CFOT - Certified Fiber Optic Technician Fiber Optic Theory Questions and Answers

Question 1

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)?

Accepted Answer

n1 must be greater than n2.

Answer

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.

Question 2

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:

Accepted Answer

Modal dispersion

Answer

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.

Question 3

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?

Accepted Answer

Rayleigh scattering

Answer

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.

Question 4

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)?

Accepted Answer

0.210

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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.

CFOT - Certified Fiber Optic Technician Practice Test

CFOT - Certified Fiber Optic Technician Practice Test

CFOT - Certified Fiber Optic Technician Fiber Optic Theory Questions and Answers