JP4008483B2 - Optical fiber communication system using optical phase conjugation. - Google Patents

Description

  The present invention relates to an optical fiber communication system using optical phase conjugation.

  By applying non-linear optics, it is possible to achieve new functions and improve characteristics that cannot be obtained by conventional optical technology. In particular, when optical phase conjugation is used, it is possible to compensate for phase fluctuations and chromatic dispersion in the transmission path.

  Conventional optical communication systems are constructed using optical components having linear optical characteristics, and are simple but have limited characteristics and functions. In the field of recent optical communication systems, a non-repeating system or an optical amplifying and repeating system extending from several hundred km to several thousand km is being realized, and its transmission speed is from several Gb / s to over 10 Gb / s. It's fast.

  In such a system, there are many problems to be solved. Among them, the influence of the chromatic dispersion of the fiber is one of the most serious and serious problems. In the above-described system, transmission characteristics deteriorate due to the influence of wavelength dispersion and the like, and as a result, the transmission speed and transmission distance are limited.

  The conventional measures against chromatic dispersion have been mainly focused on minimizing the fiber dispersion itself. As a result, fibers with zero dispersion at the transmission center wavelengths of 1.3 μm and 1.55 μm are realized.

In addition, studies on optical modulators that are not easily affected by chromatic dispersion are in progress, and modulators using LiNbO ₃ have been developed.

  Furthermore, researches are underway to apply reverse chirping to the transmitted signal light in advance and compensate by chromatic dispersion of the transmission line, or to perform optical or electrical dispersion compensation in the receiver.

  As described above, research on chromatic dispersion is being conducted in all transmitters, transmission lines, and receivers, reflecting the seriousness of the problem.

When the signal light is an optical pulse (including a pulse train composed of a plurality of optical pulses) whose intensity is modulated or amplitude-modulated, the pulse waveform may be distorted due to a cause other than chromatic dispersion. Examples of this are:
(1) Waveform distortion due to the synergistic effect of wavelength dispersion and optical Kerr effect,
(2) ASE (Amplified Spontaneous) of optical amplifier in optical amplification multi-relay transmission
Emisson) Waveform distortion due to random phase fluctuations due to noise accumulation,
Etc. are considered to be prominent. The present invention is to deal with the waveform distortion (1) among these.

Consider the case where an optical pulse propagates through a dispersion medium. When a non-chirped pulse passes through the dispersion medium, if it is a normal dispersion medium (∂ ² β / ∂ω ² > 0), it shifts to the low frequency side at the rising edge of the pulse and shifts to the high frequency side at the falling edge To do.

On the other hand, in the case of an anomalous dispersion medium (∂ ² β / ∂ω ² <0), it shifts to the high frequency side at the rising edge of the pulse and shifts to the low frequency side at the falling edge. Here, β represents a propagation constant, and ω represents the angular frequency of light.

  In the normal dispersion medium, the longer the wavelength, the faster the group velocity, and in the anomalous dispersion medium, the shorter the wavelength, the faster the group velocity. Therefore, in either case, the pulse width increases.

  On the other hand, when the light intensity is high, the refractive index is increased by the optical Kerr effect.

Only changes. Here, n ₂ is a quantity called a nonlinear refractive index, and in the case of a silica fiber, the value is about 3.2 × 10 ⁻²⁰ m ² / W. When an optical pulse is subjected to the optical Kerr effect in a nonlinear medium,

Only the spectrum diffuses (chirps). Here, Δz is the interaction length. This phenomenon is generally referred to as self-phase modulation (SPM).
This SPM shifts to the low frequency side at the rise of the optical pulse, and shifts to the high frequency side at the fall. Due to this chirping by SPM, the influence of dispersion becomes more conspicuous, and as a result, pulse distortion becomes more significant. Therefore, when the optical pulse is subjected to the optical Kerr effect in the dispersion medium, the pulse is further diffused in the case of the normal dispersion medium than in the case of only the dispersion, but the pulse compression occurs in the case of the anomalous dispersion medium.

  Therefore, when considering the effect of chromatic dispersion described above, large pulse diffusion occurs in the case of a normal dispersion medium, and in the case of an anomalous dispersion medium, the larger effect of pulse diffusion by chromatic dispersion and pulse compression by SPM. Appears. Optical solitons are a balance between these two effects.

  In general, it is easy to think that it is convenient to add SPM pulse compression in an anomalous dispersion medium, so that it is convenient to hold a high signal SN, but recently it has become possible to transmit at a high level of optical power using an optical amplifier. With the development of dispersion-shifted fibers, it has become possible to achieve relatively small chromatic dispersion values, and it has become impossible to generally apply pulse compression.

  That is, the pulse compression effect becomes too great and a large waveform distortion occurs. In particular, in the case of an NRZ pulse, pulse compression occurs intensively at the rising and falling portions of the pulse. Therefore, in an extreme waveform change or in an extreme case, the falling portion overtakes the rising portion, and the pulse is 3 Some things break up. In addition, in the case of long-distance optical amplification multi-relay transmission, there is a problem that four-wave mixing occurs between signal light as excitation light and spontaneous emission light from an optical amplifier, and the influence thereof becomes significant.

  Therefore, an object of the present invention is to provide an optical fiber communication system in which waveform distortion due to a synergistic effect of chromatic dispersion and the optical Kerr effect is suppressed.

  Another object of the present invention is to maintain an optimal reception state regardless of polarization fluctuations in the system of the present invention.

  Still another object of the present invention is to optimize the supervisory control in the system of the present invention.

  Another object of the present invention is to adapt the inventive system to wavelength division multiplexing (WDM).

  According to the present invention, a plurality of optical transmitters that respectively transmit a plurality of signal lights having different wavelengths, a first optical fiber that transmits the plurality of signal lights, and the first optical fiber are supplied from the first optical fiber. A plurality of phase conjugate light generators that respectively receive the plurality of signal lights and generate phase conjugate light corresponding to at least one of the plurality of signal lights, and the phase conjugate light output from each of the phase conjugate light generators A plurality of optical filters that respectively transmit light, a plurality of second optical fibers that respectively transmit the phase conjugate light from each of the optical filters, and a plurality that receive the phase conjugate light from each of the second optical fibers When the first optical fiber and each of the second optical fibers are divided into the same number, it corresponds when counting from the phase conjugate light generator in order in each divided section. Of chromatic dispersion The average value is set to the same sign and approximately inversely proportional to the length of each divided section, and the average value of the product of optical frequency, signal light intensity and nonlinear refractive index in each divided section is the length of each divided section. An optical fiber communication system is provided that is set to be approximately inversely proportional to.

  According to the present invention, since the optical fiber communication system is configured as described above, there is an effect that the waveform distortion due to the synergistic effect of the chromatic dispersion and the optical Kerr effect can be compensated. In the following description, it is assumed that the optical fiber is a single mode fiber in order to be applied to a high-speed system.

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

FIG. 1 is a diagram showing a basic configuration of an optical fiber communication system according to the present invention. The transmitter 2 generates a signal light by performing modulation based on the transmission data, the signal light E _S is the first optical fiber as a probe light SMF1 (length L _1, dispersion D _1, nonlinear refractive index n ₂₁ ) And then input to a phase conjugate light generator (PC) 6 in the middle of all transmission paths.

The phase conjugate light generator 6 converts the signal light Es into the phase conjugate light E _C using the pump light E ₀ , which is converted into the second optical fiber SMF2 (length L ₂ , dispersion D ₂ , nonlinear refractive index n _22. ) To the receiver 4.

  In the receiver 4, the phase conjugate light is received by the light receiver and signal detection is performed. Signal detection is performed, for example, by direct optical detection or optical heterodyne detection after phase conjugate light is extracted by a bandpass filter. Thereby, the transmission data is reproduced.

  The optical fiber used here is, for example, a silica fiber, and a typical example is a 1.3 μm zero-dispersion fiber or a 1.55 μm dispersion-shifted fiber generally used in optical communication. The signal light may be frequency multiplexed signal light of output signal light from a plurality of light sources having different frequencies.

  The phase conjugate light generator 6 includes a second-order or third-order nonlinear optical medium and means for supplying signal light and pump light to the medium. When a second-order nonlinear optical medium is used, phase conjugate light is generated by a parametric effect, and when a third-order nonlinear optical medium is used, phase conjugate light is generated by degenerate or non-degenerate four-wave mixing. appear.

  As the third-order nonlinear optical medium, for example, a silica optical fiber can be used. In this case, excellent phase conjugate light can be obtained by making the wavelength of the pump light in the four-wave mixing substantially coincide with the zero dispersion wavelength of the fiber. Can be generated.

  FIG. 2 is a block diagram illustrating an example of a phase conjugate light generator. This phase conjugate light generator includes an optical fiber 121 as a nonlinear optical medium, a laser diode 122 as a pump light source, and an optical coupler 123 as an optical means that adds signal light and pump light to the optical fiber 121 and supplies them. It has.

  The optical fiber 121 is preferably a single mode fiber. In this case, when non-degenerate four-wave mixing is caused by slightly differentiating the wavelength of the signal light and the wavelength of the excitation light, the wavelength that gives zero dispersion of the optical fiber 121 is the wavelength of the pump light (the laser diode 122). It should be matched with the oscillation wavelength.

  The optical coupler 123 has four ports 123A, 123B, 123C, and 123D. The first optical fiber SMF1 of FIG. 1 is connected to the port 123A, the laser diode 122 is connected to the port 123B, the first end of the optical fiber 121 is connected to the port 123C, and the port 123D is set to the dead end. ing. The second end of the optical fiber 121 is connected to the second optical fiber SMF in FIG.

  In the present specification, the term “connection” means an operational connection, including a case where the optical connection is directly made, and a case where the connection is made via an optical element such as an optical filter or an optical isolator. And the case of being connected after appropriately adjusting the polarization state.

  The optical coupler 123 functions to output the light supplied to the ports 123A and 123B from the port 123C. Examples of the optical coupler 123 include a fiber fusion type, a half mirror, an optical multiplexer, and a polarization beam. A splitter or the like is used.

  According to this configuration, the signal light supplied to the port 123A of the optical coupler 123 and the pump light supplied to the port 123B can be added and supplied to the optical fiber 121, which is a nonlinear optical medium. Thus, the signal light can be converted with phase conjugate light.

  FIG. 3 is a block diagram showing another example of the phase conjugate light generator. This phase conjugate light generator is characterized in that a polarization scrambler (polarization scrambler) 124 is provided between the laser diode 122 and the port 123B of the optical coupler 123, in contrast to the example of FIG. .

  In general, there are two polarization modes whose polarization planes are orthogonal to each other in the polarization mode of a single-mode fiber, and these two polarization modes are combined by the influence of various disturbances. The polarization state of the light supplied to one end does not match the polarization state of the light output from the second end of the fiber. Therefore, when a single mode fiber is used as the transmission path, the polarization state of the signal light supplied to the phase conjugate light generator varies with time due to environmental changes or the like.

  On the other hand, the conversion efficiency from signal light to phase conjugate light in the phase conjugate light generator depends on the relationship between the polarization state of the signal light supplied to the phase conjugate light generator and the polarization state of the pump light.

  In the example of FIG. 3, since the pump light from the laser diode 122 is combined with the signal light via the polarization scrambler 124, the polarization state of the supplied signal light varies with time. However, stable operation of various optical devices can be realized.

The polarization scrambler 124 is normally configured using a combination of a half-wave plate and a quarter-wave plate, a LiNbO ₃ phase modulator, and the like. For example, the pump light output from the laser diode 122 is substantially linearly polarized light. In some cases, it functions to rotate its plane of polarization.

  In the example shown in FIG. 3, the polarization scrambler 124 is made to act on the pump light output from the laser diode 122, but the port 123A of the optical coupler 123 and the first optical fiber SMF1 in FIG. A polarization scrambler may be arranged between or in the transmitter so that the polarization scrambler acts on the signal light.

  Next, the principle of the present invention will be described. The propagation of signal light E (x, y, z, t) = F (x, y) φ (z, t) exp [i (ωt−kz)] in optical fiber transmission is generally described by the following nonlinear wave equation. Is possible. Here, F (x, y) represents a lateral mode distribution, φ (z, t) represents a complex envelope of light, and this φ (z, t) changes sufficiently slowly compared to the frequency ω of light. Assume that.

Where T = t−β ₁ z (β ₁ is the propagation constant), α is the loss of the fiber, β ₂ is the chromatic dispersion of the fiber,

Represents the coefficient of the optical Kerr effect in the fiber. Here, n ₂ and A _eff represent the nonlinear refractive index and effective core area of the fiber, respectively. c is the speed of light in vacuum. Here, up to the first order dispersion was considered, and higher order dispersion was omitted. Further, α, β ₂ , and γ are functions of z, and are expressed as α (z), β ₂ (z), and γ (z), respectively. Further, the position of the phase conjugate light generator is the origin (z = 0). Here, the following normalization function is introduced.

here,

Represents an amplitude. When α (z)> 0, the transmission path has a loss, and when α (z) <0, it represents a gain. A (z) ≡A (0) represents the case of no loss. A (z) ² = P (z) corresponds to the optical power. Substituting Equations (5) and (6) into Equation (3) yields the following evolution equation.

Here, the following conversion is performed.

As a result, equation (7) can be converted as follows.

Here, sgn [β ₂ ] ≡ ± 1 takes +1 when β ₂ > 0, that is, normal dispersion, and −1 when β ₂ <0, ie, abnormal dispersion. If equation (9) holds, the complex conjugate also holds, and the following equation is obtained.

  The complex conjugate light u * follows the same evolution equation as that for u. However, the propagation direction at that time is reversed. This operation is exactly the operation of a phase conjugate light generator. In the transmission type phase conjugate light generator, the above is equivalent to inverting the phase shift due to GVD (group velocity dispersion) and SPM.

Now consider the system of FIG. In long-distance transmission, the transmission line loss is compensated by optical amplification relay. A phase conjugate light generator is disposed between the transmission path I (length L ₁ ) and the transmission path II (length L ₂ ) corresponding to the optical fibers SMF1 and SMF2 in FIG.

In the normalized coordinates (ζ axis), the phase conjugate light generator is placed at the midpoint ζ = 0, and the receiver is placed at ζ = ζ ₀ . In the transmission line I (−ζ ₀ <ζ <0), u (ζ) follows the evolution equation (9). The phase conjugate light generator converts u (0) into phase conjugate light u * (0). u * (ζ) propagates in the transmission line II (0 <ζ <ζ ₀ ) according to the evolution equation (10).

  At this time, within the normalized distance dζ at any two points −ζ and ζ that are symmetrical with respect to the position (ζ = 0) of the phase conjugate light generator on the ζ axis, If the values of the parameters are set so that the term coefficients are equal, u * at −ζ becomes the phase conjugate light of u at ζ. That is, the following two expressions are the conditions.

Equation (11) shows that the dispersion signs of the transmission lines I and II need to be equal, which matches the conditions for dispersion compensation. Considering that γ> 0 and A (z) ² > 0 in the fiber, the above conditions can be summarized as follows.

  The sign of the phase shift due to GVD and SPM in (−ζ) in the transmission line I is inverted by the phase conjugate light generator. Therefore, the waveform distortion due to this phase shift is compensated by the distortion due to the phase shift at (ζ) in the transmission line II. Thus, if compensation by the above settings is repeated for each small section, compensation over the entire length becomes possible.

  Next, the compensation condition is described in terms of the z coordinate. From equation (13)

Get. That is, the condition is that the ratio of the chromatic dispersion to the product of the nonlinear constant and the optical power in each section is equal. Here, −z ₁ and z ₂ are two points that satisfy the following expression.

Equations (16) and (17) are obtained from equations (14) and (15).

dz ₁ and dz ₂ are the lengths of the small sections at −z ₁ and z ₂ , respectively, and each section length is inversely proportional to the dispersion in the section or inversely proportional to the product of the nonlinear constant and the optical power. Here, considering the relationship between the dispersion β ₂ and the dispersion parameter D, D = − (2πc / λ ₂ ) β ₂ , the following relationship is obtained from the equations (16) and (17). D is a function of z and is also expressed as D (z).

It can be seen that the condition for compensation is that the increase in one of the two positions symmetric with respect to the phase conjugate light generator and the decrease in the other are equal for both the dispersion and nonlinear effects.

Equations (18) and (19) are necessary conditions for compensation, and indicate that the total dispersion amount and the total amount of the optical Kerr effect in each corresponding small section are equal. Here, considering the expression (4) and I = P / A _eff represents the light intensity, the product of the dispersion value, nonlinear refractive index, and light intensity of each small section of the transmission path I and transmission path II is the length of the section. It is shown that compensation is possible if the ratio is set to be inversely proportional to the ratio and the ratio is set to be equal.

  In particular, when α, D, and γ are constant and the power fluctuation is small, if the equations (18) and (19) are integrated,

Here, consider a method of providing a gain for compensating for the loss. First, it is possible to use a distributed constant gain medium as a transmission line. For example, a Raman amplifier, a doped fiber amplifier diluted with Er ³⁺ ions, or the like can be considered.

  In the present invention, the ratio between the optical Kerr effect and the dispersion value is controlled. By providing the same value of the optical Kerr effect and dispersion ratio at equivalently symmetrical positions with respect to the phase conjugate light generator, complete compensation can be realized.

  In order to increase this ratio along the transmission line, the dispersion should be gradually decreased or the optical Kerr effect should be gradually increased. It is possible to change the value of the dispersion by the design of the fiber. For example, the above-mentioned ratio can be changed by changing the zero dispersion wavelength of the dispersion-shifted fiber (DSF), or changing the relative refractive index difference between the core and cladding of the fiber and the core diameter.

  On the other hand, the optical Kerr effect can be changed by changing the nonlinear refractive index or changing the light intensity. That is, an optical fiber applicable to the present invention can be manufactured by continuously changing at least one fiber parameter selected from loss, nonlinear refractive index, mode field diameter, and dispersion. Here, a method for changing the light intensity is considered.

For example, in order to increase the light intensity along a lossy transmission line, the effective core area A _eff may be gradually decreased within a range where the loss does not change so much. For example, if the mode field diameter (MFD) is halved, the light intensity is about 4 times.

  For a larger loss, the MFD must be further reduced. However, if the MFD is made too small, the loss increases and no effect is obtained. The practical minimum value of MFD is at most 2 to 3 μm.

  Considering that the MFD of the 1.3 μm zero-dispersion SMF is about 10 μm and the MFD of the 1.55 μm DSF is about 8 μm, the loss that can be handled only by the MFD is about 7 dB for the SMF and about 6 dB for the DSF.

  Even when there is a larger loss, the present invention may be implemented by combining the effect of reducing the core diameter and the effect of reducing the dispersion value. For example, if the dispersion value can be halved, a distribution satisfying the equation (14) can be realized even when there is a further loss of 3 dB.

FIG. 5 is a diagram showing a first embodiment of the present invention. Here, the minute sections 1j (length Δz _1j ) and 2j (long) at symmetrical positions z _1j and z _2j (defined by the equation (15)) of each transmission line with the phase conjugate light generator 6 interposed therebetween. Each parameter of Δz ₂ j)

Set to be. D _1j , ω ₁ , n _21j , <I _1j > are the dispersion parameter, optical frequency, nonlinear refractive index, and average intensity in section 1 j, respectively, and D _2j , ω ₂ , n ₂₂ j, <I _2j > are sections The dispersion parameter, optical frequency, nonlinear refractive index, and average intensity at 2j.

A specific example will be described. Now, it is assumed that the dispersion of the optical fiber SMF1 is constant at D ₁ = −30 ps / nm / km, and the dispersion of the optical fiber SMF2 is constant at D ₂ = −0.3 ps / nm / km. At this time, L ₁ / L ₂ = D ₂ / D ₁ = 1/100 from the equation (20).

Therefore, for example, when the total length of the optical fiber SMF2 is L ₂ = 50 km, L ₁ = 500 m. This indicates that transmission with no distortion of 50 km becomes possible by predistorting the waveform with a 500 m fiber.

  Alternatively, a plurality of fibers having different fiber parameters may be arranged in tandem so as to satisfy the expressions (22) and (23), and each fiber may be spliced and connected.

FIG. 6 is a diagram showing a second embodiment of the present invention. Here, a case where the present invention is applied to multi-relay transmission using an optical amplifier is shown. Now, as the optical fiber SMF2 is transmission path, the middle of the (N-1) optical amplifiers A-1, · · ·, A- (N-1) the relays at intervals l ₂ overall length L ₂ = Nl ₂ optical amplification repeater transmission. At this time, as shown in FIG. 6, the optical fiber SMF1 is also equally divided into N in the same way as the number of relays, and the distance of each section is l ₁ and the total length is L ₁ .

At this time, since the ratio of L ₁ and L ₂ (ratio of l ₁ and l ₂ ) is proportional to the reciprocal of the dispersion of each fiber, L ₁ = (D ₂ / D ₁ ) L ₂ (l ₁ = (D ₂ / D ₁₎ l ₂₎ to. Regarding the optical Kerr effect, Equation (23) is established in each corresponding minute section in the corresponding section with respect to the phase conjugate light generator 6.

  For example, in the case of the above-described dispersion value, in the transmission in the relay section 50 km, the above setting is performed by dividing the optical fiber SMF1 every 500 m. Therefore, for example, if a fiber having a total length of 20 km divided by 40 for every 500 m is used as the optical fiber SMF1, transmission of a total length of 2000 km by 39 relays every 50 km after the phase conjugate light generator 6 becomes possible.

  In this case, it goes without saying that the length of each section of the optical fiber SMF2 corresponding to each section of the length 500 m of the optical fiber SMF1 is different. The corresponding section is defined by the equation (15), and the section of large dispersion among the sections of the optical fiber SMF1 covers a longer section of the optical fiber SMF2.

Here, the division is performed at equal intervals. However, since the equations (22) and (23) only need to be established for each corresponding section, it is not particularly necessary to have equal intervals. In particular, since the optical fiber SMF1 is not provided with an optical amplifier that compensates for loss, it may be difficult to satisfy the conditions with realistic dispersion and power. In such a case, the requirement for dispersion and power can be relaxed by increasing L ₁ as the intensity decreases due to loss without dividing L ₁ at equal intervals.

  Also, the dispersion of the optical fiber SMF2 is not made constant. For example, by dividing each relay section to increase the dispersion relatively in the high power portion and relatively reduce the dispersion in the low power portion, The effect of loss can be reduced. By such a method, it is possible to ease the requirements for dispersion and power in the optical fiber SMF1.

  Needless to say, the finer the division in such a method, the more effective, but in practice, even a few divisions are sufficiently effective. The required number of divisions depends on the transmission speed and transmission distance.

  In the embodiment of FIG. 6, the optical fiber SMF2 is optically amplified and relayed, but the optical fiber SMF1 may be compensated by the optical fiber SMF2 by the same method after optically amplified and relayed. An example is shown in FIG.

  FIG. 7 is a diagram showing a third embodiment of the present invention. Here, the number of relays is made the same before and after the phase conjugate light generator 6 and is set so that the expressions (22) and (23) are established in a symmetric section with respect to the phase conjugate light generator 6. Specifically, N optical amplifiers A1-1,..., A1-N are provided in the middle of the first optical fiber SMF1, and N optical amplifiers are also provided in the middle of the second optical fiber SMF2. , A2-N are provided.

In this embodiment, since L ₁ can be lengthened, L ₂ is also lengthened correspondingly, and long-distance transmission is possible.

  At this time, as described above, the dispersion in the optical fiber SMF2 is not made constant. For example, each relay section is divided so that the dispersion is relatively large in the high power portion and relatively small in the low power portion. Equivalently, the loss effect can be reduced.

  FIG. 8 is a diagram showing a fourth embodiment of the present invention. Here, in the transmission using the average intensity approximation, an application in the case where the dispersion and the optical Kerr effect are not constant in the transmission path is shown. First, for the average value of the dispersion parameter,

And the average value of the product of nonlinear refractive index and light intensity,

Set so that. This allows approximate compensation.

As shown in FIG. 8, with respect to the compensation residue, the dispersion D ₃ of the third optical fiber SMF 3 having a length L ₃ provided between the optical fiber SMF 2 and the receiver 4 and the optical car By adjusting the effect n ₂₃ I ₃ appropriately, almost complete compensation is possible.

FIG. 9 is a diagram showing a fifth embodiment of the present invention. In this embodiment, the fourth embodiment of FIG. 8 is applied to an optical amplification multi-relay transmission system. In this case, a plurality of repeaters may be provided before and after the phase conjugate light generator 6 and set so that the expressions (22) and (23) are satisfied in the corresponding section with respect to the phase conjugate light generator 6. However, as shown in Japanese Patent Application No. 5-221856, a certain amount of compensation is possible even if the above equation is established for the average value over the entire length. In order to further improve the degree of compensation, the third optical fiber SMF3 is used in the same manner as in the fourth embodiment of FIG. 8, and the dispersion D ₃ and the optical Kerr effect n ₂₃ I ₃ are adjusted. .

  By the way, in an actual long-distance transmission system, the dispersion value fluctuates depending on the surrounding environment. In particular, the influence of the fluctuation of the dispersion value due to the temperature fluctuation is large, and this is particularly noticeable in the case of a system set to a small dispersion value near zero dispersion.

The dispersion value near zero dispersion can be changed according to the slope of the second-order dispersion (about 0.08 ps / nm 2 / km) by changing the wavelength of the signal light. On the other hand, in a system that generates phase conjugate light using four-wave mixing, if the angular frequency of the phase conjugate light is ω _C , the angular frequency of the pump light is ω _P , and the angular frequency of the signal light is ω _S , then ω _C Since there is a relationship of = 2ω _P −ω _S , it is possible to change ω _C by changing ω _S or ω _P.

In this way, by adjusting ω _S at the transmitter or by adjusting ω _P at the phase conjugate light generator 6 by a control signal sent from a terminal station (not shown), it is possible to always perform optimal transmission according to dispersion fluctuations. It can be carried out.

FIG. 10 is a diagram showing the frequency arrangement of the signal light, the pump light, and the phase conjugate light with respect to the zero dispersion wavelengths ω ₁₀ and ω ₂₀ of the optical fibers SMF1 and SMF2.

If, when the dispersion curve of the two fibers are shifted in the same direction by a change in environment (see the chain line in the figure), the but it is to shift the omega _S and omega _C in the same direction, with respect to changes in the omega _S omega _C so shifted in the opposite direction, it is preferable to vary by the same the omega _P while changing the omega _S in the same direction as the omega _S. In the simplest case as shown in FIG. 10, ω _S and ω _P may be shifted in the same direction by the same magnitude (Δω) (ω _C + Δω = 2 (ω _P + Δω) − (ω _S + Δω)). . In reality, fluctuations in dispersion are not simple, and will be corrected accordingly. Actually, the optimum state is obtained by finely adjusting ω _S and ω _P while monitoring the received waveform at the terminal station.

  FIG. 11 is a diagram showing a sixth embodiment of the present invention. This embodiment basically uses the average intensity, but controls the dispersion shown in FIG. 7 in order to mitigate the influence of the change (decrease) in power between the optical amplifiers.

  Specifically, each relay section is divided into several parts, and the dispersion value is gradually reduced toward the transmission direction. An example is shown in FIG.

Here, in the case where the average dispersion of the transmission path is set to D ₁ = −30 ps / nm / km and D ₂ = −0.30 ps / nm / km, each relay section is divided into three and toward the transmission direction − An example is shown in which 0.35, -0.30, and -0.25 ps / nm / km are set.

  At this time, if the relay section is 51 km, for example, it will be divided every 17 km, and the slope of dispersion will be about -0.04 dB / km. Therefore, for example, if the loss of the fiber is -0.20 dB / km, the change in the ratio expressed by the equation (23) can be reduced to about -0.16 dB / km.

  Thereby, a state equivalent to a state with a smaller loss can be realized. Therefore, it is possible to enlarge the relay section of the optical amplifier. In addition, the distortion compensation effect can be improved even at the same relay interval.

  FIG. 13 is a diagram showing a seventh embodiment of the present invention. In this embodiment, when the present invention is applied to an optical amplification multi-relay transmission system, the ratio of the nonlinear effect to the dispersion is made constant in the optical fiber SMF1.

That is, the optical fiber SMF1 is divided into a plurality of sections so that the sum of the dispersion values D _1j Δz _1j in each section j matches the total chromatic dispersion of the optical fiber SMF2, and the nonlinear effect in each section j The value of the dispersion ratio (分散 n ₂₁ I _Sj / D _1j ) is set constant. On the other hand, the optical fiber SMF2 performs optical amplification multi-relay transmission using average value approximation. The total amount of nonlinear effects in the optical fiber SMF1 is made to coincide with the total amount of average values of nonlinear effects in the optical fiber SMF2. The optical fiber SMF2 may be set in the same manner as the optical fiber SMF1.

Since the decrease in I _Sj due to loss can be compensated by gradually decreasing D _1j , the ratio of the nonlinear effect to the dispersion can be made constant. Further, by increasing the length of the section Δz _1j in a manner inversely proportional to the loss, the dispersion value in each section can be made constant. That is, n ₂₁ I _Sj Δz _1j is made constant, and D _1j Δz _1j is made constant.

  In this embodiment, the number of divisions of the optical fiber SMF1 is the same as the number of relays in the optical fiber SMF2. However, in such an average value approximation, in practice, the number of divisions of the optical fiber SMF1 is set to the relay of the optical fiber 2. Even if it is less than the number, the effect can be obtained. That is, an average value for several divisions in the same number of divisions is substituted. The effect at this time depends on the transmission speed and the transmission distance.

FIG. 14 is a diagram showing an eighth embodiment of the present invention. In this embodiment, the light output from the phase conjugate light generator 6 is branched into two by an optical coupler 8 or an optical switch (not shown) instead of the optical coupler 8, and one of the branched lights is received by a receiver 4 by an optical fiber SMF2 (length L ₂ ). The other branched light is transmitted to the receiver 4 (# 2) through the optical fiber SMF3 (length L ₃ ).

  The optical fiber SMF2 is provided with optical amplifiers A2-1, 2,..., N2, and the optical fiber SMF3 is provided with optical amplifiers A3-1, 2,.

  As in this embodiment, even when the present invention is applied to the branching of the transmission path, the dispersion from the light from the nonlinear optical medium 6 is matched to the distance to each receiver 4 (# 1, # 2). And can be transmitted depending on the light intensity.

  FIG. 15 is a diagram showing a ninth embodiment of the present invention. In this embodiment, additional compensation is performed using a plurality of third fibers in wavelength multiplexing transmission. In the figure, 10 (# 1, # 2,..., #N) denotes an optical filter that performs channel selection for phase conjugate light transmitted by the optical fiber SMF2. The light output from each optical filter 10 (# 1, # 2..., #N) is received by a receiver 4 (# 1) via optical fibers SMF3-1,. , # 2,..., #N).

The N-channel wavelength multiplexed signal light E _S1 , E _S2 ,..., E _SN (frequency: ω _S1 , ω _S2 ,..., Ω _SN ) is transmitted by the optical fiber SMF1, and then the phase conjugate light generator 6 To N-channel wavelength multiplexed phase conjugate light E _C1 , E _C2 ,..., E _CN (frequency: ω _C1 , ω _C2 ,..., Ω _CN ) Receive by machine.

  At this time, the dispersion of each channel in the optical fibers SMF1 and SMF2 is as shown in FIG.

  In dispersion compensation using a phase conjugate light generator, since the signs of dispersion need to be the same before and after the phase conjugate light generator, the frequency arrangement is as shown in FIG. 16 for zero dispersion. In the example shown in the figure, conversion is from normal distribution to normal distribution. In this case, the absolute value of dispersion for the first channel is the minimum value in the optical fiber SMF1, whereas the absolute value of dispersion for the Nth channel is the minimum value in the optical fiber SMF2.

  Therefore, in principle, it is difficult to perform complete dispersion compensation for all channels simultaneously. In the ninth embodiment of FIG. 15, in such a case, the output of the optical fiber SMF2 is branched and then the frequency is selected for each channel, and then the third fiber SMF3 that matches the residual compensation amount for each channel. −1, 2,..., N are used for additional compensation.

The tenth embodiment of the present invention shown in FIG. 17 is for ideally compensating all channels equally. Here, the signal light different fiber SMF11,12 for each channel, ..., transmitted in 1N, this time, is transmitted in the intensity _{(I 11, I 12, I} 1N) commensurate with different dispersion. The phase conjugate light generators 6 (# 1), (# 2)..., (#N) for each channel of the output light of the optical fiber SMF1, or one phase conjugate light generator (not shown) for all channels at once. Are converted into phase conjugate light, transmitted through a common optical fiber SMF2, and received in the same manner as in the ninth embodiment of FIG.

  However, an optical multiplexer that multiplexes a plurality of signal lights or phase conjugate lights is not shown. Here, the third optical fiber SMF3 is unnecessary. In this case, the dispersion of each channel and the setting of the nonlinear effect may be performed by any of the methods described so far.

  By the way, since the phase conjugate light generator has polarization dependence, the conversion efficiency varies depending on the polarization state of the signal light, and the system characteristics become unstable. In addition, many optical components used in phase conjugate light generators and optical amplifiers have polarization dependence, and the signal level becomes unstable when they are connected in multiple stages.

  In order to suppress this, polarization diversity or polarization active control may be applied, or polarization scrambling may be performed on signal light or pump light. In particular, the method of performing polarization scrambling of signal light at the transmitter is simple in configuration and is promising from the viewpoint of eliminating the influence of various polarization dependencies that are currently problematic in long-distance transmission. .

  FIG. 18 is a diagram showing an eleventh embodiment of the present invention. This optical fiber communication system is characterized in that a polarization maintaining fiber (PMF) is used as the first optical fiber SMF1 as compared with the second embodiment of FIG.

  The transmitter 2 outputs signal light that is substantially linearly polarized. Generally, a polarization maintaining fiber has at least one principal axis, and linearly polarized light having a polarization plane parallel to the principal axis can be transmitted while maintaining the polarization plane.

  The signal light from the transmitter 2 is supplied to the first optical fiber SMF1 so that its plane of polarization is parallel to the main axis of the first optical fiber SMF1. The first optical fiber SMF1 and the phase conjugate light generator 6 are connected to each other so that the polarization plane of the signal light output from the first optical fiber SMF1 matches the polarization plane of the pump light in the phase conjugate light generator 6. Is done.

  The conditions for compensating the waveform distortion due to the synergistic effect of chromatic dispersion and the optical Kerr effect are the same as those in the second embodiment of FIG.

  The generation efficiency of phase conjugate light generated by four-wave mixing (FWM) or optical parametric amplification depends on the polarization states of the input signal light and the pump light. In the present embodiment, since the polarization state of the signal light input to the phase conjugate light generator 6 is fixed, the phase conjugate light generator 6 can generate phase conjugate light stably and with high efficiency.

  Desirably, the transmitter 2, the first optical fiber SMF1, and the phase conjugate light generator 6 are arranged in the transmitting station, the second optical fiber SMF2 is used as a transmission line, and the receiver 4 is arranged in the receiving station. .

  FIG. 19 shows the twelfth embodiment of the present invention. In this embodiment, (N-1) optical amplifiers A-1,..., A- (N-1) are provided in the middle of the first optical fiber SMF1, and receive the phase conjugate light generator 6 and the receiver. The machine 4 is connected by a second optical fiber SMF2.

  The first optical fiber SMF1 is composed of a polarization maintaining fiber, and it is assumed here that the principal axis directions of the respective fiber sections coincide with each other for convenience. Polarizers 12-1,..., 12- (N-1) are respectively provided on the signal light output sides of the optical amplifiers A-1, ..., A- (N-1). Yes. The polarization main axis of each polarizer is set to be substantially parallel to the plane of polarization of the signal light output from each optical amplifier. In other words, the polarization main axis of each polarizer is arranged so as to be substantially parallel to the main axis of each fiber section.

  Desirably, the transmitter 2 is disposed in the transmitting station, the first optical fiber SMF1 is used as a transmission path, and the phase conjugate light generator 6, the second optical fiber SMF2, and the receiver 4 are disposed in the receiving station. Yes.

  When the first optical fiber SMF1 made of a polarization-maintaining fiber is used as a transmission line, the length is normally 10 km or more, so that signal light that is substantially linearly polarized is supplied to the input end of the first optical fiber SMF1. Even so, the linear polarization state of the signal light may be lost at the output end of the first optical fiber SMF1. Therefore, in this embodiment, the polarization state of the signal light is improved on the signal light output side of each optical amplifier. Therefore, the polarizers 12-1,..., 12- (N-1) may be provided on the signal light input side of the optical amplifiers A-1,. . Also, the polarizer need not be added to every optical amplifier.

  Also in this embodiment, as in the eleventh embodiment of FIG. 18, it is possible to maintain an optimal reception state regardless of polarization fluctuations.

  FIG. 20 is a diagram showing still another example of the phase conjugate light generator. This phase conjugate light generator is characterized in that it has two laser diodes 122A and 122B as pump light sources, in contrast to the phase conjugate light generator of FIG.

  The laser diodes 122A and 122B respectively output first and second pump lights that are substantially linearly polarized waves. The first and second pump lights are combined by the polarization coupler 125 so that their polarization planes are orthogonal to each other, and are supplied to the optical fiber 121 that is a nonlinear optical medium via the optical coupler 123.

  Desirably, the first and second pump lights have different optical frequencies, and the difference between them is set equal to or greater than the frequency corresponding to the transmission speed of the signal light. Desirably, the first and second pump lights have substantially the same amplitude.

  According to the configuration of the phase conjugate light generator, phase conjugate light is always generated in the optical fiber 121 that is a nonlinear optical medium, so that an optimal reception state can be maintained regardless of the polarization fluctuation of the signal light.

  FIG. 21 is a diagram showing a thirteenth embodiment of the present invention. The optical transmitter 2, the first optical fiber SMF1, and the phase conjugate light generator 6 are included in the transmission station ST. The second optical fiber SMF2 is used as a transmission line, and the receiver 4 is included in the receiving station RT.

  The receiving station RT further includes a monitor circuit 14 that monitors a parameter indicating the quality of reproduction of transmission information in the optical receiver 4. The monitor circuit 14 outputs a monitor signal.

  The transmitting station ST further includes a controller (feedback means) 16. The controller 16 receives the monitor signal from the monitor circuit 14, and the wavelength or power of the signal light in the optical transmitter 2 or the phase conjugate light generator 6 so that the parameter monitored in the monitor circuit 14 becomes an optimum value. Feedback control of the wavelength or power of the pump light is performed.

  For example, by controlling the wavelength of the signal light and / or pump light, the dispersion of the transmission line is maintained at an optimum value, and by controlling the power of the signal light and / or pump light, a waveform resulting from the synergistic effect of the dispersion and the optical Kerr effect Distortion is compensated appropriately.

  FIG. 22 is a diagram showing a fourteenth embodiment of the present invention. The optical transmitter 2 is included in the transmission station ST, and the first optical fiber SMF1 is used as a transmission path. The receiving station RT includes a phase conjugate light generator 6, a second optical fiber SMF 2, an optical receiver 4 and a monitor circuit 14.

  When the control target is the wavelength or power of the pump light in the phase conjugate light generator 6, the controller 16 is included in the receiving station RT, and the control target is the wavelength or power of the signal light in the optical transmitter 2. In this case, it is included in the transmitting station ST.

  In the system of FIG. 21 or FIG. 22, the monitor signal supplied from the monitor circuit 14 to the controller 16 can be transmitted through the transmission path of this system. For example, bidirectional transmission may be performed, and a low-speed monitoring signal may be superimposed on signal light in the reverse direction.

  Next, several embodiments for solving the problem described with reference to FIG. 16 when the present invention is adapted to wavelength division multiplexing (WDM) will be described.

FIG. 23 is a diagram showing a fifteenth embodiment of the present invention. The transmitters 2 (# 1, # 2,..., #N) output signal lights E _S1 , E _{S2 ,.} The optical frequencies of these signal lights are ω _S1 , ω _{S2 ,.}

  These signal lights are transmitted through a plurality of first optical fibers SMF11, SMF12,..., SMF1N, and are added and branched by an optical multi / demultiplexer 18 comprising a star coupler or the like.

  The branched signal lights are respectively supplied to phase conjugate light generators 6 (# 1, # 2,..., #M). The phase conjugate light generator 6 (# 1, # 2,..., #M) generates phase conjugate light corresponding to at least one of the plurality of supplied signal lights. The generated phase conjugate light is transmitted through the optical filter 20 (# 1, # 2,..., #M) and then transmitted through the plurality of second optical fibers SMF21, SMF22,. # 1, # 2..., #M).

Phase conjugate light transmitted by a plurality of second optical _{fibers, E 'C1, E' C} 2, ···, indicated by E _'CM.

Each length of the first optical fiber SMF1j (j = 1, 2,..., N) is L _1j , dispersion is D _1j , nonlinear coefficient is γ _1j , and the power of each signal light is P _1j . Suppose there is. Each length of the second optical fiber SMF2k (k = 1, 2,..., M) is L _2k , dispersion is D _2k , nonlinear coefficient is γ _2k , and the power of each phase conjugate light is _Let P _2k .

  At this time, each parameter is set so that the following two conditions are satisfied.

D _1j L _1j = D _2k L _2k = (constant)
γ _1j P _1j / D _1j = γ _2k P _2k / D _2k = (constant)
Here, the meaning of “constant” includes that the average value in an arbitrary section in each fiber is constant.

Here, compensation of waveform distortion by each second optical fiber SMF2k is set to be optimized for phase conjugate light passing through the band of the optical filter 20 (#k). Further, the channel E ′ _Ck extracted by the combination of the phase conjugate light generator 6 (#k) and the optical filter 20 (#k) is included in the band of any one channel of signal light or an optical filter in the vicinity thereof. This is phase conjugate light of a plurality of channels.

  The channel that transmits each optical filter can be arbitrarily set by controlling the wavelength of pump light and / or controlling the transmission wavelength of the optical filter in the phase conjugate light generator.

  For example, this system functions as a distribution system when the second optical fiber is used as a transmission line, and when the second optical fiber is in a receiving station or a repeater, channel switching (crossing) is performed. Functions as a system.

  FIG. 24 is a diagram showing a sixteenth embodiment of the present invention. This system uses a common first optical fiber SMF1 for a plurality of optical transmitters 2 (# 1, # 2,..., #N) as compared with the fifteenth embodiment of FIG. It is characterized in that

  With this change, the input end of the first optical fiber SMF1 is connected to each optical transmitter 2 (#j) via the optical multiplexer 22, and the output end is connected to each phase conjugate light generator via the optical demultiplexer 24. 6 (#k).

  The dispersion in the common first optical fiber SMF1 is made substantially constant for all channels. For example, as the first optical fiber SMF1, a dispersion-shifted fiber having a large dispersion, a 1.3 μm band zero dispersion fiber for 1.55 μm band signal light, and a 1.55 μm band zero dispersion fiber for 1.3 μm band signal light are used. By using it, the above-mentioned conditions can be satisfied.

  With respect to such a common first optical fiber SMF1, each second optical fiber SMF2k satisfies the conditions of the present invention, so that an optimum reception state can be obtained for each channel.

  FIG. 25 is a diagram showing a seventeenth embodiment of the present invention. Here, a combination of N optical fibers SMF11 ′, SMF12,..., SMF1N ′ having relatively large dispersion and a common optical fiber SMF1 ′ having relatively small dispersion is used as the first optical fiber. It has been.

  The optical fibers SMF11 ′, SMF12 ′,..., SMF1N ′ and the optical fiber SMF1 ′ are connected by an optical multiplexer 22, and the optical fiber SMF1 ′ and each phase conjugate light generator 6 (#k) are optically connected. They are connected by a multiplexer.

  Also in this system, by satisfying predetermined conditions for the first optical fiber and the second optical fiber, the waveform distortion can be satisfactorily compensated for each channel, and an optimum reception state can be obtained.

  FIG. 26 is a diagram illustrating an example of a channel selector. Here, the channel selector 26 is provided in association with each optical transmitter 2 (#j).

  The channel selector 26 generates a control signal based on the data from the optical transmitter 2 (#j). A control signal from the channel selector 26 is supplied to the controller 28.

  The controller 28 selects the signal light of a desired channel based on the supplied control signal, and determines the wavelength of the pump light in the phase conjugate light generator 6 (#k) and the characteristics of the optical filter 20 (#k). Control at least one.

  FIG. 27 is a diagram showing another example of the channel selector. Here, the channel selector 26 is provided in association with each optical receiver 4 (#k), and the channel selector 26 generates a control signal based on the data from the optical receiver 4 (#k).

  The controller 28 selects the signal light of the desired channel based on the control signal supplied from the channel selector 26, the wavelength of the pump light in the phase conjugate light generator 6 (#k), and the optical filter 20 (#k). Control at least one of the characteristics of

  Next, the results of a demonstration experiment conducted to confirm the effectiveness of the present invention will be described.

  Referring to FIG. 28, a block diagram of the system used in the demonstration experiment is shown. This system substantially corresponds to the sixth embodiment of FIG.

  The transmitter corresponds to the transmitter 2 of FIG. 11, the fiber compensator corresponds to the first optical fiber SMF1 of FIG. 11, and the phase conjugate light generator (Phase controller) of FIG. Corresponding to the phase conjugate light generator 6, the dispersion shifted fiber (DSF-1, 2,..., 46) and the erbium doped fiber amplifier (EDFA 1, 2,..., 45) are the second light in FIG. Corresponding to the fiber SMF2, the receiver (Receiver) corresponds to the receiver 4 of FIG.

Two 3-electrode λ / 4 shift type DFB-LDs (distributed feedback laser diodes) were used as light sources in the transmitter. Time-division multiplexed 20 Gb / s signal light E _S (wavelength λ _S = 1551 nm) is generated by time-division multiplexing 10 Gb / s 2-channel RZ signals having a pulse width (FWHM) of about 40 ps. It was.

To generate a 10 Gb / s RZ pulse, the first LiNbO ₃ modulator (LN-1) is used to intensity modulate E _S with a 10-GHz sine wave, and then a second LiNbO ₃ modulator ( LN-2) was used to modulate the intensity with a 10 Gb / s NRZ data signal (PN: 223-1).

  The modulated ES is input to the two-stage DD-DCFs 1 and 2 with the power P1, and the waveform is compensated in advance.

  Here, “DD-DCF” represents a dispersion-decreasing dispersion-compensating fiber (DD-DCF).

  Each DD-DCF is configured by splicing five DCFs to each other. Each loss of DD-DCF was 0.46 dB / km, and the mode field diameter of each DCF was set to about 4 μm.

In order to approximately satisfy the condition of equation (14), the dispersion parameter D ₁ should be reduced as the average optical power in each of the DCFs decreases. To that end, the length and D ₁ of each of the five DCFs were set as shown in the table.

  The length of each DD-DCF was 13.7 km and the total dispersion of each was −662.8 ps / nm.

Note that two optical amplifiers were cascaded to set the power of light input to each DD-DCF to P ₁ .

Next, the phase conjugate light generator generates E _S previously compensated (distorted) by a non-degenerate forward FWM using pump light E _P with a wavelength λ _P = 1554 nm in a 20 km DSF. It was converted into phase conjugate light E _C (wavelength λ _C = 1557 nm) propagating in the same direction. The conversion efficiency from E _S to E _C was −12 dB.

  Next, the phase conjugate light Ec is a 3036 km transmission line composed of 46 DSFs (0.21 dB / km loss) cascaded and 45 EDFAs (each noise figure is about 6 dB) provided between them. Was supplied to.

The average dispersion at λ _C of this transmission line was minus 0.44 ps / nm / km. Therefore, the difference between the total dispersion in the two-stage DD-DCF and the total dispersion in the transmission line was about 10 ps / m.

  The length of each DSF was 66 km, and the optical input power P2 to each DSF was set to +6 dBm.

The optimum value of P ₁ was +16 dBm under the above conditions. The non-linear constant γ1 of DD-DCF was estimated to be about 18.0 W ⁻¹ km ⁻¹ .

In order to suppress stimulated Brillouin scattering (SBS), E _S and E _P were frequency modulated by sinusoidal signals of 500-kHz and 150-kHz, respectively. At the receiver, E _C was time division demultiplexed and the bit error rate (BER) measured by using a third LiNbO ₃ modulator (LN-3) and a phase-locked loop (PLL).

  For comparison, a 1518 km transmission experiment using one DD-DCF and 23 DSFs was also performed.

FIG. 29 shows the measured BER characteristics. Even after 3036 km transmission, signal detection was possible with a BER less than 10 ^-9 . The power penalty of 4.8 dB at a BER of 10 ⁻⁹ was due to S / N degradation from theoretical values such as noise of EDFA. In this experiment, λ _C was detuned by about 1.5 nm from a wavelength λ _G ≈1558.5 nm that gives a gain peak in each EDFA. If λ _C can be matched with λ _G , higher S / N characteristics can be obtained. In the 1518 km transmission experiment, the penalty was about 1.2 dB.

FIGS. 30A to 30E show changes in the detected waveform in the 3036 km transmission experiment. (A) is the output waveform of the transmitter, (b) is the output waveform of the phase conjugate light generator, (c) is the waveform after 1518 km transmission, (d) is the waveform after 2706 km transmission, and (e) is after 3036 km transmission. The waveforms are shown respectively. It can be seen that the pre-distorted waveform is gradually improved with the propagation of EC. The residual waveform distortion in (e) was due to incomplete compensation conditions. That is, in this demonstration experiment, the EDFA interval (DSF length; 66 km) was not sufficiently shorter than the nonlinear length defined by (γ ₂ P ₂ ) ⁻¹ , so that the waveform improvement was not complete. Is.

  Therefore, in the present invention, when a plurality of optical amplifiers are used, it is desirable to set these intervals shorter than the nonlinear length.

  Further, the compensation can be further improved by making the number of divisions of the DCF in the DD-DCF larger than 5 in the experiment.

Referring to FIGS. 31A, 31B, and 31C, there are shown diagrams of optical power P, dispersion β ₂ and nonlinear effect γP / β ₂ in the system of FIG. The position of the phase conjugate light generator is the origin O.

  In order to ensure the clarity of the drawing, it should be noted that the scale of each horizontal axis indicating the distance is different between the left side and the right side of the origin.

From (C) of FIG. 31, the nonlinear effect γP / β ₂ is substantially constant and the same value on the upstream side and downstream side of the phase conjugate light generator, and the present invention is applied in a limited manner. I understand.

  Referring to FIG. 32, there is shown a configuration in which a dispersion compensator (DC) 30 is added to the basic configuration of FIG. In the illustrated example, the dispersion compensator 30 is inserted in the middle of the second optical fiber SMF2. However, the dispersion compensator 30 is not limited to this example, and the dispersion compensator 30 includes the first optical fiber SMF1 and the phase conjugate. It is only necessary to be provided on the optical path including the light generator 6 and the second optical fiber SMF2.

  With reference to (A) and (B) of FIG. 33, the expansion of the transmission distance by adding the dispersion compensator 30 will be described. In each of FIGS. 33A and 33B, the horizontal axis represents the distance, and the vertical axis represents the integral (−∫Ddz) regarding the distance of the dispersion parameter.

  When the present invention is applied in a limited manner, the total dispersion of the first optical fiber SMF1 and the total dispersion of the second optical fiber SMF2 are substantially equal as shown in FIG.

The sign of the dispersion value of the dispersion compensator 30 is set opposite to the sign of the dispersion value of the first and second optical fibers. Therefore, when the dispersion compensator 30 is provided in the middle of the second optical fiber SMF2 as shown in FIG. 32, for example, at the midpoint, as shown in FIG. 33B, the second optical fiber is provided. The distance L ₂ of SMF ₂ is expanded to L ₂ ′.

  As the dispersion compensator 30, a dispersion compensating fiber can be used. When the first and second optical fibers SMF1 and SMF2 have normal dispersion values and the wavelength of the signal light is in the 1.55 μm band, the dispersion compensator 30 has a wavelength in the vicinity of 1.3 μm. A dispersion compensating fiber that provides zero dispersion is desirable.

Now, assuming that the dispersion per unit length of the dispersion compensating fiber is −D ₃ and the length is l ₃ , the value m = D ₃ l ₃ / D ₂ L ₂ (0 ≦ m <1) representing the degree of dispersion compensation. ). In this case, the total dispersion of the second optical fiber SMF2 is _{D 2 L 2 -D 3 l 3} .

'Because it is -D ₃ l _3, L _2' total dispersion of the first optical fiber SMF1 is D1 L1, conditions of dispersion compensation _{D 1 L 1 = D 2 L} 2 = D 2 L 2 is expressed by the following equation Given in.

L ₂ ′ = L ₂ (1 + m) = D ₁ L ₁ (1 + m) / D ₂
When the nonlinear effect is also compensated at the same time, the optical input power P ₁ ′ to the first optical fiber SMF1 is preferably made substantially equal to P ₁ (1 + m).

  The optimization of the dispersion value of the dispersion compensator 30 and the optimization of the input optical power to the first optical fiber SMF1 can be performed, for example, so that the reproduction quality of transmission information in the receiver 4 is the best.

As the dispersion compensator 30, although not shown, a plurality of dispersion compensators 30j can be used. Each dispersion compensator 30j has a dispersion value -D _3j (j is a natural number) of the opposite sign to the dispersion value of the transmission line (SMF1, 2). In this case, L ₂ ′ is given by

L ₂ ′ = L ₂ (1 + Σm _j ) = D ₁ L ₁ (1 + Σm _j ) / D ₂
Here, m _j = D _3j l _3j / D ₂ L ₂ , and l _3j is the length of each dispersion compensator 30j.

  As is clear from the results of the above-described demonstration experiment, if the compensation condition is satisfied only in the first optical fiber SMF1, compensation can be made even if the dispersion of the second optical fiber SMF2 is constant. In this case, as described above, it is possible to satisfactorily compensate by setting the relay interval of the optical amplifier in the second optical fiber SMF2 to be shorter than the nonlinear length.

  The configuration of FIG. 32 is obtained by extending the transmission distance that can be compensated by the dispersion compensator 30 based on this principle.

  The effect of the configuration shown in FIG. 32 is particularly remarkable in long-distance transmission over several thousand kilometers such as undersea transmission. The reason will be explained.

  In the compensation using the phase conjugate light generator, it is necessary to make the waveform distortion in the fibers before and after that the same. For this reason, the waveform is most distorted immediately before and after the phase conjugate light generator. Accordingly, the spectrum is most widened in the phase conjugate light generator.

  On the other hand, noise is added from the phase conjugate light generator and the optical amplifier, and the S / N degradation due to this noise is larger as the spectrum is wider. Therefore, designing the system so that the spectrum spread immediately before and after the phase conjugate light generator is reduced is very effective in extending the transmission distance.

  In this sense, it is effective to reduce the dispersion value of the transmission line. For example, in the configuration of FIG. 32, when the transmitter 2, the first optical fiber SMF1, and the phase conjugate light generator 6 are provided in the transmission station and the second optical fiber SMF2 is used as a transmission line, m = 0.5. That is, half of the transmission line dispersion is compensated by the dispersion compensator 30. As a result, it is possible to reduce the dispersion value or number of DD-DCFs required to compensate for transmission lines having the same length.

In this case, L ₂ ′ = 2 × L ₂ , and a transmission line twice as long as that when the dispersion compensator 30 is not used can be compensated by the same first optical fiber SMF 1. In other words, since the length of the first optical fiber SMF1 required for compensation of the transmission path having the same length is halved, the waveform distortion can be halved. In addition, the number of DD-DCFs can be reduced.

  FIG. 34 shows the configuration of a dispersion compensator using a fiber grating FG. The dispersion compensator of FIG. 34 can be used as the dispersion compensator 30 of FIG.

Optical pulses having wavelengths λ ₁ and λ ₂ at both edges of the optical pulse are supplied to the fiber grating FG through the optical circulator OC. The grating pitch of the fiber grating FG has a predetermined distribution. Light with a wavelength λ ₁ is Bragg reflected at a position relatively close to the optical circulator OC, and light with a wavelength λ ₂ is Bragg reflected at a relatively far position. Is done. Thereby, the optical pulse is compressed, and dispersion compensation can be performed by taking out the Bragg reflected light from the fiber grating through the optical circulator OC.

  For example, when the dispersion compensator 30 of FIG. 32 is applied to a system in which an optical amplifier Aj (j is a natural number) is provided in the middle of the second optical fiber SMF2 as in the second embodiment of FIG. The dispersion compensator 30 is preferably disposed immediately before the optical amplifier A-j. This is because when the dispersion compensator 30 is a dispersion compensating fiber, the nonlinear effect in the dispersion compensating fiber can be reduced as the input optical power to the dispersion compensating fiber is reduced.

  When the dispersion compensator 30 is applied to the ninth embodiment of FIG. 15, the dispersion compensator 30 is provided in the middle of the optical fiber SMF1, the optical fiber SMF2, or the optical fibers SMF3-1, 2,. It is done.

  When the dispersion compensator 30 is applied to the tenth embodiment of FIG. 17, the dispersion compensator 30 is provided in the middle of the optical fibers SMF11, 12,..., 1N or the optical fiber SMF2.

  When the dispersion compensator 30 is applied to the eleventh embodiment of FIG. 18 or the twelfth embodiment of FIG. 19, the dispersion compensator 30 is provided in the middle of the polarization maintaining fiber SMF1 (PMF) or the optical fiber SMF2.

  When the dispersion compensator 30 is applied to the fifteenth embodiment of FIG. 23, the dispersion compensator 30 is in the middle of the optical fibers SMF11, 12,..., 1N or the optical fibers SMF21, 22,. Is provided.

  When the dispersion compensator 30 is applied to the sixteenth embodiment of FIG. 24, the dispersion compensator 30 is provided in the middle of the optical fiber SMF1, or the optical fibers SMF21, 22,.

  When the dispersion compensator 30 is applied to the seventeenth embodiment shown in FIG. 25, the dispersion compensator 30 includes optical fibers SMF11 ′, 12 ′,..., 1N ′ or optical fibers SMF21, 22,. It is provided in the middle of 2M.

It is a figure which shows the basic composition of this invention. It is a block diagram which shows the example of a phase conjugate light generator. It is a block diagram which shows the other example of a phase conjugate light generator. It is a principle explanatory view of the present invention. It is a block diagram of the optical fiber communication system which shows 1st Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 2nd Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 3rd Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 4th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 5th Embodiment of this invention. It is a figure which shows the frequency arrangement | positioning of each light with respect to the zero dispersion wavelength of two fibers. It is a block diagram of the optical fiber communication system which shows 6th Embodiment of this invention. It is a figure which shows the example which controls dispersion | distribution in the method of average intensity | strength. It is a block diagram of the optical fiber communication system which shows 7th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 8th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 9th Embodiment of this invention. It is a figure which shows the frequency arrangement | positioning in 9th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 10th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 11th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 12th Embodiment of this invention. It is a block diagram which shows the further another example of a phase conjugate light generator. It is a block diagram of the optical fiber communication system which shows 13th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 14th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 15th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 16th Embodiment of this invention. It is a block diagram of the optical fiber communication system which shows 17th Embodiment of this invention. It is a figure which shows the example of a channel selector. It is a figure which shows the other example of a channel selector. It is a block diagram of the system used in the verification experiment. It is a figure which shows the BER (bit error rate) characteristic in the system of FIG. It is a figure which shows the change of the waveform in the system of FIG. It is a figure which shows diagrams, such as power in the system of FIG. It is a figure which shows the structure which added the dispersion compensator. It is a figure for demonstrating the expansion of the transmission distance in FIG. It is a figure which shows the dispersion compensator using a fiber grating.

Explanation of symbols

2 Transmitter 4 Receiver 6 Phase conjugate light generator

Claims (7)

A plurality of optical transmitters that respectively transmit a plurality of signal lights having different wavelengths;
A first optical fiber for transmitting the plurality of signal lights;
A plurality of phase conjugate light generators respectively receiving the plurality of signal lights supplied from the first optical fiber and generating phase conjugate light corresponding to at least one of the plurality of signal lights;
A plurality of optical filters that respectively transmit the phase conjugate light output from the phase conjugate light generators;
A plurality of second optical fibers respectively transmitting the phase conjugate light from each of the optical filters;
A plurality of optical receivers for receiving the phase conjugate light from each of the second optical fibers,
When the first optical fiber and the second optical fibers are divided into the same number, the average value of the chromatic dispersion in the corresponding section when counting in order from the phase conjugate light generator in each divided section Is set to a value substantially the inverse of the length of each divided section and the average value of the product of optical frequency, signal light intensity and nonlinear refractive index in each divided section is approximately the length of each divided section. An optical fiber communication system set to be inversely proportional. There are a plurality of the first optical fibers corresponding to the plurality of optical transmitters,
2. The optical fiber communication system according to claim 1, further comprising means for adding the plurality of signal lights from the plurality of first optical fibers together and branching and outputting the plurality of signal lights. The first optical fiber is common to the plurality of optical transmitters;
2. The optical fiber communication system according to claim 1, further comprising means for adding the plurality of signal lights from the plurality of optical transmitters and supplying the signal light to the common first optical fiber. Each of the phase conjugate light generators includes a nonlinear optical medium to which the signal light is supplied, a pump light source that outputs pump light, and an optical unit that supplies the pump light to the nonlinear optical medium,
A channel selector for generating a control signal;
The optical fiber communication system according to claim 1, further comprising means for selecting a desired signal light by controlling at least one of a wavelength of each pump light and a characteristic of each optical filter based on the control signal.   5. The optical fiber communication system according to claim 4, wherein a plurality of channel selectors are provided in association with the plurality of optical transmitters.   5. The optical fiber communication system according to claim 4, wherein a plurality of the channel selectors are provided in association with the plurality of optical receivers.   The optical fiber communication system according to claim 1, further comprising a dispersion compensator provided on an optical path including the first optical fiber, the phase conjugate light generator, and the second optical fiber.