JPH0738497A - Optical soliton repeater - Google Patents

Abstract

PURPOSE:To extend a relay interval by providing an optical intensity modulator to either of an incident side or a radiation side of an optical amplifier and a clock signal extract section to the optical intensity modulator. CONSTITUTION:N-stages and R-stages of repeaters 8 are provided in an optical soliton transmission line, and each repeater uses an optical filter 4 and an optical modulator 5 to apply control to an optical soliton pulse whose strength is recovered by an optical amplifier 3 without conversion of the light. In this case, a clock extract section 6 extracts a clock signal from an arrived optical soliton pulse train and makes the phase and modulation frequency of the optical modulator 5 coincident with the frequency and phase of the optical soliton pulse train based on the clock signal to execute intensity modulation synchronously with the optical soliton signal. After the optical amplification is applied to the optical soliton pulse the same as that of the 1st stage, the optical soliton pulse is made incident in the R-th stage of the repeater 8. Thus, the noise in the transmission line is eliminated.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention can extend the repeater interval by eliminating the noise of the optical soliton pulse in the optical soliton transmission line, and can economically realize the long distance and ultra large capacity optical communication. The present invention relates to a repeater for an optical soliton transmission line that can be easily maintained.

[0002]

2. Description of the Related Art In order to increase communication capacity in optical communication, it is necessary to narrow the pulse width and pulse interval of optical pulses to increase the transmission density of optical pulses per unit time. However, when the pulse width is narrowed, the frequency spectrum of the optical pulse spreads in inverse proportion to it, and the light propagating in the optical fiber has a different propagation speed depending on its frequency (that is, wavelength) (called group velocity dispersion). As it does, the waveform of the optical pulse spreads. That is, in the optical communication using a normal optical pulse,
Due to the effect of group velocity dispersion existing in the optical fiber, the pulse width of the optical pulse broadens, and the extent of the broadening becomes more remarkable as the pulse width becomes narrower. Therefore, with the current technology, it is difficult to further increase the transmission capacity, and there is a limit to increase the capacity of optical communication.

In the group velocity dispersion of the currently used optical fiber mainly composed of quartz, the wavelength is about 1.3 μm.
(Approximately 1.5μ for an optical fiber with a devised structure.
In the wavelength band of longer wavelength than m), there is an anomalous dispersion characteristic in which the propagation speed becomes slower as the wavelength of light becomes longer. On the other hand, when a high-intensity optical pulse propagates in an optical fiber, the wavelength shifts to the long wavelength side in the front side of the optical pulse and to the short wavelength side in the rear side due to the self-phase modulation effect, which is one of the nonlinear optical effects. . When anomalous dispersion and self-phase modulation effect occur at the same time in this optical fiber, the pulse width is compressed because the front side of the optical pulse is delayed and the rear side proceeds, and the effect of this compression and the effect of spreading due to group velocity dispersion are balanced. By the way, the light pulse becomes stable. This is an optical soliton.

Further, since the practically used optical fiber has a slight optical loss, the intensity of the light propagating in the optical fiber is attenuated as it propagates. Since the amount of light attenuation varies depending on the wavelength, in the case of performing long-distance optical communication, it is preferable to use the wavelength band (minimum loss wavelength region) near the 1.55 μm band where the attenuation of light intensity is the smallest. preferable. The optical loss in this wavelength band of 1.55 μm is 0.2 dB (about 5%) per 1 km. This 1.
The 55 μm band is the anomalous dispersion region as described above, and the optical soliton is formed in the lowest loss wavelength region of this anomalous dispersion region, so it is also effective for long-distance optical communication (Reference 1: A.Ha.
segawa: Appl.Opt.23, p.3302 (1984)). As described above, the optical soliton is characterized by propagating in the optical fiber without changing the waveform by balancing the spread due to the group velocity dispersion and the compression due to the nonlinear optical effect (optical Kerr effect). Therefore, the transmission method using the optical soliton is a promising method for realizing long-distance, large-capacity optical communication, and many experiments have been conducted and studied.

In order to compensate for the decrease in the intensity of optical soliton pulses caused by optical loss, an erbium-doped optical fiber amplifier (ED) recently developed for optical soliton transmission.
FA) is used (Reference 2: M. Nakazawa et.al .:
Electron. Lett, Vol. 25, p. 199 (Feb. 1989)). In addition, 1.
As an optical amplifier used in the vicinity of the 5 μm band, one using a semiconductor laser other than the above-mentioned EDFA has been studied. An optical soliton transmission method using such an optical amplifier is called a dynamic soliton transmission method.
If the dynamic soliton transmission method that performs optical amplification by this EDFA is applied to an optical transmission line, it is possible to communicate over a distance of several thousand km to 10,000 km with a transmission capacity of 10 Gbit / s (Reference 3: M. Nakazawa). et.al .: IEEE J. Quantum Electro
n.Vol.26, p.2095 (1990)). In addition, even in optical communication with a low transmission capacity that does not use ordinary optical solitons, if EDFA is used, communication over a distance of several thousand km to 10,000 km is possible (Reference 4: H. Taga et.al. : OFC / IOOC '93, PD1, (Fe
b.1993), San Jose).

[0006]

By the way, when performing long-distance optical communication, an interval (relay interval) at which optical amplifiers are installed.
Has a problem that it can only be extended to several tens of kilometers. This is due to the increased noise
This is because the / N ratio deteriorates. Therefore, it is necessary to install a large number of optical amplifiers in the optical transmission line, which is disadvantageous in that installation and maintenance costs are high. Further, there is no means for removing these noises, and the relay interval must be limited to be short. This is the same whether an optical soliton pulse is used or an ordinary optical pulse that is not an optical soliton is used.

There are the following two reasons why the noise increases when the relay interval is extended. First, one is that the amplifier spontaneous emission noise (ASE) increases. For example, as described above, the amount of attenuation of light propagating in the optical fiber is very small, about 0.2 dB per 1 km in the 1.55 μm band,
At a distance of 100 km, attenuation of 20 dB (that is, attenuation to 1/100) will occur. Therefore, it is necessary to increase the amplification degree of the optical amplifier, but increasing the amplification degree is not preferable because ASE increases.

The other is that the number of dissipative waves (non-soliton components: waves that propagate and spread as they propagate) that do not form optical solitons increases. In order to form an optical soliton, it is necessary that the waveform of the optical pulse is of the sech (Note: hyperbolic secant function) type, and the group velocity dispersion and the self-phase modulation effect are balanced. If these conditions are deviated, a non-soliton component is generated from a part of the optical pulse and becomes noise. In the dynamic soliton transmission method, this non-soliton component has a characteristic of increasing in proportion to the square of the relay interval. That is, the relay interval is La,
Letting Lc be a characteristic distance of a soliton called a soliton period, it is proportional to (La / Lc) ² . Since this Lc becomes shorter in inverse proportion to the square of the pulse width, it becomes particularly remarkable when the optical pulse width is narrowed in order to increase the communication capacity. As described above, when the relay interval is extended, the SN ratio is deteriorated due to the accumulation of these noises and long-distance transmission cannot be performed. Therefore, there is a problem that the relay interval must be limited to be short. there were.

The present invention has been made in view of the above circumstances, and by removing the noise of the optical soliton pulse in the optical soliton transmission line, the repeater interval can be extended, and therefore, the long distance, An object of the present invention is to provide an optical soliton repeater device that can economically realize ultra-high capacity optical communication and that is easy to maintain and manage.

[0010]

In order to solve the above problems, the present invention employs the following optical soliton repeater.
That is, the optical soliton repeater according to claim 1 is characterized in that a light intensity modulator is provided on either the incident side or the emission side of the optical amplifier, and the clock signal extraction unit is provided in the light intensity modulator. There is.

An optical soliton repeater apparatus according to a second aspect of the invention is characterized in that a band transmission type optical filter is provided on either the incident side or the emitting side of the optical amplifier.

[0012]

In the optical soliton repeater of the present invention, the fact that the noise caused by the ASE noise and the non-soliton component of the optical pulse is a dissipative wave is utilized. That is, in the optical soliton repeater according to claim 1 of the present invention, the optical intensity modulator is provided on either the incident side or the outgoing side of the optical amplifier, and the clock signal extraction unit is provided on the optical intensity modulator. By
For an optical soliton pulse propagating in an optical fiber,
Intensity modulation synchronized with the optical soliton pulse is performed to remove noise not synchronized with the optical soliton pulse. This makes it possible to extend the relay interval by eliminating the accumulation of optical noise, which is a main factor limiting the relay interval.

Further, in the optical soliton repeater according to the present invention, a band transmission type optical filter is provided on either the incident side or the emission side of the optical amplifier, so that an optical signal of a predetermined wavelength band is transmitted. Then, the other optical signals are removed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a configuration diagram showing an optical soliton transmission line using the optical soliton repeater of the present embodiment. In FIG. 1, 1 is an optical soliton generator, 2 is an optical fiber for transmission,
3 is an optical amplifier, 4 is an optical filter connected to the N (R) stage optical amplifier 3, 5 is an optical modulator connected to the optical filter 4, and 6 is a clock signal extraction unit connected to the optical modulator 5. , 7 are light receivers. Then, the optical amplifier 3, the optical filter 4, the optical modulator 5, and the clock signal extraction unit 6 constitute a relay device 8 of this embodiment. Here, the repeaters 8 are provided at the N and R stages in the optical soliton transmission line. Further, in FIG. 1, La is a relay interval (interval between the optical amplifiers 3), which is equal to the length of the transmission optical fiber 2.

The optical soliton generator 1 generates an optical trison pulse having a coded (information-carrying) wavelength band of 1.55 μm. For example, a semiconductor laser having a DFB structure, an optical modulator, an optical amplifier, etc. It consists of
The transmission optical fiber 2 is, for example, a silica-based single mode optical fiber having a group velocity dispersion characteristic and a light loss wavelength characteristic in which a zero dispersion region exists near a wavelength band of 1.5 μm. The optical amplifier 3 amplifies the optical soliton pulse signal as light, and may be an erbium-doped optical fiber amplifier (EDFA), a semiconductor optical amplifier, or the like.

The optical filter 4 is a band-pass filter of light (a bandpass filter), and for example, an interference multilayer filter, a Fabry-Perot resonator, etc. can be used. The optical modulator 5 modulates the light intensity, and for example, a Mach-Zehnder type modulator made of lithium niobate (LiNbO ₃ ), an electroabsorption type semiconductor modulator, a semiconductor optical amplifier or the like can be used. it can. The clock signal extraction unit 6 extracts a clock signal from the optical soliton pulse train and outputs the clock signal to the optical modulator 5
Is supplied to.

As the light receiver 7, P having a high-speed response characteristic is used.
A photodiode having an IN structure or the like can be used. Since the optical filter 4 is also included in the optical amplifier 2 as a component, when the optical filter in the optical amplifier 2 is used, the independent optical filter 4 may not be used.

Next, an optical soliton transmission method using the repeater 8 will be described. First, the optical soliton generator 1
Optical trison pulse is generated by the optical soliton pulse, and the optical soliton pulse is incident on the optical fiber 2 for transmission.
Propagate inside. Since the intensity of the propagating optical signal decreases due to the optical loss of the optical fiber 2, the intensity of the optical soliton pulse is restored by the optical amplifier 3 (first-stage optical amplification operation). After repeating this operation N-1 times (here, about 1 to 10 times), this optical trison pulse is repeated N times.
It is made incident on the relay device 8 of the stage.

In the repeater 8, the optical filter 4 and the optical modulator 5 control the optical soliton pulse whose intensity is restored by the optical amplifier 3 as it is. At this time, the clock signal extraction unit 6 extracts a clock signal from the arrived optical soliton pulse train and matches the modulation frequency and phase of the optical modulator 5 with the frequency and phase of the optical soliton pulse train with reference to the clock signal. , Intensity modulation synchronized with the optical soliton signal is performed. After that, the optical trison pulse is subjected to the same optical amplification operation in the N + 1th stage to the R-1th stage as in the first stage, and then this optical trison pulse is incident on the repeater device 8 of the Rth stage to be converted into an optical soliton signal again. Performs synchronized intensity modulation. This optical trison pulse propagates a required distance, and then the optical soliton signal is received by the photodetector 7.

It should be noted that the optical filter 4 and the optical modulator 5 can obtain the same effect even if the installation order of these is changed. In addition, the same effect can be obtained when the optical filter 4 and the optical modulator 5 are installed on either the incident side or the emitting side of the optical amplifier 3, or when the optical amplifier 3 is installed so as to be scissored. . The input signal light to the clock signal extraction unit 6 can be taken from any position such as the incident side of the optical amplifier 3, the incident side of the optical filter 4, the incident side of the optical modulator 5, or the emitting side thereof. .

Now, comparing the case where the repeater 8 of this embodiment is used in the optical soliton transmission line and the case where it is not used (when the soliton control by the optical modulator 5 is not performed) In case of 10Gbit / s communication capacity, La
Was about 20 to 30 km, but 10 when used.
I was able to extend it to about 0 km. In this case, the total length of the optical fiber from the optical soliton generator 1 to the light receiver 7 could be 10,000 km or more. Further, when the communication capacity is further increased to 20 Gbit / s, 40 Gbit / s, etc., the same effect is obtained.

FIG. 2 is a diagram showing the principle of noise removal in the relay device of this embodiment. The ASE generated from the optical amplifier 3 and the dissipative wave due to the non-soliton component of the optical pulse are continuous noises having a uniform intensity in time. As shown in FIG. 2, this noise passes through the optical modulator 5 and is modulated so that the light intensity changes with time as shown by the waveform (a) in the figure. At this time, the peak of the modulation has the same light intensity as the input, but the light of the trough of the modulation becomes weaker, so that the noise is reduced as a whole. Further, the noise part of the waveform of (a) spreads due to the group velocity dispersion of the optical fiber 2 while being transmitted through the transmission line, but the part of the optical soliton pulse does not spread. Therefore, after propagation, the optical signal has a waveform as shown in (a). This optical signal is re-modulated by the next optical modulator 5 to become an optical signal having a waveform as shown in (c), and the noise portion is reduced as described above. Here, it is important to match the center of the optical soliton pulse with the peak of the modulation waveform, and the clock signal extraction unit 6 performs this function.

FIG. 3 is a graph showing the result of analyzing the effect of noise removal by the relay device 8 of this embodiment. As is clear from the figure, when the relay device 8 is not used (broken line)
Then, noise is accumulated exponentially, but it is understood that the noise is suppressed to a certain amount or less when the relay device 8 is used.

FIG. 4 is a diagram showing the result of a simulation analysis by a computer performed for confirming the effect of the relay device 8 of this embodiment. Here, the interval between the optical amplifiers 3,
That is, the length of the optical fiber 2 is 100 km, and 10
The change of the waveform for each propagation distance when an optical soliton pulse corresponding to Gbit / s is propagated through the optical fiber 2 is shown every 1000 km including the input waveform up to 20,000 km. As can be seen from the figure, the optical soliton pulse is 20,000 km.
It can be seen that the signal propagates in a stable state up to. On the other hand, when the repeater 8 is not used, the optical soliton pulse is 100
It was confirmed that the light cannot propagate in the optical fiber 2 of km, and it can propagate when the distance between the optical amplifiers 3 is 20 to 30 km or less.

[0025]

As described above, according to the first aspect of the present invention.
According to the optical soliton repeater described, the light intensity modulator is provided on either the incident side or the emission side of the optical amplifier, and the clock signal extraction unit is provided on the light intensity modulator, so that the light propagates in the optical fiber. For the optical soliton pulse, intensity modulation synchronized with the optical soliton pulse can be performed to remove noise not synchronized with the optical soliton pulse. Therefore, the accumulation of optical noise, which is a main factor limiting the relay interval, can be removed, and the relay interval can be extended.

Further, according to the optical soliton repeater of the present invention, since the band-pass type optical filter is provided on either the incident side or the emitting side of the optical amplifier, the optical signal in the predetermined wavelength band is transmitted. It is possible to pass the light and remove other optical signals.

As described above, the installation interval of the optical amplifiers can be extended to 2 to 3 times that of the conventional one, and thus the number of optical amplifiers required when constructing a system for long-distance optical communication is halved. Therefore, it is possible to provide a repeater for an optical soliton transmission line, which can significantly reduce the construction and maintenance costs of equipment.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an optical soliton transmission line using a repeater according to an embodiment of the present invention.

FIG. 2 is a diagram showing a principle of noise removal of the relay device according to the embodiment of the present invention.

FIG. 3 is a diagram showing a result of analyzing an effect of noise reduction by the relay device according to the embodiment of the present invention.

FIG. 4 is a diagram showing a result of a simulation analysis of the relay device according to the embodiment of the present invention.

[Explanation of symbols] 1 optical soliton generator 2 optical fiber for transmission 3 optical amplifier 4 optical filter (band-pass filter) 5 optical modulator (light intensity modulator) 6 clock signal extraction unit 7 light receiver 8 repeater La repeater distance

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Claims (2)

[Claims] 1. A repeater device for optical soliton transmission comprising an optical amplifier, wherein an optical intensity modulator is provided on either an incident side or an outgoing side of the optical amplifier, and a clock signal is provided to the optical intensity modulator. An optical soliton repeater characterized by having an extraction unit. 2. The optical soliton repeater according to claim 1, wherein a band transmission optical filter is provided on either the incident side or the outgoing side of the optical amplifier.