Optical Fiber MCQ Quiz - Objective Question with Answer for Optical Fiber - Download Free PDF

Concept:

The number of propagating modes in a multimode step-index fiber is approximately given by, \( M = \frac{V^2}{2} \)

Where V is the normalized frequency (V-number), and is given by, \( V = \frac{2\pi a}{\lambda} \cdot NA \)

NA is the numerical aperture and is defined as, \( NA = n_1 \cdot \sqrt{2\Delta} \)

Where, n₁ = core refractive index, Δ = relative refractive index difference, λ = operating wavelength, a = core radius

Calculation:

Given:

n₁ = 1.5, Δ = 2% = 0.02, λ = 1300 nm = 1.3 × 10^-6 m, M = 1000

Calculate NA: \( NA = 1.5 \cdot \sqrt{2 \cdot 0.02} = 1.5 \cdot \sqrt{0.04} = 1.5 \cdot 0.2 = 0.3 \)

Now, \( M = \frac{V^2}{2} \Rightarrow V^2 = 2M = 2000 \Rightarrow V = \sqrt{2000} \approx 44.72 \)

From, \( V = \frac{2\pi a}{\lambda} \cdot NA \)

\( 44.72 = \frac{2\pi a}{1.3 \times 10^{-6}} \cdot 0.3 \Rightarrow a = \frac{44.72 \cdot 1.3 \times 10^{-6}}{2\pi \cdot 0.3} \approx \frac{58.136 \times 10^{-6}}{1.884} \approx 30.85 \times 10^{-6}~m \)

Core diameter = 2a ≈ \( 61.7~\mu m \)

Correct Option:

The closest answer is: 3) 62 µm

Concept:

The maximum digital transmission rate over an optical fiber is limited by pulse dispersion, which spreads the signal in time and can cause inter-symbol interference (ISI).

For Return-to-Zero (RZ) data transmission, the bit interval must be at least equal to the total pulse spreading to avoid overlap between bits.

Calculation:

Given:

Fiber length = 10 km, Pulse spreading constant = 10 ns/km

Total pulse spread = \(10~\text{km} \times 10~\text{ns/km} = 100~\text{ns}\)

In unipolar RZ transmission, the maximum bit rate is the reciprocal of the bit period.

\( \text{Bit rate} = \frac{1}{\text{Bit interval}} = \frac{1}{100~\text{ns}} = 10~\text{Mbps} \)

The correct answer is Communication.

Key Points

• Optical fiber is a key component in modern telecommunications, allowing for the transmission of data over long distances at high speeds.
• Use of Optical Fiber in Communication:
  • Data Transmission: Optical fibers transmit information as light pulses. An electrical signal is converted into a light signal using a transmitter, and this light travels through the fiber to its destination. At the other end, a receiver converts the light signal back into an electrical signal.
  • Bandwidth: Fiber optics can handle high data rates (bandwidths). Modern optical fibers can carry up to 100 terabits of data per second.
  • Long Distance Communication: Due to low attenuation (signal loss) in optical fibers, data can travel for many kilometers without significant degradation. Repeaters or amplifiers can be placed at intervals to boost the signal for even longer distances.
  • Telephony and Internet: Optical fiber is the backbone of modern telecommunication systems, including internet and telephone networks. It facilitates high-speed internet connections, video conferencing, and other data-intensive applications.
  • Security: Optical communication is more secure than electrical communication. It's difficult to tap into a fiber optic cable without being detected.
  • Multiplexing: Techniques like wavelength-division multiplexing allow multiple channels (each at a different light wavelength) to be sent down the same fiber strand, increasing the amount of data that can be sent.

Additional Information

Optical Fibers:

• Construction: An optical fiber consists of a core surrounded by a cladding layer, both made of dielectric materials. The core is where the light signal travels, and the cladding layer ensures total internal reflection of the light within the core.
• Types:
  • Single-mode fiber: Has a small core and is used for long-distance communication.
  • Multimode fiber: Has a larger core and is used for shorter distances.
• Advantages:
  • Reduced attenuation compared to electrical cables.
  • Resistance to electromagnetic interference.
  • High bandwidth potential.
  • Thin and lightweight.
• Applications Beyond Communication:
  • Medical: Used in endoscopy and laser surgeries.
  • Military: Secure communication and sensing.
  • Sensors: For measuring parameters like temperature, strain, and vibration.
• Challenges:
  • Fiber optics are delicate and can break if bent too sharply.
  • They require specialized equipment for splicing and connecting.
  • Initially, the infrastructure investment can be higher compared to traditional copper systems.

Optical fibers work on the principle of total internal reflection. Optical fiber is a long, thin strand of very pure glass about the diameter of human hair.

Optical fibers are arranged in bundles called optical cables and are used to transmit light signals over long distances.

Material dispersion is a function of the wavelength (λ) because different materials have different refractive indexes at different wavelengths.

This causes the speed of light to vary with wavelength, resulting in different propagation constants for different wavelengths.

Material dispersion is also called chromatic dispersion. Material dispersion exists due to changes in the index of refraction for different wavelengths.

Consider a fiber cable carrying an optical signal equally with various modes and each mode contains all the spectral components in the wavelength band. 

All the spectral components travel independently and they observe different time delays and group delays in the direction of propagation.

The velocity at which the energy in a pulse travels along the fiber is known as group velocity. Group velocity is given by

\(\rm\frac{1}{v_s} = −\frac{λ^2}{2 \pi c} \frac{d b}{d λ}\)

Hence option 3 is correct.

Concept: 

The path followed by the light in a fiber is called the mode of the fiber. In a single-mode fiber, the light passes through one path only whereas in the case of multi-mode fiber, The number of modes propagating in a graded-index fiber can be estimated as:

\(M =(\frac{α}{α +2 }) \frac{{{V^2}}}{2}\)

Where α represents the refractive index profile and V is the mode volume given as

\(V = \frac{{2\pi a}}{\lambda }\sqrt {n_1^2 - n_2^2} =\frac{{2\pi a}}{\lambda }N.A.\)

where a is the fiber core radius, λ is the wavelength of the optical source and N.A. is the numerical aperture.

Calculation:

Putting the values given in the question, we get (Assume fiber core radius, a = 25 μm)

\(V =\frac{{2\pi a}}{\lambda }N.A. =\frac{{2\pi \times 25 \times 10^{-6}}}{1310 \times 10^{-9} }\times 0.22 = 26.38\)

\(M =(\frac{2}{2 +2 }) \frac{{{V^2}}}{2} = \frac{V^2}{4} = \frac{26.38^2}{4} = 174\)

Hence, the total number of guided modes are 174.

Additional Information

Numerical aperture is considered as a light-gathering capacity of an optical fiber given by

\(N.A. =\sqrt {n_1^2 - n_2^2}\), 

where n1 is the refractive index of the core and n2 is the refractive index of the cladding.

Number of modes for step-index fiber, N = \(V^2 \over 2\)

The correct answer is total internal reflection.

Key Points

• Optical fiber:
  • The working of an optical fiber is based on total internal reflection. Hence, option 2 is correct. 
  • Optical fibers consist of many long high-quality composite glass/quartz fibers. Each fiber consists of a core and cladding.
  • The refractive index of the core (μ1) material is higher than that of the cladding (μ2).
  • When the light is incident on one end of the fiber at a small angle, the light passes inside, undergoes repeated total internal reflections along with the fiber, and finally comes out.
  • The angle of incidence is always larger than the critical angle of the core material concerning its cladding.
  • Even if the fiber is bent, the light can easily travel through along the fiber.
• 

Step-index fiber:

1. The refractive index of the core is uniform throughout and undergoes on abrupt change at the core-cladding boundary.

2. The path of light propagation is zig-zag in a manner

Additional Information

Optical fibres are transparent fibres and act as a light pipe to transmit light between its two ends. They are made up of silicon dioxide.

Refracting index is changing between the cladding and the core in optical fibre

Graded-index fiber:

1. The refractive index of the core is made to vary gradually such that it is maximum at the center of the core.

2. The path of light is helical in manner.

This type of fiber optics works when the wavelength is much smaller than the core radius.

So, the refractive index of the core remains constant for step-index fiber.

Difference between step-index and graded-index fiber.

The data rate for optical fiber is 1 Gbps.

• In an optical fiber, the information is passed through light, which must not escape outside of it.
• This phenomenon of confining the light inside the optical fiber is termed as Total internal reflection.
• For this, the construction and material used to ensure that the total internal reflection of light takes place to prevent the escape of it.

The data rate of different communication systems is:

• Single-mode step indexed fibers are widely used for wideband communications are preferred for long-distance communication.
• Single-mode step-index fiber is used to eliminate modal dispersion during optical communication.
• In this fiber, a light ray can travel on only one path so minimum refraction takes place hence, no pulse spreading permits high pulse repetition rates.

Advantages of single-mode fiber:

1) Low signal loss

2) No modal dispersion

3) Does not suffer from modal dispersion

4) Can be used for higher bandwidth applications

5) Long-distance applications

6) Cable TV ends

7) High speed local and wide area network

Single-mode means the fiber enables one type of light mode to be propagated at a time. This is explained with the help of the following diagram:

Single-mode fiber core diameter is much smaller than multimode fiber.

2) For single-mode fiber, the B.W ranges from 50 to 100 GHz/km

Multimode fibers:

Fibers that carry more than one mode are called multimode fibers. There are two types of multimode fibers:

1) Step Index

2) Graded Index

The comparison of the refractive index profile for step and graded fibers are respectively shown as:

The multimode step-index multimode fiber suffers from Modal dispersion.

• Rays of light enter the fiber with different angles to the fiber axis. The limit is the fiber’s acceptance angle.
• Rays that enter with a shallow angle travel a more direct path and arrive sooner than those that enter at steeper angles.
• This arrival of different modes of light at different times is called modal dispersion.

• Optical fibers are cylindrical solid glass material acting as waveguides made of two concentric layers of very pure glass.
• The core (the interior layer) with refractive index n₁ serves as the medium for light propagation, while the cladding (the exterior layer) has a lower refractive index n₂ where n₁ > n₂ assuring that light rays are reflected the core.
• Since the cladding does not absorb any light from the core, the light wave can travel great distances.

Explanation:

• The working principle of optical fibers is Total Internal Reflection.
• Optical fiber mostly used for communication purposes with negligible loss of energy.
• The “Total Internal Reflection” of light is the boundary between transparent media of two different refractive indices.
• At present, Optical fiber cables are used for communication like sending images, voice messages, etc.
• The designing of this cable is done with Plastic or glass so that data can be transmitted effectively and quickly than other modes of communications

Optical Fiber:

• In an optical fiber, the information is passed through light, which must not escape outside of it.
• This phenomenon of confining the light inside the optical fiber is termed Total internal reflection.
• For this, the construction and material used to ensure that the total internal reflection of light takes place to prevent the escape of it.

• In any type of optical fiber, the refractive index of the core is always greater than the refractive index of the cladding.
• It is this property of core and cladding which makes light propagate inside the fiber. 
• Thus, In a step-index optical fiber refractive index of the core is higher than the cladding

Principle:

• When light travels from a high refractive index medium to a low-refractive-index medium, it is refracted away from the normal as shown:

• At a certain angle θi, there is no refracted wave and the wave is totally internally reflected  (θr = 90°).
• This angle is called a critical angle.
• Inside an optical fiber, we have a high refractive index core (n1) and low refractive index cladding (n2).
• This results in the propagation of waves inside a fiber through total internal reflection phenomenon.

Graded index fiber is used to minimize the dispersion. This is because, in the graded-index fiber, the profile of the refractive index is parabolic and due to this refocusing of the signal within the core is increased, which eventually increases the data rate. 

a is the radius of core.

Important Points:

Difference between step index and graded-index fiber.

Concept:
Fiber Optics or Optical Fiber:

• It refers to the technology that transmits information as light pulses along a glass or plastic fiber.   
• They work on the principle of total internal reflection which refers to a phenomenon of bouncing back of the light to the original denser medium from a rarer medium when the angle of refraction becomes more than 90 degrees. Hence, statement 2 is correct.
• They find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths (data transfer rates) than electrical cables.
• And because fiber-optic cables are non-metallic, they are not affected by electromagnetic interference (i.e. weather) that can reduce the speed of transmission.
• Fiber cables are also safer as they do not carry a current and therefore cannot generate a spark.
• Optical fiber cables have become one of the key points in the 5G network. Hence, statement 1 is correct.
  • India’s digitization efforts to transform education, health care, and agriculture, including taking optical fiber to every village as it is fundamental in the nationwide launch of 5G services.
  • 5G will allow for much faster Internet speeds and lower latency compared to 4G.
  • Peak speeds could touch 10 Gbps, compared to the 100 Mbps for 4G.
  • While 4G latency is 10-100 milliseconds, on 5G it is expected to be under 1 millisecond.

Being a single wavelength light source with uniform phase:

• Laser light travels smoothly with very little dispersion.
• Makes it ideal for long-distance communication.

• For fiber optics with glass fibers, light in the infrared region is widely used which has wavelengths longer than visible light, typically of 850, 1300, and 1550 nm.
• Transmitters (Lasers or LEDs) emit in infrared regions and are used as a light source for fiber transmission.
• The laser acts as the best source in optical communication because it is a single wavelength light source.
• Ordinary light contains many different wavelengths of light, differences emerge in the speed of the transmission, reducing the number of signals that can be transmitted in any set time.
• Receivers (photodetectors) at these particular wavelengths are also easy to construct.

Notes:

• We use infrared because the attenuation of the fiber is much less at those wavelengths.
• The attenuation of glass optical fiber is caused by two factors, absorption, and scattering.
• Absorption occurs in several specific wavelengths called water bands due to the absorption by minute amounts of water vapor in the glass.

Optical communication utilizes the principle of total internal reflection by which light signals can be transmitted from one place to another with a negligible loss of energy

Fiber optic cable has many advantages over co-axial cable like high bandwidth, low loss, more security, etc.

The advantages of optical fibers are:

• Optical fibers have greater information-carrying capacity due to large bandwidth
• Optical fibers are free from electromagnetic interference and offer high signal security
• Optical fibers suffer less attenuation than coaxial cable and twisted wire cables.