JP4693644B2 - Wavelength division multiplexing optical modulation method and apparatus - Google Patents

Description

  The present invention relates to a wavelength division multiplexing optical modulation method used in a wavelength division multiplexing (WDM) transmission system that multiplexes signal light of a plurality of wavelengths into the same optical fiber to realize a large capacity of optical communication.

  In a conventional WDM transmission system in optical communication, the optical transmitters prepare lasers (for example, distributed feedback laser diodes: DFB-LDs, which are semiconductor lasers) that oscillate at different wavelengths, as many as the number of channels. The light of each wavelength is modulated by a transmission data signal sequence using an external optical modulator. At the optical receiving end, the wavelength multiplexed light is demultiplexed into modulated signal light of each wavelength using a wavelength filter or the like, and the signal light of each wavelength is individually demodulated and detected. As a modulation / demodulation method, an intensity modulation-direct detection method in which 0 and 1 of a transmission data signal are encoded and transmitted as light intensity, a phase modulation method in which 0 and 1 of a transmission data signal are encoded and transmitted as an optical phase, There is a delay phase modulation system that encodes 0 and 1 of the transmission data signal as a phase change of light. In order to economically construct a WDM transmission system, it is important to expand the transmission capacity of one fiber by narrowing the wavelength interval of the WDM signal and increasing the frequency utilization efficiency.

Further, in the conventional WDM transmission system, since it is encoded by a high-speed transmission data sequence such as 10 Gbps or 40 Gbps, the signal light of each wavelength has a spread on the optical frequency spectrum of about the modulation speed frequency band. Therefore, if the wavelength interval between channels is narrowed, the optical spectrum of each wavelength overlaps with each other, and as shown in FIG. In addition, linear crosstalk occurs in which the detection signal of the own channel (for example, λ ₁ ) varies due to the signal light of the adjacent channel (for example, λ ₂ ). For example, the signal of the adjacent wavelength is mixed without being filtered due to the characteristics of the separation filter, so that the detection signal quality is deteriorated. For this reason, a method has been proposed in which linear crosstalk from an adjacent channel that cannot be completely filtered by a separation filter is removed by performing an electrical calculation after optical / electrical conversion of signal light at a receiving end (Non-Patent Document 1).

M. J. Minardi. Et al., “Adaptive Crosstalk Cancellation in Dense Wavelength Division Multiplexing Networks”, Electronics Letters, 13th August 1992, Vol.28, No.17, pp.1621-1622

  In a WDM transmission system, for example, specifically, a beat in which a signal of an adjacent wavelength and a signal of its own wavelength vibrate at the frequency difference is formed by such linear crosstalk. The light of each wavelength is output light from a different semiconductor laser, and the phase relationship between them is random, so that the phase of the beat varies randomly with time. Therefore, there is a problem that the beat is randomly superimposed on the eye pattern of the desired detection signal, and the transmission characteristics are deteriorated by closing the eyes of the eye pattern. Furthermore, if the frequency interval between the modulation frequency band and the signal light is narrowed in order to increase the frequency utilization efficiency, the modulation frequency band and the beat component frequency will be close, and the receiving end electrical filter band and the signal light frequency band will be close. Therefore, the beat component cannot be removed. For this reason, as described above, in the method of removing linear crosstalk by electrical calculation at the receiving end, the phase relationship of the transmission signal light is random, so that the beat is superimposed on the eye pattern of the detection signal. That is, due to the occurrence of the linear crosstalk, the narrowing of the wavelength interval is limited, and the WDM spectrum utilization efficiency is limited.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a wavelength division multiplexing optical modulation method and apparatus for improving spectrum use efficiency in WDM transmission.

The present invention, in order to achieve the above object, the invention according to claim 1, as an integer of 2 or more and M, respectively encoded by the M transmission data sequence the M light of different wavelengths from each other a method for multiplexing and transmitting the M-number of light said encoded, the k as an integer of 1 to M, the light of the wavelength of the k is any one of said M light of the wavelength When encoding with the k-th transmission data sequence corresponding to light , the reception side uses one or more transmission data sequences corresponding to light of one or more wavelengths adjacent to the k-th wavelength among the plurality of lights. to reduce cross-talk component, the step of encoding with all of one or more transmission data sequence corresponding to one or more wavelengths of light to the adjacent transmission data sequence of the k-th light having a wavelength of the first k It is characterized by providing.

Further, the invention according to claim 2, in the method of claim 1, the step of pre Kifu-coding includes the step of encoding the optical wavelength of the first k in transmission data sequence of the first k And encoding the k-th wavelength light with one or more transmission data sequences corresponding to the adjacent one or more wavelengths of light .

The invention according to claim 3 corresponds to the light of the kth wavelength or the light of one or more adjacent wavelengths encoded by the kth transmission data sequence in the method of claim 2. The method further comprises the step of adjusting the intensity or phase of the light of the k-th wavelength encoded with one or more transmission data series.

According to a fourth aspect of the present invention, in the method according to any one of the first to third aspects, the beat component oscillates at a frequency difference between the light of the k-th wavelength and the light of one or more adjacent wavelengths. The method further comprises the step of temporally stabilizing the phase of.

Further, according to claim 5 invention, as two or more integer M, the M number of light wavelengths different from each other generated from the multi-wavelength light generating means of the M which respectively encoded in the M transmission data sequence An apparatus comprising a modulation means and an optical multiplexing means for multiplexing the encoded M light beams , wherein k is an integer between 1 and M, and the modulation means includes: a modulating means for encoding the light of the wavelength of the k either by the k transmission data sequences corresponding to the light of the wavelength, the wavelength of the k-th of the plurality of light receiving side In order to reduce crosstalk components due to one or more transmission data sequences corresponding to adjacent one or more wavelengths of light, the kth wavelength of light is converted to the kth transmission data sequence and the one or more adjacent wavelengths. Encoded with all of one or more transmission data sequences corresponding to the light of It is characterized by comprising modulation means for

The invention according to claim 6 is the apparatus according to claim 5, wherein the modulating means encodes the light of the kth wavelength with the kth transmission data sequence, And a modulator that encodes the light of the kth wavelength with one or more transmission data sequences corresponding to the light of the one or more adjacent wavelengths .

The invention according to claim 7 is the apparatus according to claim 6, and corresponds to the light of the kth wavelength or the light of one or more adjacent wavelengths encoded with the kth transmission data sequence. And adjusting means for adjusting the intensity or phase of the light of the k-th wavelength encoded with one or more transmission data series.

The invention according to claim 8 is the apparatus according to any one of claims 5 to 7 , wherein the apparatus vibrates with a frequency difference between the light of the kth wavelength and the light of one or more adjacent wavelengths. It further comprises phase stabilization means for temporally stabilizing the phase of the beat component.

The invention according to claim 9 is the apparatus according to claim 8, wherein the phase stabilizing means is connected to an output side of the optical multiplexing means and branches the multiplexed light. means, for each wavelength of the branched light, and the phase feedback means for feeding back the phase difference between the clock component of the beat component the transmission data sequence, based on said feedback phase difference, the M And a phase control means for controlling each phase of the light .

The invention according to claim 10 is the apparatus according to claims 5 to 9, wherein the multi-wavelength light generating means outputs the M lights with their phases synchronized. It further comprises light generating means.

  As described above, according to the present invention, the frequency interval can be narrowed by reducing the fluctuation of the detection signal due to crosstalk that the main signal light receives from the adjacent signal light at the receiving end.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In the WDM transmitter, light of each wavelength λ ₁ , λ ₂ , λ ₃ ... Is encoded by a plurality of transmission data series D ₁ , D ₂ , D ₃ . Wavelength-multiplexed by multi-wavelength signal optical multiplexing means such as a type grating (AWG) and transmitted. Transmission data series D ₁ , D ₂ , D ₃ ... Are assigned to wavelengths λ ₁ , λ ₂ , λ ₃ . At the receiving end, after separating by a wavelength separation filter or the like and extracting the signal light of each wavelength, a received data series is obtained from the detected electrical signal obtained by demodulation and detection.

Here, as shown in FIG. 2A, since the signal light spectrum of each wavelength has a spectrum spread corresponding to the modulation speed, the spectrum of the WDM transmission signal light has an overlapping portion. Accordingly, the signal light having the wavelength λ _A obtained by separating the signal light having the wavelength λ _A (for example, λ ₂ in FIG. 2) at the receiving end is equal to the adjacent wavelength λ _B (for example, λ ₁ or λ in FIG. 2). ₃ ) The signal light component is included. This is shown in FIG.

When this is expressed using a simple mathematical formula, the electric field of the signal light having the wavelength λ _A after wavelength separation is added to the adjacent wavelength in addition to the component E _a = d _a E ₀ cos (ω _a t + φ _a ) of the wavelength λ _A The signal light component ΔE _b = αd _b E ₀ cos (ω _b t + φ _b ) of λ _B has a crosstalk component. Here, d _a and d _b are optical field amplitudes and / or phases depending on the transmission data series D _A and D _B , ω _a and ω _b are the angular frequencies of the wavelengths λ _A and λ _B , t is time, and φ _A , phi _B represents the initial phase of each wavelength signal light, respectively. Α (α <1) is a parameter determined by characteristics of a wavelength separation filter or the like, and is a crosstalk coefficient representing the magnitude and phase of a crosstalk component. Again, to represent the optical electric field to be input to the receiver of the signal light having a wavelength lambda _A using equation is expressed as a sum of the main signal light component and the crosstalk component of the wavelength lambda _B of the wavelength lambda _A.

  A photocurrent obtained by detecting this with an optical receiver is obtained by squaring the equation (1-1).

Here, only an actual frequency component is described as an electric signal, and components in the optical frequency region beyond that are ignored. From the equation (1-2), the detection signal is the reception signal represented by the second term in addition to the main reception detection signal represented by the product of the optical electric fields of the signal light of the reception wavelength λ _A represented by the first term. _A beat component expressed by the product of the signal light electric field of wavelength λ _{A and} the signal light electric field of adjacent wavelength λ _B , and a crosstalk component expressed by the product of the optical fields of the signal light of adjacent wavelength λ _B represented by the third term including. FIG. 3 schematically shows the time waveforms of the main received signal (solid line) and the beat component (dotted line) as sampling waveforms. In the conventional WDM, since the phase difference φ _a −φ _b between the received wavelength and the adjacent wavelength fluctuates randomly in time, the phase of the beat component is determined by the phase difference φ _a −φ _b included in the cosine function of the second term. Varies randomly in time, and the phase relationship with the main received signal varies randomly as shown in FIG. As a result, the time-sampling waveform of the received signal measured in a state where the phase of the adjacent wavelength signal light is not controlled and the beat phase is random as in the prior art is as shown in FIG. The eye pattern is closed and error-free transmission is not possible. On the other hand, as shown in FIG. 3B, by applying a method for temporally stabilizing the phase of the beat component, it is possible to suppress a phenomenon in which the main reception detection signal varies randomly depending on the beat component. Here, the details of the method of temporally stabilizing the phase of the beat component are described in the fifth and sixth embodiments.

  FIG. 4B is a time sampling waveform of the detection signal when the beat component phase and the data clock phase are synchronized. By adjusting the phase of the beat component and the phase of the data clock so that the beat component does not reduce the level of the main received signal at the data identification point, the eye pattern opens and error-free transmission can be realized.

_A signal light of a certain wavelength λA obtained by demultiplexing the WDM signal light into signal light of each wavelength by a wavelength separation filter is detected by an optical receiver. At this time, since the signal light input to the optical receiver includes the signal light having the wavelength λ _B , the detection signal has a beat component that vibrates at a frequency corresponding to the wavelength interval of the WDM signal light and the adjacent wavelength λ _B. A received signal is included. When the frequency interval of the WDM signal light is larger than the modulation frequency band of the signal light, the above-mentioned beat component can be removed by making the filter band of the electric filter at the receiving end equal to the modulation frequency band. The signal received by the received signal of wavelength λ _B remains. If the eye pattern of the detection signal at this time is schematically shown, the eye of the eye pattern is closed as shown in FIG. 5A, and a good detection signal cannot be obtained.

Therefore, as shown in FIG. 6, at the transmitting end, λ ₁ , λ ₂ , λ ₃ ,... Λ _k ..., So as to cancel the fluctuations received by the optical signal of the adjacent wavelength λ _B using the light of wavelength λ _A a plurality of transmission data sequence _D 1 corresponding to _{λ M, D 2, D 3} , ... D k ..., encodes the optical amplitude and / or phase of the lambda _k using _{D M.} Here, for the adjacent two wavelengths λ _k and λ _{k + 1} , received photocurrents I _k and I _{k + 1} of each wavelength signal light obtained by demultiplexing with the wavelength separation filter are expressed by the following equations. However, the transmission optical fields of the wavelengths λ _k and λ _{k + 1} encoded by the transmission data signals D _k and D _{k + 1} are d _k and d _{k + 1} , their complex conjugates are d ^* _k and d ^* _{k + 1} , and the adjacent wavelengths λ _{k ±} the crosstalk component alpha _± be mixed from _1, the component from the own wavelength lambda _k and alpha _0. See Example 5 for details of d _k , α _± .

However, here, the beat component due to the adjacent wavelength is ignored because it is removed by the electric filter. As expressed by the matrix of the transmission electric field d _k and the crosstalk component α _± of each wavelength as in the previous equation, the inverse matrix of this matrix is the transmission signal of each wavelength at the transmission end such that a desired reception photocurrent is obtained at the reception end. Will surface. The inverse matrix of the equation (2-1) can be expressed as the following equation (2-2).

At this time, the reception electric field to focus the signal light of the wavelength lambda _k offsetting changes due to the adjacent wavelengths can be expressed as the following (2-3) equation.

Assuming an intensity modulation-direct detection method, the desired received photocurrent I _k of the conventional wavelength λ _k is generally defined as “0” or “1”, but a real number solution is obtained as d _{k, n.} In order to exist, “0” or “1” is defined as “0+ | α ₊ | ² / | α ₀ | ² ” or “1+ | α ₊ | ² / | α ₀ | ² ”. Accordingly, offset the changes incurred by encoding the also depends on D _{k + 1 λ k} well D _k, the signal light of the detection signal is lambda _{k + 1} of the lambda _k at the receiving end as shown in Table 2-1 it can. The detection signal at the receiving end at this time does not depend on the value of D _{k + 1} as shown in Table 2-2. Further, when the eye pattern of the signal light having the wavelength λ _A detected at the receiving end at this time is schematically shown, an eye pattern with open eyes can be realized as shown in FIG.

  Up to this point, the case of two wavelengths has been considered, but when the equation (2-1) is expanded to the M wavelength, it can be expressed by the following equation.

Similar to the case of two wavelengths, by taking the inverse matrix of equation (2-4), the transmission data _Dk of each wavelength at the transmitting end that represents a desired received photocurrent at the receiving end is represented.

Thus, if coding the light intensity and / or phase of the lambda _k using a plurality of data series in order to compensate the received waveform of the lambda _k, adjacent wavelength _{λ k ± m (m = 1,2} , ... ) Cancels the fluctuations caused by the optical signal and maintains an open state of the eye pattern, thereby realizing error-free transmission. Here, the intensity modulation-direct detection method is assumed, but the optical electric field d _k includes various characteristics of light such as optical amplitude, optical phase, and polarization. The method according to the present invention can also be applied to the modulation method.

In the WDM transmitter, light of each wavelength λ ₁ , λ ₂ , λ ₃ ... Is encoded by a plurality of transmission data series D ₁ , D ₂ , D ₃ . Wavelength multiplexed by a multi-wavelength signal optical multiplexing means such as a type grating (AWG) or an interleaver, and then transmitted. Therefore, the transmission data series D ₁ , D ₂ , D ₃ ... Are assigned to the wavelengths λ ₁ , λ ₂ , λ ₃ . At the receiving end, after being separated into each wavelength signal light by the multi-wavelength signal light demultiplexing means, a received data series is obtained by demodulation and detection. When the frequency interval between the modulation frequency band and the signal light is narrowed, the spectral overlap of the signal light of each wavelength of the WDM transmission signal light is large when the frequency utilization efficiency is increased. The received signal light includes a lot of signal light components of adjacent wavelengths. Accordingly, since the modulation frequency band is close to the beat component frequency, and the frequency band of the signal light is close to the frequency band of the electric filter at the receiving end, the frequency of the signal light is also removed. It is not possible to remove only the ingredients. Further, in this case, in addition to the beat component, as described in the second embodiment, the reception signal of the wavelength λ _A is also subject to fluctuation due to the detection signal of the adjacent wavelength λ _B. With these two effects, as shown in FIG. 7A, the eye pattern obtained when detecting the signal light having the wavelength at the receiving end is closed, and error-free transmission cannot be realized.

Therefore, as shown in FIG. 8B, the phase of the beat component and the phase of the data clock are synchronized to adjust the phase therebetween, and as shown in FIG. 8A, the wavelength λ _{A is} further increased. _A plurality of transmission data series D corresponding to λ _A and λ _B at the transmitting end so as to cancel out the fluctuations caused by the optical signal of the adjacent wavelength λ _B (for example, λ _{k + 1} ) by the optical modulation means (for example, λ _k ) By encoding the optical amplitude and / or phase of λ _A using _{A 1} and D _B , the eye pattern is widened at the data identification point, and error-free transmission is realized.

Using d _An and d _Bn depending on the transmission data series D _A and D _B , the signal light electric fields of wavelengths λ _A and λ _B are d _{A, n} E ₀ Exp (i (ω _A t + φ _A )), d _B, Assume that encoding is performed such that _n E ₀ Exp (i (ω _B t + φ _B )) is transmitted. The received photocurrents I _k and I _{k + 1} at the receiving ends of the wavelength signal lights λ _k and λ _{k + 1} are α _{k for} the crosstalk component mixed from the adjacent wavelength λ _k , and Δω for each frequency difference between the wavelengths λ _k and λ _{k + 1} . If the 1-bit time interval of the transmission data series is ΔT, it can be expressed by the following equation.

Here, the first term on the right side of the equation (3-1) represents the detection signal of the received wavelength itself and the detection signal of the adjacent wavelength itself, and the second term and the subsequent items on the right side are detection signals of the received wavelength and beat components of the adjacent wavelength. Represents. The frequency difference Δω and the initial phase difference φ _k −φ _{k + 1} of the received wavelength and its adjacent wavelengths are stabilized in time, and the phases of the beat signal and the data clock are synchronized as shown in FIG. Thus, it is possible to reduce detection signal deterioration due to reception of beat components. Similarly to the second embodiment, by applying the inverse matrix of the matrix of the first term on the right side of the equation (3-1) to the data code vectors [d ₁ , d ₂ , d ₃ . By encoding the optical field intensity and / or phase of each wavelength with the data code converted into [D _1n , D _2n , D _3n ...], Fluctuation due to detection signal crosstalk of adjacent wavelengths themselves can be reduced. The resulting time sampling waveform is shown in FIG. It is possible to open the eyes of the eye pattern by canceling the fluctuations caused by the optical signal of the adjacent wavelength λ _{k ± m} (m = 1, 2,...). Further, details of a method for temporally stabilizing the frequency difference Δω and the initial phase difference φ _A −φ _B and a method for synchronizing the phases of the beat signal and the data clock are described in the fifth and sixth embodiments.

Since d _k d ^* _k includes various characteristics of light such as light intensity, light phase, and polarization, the above description does not depend on the modulation method, but can be applied to all modulation methods.

In this embodiment, when the light of the transmission wavelength is encoded by the transmission data, by encoding the transmission data of the adjacent wavelength, the beat component with the adjacent wavelength and the detection signal of the adjacent wavelength itself are encoded. The method of reducing the detection signal fluctuation | variation of the receiving wavelength by FIG. Here, by encoding the optical field amplitude and / or phase of the transmission wavelength in consideration of the transmission data of the adjacent wavelength, the fluctuation of the detection signal of the reception wavelength itself and the fluctuation due to the beat component of the adjacent wavelength signal are canceled out. . In this embodiment, the case of two wavelengths will be described as an example. As shown in the following equation (4-1), each wavelength is encoded using data of the adjacent wavelength. Here, d _{k, n} is a complex number representing the envelope of the optical field amplitude and phase in the n-th bit of a wavelength λ _k, β _k is a complex number representing the crosstalk from the wavelength lambda _k, in the third embodiment α _k is normalized (β _k = α _k / α ₀ ).

  When this wavelength signal light is detected by being multiplexed / demultiplexed, the received photocurrent at that time can be expressed as in equation (4-2).

Here, Δω represents the angular frequency difference between the wavelengths λ _k and λ _{k + 1} , ΔT represents the time interval between n bits and n + 1 bits, and the subscript * represents the complex conjugate. Accordingly, the reception light current _{I 1} of the wavelength lambda ₁ can be expressed by (4-3) below.

If ΔωΔT (n + 1/2) + φ _k −φ _{k + 1} = 2mπ (m is an integer), the term d ₂ that is a crosstalk component from the adjacent wavelength disappears, and the crosstalk component can be canceled (4- 4) From the equation. The initial phase may be adjusted in advance so that φ _k −φ _{k + 1} = 2mπ−ΔωΔT (n + 1/2). If the condition satisfying ΔωΔT = 2mπ, that is, the WDM carrier frequency interval is an integer multiple of the bit rate, a constant initial phase difference φ _k −φ _{k + 1} = 2 (n−1 / 2) π may be sufficient.

  In this way, when the light of the transmission wavelength is encoded, the beat component with the adjacent wavelength mixed in the reception wavelength is canceled by the detection signal of the reception wavelength itself by pre-compensating with the transmission data of the adjacent wavelength. The time sampling waveform shown in FIG. It can be seen that the eyes of the eye pattern are opened compared to the case where the crosstalk component and the beat component shown in FIG.

  Further, when the WDM transmission signal is an M channel, a high-order term of β remains, but the value can be considered to be almost negligible, and can be applied to a WDM transmission system having two or more channels.

The operation of this embodiment will be described with reference to FIG. The phase-locked multi-wavelength light generating means of the optical transmission unit of this embodiment generates light of wavelengths λ ₁ , λ ₂ ,... Λ _k , λ _M , and is demultiplexed by the multi-wavelength signal light demultiplexing means. transmission data sequence _D 1 in the light modulating _{means, D 2, ... D k ...} , are encoded by the _{D M.} The encoded wavelength signal lights are multiplexed by the multi-wavelength signal optical multiplexing means and output as optical multiplexed signal light.

Here, as shown in FIG. 2, since the signal light spectrum of each wavelength has a spectrum spread corresponding to the modulation speed, the spectrum of the WDM transmission signal light overlaps each other, and the received signal is obtained by separating the signal light of a certain wavelength. The light includes signal light components of adjacent wavelengths. For example, given the detection signal of the wavelength lambda _1, the electric field of the wavelength signal light having a wavelength lambda ₁ to be input to the detection unit can be expressed by (5-1) below.

In addition, the electric field of the signal light having the wavelength λ ₂ that is input as crosstalk by the signal light detection unit having λ ₁ can be expressed by Equation (5-2).

Here, d ₁ and d ₂ are complex numbers depending on each transmission data, ω ₁ and ω ₂ are angular frequencies of each wavelength signal light, t is time, k ₁ and k ₂ are wave numbers of each wavelength signal light, L ₁ , L ₂ are optical path lengths from the demultiplexing means to the multiplexing means shown in FIG. 10 for each wavelength, φ ₁ , φ ₂ are initial phases of the respective wavelength signal lights, and α is a crosstalk coefficient (α <1). ing. The electric field of the beat light that vibrates at the optical frequency difference between both wavelengths that appears when the wavelength signal light detected by the signal light detection means having the wavelength λ ₁ and the adjacent wavelength signal light is detected is Δω = ω ₁ −ω ₂ , If Δφ = φ ₁ −φ ₂ , it can be expressed by the equation (5-3).

In the phase-locked multi-wavelength light that is the light source at the transmission end, the frequency and the initial phase between wavelengths are temporally stable, and even if Δω and Δφ are always constant, L ₁ , L ₂ becomes unstable in time, and the fluctuation of the optical path length becomes a problem. This problem is that the phase of the signal light that vibrates due to the difference between the optical frequencies of the two wavelengths appearing when the wavelength signal light detected by the signal light detection means having the wavelength λ ₁ and the adjacent wavelength signal light is detected is the transmission data clock. Since it is asynchronous with the phase and is randomly superimposed on the data detection signal as shown in FIG. 3A of the first embodiment, the eye pattern of the received signal deteriorates as shown in FIG. Therefore, by integrating the optical circuit surrounded by the dotted line in FIG. 10, (L ₁ -L ₂ ) can be temporally stabilized, and both appearing when a certain wavelength signal light and its adjacent wavelength signal light are detected. The above-mentioned problem can be solved by stabilizing the phase of the beat light oscillating with the optical frequency difference of the wavelength and synchronizing it with the phase of the data clock. As shown in FIG. 4B, even when beat light oscillating at an optical frequency with the adjacent wavelength is mixed into a certain reception wavelength, the eye pattern is kept open and error-free transmission is realized. Can do.

Here, an optical circuit from the multi-wavelength signal light demultiplexing means to the multi-wavelength signal light multiplexing means is used by using, for example, a PLC (planar optical circuit) waveguide technology that is an optical integrated circuit. The optical path lengths L ₁ , L ₂ ,... L _k , L _n can be temporally stabilized.

For example, an AWG (arrayed waveguide grating) multiplexer / demultiplexer applying the PLC waveguide technology of SiO ₂ and GaAs to the multi-wavelength signal optical demultiplexing means and the multi-wavelength signal optical multiplexing means, and an EO such as LiNbO ₃ as the optical modulation means A PLC technology that integrates a phase / intensity modulator such as a crystal, SOA (semiconductor amplifier), EAM (electro-absorption modulator) on a substrate such as SiO ₂ or GaAs can be used. Thereby, the optical circuit from the multi-wavelength signal light demultiplexing means to the multi-wavelength signal light multiplexing means can be integrated, and the fluctuation of the optical path length between each wavelength signal light due to thermal and mechanical fluctuations can be canceled, (L ₁ -L ₂ ) can be temporally stabilized. Therefore, it is possible to synchronize the phase of the beat light and the phase of the data clock which are oscillated by the optical frequency difference between the two wavelengths appearing when the signal light of a certain wavelength and its adjacent wavelength is detected.

  Note that the phase-locked multi-wavelength light generating means shown in FIG. 10 can use, for example, a mode-locked laser, supercontinuum light, sideband light obtained by optically modulating a single wavelength laser, or the like. In addition, as the phase-locked multi-wavelength light generating means, single wavelength lasers are individually prepared for the number of WDM wavelength signal lights, and light injection locking is performed on them using a multi-wavelength light source, and the phase of each wavelength signal light is synchronized. There is also a method for synchronizing them.

The operation of this embodiment will be described with reference to FIG. The multiwavelength signal light at the optical transmission end of this embodiment is input to the phase shift means, and the phase is shifted by Φ. Transmission data series D ₁ , D ₂ ,... D _k , D _n through the phase-shifted signal delay means are encoded by being input to the optical modulation means. The encoded wavelength signal lights are combined into the multi-wavelength signal optical combining means and output as optical multiplexed signal light. Part of the optical multiplexed signal light is input to the phase feedback means by the optical branching means, and the others are output as WDM transmission signals. The optical multiplexed signal light input to the phase feedback means is demultiplexed into two wavelengths or multiple wavelengths including adjacent wavelengths in the phase feedback means, and each of them is subjected to optical / electrical conversion, and a beat component that vibrates at the optical frequency difference between them. Get. By frequency mixing component oscillating in the beat component and the data clock, the phase [Delta] [phi ₁ of the beat _{component, Δφ 2, ... Δφ k ...} , the difference between the phase θ of the [Delta] [phi _n and data clock (Δφ ₁ -θ), Δφ ₂₋ θ),... (Δφ _k −θ)... (Δφ _n −θ) are detected. The detected these phase difference signals are input to the phase control means, -Derutafai ₁ the phase shift amount Φ of the phase shifting _{means, -ΔΦ 2, ... -ΔΦ k ...} , is changed by -ΔΦ _n. Phase change amount .DELTA..PHI ₁ phase shifting _{means, ΔΦ 2, ... ΔΦ k ...} , ΔΦ n _{is, Δφ 1 -θ = 0, Δφ} 2 -θ = 0, ... Δφ k -θ = 0 ..., Δφ n -θ = The phase difference between the beat signal obtained by detecting the signal light of a certain wavelength and its adjacent wavelength and the data clock is temporally stabilized and the phases can be synchronized.

  FIG. 12 shows an eye pattern detected by the optical receiving means of the receiving unit when the phase of the beat component of both wavelengths and the phase of the data clock appear when detecting an optical signal of a certain transmission wavelength and its adjacent wavelength. This is shown schematically. As shown in FIG. 12, the signal delay means of FIG. 11 is set in advance so that the amplitude of the signal light that vibrates due to the optical frequency difference between a certain reception wavelength and its adjacent wavelength is maximized at the data identification point.

  In this way, even when an optical signal that vibrates at an optical frequency with the adjacent wavelength is mixed into a certain reception wavelength, the eye pattern opening is maintained, and error-free transmission can be realized.

  The optical phase shift means of the present embodiment is, for example, a piezo-driven moving stage that shifts the optical phase using a change in optical path length, or an electroacoustic optical that shifts the optical phase using a change in instantaneous frequency. A frequency optical modulator typified by an element (AOM), a phase optical modulator typified by an LN electro-optical element that shifts an optical phase by utilizing a change in the refractive index of a substance due to an electro-optic effect, and the like are used. it can.

When a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A and any transmission data sequence D _B different from the transmission data sequence D _A is transmitted at a wavelength λ _B , the transmission data sequence D _A is used. _A precompensated signal light of wavelength λ _A in which the optical field amplitude and / or phase is encoded using the transmission data sequence D _B to the main optical signal of wavelength λ _A in which the optical electric field amplitude and / or phase is encoded By using a specific example, it is possible to reduce the fluctuation that the detection signal obtained by extracting and detecting the λ _A optical signal separated by the wavelength at the receiving end can be affected by the λ _B optical signal. To do.

First, a mechanism that causes crosstalk in WDM transmission will be described. In general, the optical field of the signal light of the k-th channel is expressed as d _{k, n} (t) E ₀ exp [iω _k t + φ _k ] + c. c. Can be defined. Here, d _{k, n} (t) is a complex number representing the amplitude and phase of the optical electric field, E ₀ is the optical amplitude common to all channels, ω _k is the optical angular frequency of each wavelength, and φ _k is the initial phase of each wavelength. Represents. Here, the description will be made assuming non-return-to-zero (NRZ) signal light, but the same description can be applied to other signal light such as return-to-zero (RZ). For simplicity, it is approximated that both the optical amplitude and phase are modulated stepwise. The optical electric field ε _k (ω) obtained by Fourier transform is given by the following equation. However, the complex conjugate part is ignored here. Assume an n-bit transmission data string, an M channel, and a 1-bit time interval ΔT.

All these channels are wavelength multiplexed and transmitted. The optical field ε ′ _j (ω) of the signal light having the wavelength k separated at the receiving end acts on the transmission characteristic h _j (ω) of the transmission line / demultiplexing filter on the sum of the transmission optical signal optical fields of the previous wavelength. Can be obtained.

  However, the signal light of other wavelengths mixed in the reception wavelength is assumed to be only adjacent wavelengths, and other than that is assumed to be negligibly small. The time function of the optical electric field is obtained by inverse Fourier transform of this frequency function.

Here, pf _{i, k} (t) is expressed by the following equation, and represents an optical electric field envelope of the signal light having the wavelength k mixed in the receiver having the wavelength j, and depends on the characteristics of the wavelength demultiplexing filter.

In the case of direct detection, the photocurrent I _j output from the optical receiver having the wavelength j is given by E ′ _j (t) E ′ _j ^* (t), and is given by the following equation. However, it is assumed that inter-bit symbol interference is sufficiently small, and pf _{j, k} (t) is negligibly small at t <0 or ΔT <t.

Here, assuming that it is identified at the center of the bit of this photocurrent, t = (n + 1/2) ΔT is substituted to obtain the detection signal _{Im, n} .

This is a mathematical description of the crosstalk generation mechanism. The crosstalk that the received signal of each channel receives from the other channels is based on the beat components of the received wavelength signal light and the adjacent wavelength signal light, and the detection signal of the adjacent wavelength itself. I understand that Here, it is assumed that the optical electric field of the signal light of each wavelength is given by d _{k + 1, n} (t) E ₀ exp [iω _k t + φ _k ] + c.c., And this d _{k, n} (t) Are parameters representing the amplitude and phase of the optical field of each wavelength. In the conventional WDM transmitter, the value of the transmission data series D _k is individually encoded in d _{k, n} (t).

For example, in the direct modulation scheme, when the transmission data sequence of channel k is 0110001... And the transmission data sequence of channel k + 1 is 11011000..., D _{k + 1} so that the amplitude of d _{k, n} (t) becomes 0110001. _{, N} (t) have been modulated using the individual optical modulators so that the optical field amplitudes of the wavelengths λ _k , λ _{k + 1} are respectively adjusted so that the amplitude of _n (t) becomes 11011000. In this case, since the first bit of channel k is “0”, it is ideal that the photocurrent detected at the receiving end is also 0, but as can be seen from equation (7-6), the bit of channel k + 1 Is “0”, the detected photocurrent is 0, but when the bit of channel k + 1 is “1”, the detected photocurrent is 2 | α ₊ | ² , and the channel k + 1 has the bit pattern of channel k + 1. A phenomenon (crosstalk) in which the detected photocurrent fluctuates occurs. Further, when the reception channel k is “1” and the channel k + 1 is “0”, the detection signal is “α ² ₀ ”, but when the channel k + 1 is “1”, α ² ₀ + 2α ₀ depending on the beat component. a + cos [ΔωΔT _n + φ _k −φ _k ] + α ² ₊ ”. Here, the description is limited to the direct modulation method, but the optical electric field d _k includes various characteristics of light such as optical amplitude, optical phase, and polarization. Similar explanations apply to modulation schemes.

In the present invention, the main signal light encoded in advance by the transmission data sequence D _{k having} the wavelength λ _k is added to the other main signal light so as to cancel the variation in the detection signal of the reception wavelength due to the data pattern of the other channel. A compensation signal light having a wavelength λ _k encoded by a channel, for example, a transmission data sequence D _{k + 1} having a wavelength _{k + 1} is superimposed. The influence of the detection signal from the channel k + 1 can be understood from the equation (7-6). From the first term, the detection signal of the adjacent wavelength itself is detected.

  From the second term, received wavelength-adjacent wavelength beat component

I understand that Thus, for example, in encoding the light of wavelength _{λ k, D k, n =} d k, encoded as _{n -α + / α 0 d k} + 1, n. In this case, the first term is

Here, if ΔωΔT (n + 1/2) + φ _k −φ _{k + 1} = 2πm (m: integer) is satisfied,

Thus, the crosstalk from channel k + 1 can be canceled.

FIG. 13 shows a configuration example of this method. Each wavelength is generated from, for example, the same light source, separated, and input to each light modulation unit. In each optical modulation means, the light of wavelength λ _k is the main signal light S _k encoded with the respective transmission data series D _k and the compensation signal light encoded with a transmission data series different from the main signal light S _{k. , K-2} , _{Ck, k-1} , _{Ck, k + 1} , _{Ck, k + 2} . Table 3 defines coefficients representing the amplitude and / or phase of the optical electric field of the compensation signal light. Note that the optical electric field amplitude and / or phase of the compensation signal light is a value normalized by the optical electric field amplitude and / or phase of the main signal light. However, here, the optical electric field is described using phasor display, and the complex conjugate component is excluded. _{β k, γ k, δ k} ... is a complex number, the transmission path is a parameter that depends on the transmission characteristics of such demultiplexing filter.

  Express transmission data as D'k, n as

  When the crosstalk component from the adjacent wavelength is considered, the received electric field can be expressed by the following equation.

In the photocurrent I _k which is the square of the absolute value of the received electric field at this time, the crosstalk component from the adjacent wavelength is completely canceled, that is, only the components of the main signal d _{k, n} remain in Table 3. The coefficient of the compensation signal light may be associated.

Here, for the sake of simplicity, in the case where only the compensation signal light C _{k, k−1} , C _{k, k + 1} is superimposed on the main signal light S _k , the effect of reducing the crosstalk components from both adjacent wavelengths is obtained. Here, it is assumed that only the compensation signal light of both adjacent wavelengths is used, but by adding more compensation signal light with C _{k + 2} , C _{k + 3} ,..., C _k−2 , C _k−3 ,. In some cases, fluctuations in the received signal due to signal light crosstalk having a wavelength of λ are more effectively reduced. The optical field of the signal light in which each wavelength λ _k is encoded by the transmission data series D _k is _expressed as d _{k, n} (t) E ₀ exp [iω using the complex number d _{k, n} encoded by D _k . _k t + φ _k ] + c. c. Can be defined as However, E ₀ is the optical amplitude shared by all channels, ω _k is the optical frequency of each wavelength, and φ _k is the initial phase of each wavelength. At this time, according to Table 3, the optical electric field amplitude / phase of the wavelength λ _{k including} the compensation signal light is D _{k, n} (t) E ₀ exp [iω _k t + φ _k using the complex number D _{k, n} (t). ] + C. c. In the following description, it is assumed that D _{k, n} (t) is defined by the following equation.

This equation indicates that the optical signal light of the wavelength λ _k is the signal light d _{k, n} (t) E ₀ exp [iω _k t + φ _k ] + c. c. Signal light for compensating for fluctuations from λ _{k + 1}

And compensation signal light for compensating for crosstalk from the wavelength λ _k−1

It shows that it consists of. The effect of reducing crosstalk from the signal lights of both adjacent wavelengths by superimposing these compensation signal lights is shown below. It can be seen that the photocurrent detected after wavelength separation using a wavelength demultiplexing filter or the like at the receiving end is expressed by the following equation from Equation (7-6).

Here, α ₀ , α ₊ , α ₋ are parameters representing transfer characteristics such as a transmission line and a demultiplexing filter. The relationship between these parameters and β ₊ , β ₋ is β ₊ = α ₊ / α ₀ , β = alpha _- / alpha to ₀ and becomes to predetermined wavelength separation and phase adjuster. For example, the separation ratio of the wavelength separation means for separating the main signal light S _k having the wavelength λ _k and the compensation signal lights C _{k, k−1} , C _{k, k + 1} ... Is measured in advance from the reception spectrum of the wavelength λ _k. By setting the obtained spectral intensity ratio of each wavelength, the relationship of β ₊ = α ₊ / α ₀ and β = α ₋ / α ₀ is realized. At this time, generally the photocurrent of the k-th channel is expressed by the following equation.

Here, when the phase difference φ _k −φ _{k + 1} is controlled so as to satisfy ΔωΔT (n + 1/2) + φ _k −φ _{k + 1} = 2πm (m: integer), the following equation is simplified. When the optical frequency interval of the WDM carrier is an integral multiple of the data clock, ΔωΔt = 2πl (l: integer) is satisfied, so that the phase difference φ _k −φ _{k + 1} can be fixed.

In Expression (7-10), it can be seen that the first-order terms of β ₊ and β ₋ are canceled, and crosstalk from other transmission data sequences is reduced.

If R _XT is defined as the ratio of the optical field of the crosstalk component to the desired photocurrent component, it can be obtained from equation (7-1) when the compensation light is not superimposed.

  On the other hand, when the signal light for compensation is superimposed, it is obtained from Expression (7-10).

It can be seen that the crosstalk is reduced as compared with the equation (7-11) in the order of β. Here, an example of superimposing only both sides as the compensation signal light has been shown. However, as shown in Table 3, there is a possibility that crosstalk can be reduced with higher accuracy by superimposing more compensation signal light. There is. In Expression (7-10), the second-order terms of β ₊ and β ₋ still remain, and high-order compensation signal lights C _{k + 2} , C _{k + 3} ,..., C _k−2 so as to cancel them. , C _k-3 ,... Can be superimposed.

  The intensity modulation-direct detection method has been described above. In this method, the optical electric field dk includes various characteristics of light such as optical amplitude, optical phase, and polarization. The method according to the present invention, in which the compensation signal light is superimposed, is effective in reducing crosstalk, regardless of the modulation method or the reception method, such as the phase modulation method or the differential phase modulation method. Here, the explanation is limited to the case of reducing the crosstalk from only both sides, but in general, for the reduction of crosstalk from signal light of wavelengths other than both sides, the transmission signal is similarly pre-compensated. A method of superimposing compensation signal light for this purpose is effective.

In Expression (7-10), an optical electric field that is −β times that of the main signal light is superimposed as the compensation signal light for both adjacent signals. This means that the optical electric field phase of the compensation signal light is reversed from that of the main signal light. However, β is given by transfer characteristics such as a transmission line and a receiving end demultiplexing filter. The channel k + 1 is "0" / "1" in the case of the channel k + 1 of the inversion bit pattern "1", "0" main signal a compensation signal light encoded by a compensation signal light having a wavelength lambda _k It is also considered that they are superimposed in the same phase as the light.

In addition, β ₊ and β ₋ in Expression (7-10) are determined by the filter characteristics and transmission path dispersion characteristics of the receiving end, and the optical field amplitude and phase of the optimum compensation signal light are adjusted accordingly. It needs to be adjusted. There is also a method including means for adjusting the optical electric field amplitude and phase of each compensation signal light, an example of which is shown in FIG. Here, the light intensity / phase of the compensation signal light is adjusted by the light intensity / phase adjusting means. FIG. 14 shows a method of adjusting the light intensity / phase before encoding, but the same effect can be obtained even if the adjustment is performed after encoding. In FIG. 14, a means for adjusting the light intensity and phase of each wavelength is separately provided. However, in the method shown in FIG. Here, although only the compensation signal light is adjusted, a method of simultaneously adjusting the main signal light is also conceivable.

Furthermore, since the optimum conditions of the optical amplitude and phase of the compensation signal light change due to time changes such as the transmission line dispersion characteristics and the characteristics of the receiving end branching means, these crosstalk compensation effects are monitored in FIG. A method of feedback control is shown to be sufficiently reduced. As a state parameter of each channel in FIG. 15, there is a method of monitoring the change information of the dispersion characteristic of the transmission path and the receiving end filter characteristic and feedback-controlling the optical amplitude and phase of the compensation signal light. Further, a method of monitoring the detection signal quality of each wavelength at the receiving end and performing feedback control on the optical amplitude and phase of the compensation signal light so as to improve the quality is conceivable. As described above, as each channel state parameter in FIG. 15, all information related to α ₀ , α ₊ , α ₋ and the like in Expression (7-1), such as information on the transmission path state and information on the detection signal, are included. As a candidate.

  A method of directly embodying the image of the equation (7-1) is a method of generating the compensation signal light and superimposing it on the main signal light. The WDM transmission signal light of the equation (7-1) For example, each transmission data sequence is calculated using an electric circuit, etc., and an encoded electric signal for encoding light of each wavelength is generated, and each encoded electric signal is used to generate each encoded data signal. There is also a method for encoding light of a wavelength.

Further, as can be seen from Equation (7-1), the optical power ratio between the compensation signal light and the main signal is | β _± | ² , which is the light with respect to the main signal component of the crosstalk light component mixed at the receiving end. It can be seen that the power ratio is equal to | α _± | ² / | α _{0 |} ² .

When a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A and any transmission data sequence D _B different from D _A is transmitted at a wavelength λ _B , the optical electric field of light input from the optical input port using the amplitude or phase or multiple input light modulator the amplitude and phase can be modulated by an electrical signal input from the two or more electrical input port, and inputs the wavelength lambda _a from the light input port, a transmission data sequence D _a by inputting the transmission data sequence D _B from the electrical signal input port, to encode the optical electric field amplitude and / or phase and transmission data sequence D _a by transmission data sequence D _B of the wavelength lambda _a, using FIG. I will explain.

FIG. 16 shows an example of the encoding method of the present embodiment. The multi-input optical modulator shown in FIG. 16 is a modulator having an optical input port and a plurality of electrical input ports, and an electrical signal from each electrical input port is input to the optical modulator. Signal light of the wavelength lambda _A input from the optical input port is input to the modulator is demultiplexed. The signal light of wavelength λ _A input to each optical modulator has its optical electric field amplitude and / or phase encoded by transmission data series D _A and D _B , combined, and then output from the optical output port. The In this way, using the multi-input modulator, the signal light having the wavelength λ _A is encoded by the transmission data series D _A and D _B.

  As the multi-input optical modulator, for example, an optical SSB (Single Side Band) modulator can be applied.

In the optical modulation means described in the second to seventh embodiments, a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A , and one of the different transmission data sequences D _B is transmitted at a wavelength λ _Bi when D _A. in this case, input light of wavelength lambda _a two or more light modulators arranged in parallel, encodes the wavelength lambda _a by transmission data sequence D _a using one of the optical modulators, other light modulation by encoding the wavelength lambda _a by transmission data sequence D _B with vessels, reduce the variation detection signal of the signal light of the wavelength lambda _a separated for each wavelength at the reception end receives the signal light of the wavelength lambda _B It shows that precompensated signal light can be generated.

Here, for the sake of simplicity, description will be made only in the case of two channels, but the same method can be applied even if the number of channels increases. Focusing on signal light of a certain wavelength λ _A , description will be given with reference to FIG. The light of wavelength λ _A is encoded with its optical electric field amplitude and / or phase by transmission data D _A and transmission data D _B using a plurality of optical modulation means arranged in parallel as shown in FIG. The The light of wavelength λ _A is separated into _two lights A ₁ and A ₂ at point a and is input to the light modulator M _A1 and the light modulator M _A2 , respectively. Optical electric field amplitude and / or phase of light A ₁ is encoded by the transmission data sequence D _A by the light modulation means M _A1, the main signal light is generated. On the other hand, the optical electric field amplitude and / or phase of the light A ₂ is encoded by the transmission data series D _B transmitted by the light of the wavelength λ _B by the optical modulator M _A2 to generate the compensation signal light. The main signal light and the compensation signal light are combined at point b, and transmission signal light that has been precompensated so as to reduce fluctuations due to adjacent wavelengths is generated. Here, by adjusting the branching ratio at the point a and the coupling ratio at the point b, a desired optical power ratio between the compensation signal light and the main signal light can be realized.

  Further, when the branching ratio and the coupling ratio cannot be adjusted, it may be possible to adjust by each modulation means as described below. For example, assuming a Mach-Zehnder type modulator as the modulation means, the optical power ratio or optical phase of the main signal light and compensation signal light is adjusted by adjusting the bias point of each modulator and the amplitude of the drive signal. it can.

FIGS. 18 and 19 show the relationship between the applied voltage of the Mach-Zehnder optical modulator, the transmission data series, and the optical output. In general, in the intensity modulation method, the bias voltage of the optical modulator is such that the transmittance is 1 when the transmission data sequence is “1” and the transmittance is 0 when the transmission data sequence is “0”. While adjusting the modulation voltage, wherein as shown in FIG. 18, as transmission data sequence D _B becomes transmittance and ρ when "1", when the transmission data sequence D _B is "0" The bias voltage and modulation drive voltage of the optical modulator are adjusted so that the transmittance is zero. The transmittance depends on the bias voltage and the modulator driving voltage, and ρ can be adjusted by adjusting them. Therefore, if this method is used, the optical power ratio between the main signal light and the compensation signal light can be arbitrarily adjusted.

Further, as shown in FIG. 19, as transmission data sequence D _A is the transmittance and σ at "1", the transmittance is ρ when transmission data sequence D _A is "0" Thus, the bias voltage and modulation drive voltage of the optical modulator are adjusted. By this method, modulated light having an optical power bias of an arbitrary magnitude and an arbitrary optical power amplitude can be generated.

By using the above method, it is possible to arbitrarily adjust the optical power ratio between the main signal generated by the optical modulators arranged in parallel and the signal light for compensation. For example, the bias voltage and the modulator drive voltage amplitude of the optical modulator M _A1, transmission data sequence D _A is such that the transmittance is 1 when "1", transmission data sequence D _A is "0" Sometimes, the transmittance is adjusted to be zero. On the other hand, the bias voltage and the modulator drive voltage amplitude of M _A2, using the inverted data sequence of the transmission data sequence D _B, as transmission data sequence D _B is the transmittance 0 when "1", its transmittance when transmission data sequence D _B is "0" is adjusted to be beta. At this time, the signal light obtained by multiplexing the compensation signal light and the main signal light, as shown in FIG. 20 (a), the signal light encoded by the transmission data sequence D _A is, transmission data sequence D _{When B} is “1”, precompensated signal light whose optical power is reduced by ρ is obtained. Also, a main signal light generated under the same bias voltage and modulator drive voltage amplitude conditions, a modulator drive voltage amplitude condition where the transmittance is ρ, and a bias voltage condition where the optical phase of the main signal light is inverted. in the case of multiplexing the compensation signal light encoded by the transmission data sequence D _B, as shown in FIG. 20 (b), the signal light encoded by the transmission data sequence D _a is, transmission data sequence D _B is the signal light whose optical power has been pre-compensated such that small as ρ when "0" is obtained.

Similarly, transmittance 0 when the transmission data D _B is "0", such that the transmission data D _B is the transmittance ρ when "1", to adjust the bias voltage of the optical modulator M ₂ Thus, an optical signal in which transmission data is inverted can be generated. The compensation signal light and the main signal light by the optical phase is combined so that the phase relationship, the signal light encoded by the transmission data sequence D _A is, transmission data sequence D _B is "0" In this case, it is possible to realize a function of obtaining precompensated signal light whose optical electric field amplitude is reduced by ρ.

FIG. 21 shows an example in which an optical phase shifter is inserted in the optical path of the light A ₂ so that the optical phase difference between the main signal light and the compensation signal light can be adjusted in advance.

For example, by adjusting the optical phase shifter so that the optical phase difference between the optical path A ₁ and the optical path A ₂ is inverted in advance, and adjusting the modulator driving voltage of the optical modulator M _A2 so that the transmittance becomes ρ, When the transmission data sequence D _B is “0”, the pre-compensated signal light whose optical field amplitude is reduced by ρ is obtained for the signal light of the wavelength λ _A encoded by the transmission data sequence D _A. Similarly, the optical phase shifter is adjusted in advance so that the optical phase difference between the optical paths A ₁ and A ₂ is in phase, and the modulator driving voltage of the optical modulator M _A2 is adjusted so that the transmittance is ρ. Thus, the signal light having the wavelength λ _A encoded by the transmission data series D _A is pre-compensated so that the optical electric field amplitude is reduced by ρ when the inverted data series of the transmission data series D _B is “1”. Signal light is obtained.

If the function shown in the present embodiment is used, pre-compensation is performed so that the detection signal of the signal light having the wavelength λ _A separated for each wavelength at the receiving end is reduced by the signal light having the wavelength λ _B. Signal light can be generated.

In the present embodiment, in the optical modulation means described in the second to seventh embodiments, a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A , and any one transmission data sequence D _B different from D _A has a wavelength when sent in lambda _B, enter the light of the wavelength lambda _a two or more light modulators arranged in series, codes wavelength lambda _a by transmission data sequence D _a using one of the optical modulator And the wavelength λ _A is encoded by the transmission sequence D _B using the other optical modulator, so that the detection signal of the signal light of the wavelength λ _A separated for each wavelength at the receiving end becomes the signal light of the wavelength λ _B It can be shown that precompensated signal light can be generated so as to reduce fluctuations experienced by.

FIG. 22 shows the light modulation means of this embodiment. In the present embodiment, as shown in FIG. 22, a plurality of optical modulators arranged in series are used, and the optical field amplitude and / or optical phase of the light of wavelength λ _A is changed by the transmission data series D _A and D _B. Encoded. Light of the wavelength lambda _A, the optical electric field amplitude and / or the optical phase of the signal light of the wavelength lambda _A in the optical modulator M _A1 by the transmission data sequence D _A is coded, the main signal light. On the other hand, the optical signal amplitude and / or optical phase of the main signal light is encoded by the transmission data series D _B in the optical modulator M _A2 , and the main signal light of wavelength λ _A is received by the signal light of wavelength λ _B at the receiving end. The transmission signal light is pre-compensated so as to reduce the received fluctuation. Here, as a method of adjusting the optical power for generating the desired precompensated transmission signal light by the optical modulator M _A2 , assuming a Mach-Zehnder type optical modulator as the optical modulation means, for example, the bias voltage of the modulator In addition, there is a method capable of adjusting the optical power or the optical phase by adjusting the amplitude of the drive signal. Here also, the optical modulator M _A1 and the optical modulator M phase difference corresponding to the optical path length difference of _A2, transmission data sequence D transmission data sequence by an electrical signal with a phase delay unit phaser etc. _B D _A advance combined by giving a phase of the electrical signals of the transmission data sequence D _B and.

Here, an example is shown in which signal light of wavelength λ _A is encoded by transmission data sequences D _A and D _B. FIG. 23 shows the relationship between the bias voltage, the modulation voltage, and the optical output in the optical modulators M _A1 and M _A2 . In the optical modulator M _A1 as shown in FIG. 23 (a), the as transmission data sequence D _A signal light of the wavelength lambda _A that has been entered transmittance is 1 when "1", transmission data sequence D _a adjusts the bias voltage and the modulation voltage of the optical modulator as a transmissivity of ρ when "0". In the optical modulator M _A2, the signal light of the optical modulator M _A1 encoded wavelength lambda _A, as transmission data sequence D _B transmittance becomes 1 when "1", whereas, the transmission data series D _B adjusts the bias voltage of the optical modulator as the transmittance becomes σ (σ <1) and the modulation voltage when "0". The light output at this time is as shown in FIG. 23 (b), transmission data sequence D _B is the optical power σ by a factor becomes smaller as a signal light when "0" is obtained. In this way, the light output from the optical modulator M _A1 can be multiplied by 1 or σ times by the optical modulator M _A2 , so that the modulated light having an optical power bias of an arbitrary size and an arbitrary optical power amplitude Can be generated. Furthermore, transmission in the optical modulator M _A2, as transmission data sequence D _B becomes transmittance sigma and (sigma <1) when the "1", whereas, when the transmission data sequence D _B is "0" adjusting the bias voltage and the modulation voltage of the optical modulator such that the rate 1, transmission data sequence D _B is the optical power σ by a factor becomes smaller as a signal light at the time of "1" is obtained.

Illustrates an optical modulation method of this embodiment in FIG. 24, the following focuses on the signal light of the wavelength lambda _A, illustrating the present embodiment. When a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A and at least one transmission data sequence D _Bi different from D _A is transmitted at the wavelength λ _Bi , the transmission data sequences D _A and D _Bi are is input to the electric circuit, they use the modulator driving electrical waveform obtained by electrical operation, encodes the light of the wavelength lambda _a. In this electrical circuit, transmission data series D _A and D _Bi transmitted at wavelength λ _A are input, and transmission data is reduced so as to reduce fluctuations received by the detection signal of wavelength λ _A from the signal light of wavelength λ _B. sequence D sends _B based on a data series D _a is processed, it is output as the modulator driving electrical waveform. As shown in the second, third, seventh, etc., the electric circuit may be designed so that various transmission signal lights can be generated. The light intensity and / or phase of the wavelength λ _A is encoded by the electric signal D ′ _A output from the electric circuit. Here, when the modulation voltage-optical output characteristic M (V) of the optical modulator is non-linear, a modulator driving electric waveform may be generated in consideration of the non-linear characteristic. With this configuration, it is possible to generate an arbitrary transmission waveform that pre-compensates a detection signal received from signal light of another wavelength.

Here, a specific description will be given assuming two channels, intensity modulation, and NRZ. When the transmission data D _B is "1", it is compared with the case of "0", the detection signal of the wavelength lambda _A is assumed to increase by [rho. However, the transmission data D _A is "1", the transmission data D _B has a 1 the intensity of the detection signal when the "0". In this case, when the transmission data series D _A is “0”, the data pattern “0” / “1” of the transmission data series D _B is changed to “ρ” / “0”, and “1” of the transmission data series D _A is set. in the case of "a" 1 "/" If generating a transmission signal light of 1-[rho ", the detection signal of the lambda _a regardless of the transmit data D _B is" [rho "/" 1 ". In the electric circuit, [D _A , D _B ] = [0, 0], [0, 1], [1, 0], [1] in consideration of the inverse function of the modulation voltage-optical output characteristic of the optical modulator. , 1], M ⁻¹ (ρ), M ⁻¹ (0), M ⁻¹ (0), and M ⁻¹ (1−ρ) are output. An example of the modulation voltage, bias voltage, and optical output in the optical modulator is shown in FIG. Here, a specific description has been given by taking the case of two-wavelength intensity modulation-NRZ as an example, but the same applies even when the number of wavelengths is increased to M wavelengths. In this case, the light of each wavelength may be encoded using all the data series, but in general, the adjacent channels cause the main detection signal to be affected. Effects can be expected. In the above description, the description is limited to the intensity modulation method. However, since the optical electric field dk includes various characteristics of light such as optical amplitude, optical phase, and polarization, a phase modulation method that modulates using these characteristics, etc. Similar methods can be applied to other modulation schemes, and any precompensated transmission signal light can be obtained.

Further, when a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A , and at least two transmission data sequences D _Bi and D _Ci different from D _A are transmitted at wavelengths λ _Bi and λ _Ci , respectively, _A modulator driving voltage waveform D ′ _A is generated based on the transmission data series D _Bi and the transmission data series D _A , and light of wavelength λ _A is input to two or more optical modulators arranged in series and / or in parallel Then, using one of the optical modulators, at least one of the optical electric field amplitude and / or optical phase of the light of wavelength λ _A is encoded by the modulator driving voltage waveform D ′ _A and transmitted using the other optical modulator. the method of encoding at least one of the optical field amplitude and / or the optical phase of the light of the wavelength lambda _a by the data D _C, will be described with reference to FIGS. 26 and 27. Generates transmission data sequence D modulator driving voltage waveform D _'A obtained by processing the transmission data sequence D _A by _B, thereby inputting the electrical input port of the optical modulator. In order to further reduce the variation received from the wavelength lambda _C at the receiving end, an example of a method of generating a transmission signal light precompensation using a data series D _C.

In the electric circuit, electric signals of the transmission data series D _A and D _Bi are input, and transmission is performed based on the transmission data series D _B so as to reduce fluctuation that the detection signal of the wavelength λ _A receives from the signal light of the wavelength λ _B. data series D _A is processed, it is output as the modulator driving voltage waveform. As for the processing method, an electric circuit may be designed so that various transmission signal lights can be generated as shown in the second, third, seventh embodiments. Here, when the modulation voltage-optical output characteristic M (V) of the optical modulator is non-linear, a modulator driving electric waveform may be generated in consideration of the non-linear characteristic. In FIG. 27, the light of wavelength λ _A is encoded by the modulator driving voltage waveform D ′ _A using the optical modulator M _A1 . Further, the signal light outputting optical modulator M _A1 is input to the optical modulator M _A2, the detection signal by the transmission data sequence D _C is encoded to reduce the variation received from the wavelength lambda _C.

In FIG. 27, the light of wavelength λ _A is encoded by the modulator driving voltage waveform D ′ _A using the optical modulator M _A1 . Further, the wavelength λ _A divided at the point a is encoded by the modulator driving voltage waveform D ′ _A in the optical modulator M _A1 . On the other hand, the signal light for compensation is generated such that the wavelength λ _A is reduced by the optical modulator M _A2 by the transmission data sequence D _C according to the wavelength λ _C. At point b, they are combined to generate transmission signal light that has been precompensated so as to reduce fluctuations from the wavelengths λ _Bi and λ _Ci . There is also a method in which DC _Ci is input to the electric input port of M _A2 after generating a modulator M _A2 drive voltage waveform by an electric circuit. In the method of this embodiment, it is possible to generate an arbitrary transmission signal light that has been pre-compensated so as to reduce the fluctuation that the reception signal receives from the signal light of other wavelengths by using various electric signal processing techniques.

Further, when a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A , and at least two transmission data sequences D _Bi and D _Ci different from D _A are transmitted at wavelengths λ _Bi and λ _Ci , respectively, A modulator drive voltage waveform is generated based on the transmission data series D _Bi and the transmission data series D _Ci, and light of wavelength λ _A is input to two or more optical modulators arranged in series and / or in parallel. The wavelength λ _A is encoded by the transmission data sequence D _A using one of the optical modulators, and the wavelength λ _A is encoded by the modulator driving voltage waveform using the other optical modulator. _A method of reducing the fluctuation that the detection signal of λ _A receives from the signal light of wavelengths λ _B and λ _C will be described with reference to FIGS.

In the electric circuit, electric signals of the transmission data series D _Bi and D _Ci are input, and a compensation signal light is generated so as to reduce the fluctuation that the detection signal of the wavelength λ _A receives from the signal light of the wavelengths λ _Bi and λ _Ci. to generate a modulator driving voltage waveform based on the transmission data sequence D _B and transmission data sequence D _a. Using the optical modulator M _A2 , the light of wavelength λ _A is encoded by the modulator driving voltage waveform, and the compensation signal light is generated. What is necessary is just to design an electric circuit so that various signal light for a compensation as shown in Example 2, 3, 7 etc. can be produced | generated. Here, when the modulation voltage-optical output characteristic M (V) of the optical modulator is non-linear, a modulator driving electric waveform may be generated in consideration of the non-linear characteristic. In Figure 28, light of the wavelength lambda _A is encoded by the transmission data sequence D _A using an optical modulator M _A1, furthermore, sign light of the wavelength lambda _A by a modulator driving voltage waveform in the optical modulator M _A2 It becomes. On the other hand, in FIG. 29, the light of wavelength lambda _A is separated into two light _{A 1} and _{A 2} in a point, the light of _{A 1} is encoded by the transmission data sequence _{D A} in the optical modulator _{M A1,} A _{The second} light is encoded by the modulator driving voltage waveform in the optical modulator _MA2 . As a result, it is possible to reduce the fluctuation that the detection signal of wavelength λ _A receives from the signal light of wavelengths λ _B and λ _C. Also, in terms of generating the modulator M _A1 driving voltage waveform by an electrical circuit D _A, there is a method of inputting the electrical input ports of the M _A1. In the method of this embodiment, it is possible to generate an arbitrary transmission signal light that has been pre-compensated so as to reduce the fluctuation that the reception signal receives from the signal light of other wavelengths by using various electric signal processing techniques.

  Example of superimposing compensation signals using an optical modulator when the number of compensation signals to be superimposed on the main signal increases due to changes in the influence of crosstalk due to differences in signal modulation speed, signal wavelength interval, demultiplexing filter width, etc. Then, the number of optical modulators increases, which is not economical. In the same case, in the example of superimposing using the electric circuit, the arithmetic processing speed of the electric circuit increases, and the number of compensation signals is limited by the limit of the arithmetic processing speed. On the other hand, in the method of superimposing compensation signals using an optical modulator and an electric circuit, when the number of compensation signals superimposed on the main signal increases, compensation is performed within the range of the arithmetic processing speed of the electric circuit. If the number of signals for use is determined, the number of optical modulators can be reduced, and the crosstalk can be reduced more economically than in the case where the compensation signals are superimposed using an optical modulator.

When a certain transmission data sequence D _A is transmitted at a certain wavelength λ _A and at least one transmission data sequence D _B different from D _A is transmitted at the wavelength λ _B , a part of light of the wavelength λ _A is transmitted as transmission data. the method of generating a compensation signal light having a wavelength lambda _a by entering the modulator for encoding the sequence D _B, will be described with reference to FIG. 30. Light ... λ _k-1 of the plurality of wavelengths in the wavelength separation and phase adjusting means shown in FIG _{30, λ k, λ k +} 1 ... are input, each output port _{... P k-1, P k} , the P _{k + 1 ...} mainly each wavelength ... λ _{k-1 to,} lambda _k, although lambda _{k + 1 ...} of the light is output, the output is also superimposed light of a different wavelength from that in addition to it. For example, assume that light of a plurality of wavelengths is output to each output port as shown in FIG. Numerical values in Table 4, each port _{... P k-1, P k} , P k + 1 ... wave ... _{λ k-1} is the main component of the output light of, lambda _k, normalized by lambda _{k + 1 ...} optical field amplitude It was defined as the optical electric field of each wavelength. However, here, the optical electric field is described using phasor display, and the complex conjugate component is excluded.

Output lights from the respective ports... P _k−1 , P _k , P _{k + 1} ... Are respectively input to individual modulation means, and encoded by transmission data sequences _Dk−1 , D _k , D _{k + 1} . The encoded optical signals are combined into transmission signal light. In this way, transmission data sequence _{... D k-1, D k} , D k + 1 ... each wavelength ... _{λ k-1} encoded by, lambda _k, and lambda _{k + 1 ...} main signal light components, respectively transmission data sequence ... D _k−1 , D _k , D _{k + 1} ... Intensity ratios and phase differences with respect to the signal light components for compensation of wavelengths .lambda. _K-1 , .lamda. _K , .lambda. _{K + 1} . It can be set freely depending on the design of the phase adjusting means. It is also desirable that the wavelength separation / phase adjustment means be designed so that the intensity ratio and the phase difference can be freely adjusted. In this transmission signal light generation method, it is possible to generate arbitrary transmission signal light that has been pre-compensated so as to reduce fluctuations received signals receive from signal light of other wavelengths.

In the following, it is mathematically shown that this method can generate the WDM transmission signal light described by Expression (7-10). Here, ignored as 0 all the coefficients other than beta _± For _{simplicity, β 1+ = β 2+ = β} 3+ = ... = β +, β 1- = β 2- = β 3- = ... β - and the However, even if these are individual values and the coefficients other than b _± are not 0, the same following argument holds.

  The output light from the wavelength separation / phase adjustment means in FIG. 30 is expressed as follows using mathematical formulas.

However,... E _k−1 , E _k , E _{k + 1} ,... Represent the optical electric fields output to the respective ports... P _k−1 , P _k , P _{k + 1} , ... ω _k−1 , ω _k , ω _{k + 1} ,. ... the wavelength _{... λ k-1, λ k} , the λ _{k + 1 ...} optical angular frequency _{of, φ k-1, φ k} , φ k + 1, ... the wavelength _{... λ k-1, λ k} , λ _{k + 1 ...} initial phase of the Represents. This is input to the modulator, and the light in which the optical electric field amplitude and phase are encoded by the transmission data series... D _k−1 , D _k , D _{k + 1} .

  Therefore, the optical electric field obtained by combining these is given as the sum of the components of the vector.

  This is WDM signal light represented by Expression (7-10), and arbitrary compensation signal light can be superimposed by this method. For example, the light intensity separation ratio of wavelength separation means represented by AWG or optical wavelength filter is set in advance so as to be the light intensity of the main signal light and crosstalk light of the reception wavelength, and the phase represented by the phase compensator By setting the apparatus in advance such that the phase of the compensation signal light is inverted with respect to the phase of the main signal light by the adjusting means, an apparatus configuration that can superimpose any compensation signal light is obtained.

  Although the present invention has been specifically described with reference to some embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. The embodiments illustrated herein can be modified in configuration and details without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a schematic diagram showing the mode of the linear crosstalk in the multiwavelength signal light demultiplexing means of a receiving part. FIG. 2A is a schematic diagram showing the state of linear crosstalk on the optical spectrum, FIG. 2A shows the spread of the spectrum depending on the modulation speed of the signal light spectrum of each wavelength, and FIG. 2B shows the spectrum overlap. Shows the linear crosstalk by. It is a figure which shows the mode of the time waveform of a main received signal (solid line) and a beat component (dotted line), and Fig.3 (a) shows the case where the phase of a beat component changes randomly with respect to the phase of a main received signal. FIG. 3B shows a case where the phase of the beat component is stabilized in terms of time. FIG. 4 (a) shows a measurement waveform of a received signal, FIG. 4 (a) shows a case where the phase of the beat component is measured in a random state, and FIG. 4 (b) shows the phase of the beat component as the phase of the data clock. The case where it measured in synchronization is shown. FIG. 5A is a diagram showing a state of an eye pattern of a reception detection signal. FIG. 5A shows a case where signals of adjacent wavelengths are mixed, and FIG. 5B shows that signals of adjacent wavelengths are canceled out. The case where the transmission signal is encoded is shown in FIG. It is a functional block diagram which shows the structural example which encodes each transmission wavelength using the transmission data series of an adjacent wavelength in a transmission part according to one Example of this invention. FIGS. 7A and 7B are diagrams showing an eye pattern of a reception detection signal. FIG. 7A shows a case where a fluctuation is caused by a signal of an adjacent wavelength in addition to a beat component, and FIG. The transmission signal is encoded so that the signal of the adjacent wavelength is canceled by synchronizing with the phase of the data clock. According to one embodiment of the present invention, the transmission unit encodes each transmission wavelength using a transmission data sequence of adjacent wavelengths (FIG. 8A), and synchronizes the phase of the beat component with the phase of the data clock (FIG. 8). (B)) It is a figure explaining a structural example. FIG. 9A shows a state of an eye pattern of a reception detection signal. FIG. 9A shows a case where a fluctuation is caused by a signal of an adjacent wavelength in addition to a beat component, and FIG. This shows a case where a transmission signal is encoded so that fluctuations due to beat components and signals of adjacent wavelengths are canceled out. It is a figure explaining the structural example which integrates from the multiwavelength signal optical multiplexing means of a transmission part to a multiwavelength signal optical multiplexing means in order to stabilize the phase of a beat component according to one Example of this invention. FIG. 6 is a functional block diagram illustrating a configuration example in which the phase of a beat component is fed back and synchronized with the phase of a data clock according to an embodiment of the present invention. It is a figure which shows the mode of the eye pattern of a reception detection signal when synchronizing the phase of a beat component and a data clock according to one Example of this invention. FIG. 6 is a functional block diagram illustrating a configuration example of superimposing precompensation signal light based on a transmission data sequence of adjacent wavelengths at the transmission end so that fluctuations due to signals of adjacent wavelengths are canceled at the reception end according to an embodiment of the present invention. . FIG. 6 is a functional block diagram illustrating a configuration example in which each compensation signal light is adjusted at the transmission end in accordance with the filter characteristics and transmission path dispersion characteristics at the reception end according to an embodiment of the present invention. FIG. 6 is a functional block diagram showing a configuration example of adjusting compensation signal light at a transmission end by feeding back channel parameters related to a transmission path and / or a reception unit according to an embodiment of the present invention. It is a figure explaining the structural example which superimposes the pre-compensation signal light by the transmission data series of another wavelength at a transmission end using a multi-input optical modulator according to one Example of this invention. It is a figure explaining the structural example which superimposes the pre-compensation signal light by the transmission data series of another wavelength at a transmission end using the some optical modulation means arrange | positioned in parallel according to one Example of this invention. It is a figure explaining the relationship between the applied voltage of the optical modulator which adjusts the amplitude of pre-compensation signal light, a transmission data series, and an optical output according to one Example of this invention. It is a figure explaining the relationship between the applied voltage and transmission data series of an optical modulator which adjusts the amplitude of the self-wavelength signal light, and an optical output according to one Example of this invention. FIG. 20A is a diagram showing an optical output obtained by combining the self-wavelength signal light and the compensation signal light according to one embodiment of the present invention. FIG. 20A shows the optical output when the transmission data series of other wavelengths is “1”. FIG. 20B shows a signal that has been precompensated so that the optical output is reduced by ρ when the transmission data series of other wavelengths is “0”. Showing light. The figure explaining the example of a structure which superimposes the precompensation signal light by the transmission data series of another wavelength in the transmission end using the some optical modulation means and optical phase shifter which are arrange | positioned in parallel according to one Example of this invention. is there. It is a figure explaining the structural example which superimposes the pre-compensation signal light by the transmission data series of another wavelength in a transmission end using the some optical modulation means arrange | positioned in series according to one Example of this invention. FIG. 23 is a diagram for explaining the relationship between the applied voltage and transmission data sequence of the two optical modulators of FIG. 22 and the optical output according to an embodiment of the present invention; FIG. FIG. 23B shows a state where the transmittance is adjusted to be σ when the transmission data series of other wavelengths is “0”. It shows how it works. FIG. 6 is a diagram illustrating a configuration example in which pre-compensation signal light based on a transmission data sequence of another wavelength is superimposed at a transmission end by generating a drive electric signal of a modulator using an electric circuit according to an embodiment of the present invention. . FIG. 25 is a diagram illustrating the relationship between the modulation voltage and bias voltage and the optical output in the optical modulator of FIG. 24 according to one embodiment of the present invention. It is a figure explaining the example of a structure which superimposes the pre-compensation signal light by the transmission data series of another wavelength in a transmission end using the some optical modulation means and electric circuit which are arrange | positioned in series according to one Example of this invention. . It is a figure explaining the example of a structure which superimposes the precompensation signal light by the transmission data series of another wavelength in a transmission end using the some optical modulation means and electric circuit which are arrange | positioned in parallel according to one Example of this invention. . The figure explaining another structural example which superimposes the pre-compensation signal light by the transmission data series of another wavelength in the transmission end using the some optical modulation means and electric circuit which are arrange | positioned in series according to one Example of this invention. It is. The figure explaining another structural example which superimposes the pre-compensation signal light by the transmission data series of another wavelength in a transmission end using the some optical modulation means and electric circuit which are arrange | positioned in parallel according to one Example of this invention. It is. FIG. 4 is a functional block diagram showing a configuration example of generating self-wavelength compensation signal light by inputting self-wavelength signal light to a modulator of other wavelength signal light according to an embodiment of the present invention.

Claims (10)

In a method in which M is an integer of 2 or more, M lights having different wavelengths are encoded with M transmission data sequences, and the encoded M lights are multiplexed and transmitted.
The k as an integer of 1 to M, when encoded with transmission data sequence of the k corresponding light of the wavelength of the k is any one of the M light in the light of the wavelength, the receiving side In order to reduce crosstalk components due to one or more transmission data sequences corresponding to light of one or more wavelengths adjacent to the kth wavelength of the plurality of lights ,
Method characterized by comprising the step of encoding with all of one or more transmission data sequence corresponding to the light of one or more wavelengths that the adjacent transmission data sequence of the k-th light having a wavelength of the first k. The method of claim 1, wherein
Step of pre-Kifu-coding is,
A step of encoding the optical wavelength of the first k in transmission data sequence of the first k,
Encoding the k-th wavelength light with one or more transmission data sequences corresponding to the adjacent one or more wavelengths of light . The method of claim 2, wherein
Intensity of the light of the kth wavelength encoded with one or more transmission data sequences corresponding to the light of the kth wavelength encoded with the kth transmission data sequence or the light of one or more adjacent wavelengths. Or a step of adjusting the phase. The method according to any of claims 1 to 3,
The method further comprising the step of temporally stabilizing the phase of a beat component that vibrates at a frequency difference between the light of the kth wavelength and the light of the one or more adjacent wavelengths. The M as an integer of 2 or more, and M modulation means for respectively coding with M transmit data series M optical wavelengths different from each other generated from the multi-wavelength light generating means, the coded M items of An optical multiplexing means for multiplexing light ,
As the modulation means,
The k as an integer of 1 to M, there a modulation means for encoding in the transmission data sequence of the k corresponding light of the wavelength of the k is any one of the M light in the light of the wavelength In order to reduce crosstalk components due to one or more transmission data sequences corresponding to light of one or more wavelengths adjacent to the kth wavelength of the plurality of lights on the receiving side, the kth wavelength apparatus characterized by comprising a modulation means for encoding on all transmission data sequence transmitted data sequence with one or more corresponding to the adjacent one or more wavelengths of light of the first k. The apparatus of claim 5, comprising:
The modulating means includes
A modulator for encoding the light of the wavelength of the k-th in the transmission data sequence of the first k,
A modulator that encodes the k-th wavelength light with one or more transmission data sequences corresponding to the one or more adjacent wavelengths of light . The apparatus according to claim 6, comprising:
Intensity of the light of the k-th wavelength encoded with one or more transmission data sequences corresponding to the light of the k-th wavelength encoded with the k-th transmission data sequence or the light with one or more adjacent wavelengths Or an adjusting means for adjusting the phase. An apparatus according to any one of claims 5 to 7 ,
The apparatus further comprises phase stabilizing means for temporally stabilizing the phase of a beat component that vibrates at a frequency difference between the light of the kth wavelength and the light of one or more adjacent wavelengths. The apparatus according to claim 8, comprising:
The phase stabilizing means includes
Branching means for branching the multiplexed light connected to the output side of the optical multiplexing means ;
Phase feedback means for feeding back the phase difference between the beat component and the clock component of the transmission data sequence for each wavelength of the branched light ;
An apparatus comprising: phase control means for controlling the phase of each of the M lights based on the phase difference fed back. An apparatus according to claims 5-9,
As the multi-wavelength light generating means,
Multi-wavelength light generating means for outputting the M lights in synchronism with each phase.

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