# Optical Communications Questions and Answers – Fiber Cutoff Wavelength Measurements

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This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Fiber Cutoff Wavelength Measurements”.

1. A multimode fiber has many cutoff wavelengths.
a) False
b) True

Explanation: A multimode fiber has many cutoff wavelengths. It is because the number of bound propagating modes is usually large.

2. What does ‘a’ stands for in the given equation?

Mg=(πa/λ)2(n12 - n22)

b) Constant
c) Coefficient of refraction
d) Density

Explanation: The above equation gives the number of guided modes for a parabolic refractive index graded fiber, where a is the core radius and n1, n2 are the core and cladding indices respectively.

3. The _________ wavelength is defined as the wavelength greater than which the ratio of the total power and the fundamental mode power has to be decreased to less than 0.1dB.
a) Magnetic
b) Quasi
c) Cut-off
d) EIA

Explanation: The cut off wavelength is usually measured by increasing the signal wavelength in a fixed length of fiber until the mode is undetectable. It is usually called a effective cut-off wavelength.

4. How many methods are used to determine the effective cutoff wavelength?
a) Two
b) One
c) Four
d) Three

Explanation: Three methods are usually used for the determination of the effective cutoff wavelength. These are bending-reference technique, power step method and alternative test method.

5. What is the name of the test used to determine the efficient values of the effective cutoff wavelength?
a) Round robin test
b) Mandarin test
c) Hough Werner test
d) Fulton test

Explanation: Round robin test is an effective method to determine the efficient values for the cutoff wavelength. It shows that in some methods, the values through round robin test are the same.

6. The effective cutoff wavelength for a cabled single mode fiber will be generally smaller than that of the un-cabled fiber.
a) True
b) False

Explanation: The effective cutoff wavelength for a cabled single mode fiber is always smaller than that of the un-cabled fiber. This is usually because of the bend effects.

7. How many bend effects are produced in the fiber?
a) One
b) Three
c) Two
d) Four

Explanation: Usually, two bend effects are produced. They are macro-bending and micro-bending. These effects incarcerate certain changes in the fiber efficiency.

8. _______________ method does not require a leaky mode correction factor or equal mode excitation.
a) Bending-reference
b) Power step method
c) Alternative test method
d) Refracted near-field method

Explanation: Refracted near-field method is complementary to the transmitted near-field method. It has the advantage that it does not require a leaky mode correction factor. Moreover, it provides the relative refractive index directly without recourse to external calibration.

9. The _______ method is the most commonly used method for the determination of the fiber refractive index profile.
a) Refracted near-field method
b) Bending-reference
c) Power step method
d) Alternative test method