1. Field of the Invention
The present invention relates to an optical fiber and an optical transmission system.
2. Description of the Related Art
Optical transmission systems including a single-mode optical fiber as a signal light transmission path are demanded to have less time (delay time, or “latency”) for signal light to transmit from a transmitter to a receiver. This demand has been increasing in recent years. For example, a very small difference in delay time on the order of milliseconds or less in financial transactions using an optical transmission system may influence an enormous financial benefit.
The latency TL [s] of signal light in an optical fiber transmission path having a transmission length L [m] is expressed by Equation (1):
where c denotes the speed (3×108 [m/s]) of light in vacuum space, vg denotes the group velocity of signal light in the optical fiber transmission path, and ng denotes the group index of the optical fiber. Equation (1) implies that an optical fiber having a low group index ng is suitable to reduce the latency TL.
An ITU-T Recommendation G.652 compliant standard single-mode fiber (SSMF) has a group index ng of 1.4679. On the other hand, an optical fiber having a low group index ng of 1.4620 is described in John A. Jay, “Low Signal Latency in Optical Fiber Networks”, Proceedings of the 60th IWCS Conference, pp. 429-437 (Non Patent Literature 1).
Accordingly, it is an object of the present invention to provide an optical fiber capable of reducing latency of signal light (hereinafter, referred to as “signal latency”) and an optical transmission system including the optical fiber.
A first aspect of the present invention provides an optical fiber including a core and a cladding that surrounds the core. The optical fiber has a group index of 1.465 or less at a wavelength of 1550 nm and an absolute value of chromatic dispersion of 4 ps/nm/km or less at a wavelength of 1550 nm. A second aspect of the present invention provides an optical transmission system including the optical fiber according to the first aspect of the present invention as a signal light transmission path.
According to the present invention, signal latency can be reduced.
The optical fiber described in Non Patent Literature 1 is an ITU-T Recommendation G.652 compliant single-mode fiber and has a chromatic dispersion of approximately 17 ps/nm/km at a wavelength of 1550 nm. Dispersion of transmission optical fiber causes linear noise that is a contributor to degradation in the quality of signal light and, accordingly, has to be compensated by a dispersion compensation module. The inventor has found that signal latency cannot be reduced merely by reducing a group index ng of an optical fiber used as a transmission path in an optical transmission system.
Examples of the dispersion compensation module include a dispersion compensating optical fiber (DCF) that has dispersion of a different sign from dispersion of a transmission optical fiber and has a large absolute value of the dispersion. Although the transmission optical fiber has a length ranging from, for example, 80 km to 100 km per span, the DCF has a length ranging from a few kilometers to several tens of kilometers per span. Since the transmission optical fiber and the DCF are connected in series, signal latency increases depending on the length of the DCF.
A digital signal processor (DSP) typically represented by digital coherent technology may be used as a dispersion compensation module. The DSP, serving as a dispersion compensation module, is included in a receiver and is configured to equalize waveform distortion of signal light caused by dispersion in a transmission optical fiber. To equalize the waveform distortion of signal light caused by a large dispersion, the number of taps in the DSP has to be increased. Signal latency increases depending on the number of taps in the DSP.
Embodiments of the present invention will be described in detail with reference to the accompanying drawings.
An optical fiber according to an embodiment of the present invention is suitably used as the optical fibers 41 to 43. The optical fiber according to the embodiment has a group index ng of 1.465 or less at a wavelength of 1550 nm and an absolute value of chromatic dispersion of 4 ps/nm/km or less at a wavelength of 1550 nm. The group index ng of the optical fiber is expressed by Equations (2) and (3):
where neff denotes the effective refractive index of a propagation mode qualitatively obtained by weighting the refractive index of a core of the optical fiber and the refractive index of a cladding thereof with optical power distribution of propagated light, ω denotes the angular frequency of light, β denotes the propagation constant of the propagation mode, k denotes the wave number of light, and λ denotes the wavelength of light.
If the group index ng is less than or equal to 1.465, a latency of 10 μs can be reduced per 1,000-km length as compared with the SSMF. Furthermore, the group index ng of the optical fiber is preferably less than or equal to 1.462. If the group index ng is 1.462 or less, a latency of 20 μs can be reduced per 1,000-km length as compared with the SSMF.
The optical fiber according to the embodiment of the present invention preferably includes two or more layered cores and a cladding.
A relative refractive index difference Δ1 [%] between the first core and the cladding is expressed by Equation (4).
A relative refractive index difference Δ2 [%] between the second core and the cladding is expressed by Equation (5).
A relative refractive index difference Δ0 [%] between the first core and pure silica is expressed by Equation (6).
The magnitude relationship between the refractive indices of regions in the optical fiber illustrated in
Preferably, the relative refractive index difference Δ0 between the first core and pure silica ranges from −0.1% to 0.1%. Reducing the refractive index of the first core through which most of signal light passes can reduce the group index ng. Furthermore, the core is preferably not doped with Ge. To negatively increase the relative refractive index difference Δ0, the first core would have to be doped with a large amount of F, thus leading to an increase in attenuation. It is not preferable from the viewpoint of manufacturability.
The above-described results obtained from
Table I describes the specifications of optical fibers according to Examples 1 to 11. Table II describes the characteristics of the optical fibers according to Examples 1 to 11. These tables also describe the specifications and characteristics of a related-art single-mode optical fiber (SMF) according to Comparative Example 1 and those of a related-art dispersion shifted optical fiber (DSF) according to Comparative Example 2.
As described above, since the group index ng of the optical fiber according to the embodiment of the present invention is low, 1.465 or less, signal latency can be reduced. In addition, since the absolute value of optical fiber chromatic dispersion in this embodiment is small, 4 ps/nm/km or less, it is unnecessary to provide a dispersion compensation module, which gives a signal latency, or it needs a dispersion compensation module which gives little signal latency. The optical fiber according to the embodiment of the present invention and the optical transmission system including the optical fiber as a signal light transmission path can reduce signal latency.
Number | Date | Country | Kind |
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2013-156476 | Jul 2013 | JP | national |