1. The development of multi core single mode fiber
A typical optical fiber is single mode fiber structure, the outer cladding / inner cladding surrounding the core constitutes a waveguide. A common multi core single mode fiber includes a number of cores, and each core has their own single mode optical fiber inner cladding. Thus, each core is a fiber optic of the waveguide, i.e., one single mode fiber optic cable’s function is equal to a plurality of single core fiber. The obvious advantage of this fiber is low cost, low production costs of about 50% compared with ordinary fiber. In addition, Fiber optic cables can increase integration density, but also can reduce construction costs. Back in the late 1970s, foreigner proposed the idea which uses multi-core optical fiber manufacturing high density fiber. However, because of the imperfect manufacturing technology, fiber affected by the residual stress, low mechanical strength, poor reliability, has not developed into practical and commercialization. Into the 1990s, fiber optic communication in the FTTH ( fiber to the home ) development has encountered higher cost of obstacles, and the intense competition in the copper. To overcome this obstacle, we must significantly reduce the cost of fiber optic cable manufacturing, and requires the development of high-density large armored fiber optic cable, in order to facilitate the laying of fiber optic cable and reducing installation costs. Thus, in 1994, France Telecom proposed a new concept of multi core single mode fiber and design a 4 core fiber optic cable. In July 1994 made more than 100 kilometers Single Mode Fiber Cable, and use different core and structures fiber optic cables and non-fiber ribbon cable into the cable experiment. Compared with the common Single Mode Fiber, the cable density increase many times. Initially confirmed that the proposed multi core single mode fiber can reduce manufacturing costs while addressing the fiber optic cable and the development of high- density optical fiber optic cable. Since then, France Telecom and Alcatel have carried out a 4 core single mode optical fiber research and development , studied from optical design, perform manufacture, drawing techniques, optical properties, a 4-pin cable, and mechanical connections from each a core of separation and termination of ordinary single mode fiber and other aspects of a comprehensive. Currently, the cost per meter per core for $ 0.03, while the cost per meter of ordinary single mode fiber is $ 0.055. We can say that multi-core single mode fiber forward practical development. This article focuses on the structure of multi-core single mode optical fiber design, manufacturing processes, and fiber properties, but also a brief introduction to the new fiber optic FTTH applications in the technical and economic advantages.
2. Design of multi core single mode fiber
To meet the needs of FTTH systems. The design of multi-core fiber should meet the following key requirements: 1.31 um and 1.55um wavelength dual-window work; Crosstalk between each core is greater than -35 dB: precise geometry; Easy identification of each core and ordinary Single mode Fiber Optic Cable. Further, since the optical fiber length shorter in FTTH network (≤ l0 km), so the requirements for dispersion and attenuation of the fiber is relatively relaxed, there are strict requirements on bending and micro bending losses. Each of the core is respectively located on the vertices of a square, the center of the square is the central axis of the multi core single mode fiber. Each core is a single mode optical fiber waveguide, cutoff wavelength 1.3 um. In order to reduce the bending loss and crosstalk, reinforce fiber molded capacity constraints, therefore compared with the conventional G652 optical fiber, the refractive index difference has increased. 3 core is a simple step structure, a refractive index difference 0.0062; another annular core refractive index profile, the refractive index difference 0.0l, this core is provided in order to facilitate identification of each of the cores.
The main advantage of multi core Single Mode Fiber Cable
1. Attenuation coefficient
By the testing and research of nearly 200km long fiber, the attenuation coefficient of the multi core single mode optical fiber reached levels close to the corresponding single core fiber.
2. Bending and micro-bending loss characteristics
Bending and micro bending loss characteristics of the multi-core single mode fiber optic are good, negligible additional loss introduced. This is due to the higher refractive index and limits the ability of a strong mold. Multi core single mode fiber optic in the l310nm and 1550nm wavelength mode field diameter was 8.4 ± 0.21un and 9.75 ± 0.25um, slightly smaller than the ordinary single mode optical fiber.
3. Crosstalk characteristics
To 30 g of tension around 15 cm in diameter optical fiber on ferrule l0 km optical fiber with characteristic measurement, crosstalk between each core at l 310 nm wavelength, less than-70dB; at 1 550 nm wavelength, less than a 50 dB, are excellent -35 dB in the required index. Even slim to the 30 km, between 0-30g tension variations, changes, crosstalk is also only a few dB.
4. Mechanical strength
With 10m length test, the average intensity of 70N. The conventional 125um fiber diameter of 60 N, it is because of a large cross-sectional area of multi-core single mode fiber. Fracture stress 4.6 GPa, 94% for the conventional 125 um diameter fiber.