JPH10135930A - Optical signal transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid the effects of 4-light-wave mixed noise, to extend a light wavelength multiplex number and to effectively utilize an optical transmission line by orthogonally crossing the polarization planes with each other for at least two optical pulse signals among the optical pulse signals. SOLUTION: Output light from respective light sources 1-1 to 1-8 is intensity- modulated in respective intensity modulation circuits 2-1 to 2-8. Then, for intensity-modulated optical signals, a polarization state is adjusted by polarization state control parts 3-1 to 3-8, provided with a means for orthogonally crossing the polarization planes with each other for the optical pulse signals of at least two wavelengths among the optical pulse signals. Thereafter, the optical pulse signals are outputted by pulse position control parts 4-1 to 4-8, capable of adjusting the pulse positions of the respective optical pulse signals, connected to one optical fiber by a light wavelength multiplex part 5 and sent out through a light amplifier 6 to an optical fiber transmission line 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for optical communication.
The present invention is applied to an optical wavelength division multiplex transmission device. The present invention relates to a technology for extending the number of optical wavelength multiplexes.

[0002]

2. Description of the Related Art In optical communication, there is a so-called wavelength multiplexing technique of simultaneously transmitting optical signals of a plurality of wavelengths and assigning different information to each wavelength in order to effectively use an optical transmission line. In a wavelength division multiplexing communication system using a dispersion-shifted fiber in the wavelength division multiplexing technique, deterioration of characteristics due to nonlinear effects (four-wave mixing noise) of the optical fiber becomes an important problem. Four-wave mixing noise refers to a phenomenon in which when two or more optical signals having different wavelengths are multiplexed, an optical signal having a different wavelength from the two optical signals is generated.

FIG. 4 shows the principle of a conventional wavelength division multiplexing transmission system for suppressing a nonlinear effect (four-wave mixing noise).
This will be described with reference to FIG. FIG. 4 is a diagram for explaining a conventional wavelength division multiplexing transmission system. This is K. Sekine, N. Kikuch
i, S.Sasaki and H.Ikeda, "FWM crosstalk suppression u
sing bit-phase arranged RZ signals in WDM system
s ", Proc. OECC '96, 17B2-4, pp. 114-115, 1996.

Here, a case will be described in which light of three wavelengths is multiplexed and transmitted. The intervals between the three wavelengths are equal, λ3−λ2 = λ2−λ1, and an RZ (Return to Zero) coding scheme is used. As a coding method often used in optical communication, there is a coding method of NRZ (Non Return to Zero). N
When RZ is ON-ON code continuous, ON-ON
In the case of RZ, the optical power is always zero between ON and ON, even if the code is ON-ON.
Z is more advantageous.

It is assumed that the relative time lag between the signal of λ1 and the signal of λ2 is zero, and the relative time lag between λ1 and λ3 is half of one bit of the signal. Four-wave mixing noise, which is a non-linear effect in an optical fiber, occurs when two or more lights are emitting at the same time, and its frequency f _FWM is expressed as f _FWM = f _i + f _j -f _k ( k ≠ i, j). Where f _i , f _j , f _k
Is the frequency of the signal light. Accordingly, since the wavelength intervals are equal, if λ1 and λ2 emit light simultaneously (ON), a frequency component (FWM) due to four-wave mixing noise occurs at the wavelength of λ3. However, since the signal of λ3 is shifted by a half bit time, it is not affected by the frequency component of the four-wave mixing noise at the time of signal identification. If λ2 and λ3 emit light at the same time, a frequency component due to four-wave mixing noise occurs at the wavelength of λ1, but since the signals of λ2 and λ3 do not emit light at the wavelength of λ1 at the same time, No frequency component is generated due to four-wave mixing noise.

In this example, there has been described a case where there is no time lag between λ1 and λ2, and the time lag between λ1 and λ3 is half of one bit signal. However, there is no relative time lag between the signals λ1 and λ3. The same can be said for the case where the relative time shift between λ1 and λ2 is half of one bit of the signal. That is, in a state where each signal light is emitted, by preventing the frequency component due to the four-wave mixing noise from being generated at the signal wavelength, it is possible to prevent the deterioration of the signal due to the four-wave mixing noise.

[0007] Even when the number of wavelengths is set to three or more, similarly to the above example, it is possible to prevent a frequency component due to four-wave mixing noise from being generated in the frequency of the emitted signal light. FIG. 5 shows an optimal relative time shift arrangement when the number of wavelengths is increased. FIG. 5 is a diagram showing an optimum arrangement for the number of multiplexed wavelengths in the conventional WDM transmission system.
Here, “0” indicates that the relative time shift is zero, and “π” indicates that the relative time shift is half of one bit.

[0008]

In the prior art, as shown in FIG. 5, when the number of multiplexed wavelengths is larger than "7", the frequency of light emission is prevented from generating a frequency component due to four-wave mixing noise. Therefore, the number of multiplexed wavelengths cannot be increased to “8” or more.

That is, a wavelength at which an optical signal can be optically multiplexed and transmitted without error while suppressing the coexistence of the frequency component due to the four-wave mixing noise and the optical signal without suppressing the generation of the frequency component due to the four-wave mixing noise. "7" is the largest as the number.

[0010] However, the demand for optical communication is increasing, and it is desired to expand the number of multiplexed optical wavelengths from the viewpoint of effective use of optical transmission lines.

The present invention has been made in view of such a background, and an object of the present invention is to provide an optical signal transmission device capable of expanding the number of optical wavelength multiplexes. An object of the present invention is to provide an optical signal transmission device that can effectively use an optical transmission line. An object of the present invention is to provide an optical signal transmission system that effectively uses an optical transmission line and has no signal error.

[0012]

SUMMARY OF THE INVENTION A first aspect of the present invention is an optical signal transmitting apparatus, which comprises a plurality of mutually different wavelengths (λ).
1, .lambda.2,..., .Lambda.n), a plurality of optical transmitters (11-1 to 11-n) for transmitting optical pulse signals, and means (5) for wavelength multiplexing the output optical pulse signals of the optical transmitters. It is an optical signal transmission device provided with.

Here, a feature of the present invention is that a polarization state control means (3-1 to 3-n) for respectively changing the polarization state of the output optical pulse signals of the plurality of optical transmitters is provided. The polarization state control means includes means for controlling the polarization planes of optical pulse signals of at least two wavelengths among optical pulse signals that cause four-wave mixing noise so that their polarization planes are orthogonal to each other. It is in.

That is, the literature (K. Inoue, IEEE Photonics
Technology Letters, vol. 3, No. 6, pp. 560-563, 1991), when the polarization states of two or more wavelength multiplexed signal lights are made orthogonal to each other, Generation of frequency components due to four-wave mixing noise, which is a non-linear effect in the optical fiber, can be greatly suppressed. As a specific numerical value, the generation efficiency of four-wave mixing noise can be reduced to ４ or less.

In the present invention, by utilizing this property and controlling the polarization state of the optical pulse signal, at least two optical pulse signals among the optical pulse signals which cause the generation of frequency components due to four-wave mixing noise are obtained. The most important feature is to suppress the generation of frequency components due to four-wave mixing noise by making the polarization planes orthogonal to each other.

It is preferable that a delay means is provided for delaying the phase of the optical pulse signal so as to avoid four-wave mixing noise generated by another optical pulse signal.

[0017] The polarization state control means may be configured so that the polarization planes of a plurality of optical pulse signals having different wavelengths are alternately orthogonal to each other, or a combination of a plurality of wavelengths among the plurality of wavelengths. Accordingly, the polarization plane of an optical pulse signal of one wavelength that causes four-wave mixing noise can be orthogonalized.

A second aspect of the present invention is an optical signal transmission system, which is characterized by the optical signal transmission device and an optical fiber transmission line for transmitting an output optical signal of the optical signal device. An optical amplifier provided in the optical fiber transmission line, an optical wavelength separating means for separating an output optical signal of the optical fiber transmission line into the plurality of wavelengths, and an optical receiver provided for each of the plurality of wavelengths. It is located with.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

[0020]

【Example】

(First Embodiment) A first embodiment of the present invention will be described with reference to FIG. FIG. 1 is an overall configuration diagram of the first embodiment of the present invention.

The present invention relates to an optical signal transmitting apparatus, comprising: optical transmitters 11-1 to 11-8 for transmitting optical pulse signals having mutually different wavelengths λ1, λ2,. An optical signal transmitting apparatus including an optical wavelength multiplexing unit 5 as means for wavelength multiplexing the output optical pulse signals of -1 to 11-8.

Here, a feature of the present invention is that a polarization state control unit as polarization state control means for changing the polarization state of each of the output optical pulse signals of the optical transmitters 11-1 to 11-8. 3-1 to 3-8, and the polarization state control units 3-1 to 3-8 are provided for the optical pulse signals of at least two wavelengths among the optical pulse signals which cause the four-wave mixing noise. Means for controlling the polarization planes to be orthogonal to each other is provided in the internal circuit.

Pulse position control units 4-1 to 4- as delay means for delaying the phase of the optical pulse signal so as to avoid four-wave mixing noise generated by another optical pulse signal.
8 is provided.

In the first embodiment of the present invention, the polarization state control unit 3
In (1) to (3) to (8), the polarization planes of the plurality of optical pulse signals having different wavelengths are alternately orthogonalized.

Next, the operation of the first embodiment of the present invention will be described. In the first embodiment of the present invention, a case where the number of multiplexed wavelengths is "8" will be described. Here, 1-1 to 1-8 are light sources, and 2-1-1.
1 to 2-8 are intensity modulation circuits (IM), 3-1 to 3-8 are polarization state control units (PC), 4-1 to 4-8 are pulse position control units (D), and 5 is an optical wavelength. A multiplexing unit, 6 is an optical amplifier, 7 is an optical fiber transmission line, 8 is an optical amplifier as an optical preamplifier, 9 is an optical wavelength separation unit, and 10-1 to 10-8 are optical receivers.

In the first embodiment of the present invention, it is assumed that the wavelength intervals are equal. Output light output from each of the light sources 1-1 to 1-8 is intensity-modulated by an RZ code by intensity modulation circuits 2-1 to 2-8. Each intensity modulation circuit 2-1 to 2-8
In (2), the polarization state of the optical signal intensity-modulated by the RZ code is adjusted by the polarization state control units 3-1 to 3-8.

The optical pulse signals output from the polarization state controllers 3-1 to 3-8 are input to pulse position controllers 4-1 to 4-8. The pulse position controllers 4-1 to 4-8 can adjust the pulse position of each optical pulse signal. The optical pulse signals output from the pulse position controllers 4-1 to 4-8 are coupled to one optical fiber by an optical wavelength multiplexing unit 5 composed of an arrayed waveguide grating type optical filter, and the optical amplifier 6 And then sent out to the optical fiber transmission line 7. On the receiving side, the received wavelength-division multiplexed signal is collectively amplified by an optical amplifier 8, and then separated into signal lights of respective wavelengths by an optical wavelength separation unit 9 composed of an arrayed waveguide grating optical filter. -1 to 10
Receive at -8. Optical wavelength multiplexing unit 5 and optical wavelength separating unit 9
Can be constituted by an optical coupler.

Referring to FIG. 2, the polarization state control by the polarization state control units 4-1 to 4-8 and the pulse position control by the pulse position control units 3-1 to 3-8 according to the first embodiment of the present invention will be described. explain. FIG. 2 is a diagram showing a controlled polarization state and a controlled pulse position according to the first embodiment of the present invention. In the example of FIG. 2, λ1, λ2, λ3, λ4 are ON at the same timing.
Λ5, λ6, λ7, λ8 are also in the ON state at the same timing.

Λ1, λ2, λ3, λ4 and λ5, λ6, λ
At 7 and λ8, the ON timing is shifted by half the time of one bit. In the drawing, the phase shift is represented by “0” and “π” of the phase φ.
The signals of λ1, λ2, λ3, λ4, λ5, λ6, λ
It can be seen that the signals 7 and λ8 are not simultaneously turned on.

Therefore, in the following, λ1, λ2, λ3,
Consider only the four wavelengths of λ4. Λ1, λ2, λ3, λ by the polarization state control units 3-1 to 3-4.
4 is the same as λ1 and λ3, λ2 and λ4
Are in the same polarization state, and λ1 and λ2 have orthogonal polarization states. Here, literature (K. Inoue, IEEE Photonics
As shown in Technology Letters, vol. 3, No. 6, pp. 560-563, 1991, the polarization state of four wavelength multiplexed signal lights is changed (↑, ↓, ↑, ↓). In this case, it is possible to greatly suppress the occurrence of four-wave mixing noise, which is a nonlinear effect in the optical fiber, in the four signal light components. As a specific numerical value, the generation efficiency of four-wave mixing noise is 1
/ 4 or less. Here, ↑ and ↓ represent the polarization state, λ1 and λ3 have the same polarization state, and λ
2 and λ4 are the same, and λ1 and λ2 are orthogonal to each other.

That is, the pulse position control units 4-1 to 4-8
And four wavelength signals λ1, λ2, λ3, λ4 and λ
The four wavelength signals of 5, λ6, λ7 and λ8 are simultaneously O
Since there is no N state, four-wave mixing noise does not occur in a range where the time lag is maintained between these two sets of wavelengths. Further, for the four wavelength signals λ1, λ2, λ3, and λ4, the polarization states of adjacent wavelengths are orthogonalized by the polarization state control units 3-1 to 3-8, so that the same frequency as the signal light component is used. The frequency component of the four-wave mixing noise appearing in (1) can be reduced to 1/4 or less of that in the case where the polarization states match. Generation of four-wave mixing noise due to four wavelengths of λ5, λ6, λ7, and λ8 can be similarly suppressed.

As described above, the occurrence of the frequency component of the four-wave mixing noise on the signal light frequency component can be suppressed by controlling the timing and controlling the polarization state. In the above example, λ1, λ2, λ3, λ4 and λ5, λ6, λ
Although the ON timing of the λ7 and λ8 is shifted by half the time of one bit, the timing control of the ON state is divided into two sets of λ1, λ3, λ5 and λ7 and λ2, λ4, λ6 and λ8. Is performed, and the polarization state of the four wavelengths is set to (↑, ↓, ↑, ↓), thereby suppressing the generation of four-wave mixing noise.

That is, in FIG. 2, the frequency components FWM ₁ , FWM ₅ , FWM ₉ , and FWM ₁₃ of the four-wave mixing noise
Are based on the optical pulse signals of λ1 and λ3, respectively. Further, the frequency component FWM ₂ of the four-wave mixing noise,
FWM ₆ , FWM ₁₀ , and FWM ₁₄ are based on λ2 and λ4 optical pulse signals, respectively. Further, frequency components FWM ₄ , FWM ₈ , and FWM of four-wave mixing noise
Numeral ₁₂ is based on the optical pulse signals of λ6 and λ8, respectively. Also, the frequency component FW of the four-wave mixing noise
M ₃ , FWM ₇ , and FWM ₁₁ are λ5 and λ7, respectively.
This is due to the light pulse signal of FIG.

The frequency components of the four-wave mixing noise are as follows:
It is generated by two optical pulse signals having the same polarization plane, but appears at positions where they do not affect each other. The frequency component of four-wave mixing noise generated by two optical pulse signals having different polarization planes is suppressed, and does not affect the optical pulse signal. Therefore, according to the present invention, it is only necessary to consider the arrangement of two optical pulse signals having the same polarization plane, so that the degree of freedom in signal arrangement is expanded and the number of optical multiplexes can be expanded.

In the first embodiment of the present invention, the pulse position controllers 4-1 to 4-4 are provided after the polarization state controllers 3-1 to 3-8.
8 are arranged, but this arrangement order can be reversed.

In the first embodiment of the present invention, the pulse position controllers 4-1 to 4-8 are described to control the pulse position of the optical signal.
It is also possible to electrically control the pulse position by arranging at the -8 input.

In the first embodiment of the present invention, the wavelength intervals are set at equal intervals. However, when the wavelength intervals are arranged at irregular intervals, the frequency component generated by the four-wave mixing noise differs from the signal light frequency.
The influence of four-wave mixing noise can be further avoided as compared with the case of the equally-spaced wavelength arrangement. In some cases, it is defined by an optical frequency interval corresponding to the wavelength interval.

(Second Embodiment) The second embodiment of the present invention shows an example in which polarization control and pulse position control different from those of the first embodiment of the present invention are performed. The device configuration is common to the first embodiment of the present invention. FIG. 3 is a diagram showing controlled polarization states and controlled pulse positions according to the second embodiment of the present invention. FIG.
In the example of λ1, λ2, λ5, and λ7
In the N state, λ3, λ4, λ6, and λ8 are also ON at the same timing. λ1, λ2, λ5,
At λ7 and λ3, λ4, λ6, and λ8, the ON timing is shifted by half the time of one bit.
In the drawing, the phase shift is represented by “0” and “π” of the phase φ. The above pulse position control units 4-1 to 4-
8, λ1, λ2, λ5, λ
7 and the signals of λ3, λ4, λ6, λ8
It turns out that it does not become N state. As for the polarization state, the polarization states other than λ6 are the same.

That is, in FIG. 3, the frequency components FWM ₂₁ , FWM ₂₇ , FWM ₃₃ , and FW of the four-wave mixing noise are shown.
M ₃₉ , FWM ₂₂ , FWM ₂₈ , FWM ₃₄ , and FWM ₄₀ are based on light pulse signals of λ1, λ2, λ5, and λ7, respectively. Further, the frequency component FWM of the four-wave mixing noise
₂₃ , FWM ₂₉ , FWM ₃₅ and FWM ₄₁ are respectively λ1,
This is based on light pulse signals of λ2 and λ5. Also, 4
Frequency components FWM ₂₄ , FWM ₃₀ , FWM of light wave mixing noise
₃₆ , FWM ₂₅ , FWM ₃₁ , and FWM ₃₇ are based on λ3 and λ4 optical pulse signals, respectively. The frequency components FWM ₂₆ , FWM ₃₂ , and FWM ₃₈ of the four-wave mixing noise are based on λ3, λ4, and λ8 light pulse signals, respectively.

The frequency components of the four-wave mixing noise are:
It is generated by two or three optical pulse signals having the same polarization plane, but appears at positions where they do not affect each other. The frequency component of four-wave mixing noise generated by making the polarization plane of one optical pulse signal of two or three optical pulse signals orthogonal to each other is suppressed, and does not affect the optical pulse signal. Therefore, according to the present invention, it is only necessary to consider the arrangement of two optical pulse signals having the same polarization plane, so that the degree of freedom in signal arrangement is expanded and the number of optical multiplexes can be expanded.

As an advantage of the second embodiment of the present invention, as shown in FIG. 3, the polarization state of one wavelength (the polarization state of λ6)
, The influence of four-wave mixing noise can be avoided, so that the polarization control can be simplified.

(Summary of Embodiment) For a signal intensity-modulated by the RZ code, the number of wavelength multiplexing can be made larger than "7" by using the control of the ON timing and the control of the polarization state.

The optical receivers 10-1 to 10-8 shown in FIG.
Can be realized by using an existing configuration that receives an optical signal without depending on the polarization plane.

The optical signal transmitting device shown in the first and second embodiments of the present invention is provided on the optical fiber transmission line 7 for transmitting the output optical signal of the optical signal device, and provided on the optical fiber transmission line 7. Optical amplifiers 6 and 8, an optical wavelength separating section 9 for separating the output optical signal of the optical fiber transmission line 7 into wavelengths λ1, λ2,..., Λ8, and provided for each of the wavelengths λ1, λ2,. It is configured as an optical signal transmission system including the optical receivers 10-1 to 10-8.

[0045]

As described above, according to the present invention,
The number of optical wavelength multiplexes can be expanded. This makes it possible to realize an optical signal transmission device that can effectively use the optical transmission path. Furthermore, an optical signal transmission system that effectively uses an optical transmission line and has no signal error can be realized.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a controlled polarization state and a controlled pulse position according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a controlled polarization state and a controlled pulse position according to a second embodiment of the present invention.

FIG. 4 is a diagram for explaining a conventional wavelength division multiplexing transmission system.

FIG. 5 is a diagram showing an optimum arrangement for the number of multiplexed wavelengths in a conventional wavelength multiplex transmission system.

[Explanation of symbols]

 1-1 to 1-8 Light source 2-1 to 2-8 Intensity modulation circuit 3-1 to 3-8 Polarization state control unit 4-1 to 4-8 Pulse position control unit 5 Optical wavelength multiplexing unit 6, 8 light Amplifier 7 Optical fiber transmission line 9 Optical wavelength separation unit 10-1 to 10-8 Optical receiver 11-1 to 11-8 Optical transmitter

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/135 10/13 10/12 10/28 10/26 10/04 10/06

Claims (5)

[Claims] A plurality of mutually different wavelengths (λ1, λ2,
.., Λn) and a means (5) for wavelength-multiplexing the output optical pulse signals of the optical transmitters. In the signal transmission device, polarization state control means (3-1 to 3-n) for respectively changing the polarization states of output optical pulse signals of the plurality of optical transmitters.
The polarization state control means includes means for controlling the polarization planes of optical pulse signals of at least two wavelengths among optical pulse signals that cause four-wave mixing noise so that their polarization planes are orthogonal to each other. An optical signal transmitting apparatus characterized in that: 2. The optical signal transmitting apparatus according to claim 1, further comprising delay means for delaying the phase of the optical pulse signal so as to avoid four-wave mixing noise generated by another optical pulse signal. 3. The optical signal transmitting apparatus according to claim 1, wherein said polarization state control means alternately orthogonalizes the planes of polarization of a plurality of optical pulse signals having different wavelengths. 4. The optical signal transmitting apparatus according to claim 1, wherein a polarization plane of an optical pulse signal of one wavelength that causes four-wave mixing noise is orthogonalized in combination with a plurality of wavelengths among the plurality of wavelengths. 5. The optical signal transmission device according to claim 1, an optical fiber transmission line for transmitting an output optical signal of the optical signal device, and an optical fiber provided in the optical fiber transmission line. An optical signal transmission system comprising: an amplifier; optical wavelength separation means for separating an output optical signal of the optical fiber transmission line into the plurality of wavelengths; and an optical receiver provided for each of the plurality of wavelengths.