JP4131833B2 - Optical amplifier and optical repeater transmission system using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical amplifying device that enables ultrahigh-speed and ultra-large-capacity transmission using optical pulses and an optical repeater transmission system using the same.
[0002]
[Prior art]
Conventionally, an optical parametric amplifier is known as an optical amplifier for amplifying an optical signal transmitted through an optical fiber transmission line. Regarding the effect of removing intensity noise of the “1” level of the optical signal by this optical parametric amplifier, the experimental results are shown in the following Non-Patent Document 1, and the effect is confirmed. In the output of this parametric optical amplifier, the noise at the “1” level of the optical signal is suppressed by the non-linear saturation characteristic of the parametric optical amplifier, and therefore includes the noise that the optical signal had when it was input to the parametric optical amplifier. There will be no. That is, by using an optical parametric amplifier instead of a linear repeater, the influence of noise remains in one repeat section and does not accumulate. That is, the degradation of the signal S / N ratio can be reduced as compared with the case where a linear repeater is used.
The optical parametric amplifier itself also generates noise (spontaneously emitted optical noise: ASE), and this noise is added to the output signal, but this noise component also gains as it propagates through the optical fiber for optical parametric amplification. It will be suppressed because of saturation.
[0003]
The relationship between the excitation light input to the optical parametric amplifier and the polarization of the signal light will be described below. When a single-mode optical fiber is used as the medium for the parametric amplifier, the signal light and control light propagating in the single-mode optical fiber change as the polarization state propagates, and the principal axis direction is not fixed. The intensity of the four-wave mixed light generated depends on whether the polarization relationship between the control light and the control light is parallel or orthogonal at the incident end of the optical parametric amplifier, and the polarization of the signal light and control light is orthogonal. It is known that almost no four-wave mixed light is generated when incident. This phenomenon is described in detail in Non-Patent Document 2.
[0004]
In order to avoid the influence of the fluctuation of the polarization in the optical fiber, the following method can be used.
1) The polarization of the pumping light incident on the optical parametric amplifier is fixed, and the signal light is matched with the polarization of the signal light and the pumping light using a polarization controller.
2) A light source that combines two orthogonally polarized waves that are orthogonal to each other is used as excitation light.
Of these methods, if the speed of the polarization fluctuation of the signal light is slow, the polarization of the signal light and the excitation light can be matched by using a commercially available polarization automatic controller. On the other hand, in the method 2), the efficiency of the four-wave mixing output is an output that does not depend on the polarization state of the signal light, and a polarization-independent operation can be realized. Details of this polarization-independent operation are described in Non-Patent Document 3.
The conventional optical parametric amplifier described above has a problem that the waveform distortion in the fiber is large and a large pumping power is required.
[0005]
[Non-Patent Document 1]
K. Inoue and T. Mukai, "Experimental study on noise characteristics of a gain-saturated fiber optical parametric amplifier," IEEE Journal of Lightwave Technology, vol.20, No.6, pp.969-974, 2002.
[Non-Patent Document 2]
K. Inoue, "Polarization effect on four-wave mixing efficiency In a single-mode fiber," IEEE Journal of Quantum Electronics, vol.28, No.4, pp.883-894, 1992.
[Non-Patent Document 3]
RMJopson, AHGnauck, and RMDerosier, "Compensation of fiber chromatic dispersion by spectral inversion," Electron. Lett., Vol. 29, pp. 576-578, 1993.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to enable an optical relay transmission distance of short pulse and high bit rate to be extended by eliminating the influence of S / N degradation due to fluctuations in signal light intensity that could not be removed by conventional optical amplifiers. An object of the present invention is to provide an amplifier and an optical repeater transmission system using the same.
[0007]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems. The invention according to claim 1 is directed to a first light having zero dispersion at a wavelength λp different from the wavelength λs of the signal light (where λp <2λs). and the fiber, the incident on the first excitation light source, the first of said output light and the signal light of the excitation source combined to the first optical fiber wavelength of the output light is set to the wavelength λp A first optical filter that extracts only the component of the wavelength λs from the output light of the first optical fiber, and a second optical fiber in which the zero dispersion wavelength is set equal to the wavelength λs And an optical clock generator that outputs an optical clock having a wavelength λclk (where λclk> λs / 2), and the optical clock and the output light of the first optical filter are combined into the second optical fiber. Incident second multiplexing means and the second optical fiber From the output light, a second optical filter for extracting only a component of the wavelength RamudaFWM satisfies the relationship 2 / λs = 1 / λclk + 1 / λFWM using said wavelength [lambda] s and the wavelength Ramudaclk, the wavelength [lambda] s and said wavelength RamudaFWM A third optical fiber having zero dispersion at a wavelength λp2 that satisfies the relationship 2 / λp2 = 1 / λs + 1 / λFWM, a second pumping light source in which the wavelength of output light is set to the wavelength λp2 , and From the output light of the third optical fiber, the third combining means that combines the output light of the second excitation light source and the output light of the second optical filter and enters the third optical fiber And a third optical filter that extracts only the component of the wavelength λs .
[0008]
The invention according to claim 2, wherein the first to place the third optical fiber, light of claim 1, periodically poled is characterized by using a ferroelectric having a structure inverted It is an amplification device.
[0009]
According to a third aspect of the present invention, there is provided an optical pulse transmission method for transmitting an optical pulse signal train using a transmission optical fiber, wherein the optical amplifying device according to the first or second aspect is disposed at every predetermined distance. Then, the optical pulse transmission system is characterized in that the optical pulse signal train is relay-transmitted.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an optical repeater transmission system using an optical parametric amplifier according to a first embodiment of the present invention. In this figure, 101 is an optical signal transmitter, 102-1, 102-2,... Are optical transmission lines, 103-1, 103-2,... Are optical amplifiers, 104-1, 104-2,. ... Are optical parametric amplifiers, and 105 is an optical signal receiver.
[0012]
FIG. 2 is a diagram showing the configuration of the optical parametric amplifiers 104-1, 104-2,..., 201 is a signal input port, 202 is a pumping light source, 203 is a multiplexer, and 204 is a nonlinear optical medium (optical fiber). , 205 is an optical filter, and 206 is an output port. As the optical parametric amplifiers 104-1, 104-2,..., Four-wave mixing in an optical fiber is used in this embodiment. FIG. 3 shows the relationship between the wavelength dispersion characteristics of the optical fiber 204 and the wavelengths of the signal light and the pumping light. The wavelength λp of the pumping light is set to the zero dispersion wavelength of the optical fiber, and the wavelength λs of the signal light is set to a wavelength somewhat away from λp. Here, when the intensity of the excitation light is increased, four-wave mixed light is generated at a wavelength λFWM at a position substantially symmetrical to λs with the wavelength λp as the center as shown in FIG. These three wavelengths have phase matching conditions
2 / λp = 1 / λs + 1 / λFWM
Is established.
[0013]
When the excitation light intensity is further increased, the intensity of the four-wave mixed light gradually increases, and when it becomes approximately the same as the signal light intensity, a conversion from the four-wave mixed light to the signal light intensity occurs via the excitation light. Light and four-wave mixed light intensity increase simultaneously. At this time, the signal light is amplified (parametric amplification). FIG. 5 shows a characteristic example of the change in the output signal light intensity when the input signal light intensity is changed. When the input signal light intensity is increased, the output signal light intensity is saturated, and when the input signal light intensity is further increased, the output signal light intensity is decreased. By utilizing this output saturation characteristic, it is possible to suppress intensity fluctuation of the “1” level of the optical signal. FIG. 6 shows the operation of suppressing the intensity noise of the optical signal using the optical parametric amplifiers 104-1, 104-2,. If the average of the output light '1' level is set to the maximum output point of the optical parametric amplifiers 104-1, 104-2, ..., the intensity noise component of the signal light can be suppressed by the saturation characteristics. is there.
[0014]
As the characteristics of the optical fiber used for the optical parametric amplifiers 104-1, 104-2,..., A fiber having a large nonlinear optical coefficient is advantageous because it has a small excitation light intensity and can exhibit an amplification effect. In recent years, a photonic crystal fiber or the like having a large nonlinear optical coefficient has been developed, and this fiber can also be applied. Further, the dispersion characteristic of the optical parametric amplifier fiber includes a dispersion characteristic suitable for the type of signal to be transmitted. When transmitting a high-speed time division multiplexing (TDM) signal at one wavelength, the zero dispersion wavelength of the optical parametric amplifier fiber is set to the wavelength of the pumping light source as shown in FIG. When a fiber having a small characteristic is used, amplification with the constant gain within the signal spectrum can be performed most efficiently. On the other hand, when transmitting a wavelength division multiplexing (WDM) signal, the zero-dispersion wavelength of the optical parametric amplifier fiber is set to the wavelength of the pumping light source as shown in FIG. Use of a characteristic fiber can eliminate the influence of crosstalk between WDM signals.
[0015]
As the nonlinear medium for the optical parametric amplifier, a semiconductor optical amplifier (SOA) can be used. However, in the case of the SOA, the gain band is determined by the carrier life and is 10 GHz at most. Cannot be used. In contrast, since the fiber has a high-speed nonlinear response of 1 ps or less, intensity fluctuation can be suppressed even for a high-speed optical signal.
Further, since the gains of the optical parametric amplifiers 104-1, 104-2,... Are 40 dB or more, when the output saturation characteristics can be ensured, the optical amplifiers 103-1, 103-2,. Can be omitted.
[0016]
(Second Embodiment)
FIG. 8 is a diagram showing a second embodiment of the present invention, in which 801 is an optical signal transmitter, 802-1, 802-2,... Are optical transmission lines, 803-1, 803-2,. Are optical amplifiers, 804-1, 804-2, ... are optical parametric amplifiers, 805-1, 805-2, ... are '0' level shapers, 806-1, 806-2, ... .. is an optical clock generator, and 807 is an optical signal receiver.
[0017]
The present embodiment is configured to suppress noise of “0” level of the optical signal. First, the optical signal is the same as that of the first embodiment until the intensity noise of “1” level is suppressed by the optical parametric amplifiers 805-1, 805-2,. The outputs of the optical parametric amplifiers 804-1, 804-2,... Are suppressed to “0” level noise by the “0” level shapers 805-1, 805-2,. The configuration of the '0' level shapers 805-1, 805-2,... Is shown in FIG. 901 is a signal input port, 902 is an optical clock input port, 903 is a control light input port, 904 is a multiplexer, 905 is an optical fiber for four-wave mixing light generation, 906 is an optical filter, 907 is a multiplexer, and 908 is An optical fiber for generating four-wave mixing light, 909 is an optical filter, and 910 is an output port.
[0018]
“0” level shapers 805-1, 805-2,... Are two-stage cascaded wavelength converters using four-wave mixing and the operation principle is shown in FIG. First, in the first stage, output pulses (wavelength λclk) and signal light (wavelength λs) of the optical clock generators 806-1, 806-2,... Are input to the four-wave mixed light generation optical fiber 905. (FIG. 10A), four-wave mixed light (wavelength λ FWM) is generated (FIG. 10B). The zero dispersion wavelength of the four-wave mixing light generating optical fiber 905 is set equal to the signal light wavelength λs. Therefore, the relationship between λclk, λs, and λFWM is
2 / λs = 1 / λclk + 1 / λFWM
It has become. In such a configuration, the intensity of the four-wave mixed light is proportional to the square of the signal light intensity. In other words, the “0” level of the signal light has an output proportional to the intensity product of the square of the light intensity and the optical clock, so that the generated four-wave mixed light has a “1” level and a “0” level. The intensity difference, that is, the extinction ratio is improved compared to the input signal light.
[0019]
From the output light of the four-wave mixed light generating optical fiber 905, only the four-wave mixed light is extracted by the optical filter 906 (FIG. 10C). Next, the four-wave mixing light and the cw excitation light (wavelength λp2) are combined and input to the second four-wave mixing light generating optical fiber 908 (FIG. 10 (d)). At this time, an optical amplifier that amplifies the intensity of the four-wave mixed light may be inserted. The zero dispersion wavelength and the pumping light wavelength λp2 of the optical fiber 908 are equal and set so as to satisfy the following conditions.
2 / λp2 = 1 / λsig + 1 / λFWM
With this setting, the wavelength λsig of the second four-wave mixed light output from the optical fiber 908 becomes equal to the original signal light wavelength λs (FIG. 10 (e)). By extracting with the optical filter 909, a signal with improved extinction ratio can be extracted.
[0020]
(Third embodiment)
FIG. 11 shows a non-linear optical element constituting an optical parametric amplifier in the third embodiment of the present invention. Although the operation of this embodiment is the same as that of the first example, a domain inversion LiNbO 3 element is shown as a ferroelectric whose polarization is periodically inverted instead of an optical fiber as a nonlinear optical element. As shown in FIG. 11A, this element realizes quasi phase matching (QPM) by a structure in which the polarization is periodically inverted, and enables highly efficient parametric light amplification. In this element, firstly, second harmonic light of incident excitation light (wavelength λp) is generated (Second Harmonic Generation: SHG). At this time, the phase velocity difference between the incident signal light and the generated SH light is periodically compensated by the domain inversion structure. This is because the domain inversion reverses the polarity of the polarization and reverses the phase velocity relationship.
[0021]
The graph of FIG. 11B shows the relationship between the propagation distance of the SH light propagating in the domain inversion element and the generated SH light intensity. The curve A in the graph has no domain inversion structure, and the curve B is It is a graph which shows the relationship of propagation distance -SH light intensity in the case of having a domain inversion structure. In the case of having a domain inversion structure, the SH light intensity increases monotonously with the propagation distance as shown by curve B. Output light (wavelength λout) is generated by the difference frequency generation (DFG) between the SH light and the signal light (wavelength λs). At this time, the relationship between the wavelengths of the excitation light, the signal light, and the output light is
1 / λout = 2 / λp−1 / λs
From
2 / λp = 1 / λs + 1 / λout
Thus, as shown in FIG. 11 (c), this is the same as the wavelength relationship in the case of four-wave mixing. However, in the case of a QPM element, since the wavelength conversion efficiency per unit length is extremely high, it is possible to obtain sufficient efficiency even with an element length of about 1 cm, and the apparatus can be downsized and stably operated.
In FIG. 11, a waveguide structure element is shown as the domain-inverted LiNbO 3 element, but a bulk crystal may be used instead of the waveguide.
[0022]
【The invention's effect】
As described above, according to the present invention, intensity noise of an optical signal that cannot be removed by a conventional optical amplifying device can be suppressed and accumulation of noise can be prevented, so that it is used in a multistage relay transmission system. Therefore, it is possible to increase the relay distance, so that a large effect can be expected in the expansion of the operation margin of the system and the economy.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an optical repeater transmission system using an optical parametric amplifier according to a first embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of an optical parametric amplifier 104 in the same embodiment.
FIG. 3 is a waveform diagram for explaining the operation of the optical fiber 204 of the optical parametric amplifier 104;
FIG. 4 is a waveform diagram for explaining the operation of the optical fiber 204 of the optical parametric amplifier 104;
FIG. 5 is a diagram showing input / output characteristics of the optical parametric amplifier 104;
6 is a waveform diagram for explaining a noise suppression operation of the optical parametric amplifier 104. FIG.
FIG. 7 is a diagram showing characteristics of an optical fiber 204 of the optical parametric amplifier 104. FIG.
FIG. 8 is a block diagram showing a configuration of an optical repeater transmission system using an optical parametric amplifier and a “0” level shaper according to a second embodiment of the present invention.
FIG. 9 is a block diagram showing a configuration of a “0” level shaper according to the same embodiment;
FIG. 10 is a waveform diagram for explaining the operation of the “0” level shaper according to the same embodiment;
FIG. 11 is an explanatory diagram for explaining a third embodiment of the present invention.
[Explanation of symbols]
104-1, 104-2, 804-1, 804-2 ... optical parametric amplifier 202 ... pumping light sources 203, 904, 907 ... multiplexers 204, 905, 908 ... optical fibers 205, 906, 909 ... optical filters 805- 1, 805-2 ... '0' level shapers 806-1, 806-2 ... optical clock generators

Claims (3)

A first optical fiber having zero dispersion at a wavelength λp (where λp <2λs) different from the wavelength λs of the signal light ;
A first excitation light source in which the wavelength of output light is set to the wavelength λp ;
A first multiplexing unit which is incident on the first optical fiber by multiplexing the output light and the signal light of the first excitation light source,
A first optical filter that extracts only the component of the wavelength λs from the output light of the first optical fiber;
A second optical fiber in which a zero dispersion wavelength is set equal to the wavelength λs ;
An optical clock generator for outputting an optical clock having a wavelength λclk (where λclk> λs / 2) ;
Second combining means for combining the optical clock and the output light of the first optical filter and entering the second optical fiber;
From the output light of the second optical fiber , using the wavelength λs and the wavelength λclk
2 / λs = 1 / λclk + 1 / λFWM
A second optical filter that extracts only the component of the wavelength λFWM that satisfies the relationship :
Using the wavelength λs and the wavelength λFWM
2 / λp2 = 1 / λs + 1 / λFWM
A third optical fiber having zero dispersion at a wavelength λp2 satisfying the relationship :
A second excitation light source in which the wavelength of the output light is set to the wavelength λp2 ,
Third combining means for combining the output light of the second excitation light source and the output light of the second optical filter and entering the third optical fiber;
A third optical filter that extracts only the component of the wavelength λs from the output light of the third optical fiber;
An optical amplifying device comprising: 2. The optical amplifying apparatus according to claim 1 , wherein a ferroelectric material having a structure in which polarization is periodically inverted is used instead of the first to third optical fibers. 3. An optical pulse transmission system for transmitting an optical pulse signal train using a transmission optical fiber, wherein the optical pulse signal train is relayed by arranging the optical amplifying device according to claim 1 at every predetermined distance. An optical pulse transmission system characterized by transmitting.

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