1. What is an Optical Fiber and also mention its type?

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Optical fiber is the technology associated with data transmission using light pulses travelling along with a long fiber which is usually made of plastic or glass. Metal wires are preferred for transmission in optical fiber communication as signals travel with fewer damages.

The types of optical fibers depend on the refractive index, materials used, and mode of propagation of light.
The classification based on the refractive index is as follows:
 Step Index Fibers
 Graded Index Fibers
The classification based on the materials used is as follows:
 Plastic Optical Fibers
 Glass Fibers
The classification based on the mode of propagation of light is as follows:
 Single-Mode Fibers
 Multimode Fibers
The mode of propagation and refractive index of the core is used to form four combination types of optic fibers as follows:
 Step index-single mode fibers
 Graded index-Single mode fibers
 Step index-Multimode fibers
 Graded index-Multimode fibers

2.What is a Optical Fiber Communication System and what are its basic elements?

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The technique of transmitting data from one location to another by transmitting infrared light pulses using optical fiber is known as fiber optic communication. Here, the light is in the form of a carrier signal that is changed to hold the data.
Basic elements of optical Fiber Communication System are:
1. Transmitter(Compact Light Source)
2. Optical Fiber Cable
3. Receiver(Photo Detector)

3State Advantages of Optical Fiber Communication.

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a)Enormous Potential Bandwidth
b)Signal Security
c)Electrical Isolation
d)Low Transmission Loss

4.What are the characteristics of light ?

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a) Intensity
b) Wavelength
c) Spectral Width
d) Polarization
Intensity, wavelength and spectral width treat light as a scalar quantity but polarization treats light as vector quantity.

5.State Snell’s law.

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A law stating that the ratio of the sines of the angles of incidence and refraction of a wave are constant when it passes between two given media.

6What do you mean by Total Internal Reflection?

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Total Internal Reflection (TIR) is a phenomenon , by which light experiences complete reflection at an interface between two media. i.e When the light ray is travelling from denser medium to less denser medium, the angle of incidence becomes greater than the critical angle no refraction will take place and the light rays are reflected back in the same medium.

7. What are the types of Modes in Planar guide(Optical fiber) ?

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There are 3 types of modes waveguide modes :
a) Transverse Electric wave
b) Transverse Magnetic wave
c) Transverse Electromagnetic wave (hybrid wave propagation)

8.Define Critical Angle.

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Critical angle is the minimum angle of incident beyond which the total internal reflection occurs for light travelling from a medium of higher to one of lower refractive index.

9.Define V-number.

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V-number is defined by : V = 2πa/λ. N A. It is a dimensionless number gives a measure of the number of modes a fiber can support.

10.What are the ways in which a light ray can be launched inside a fiber?

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We can launch a ray inside a fiber in two ways:
i) Meridinal Ray : When the Ray which we launch in the fiber and axis of the fiber lies in the same plane then such a ray is known as meridinal ray.

Ray : These Rays are not confined to a single plan but instead tends to follow a helical type path along the fiber.

11. Why is cladding necessary for an optical fiber?

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Cladding is necessary for an optical fiber for the following reasons:
• Provide proper light guidance inside the core.
• And to avoid leakage of light from the fiber.

12.Define Acceptance Angle , Numerical Aperture

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Acceptance Angle : The maximum incidence angle of a light ray which can be used for injecting light into a fiber core or waveguide is known as acceptance angle

Numerical Aperture: For an optical fiber with core refractive index of n1 and cladding refractive index of n2 , Numerical Aperture(NA) is defined as :

13.Diffrence between multimode fiber and Single mode fiber?

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PARAMETER	SINGLE-MODE OPTICAL FIBER	MULTIMODE OPTICAL FIBER
Number of propagating mode	Only one	More than one
Core diameter	Small	Large
Optical source	LASER	LED
Bandwidth	More	Less
Handling and coupling	Difficult	Easy
Initial cost of fiber	Low	High
Attenuation	Less	More
Transmission Rate	High	Low

1.What are the mechanisms of losses occurs in optical fiber?

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The mechanisms of losses that occur in an optical fiber are:
1. Absorption (fiber material)
2. Scattering ( fiber material + structural imperfection)
3. Radiative effects
4. Pulse broadening.

2.AWhat is the difference between intermodal and intramodal dispersion?

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Intermodal dispersion: Dispersion caused by the delay between different modes. Typically, it is the delay between the shortest path (zero mode) and the longest path (critical mode).
Intramodal dispersion: Dispersion that is independent of modes, related to the line width of the source and caused by variations in the refractive index as a function of wavelength.

3.Define Attenuation loss and Bending loss.

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Attenuation loss(or path propagation loss) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space.
Attenuation occurring as a result of either a bend in an optical fiber that exceeds the minimum bend radius or an abrupt discontinuity in the core/cladding interface is called bending loss.

4.What is macro bend and micro bend losses?

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Macro bend losses occur when radius of curvature is large compared to the fiber diameter. Micro bend losses are small scale fluctuation in radius of curvature of fiber axis

5.What is polarization?

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Polarization is the electric field orientation of a light signal. Also, it can vary significantly along the length of a fiber polarization state. Thus, it is a fundamental property of an optical signal.

1.Why do you prefer laser diode over LEDs for communication applications?

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• It allows high intensity radiation.
• Because laser diode has narrow spectral width of its laser source which is preferable.

2. What are the difference between LED and Laser diode?

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LED	LASER
Optical output is incoherent.	Optical output is nearly coherent.
No optical cavity exists for wavelength selectivity.	The optical energy is produced in an optical resonant cavity.
No spatial and temporal coherence	Optical energy has spatial and temporal coherence.
The output radiation has a broad spectral width	The output beam has very directional and narrow spectral width.

3.State the essential components of a laser?

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The major components of lasers are:
• Amplifying medium
• An optical feedback
• Finally, the pumping source.

4.Give some important photo detector materials.

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The important materials are:
1. InGaAs
2. InAlGaAs.

5.What is Quantum Efficiency?

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As we know re combination could be radiative and non radiative . also energy photon generated may or may not enter the fiber so there are two types of efficiencies
1. Internal Quantum Efficiency
2. External Quantum Efficiency

6.List the advantages and disadvantages of LED

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Advantages
    • Simpler fabrication
    • Low cost
    • Less temperature dependent
    • Linearity
Disadvantages
    • Incoherent source of light
    • Usually lower modulation Bandwidth
    • Harmonic distortion
    • Large Spectral

7.What is Laser Diode?

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Lasers comes in many forms but for optical fiber communication “Laser diodes” are used exclusively.Laser diodes have response times less than 1ns and spectral widths of 2nm or less. Lasers are used in systems in which requirements are more than 200MHz

1.What are the desirable characteristics of optical sources?

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 Emission within the low loss window of the fiber.
 Narrow spectral width
 Ease and linearity of modulation
 High reliability

2. What are the possible noises that occur in optical receivers?

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The possible noises that occur in optical receivers are as follows:

1. Photo detector noise
2. Quantum noise
3. Dark current noise ( bulk dark current, surface dark current )
4. Amplifier noise
5. Lastly the thermal noise or Johnson noise.

3. Define fiber splicing.

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Fiber splicing is a Permanent joint formed between two individual optical fibers in the field or factory.

4. What is a fiber coupler?

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Optical fiber coupler is a device that distributes light from a main fiber into one or more branch fibers. Thus, we use a directional coupler to combine and split optical signals in an optical network.

5.What is Optical signal to noise ratio (OSNR)?

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It is the ratio of signal power to noise power, over a specific spectral bandwidth, at any point in an optical link. Noise power can be defined as any undesirable signal interference.

6. What is dark current noise?

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The dark current noise arises from electrons/holes which are thermally generated in the PN junction of the photodiode. Thus, this current continues to flow through the bias circuit of the device when no light is incident on the photodiode

1.What are leaky modes in optical fiber?

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A leaky mode in an optical fiber is a mode having electric field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond the finite distance

2. What is Eye Diagrams?

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The eye diagram is a powerful measurement tool for assessing the data handling ability of a digital transmission.
The eye pattern measurements are done in time domain and allow the effects of waveform distortion to be shown immediately on the display of standard BER test equipment

3.Define Point to Point fiber optic data links

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A P2P or point to point fiber optic data link consists of an optical transmitter to transmit the data, optical fiber as the medium, and an optical receiver. There will be splicing points in between the fiber optic link system, there will be fiber optic connectors at transmitter and receiver points

4.What do you mean by WDM?

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WDM is wavelength division multiplexing .The optical beam consists of different wavelengths and several channel information is transmitted over a single channel.

5. What is Power Penalties?

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The ratio of the reduced received power (or power received due to signal impairments) to the ideal received power is known as power penalties

6. What is Neighborhood processing?

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In neighborhood processing we consider the value of each pixel along with the values of the pixels in the immediate neighborhood, to arrive at the new value of the pixel.< br>

7. . What is a quantum limit?

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It is possible that we find the minimum optical power that we receive, required for a specific bit error rate performance in a digital system. Hence,we refer to this minimum received power level as the quantum limit

8. What is the cutoff wavelength of fiber?

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We define the cutoff wavelength as the minimum value of wavelength that can be transmitted through the fiber. Additionally, the wavelength greater than the cutoff wavelength can be transmitted.

9. What is BER(Bit Error Ratio)?

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It is an approach to divide the number .No .of. Errors occurring over a certain time interval t by the number N of pulses transmitted during this interval

10. What is BER(Bit Error Ratio)?

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Avalanche Photodiode: An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity.
PIN Photodiode: PIN photodiode is a kind of photo detector; it can convert optical signals into electrical signals.

1. Define Point to Point fiber optic data links

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A P2P or point to point fiber optic data link consists of an optical transmitter to transmit
the data, optical fiber as the medium, and an optical receiver. There will be splicing points in between the fiber optic link system, there will be fiber optic connectors at transmitter and receiver points.

2. List Down the application of radio-over-fiber links.

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1. Access to dead zones

An important application of RoF is its use to provide wireless coverage in the area where wireless backhaul link is not possible.
2. FTTA (Fiber to the Antenna)

By using an optical connection directly to the antenna, the equipment vendor can gain several advantages like low line losses, immunity to lightening strikes/electric discharges etc.

3.What is the purpose of rise time budget analysis?

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Rise time budget: A rise-time budget analysis is a convenient method to determine the dispersion limitation of an optical link. The power budget involves the power level calculations from the transmitter to the receiver.

4.What are the key parameters required for analyzing the optical ?

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Along with the primary design criteria, there are some additional parameters which facilitate better design and quality analysis of the optical link. These factors consist of the scheme of modulation, the system fidelity, cost, upgradeability, commercial availability etc.

5.What is Fiber Optic Communication System Design Considerations?

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When designing a fiber optic communication system some of the following factor must be taken into consideration:
• Power Budget
• Bandwidth and Riser Time Budgets
• Fiber Connectors
• Fiber Optic Couplers
• Wavelength Division Multiplexers(WDM)