51. A single-mode fiber has a beat length of 4cm at 1200nm. What is birefringence?
2 × 10-5
1.2 × 10-5
3 × 10-5
2 × 10-6
Answer: 3. 3 × 10-5
Explanation:
Birefringence is defined as
Bf = (ny – nx) = λ/Lp
Here,
λ=wavelength = 12 × 10−1
Lp = beat length = 4cm = 4 × 10-4
Bf = 12 × 10−1/ 4 × 10-4
Bf = 3 × 10-5
52. How many propagation modes are present in single-mode fibers?
One
Two
Three
Five
Answer: 2. Two
Explanation:
For a given optical fiber, the number of modes depends on the dimensions of the cable and the variations of the indices of refraction of both core and cladding across the cross-section.
In any ordinary single-mode fiber there are two independent, degenerate propagation modes. These modes are very similar, but their polarization planes are orthogonal. These may be chosen arbitrarily as the horizontal (H) and the vertical (V) polarizations.
Either one of these two polarization modes constitutes the fundamental HE11 mode. In general, the electric field of the light propagating along the fiber is a linear superposition of these two polarization modes and depends on the polarization of the light at the launching point into the fiber.
53. The main advantages of single-mode fiber are?
Higher speed
No Modal Dispersion
Higher Data Rate
All of the above
Answer:4. All of the above
Explanation:
Advantages of single Mode Fiber
Single-mode fiber does not face modal dispersion, modal noise, and other effects that arise with the multimode transmission. They are free from intermodal dispersion. The intermodal dispersion is the time difference between the entry of the pulse at one end of optical fiber and the arrival of the pulse at the other end of the fiber.
Single-mode fiber cable carries signals at much higher speeds than multimode fibers. They are the standard choice for high data rates or long-distance telecommunications (> a couple of Kms) which use diodes as the source.
Single-mode systems with a 10-µm core are found in long-distance telecommunications applications where ultimate performance is the leading criteria. The most important optical parameters for signal analysis are the operating wavelength, the attenuation in dB/km, and the chromatic dispersion.
Single-mode fibers do not suffer from modal dispersion (because there’s only one mode of propagation). Likewise, single-mode fiber links do not suffer from differential mode delay, modal noise, or mode partition noise.
Laser-diode RIN noise is less of an issue for single-mode fiber than for multimode because of the generally higher quality of single-mode connectors, which admit fewer Reflection.
54. The main disadvantage of single-mode fiber over multimode fiber is
Difficult to achieve end to end connection
Expensive equipements
Difficult to launch light
All of the above
Answer: 3. Hondros and debye
Explanation:
Disadvantages of Single-mode fiber
Due to the smaller size of the core in single-mode fiber, coupling light requires many tolerances than the coupling light in multimode fiber. Recent developments indicate that these tighter tolerances can be achievable.
Single-mode fiber components and equipment are very expensive and hence multimode counter parts are widely used.
It is difficult to launch the light through the fiber.
It is difficult to achieve end-to-end connections of similar fibers.
The prime disadvantage of single-mode fiber is the difficulty of launching signals into it. A single-mode fiber core spans only 10 µm, much less than a multimode fiber core. LED sources generally do not have a narrow enough beamwidth to couple into such a small core area.
The only choice for single-mode sources is a pinpoint source, like a laser diode. Unfortunately, laser-diodes and their associated output power control circuitry are significantly more expensive than LED sources.
The small core diameter of a single-mode fiber and its attendant requirements for mechanical precision in all coupling components also increase the cost of cable connectors, the cost of optical packages, and the amount of time and energy required in the field to install, test, and maintain the optical connections. Single-mode systems cost big bucks.
55. Multimode Fiber carries more than _______
One mode
Two mode
Three Mode
Ten Mode
Answer:1. One mode
Explanation:
Fibers that carry more than one mode are called multimode fibers. Multimode fiber has a bigger diameter. The range is around 50 to 100 microns but 62.5 micrometers is usually used. In most of the applications, two fibers are used in multimode fibers and Wavelength Division Multiplexing is not normally used on multi-mode fibers. Plastic optic fibers promise performances similar
A multi-mode cable has a larger core than single-mode fiber. This core allows light to flow on multiple paths along with the core, allowing a small impulse light pulse to be smeared into a wide out- put pulse. This dispersion degrades the rate at which light pulses can be transmitted.
56. Wavelength Division Multiplexing is not normally used in ________
Single Mode Fiber
Multimode Fiber
Both 1 and 2
None of the above
Answer:1. Multimode Fiber
Explanation:
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology that multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light.
This technique enables bidirectional communications over a single strand of fiber, also called wavelength-division duplexing, as well as multiplication of capacity.
Wavelength Division Multiplexing is not normally used in Multimode Fiber. Most WDM systems operate on single-mode fiber optic cables which have a core diameter of 9 µm.
57. The diameter range of Multimode fiber is _____
10 to 20 micron
20 to 30 micron
40 to 50 micron
50 to 100 micron
Answer:1. Multimode Fiber
Explanation:
Fibers that carry more than one mode are called multimode fibers. Multimode fiber has a bigger diameter. The range is around 50 to 100 microns but 62.5 micrometers is usually used. In most applications, two fibers are used | in multimode fibers and Wavelength Division Multiplexing is not normally used on multi-mode fibers.
58. Multimode fiber is commonly used in ______
Short Distance
Long Distance
Medium Distance
All of the above
Answer:1. Short Distance
Explanation:
Multimode fiber is commonly used in short-distance (usually less than 2 km) communication applications. Because signal quality drops as distance increases, multimode fiber is generally used for short-distance communications across local area networks (LANs).
59. The core size of Multimode fiber is _____ as compared to single-mode Fiber
Small
Large
Equal
None of the above
Answer:2. Large
Explanation:
Multimode fiber’s core size is larger than single-mode fiber and is, therefore, easier to connect, with greater tolerance of components with less precision.
Multimode fibers can propagate over 100 modes. The number of modes propagated depends on the core size and numerical aperture (NA). As the core size and NA increase. the number of modes increases. Typical values of fiber core size and NA are 50 to 100 um and 0.20 to 0.29. respectively.
60. Which of the following are the types of Multimode Fiber?
Step Index Multimode Fiber
Graded Step Index Multimode Fiber
Both 1 and 2
None of the above
Answer:3. Both 1 and 2
Explanation:
There are two types of multimode fibers:
(I) step-index fiber
(ii) graded-index fiber.
They differ by the index of refraction profiles of their core and cladding.
Step Index Fiber:- A step-index fiber is an optical fiber that has a core and cladding with different but uniform refractive indexes.
Graded Index Fiber:- Graded index multimode fiber is a type of optical fiber where the refractive index is higher at the axis of the core and then it decreases gradually towards the core-cladding interface. That is to say, the refractive index of a graded-index fiber gradually decreases from its center, and eventually decreases to the same value as the cladding at the core edge.
