Optimizing the bending loss coefficient of mini bare fiber single tube cables requires in-depth exploration at the structural design level. Low-loss transmission under bending can be achieved by manipulating the optical field distribution, suppressing mode coupling, and enhancing mechanical confinement. The key principle lies in balancing structural parameters and material properties to ensure that the optical signal maintains total internal reflection during bending, thereby reducing energy leakage caused by mode field deformation.
The refractive index difference between the core and cladding is crucial to the bending resistance of mini bare fiber single tube cables. Increasing the refractive index difference between the core and cladding strengthens the confinement of the optical beam within the core and reduces the tendency of the mode field to diffuse outward during bending. For example, using high-purity quartz glass as the base material and manipulating the cladding refractive index through fluorine or germanium doping to create a graded refractive index profile can further constrain the mode field distribution. This design also reduces bend sensitivity through optimized geometric symmetry, ensuring consistent performance at different bend angles.
Reducing the core diameter is a direct means of reducing bending loss in mini bare fiber single tube cables. When the core diameter approaches single-mode transmission, higher-order modes are effectively suppressed, leaving only the fundamental mode propagating within the core, thereby reducing bend-induced mode coupling losses. For example, reducing the core diameter to less than half its traditional value can significantly reduce losses at a specific bend radius. However, this reduction in core diameter requires simultaneous optimization of doping concentration and distribution to maintain the effective mode field area and avoid enhancing nonlinear effects due to a reduced mode field.
Introducing air holes or low-refractive-index trenches is an effective structural innovation for mini bare fiber single tube cables to suppress mode field deformation. Designing a periodic arrangement of air holes or low-refractive-index annular trenches around the core creates a photonic bandgap or leakage channel, forcing the beam energy to be concentrated in the core region. For example, a rectangular lattice-arranged air hole structure maintains a Gaussian distribution of the mode field during bending while keeping fundamental mode bending losses to extremely low levels. Through precise control of geometric parameters, this structure also balances mode field area and bending losses, achieving a large mode field with low losses.
Multimode interference suppression technology uses structural design to block the transmission path of higher-order modes. For example, an asymmetric refractive index profile is designed at the junction of the core and cladding to rapidly attenuate higher-order modes during bending due to phase mismatch. This design can increase the bending loss of the second-order mode to over a hundred times that of the fundamental mode, thereby ensuring stable single-mode operation. Furthermore, by optimizing the groove position and width, the mode field distribution can be further manipulated, ensuring that the electric field of the fundamental mode maintains a Gaussian distribution even when bent, thus safeguarding beam quality.
Flexible coating and sheath design are important supplements to the external constraints of mini bare fiber single tube cables. High-elastic modulus acrylic or silicone coatings can absorb external stress and reduce microbending losses. For example, adding elastic buffer material between the optical fiber and the metal layer can dissipate dynamic stress caused by environmental factors such as wind vibration and icing. Furthermore, a flexible sheath allows connectors to bend freely in confined spaces without compromising transmission performance, making it suitable for high-density cabling scenarios such as stacked interfaces.
Systematic optimization of structural parameters requires a combination of simulation and experimental verification. Finite element method simulations of bending loss characteristics under different structural parameters can quickly identify the optimal design combination. For example, parameters such as air hole spacing and trench depth can be adjusted for specific application scenarios to achieve the optimal balance between loss and mode field area at the target bend radius. During the actual drawing process, the preform stacking accuracy and drawing tension must be strictly controlled to ensure geometric consistency of the fiber structure and avoid performance fluctuations caused by manufacturing errors.
Optimizing the bending loss coefficient of a mini bare fiber single tube cable is a multi-dimensional, collaborative problem involving materials, geometry, and process. Through refractive index difference control, core diameter optimization, air hole structure design, multimode interference suppression, flexible coating application, and parameter system verification, the fiber's bending resistance can be significantly improved, meeting the high-density, low-loss cabling requirements of scenarios such as 5G fronthaul and data centers.
