Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1998-12-08
2001-09-18
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Plural
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06292603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion compensation device capable of compensating chromatic dispersion caused by optical fibers in a multiple-wavelength light transmission system so as to reduce waveform distortions due to the chromatic dispersion.
2. Description of the Prior Art
Recent years have seen the wide use of wavelength multiplexing technology as a measure to increase the amount of transmission through optical fibers. In the wavelength multiplexing technology, N light sources each of which can emit light with a different wavelength are disposed and each light is modulated with a data signal having a bit rate B, so that the amount of transmission is increased up to N*B. Recently, by using the wavelength multiplexing technology and light amplification technology, a system capable of transmitting lightwave signals with an amount of transmission of more than 100 Gbits at distances of up to 10,000 km is proceeding toward practical utilization.
Conventionally, in a 1.55 &mgr;m-wavelength-range optical fiber transmission system, in order to reduce the occurrence of waveform distortions due to the chromatic dispersion caused by optical fibers to a minimum, dispersion shifted fibers or DSFs, which are designed so that the chromatic dispersion is zero at a wavelength of 1.55 &mgr;m, have been used. If such prior art dispersion shifted fibers are used for a multiple-wavelength light transmission system, as previously mentioned, with a large amount of transmission, the system can be brought into a state in which the propagation constants for wavelengths which are next to each other are of almost equal, that is, phase matching is established, so that a unnecessary signal is generated due to the four-wave mixing. Although a description of the four-wave mixing will be omitted hereafter because an explanation for the four-wave mixing is given in detail by for example Govind P. Agrawal, “Nonlinear Fiber Optics”, Academic Press, 1989, it should be noted that the four-wave mixing causes serious degradation of the transmission characteristic.
In order to prevent the four-wave mixing, nonzero dispersion shifted fibers whose zero dispersion wavelength is forced to deviate from a lightwave signal wavelength range are now going to use in multiple-wavelength light transmission systems. For example, in a case where multiple-wavelength light with a wavelength range of 1,550 nm to 1,560 nm is transmitted via a nonzero dispersion shifted fiber whose zero dispersion wavelength is 1,580 nm and whose dispersion slope is 0.1 ps
m
2
/km, a light component with a wavelength of 1,550 nm is transmitted while it undergoes −3 ps
m/km of dispersion and a light component with a wavelength of 1,560 nm is transmitted while it undergoes −2 ps
m/km of dispersion. Therefore, in this case, phase matching is not established adequately and hence this makes it difficult for the four-wave mixing to occur.
When such multiple-wavelength light is transmitted over great distances by way of a nonzero dispersion shifter fiber, each light component undergoes a cumulative, negative amount of dispersion. To reduce the average of the amounts of dispersion which all light components undergo to zero, so-called “dispersion management” is carried out. To do dispersion management, some 1.3 &mgr;m zero-dispersion fibers of an appropriate length, each of which provides a certain amount of dispersion of +17 ps
m/km for light with a wavelength of 1.55 &mgr;m, are inserted at some midpoints in the transmission path. However, depending on the wavelength, a light component has an amount of residual dispersion, which has not been canceled, due to the gradient of the dispersion characteristic of nonzero dispersion shifted fibers, i.e. dispersion slope. For example, in a nonzero dispersion shifted fiber 10,000 km long in which dispersion management is carried out such that the dispersion for light with a wavelength of 1,555 nm is zero, a light component with a wavelength of 1,550 nm has a certain amount of residual dispersion of (1,555−1,550)*0.1*10,000 km = −5,000 ps
m and a light component with, a wavelength of 1,560 nm has a certain amount of residual dispersion of (1,560−1,555)*0.1*10,000 km=+5000 ps
m.
A sending terminal station performs a dispersion compensation operation on each light component with a certain wavelength in multiple-wavelength light including light components each having such residual dispersion, using a dispersion compensation fiber. Dispersion compensation that is carried out at a sending terminal station is called pre dispersion compensation. On the other hand, dispersion compensation that is carried out at a receiving terminal station is called post dispersion compensation. Sharing a needed amount of dispersion compensation half and half between the sending terminal station and the receiving terminal station is effective in compensating the residual dispersion in each light component, as disclosed by M. I. Hayee et al., “Pre-and Post compensation of dispersion and nonlinearities in 10-Gb/s WDM systems”, IEEE Photonics Technology Letters, Vol. 9, No. 9, pp. 1271, 1997.
Referring now to
FIG. 9
, there is illustrated a block diagram showing the structure of an example of a wavelength multiplexing sending terminal station including eight optical sources and a plurality of prior art dispersion compensation devices each for performing pre dispersion compensation. In the figure, reference numerals
100
a
to
100
h
denote optical sources (or optical senders), i.e. OSs for sending out lightwave signals with wavelengths of &lgr;
−4
, &lgr;
−3
, &lgr;
−2
, &lgr;
−1
, &lgr;
+1
, &lgr;
+2
, &lgr;
+3
, &lgr;
+4
, respectively,
102
a
to
102
g
denote dispersion compensation fibers each for providing a positive amount of dispersion for light of a wavelength which lies in a 1.55 &mgr;m wavelength range,
103
a
to
103
g
denote dispersion compensation fibers each for providing a negative amount of dispersion for light of a wavelength which lies in a 1.55 &mgr;m wavelength range,
104
a
to
104
i
denote light amplifiers, and
105
denotes an optical multiplexer. Preferably, an array type waveguide grating or AWG is used as the optical multiplexer
105
. In addition, reference numeral
106
denotes a transmission fiber in which dispersion management is carried out. A single mode fiber or SMF whose zero dispersion wavelength is typically 1.3 &mgr;m is used as each of the plurality of dispersion compensation fibers
102
a
to
102
g
for providing a positive amount of dispersion. The amount of dispersion per a loss of 1 dB provided by one single mode fiber can be in the range of +80 ps
m to +100 ps
m. On the other hand, each of the plurality of dispersion compensation fibers
103
a
to
103
g
can produce a certain negative amount of dispersion of −240 ps
m per a loss of 1 dB. In this specification, a dispersion compensation fiber for providing a negative amount of dispersion is simply referred to as a DCF. The reason why when comparing the path for a lightwave signal with a wavelength of &lgr;
+4
with the path for a lightwave signal with a wavelength of &lgr;
−4
in the system as shown in
FIG. 9
, for example, the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of &lgr;
+4
is less than the number of light amplifiers disposed on the path for the lightwave signal with a wavelength of &lgr;
−4
is that the amount of dispersion per a loss of 1 dB caused by one DCF is greater than that caused by one SMF, that is, the dispersion efficiency of one DCF is greater than that of one SMF.
A description will be made as to the operation of the sending terminal station. The sending terminal station can provide a certain amount of dispersion for each lightwave signal of a certain wavelength using the plurality of dispersion compensation fibers
102
a
to
102
g
and the plurality of dispersion
Goto Koji
Kitayama Tadayoshi
Mizuochi Takashi
Tanaka Hideaki
Font Frank G.
Kokusai Denshin Denwa Co. Ltd.
Natividad Phil
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