JPH11205226A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress accumulated wavelength dispersion without a dispersion compensation fiber. SOLUTION: An optical transmission terminal station 10 outputs a signal light with a λ1 longer than a zero dispersion wavelength 20 of transmission optical fibers 16-1 to 16-3 to the transmission optical fiber 16-1. The signal light propagated through the transmission optical fiber 16-1 is made incident onto a wavelength converter 18-1. The wavelength converter 18-1 applies wavelength conversion to the incident signal light with the wavelength λ2 into a wavelength λ2 shorter than the zero dispersion wavelength λ1 and outputs it to the transmission optical fiber 16-2. The signal light propagated through the transmission optical fiber 16-2 is made incident onto a wavelength converter 18-2. The wavelength converter 18-2 applies wavelength conversion to the incident signal light with a wavelength λ2 into the signal with the wavelength λ2 conversely to the case with the wavelength converter 18-1 and outputs the resulting signal to the transmission optical fiber 16-3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system, and more specifically, to an optical transmission system that suppresses deterioration of transmission characteristics due to chromatic dispersion.

[0002]

2. Description of the Related Art In a conventional optical fiber transmission system, a signal wavelength is slightly shifted from a zero-dispersion wavelength, and is set to a wavelength that causes considerable chromatic dispersion in order to avoid deterioration of transmission characteristics due to nonlinearity of the optical fiber. . Then, a method of using a dispersion map in which positive dispersion fibers and negative dispersion fibers are alternately combined or disposing a dispersion compensating fiber at an appropriate position to keep the accumulated chromatic dispersion within a predetermined value has been considered.

[0003]

In practice, one type of transmission optical fiber can be used by a method using a special dispersion map in which two types of transmission optical fibers of positive dispersion and negative dispersion are alternately combined. Since the method using the compensating fiber is more preferable in terms of simplicity and cost of the system configuration, the latter configuration is usually adopted.

However, in consideration of further increasing the transmission speed, the configuration using the dispersion compensating fiber has the following problems. That is, if the insertion interval of the dispersion compensating fiber is lengthened and the accumulated chromatic dispersion is greatly compensated at once, waveform distortion occurs. Conversely, if the optical transmission line is configured to shorten the insertion interval of the dispersion compensating fiber and frequently compensate for the accumulated chromatic dispersion so that the accumulated chromatic dispersion is always small, the effect of four-photon mixing (FWM) appears. Transmission characteristics deteriorate. Furthermore, since the effective area of the core of the dispersion compensating fiber is small, the nonlinear effect increases, which also deteriorates the transmission characteristics.

An object of the present invention is to provide an optical transmission system having a novel configuration capable of suppressing accumulated chromatic dispersion to a small level.

[0006]

According to the present invention, there is provided first wavelength conversion means for converting the wavelength of signal light transmitted through an optical transmission medium into a wavelength longer than the zero dispersion wavelength of the optical transmission medium; Second wavelength converting means for converting the wavelength of the signal light transmitted through the medium to a wavelength shorter than the zero dispersion wavelength is disposed at an appropriate position in the optical transmission line. Thereby, the normal dispersion region and the abnormal dispersion region can be appropriately arranged (preferably alternately), and the accumulated chromatic dispersion can be suppressed to a small value. Further, by leaving sufficient accumulated chromatic dispersion, for example, by taking a sufficiently large interval between the first and second wavelength conversion means, or by sufficiently separating the wavelength of the signal light on the transmission medium from the zero dispersion wavelength, The effect of four-photon mixing can be reduced.

Since the wavelength converting means is used, the wavelength of the signal light does not become constant on all transmission media, but by arranging the optical amplifying means at a position where the wavelength of the signal light becomes a specific value, the same amplification band characteristic can be obtained. It is enough to prepare optical amplification means with
Construction and maintenance are facilitated and their costs can be reduced.

[0008] Further, by adopting a two-stage wavelength conversion structure in which an optical amplifying means is disposed between two wavelength converters, an optical amplifying means having a desired amplification band can be used.
The number of types of optical amplification means can be reduced to one or a small number, construction and maintenance become easy, and their cost can be reduced.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1A is a schematic block diagram of an optical transmission system according to an embodiment of the present invention, and FIG.
The change with respect to the distance of the accumulated chromatic dispersion is shown. FIG.
The vertical axis of (b) shows the accumulated chromatic dispersion, and the horizontal axis shows the distance.

In the optical fiber transmission line 14 between the optical transmitting terminal 10 and the optical receiving terminal 12, a transmission optical fiber 16-
Wavelength converters 18-1 and 18-2 are inserted between 1 to 16-3. All of the transmission optical fibers 16-1 to 16-3 have the same structure, and the zero dispersion wavelength is λ0. FIG. 2 shows the wavelength dispersion characteristics of the transmission optical fibers 16-1 to 16-3. The vertical axis represents chromatic dispersion, and the horizontal axis represents wavelength.

The optical transmitting terminal 10 outputs a signal light to the transmission optical fiber 16-1 at a wavelength λ1 longer than the zero dispersion wavelength λ0. At the wavelength λ1, the chromatic dispersion of the transmission optical fiber 16-1 is positive as shown in FIG.
As shown in (b), the cumulative chromatic dispersion increases as b.

The signal light propagating through the transmission optical fiber 16-1 enters the wavelength converter 18-1. Wavelength converter 18
-1 indicates that the incident signal light of wavelength λ1 is converted to zero dispersion wavelength λ.
The wavelength is converted to a wavelength λ2 shorter than 0 and output to the transmission optical fiber 16-2. As shown in FIG. 2, the chromatic dispersion of the transmission optical fiber 16-2 becomes negative at the wavelength λ2, so that the accumulated chromatic dispersion is transmitted through the transmission optical fiber 16-2 as shown in FIG. In the meantime.

The signal light transmitted through the transmission optical fiber 16-2 enters the wavelength converter 18-2. Wavelength converter 18
-2 is the incident wavelength λ, contrary to the wavelength converter 18-1.
The signal light of No. 2 is converted into a wavelength λ1 and output to the transmission optical fiber 16-3. As described above, the chromatic dispersion of the transmission optical fiber 16-3 is positive at the wavelength λ1, and the accumulated chromatic dispersion is, as shown in FIG. 1B, during transmission of the transmission optical fiber 16-3. To increase.

As described above, in this embodiment, the wavelength of the signal light is alternately converted into the wavelength λ1 longer than the zero-dispersion wavelength and the wavelength λ2 shorter than the zero-dispersion wavelength λ0 at appropriate intervals, thereby reducing the accumulated chromatic dispersion. It can be restricted within a predetermined range. Conventional dispersion compensating fibers are no longer needed.

When optical amplification is required, an optical amplifier is inserted at an appropriate place. In that case, an optical amplifier having an optical amplification band covering both the wavelength λ1 and the wavelength λ2 may be used. However, when an optical amplifier having such a wide optical amplification band is not available or is difficult to obtain. In such a case, an optical amplifier capable of optically amplifying the wavelength λ1 may be arranged on the input side of the wavelength converter 18-1 and the output side of the wavelength converter 18-2. It may be arranged on the output side of the wavelength converter 18-2 and the input side of the wavelength converter 18-2.

More generally, the wavelength converters 18-1 and 18-1
The wavelength conversion in 8-2 may have a two-stage configuration, and an optical amplifier may be incorporated in the middle. The wavelength converter 18 in that case
FIG. 3 shows a schematic block diagram of the components 1 and 18-2. 20
Is a wavelength converter that converts the wavelength of the input light having the wavelength λ1 (or λ2) into the wavelength λp, 22 is an optical amplifier that has the wavelength λp in the optical amplification band and optically amplifies the output light of the wavelength converter 20; Reference numeral 24 denotes a signal light output from the optical amplifier 22 having a wavelength λ.
This is a wavelength converter that converts the wavelength from p to wavelength λ2 (or λ1). In this configuration, as the optical amplifier 22, the wavelength λ1 of the signal light on the transmission optical fibers 16-1 to 16-3 is used.
There is an advantage that a preferable optical amplification band can be used irrespective of λ2.

In the wavelength converter 18-1, the wavelength converter 20
Converts the wavelength λ1 of the input light into the wavelength λp. Optical amplifier 2
2 optically amplifies the output light of the wavelength converter 20,
Reference numeral 4 converts the wavelength of the signal light having the wavelength λp output from the optical amplifier 22 into the wavelength λ2.

In the wavelength converter 18-2, the wavelength converter 20
Converts the wavelength λ2 of the input light into the wavelength λp. Optical amplifier 2
2 optically amplifies the output light of the wavelength converter 20,
Reference numeral 4 converts the wavelength of the signal light having the wavelength λp output from the optical amplifier 22 into the wavelength λ1.

As described above, in this embodiment, the accumulated chromatic dispersion can be limited within a predetermined value without using the dispersion compensating fiber, and the problem caused by using the dispersion compensating fiber is completely eliminated. Solvable. However, the present invention does not exclude the partial use of the dispersion compensating fiber, and also includes the use of the dispersion compensating fiber in the scope of the present invention.

In this embodiment, the wavelengths λ1 and λ2 of the signal light on the transmission optical fibers 16-1 to 16-3 are:
It can be selected as appropriate. For example, the transmission optical fiber 16-1
The wavelength of the signal light above does not need to be the same as the wavelength of the signal light on the transmission optical fiber 16-3. This allows
The dispersion slope in each of the transmission optical fibers 16-1 to 16-3 can be appropriately selected. For example, the dispersion slope of the entire optical transmission line can be made zero. Needless to say, the transmission distances of the transmission optical fibers 16-1 to 16-3 need not be the same.

Wavelength converters 18-1, 18-2, 20, 2
4 is, for example, a DFG (Difference Frequency
The present invention can be realized by using a nonlinear optical effect element such as frequency generation and four-photon mixing (FWM), an electroacoustic frequency shifter, an electroabsorption modulator, and a semiconductor laser amplifier, or a combination thereof. A wavelength shift of 20 to 30 nm can be realized. Even after the transmission system is laid, the chromatic dispersion characteristics of the entire transmission system can be adjusted by changing the wavelength after wavelength conversion, for example, by changing the pump light wavelength of the nonlinear optical effect element.

Although the case of transmitting signal light of a single wavelength has been described as an example, it is obvious that the present embodiment can be applied to a wavelength division multiplex transmission system. In this embodiment, the dispersion slope of the entire transmission line can be made zero, so that the wavelength dependence of the transmission characteristics in the wavelength division multiplex transmission system can be reduced,
The degree of wavelength multiplexing and transmission characteristics can be improved.

[0024]

As can be easily understood from the above description, according to the present invention, the accumulated chromatic dispersion can be suppressed with a very simple structure. Since the dispersion compensating fiber is not used or can be reduced, the nonlinear effect of the entire transmission system can be suppressed, and the waveform deterioration due to the dispersion compensating fiber also decreases. Even after the transmission system is laid, the chromatic dispersion characteristics of the entire transmission system can be changed by replacing the wavelength conversion means or changing the characteristics.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention (a) and a schematic diagram of accumulated chromatic dispersion (b).

FIG. 2 is a schematic diagram of the wavelength dispersion characteristics of the transmission optical fibers 16-1 to 16-3.

FIG. 3 is a configuration example of wavelength converters 18-1 and 18-2.

[Explanation of symbols]

 10: Optical transmitting terminal 12: Optical receiving terminal 14: Optical fiber transmission line 16-1 to 16-3: Transmission optical fiber 18-1, 18-2: Wavelength converter 20: Wavelength converter 22: Optical amplifier 24: wavelength converter

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/12

Claims (5)

[Claims] 1. A plurality of optical transmission media, one or more first wavelength converting means for converting the wavelength of signal light transmitted through the optical transmission medium into a wavelength longer than the zero dispersion wavelength of the optical transmission medium, An optical transmission system, comprising: one or more second wavelength converting means for converting a wavelength of signal light transmitted through the optical transmission medium into a wavelength shorter than the zero dispersion wavelength. 2. The optical transmission system according to claim 1, wherein said first wavelength converting means and said second wavelength converting means are arranged alternately. 3. A method according to claim 1, wherein at least one of said first wavelength converting means and said second wavelength converting means performs optical amplification on an input signal light, and outputs a signal light output from said optical amplification means on a predetermined basis. The optical transmission system according to claim 1, further comprising a wavelength converter that converts the wavelength to the wavelength of the wavelength. 4. A wavelength converter for converting the wavelength of an input signal light into a predetermined wavelength, and a signal output from the wavelength converter, wherein at least one of the first wavelength conversion unit and the second wavelength conversion unit converts the wavelength of the input signal light into a predetermined wavelength. The optical transmission system according to claim 1, further comprising an optical amplification unit configured to amplify light. 5. At least one of said first wavelength converting means and said second wavelength converting means converts an optical amplifying means and an input signal light into a wavelength band included in an optical amplifying band of said optical amplifying means. 3. The optical transmission system according to claim 1, comprising: a first wavelength converter that converts the signal light output from the optical amplifying unit to a predetermined wavelength.