Optical fiber numerical aperture (NA) and system implications Optical fibers typically have a numerical aperture (NA) in the range 0.15 to 0.4. If a fiber has a higher NA value within this range, what happens to attenuation and bandwidth?

Introduction / Context:
Numerical aperture (NA) quantifies the light-gathering ability of an optical fiber. In step-index multimode fibers, NA is directly related to the refractive index contrast between core and cladding and determines how many modes can propagate. Understanding how NA affects attenuation and bandwidth is essential when selecting fibers for short-distance links versus long-haul, high-speed systems.

Given Data / Assumptions:

• Fiber NA range considered: approximately 0.15 to 0.4.
• Context: conventional multimode step-index or graded-index fibers used for communications and links.
• Attenuation (loss) and bandwidth are the performance metrics of interest.

Concept / Approach:
Higher NA implies a larger acceptance cone, allowing more high-angle rays to enter and be guided. This increases the number of supported modes (mode count). More modes generally produce greater intermodal dispersion, which spreads pulses in time and reduces bandwidth. Additionally, higher index contrast (which yields higher NA) can increase scattering and microbending sensitivity, often raising attenuation slightly compared to lower-NA designs optimized for long distances.

Step-by-Step Solution:

Verification / Alternative check:
Graded-index multimode fibers with lower effective NA exhibit better bandwidth-distance products than comparable high-NA step-index fibers. Conversely, plastic optical fibers with very high NA are easy to couple but have modest bandwidth and higher attenuation—consistent with the stated trend.

Why Other Options Are Wrong:

• Reduced losses: higher NA does not generally reduce attenuation in communications-grade fibers.
• High bandwidth: contradicts the dispersion increase with higher mode count.
• Reduced losses and low bandwidth: only bandwidth trend aligns; the loss trend does not.

Common Pitfalls:
Confusing coupling efficiency (which improves with higher NA) with end-to-end link attenuation; overlooking that many-system bandwidth is dispersion-limited, not only power-limited.

Final Answer:
more losses and low bandwidth