A telecom engineer designing a transatlantic undersea cable must choose between multimode and single-mode fiber. Which choice is correct, and why?
AMultimode fiber; its larger core makes alignment easier and more reliable over long distances
BSingle-mode fiber; its single propagation path eliminates modal dispersion, preserving signal integrity over thousands of kilometers
CStep-index multimode fiber; the sharp index boundary provides stronger total internal reflection than graded designs
DGraded-index fiber; its continuously varying refractive index eliminates the need for cladding altogether
Single-mode fiber is the universal choice for long-haul links because its tiny core (~8–10 μm) permits only one propagation mode, eliminating modal dispersion entirely. Multimode fibers support many ray paths that arrive at different times, spreading pulses over long distances until the signal becomes unreadable. Graded-index reduces (but does not eliminate) this spreading and suits campus-length runs. All fiber types require cladding — option 3 misunderstands the basic design.
Question 2 Multiple Choice
What causes modal dispersion in a step-index multimode fiber?
AThe cladding selectively absorbs certain wavelengths, attenuating higher-order modes more than lower-order ones
BDifferent rays travel at different angles and thus traverse different path lengths, arriving at the far end spread out in time
DImpurities in the silica core scatter light into random directions, creating multiple arrival times
In a step-index fiber, total internal reflection preserves all ray angles — rays entering at steeper angles bounce more frequently and travel longer zigzag paths than rays traveling near the axis. These different paths (modes) arrive at the far end at slightly different times, broadening the pulse. Graded-index fiber compensates by varying the refractive index so off-axis rays pass through lower-density material and travel faster, nearly equalizing arrival times.
Question 3 True / False
In an optical fiber, the cladding should have a higher refractive index than the core to ensure total internal reflection occurs at the boundary.
TTrue
FFalse
Answer: False
This is backwards. Total internal reflection requires light to travel from a denser medium (higher refractive index) toward a less dense medium (lower refractive index) at an angle exceeding the critical angle. Therefore the core must have the higher refractive index and the cladding must have the lower one. If the cladding were denser than the core, light would refract outward at the boundary rather than reflect inward, and the fiber would not guide light.
Question 4 True / False
A graded-index fiber reduces modal dispersion compared to a step-index fiber because rays traveling near the outer edge of the core move faster through the lower-index material, compensating for their longer zigzag paths.
TTrue
FFalse
Answer: True
Exactly right. The refractive index in a graded-index fiber decreases smoothly from the core center outward. Since the speed of light in a medium increases as refractive index decreases, rays traveling near the edge move faster than rays near the center. This speed advantage nearly compensates for the longer path those edge rays travel, causing all modes to arrive at nearly the same time — dramatically reducing pulse spreading compared to a step-index design.
Question 5 Short Answer
Why does making an optical fiber's core extremely narrow (on the order of the wavelength of light) result in single-mode operation, and what practical tradeoff does this create?
Think about your answer, then reveal below.
Model answer: When the core diameter is comparable to the wavelength of light, only one spatial propagation mode fits geometrically — there is no room for the additional off-axis paths that multimode fibers support. This eliminates modal dispersion entirely. The practical tradeoff is coupling difficulty: a core of only ~8–10 μm is extremely hard to align precisely, requiring high-precision connectors and splicing equipment, making single-mode fiber more expensive to install than multimode.
Single-mode operation is a consequence of wave optics, not just geometric optics. The core is so small that higher-order modes are cut off — they cannot propagate stably and radiate out into the cladding. The benefit (zero modal dispersion) is why all long-haul telecommunications infrastructure uses single-mode fiber; the cost (alignment difficulty) is why data centers and buildings use multimode for short runs where the dispersion penalty is negligible.