Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-10-23
2003-08-12
Kim, Ellen E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06606437
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an optical transmission line, which is used for example for wavelength division multiplexed optical transmission, a negative dispersion optical fiber to be used in the optical transmission line, and an optical transmission system which uses the optical transmission line.
2. Discussion of the Background
Generally, a single mode optical fiber (shall be referred to hereinafter as “SMF”) is used in an optical transmission line, and this SMF has a zero dispersion wavelength in the 1310 nm band. Though the use of this SMF for optical transmission in the 1550 nm band is being considered, the SMF has a positive dispersion value and a positive dispersion slope in the 1550 nm band. Thus when the SMF is used singly for wavelength division multiplexed optical transmission in the 1550 nm band, the ill effect of wavelength dispersion occurs. Therefore, in order to compensate for this wavelength dispersion, active research is being carried out on module-type wavelength dispersion compensation optical fibers for short length use (this type of module-type dispersion compensated optical fiber shall be referred to hereinafter as “DCF”). Such a DCF is disclosed for example in Japanese Laid-open Patent Publication No. Hei 6-11620.
As an example of a DCF, a DCF has been developed with which the refractive index of the center core is made high to make the dispersion value a large negative value and thereby achieve a high figure of merit (FOM) (FOM=approximately 200).
Also, known forms of refractive index profiles of DCF's include single peak type profiles, such as those of matched cladding type fibers, and multiple cladding type fibers, such as W-type fibers. The abovementioned single peak type DCF has a positive dispersion slope. Thus when this type of DCF is connected to an SMF, though the dispersion at a single wavelength will be compensated, the dispersion slope will increase further. This type of DCF is therefore unsuitable for wavelength division multiplexed transmission (shall be referred to hereinafter as “WDM transmission”). Meanwhile, W-type and other types of multiple cladding type DCF's are optical fibers that can compensate the dispersion and dispersion slope of an SMF. These fibers have thus been attracting attention in that they provide an arrangement suitable for WDM transmission when connected with an SMF.
That is, a slope compensation type dispersion compensation fiber (DFCF), which can compensate the dispersion value and the dispersion slope at the same time, is being demanded for compensation of the wavelength dispersion of an SMF. A high FOM and the control of the compensation factor described below are required of such a DCF.
The dispersion compensation performance that is exhibited when a DCF is connected with an ordinary SMF can be understood readily when expressed by the compensation factor as follows:
Compensation factor (%)={(
S
DCF
/S
SMF
)/(
D
DCF
/D
SMF
)} (1)
In equation (1), SDCF is the dispersion slope of the DCF, SSMF is the dispersion slope of the SMF, DDCF is the dispersion value of the DCF, and DSMF is the dispersion value of the SMF. The above values are values within the bandwidth of SMF dispersion compensation by the DCF (conventionally, a bandwidth of 1520 to 1570 nm) or values at an arbitrary wavelength within this wavelength band. With regard to the above equation, wide bandwidth zero dispersion can be accomplished more successfully the closer the compensation factor is to 100%. An optimal design for this DCF is proposed in Japanese Laid-open Patent Publication No. Hei 8-136758.
However, such a DCF aimed at short lengths is effective only for dispersion compensation of SMF's that have been installed presently and cannot comprise a new fiber line just by itself. Due to the nature of its profile, the above-described DCF cannot maintain the low nonlinearity that is the excellent feature of SMF's. That is, a DCF is aimed at compensating the dispersion value or dispersion slope of an SMF with as short a length as possible. A DCF is thus generally small in MFD and large in &Dgr;
1
, and such a DCF tends to be extremely likely to give rise to nonlinear phenomena.
Recently, line-type dispersion compensation optical fibers with dispersion characteristics that are inverse to those of the SMF (this type of line-type dispersion compensation optical fiber shall be referred to hereinafter as “RDF”) are being considered as optical fibers of low nonlinearity that can compensate the dispersion and dispersion slope efficiently. RDF's are described for example in ECOC '97 Vol.1 p.127 and Japanese Laid-open Patent Publication No. Hei 10-319920.
The above-described conventional DCF's and RDF's are designed only for compensation in the 1520 nm to 1570 nm band (shall be referred to hereinafter as the “C-band”).
Recently, the use of a wavelength band of 1570 nm or more, or to be more specific, the use of the 1570m to 1620 nm band (shall be referred to hereinafter as the “L-band”) for wavelength division multiplexed optical transmission is being considered. For example, optical amplifiers that can amplify light of the L-band are being developed. Expansion of the wavelength band of wavelength division multiplexed optical transmission by performing wavelength division multiplexed optical transmission using both this L-band and the abovementioned C-band is being considered.
However at present, dispersion compensation optical fibers for compensation in the L-band have not been proposed and optical transmission lines for performing wavelength multiplexed optical transmission in the wavelength band of the L-band have not been realized.
Though optical transmission lines that are comprised of SMF's and dispersion compensation optical fibers have merits, such as {circle around (1)} to {circle around (3)} given below, since such transmission lines were targeted mainly at the C-band, they are not suitable as L-band wavelength division multiplexed optical transmission lines. The abovementioned merits include the following: {circle around (1)} SMF of low nonlinearity and low loss can be used. {circle around (2)} The dispersion in the C-band becomes flat. {circle around (3)} Since the line has a large local dispersion (dispersion value per unit length), the occurrence of four-wave mixing (shall be referred to hereinafter as “FWM”), which becomes prominent near zero dispersion, can be restricted.
FIG. 13
is a conceptual diagram of the dispersion characteristics of the condition where an SMF is connected to an RDF for C-band compensation. As shown in
FIG. 13
, an optical transmission line, in which an SMF is connected to a C-band compensation RDF, has a large negative dispersion and dispersion slope in the L-band. Thus when an L-band optical signal is transmitted through an optical transmission line for the C-band, the distortion of the signal waveform due to dispersion becomes a large obstacle that makes WDM transmission in the L-band difficult. The same can be said for a DCF for C-band compensation.
An optical transmission line and an optical transmission system, with which wavelength division multiplexed optical transmission can be performed using both wavelength bands of the L-band and the C-band as mentioned above, were thus difficult to realize.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems. That is, a final object of this invention is to present a wavelength division multiplexed optical transmission system with which wavelength division multiplexed optical transmission can be performed using both wavelength bands of the L-band and the C-band. In order to achieve this purpose, this invention first provides an optical transmission line that enables high-quality wavelength division multiplexed optical transmission in the L-band and an optical transmission line that enables high-quality wavelength division multiplexed optical transmission in the C-band. In order to realize these optical
Kokura Kunio
Mukasa Kazunori
Kim Ellen E.
The Furukawa Electric Co. Ltd.
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