1What is the fundamental principle behind light propagation in an optical fiber?
A.Diffraction
B.Total Internal Reflection
C.Polarization
D.Refraction
Correct Answer: Total Internal Reflection
Explanation:
Optical fibers work on the principle of Total Internal Reflection (TIR), where light traveling from a denser medium (core) to a rarer medium (cladding) is reflected back into the denser medium if the angle of incidence exceeds the critical angle.
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2In an optical fiber, what is the relationship between the refractive index of the core () and the refractive index of the cladding ()?
A.
B.
C.
D.
Correct Answer:
Explanation:
For Total Internal Reflection to occur, light must travel from a medium of higher refractive index to a medium of lower refractive index. Therefore, the core index must be greater than the cladding index .
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3Which component of the optical fiber protects the core and cladding from moisture and mechanical damage?
A.Cladding
B.Core
C.Buffer jacket
D.Sheath
Correct Answer: Buffer jacket
Explanation:
The buffer jacket (or coating) is a plastic coating that protects the fiber from moisture, physical damage, and absorbs mechanical shock.
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4Why is an optical fiber considered a dielectric waveguide?
A.It conducts electricity
B.It generates magnetic fields
C.It is made of non-conducting transparent materials
D.It is made of metal
Correct Answer: It is made of non-conducting transparent materials
Explanation:
Optical fibers are made of dielectric (non-conducting) materials like glass (silica) or plastic, which guide electromagnetic waves (light) through them.
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5The critical angle at the core-cladding interface is given by which formula?
A.
B.
C.
D.
Correct Answer:
Explanation:
From Snell's law, at the critical angle, the angle of refraction is . , leading to .
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6What is the 'Acceptance Angle' of an optical fiber?
A.The angle at which the fiber breaks
B.The critical angle inside the core
C.The angle of refraction in the cladding
D.The maximum angle of incidence at the input end for which light is propagated
Correct Answer: The maximum angle of incidence at the input end for which light is propagated
Explanation:
The acceptance angle is the maximum angle relative to the fiber axis at which light may enter the fiber to be guided by total internal reflection.
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7Calculate the Numerical Aperture (NA) of a fiber with core refractive index and cladding refractive index .
A.0.244
B.0.156
C.1.49
D.0.02
Correct Answer: 0.244
Explanation:
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8The Numerical Aperture (NA) represents which characteristic of the optical fiber?
A.The speed of light in the fiber
B.The attenuation of the fiber
C.The light gathering power of the fiber
D.The diameter of the core
Correct Answer: The light gathering power of the fiber
Explanation:
Numerical Aperture is a measure of the ability of the fiber to collect or gather light. A higher NA implies a larger acceptance angle and more light gathering capacity.
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9The fractional refractive index change is defined as:
A.
B.
C.
D.
Correct Answer:
Explanation:
The relative refractive index difference (or fractional change) is defined as .
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10Which relation correctly connects Numerical Aperture (NA) and fractional refractive index change ()?
A.
B.
C.
D.
Correct Answer:
Explanation:
Since and , approximating gives .
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11What is the V-number (or normalized frequency) of an optical fiber?
A.The velocity of light in the core
B.A measure of fiber attenuation
C.The voltage drop across the fiber
D.A parameter determining the number of modes
Correct Answer: A parameter determining the number of modes
Explanation:
The V-number is a dimensionless parameter that determines the number of modes a fiber can support. It depends on the fiber dimensions, refractive indices, and wavelength.
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12The formula for the V-number is given by:
A.
B.
C.
D.
Correct Answer:
Explanation:
The V-number is defined as , where is the core radius.
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13For a Step Index fiber to support only a single mode, the V-number must be:
A.
B.
C.
D.
Correct Answer:
Explanation:
The cut-off value for single-mode operation in a step-index fiber is . If is less than this value, only the fundamental mode propagates.
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14What is the approximate number of modes () in a Step Index Multimode fiber for a large V-number?
A.
B.
C.
D.
Correct Answer:
Explanation:
For a step-index multimode fiber with a large V-number, the total number of guided modes is approximately .
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15What distinguishes a Step Index fiber from a Graded Index (GRIN) fiber?
A.Step index is made of plastic; GRIN is made of glass.
B.GRIN fibers cannot support total internal reflection.
C.Step index has no cladding.
D.Step index has a uniform core refractive index; GRIN has a varying core index.
Correct Answer: Step index has a uniform core refractive index; GRIN has a varying core index.
Explanation:
In Step Index fibers, the refractive index is constant throughout the core and changes abruptly at the cladding. In Graded Index fibers, the refractive index decreases continuously from the axis to the cladding interface.
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16In a Graded Index (GRIN) fiber, the trajectory of a light ray is:
A.Zig-zag straight lines
B.Random
C.Strictly straight along the axis
D.Sinusoidal or helical
Correct Answer: Sinusoidal or helical
Explanation:
Due to the continuous variation of the refractive index in a GRIN fiber, light rays are continuously refracted, resulting in a curved (sinusoidal) path rather than zig-zag reflection.
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17What is the primary advantage of Graded Index fibers over Multimode Step Index fibers?
A.Higher tensile strength
B.Reduced intermodal dispersion
C.Easier splicing
D.Lower cost
Correct Answer: Reduced intermodal dispersion
Explanation:
The varying refractive index in GRIN fibers allows rays traveling different path lengths to travel at different speeds, arriving at the output at nearly the same time, thus significantly reducing intermodal dispersion (pulse spreading).
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18The number of modes in a Graded Index fiber is approximately:
A.
B.
C.
D.
Correct Answer:
Explanation:
For a parabolic refractive index profile in a GRIN fiber, the number of modes is approximately half that of a step-index fiber with the same parameters, i.e., .
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19If the core diameter of a fiber is reduced significantly (to near the wavelength of light), it becomes a:
A.Single Mode fiber
B.Multimode Graded Index fiber
C.Plastic Optical fiber
D.Multimode Step Index fiber
Correct Answer: Single Mode fiber
Explanation:
Small core diameters (typically 8-10 microns) restrict the fiber to support only the fundamental mode, making it a Single Mode fiber.
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20Which of the following causes signal attenuation in optical fibers?
A.Total Internal Reflection
B.Absorption, Scattering, and Bending
C.Refraction
D.Dispersion
Correct Answer: Absorption, Scattering, and Bending
Explanation:
Attenuation (loss of signal power) is primarily caused by material absorption, Rayleigh scattering, and bending losses (macro and micro-bending).
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21Rayleigh scattering loss in optical fibers is inversely proportional to:
A.
B.
C.
D.
Correct Answer:
Explanation:
Rayleigh scattering, caused by microscopic density fluctuations, scales as . This implies shorter wavelengths scatter much more than longer wavelengths.
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22Attenuation () in optical fibers is usually expressed in units of:
A.Joules
B.dB/km
C.Decibels (dB)
D.Watts
Correct Answer: dB/km
Explanation:
Attenuation is the rate of loss of signal power per unit length, expressed in decibels per kilometer (dB/km).
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23If the input power is and output power is over a length , the attenuation is given by:
A.
B.
C.
D.
Correct Answer:
Explanation:
The standard formula for attenuation in dB/km is .
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24What is 'Dispersion' in the context of optical fibers?
A.The breaking of the fiber
B.The broadening of light pulses as they travel
C.The reflection of light at the cladding
D.The loss of optical power
Correct Answer: The broadening of light pulses as they travel
Explanation:
Dispersion refers to the spreading of optical pulses in time as they propagate along the fiber, which limits the bandwidth and information-carrying capacity.
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25Which type of dispersion is dominant in Multimode Step Index fibers?
A.Material dispersion
B.Polarization mode dispersion
C.Waveguide dispersion
D.Intermodal dispersion
Correct Answer: Intermodal dispersion
Explanation:
Intermodal dispersion occurs because different modes travel different path lengths (zig-zag angles), causing them to arrive at different times. This is most significant in Multimode Step Index fibers.
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26A fiber has an acceptance angle of in air. What is its Numerical Aperture?
A.0.500
B.0.866
C.0.577
D.1.00
Correct Answer: 0.500
Explanation:
.
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27Which loss mechanism is caused by the presence of hydroxyl () ions in the silica material?
A.Absorption loss
B.Waveguide dispersion
C.Rayleigh scattering
D.Microbending
Correct Answer: Absorption loss
Explanation:
Impurity absorption, specifically caused by water molecules ( ions) trapped in the glass, creates specific absorption peaks in the transmission spectrum.
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28Microbending loss is caused by:
A.Bending the fiber into a coil
B.Wrapping the fiber around a spool
C.Improper splicing
D.Microscopic irregularities in the core-cladding interface
Correct Answer: Microscopic irregularities in the core-cladding interface
Explanation:
Microbending losses arise from microscopic imperfections or small perturbations in the fiber geometry/interface that cause mode coupling and radiation of power.
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29Macrobending loss occurs when:
A.The fiber is bent with a radius of curvature comparable to the fiber diameter
B.The fiber has impurities
C.The fiber is straight
D.The fiber is bent with a large radius relative to the fiber diameter
Correct Answer: The fiber is bent with a radius of curvature comparable to the fiber diameter
Explanation:
Macrobending refers to large-scale bends (visible to the eye). If the bend radius is small enough (tighter bend), the angle of incidence at the bend may fall below the critical angle, causing light leakage.
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30What is the refractive index profile of a Step Index fiber?
A.Exponentially increasing in the core
B.Linearly decreasing in the core
C.Parabolic in the core
D.Constant in the core, abrupt change at cladding
Correct Answer: Constant in the core, abrupt change at cladding
Explanation:
A Step Index fiber has a uniform refractive index throughout the core, which drops strictly to at the core-cladding boundary.
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31Meridional rays in an optical fiber:
A.Never pass through the fiber axis
B.Are absorbed by the cladding
C.Always pass through the fiber axis
D.Travel in a helical path
Correct Answer: Always pass through the fiber axis
Explanation:
Meridional rays are rays that are confined to a plane containing the optical axis of the fiber and always cross the axis during propagation.
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32Skew rays in an optical fiber:
A.Do not intersect the fiber axis and follow a helical path
B.Are not guided by the fiber
C.Intersect the fiber axis
D.Only exist in Single Mode fibers
Correct Answer: Do not intersect the fiber axis and follow a helical path
Explanation:
Skew rays travel through the fiber without intersecting the optical axis, following a helical-like path around the axis.
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33Which of the following is a key application of optical fibers in medicine?
A.Endoscopy
B.Ultrasound
C.MRI scans
D.X-rays
Correct Answer: Endoscopy
Explanation:
Fiber optics are used in endoscopes (fiberoscopes) to transmit light into and images out of the body for minimally invasive viewing of internal organs.
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34Why are optical fibers immune to Electromagnetic Interference (EMI)?
A.They transmit photons (light) not electrons
B.They operate at low frequencies
C.They have high resistance
D.They are shielded by lead
Correct Answer: They transmit photons (light) not electrons
Explanation:
Since optical fibers use light signals and constitute a dielectric (non-conducting) medium, they are not affected by electromagnetic fields, radio waves, or lightning.
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35The transmission bandwidth of a Single Mode fiber is:
A.Zero
B.Higher than Multimode fibers
C.Lower than Multimode Step Index fiber
D.Lower than Multimode Graded Index fiber
Correct Answer: Higher than Multimode fibers
Explanation:
Single mode fibers eliminate intermodal dispersion, allowing for extremely high bandwidths and long-distance transmission compared to multimode fibers.
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36Given and , calculate the Numerical Aperture.
A.0.300
B.0.015
C.0.150
D.0.212
Correct Answer: 0.212
Explanation:
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37Which wavelength window is commonly used in modern long-distance optical fiber communication due to minimum attenuation?
A.400 nm
B.850 nm
C.1550 nm
D.633 nm
Correct Answer: 1550 nm
Explanation:
1550 nm is the wavelength region where silica fibers exhibit the lowest attenuation (approx 0.2 dB/km).
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38What is the function of a fiber optic sensor?
A.To sense physical quantities like temperature or pressure by modulating light properties
B.To convert light to electricity only
C.To amplify light signals
D.To store data
Correct Answer: To sense physical quantities like temperature or pressure by modulating light properties
Explanation:
Fiber optic sensors use the fiber to detect changes in external parameters (temperature, pressure, strain) which modulate the intensity, phase, or polarization of the light passing through.
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39An active fiber optic sensor:
A.Cannot measure temperature
B.Uses the fiber only to guide light to a remote sensor
C.Uses the fiber itself as the sensing element
D.Does not require a light source
Correct Answer: Uses the fiber itself as the sensing element
Explanation:
Also known as intrinsic sensors, active sensors use the optical fiber itself to sense the perturbation, whereas passive (extrinsic) sensors use the fiber only to transport light.
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40If the V-number of a step index fiber is 10, the approximate number of modes is:
A.100
B.50
C.20
D.10
Correct Answer: 50
Explanation:
For a step index fiber, the number of modes .
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41Intramodal dispersion consists of:
A.Absorption and Reflection
B.Intermodal and Material dispersion
C.Material and Waveguide dispersion
D.Bending and Scattering
Correct Answer: Material and Waveguide dispersion
Explanation:
Intramodal (chromatic) dispersion occurs within a single mode and is composed of material dispersion (refractive index varies with ) and waveguide dispersion (mode propagation constant varies with ).
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42Which material is most commonly used for the core of low-loss optical fibers?
A.Copper
B.Pure Silica () doped with Ge or P
C.Plastic
D.Diamond
Correct Answer: Pure Silica () doped with Ge or P
Explanation:
Silica glass doped with Germanium or Phosphorus is used to increase the refractive index of the core while maintaining low loss.
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43The cone of light that can be accepted by the fiber is called the:
A.Dispersion cone
B.Acceptance cone
C.Refraction cone
D.Critical cone
Correct Answer: Acceptance cone
Explanation:
The acceptance cone is the solid angle corresponding to the acceptance angle rotated around the fiber axis.
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44In the context of fiber optics, what does 'Index Profile' refer to?
A.The variation of light intensity
B.The length of the fiber
C.The variation of refractive index with radial distance from the axis
D.The manufacturing cost
Correct Answer: The variation of refractive index with radial distance from the axis
Explanation:
The index profile describes how the refractive index changes as a function of the distance from the center of the fiber core.
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45Calculate the critical angle for a fiber with core index 1.5 and cladding index 1.45.
A.
B.
C.
D.
Correct Answer:
Explanation:
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46Which type of fiber is best suited for short-distance, low-bandwidth data links?
A.Plastic Optical Fiber (POF)
B.Photonic Crystal Fiber
C.Dispersion Shifted Fiber
D.Single Mode Silica Fiber
Correct Answer: Plastic Optical Fiber (POF)
Explanation:
Plastic fibers have higher attenuation and lower bandwidth but are cheaper, more robust, and easier to connect, making them ideal for short-range applications (e.g., within a car or home audio).
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47If the numerical aperture of a fiber is increased, what happens to the bandwidth?
A.It becomes infinite
B.It increases
C.It remains the same
D.It decreases
Correct Answer: It decreases
Explanation:
A higher NA means more modes are accepted (in multimode fiber), which increases intermodal dispersion, thereby reducing the bandwidth.
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48A fiber has a core diameter of and . It operates at . What is the V-number?
A.31.4
B.62.8
C.15.7
D.10
Correct Answer: 31.4
Explanation:
(Note: using diameter in formula ).
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49Total internal reflection occurs only when the angle of incidence is:
A.Zero
B.Less than the critical angle
C.Greater than the critical angle
D.Equal to the critical angle
Correct Answer: Greater than the critical angle
Explanation:
For TIR to occur, the incident ray must strike the boundary at an angle greater than the critical angle.
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50Which of the following acts as a displacement sensor using fiber optics?
A.Microbending sensor
B.Photodiode
C.Laser diode
D.Thermocouple
Correct Answer: Microbending sensor
Explanation:
A microbending sensor detects displacement or pressure by measuring the loss of light caused by bending the fiber between corrugated plates.