Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-01-31
2003-10-14
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S024000, C372S006000
Reexamination Certificate
active
06633697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Raman amplification method used in optical communication system and an optical transmission method using such a method.
2. Related Background Art
A frequency component (wavelength component) of an optical signal intensity of which is modulated and which has been used in existing optical communication system has a certain width. On the other hand, an optical fiber has a dispersion property that a propagating speed is changed depending upon a wavelength. Due to these two properties, when the optical signal is propagated through the optical fiber, a signal wave form is distorted because of difference in propagating speed between the wavelength components. When a pulse is inputted as the optical signal, since a pulse width is widened after propagation, this phenomenon is called as “pulse broadening due to dispersion” (for example, refer to Foundation Of Optical Waveguide written by Katsunari Okamoto and published From Corona Co.).
Although digital format is resistant to the change of wave form than analog format, error in detection is considerably increased if the overlap with adjacent bits become greater due to pulse broadening. To avoid this, in the prior arts, a wavelength having smaller (near zero) dispersion has been used to suppress the pulse broadening or spread pulse has been returned to the original form by delaying the wavelength component preceding through medium having dispersion opposite to that of the transmission line and by hastening the delayed wavelength component.
However, in the recent optical communication system, due to high output of the optical signal and multiplexing of the wavelength, a non-linear phenomenon within the optical fiber has become noticeable and distortion of the wave form could not be coped with only in the view point of dispersion. Main non-linear phenomena in question include self phase modulation (SPM), cross phase modulation (XPM) and four wave mixing (FWM). In SPM and XPM, phase of light is changed by a little change in refractive index of the optical fiber caused in accordance with light intensity. Since the change in phase causes instantaneous change in frequency and the changed amount is not constant, non-reversible wave form distortion is generated by the dispersion property of the optical fiber. The FWM is a phenomenon in which, when a polarization field is induced by plural inputted lights having different frequency, components different from the frequencies of the inputted lights are created, thereby generating light having new frequency. The FWM becomes noticeable particularly when the dispersion is near zero. If the light generated by the FWM coincides with the wavelength used as the signal, the error in detection will be increased.
As means for preventing deterioration of the transmission property based on such non-linearity of the optical fiber, there are two approaches. First approach is a method for reducing light intensity within the optical fiber to decrease the non-linear effect, and a second approach is a method for using a transmission method utilizing the non-linear effect. The former method can be realized by merely lowering an input level to the optical fiber or by utilizing an optical fiber having a large mode field diameter. The latter method can be realized by utilizing optical soliton. However, even when these methods are used, there remain the following problems.
If the input level to the optical fiber is lowered, since a signal
oise ratio (S/N ratio) at a receiving side is decreased, the error in detection will be increased. This can be interpreted so that a transmittable distance becomes shorter. Since the optical fiber having the large mode field diameter has a large dispersion slope (wavelength dependency of dispersion), it is difficult to set optimum dispersion with respect to all of channels in which the wavelengths are multiplexed. In the optical soliton communication system, due to perturbation (such as transmission loss or unevenness of dispersion) existing in the actual transmission line, dispersive wave out of the soliton condition are generated, which deteriorate the transmission property. As mentioned above, although the existing optical communication systems must be designed in careful consideration of several limitation factors, if there is no loss in the optical fiber as the optical transmission line, such limitations will be greatly relaxed. For example, in the transmission line having no loss, since there is no deterioration of the S/N ratio based on the propagation loss, the limitation factors caused by lowering the input level to the optical fiber are relaxed. Further, when the transmission line having no loss is applied to the optical soliton system, generation of the dispersive wave is greatly reduced. As one of conventional optical transmission lines most approaching to the transmission line having no loss, there is an optical transmission line in which loss is compensated by a Raman amplification.
A Raman amplification method utilizing Raman scattering of an optical fiber has advantages that the optical transmission line itself becomes an amplifier fiber and that any wavelength band can be amplified. In case of a silica-based optical fiber, peak of gain is generated at a long wavelength side greater than a wavelength of a pump light, i.e., in a frequency band having smaller frequency (than that of pump light) by about 13 THz. For example, 13 THz is a difference between wavelengths of 1450 nm and 1547 nm. Wavelength difference or frequency difference between the pump light and the gain peak is called as “Raman shift” which is a value depending upon composition of the optical fiber.
In general, in the Raman amplification method for communication, as shown in
FIG. 21
, a backward pumping scheme in which the pump light and the optical signal are propagated in opposite directions is adopted. Since a mechanism for generating Raman gain is operated at very high speed, in a forward pumping scheme in which the pump light and the optical signal are propagated in the same directions, fluctuation of intensity of the pump light is overlapped with the signal wave form as it is, with the result that the transmission property is deteriorated greatly. This is also described in Japanese Patent Application Laid-open No. 9-318981.
FIGS. 22
to
27
show general properties of intensity distribution of the pump light and optical signal along a longitudinal direction within the amplifier fiber of the Raman amplification method utilizing the conventional backward pumping scheme (regarding calculating methods, refer to “Nonlinear Fiber Optics”, Chap. 8, written by G. P. Agrawal and published from Academic Press, “Applied Optics”, Vol. 11, pp. 2489-2494, written by R. G. Smith and published in 1972, and “J. Quantum Electron”, Vol. QE-14, pp. 347-352, written by J. Auyeung and A. Yariv and published in 1978).
As Raman amplifiers utilizing the Raman amplification method, there are a distributed type in which the optical transmission line is used as the amplifier fiber, and a lumped type in which the amplifier fiber is provided independently from the optical transmission line. In the following explanation, the distributed type will be described. However, also in the lumped type, since performance of the optical signal and pump light in the amplifier fiber can be expressed by the same formula, the same effect can be achieved, although parameter values are different.
FIG. 22
is a graph showing change in pump light power and change in optical signal power. In this graph, a curve a indicates the optical signal power when incident power of the pump light is 100 mW (curve {circle around (1)}), a curve b indicates the optical signal power when incident-power of the pump light is 200 mW (curve {circle around (2)}), and a curve c indicates the optical signal power when incident power of the pump light is 300 mW (curve {circle around (3)}). In the graph, a curve d indicates the optical signal power when the pump light is not inputted.
Emori Yoshihiro
Namiki Shu
Healy Brian
Ther Furukawa Electric Co., Ltd.
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