JPH10224298A - Optical communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress transmission deterioration due to nonlinear effects by making the wavelength interval of signal light that is initially arranged at both ends of zero dispersion wavelength, at least larger than a series of adjacent wavelength intervals. SOLUTION: Signal lights λ1 to λ8 are arranged at both side regions of this wavelength λ0 as zero division wave length λ0 in signal light that has undergone wavelength division multiplexing. At least λ4 and λ5 which are arranged at both sides of the λ0 are not used as signal light, and the wavelengths (λ1 to λ3 and λ6 to λ8 ) in its outer wavelength regions are undergone division multiplexing and transmitted. Furthermore, the wavelengths of λ3 and λ6 which are arranged at both sides of λ4 and λ5 are sometimes not used for transmission of signal light. Thus, because signal light to be propagated is transmitted in wavelength regions that are away from λ0 to that degrees, the effect of transmission deterioration due to nonlinear effects is reduced. The range that is removed from a transmission area of an optical signal of both sides of λ0 is decided by the wavelength intervals of adjacent optical signals and the number of relays.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication method capable of suppressing transmission deterioration due to a nonlinear refractive index of an optical fiber or a fiber amplifier.

[0002]

2. Description of the Related Art In recent years, wavelength division multiplexing transmission has attracted attention as a method for realizing a larger capacity of optical communication. However, wavelength multiplexed signals are converted to erbium-doped fiber amplifiers (E
When performing long-distance relay transmission using DFA), transmission degradation due to four-wave mixing occurring in the fiber becomes a problem.

The following method has been proposed to suppress transmission deterioration due to four-wave mixing. The first method is to distribute the dispersion value of the fiber in the transmission line to keep the signal wavelength away from the zero dispersion wavelength (1994 IEICE Spring Conference: B-1051). The second method is to multiplex. To disperse transmission degradation due to four-wave mixing by shifting a plurality of wavelength intervals (IEEE PHOTONICS LETT)
ERS, VOL. 6, NO. 11, NOV. 1994
pp1374-1376), and a third method (ECOC '96 ThB.3.1) for reducing the noise generated by four-wave mixing by lowering the output after EDFA has been proposed.

[0004]

In the first method for suppressing transmission deterioration due to four-wave mixing, since the optical fiber cables having different characteristics are combined and laid, the cable configuration becomes complicated and the cost increases. Also, even if the fiber dispersion in the transmission line is distributed and the zero-dispersion wavelength is kept away from the signal wavelength, the signal wavelength approaches the zero-dispersion wavelength in the amplification stage, and the signal is amplified in the close state. It cannot be ruled out. In the second method, it is difficult to determine the optimum distribution, and the method becomes more complicated as the number of signal wavelengths increases. Third
The method of (1) has a problem that the number of times of amplification increases because the amplification interval becomes short, which is not suitable for long-distance transmission. Therefore, an object of the present invention is to provide an optical communication method capable of solving such a problem and suppressing nonlinear effects, and capable of performing wideband wavelength division multiplex transmission.

[0005]

SUMMARY OF THE INVENTION The present invention relates to an optical communication method for dividing a plurality of signal lights into predetermined wavelength intervals and multiplexing them on an optical fiber having zero chromatic dispersion in a plurality of signal light wavelength bands. Characterized in that the wavelength spacing of the signal light initially arranged on both sides of the zero-dispersion wavelength is at least greater than a series of adjacent wavelength intervals, and the wavelength interval is twice the series of adjacent wavelength intervals. It is preferable to make the above.

According to the present invention, the signal light arranged on both sides of the wavelength at which the chromatic dispersion is zero is transmitted with a larger interval than the wavelength interval of a series of adjacent signal lights, so that the nonlinear refraction is achieved. Transmission degradation due to the rate can be significantly suppressed. By doing so, transmission degradation due to the non-linear effect can be suppressed, and the wavelength region on both sides of the zero dispersion wavelength can be used, so that the transmission band can be used effectively.

Further, the present invention provides an optical communication system for multiplex-transmitting a plurality of signal lights at predetermined wavelength intervals to an optical transmission line having an optical fiber and an optical amplifier having zero chromatic dispersion in a plurality of signal light wavelength bands. The method is characterized in that a zero-dispersion wavelength is located at a valley of a gain curve of the optical amplifier.

According to the present invention, since the wavelength near the zero dispersion at which the non-linear effect is remarkable appears at the valley of the gain curve which is difficult to use as an optical amplifier, the wavelength band of the optical amplifier and the optical transmission line is set. Can be used effectively. By arranging in this manner, there is no waste even if the wavelength intervals of the optical signals initially arranged on both sides of the zero-dispersion wavelength are made larger than a series of adjacent wavelength intervals.

[0009]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a schematic diagram showing the configuration of an entire optical communication system according to the present embodiment. The wavelength division multiplexed signal light is emitted from an optical transmitter 1 and is transmitted while being relayed by an optical amplifier 3. The light propagates through the fiber 2 and is sent to the optical receiver 4. The optical fiber 2 has a characteristic that the wavelength dispersion at which the transmission loss is minimized, for example, the chromatic dispersion at 1.55 μm is almost zero, and when the dispersion-shifted optical fiber 2-0 having a step-like refractive index distribution is used, or FIG. As shown in the figure, a single mode optical fiber 2-1 having a positive chromatic dispersion at a wavelength of 1.55 μm and a dispersion shifted optical fiber 2- having a negative chromatic dispersion at a wavelength of 1.55 μm 2-
2 may be connected so that the total chromatic dispersion becomes zero.

FIG. 3 is a diagram showing each signal light wavelength-division multiplexed by the optical transmission device 1 and their wavelength relationship.
Zero dispersion wavelength is lambda _0, the wavelength ·· λ ₁ ·· λ ₈ ·· of the signal light regions on both sides of the wavelength lambda ₀ represents a case where it is disposed. In the present invention, at least the wavelengths of λ ₄ and λ ₅ arranged on both sides of λ ₀ are not used as signal light,
Wavelengths in the wavelength region outside of 4 and λ5 (λ ₁ to λ ₃ and λ ₆ to
λ ₈ ) is transmitted after being divided and multiplexed. Further, wavelengths of λ ₃ and λ ₆ arranged on both outer sides of λ ₄ and λ ₅ may not be used for signal light transmission.
With this configuration, the propagating signal light is transmitted in a wavelength region farther from λ ₀ , so that the influence of transmission deterioration due to the nonlinear effect is reduced. The range in which both sides of λ ₀ are excluded from the optical signal transmission area is determined by the wavelength interval and the number of relays of adjacent optical signals.

FIG. 3 shows a case in which adjacent wavelength intervals of signal light are equal to each other, and adopting such a method simplifies signal processing in the optical transmitting and receiving apparatus 4. On the other hand, FIG. 4 shows a case in which adjacent wavelength intervals of signal light are not equal to each other, and using such a method has the advantage that nonlinear effects can be dispersed.

When light propagates through an optical fiber, light of a new wavelength is generated near the incident wavelength due to the nonlinear effect. Generally, the magnitude of the nonlinear effect of an optical fiber is expressed in a form proportional to n ₂ · p / Aeff.
Here, n ₂ is the nonlinear refractive index of the optical fiber, p is the optical power, and Aeff is the effective core area of the optical fiber.

On the other hand, as the number of multiplexed wavelengths of signal light propagating in an optical fiber increases, the wavelength of each signal light comes closer. When such approaching signal light is directly propagated as in the conventional case, the signal light near the zero dispersion wavelength λ ₀ generates light of a new wavelength due to the nonlinear effect, and these lights interfere with the nearby optical signal. As a result, the transmission signal is degraded. Further, since the signal light immediately after the optical transmitter 1 or the optical amplifier 3 is amplified to a large power, the transmission deterioration due to the non-linear effect becomes particularly remarkable, and increases with each relay.

[0015] The present invention, as shown in FIG. 3, .lambda.4 disposed on both sides of at least the zero dispersion wavelength lambda ₀ and lambda ₅
Is a method of dividing and multiplexing the wavelength regions (λ _{1 to} λ ₃ and λ _{6 to} λ ₈ ) outside λ ₄ and λ ₅ without using the wavelength as the signal light and transmitting the light. By removing the vicinity of both sides of the wavelength λ ₀ where the nonlinear effect appears most from the transmission band of the signal light, it is possible to suppress most of the transmission deterioration due to the nonlinear effect.

On the other hand, the optical amplifier 3 shown in FIG. 1 or FIG. 2 is usually constituted by supplying pumping light to an optical fiber having erbium added to the core, and the signal light enters the optical amplifier 3 in this state to amplify it. I do. The gain curve of this type of optical amplifier 3 has flat portions B and C on both sides of a valley A, for example, as shown in FIG. The valley A of this curve is not normally used as an amplification band because waveform distortion occurs. Therefore, a configuration is adopted in which the wavelength near zero dispersion at which the nonlinear effect appears remarkably coincides with the valley A of the gain curve which is difficult to use as the optical amplifier 3. According to the present invention, since the wavelength near zero dispersion is not used as the transmission band of the signal light, the wavelength bands of the flat portions B and C of the gain curve of the optical amplifier 3 and the wavelength band of the optical transmission line 2 are effectively used without waste. It is possible to do.

[0017]

Embodiment 1 FIG. 6A shows an optical transmission apparatus 1 according to the present embodiment.
FIG. 3 is a diagram showing each wavelength division multiplexed signal light and their wavelength relationship, wherein the zero dispersion wavelength at 1.55 μm is λ ₀ , and signal light wavelengths λ _{1 to} λ ₁₀ are arranged on both sides of λ _0. It shows the case where it was done. In the present embodiment, at least the wavelengths λ ₄ to λ ₇ arranged on both sides of λ ₀ are not used as signal light, and the wavelengths in the wavelength ranges λ ₁ to λ ₃ and λ ₈ to λ ₁₀ are divided and multiplexed. Transmitted. The optical signal interval is 1 nm. The output power of the optical transmission device 1 is 8.0 mW.

Such a signal light is distributed to a dispersion-shifted optical fiber: 405 km and a single-mode optical fiber: 45 km.
, And the transmission characteristics were measured by the eye pattern in the optical receiver 4.

Here, the wavelength of dispersion-shifted optical fiber is 1.
The dispersion at 55 μm is −2 ps / nm / km, the dispersion slope is 0.01 ps / nm ² / km, and the dispersion at a wavelength of 1.55 μm of the single mode optical fiber is +
18ps / nm / km, dispersion slope 0.058ps
/ Nm ² / km. Accordingly, the dispersion of the entire optical transmission fiber is designed to be zero at a wavelength of 1.55 μm.

FIGS. 7 and 8 show the eye patterns of the optical signals transmitted through these optical fibers. FIG. 7A is a diagram showing an eye pattern by an optical signal of wavelength λ ₃ ,
Since λ ₃ is far from λ ₀ , it has almost no nonlinear effect, the eye is sufficiently open, and it can be seen that transmission without pulse errors can be performed. Further, FIG. 8 (a) is a diagram showing an eye pattern by the optical signal of wavelength lambda _10, lambda ₁₀ is lambda ₀
, It is hardly affected by the nonlinear effect, the eye is sufficiently open, and it can be seen that transmission without a pulse error can be performed.

[0021]

[Comparative Example 1] In contrast, FIG. 6 (b) is a diagram showing those wavelengths relationship with each signal light propagating through the same optical fiber as in Example 1, signals adjacent to both sides of lambda ₀ Light wavelength λ ₁
To [lambda] ₁₀ indicates the case where it is arranged at equal intervals. This signal light was transmitted under the same apparatus and the same conditions as in Example 1.

FIGS. 7 and 8 show the eye patterns of the signal light after transmission through the optical fiber. FIG. 7B shows the wavelength λ ₃
Is a diagram showing an eye pattern by the optical signal, lambda ₃ is lambda ₀
By influence of nonlinear effects due to lambda ₅ and lambda ₄ which are adjacent, the transmission characteristic is distorted in the shape of the eye pattern was found to be degraded. Further, FIG. 8 (b) is a diagram showing an eye pattern by the signal light of the wavelength lambda _10, lambda
₁₀ also suffers from the nonlinear effect due to λ ₇ to λ ₉ adjacent to λ ₀ , and it has been found that the shape of the eye pattern is distorted and the transmission characteristics are degraded.

[0023]

[Embodiment 2] FIG. 9 (a) is a diagram showing each signal light wavelength-division multiplexed by the optical transmission apparatus 1 according to the present embodiment and the wavelength relationship between them, and the zero dispersion wavelength at 1.55 μm is shown. λ ₀ , and shows a case where signal light wavelengths λ _{1 to} λ ₈ are arranged on both sides of λ ₀ . In this embodiment, the wavelengths of λ ₄ and λ ₅ arranged on both sides of λ ₀ are not used as signal light,
Transmitting the wavelength of the wavelength region between _{1 ~λ 3 λ 6 ~λ 8} by division multiplexing. The wavelength interval of the signal light is 2 nm. The output power of the optical transmission device 1 is 1.0 mW / ch.

The signal light is transmitted to the dispersion-shifted optical fiber 100.
The light was propagated through 0 km, and the optical receiver 4 measured the transmission characteristics using the eye pattern. Here, the dispersion of the dispersion-shifted optical fiber at a wavelength of 1.55 μm is 0 ps / nm.
/ Km, and the dispersion slope is 0.01 ps / nm ² / km. The nonlinear refractive index n ₂ of the dispersion-shifted optical fiber is 2.4 [× 10 ⁻²⁰ m ² / W], and the effective core area Aeff of the optical fiber is 80 [μm ² ].

FIG. 10A shows the eye pattern of the signal light after transmission through the optical fiber. 10 (a) is a diagram showing an eye pattern by the signal light of the wavelength lambda _6, not undergone almost non-linear effects because signal light lambda ₆ is separated from the lambda _0, Ai is sufficiently open, It was found that transmission without pulse errors was possible.

[0026]

[Comparative Example 2] In contrast, FIG. 9 (b) is a diagram showing those wavelengths relationship with each signal light propagating through the same optical fiber as in Example 2, signals adjacent to both sides of lambda ₀ Light wavelength λ ₁
Λ ₈ are arranged at equal intervals. This signal light was transmitted under the same apparatus and the same conditions as in Example 2.

FIG. 10B shows the eye pattern of the signal light after transmission through the optical fiber. FIG. 10B shows the wavelength λ _5.
Is a diagram showing an eye pattern by the signal light, lambda ₅ is lambda ₀
It was found that the eye was closed due to the nonlinear effect, the eye was closed, and the transmission characteristics deteriorated.

From these experimental results, when the condition of n ₂ · p / Aeff> 3 × 10 ^-13 (1 / ch) is satisfied, the prohibited range Δλ of λ ₀ is Δλ>
It has been found that if the prohibited range is set as 2 nm, the transmission can be performed with almost no influence of the nonlinear effect.

[0029]

The present invention is embodied in the form described above and has the following effects.

Wavelength 1.55 μ at which chromatic dispersion becomes almost zero
Since the signal light disposed on both sides of m is transmitted with a large wavelength interval, transmission deterioration due to the nonlinear refractive index can be significantly suppressed.

Further, since the wavelength near the zero dispersion of the transmission optical fiber and the wavelength near the valley of the gain curve of the relay optical amplifier are made to coincide with each other, it is possible to suppress the transmission deterioration due to the non-linear effect and to suppress the optical degradation. The bandwidth of the amplifier can be used effectively.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a configuration of an entire optical communication system according to an embodiment.

FIG. 2 is a schematic diagram illustrating a configuration of an entire other optical communication system according to the present embodiment.

FIG. 3 is a diagram illustrating wavelength division multiplexed signal lights and their wavelength relationships.

FIG. 4 is a diagram showing other wavelength division multiplexed signal lights and their wavelength relationships.

FIG. 5 is a graph showing an example of a gain curve of a fiber amplifier.

FIG. 6 is a diagram illustrating other wavelength-division multiplexed signal lights and their wavelength relationships.

FIG. 7 is a graph showing an eye pattern of signal light.

FIG. 8 is a graph showing an eye pattern of another signal light.

FIG. 9 is a diagram showing other wavelength division multiplexed signal lights and their wavelength relationships.

FIG. 10 is a graph showing an eye pattern of another signal light.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical transmission apparatus, 2 ... Optical transmission fiber, 2-0 ...
Dispersion-shifted optical fiber, 2-1 single-mode optical fiber, 2-2 other dispersion-shifted optical fiber, 3 optical amplifier, 4 optical receiver

Claims (3)

[Claims] 1. An optical communication method for multiplexing and transmitting a plurality of signal lights at predetermined wavelength intervals to an optical fiber having a chromatic dispersion of zero within a plurality of signal light wavelength bands. Wherein the wavelength interval of the signal light arranged in the optical communication system is longer than at least the series of adjacent wavelength intervals. 2. The wavelength interval of signal light initially arranged on both sides of the zero-dispersion wavelength is 2 of the series of adjacent wavelength intervals.
2. The optical communication method according to claim 1, wherein the number is twice or more. 3. An optical communication method for multiplexing and transmitting a plurality of signal lights at predetermined wavelength intervals to an optical transmission line having an optical fiber and an optical amplifier having a chromatic dispersion of zero within a plurality of signal light wavelength bands, An optical communication method, wherein a zero dispersion wavelength is arranged at a valley of a gain curve of an optical amplifier.