Development of Indoor Single-mode Optical Fiber Cable Using Bending-loss-insensitive Fiber

2.1 No accidental bending loss

Hole-assisted optical fiber (HAF) cord is promising for this application because it generates hardly any bending loss when bent with a small radius [1]. An optical fiber consists of a central core surrounded by cladding. Light is confined in the core because its refractive index is designed to be higher than that of the cladding, so it is transmitted along the fiber. However, if a fiber is bent too much, the light may leak out. The cladding of HAF contains holes, whose refractive index is less than that of glass; this increases the light confinement and greatly improves the bending characteristics (Fig. 4).

Fig. 4. Principle of HAF.

HAF-based free-bending optical cord is a kind of ultralow-bending-loss single-mode HAF cord that has already been installed in the optical fiber to the home network in Japan. However, its applications have been limited because its superior bending-loss characteristics may lead to confinement of higher-order modes, which may result in a cutoff wavelength that is longer than the communication wavelength: this could degrade the quality of long-distance transmission.

Our newly developed HAF has both low loss when bent and transmission characteristics equivalent to existing single-mode fiber [2]. Therefore, the long-distance transmission quality is not degraded, so our HAF cable is suitable for practical use in NTT’s network. The measured bending losses are shown in Fig. 5. Even when bent with a curvature radius of 5 mm, our new HAF achieved a loss of 0.1 dB or less (at 1550 nm).

Fig. 5. Bending loss characteristics.

The specifications of ITU-T G.657 A2 and B3 single-mode fiber, which are international standards for loss when an optical fiber is bent, are listed in Table 1. The HAF cord satisfies not only G.657 A2 and B3, but also G.652, which is an international standard for existing single-mode fiber.

Table 1. Specifications of single-mode HAF.

2.2 Use with existing tools

With the current optical fiber maintenance procedure, to reduce the risk of disconnecting the wrong optical fiber, a technician identifies the optical fiber cord of interest by using an optical fiber cord identifier and a nondestructive macrobending method without interrupting communication. With this method, optical fibers are identified by detecting an identification signal leaking from a bent fiber [3]. Therefore, we were anxious about whether or not light leaking from HAF could be fully detected.

We measured the identification characteristics of the 1.7-mm-diameter single-mode HAF cord when using a conventional identifier and a nondestructive macrobending method. As a result, although the detection level was not as good as with the conventional optical fiber, we confirmed that the characteristics of the new HAF cord were sufficient for its use in NTT buildings.

2.3 Performance

We have produced prototypes of the indoor optical cables containing from 2 to 32 fibers and developed new indoor optical cables that use new optical cords that achieve almost the same characteristics as conventional optical cable over longer distances.