1. Technical Field
The present invention relates to an optical fiber for optical communication, and particularly relates to an optical fiber suitable as a long distance line and Optical Fiber To The Home (FTTH) having transmission length of some tens of kilometers, and interconnection inside or outside homes in local area networks (LAN).
2. Related Art
Optical fibers are suitable for long distance communication for their wide band characteristics, and are widely used for communication in main lines of a long distance such as some tens of kilometers and above.
On the other hand, with rapid prevalence of the Internet, the amount of information transceived by individual personal computers has also increased dramatically. The widely used technology in this field has been copper electric cables such as a coaxial cable and an unshielded twisted pair (UTP) cable. Such electric cables, however, have a narrow band and so are easily affected by the electromagnetic wave noise, and so cannot easily transmit a large amount of information.
As a solution, as a technology capable of increasing the transmission capacity, FTTH has started to prevail, which uses an optical fiber also in communication between each user and a telephone station, not only to long distance communication between telephone stations. Utilizing wide band characteristics of optical fibers, The FTTH technology adopts such a system in which a single line of optical fiber is shared among a plurality of users of a group up to the vicinity of the group, and an optical signal is branched for each user and an extension of optical fiber is distributed to each user.
There are various types of optical fibers, among which a single mode optical fiber of ITU-T G. 650 standard is mainly used for long distance communication. This type of optical fiber has a small transmission loss, and comparatively cheap. The cutoff wavelength of it is about 1300 nm or below such that the signals of 1300 nm band are in the single mode. Moreover, the mode field diameter (MFD) of it is normally in the range of about 9-10 μm so as to restrain the optical non-linearity, as opposed to the MFD rating of about 8-10 μm.
The FTTH also uses a similar type of single mode optical fiber, in which an extension is drawn into the home of each user from a branch point. This is because if optical fibers widely different in MFD are connected to each other, it would cause transmission loss. Another important requirement of intra-home interconnection is bending loss. Long distance main line cables are laid in underground ducts unsusceptible to external force, and so the bending force exerted on the optical fiber main body can be assumed to correspond to only winding of a diameter of 60 mm within a terminal container (100 times at most). As opposed to this, inside and outside homes, the interconnection is pursued in the state of relatively thin codes (diameter of some millimeters) so as to be flexible and light, which is susceptible to outer force and the bending radius on the optical fiber often becomes 20 mm or smaller.
An optical fiber inherently has a characteristic of propagating signal light along a core of the optical fiber, and so being capable of transmission even in the bent state. However as the bending radius becomes small, the ratio of leak of unpropagated light from the core increases in an exponential fashion, to cause transmission loss. This is referred to as bending loss. So as to reduce the bending loss, it is effective to focus light to the core as much as possible, which is realized by reducing the MFD. With this in view, normally an optical fiber having MFD of about 6-8 μm is used, to realize bending loss of about 0.5 dB/turn at the wavelength of 1550 nm, when wound around a mandrel (cylinder) having a diameter of 20 mm for example.
However, connection loss is large when an optical fiber having MFD of about 6 μm is connected to an optical fiber having MFD of about 10 μm, and so the main line and the user line should operate in single mode optical fiber systems separate from each other. One way to solve this problem is to enlarge the cutoff wavelength. With the same MFD, the bending loss decreases in an exponential fashion as the cutoff wavelength increases. Even if the fiber cutoff wavelength defined by ITU-T G.650 is 1350 nm, the cable cutoff wavelength is generally about 1250 nm, which is about 100 nm shorter than the fiber cutoff wavelength, and so it does not pose problems in practical usage in the 1300 nm band. If the MFD is designed to be in the range of about 8-9 μm, which is close to the lower limit of G652, the bending loss will be alleviated.
The bending loss can also be reduced by using a double clad optical fiber having a small inner cladding refractive index while enlarging the MFD, as reported such as in Shojiro Kawakami and Shigeo Nishida, “Characteristics of a Doubly Clad Optical Fiber with a Low-Index Inner Cladding,” IEEE Journal of Quantum Electronics, vol. QB-10, No. 12, pp. 879-887, Dec. 1974. An optical fiber having a reduced impurity absorption loss and an optimized zero-dispersion wavelength by adopting the mentioned structure is disclosed in Japanese Patent Application Publication No. 2002-47027. With this technology, a specific refractive index difference of a low reflactive cladding is set to be about −0.021 to −0.0007%, and an MFD is set to be about 9.2 μm. Japanese Patent Application Publication No. 2006-133496 also discloses an optical fiber having an improved bending characteristic. Using this technology, the low refractive index cladding is further reduced to be in the range of −0.08 to −0.02%, and the MFD is designed to be a slightly smaller, such as in the range of 8.2 to 9.0 μm.
On the other hand, a technology of providing interconnection using an optical fiber between electronic appliances has started to prevail. A representative LAN communication standard IEEE 802.3 lists various types of optical fibers. Laser optical sources and optical receivers used in the wavelength of 1300 nm band are comparatively expensive, and so cheaper light emitting diodes (LED) and a surface emitting lasers (VCSEL) in the 850 nm band are often used. The optical fibers used for this purpose is a multimode optical fiber, which typically has a core diameter of 50 μm. The multimode optical fiber generally has different light propagation speed for each mode, and so has a parabolic shaped refractive index distribution (α type), instead of a simple step-like refractive index distribution, to reduce the difference in light propagation speed between modes. Such a device realizes a propagation speed of 500 MHz/km for example.
For the purpose of using an optical fiber having a common specification in such usages as the long distance transmission path, the FTTH, and LAN, Japanese Patent Application Publication (translation of PCT application) No. 2004-508600 discloses designing the core's refractive index distribution in substantially α shape, to reduce the mode delay in the 850 nm band to guarantee its usage in the 850 nm band. However, it fails to mention bending loss in a smaller diameter (e.g. about 20 mm).