Fiber Optic Theory Flashcards

For total internal reflection to occur within an optical fiber, what must be the relationship between the refractive index of the core (n1) and the cladding (n2)?

Answer: n1 must be greater than n2.

Total internal reflection is the guiding principle of light in an optical fiber. This phenomenon only happens when light travels from a medium with a higher refractive index to a medium with a lower refractive index at an angle of incidence greater than the critical angle. Therefore, the core must have a higher refractive index than the cladding (n1 > n2) to trap the light signal within the core.

A technician is working with a multimode fiber and notices that a light pulse is spreading out as it travels down the cable, causing the received signal to be distorted. This phenomenon, caused by different light rays taking slightly different paths of different lengths, is known as:

Answer: Modal dispersion

Modal dispersion occurs in multimode fibers because light rays (modes) enter the fiber at different angles and travel along different paths. Rays that travel a more direct path arrive sooner than those that reflect more and travel a longer path. This difference in arrival time causes the light pulse to spread, a phenomenon known as modal dispersion. Single-mode fiber eliminates this issue as it only allows one mode to propagate.

Which of the following is a primary cause of intrinsic attenuation in an optical fiber, resulting from microscopic variations in the density of the glass?

Answer: Rayleigh scattering

Rayleigh scattering is an intrinsic property of the glass fiber material itself. It is caused by small-scale, microscopic variations in the density and composition of the silica glass, which scatters the light signal. This scattering is a fundamental loss mechanism and contributes significantly to attenuation, especially at shorter wavelengths.

A fiber optic cable has a core with a refractive index of 1.480 and a cladding with a refractive index of 1.465. What is the cable's Numerical Aperture (NA)?

Answer: 0.210

The Numerical Aperture (NA) is calculated using the formula: NA = √(n1² - n2²), where n1 is the core's refractive index and n2 is the cladding's refractive index. For this question: NA = √(1.480² - 1.465²) = √(2.1904 - 2.146225) = √0.044175 ≈ 0.210. The NA represents the light-gathering ability of the fiber.

When comparing single-mode and multimode fiber, which statement is generally true?

Answer: Single-mode fiber has a smaller core diameter, allowing it to carry signals over longer distances with higher bandwidth.

Single-mode fiber has a very small core diameter (typically around 9 µm) which allows only one mode of light to propagate. This eliminates modal dispersion, resulting in lower signal loss and allowing for much higher bandwidth over longer distances compared to multimode fiber. Multimode fiber has a larger core (50 µm or 62.5 µm) and is typically used for shorter-distance applications.

Dispersion in an optical fiber causes pulse broadening. Which type of dispersion is caused by different wavelengths of light traveling at different velocities through the glass material?

Answer: Chromatic dispersion

Chromatic dispersion occurs because the refractive index of the glass fiber is slightly dependent on the wavelength of the light. Since light sources are not perfectly monochromatic and emit a small range of wavelengths, these different wavelengths travel at slightly different speeds, causing the light pulse to spread out over time. This is a limiting factor in high-speed, long-distance systems.