Single-mode optical fiber with depressed cladding for bending resistance
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Solution Overview
Problem
Conventional single mode fibers face limitations in bending resistance and mechanical reliability, particularly in indoor and narrow environments, leading to increased bending stress and potential fiber fracture, which affects the reliability and maintenance costs of FTTx networks.
Innovation Solution
A single mode fiber with a core and inner cladding composed of silica glass doped with germanium and fluorine, surrounded by a depressed cladding of pure silica glass, featuring a functionally graded material composition and structure, which reduces tensile stress and enhances mechanical properties, along with a manufacturing method using PCVD, OVD, VAD, or APVD processes to achieve improved bending resistance and compatibility with standards G652 and G657.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the mode field diameter is reduced to improve bending properties, then the MAC value decreases and bending resistance improves, but splicing loss increases when connecting to conventional single mode fibers and launching power is limited
Solution Approach 1:
The patent applies local quality by creating a depressed outer cladding layer with specific refractive index characteristics only in the outer region of the fiber, while maintaining the core and inner cladding with different properties. This localized modification improves bending resistance without affecting the core mode field diameter, thus avoiding increased splicing loss.
Solution Approach 2:
The patent uses composite material structure by combining multiple doped layers with different compositions (germanium-doped core, fluorine-doped inner cladding, and depressed outer cladding). This composite structure allows optimization of bending resistance in the outer cladding while maintaining compatible mode field characteristics in the core for low splicing loss.
2Reliability
If the cutoff wavelength is increased to improve bending properties, then the MAC value decreases and bending resistance improves, but the cutoff wavelength must remain less than or equal to 1,260 nm to transmit with the whole band, limiting the increase
Solution Approach 1:
The patent segments the cladding into an inner cladding and an outer depressed cladding layer. The depressed outer cladding layer specifically targets bending loss reduction through its refractive index profile, while the inner cladding and core maintain the cutoff wavelength characteristics needed for whole band transmission. This segmentation allows independent optimization of bending resistance and band transmission.
3Reliability
If high doping is applied to increase numerical aperture, then the bending resistance improves, but the attenuation increases
Solution Approach 1:
The patent applies local quality by concentrating the doping modifications in the outer cladding region rather than uniformly throughout the fiber. The depressed outer cladding layer with fluorine doping provides the necessary numerical aperture enhancement for bending resistance, while the core and inner cladding maintain lower doping levels to minimize attenuation.
4Reliability
If a depressed outer cladding is designed to reduce additional bending loss, then bending resistance improves, but multi-path interference occurs when the depressed cladding is too close to the core and the core deviates at fiber connection points
Solution Approach 1:
The patent introduces the inner cladding as an intermediary layer between the core and the depressed outer cladding. This intermediate layer acts as a buffer zone that prevents direct interaction between the core and the depressed outer cladding, thereby eliminating multi-path interference while allowing the depressed outer cladding to effectively reduce bending loss.
Data Source
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AI summary
A single mode fiber comprises a core, an inner cladding, a depressed cladding and an outer cladding composed of pure silica glass. The core is composed of silica glass doped with germanium and fluorine, with a diameter (a) of 8.0-8.8 µm, a relative refractive index difference (Δ1) of 0.35-0.38%, and the contribution of fluoride (ΔF) is -0.09 ± 0.02%. The inner cladding is composed of silica glass doped with germanium and fluorine, with a diameter (b) of 18-21 µm and a relative refractive index difference (Δ2) of 0 ± 0.02%. The depressed cladding is composed of silica glass doped with fluorine, with a diameter (c) of 26-36 µm and a relative refractive index difference (Δ32) at the external interface thereof is between -0.22 and -0.35%, and a relative refractive index difference (Δ31) at the internal interface thereof is between -0.20 and -0.3 5%, and Δ32≤Δ31. The fiber is able to improve the fiber bending resistance, prevent the additional stresses generated by fiber bending from passing on to the core for avoiding the increase of attenuation, thereby effectively improving the mechanical properties and service life of the fiber.