JPH10224325A - Optical delay controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical delay system which controls the bit phase in a return-to-zero(RZ) wavelength multiplex transmission system by setting an optical path difference of optical signals so as not to have equal wavelength internals between any two optical signals that are set at the same bit phases. SOLUTION: The optical signals of each wavelength which are outputted from RZ optical transmitters 1a to 1c are multiplexed by an optical multiplexer 2. The optical path difference is set by an optical delay controller 100 between the optical signals of 3-wavelength to be outputted so as not have equal wavelength internals between any two optical signals that are set at the same bit phases. These optical signals are inputted to an optical fiber transmission line 3 after the bit phases are set to the optical signals of each wavelength. The transmitted optical signals are demultiplexed in each wavelength by an optical demultiplexer 4 and then converted into the electric signals by RZ optical receivers 5a to 5c.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical information transmission using an optical fiber.

[0002]

2. Description of the Related Art A wavelength division multiplexing (WDM) system is being studied as a means for ultra-high-speed, large-capacity communication. WDM transmission is a system in which a plurality of optical signals having different wavelengths are modulated using independent information, multiplexed, and transmitted through one optical fiber. Compared to the conventional method of transmitting an optical signal of one wavelength,
Even if the transmission speeds of one wavelength are equal, it is possible to increase the transmission amount by the wavelength.

[0003] Fibers used for trunk line optical fiber transmission are generally divided into two types: dispersion fibers and dispersion shift fibers. In recent years, a dispersion-shifted fiber that is widely used in Japan and overseas is an optical fiber whose zero-dispersion wavelength is shifted to the 1.55 μm band, and the dispersion in the 1.55 μm band is made sufficiently small. Here, dispersion is a phenomenon in which light having different wavelengths is transmitted at different speeds in an optical fiber. The optical spectrum of an optical signal modulated at high speed includes different wavelength components, and these components arrive at the receiver at different times due to the influence of dispersion. As a result, the transmitted optical waveform causes large waveform distortion. If this dispersion-shifted fiber is used for wavelength division multiplexing transmission, the signal light is transmitted over a long distance while the phases of the signal light are matched, so that the generation efficiency of four-wave mixing (FWM), which is a nonlinear optical effect of the optical fiber, becomes extremely high. There's a problem. Crosstalk occurs between FWM light and signals of different wavelengths, which is a major limiting factor for transmission speed and transmission distance. The effect of FWM on wavelength multiplexing is described in, for example, Reference 1: IEEE Journal of Lightwave Technology, Vol.
3, No. 5, May, 1995, pp. 841-949.

FIG. 13 shows F generated when the number of wavelengths is two.
The state of WM light is shown. FWM occurs at a wavelength position indicated by the sum and difference of the wavelengths. When the number of wavelengths is N, the combinations are N · N · (N−1) / 2. FIG.
Reference numeral 3 denotes a case where N = 2, and the FWM light is generated at a position indicated by oblique lines. For example, when transmitting at three wavelengths,
If an optical signal of λ ₃ exists at a wavelength of λ ₂ -λ ₁ , it acts as crosstalk and causes large transmission degradation.

As a countermeasure for suppressing the generation of FWM light, a bit phase arranged RZ (Bit Phase Arranged Retur) is used.
n to zero) has been proposed. For example, Reference 2: Optoelectronics and Communication Conference Technical Digest (Optoelectronic
s and Communication Conference TechnicalDigest),
July 1996, 17B2-4.

FIG. 14 shows the configuration of a three-wavelength RZ wavelength division multiplexing transmission system. RZ optical transmitters 1a to 1c having different wavelengths
(Wavelengths λ _{1 to} λ ₃ ) are arranged. The optical signals of each wavelength are multiplexed by the optical multiplexer 2 and then output to the optical fiber 3. When the optical signals of each wavelength are multiplexed, the bit phase is set to 0 or π. The optimum bit phase arrangement is determined by the number of wavelengths. For example, in the case of three wavelengths, it is 0, 0, π or 0, π, 0 (see Reference 2). Specifically, the wavelength λ
Setting the bit phases of the optical signals _{1 to} λ ₃ to 0, 0, π can be realized by using an electrical delay as shown in FIG. After being transmitted through the optical fiber transmission line 3, the light is demultiplexed for each wavelength by the optical demultiplexer 4 and converted into electric signals by the RZ optical receivers 5a to 5c.

FIG. 15 is a conceptual diagram in which three wavelengths are arranged at equal intervals and the bit phases of the respective signals are set to 0, 0, and π.
The bit phase is the relative position of the bit slot,
π is a 1/2 bit shift. At this time, the wavelength of the FWM light generated by λ ₁ and λ ₂ is λ ₃ , but λ ₃
Since the bit phase differs from that of the optical signal, no crosstalk occurs. In addition, nine types of FWM light are originally generated at _three wavelengths. However, since the bit phases of the wavelengths λ ₁ and λ ₃ or the wavelengths λ ₂ and λ ₃ are different, no FWM light is generated. In this case, the wavelengths λ ₁ and λ ₂ And the generation of FWM light itself is also reduced.

[0008]

However, in order to realize the bit-phase-arranged RZ system, means for controlling the bit phase is required. As shown in FIG. 14, in the method of electrically controlling the bit phase by the RZ optical transmitter of each wavelength, a delay is set for each transmitter, and the number of wavelengths is added or the assignment of each bit phase is changed. There is a problem that becomes difficult. Further, when performing multi-relay transmission, the dispersion of the dispersion-shifted optical fiber affects the bit phase, and transmission degradation occurs. Since the FWM light is considered to be generated near the input of an optical fiber having a high light intensity, the wavelength interval is small, and the generation of the FWM light can be sufficiently suppressed in non-repeat transmission. For example, if the wavelength interval is 1.0 nm and 10 Gb
In it / s transmission system, dispersion shift fiber 50km
When transmitted, the group delay is about 5 ps. Since the bit slot at 10 Gbit / s is 100 ps, the bit phase shift is small. However, when the number of wavelengths increases, the wavelength interval increases, and when multi-relay transmission is performed, the dispersion is accumulated and the bit phase needs to be controlled. In particular, in the case of multiple relays, since the optical intensity of the optical signal is increased by the amplifier, there is a problem that the generation efficiency of the FWM light increases if the bit phase is broken.

An object of the present invention is to provide a practical optical delay system for controlling a bit phase in an RZ wavelength division multiplexing transmission system.

[0010]

SUMMARY OF THE INVENTION The above object includes an optical transmitter, an optical fiber transmission line, and an optical receiver, and uses a plurality of wavelengths to transmit information from the optical transmitter to the optical receiver via the optical fiber transmission line. Or in a return-to-zero (RZ) wavelength division multiplexing optical transmission system for transmitting information from an optical transmitter to an optical receiver via one or more optical repeaters including an optical fiber and an optical amplifier. This is achieved by an optical delay control device characterized in that the optical path difference between optical signals is set so that the wavelength intervals of any two optical signals are not equal to each other between the optical signals set to the phase.

Further, the setting of the optical path difference of the optical signal between the wavelengths can be more effectively achieved by performing the setting immediately after the optical multiplexer in the optical transmitter or inside the optical repeater.

The optical path difference L between the wavelengths is L = NB +
KB / M (bit slot lengths B and N are integers, M ≧ 2, 0
≤ K <M) by setting the optical path difference of the optical signal between the wavelengths.

When the group delay difference D between a plurality of wavelengths due to the group velocity dispersion value of the optical fiber and the bit slot length B are set, the optical path difference L between the wavelengths is L = NB + D (N is an integer). By setting the optical path difference of the optical signal between the wavelengths.

Further, when performing RZ wavelength division multiplexing communication using M = 2, that is, using bit phases 0 and π, the optical path difference L between wavelengths is L = NB + KB / 2, which can be easily realized.

Further, it is achieved by using a Bragg grating on the optical path of the above wavelength.

[0016] Further, this can be achieved by arranging an optical circulator and a plurality of Bragg gratings on the optical path of the above wavelength, connecting the Bragg gratings in cascade, and connecting them at a predetermined insertion interval.

Alternatively, the first, second, and second light paths
A directional coupler having third and fourth terminals and a plurality of Bragg gratings are arranged, and the plurality of Bragg gratings are set apart from the first terminal by a predetermined insertion interval, or the first and second Bragg gratings are arranged. This can be achieved by being installed at a predetermined insertion interval from the terminal.

When the Bragg grating has dispersion compensation characteristics, the object can be achieved more effectively.

Further, the object can be achieved by using an optical fiber grating as the Bragg grating.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

<Embodiment 1> FIG. 1 shows a first embodiment of the present invention, which is an example of relayless transmission. Return-to-zero (RZ) optical transmitters 1a to 1c (wavelengths λ _{1 to} λ ₃ ) are arranged. The RZ signal of this example is intensity-modulated at a duty ratio of about 50%. The optical signals of each wavelength are multiplexed by the optical multiplexer 2,
An optical signal is output. The optical path difference between the optical signals of the three wavelengths is set so that the wavelength intervals of any two optical signals are not equal to each other between the optical signals set to the same bit phase by the optical delay controller 100. For example, 3
In the bit-phase-arranged RZ scheme of wavelength, when transmission wavelengths are arranged at equal intervals, one of the optimum bit phases is 0, 0, π. After the bit phase of the optical signal of each wavelength is set, it is input to the optical fiber transmission line 3. The transmitted optical signal is demultiplexed into each wavelength by the optical demultiplexer 4, and then converted into electric signals by the RZ optical receivers 5a to 5c.

Embodiment 2 FIG. 2 shows a second embodiment of the present invention, which is an example of multi-relay transmission. Return-to-zero (RZ) optical transmitters 1a to 1c (wavelengths λ _{1 to} λ ₃ ) are arranged. The RZ signal of this example is intensity-modulated at a duty ratio of about 50%. The optical signal of each wavelength is transmitted to the optical multiplexer 2
And an optical signal is output. The optical path difference between the optical signals of the three wavelengths is set so that the wavelength intervals of any two optical signals are not equal to each other between the optical signals set to the same bit phase by the optical delay controller 100.
After the bit phase of the optical signal of each wavelength is set, it is input to the optical fiber transmission line 3. The transmitted optical signal is again set by the optical delay control device provided with the optical repeater 101 so that the wavelength intervals of any two optical signals are not equal to each other between the optical signals set to the same bit phase. At this time, there is no need to be the same as the bit phase in the preceding stage. After that, the optical signal is demultiplexed into each wavelength by the optical demultiplexer 4 and then converted into an electric signal by the RZ optical receivers 5a to 5c.

<Embodiment 3> FIG. 3 shows a third embodiment of the present invention, which is an example of relayless transmission. Return-to-zero (RZ) optical transmitters 1a to 1c (wavelengths λ _{1 to} λ ₃ ) are arranged. The RZ signal of this example is intensity-modulated at a duty ratio of about 50%. The optical signals of each wavelength are multiplexed after the optical path difference of the optical signals of each wavelength is set so that the wavelength interval of any two optical signals is not equal to each other between the optical signals set to the same bit phase. Fiber transmission line 3
Is input to The transmitted optical signal is demultiplexed into each wavelength by the optical demultiplexer 4, and then converted into electric signals by the RZ optical receivers 5a to 5c. Further, by combining the first embodiment and the third embodiment, the same effect can be obtained by setting the optical path difference of the optical signal of each wavelength.

Since the generation efficiency of the FWM light increases at the position where the light intensity is large, the bit phase of the FWM light is optimally rearranged immediately before input to the optical fiber, that is, by the optical transmitter or the optical repeater. Generation efficiency can be greatly reduced. By using the optical repeater in this way, multi-relay bit-phase-arranged RZ transmission can be performed.

<Embodiment 4> As a specific method of realizing the above-described embodiment, a method in which each wavelength passes through a different optical path or a method in which each wavelength passes through the same optical path, and a reflector is arranged on the optical path to provide an optical path difference. And so on. In the following embodiments, examples using a Bragg grating will be described more specifically.

FIG. 4 shows a fourth embodiment of the present invention, in which a Bragg grating is used for an optical delay control device. The optical delay control device 100 according to the present embodiment includes an optical circulator 6, Bragg gratings 101, 102, and 103 having reflection wavelengths of λ ₁ , λ ₂ , and λ ₃ , and a non-reflection terminator 8.
Consists of The optical multiplexer 2 outputs an optical signal multiplexed with the same bit phase. The optical signal passes through the optical circulator 6 and is input to the Bragg gratings 101 to 103. The signal light reflected by the grating returns to the optical circulator 6 again and is sent out to the optical fiber transmission line 3. In general, an optical circulator is a type of optical nonreciprocal circuit, and is used to efficiently extract an optical signal reflected by a Bragg grating. The end face on the opposite side of the grating is generally treated by a non-reflection end 8 or the like so as not to generate reflected light.

FIG. 5 shows an example of the reflection characteristics of a Bragg grating. The reflection bands of the respective wavelengths are set so as not to overlap with each other, and an optical path difference occurs because the reflection positions of the respective wavelengths are different.

For example, in a three-phase bit-phase arranged RZ system, when transmission wavelengths are arranged at equal intervals,
One of the optimal bit phases is 0,0, π. Where λ
_In order to set the bit phases of ₁ , λ ₂ and λ ₃ to 0, 0 and π, the phase difference between the optical signals of wavelength λ ₁ and λ ₂ is zero. Assuming that the optical signals of the respective wavelengths have the same bit phase in the multiplexer 2, the phase difference 0 is equal to the Bragg gratings 101 and 1.
02, the optical path difference L calculated from the reflection position of each wavelength
Is set to L = NB (N is an integer).
Here, the bit slot length B is, for example, 10 Gbit / s.
Is 2cm. Of course Bragg grating 101
Instead of and 102, a Bragg grating having a broadband reflection wavelength including λ ₁ and λ ₂ and having a sufficiently small grating length can be realized. In addition, wavelength λ ₁ and wavelength λ ₃
Can be set by setting the optical path difference L calculated from the reflection position of each wavelength in the Bragg gratings 101 and 103 to L = NB + B / 2 (N is an integer). . This gives the wavelength λ ₂
Phase difference of the wavelength lambda ₃ is also set to π and.

FIG. 6 is a conceptual diagram showing the state of the bit phase according to the first embodiment. This shows a state in which the bit phases become 0, 0, and π after three wavelength pulses having the same phase pass through the optical delay control device before the optical delay control device of the present invention. As described above, the optical delay can be realized by one optical delay control device after the optical multiplexer 2 without controlling the bit phase for each transmitter of each wavelength, and the influence of generation of FWM light and crosstalk due to FWM light can be reduced. It can be greatly reduced. Only by adding an optical delay controller to the RZ wavelength division multiplexing communication system, a bit-phase-arranged RZ system can be realized, and generation of FWM light can be reduced at low cost.

Further, in the fourth embodiment, the optical multiplexer 2 outputs pulses of the same phase bit. However, even when the phases of the respective wavelengths are not the same in the multiplexer, the optical path difference considering these may be set. A similar effect can be obtained.

The Bragg grating includes an optical fiber type and an optical waveguide type. The Bragg grating irradiates an ultraviolet ray through a phase mask to generate a minute periodic refractive index change on the order of the wavelength of light therein. Such a grating has the property of reflecting light of a wavelength determined by the grating period, and can be used for reflection-type optical filters, etc.It has a simple structure and is easy to make, and the reflection characteristics can be set freely. Is possible. In addition, a plurality of Bragg gratings can be formed in one optical fiber or an optical waveguide, and a reduction in connection loss and an improvement in Bragg grating interval accuracy can be expected.

<Embodiment 5> FIG. 7 shows a fifth embodiment of the present invention, in which a Bragg grating is used for an optical delay control device. The optical delay control device according to the second embodiment includes an optical directional coupler (optical coupler) 7 and reflection wavelengths of λ ₁ , λ ₂ , λ
Bragg gratings 101, 102 and 10 which are ₃
3, a non-reflection terminator 8. The optical signal input from the optical path 21 is bisected by the optical coupler, and
And output to the Bragg grating. The wavelengths λ ₁ , λ ₂ and λ ₃ are reflected by the Bragg grating, multiplexed by an optical coupler, and output to the optical path 23.
Although the light passes through the optical coupler twice, the power is divided into two while the phase changes ± π / 2 per optical coupler, so that the optical signal is not in principle lost by the optical delay controller. Wavelength λ
_In order to set the phase difference between the optical signal of ₁ and the wavelength λ ₂ to 0, the optical path difference L calculated from the reflection position of each wavelength in the Bragg gratings 101 and 102 is L = NB (N
Is an integer), and the optical path difference L is set to L = NB + B / 2 (N is an integer) to set the phase difference to π.

When a Bragg grating for dispersion compensation, for example, a chirped grating, is used as the Bragg grating in the fourth and fifth embodiments, dispersion compensation is also possible. A chirped grating is a chirped grating that is designed so that the spacing between the gratings gradually changes in the longitudinal direction of the grating, and obtains a certain dispersion characteristic. For example, an example in which a chirped grating is used for dispersion compensation is described in Reference 3:
Proceedings of 21st European
Proceedings of 21st European Conference on O
ptical Communication (ECOC'95)), 1995, pape
r We.B.1.7, pp585-587.

Further, in the present invention, it is possible to change only the sign of the dispersion value by simply setting the input direction of the chirped grating in reverse, regardless of whether the wavelength of each signal is in the normal dispersion region or the abnormal dispersion region. The sign of the dispersion can be freely set for each wavelength.

<Embodiments 6 and 7> FIGS. 8 and 9 show sixth and seventh embodiments of the present invention, in which the optical delay controllers of the fourth and fifth embodiments are connected in a two-stage configuration. is there. Equally spaced wavelength λ ₁
RZ optical transmitter 1a~1c having to [lambda] ₃ are arranged. The optical signals of each wavelength are multiplexed by the optical multiplexer 2, and an optical signal having the same phase is output. The optical signals of three wavelengths are input to the optical fiber transmission line after the bit phase is set to 0 or π by the optical delay control device 100. The transmitted optical signal is demultiplexed into each wavelength by the optical demultiplexer 4, and then converted into electric signals by the RZ optical receivers 5a to 5c.

In the optical delay control device according to the sixth embodiment, the optical circulators 6a and 6b are first and second stages, respectively.
The reflection wavelengths of the Bragg gratings are 101 (wavelength λ ₁ ), 102 (wavelength λ ₂ ), 103 (wavelength λ ₃ ), and 10 (wavelength λ ₃ ).
4 (wavelength λ ₁ ), 105 (wavelength λ ₂ ), 106 (wavelength λ ₃ ),
It comprises a non-reflection terminator 8. For example, an optical signal of wavelength λ ₁ is reflected by the Bragg grating 101 connected to the first-stage optical circulator 6a and then reflected by the Bragg grating 104 connected to the second-stage optical circulator 6b. The optical path difference between them can be doubled. Further, the wavelength of the Bragg grating is the first stage Bragg grating (101 to 1).
03) and the second stage Bragg grating (104-1)
The order of 06) does not need to match, and can be set freely. In addition, the optical delay control device of the present invention can be made as a small delay device by integrating it into an optical waveguide because the optical path difference of each wavelength is determined by the optical path from the input to the output of the optical delay control device. Becomes

The optical delay control device according to the seventh embodiment sets the optical circulator 6c and the reflection wavelength of the Bragg grating to 1
01 (wavelength λ ₁ ), 102 (wavelength λ ₂ ), 103 (wavelength λ ₁ )
₃ ), 104 (wavelength λ ₁ ), 105 (wavelength λ ₂ ), 106
(Wavelength λ ₃ ); For example,
The optical signal having the wavelength λ ₁ is reflected by the Bragg grating 101 connected to the optical circulator 6c, and is again transmitted through the optical circulator 6c.
Thus, the optical path difference between the wavelengths can be doubled, and the same effect as in the sixth embodiment can be obtained.

Also in the sixth and seventh embodiments, when a chirped fiber grating is used for the Bragg grating, dispersion compensation becomes possible. In the case of this configuration, if the Bragg gratings 101 (wavelength λ ₁ ) and 104 (wavelength λ ₁ ) have a dispersion compensation amount of 500 ps / nm, the total dispersion compensation amount is 1000 ps / nm, and equivalently, the dispersion compensation amount is There is a doubled effect. It is also possible to connect Bragg gratings having different dispersion amounts and dispersion signs in multiple stages for the purpose of adjusting the dispersion compensation amount. In this way, by performing multi-stage connection such as the combination of the fourth and fifth embodiments, various bit phases and dispersion compensation can be performed at each wavelength.

<Eighth Embodiment> FIG. 10 shows an eighth embodiment of the present invention.
Is an example in which. The purpose is to rearrange the bit phase collapsed by the dispersion of the optical fiber to an optimal state. In this embodiment, an optical fiber 3, an optical circulator 6, an optical amplifier 9, a Bragg grating 101 (wavelength λ ₁ ), 102 (wavelength λ ₂ ), 103 (wavelength λ ₃ ) and a non-reflection terminator 8 are provided. When the group delay difference D between the wavelengths of the fiber 3 and the bit slot length B are set, the Bragg grating is installed at a position where the optical path difference L of each wavelength is L = NB + L (N is an integer). The transmitted optical signal is transmitted from the optical fiber transmission line via an optical circulator,
It is input to the Bragg gratings 101-103.
The optical signal of each wavelength is reflected by the Bragg grating, amplified by the optical amplifier 9 via the optical circulator, and output from the optical repeater 110. At this time, the same effect can be expected even if the optical amplifier 9 is in front of the optical delay control device 100.

FIGS. 11 and 12 are conceptual diagrams for explaining the eighth embodiment. The reference for the bit phase is the wavelength λ ₂ . FIG.
In, before transmission, the bit phases of the wavelengths λ ₁ , λ ₂ , λ ₃ are set to 0, 0, π. After transmission through the optical fiber transmission line, group delay occurs between each pulse due to dispersion of the optical fiber, the bit phase becomes nearly random, and the generation efficiency of FWM light increases. By using the optical delay control device of the present invention, it is possible to set the bit phase to the optimum phase 0, 0, π again as shown in FIG. By optimally rearranging the bit phase inside the optical repeater, the generation efficiency of the FWM light can be greatly reduced. If such an optical repeater is used, multi-relay bit-phase arranged RZ transmission can be realized. Further, the same effect can be obtained by disposing the optical delay control device immediately after the optical amplifier. The optical delay control device can be multi-staged or used in combination with a dispersion compensation Bragg grating.

[0040]

According to the optical delay control device of the present invention, in the RZ wavelength division multiplexing transmission, the optical path of each optical signal is set so that the wavelength intervals of any two optical signals are not equal to each other between the optical signals set to the same bit phase. Since the difference is set, there is an effect that deterioration due to FWM light can be significantly reduced.

The setting of the optical path difference of the optical signal between the wavelengths by the optical delay controller is performed immediately after the optical multiplexer in the optical transmitter or inside the optical repeater, so that the FW is more efficiently performed.
There is an effect that M can be reduced.

Immediately after the optical multiplexer in the transmitter, the optical path difference L is located at a position where L = NB + KB / M (bit slot lengths B and N are integers, M ≧ 2, 0 ≦ K <M). By arranging the light reflector, the initial bit phase of the pulse can be controlled. As a result, it is possible to control all wavelengths at the same time, and there is an effect that expansion such as addition of the number of wavelengths and change of assignment of each bit phase can be easily performed.

In the optical repeater, when the group delay difference D between a plurality of wavelengths and the bit slot length B due to the group velocity dispersion value of the optical fiber connected between the delay devices, the optical path difference L becomes By arranging the optical reflector at a position where L = NB + D (N is an integer), it is possible to correct a bit phase shift due to dispersion of an optical fiber or to rearrange bits. As a result, the generation efficiency of the FWM light in the transmission line immediately after the optical relay can be significantly reduced,
There is an effect that multi-relay transmission is also possible. As one of the simplest examples, there is a method of arranging reflectors in order of wavelength having a large group delay caused by an optical fiber.

Further, when performing RZ wavelength division multiplexing communication using M = 2, that is, using bit phases 0 and π, the optical path difference L between wavelengths is L = NB + KB / 2, which has the effect that the realization becomes easy.

When a Bragg grating is used as the light reflector, there is an effect that setting of reflection characteristics and the like becomes easy.

Further, since the optical circulator and the Bragg fiber grating are used in the optical delay control device, there is no loss in principle, and there is an effect that the configuration can be made relatively easily. Also, there is an effect that a configuration using an optical coupler and a Bragg grating can be used in the optical delay control device.

When a Bragg grating for dispersion compensation is used for the Bragg grating, dispersion compensation is also possible, and there is an effect that transmission quality is improved.

Further, by using an optical fiber grating as the Bragg grating, the object can be effectively achieved.

Further, by configuring the optical delay control device of the present invention in multiple stages, it is possible to freely set the optical path difference of the wavelength and the amount of dispersion compensation, so that there is an effect that the transmission design becomes flexible.

[Brief description of the drawings]

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

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 5 is a diagram illustrating a reflection band of a Bragg grating according to a fourth embodiment of the present invention.

FIG. 6 is a diagram illustrating a bit phase of an optical signal according to a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 9 is a configuration diagram showing a seventh embodiment of the present invention.

FIG. 10 is a configuration diagram showing an eighth embodiment of the present invention.

FIG. 11 is a diagram illustrating a bit phase of an optical signal according to an eighth embodiment of the present invention (part 1).

FIG. 12 is a diagram illustrating a bit phase of an optical signal according to an eighth embodiment of the present invention (part 2).

FIG. 13 is a spectrum diagram showing generation of FWM light.

FIG. 14 is a configuration diagram showing a conventional bit phase arranged RZ system.

FIG. 15 is a configuration diagram showing a bit phase of an optical signal in a bit phase arranged RZ system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... RZ optical transmitter, 2 ... optical demultiplexer, 3 ... optical fiber, 4
... optical demultiplexer, 5 ... RZ optical receiver, 6 ... optical circulator, 7 ... optical coupler, 8 ... non-reflection terminator, 9 ... optical amplifier,
10: optical path delay, 11: electrical delay, 12: clock, 2
1 to 24: optical path, 100: optical delay control device, 110: optical repeater, 101 to 106: Bragg grating.

Claims (10)

[Claims] An optical transmitter, an optical fiber transmission line, and an optical receiver, for transmitting information from the optical transmitter to the optical receiver via the optical fiber transmission line using a plurality of wavelengths; A return-to-zero (RZ) wavelength division multiplexing optical transmission system for transmitting information from the optical transmitter to the optical receiver via one or more optical repeaters including the optical fiber and the optical amplifier; Wherein the optical path difference of the optical signals is set such that the wavelength intervals of any two optical signals do not become equal to each other between the optical signals set in (1). 2. The optical delay control device according to claim 1, wherein the setting of the optical path difference of the optical signal between the wavelengths is performed immediately after the optical multiplexer in the optical transmitter or inside the optical repeater. An optical delay control device characterized by the following. 3. The optical delay control device according to claim 1, wherein
The optical path difference of the optical signal between the wavelengths is set such that the optical path difference L between the wavelengths is L = NB + KB / M (bit slot lengths B and N are integers, M ≧ 2, 0 ≦ K <M). An optical delay control device characterized by the following. 4. The optical delay control device according to claim 1, wherein
When the group delay difference D between the plurality of wavelengths due to the group velocity dispersion value of the optical fiber and the bit slot length B are set, the wavelengths are set so that the optical path difference L between the wavelengths is L = NB + D (N is an integer). An optical delay control device, wherein an optical path difference between optical signals is set. 5. The optical delay control device according to claim 3, wherein M =
2. An optical delay control device, wherein 6. The optical delay control device according to claim 1, wherein an optical path difference of the optical signal between the wavelengths is set on a light path of the wavelength by using a Bragg grating. Optical delay control device. 7. The optical delay control device according to claim 1, wherein an optical circulator and a plurality of Bragg gratings are arranged on an optical path of the wavelength, and the Bragg gratings are cascaded, An optical delay control device which is connected at a predetermined insertion interval. 8. The directional coupler according to claim 1, wherein said directional coupler has first, second, third and fourth terminals on an optical path of said wavelength. A plurality of Bragg gratings are arranged, and the plurality of Bragg gratings are installed at a predetermined insertion interval from the first terminal, or are installed at a predetermined insertion interval from the first and second terminals. An optical delay control device characterized by the above. 9. An optical delay control device according to claim 6, wherein said Bragg grating has a dispersion compensation characteristic. 10. The optical delay control device according to claim 6, wherein an optical fiber grating is used as said Bragg grating.