greater length of fiber is required
• It can have a lower noise figure
• This amplifier also has a fast reaction to changes in pump power, particularly for copropagating pump and very different saturation characteristics 

Working principle :

It is based on the stimulated Raman scattering effect. Power transferred in the optical signal is known as the Raman effect and amplification. In this above method, Raman amplification provides self-phase matching between the pump and signal with a broad gain bandwidth response generated. The pumping signal can be propagated in either direction of fiber called as forward and backward pumping.

Basically, the Raman gain is dependent on several terms following  given below :

• Fiber length
• Fiber length
• Fiber attenuation
• Fiber core diameter
• Optical pump power 

The figure is shown that below a typical Raman amplifier. The circulator as well as a pump shown in the figure, it lases comprise the two key elements of the Raman optical amplifier. The circulator is basically used for injecting light back into the transmission path minimal optical loss.

Typical Raman amplifier configuration

Advantages of Raman amplifier :

• Compatible with installed SM fiber.
• Variable wavelength amplification possible.
• A very broadband operation may be possible.
• Lower crosstalk.
• Can be used extends  EDFAs full form.

Disadvantages of Raman amplifier :

• High pump power requirements.
• Sophisticated gain control needed.
• Noise is also an issue.

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 semiconductor optical amplifier

Semiconductor optical amplifier is one type of laser diode without end of the mirror. it is the latest technology that provides high-speed switching capability, high extinction ratio, gain etc. Here this article gives the advantages and disadvantages of semiconductor optical amplifier to know more details about semiconductor optical amplifier (SOA).

Advantages semiconductor optical amplifier :

• Small size
• Electrically pumped
• Smaller output power then EDFA
• Less expensive then EDFA
• It can be run with a low power laser
• It provides all types of nonlinear operation like cross-phase modulation, cross gain modulation can be conducted

Disadvantages of  semiconductor optical amplifier :

• Lower gain
• High non-linearity
• Higher noise
• Polarization dependence

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1. Advantages and disadvantages of EDFA

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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1. Advantages and disadvantage of  semiconductor optical amplifier

Erbium doped fiber amplifier

Introduction of Erbium-doped fiber amplifier :

EDFA stands for erbium-doped fiber amplifier, An optical amplifier or types are also called erbium-doped fiber amplifier or EDFA. An Erbium-doped fiber amplifier is an optical amplifier that amplified a modulated laser beam directly, without electron-optical or optoelectronic conversion. The most popular material for long-haul telecommunication application widely used in silica doped with erbium, which is known as Erbium doped fiber amplifier or EDFA. Here this article to give a brief introduction to the most deployed fiber amplifier called the erbium-doped fiber amplifier.

Working principle :

Amplification mechanism : 

The operation of an EDFA by itself normally is limited to the range of 1530 to 1560 nm region. The energy level diagram of erbium ions in silica is shown in the figure.

Erbium-doped fiber amplifier

The erbium atoms in silica are called Er³⁺ ions with three lost electrons in its orbit. The raising of ions to higher energy levels is achieved by two types of pump level.

1. Metastable state :

The metastable state is a state from which the lifetime for the transitions from this state to the ground state is very long compared with the lifetimes to the states that led to this level. 

 2. Stark splitting :

The metastable, the pump band state and ground state levels are closely spaced energy levels form the manifold called stark splitting.

Energy level diagram 

This article also gives the advantages and disadvantages of EDFA below :

Advantages of EDFA :

• Gain is an excess of 40 to 50 dB.
• Low noise 3-5 dB, it is suitable for the long haul application.
• High pump power utilization.
• It 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 1530 nm to 1560 nm region, with a relatively flat.

Disadvantages of EDFA :

• Size of EDFA is large. 
• High pump power consumption.
• It cannot be integrated with other semiconductor devices.

Semiconductor optical amplifier

Introduction to semiconductor optical amplifier :

An optical amplifier is nothing but a laser diode without feedback. Which introduce in 1990. It is mainly used the inline optical amplifier. Semiconductor optical amplifier is a laser diode without an end mirror. It is a  most important technology provides high-speed switching capability, high extinction ratio, gain etc. There are many types of optical amplifier namely semiconductor optical amplifier ( SOA ) like erbium doped optical amplifier ( EDFA ), Raman amplifier. This article will make a clearer introduction to semiconductor optical amplifier ( SOA ).

Some key point in semiconductor optical amplifier :

• Similar to laser but with nonreflecting ends and broad wavelength emission.
• In this amplifier, incoming optical signal stimulates emission of light at its own wavelength.
• The process continues through the cavity to amplify a signal.
• Semiconductor optical amplifier consumes less power and it can use fewer component.
• Active medium consists of an alloy semiconductor ( Ga, P, In, As ).
• It works in both attenuation windows, 1300 nm, and 1500 nm.
• It very broad gain spectrum.
• High fiber to fiber gain 20 dB.
• Semiconductor optical amplifier has rapid gain response 1 ps to 0.1 ns.

A basic structure of SOA :

An SOA  work in a similar way to a basic laser. As a name suggested are used to amplify the optical signal. A typical structure of InGaAsP/InP structure of given below. The structure is much the same, with two specially designed slabs of semiconductor material on top of each other, with another material between them forming the active layer. The smaller bandgap intrinsic region has a smaller refractive index than the wider bandgap p doped and n doped quasineutral region. The intrinsic region forms the core of the optical waveguide and the quasi-neutral region form the claddings. Current injection into the intrinsic region can create a large population of electron and holes.

During the operation as an optical amplifier, light is coupled into the waveguide at Z=0. As the light propagates inside the waveguide it gets amplified and finally when the light at Z=L, its power is much higher compared to what it was at Z=0.

There are two major types of Semiconductor optical amplifier is :

1. Fabry Perot amplifier ( FPA )
2. Traveling wave amplifier ( TWA )

Now here this article you have to also check it out the advantages and disadvantages of semiconductor optical amplifier below :

Advantages semiconductor optical amplifier :

• Small size.
• Electrically pumped.
• Smaller output power then EDFA.
• Less expensive then EDFA.
• It can be run with a low power laser.
• In this amplifier all types of nonlinear operation like cross-phase modulation, cross gain modulation can be conducted.

Disadvantages of  semiconductor optical amplifier :

• Lower gain.
• High non-linearity.
• Higher noise.
• Polarization dependence.

Application of semiconductor optical amplifier :

• Power booster
• Inline amplifier
• Preamplifier 

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 ''

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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.

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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.