Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-08-15
2002-07-30
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S031000, C385S038000, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S333000
Reexamination Certificate
active
06427043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for compensating for wavelength dispersions caused when transmitting wavelength division multiplexed signal light using an optical transmission path having a plurality of repeating sections, and an optical transmission system adopting such a method, and particularly to a method for compensating for wavelength dispersions and an optical transmission system capable of reducing affections of cross phase modulation caused among optical signals of respective wavelengths.
2. Related Art
Conventionally, in an optical transmission system over a long distance, an optical signal has been transmitted making use of an optical regenerating repeater which converts an optical signal into an electrical signal, to thereby perform retiming, reshaping and regenerating. However, at present, the advancement of practical use of an optical amplifier has led to the investigation of an optical amplifying-and-repeating transmission system which utilizes an optical amplifier as a linear repeater. By replacing an optical regenerating repeater with an optical amplifying-repeater, it is expected that the number of parts within the repeater is remarkably reduced, to thereby ensure reliability and to permit cost reduction. Further, as one method for realizing a large capacity of an optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission system which multiplexes two or more optical signals having wavelengths different from each other to transmit in a single optical transmission path. In a WDM optical amplifying-and-repeating transmission system provided by combining the aforementioned optical amplifying-and-repeating transmission system with the WDM optical transmission system, it is possible to collectively amplify WDM signal lights by an optical amplifier, to thereby permit realization of a lager capacity and a long distance transmission with a simple constitution (economical advantage).
In a conventional WDM optical amplifying-and-repeating transmission system, the cause of waveform distortion due to a nonlinear effect of optical transmission path includes self phase modulation (SPM), cross phase modulation (XPM), and four wave mixing (FWM). To reduce transmission characteristic degradation due to the nonlinear effect of optical transmission path, there is used a method for managing a wavelength dispersion of an optical transmission path.
For example, in an article of N. S. Bergano et al., “Wavelength division Multiplexing in Long-Haul Transmission Systems, IEEE Journal of Lightwave Technology, vol. 14, no. 6, pp. 1299-1308, 1996”, there is used an optical transmission path combining a dispersion-shifted fiber (DSF) having a length of about 900 km and having a zero-dispersion wavelength of 1,585 nm with a single-mode fiber (SMF) having a length of about 100 km and having a zero-dispersion wavelength of 1,310 nm. This optical transmission path has an averaged zero-dispersion wavelength of about 1,558 nm, and accommodates wavelengths of signal lights ranging from 1,556 nm to 1,560 nm. The distortion of a transmitted waveform such as due to four wave mixing can be mitigated, because the wavelength dispersion of the DSF is about −2 ps
m/km, in which a group velocity of signal lights and spontaneous emission light and a group of mutual signal lights are different from each other so that an interaction period of time of nonlinear effect can be shortened.
Further, to realize a WDM optical transmission system having a more larger capacity and for a longer distance, the main subjects required for an optical transmission path of the system includes: (a) lower transmission loss; (b) a larger size of nonlinear effective cross section; (c) inconsistency of the zero-dispersion wavelength of signal light with that of the optical transmission path; (d) a relatively small absolute value of an accumulated wavelength dispersion; (e) substantially longer compensation intervals for the accumulated wavelength dispersion, as compared with repeating intervals; and (f) a smaller wavelength dispersion slope or an ability of compensating for the wavelength dispersion slope.
As a countermeasure for the aforementioned subjects, it has been proposed to use, as an optical transmission path, a combination of a 1.3 &mgr;m zero dispersion fiber having a positive wavelength dispersion with a dispersion compensation fiber having a negative wavelength dispersion.
For example, in the system such as proposed in an article of M. Murakami et al, “Long-haul 16×10 WDM transmission experiment using high order fiber dispersion management technique, ECOC '98, p. 313, 1998”, there are adopted a 1.3 &mgr;m zero dispersion fiber having a positive wavelength dispersion and a dispersion compensation fiber having a negative wavelength dispersion, at the former half and the latter half of a transmission section, respectively. The optical fiber at the latter half is capable of compensating for the wavelength dispersion and dispersion slope of the optical fiber at the former half, the nonlinear effective cross section of the optical fiber at the former half is as large as about 80 &mgr;m
2
, and the transmission loss of the transmission section is as small as about 0.20 dB/km. Thus, the accumulated wavelength dispersion included in the signal lights of all wavelengths can be sufficiently reduced, so that distortion of transmitted waveform due to self phase modulation-group velocity dispersion (SPM-GVD) can be reduced.
Meantime, when it is possible to reduce the distortion of transmitted waveform due to occurrence of self phase modulation and four wave mixing in case of adopting the aforementioned known type of optical transmission path, it is very possible that cross phase modulation becomes the dominant cause of transmitted waveform distortion due to a nonlinear effect of the optical transmission path. Namely, in an optical transmission path capable of compensating for the wavelength dispersion and its slope at particular intervals, the time-wise arrangement of WDM signal lights is restored at each dispersion compensating intervals, resulting in a possibility of larger waveform distortion due to cross phase modulation.
However, substantially no proposals have been done for conducting wavelength dispersion management taking notice of cross phase modulation, to thereby reduce the degradation of transmission characteristics due to a nonlinear effect of optical transmission path.
There will be briefly explained hereinafter the occurrence of transmitted waveform distortion due to cross phase modulation.
Generally, cross phase modulation is a phenomenon in which intensity fluctuation of another signal light is transformed into phase fluctuation of a pertinent signal light, due to a nonlinear effect of an optical transmission path. Intensity fluctuation of light transmitted through an optical fiber leads to a slight change of a refractive index of the optical fiber, due to a Kerr effect attributing to the optical fiber. The velocity of propagated light changes in accordance with the refractive index change, resulting in occurrence of phase fluctuation of the propagated light itself.
FIG. 15
is a view for explaining phase fluctuation due to self phase modulation and cross phase modulation, respectively. To simplify the explanation, it is assumed that an isolated light pulse is transmitted.
Firstly, for the phase fluctuation due to self phase modulation as shown in
FIG. 15A
, the light intensity of the isolated light pulse rapidly increases at the leading edge portion of the pulse, leading to occurrence of red shift due to the Kerr effect. This red shift is a phenomenon in which the wavelength of the signal light shifts toward a longer wavelength side, i.e., into a direction where the light frequency is reduced. The red shift is also called “red chirping”. Contrary, the light intensity of the isolated light pulse rapidly decreases at the falling edge portion, leading to occurrence of blue
Fujitsu Limited
Healy Brian
Staas & Halsey , LLP
LandOfFree
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