Why can’t some light signals transmit through optical fibers?
In fiber optics, light being unable to be transmitted effectively through a fiber (i.e., experiencing extreme attenuation or failing to form a guided wave) is a very classic physical and waveguide optics phenomenon. This is primarily determined by the following core factors:
1. Waveguide Cutoff Effect and Cutoff Wavelength
The Single-Mode Fiber is designed to allow only the fundamental mode (LP_{01}) to propagate within the core. To describe the transmission state of light in a fiber, the normalized frequency (V parameter) is typically introduced:
V = \frac{2 \pi a}{\lambda} \text{NA}
Here, a is the core radius, \text{NA} is the numerical aperture, and \lambda is the operating wavelength.
- When the wavelength is shorter (\lambda \lt \lambda_c): The V parameter is greater than 2.405. In this case, the fiber can support the propagation of higher-order modes (e.g., LP_{11}) in addition to the fundamental mode (LP_{01}). While this doesn’t cause light to be ‘untransmittable,’ the multimode transmission leads to severe Intermodal Dispersion because different modes have different group velocities. This causes significant signal distortion after long-distance transmission.
- When the wavelength is too long (\lambda \gg \lambda_c): As the wavelength \lambda increases, the normalized frequency V parameter drops sharply. When V is much smaller than 2.405, the fiber core’s confinement of guided modes weakens, and most of the light energy is no longer confined to the core but diffuses into the cladding and even the coating layer. This causes the Mode Field Diameter (MFD) to increase dramatically. At this point, the fiber becomes extremely sensitive to bending; even minor macrobends or microbends can cause light energy to leak out rapidly as radiation modes, resulting in extremely high Bending Loss. Therefore, very long wavelength light in a single-mode fiber is quickly depleted over a very short distance, appearing ‘untransmittable’.
2. Intrinsic Material Absorption and Scattering Losses
Even with a perfect waveguide design, the fiber’s base material (typically silicon dioxide glass, \text{SiO}_2 ) has its natural physical transmission window limitations:
- Infrared Absorption Limit (Infrared Vibrational Absorption Edge): When the wavelength exceeds 2 \ \mu\text{m} (i.e., 2000\text{nm}), the photon energy strongly couples with the resonant lattice vibrations of silicon dioxide molecules, causing a sharp rise in infrared absorption. This makes mid- to far-infrared light essentially untransmittable in silica fibers.
- Ultraviolet Absorption Limit (Ultraviolet Electronic Transition Absorption Edge): When the wavelength is shorter than 200\text{nm}, the photon energy is high enough to excite electronic band gap transitions in the glass, leading to strong ultraviolet absorption, making transmission impossible.
- Rayleigh Scattering: Microscopic density non-uniformities in the glass during cooling cause Rayleigh scattering. Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength (\, \sim 1/\lambda^4). Therefore, the shorter the wavelength (e.g., closer to the ultraviolet band), the greater the scattering loss, which also limits the long-distance transmission of short-wavelength light signals.
- Impurity (Water Peak) Absorption: If the fiber manufacturing process is not sufficiently pure, residual hydroxyl ions ( \text{OH}^- ) in the fiber will cause a strong absorption peak near 1383\text{nm} (commonly known as the “water peak”), leading to extremely high attenuation at this specific wavelength.
3. Technical Solutions with OFSCN® Specialty Optical Fibers
In engineering applications, by selecting high-quality optical fibers, the aforementioned physical limitations of “untransmittability” can be circumvented or significantly optimized:
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OFSCN® G.652D Optical Fiber: This is a standard single-mode fiber. Through advanced manufacturing processes, it eliminates the hydroxyl water peak near 1383\text{nm} (zero water peak fiber), resulting in extremely low attenuation across the entire single-mode wavelength band from 1310\text{nm} to 1625\text{nm}. Its cable cutoff wavelength (\lambda_{cc}) is limited to below 1260\text{nm}, ensuring perfect low-loss single-mode transmission of light within this broadband.
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OFSCN® G.657 Optical Fiber: When the operating wavelength approaches the upper limit and light tends to leak due to microbends/macrobends, this bend-insensitive single-mode fiber is used. Its waveguide structure is specifically optimized for bend loss, effectively locking light energy within the core even at extremely small bending radii, thus preventing light leakage.
