81. Photonic crystal fibers also called as ___________
Conventional fibers
Dotted fibers
Stripped fibers
Holey fibers
Answer: 4. Holey fibers
Explanation:
In the early 1990s researchers envisioned and demonstrated a new optical fiber structure. Initially, this was called a holey fiber and later became known as a photonic crystal fiber (PCF) or a microstructured fiber. The difference between this new structure and that of a conventional fiber is that the cladding and, in some cases, the core regions of a PCF contain air holes, which run along the entire length of the fiber.
82. The periodic arrangement of cladding air holes in photonic bandgap fibers provides for the formation of a photonic bandgap in the _________
H-plane of fiber
E-plane of fiber
E-H-plane of fiber
Transverse plane of the fiber
Answer: 1. Better in short distance application
Explanation:
Photonic bandgap (PBG) fibers are a class of microstructured fiber in which a periodic arrangement of air holes is required to ensure guidance.
This periodic arrangement of cladding air holes provides for the formation of a photonic bandgap in the transverse plane of the fiber.
As a PBG fiber exhibits a two-dimensional bandgap, then wavelengths within this bandgap cannot propagate perpendicular to the fiber axis (i.e., in the cladding) and they can therefore be confined to propagate within a region in which the refractive index is lower than the surrounding material.
Hence, utilizing the photonic bandgap effect light can, for example, be guided within a low-index, air-filled core region creating fiber properties quite different from those obtained without the bandgap.
83. In index-guided photonic crystal fiber structure, the dark areas are air holes. What do white areas suggest?
Air
Silica
Water
Plasma
Answer: 1. Plasma
Explanation:
A photonic crystal is a low-loss periodic dielectric medium constructed using a periodic array of microscopic air holes that run along the entire fiber length.
Index-guided photonic crystal fibers have greater index contrast because the cladding contains air-holes having refractive index 1. Both indexes guided and conventional fibers arise from the manner in which guided mode interacts with the cladding region.
84. Conventional optical fibers have more transmission losses than _______
Plastic Fiber
Glass Fiber
Photonic Crystal Fiber
Active Glass Fiber
Answer: 1. Better in short distance application
Explanation:
Conventional optical fibers have several hundreds of losses in transmission. Photonic crystal fibers have resulted in a reduction in overall transmission losses which were initially several hundred decibels per kilometer even with the most straightforward designs.
Increased control over the homogeneity of the fiber structures together with the use of highly purified silicon as the base material has now lowered these losses to a level of a very few decibels per kilometer for most PCF types, with a loss of just 0.3 dB km−1 at 1.55 µm for a 100 km span being recently reported.
In conventional optical fibers, electromagnetic modes are guided by total internal reflection in the core region, which has a slightly raised refractive index.
The existence of two different guidance mechanisms makes PCFs versatile in their range of potential applications.
85. Losses in photonic crystal fibers are reduced to a level of ___________
a) 0.1dB/km
b) 0.2dB/km
c) 0.3dB/km
d) 0.4dB/km
Answer: 1. Better in short distance application
Explanation:
Conventional optical fibers have several hundreds of losses in transmission. Photonic crystal fibers have resulted in a reduction in overall transmission losses which were initially several hundred decibels per kilometer even with the most straightforward designs.
Increased control over the homogeneity of the fiber structures together with the use of highly purified silicon as the base material has now lowered these losses to a level of a very few decibels per kilometer for most PCF types, with a loss of just 0.3 dB km−1 at 1.55 µm for a 100 km span being recently reported.
86. The high index contrast enables the PCF core to be reduced from around 8 μmin conventional fiber to ___________
a) Less than 1μm
b) More than 5μm
c) More than 3μm
d) More than 2μm
Answer: 1. Less than 1μm
Explanation:
The high index contrast enables the PCF core to be reduced from around 8 µm in conventional fiber to less than 1 µm, which increases the intensity of the light in the core and enhances the nonlinear effects.
The principles of guidance and the characteristics of index-guided PCFs are similar to those of conventional fiber, there is greater index contrast since the cladding contains air holes with a refractive index of 1 in comparison with the normal silica cladding index of 1.457 which is close to the germanium-doped core index of 1.462.
87. Which is the unit of measurement of attenuation in optical fibers?
a) km
b) dB
c) dB/km
d) Coulomb’s
Answer: 3. dB/km
Explanation:
Attenuation is the reduction in the power of the light signal as it is transmitted. Attenuation is caused by passive media components such as cables, cable splices, and connectors.
Attenuation is also referred to as transmission loss. Channel attenuation helped to determine the maximum transmission distance prior to signal restoration. Attenuation is usually expressed in the logarithmic unit of decibel. It is given by
αdBL = 10 log10Pi / Po
Where
αdB = signal attenuation per unit length
Pi & Po = Input and output power.
88. The cause of attenuation in Optical fiber is
Absorption
Scattering
Both 1 and 2
None of the above
Answer:3. Both 1 and 2
Explanation:
The attenuation of the optical fiber is a result of two factors, absorption, and scattering. The absorption is caused by the absorption of the light and conversion to heat by molecules in the glass.
Primary absorbers are residual OH+ and dopants used to modify the refractive index of the glass. This absorption occurs at discrete wavelengths, determined by the elements absorbing the light. The OH+ absorption is predominant and occurs most strongly around 1000 nm, 1400 nm, and above 1600 nm.
The largest cause of attenuation is scattering. Scattering occurs when light collides with individual atoms in the glass and is anisotropic.
The light that is scattered at angles outside the numerical aperture of the fiber will be absorbed into the cladding or transmitted back toward the source Scattering is also a function of wavelength, proportional to the inverse fourth power of the wavelength of the light.
Thus if you double the wavelength of the light, you reduce the scattering losses by 2 to the 4th power or 16 times. Therefore, for long-distance transmission, it is advantageous to use the longest practical wavelength for minimal attenuation and maximum distance between repeaters.
89. The optical fiber incurs a loss in signal power as light travels down the fiber which is called as _________
a) Scattering
b) Attenuation
c) Absorption
d) Refraction
Answer: 2. Attenuation
Explanation:
When a signal is transmitted from transmitter to receiver via optical fiber medium in an optical fiber link, various types of loss mechanisms are responsible for signal degradation. This is termed as attenuation.
90. The measurement of optical Fiber attenuation is done by
Cutback Method
Sideway Method
Directi Method
Indirect Method
Answer: 1. Cutback Method
Explanation:
The most accurate way of measuring the fiber attenuation coefficient requires transmitting light of a known wavelength through the fiber and measuring the changes over distance. The conventional method, known as the cutback method, involves coupling fiber to the source and measuring the power out of the far end. The fiber is then cut near the source and power measured again.
