The two standard wavelengths used in single-mode fiber, 1310 nm and 1550 nm, are not the same; they are distinct and optimized for different transmission characteristics.

Single-mode fiber is designed to carry only a single propagation mode, which minimizes modal dispersion and allows long-distance, high-bandwidth transmission . The fiber typically operates at two primary wavelengths: 1310 nm and 1550 nm. These wavelengths are chosen because they correspond to low-loss regions in the fiber and are compatible with available laser sources . Key differences between the two wavelengths:

• 1310 nm: This wavelength has lower chromatic dispersion, making it suitable for medium-distance transmission without significant pulse broadening. It is often used in metropolitan area networks and shorter backbone links .
• 1550 nm: This wavelength experiences the lowest attenuation in silica fibers, allowing for longer-distance transmission. It is commonly used in long-haul telecommunications and undersea cables . Although both wavelengths propagate in the same single mode, they are distinct in frequency and optical properties, meaning they are not identical. Modern systems often use wavelength-division multiplexing (WDM) to transmit multiple wavelengths simultaneously over a single fiber, taking advantage of these differences to increase capacity . In summary, single-mode fiber supports multiple wavelengths, but the standard 1310 nm and 1550 nm are different wavelengths, each optimized for specific transmission distances and performance characteristics .