21. Constructive interference occurs when total phase change after two successive reflections at upper and lower interfaces is equal to? (Where m is an integer)
πm
2πm
πm/4
πm/6
Answer: 2. 2πm
Explanation:
Constructive interference occurs where the lines (representing peaks), cross over each other. In other words, when two waves are in phase, they interfere constructively. Destructive interference occurs where two waves are completely out of phase (a peak lies at the midpoint of two waves.
When measuring the reflectivity of a thin film on a substrate, we obtain constructive interference for 2δ = 2πm,
Where δ = Phase Shift
The component of phase waves which is in the x-direction is reflected in the interference between the higher and lower refractive index media. It is assumed that such an interference forms the lowest order standing wave, where the electric field is maximum at the center of the guide, decaying towards zero.
One of the best examples of constructive interference that may be observed in our day-to-day life is two speakers playing the same music while facing each other. At this time, music will appear louder and more powerful as compared to music played by a single speaker.
22. Destructive interference occurs when the maxima of two waves are _______ out of phase.
45 Degrees
90 Degrees
180 Degrees
120 Degrees
Answer: 3. 180 Degrees
Explanation:
Destructive interference occurs when the maxima of two waves are 180 degrees out of phase: a positive displacement of one wave is canceled exactly by a negative displacement of the other wave. The amplitude of the resulting wave is zero.
An example of destructive interference can be seen in. When the waves have opposite amplitudes at the point they meet they can destructively interfere, resulting in no amplitude at that point. For example, this is how noise-canceling headphones work.
23. When measuring the reflectivity of a thin film on a substrate destructive interference occurs when
δ = (2m + 1)π
2πm
2δ = (2m)π
2δ = (2m + 1)π
Answer: 4. 2δ = (2m + 1)π
Explanation:
Destructive interference occurs when the maxima of two waves are 180 degrees out of phase: a positive displacement of one wave is canceled exactly by a negative displacement of the other wave. The amplitude of the resulting wave is zero.
When measuring the reflectivity of a thin film on a substrate destructive interference occurs when 2δ = (2m + 1)π
Where
δ = Phase Shift
m = integer
An example of destructive interference can be seen in. When the waves have opposite amplitudes at the point they meet they can destructively interfere, resulting in no amplitude at that point. For example, this is how noise-canceling headphones work.
24. When light is described as an electromagnetic wave, it consists of a periodically varying electric E and magnetic field H which are oriented at an angle?
90 degrees to each other
Less than 90 degree
Greater than 90 degree
180 degree apart
Answer:1. 90 degrees to each other
Explanation:
When light is described as an electromagnetic wave, it consists of a periodically varying electric field E and magnetic field H that are oriented at right angles(90 degrees) to each other.
The traverse modes illustrate the case when the electric field is perpendicular to the direction of propagation and hence Ez = 0, but a corresponding component of the magnetic field H is in the direction of propagation.
In this instance, the modes are said to be transverse electric (TE).
Alternatively, when a component of the E field is in the direction of propagation, but Hz = 0, the modes formed are called transverse magnetic TM0.
In the case of electromagnetic waves which occur only in presence of both electric and magnetic fields, a particular change in the magnetic field will result in a proportional change in electric field and vice versa. These changes result in the formation of electromagnetic waves and for electromagnetic waves to occur both fields should be perpendicular to each other in direction of the wave travel.
25. A monochromatic wave propagates along a waveguide in the z-direction. These points of constant phase travel in constant phase travel at a phase velocity Vp is given by?
Vp=ω/c
Vp=ω/β
Vp=C/N
Vp=mass/acceleration
Answer:2. Vp=ω/β
Explanation:
The phase velocity of light is the velocity with which phase fronts propagate in a medium. It is related to the wavenumber k and the (angular) optical frequency ω
Velocity is a function of displacement. Phase velocity Vp can be concluded by keeping the phase a constant. Phase velocity Vp is a measure of angular velocity and it is given as
The group velocity of a wave is the velocity with which the overall envelope shape of the wave’s amplitudes known as the modulation or envelope of the wave propagates through space.
The wave packet does not travel at the phase velocity of the individual waves but is observed to move at a group velocity vg given by:
vg = δω/δβ
where
ω= angular frequency of the wave
β = propagation constant
The group velocity is of greatest importance in the study of the transmission characteristics of optical fibers as it relates to the propagation characteristics of observable wave groups or packets of light.
27. Multimode step-index fibers allow the propagation of a _______ number of guided modes along the channel.
Finite
Infinite
Equal
All of the above
Answer:1. Finite
Explanation:
Multimode step-index fibers allow the propagation of a finite number of guided modes along the channel.
The number of guided modes is dependent upon the physical parameters (i.e., relative refractive index difference, core radius) of the fiber and the wavelengths of the transmitted light, which are included in the normalized frequency V for the fiber.
The total number of guided modes or mode volume M, for a step-index fiber, is related to the V value for the fiber by the approximate expression:
M= V2/2
Where V = Normalize Frequency
It allows an estimate of the number of guided modes propagating in a particular multimode step-index fiber.
28. A multimode step-index fiber has a normalized frequency of 72. Estimate the number of guided modes.
a) 2846
b) 2592
c) 2432
d) 2136
Answer:2. 2592
Explanation:
Multimode step-index fibers allow the propagation of a finite number of guided modes along the channel.
The number of guided modes is dependent upon the physical parameters (i.e., relative refractive index difference, core radius) of the fiber and the wavelengths of the transmitted light, which are included in the normalized frequency V for the fiber.
The total number of guided modes or mode volume M, for a step-index fiber, is related to the V value for the fiber by the approximate expression:
M= V2/2
Here M denotes the number of modes and V denotes normalized frequency.
M = (72)2/2 = 5184/2
M = 2592
29. Skew rays follow a ___________
a) Hyperbolic path along the axis
b) Parabolic path along the axis
c) Helical path
d) Path where rays change angles at the core-cladding interface
Answer: 3. Helical path
Explanation:
Skew rays do not pass through the fiber axis. They follow the helical path in the fiber core.
The skew ray is reflected at the edges of the fiber core with an angle of 2γ.
The number of reflections of the skew rays depends upon the point of incidence and angle of incidence in the core.
If the input light is nonuniform, then skew rays follow smooth reflections, and more uniform output is obtained.
30. A graded-index fiber has a core with a parabolic refractive index profile of diameter of 30μm, NA=0.2, λ=1μm. Estimate the normalized frequency.
a) 19.32
b) 18.84
c) 16.28
d) 17.12
Answer: 2. 18.84
Explanation:
Normalized frequency is a unit of measurement of frequency equivalent to cycles/samples.
Normalized frequency for a graded-index fiber is given by
