Adjust core and cladding refractive indices to see total internal reflection animated in real time. Calculate NA, acceptance angle, attenuation, and bandwidth-distance product instantly.
The Numerical Aperture (NA) determines the maximum acceptance angle ($\theta_a$) for light entering the fiber. It depends solely on the refractive indices of the core and cladding. A larger NA means easier light coupling but can also lead to more signal distortion.
$$NA = \sqrt{n_1^2 - n_2^2}$$$n_1$ : Refractive index of the core. $n_2$ : Refractive index of the cladding. For the fiber to guide light, $n_1$ must be greater than $n_2$.
Signal power decreases exponentially with distance traveled through the fiber due to attenuation. This is a critical parameter for designing the spacing between signal repeaters or amplifiers in a long-distance network.
$$P(L) = P_0 \cdot 10^{-\alpha L/10}$$$P(L)$ : Power at length $L$. $P_0$ : Initial launched power. $\alpha$ : Attenuation coefficient (dB/km). $L$: Length of the fiber (km). This formula shows why lowering attenuation was the breakthrough that enabled global internet backbones.
Telecommunications & Internet Backbones: This is the most widespread application. Undersea and terrestrial fiber cables form the core of the global internet, carrying terabytes of data across continents with minimal loss. The simulator's attenuation parameter is constantly optimized by engineers to increase the distance between costly underwater amplifiers.
Medical Endoscopy and Surgery: Bundles of optical fibers are used in endoscopes to illuminate and transmit images from inside the body. Fibers with a high Numerical Aperture (NA) are chosen to maximize light collection, providing surgeons with a clear view during minimally invasive procedures.
Industrial Sensing and Inspection: Fibers are used to deliver light and collect data in harsh or inaccessible environments. For instance, they monitor temperature and strain inside jet engines or carry laser beams for precise welding and cutting in automated manufacturing.
Defense and Avionics: Fiber optic cables are immune to electromagnetic interference (EMI), making them ideal for use in aircraft, ships, and military vehicles for data buses and sensor networks. Their light weight and high bandwidth are critical advantages in these applications.
First, the idea that "a larger NA is always better" is an oversimplification. While a larger Numerical Aperture (NA) generally improves coupling efficiency with a light source, it comes at the cost of exciting more higher-order modes, leading to increased modal dispersion. For instance, when transmitting over 1 km of multimode fiber with NA=0.3, the pulse broadening (dispersion) becomes significantly more pronounced compared to fiber with NA=0.2, degrading signal quality in high-speed communications. Even if this effect seems negligible in short-distance wiring, it becomes impossible to ignore as the distance increases.
Next, avoid setting the "attenuation coefficient α" to unrealistic values in the simulator. For example, setting α to 0 dB/km creates a "dream fiber" with no attenuation over any distance, which lacks realism. Practical reference values are around 0.2–0.4 dB/km for single-mode fiber and 2–4 dB/km for multimode fiber. Using these values as a baseline, try to get a feel for "how much signal strength is lost over a 100 km transmission."
Finally, note that the difference between "single-mode" and "multimode" is not just about core diameter. While changing the core diameter in the simulator certainly alters the number of propagating modes, in practice, the operating wavelength is also critically important. For example, the same fiber core diameter might support single-mode propagation at a wavelength of 1.55 μm but become multimode at 0.85 μm. When adjusting parameters, always keep in mind the interrelationship between wavelength, core diameter, and refractive index difference.
Standard single-mode fiber (SMF-28): n1 = 1.4795, n2 = 1.4669, L = 80 km. Calculated NA = 0.131 gives acceptance angle θ = 7.5°. At 1550 nm wavelength with 0.19 dB/km attenuation, total path loss = 80 × 0.19 = 15.2 dB. Injected power of 0 dBm requires receiver sensitivity ≤ −15.2 dBm, achievable with modern photodiodes (−25 dBm typical). Dispersion parameter D = 17 ps/(nm·km) limits bandwidth-distance product to 625 GHz·km.