JP5077319B2 - Optical amplification transmission system - Google Patents

Description

Conventionally, in a long-distance optical transmission system, an optical signal is converted into an electric signal, and transmission is performed using an optical regenerative repeater that performs retiming, reshaping, and regenerating.

 At present, however, optical amplifiers have been put into practical use, and optical amplification repeater transmission systems using optical amplifiers as linear repeaters are being studied.

 By replacing the optical regenerative repeater with an optical amplifying repeater, the number of parts in the repeater can be greatly reduced, ensuring reliability and a significant cost reduction.

 In addition, as one of the methods for realizing a large capacity of an optical transmission system, a wavelength division multiplexing (WDM) optical transmission system that multiplexes and transmits optical signals having two or more different wavelengths on one transmission line has been attracting attention. Yes.

 In the WDM optical amplifying and repeating transmission system, which combines the WDM optical transmission system and the optical amplifying and repeating transmission system, it is possible to amplify optical signals having two or more different wavelengths by using an optical amplifier. With a configuration (economical), large capacity and long distance transmission can be realized.

A configuration example of a wavelength division multiplexing optical amplifying and repeating transmission system is shown in FIG.
For example, the WDM transmission optical system in FIG. 1 includes an optical transmission station (OS) 1, an optical reception station (OR) 2, an optical transmission line 3 connecting these transmission and reception stations, and a required part of the optical transmission line 3. The repeater 4 is composed of a plurality of optical amplifiers arranged at intervals.

 The optical transmission station 1 includes a plurality of optical transmitters (E / O) 1A that respectively output a plurality of optical signals having different wavelengths, a multiplexer 1B that wavelength-multiplexes the plurality of optical signals, and the multiplexer 1B. A post-amplifier 1C that amplifies the WDM signal light to a required level and outputs the amplified light to the optical transmission line 3 is provided.

 The optical receiver station 2 amplifies the WDM signal light of each wavelength band transmitted through the optical transmission path to a required level, and divides the output light from the preamplifier 2C into a plurality of optical signals according to the wavelength. And a plurality of optical receivers (O / E) 2A for receiving and processing a plurality of optical signals.

 The optical transmission path 3 has a plurality of relay sections connecting the optical transmission station 1 and the optical reception station 2 respectively.

 The wavelength multiplexed signal light transmitted from the optical transmission station 1 propagates through the optical transmission line 3, is optically amplified by the optical repeater 4 composed of an optical amplifier arranged at a predetermined interval in the middle, and again passes through the optical transmission line 3. Propagated and repeated to be transmitted to the optical receiving station.

 An erbium doped fiber amplifier (EDFA) is generally used as an optical amplifying repeater used in this wavelength division multiplexing transmission system.

 Recently, however, the combined use of Raman amplification has been actively studied.

 There are also repeaterless transmission systems that do not use optical repeaters.

 For repeaterless transmission systems, EDFA and distributed Raman amplification by remote-pumping remote amplification are being studied.

 In Raman amplification using an optical fiber, gain is obtained in inverse proportion to the mode field diameter of the fiber.

 Therefore, an optical fiber having a small mode field diameter is suitable for Raman amplification.

For example, 1.3 .mu.m zero dispersion fiber: negative dispersion fiber (Negative Dispersion having a wavelength dispersion and dispersion slope of opposite sign of the chromatic dispersion and dispersion slope (first derivative with respect to wavelength of the chromatic dispersion) of (Single mode fiber SMF) fiber: -D fiber) has a mode field diameter of about 5 μm and is smaller than the NZ-DSF mode field diameter of about 8 μm , so a larger Raman gain can be obtained ("Highly efficient distributed Raman amplification system in a zero-dispersion-flattened transmission"). line ", H. Kawakami et al., ThB5, OAA'99, 1999.).

In the realization of a large-capacity long-distance transmission system, the shortage of optical signal-to-noise ratio (OSNR) per wave and transmission waveform distortion due to nonlinear effects are problems.

 The OSNR per wave can be improved by applying the distributed gain Raman amplifier.

 On the other hand, transmission waveform distortion due to nonlinear effects is a problem.

 Therefore, it is important to fully consider the nonlinear effect of the optical fiber used for Raman amplification.

 When a Raman amplifier is applied, wavelength multiplexed signal light can be optically amplified over a wide band.

 On the other hand, it is important to fully consider the chromatic dispersion of the optical fiber used in the Raman amplifier.

 Furthermore, downsizing of the Raman amplifier is important.

In particular, when a Raman amplifier is applied to an optical transmission terminal station and an optical reception terminal station, it is necessary to shorten the length of the optical fiber used for the purpose and to reduce the size of the optical fiber module.

In the optical amplification transmission system according to the present invention, the transmission path for transmitting the signal light includes a first Raman amplification medium that performs distributed Raman amplification having positive dispersion with respect to the signal light wavelength, and the signal light wavelength. A second Raman amplifying medium having negative dispersion and a distributed Raman amplification having a small mode field diameter relative to the first Raman amplifying medium; and a concentrated Raman having a smaller mode field diameter than that of the second Raman amplifying medium. A third Raman amplification medium that performs amplification;
The signal light propagation direction is propagated in the order of the first, second and third Raman amplification media,
An optical amplification transmission system, wherein pump light is input from the signal light output side of the third Raman amplification medium, and Raman amplification is performed on the transmission line.

 With the configuration of the present invention, the signal light power to noise light power ratio of the signal light can be improved, transmission waveform distortion due to the fiber nonlinear effect can be reduced, and the concentrated Raman amplifier can be downsized.

Example of wavelength multiplexing transmission system configuration Transmission system configuration example 1 Relationship between signal light power and length when the transmission loss of the second fiber 3b is matched with the distributed Raman amplification gain Transmission system configuration example 2 Configuration example of optical terminal equipment Configuration example of optical amplifier Configuration example of optical amplifier Examples of supervisory control signals Example of optical loopback Example of optical loopback Example of optical loopback Relationship between signal light power and length when the transmission loss of the second fiber 3b is matched with the distributed Raman amplification gain Transmission system configuration example 2

A large Raman gain is obtained by a concentrated gain type Raman amplifier having a relatively small nonlinear effective area and an increased optical power density.

Nonlinear effective cross section of a conventional optical fiber the value of the non-linear effective cross section of the centralized gain type Raman amplifier with respect to 50 to 80 [mu] m ² is, for example, 10 [mu] m ^2.

 In order to avoid waveform distortion due to nonlinear effects such as four-wave mixing, the absolute value of chromatic dispersion per unit length is preferably a non-zero value.

 The value is, for example, 1 to 10 ps / nm / km, and depends on the signal light wavelength interval.

 Here, when the chromatic dispersion per length is large, the problem is that the accumulated chromatic dispersion (ps / nm) becomes large.

 In order to ensure a wide signal light wavelength bandwidth, the dispersion slope is preferably zero.

The primary Raman gain peak optical frequency is 13.2 TH _Z smaller than the pumping light frequency, and an effective wavelength band is 100 nm or less because a wavelength shift of approximately 100 nm occurs in the 1.55 μm band.

 It is effective to apply a Raman amplifier to the optical terminal device.

 The absolute value of the accumulated chromatic dispersion (ps / nm) of the optical fiber that generates the Raman amplification is preferably small.

 It is also desirable to compensate for chromatic dispersion in the transmission line.

 Alternatively, the absolute value of cumulative chromatic dispersion can be reduced by using in series optical fibers having different wavelength dispersions as optical fibers used in the Raman amplifier.

 For example, the 3-1 fiber has positive dispersion and the 3-2 fiber has negative dispersion.

 There is a configuration in which two optical amplifiers are directly connected, and there is also a configuration in which an optical component is inserted between the two to compensate for the loss of the optical component.

 In particular, application to an optical terminal device is effective.

 Simultaneously realizes improvement of OSNR per wave and mitigation of transmission waveform distortion due to nonlinear effects, enabling construction of large-capacity long-distance wavelength division multiplexing transmission systems.

A first embodiment is shown in FIGS.

 2 and 13 show one relay section of a transmission system using the first fiber 3a, the second fiber 3b, and the third fiber 3c as the first embodiment and using Raman amplification as the optical amplification repeater.

 This optical amplification repeater may be configured by only Raman amplification as shown in FIG. 2, or may be used in combination with other optical amplifiers such as Raman amplification and EDFA 4c as shown in FIG.

 When used together with the EDFA 4c, the EDFA 4c is provided after the wavelength synthesizer 4a.

 Table 1 shows main fiber characteristics of the first fiber 3a, the second fiber 3b, and the third fiber 3c.

 The third fiber 3c in FIGS. 2 and 13 may be provided in the repeater 4 as shown in the figure, or may be provided outside the repeater and in a gable on the transmission line.

 The wavelength multiplexed signal light having two or more different wavelengths emitted from the terminal station or the preceding optical amplification repeater has positive chromatic dispersion and dispersion slope with respect to the signal light wavelength, and its mode field diameter. Is transmitted through the first fiber 3a having a relatively large mode field diameter, and then transmitted through the second fiber 3b having a negative chromatic dispersion and a dispersion slope with respect to the signal light wavelength and a relatively small mode field diameter. Then, after transmitting the third fiber 3c having the shortest length and the smallest mode field diameter among these three types of fibers, a wavelength synthesizer 4a that combines the signal light and the excitation light is provided. As shown, the light is incident on the next-stage transmission line fiber, for example, the first-stage first fiber 3a.

 The excitation light applied to Raman amplification emitted from the excitation light source 4b is incident on the third fiber 3c via the wavelength synthesizer 4a that combines the signal light and the excitation light, and then the third fiber 3c. Then, after propagating through the second fiber 3b, the light enters the first fiber 3a.

 Here, the excitation light source 4b may be a single wave, or may be light that combines multiple wavelengths.

 In addition to correcting the dispersion value of the first fiber by the second fiber 3b alone or in combination with the third fiber, the loss generated in the second fiber is compensated by a gain due to Raman amplification generated in the second fiber.

 Further, amplification is performed so as to compensate for the loss of the entire transmission line by the gain of Raman amplification generated in the third fiber.

 With this configuration, it is possible to reduce distortion due to cross-phase modulation due to a nonlinear effect generated in the second fiber that compensates for the dispersion value of the first fiber.

 The third fiber with a small nonlinear effective cross-sectional area and a short length compensates for the loss of the entire transmission line, thereby improving the OSNR of the signal light and reducing the transmission waveform distortion due to the fiber nonlinear effect. Can be relaxed.

 Here, the third fiber has a small non-linear effective cross-sectional area and a large nonlinear effect. However, since the third fiber is substantially short, it does not cause a problem because it is difficult for the wavelength-multiplexed signal light to pass between the third fibers.

The signal light is lost in the first fiber 3a, the loss received in the first fiber in the second fiber 3b is matched with the Raman amplification gain, the change in the signal light power is reduced, and the signal light is transmitted in the third fiber 3c. Receives Raman amplification gain.
Here, FIG. 3 shows the relationship between the signal light power and the length when the transmission loss of the second fiber 3b and the distributed Raman amplification gain are matched.

 If the transmission loss of the second fiber 3b is matched with the distributed Raman amplification gain, the signal light power is hardly attenuated in the longitudinal direction.

 Therefore, when the signal light approaches or separates from signal light of other wavelengths due to the chromatic dispersion of the fiber, the amount of change in the signal light power is small, so that it deviates from the amount of frequency shift due to cross-phase modulation that occurs when approaching. The difference in the amount of frequency shift due to cross phase modulation that occurs in some cases is reduced.

 The frequency shift amount due to cross-phase modulation that occurs when approaching and the sign of frequency shift amount due to cross-phase modulation that occurs when leaving are opposite signs and cancel each other out, so the frequency shift amount due to accumulated cross-phase modulation is small, Transmission waveform distortion can be reduced.

 Since the third fiber 3c has a small core diameter, a high gain can be obtained even with a short length.

 Considering the repair, the third optical fiber has a small core diameter and is difficult to repair, so it is desirable to place it in the optical repeater.

 Furthermore, the third fiber 3c has negative dispersion with respect to the signal light wavelength, and compensating the positive dispersion amount of the first fiber 3a can reduce the dispersion compensation amount required for the second fiber 3b. This is desirable for the design of the second fiber 3b.

 Then, the first fiber 3a has a positive dispersion slope with respect to the signal light wavelength, the second fiber 3b has a negative dispersion slope with respect to the signal light wavelength, and the signal in the third fiber 3c. By having a negative dispersion slope with respect to the optical wavelength, the absolute value of the dispersion slope of the transmission line can be reduced.

 As a second embodiment, FIG. 4 shows one relay section of a transmission system using the first fiber 3a and the second fiber 3b and using Raman amplification as the optical amplification repeater.

 This optical amplifying repeater may be configured only by Raman amplification, or may be combined with other optical amplifiers such as Raman amplification and EDFA 4c.

 The wavelength multiplexed signal light having two or more different wavelengths emitted from the terminal station or the preceding optical amplification repeater has positive chromatic dispersion and dispersion slope with respect to the signal light wavelength, and its mode field diameter. Is transmitted through the first fiber 3a having a relatively large mode field diameter, and then transmitted through the second fiber 3b having a negative chromatic dispersion and dispersion slope with respect to the signal light wavelength and a relatively small mode field diameter. After being amplified to the required signal light power, it passes through the wavelength synthesizer 4a that combines the signal light and the excitation light.

 The second optical fiber 3b is a fiber having a dispersion value for compensating for the dispersion of the first fiber.

 Thereafter, the light is incident on an optical amplifier having a gain that compensates for the transmission loss of the first fiber 3a connected after the wavelength synthesizer 4a.

 The optical amplifier is, for example, an erbium-doped fiber amplifier 4c.

 The light enters the next-stage transmission line fiber, for example, the first-stage first fiber 3a.

 The excitation light applied to the Raman amplification emitted from the excitation light source 4b is incident on the second fiber 3b through the wavelength synthesizer 4a that combines the signal light and the excitation light, and propagates through the second fiber 3b. After that, the light enters the first fiber 3a.

 Here, the excitation light source 4b may be a single wave, or may be light that combines multiple wavelengths.

 By using a Raman amplifier characterized in that the pumping light propagates in the direction opposite to the propagation direction of the signal light in the optical fiber, the OSNR of the signal light is improved and the transmission waveform distortion due to the fiber nonlinear effect is reduced. Can be relaxed.

 Here, FIG. 12 shows the relationship between the signal light power and the length when the transmission loss of the second fiber 3b is matched with the distributed Raman amplification gain.

 If the transmission loss of the second fiber 3b is matched with the distributed Raman amplification gain, the signal light power is hardly attenuated in the longitudinal direction.

 Therefore, when the signal light approaches or separates from signal light of other wavelengths due to the chromatic dispersion of the fiber, the amount of change in the signal light power is small, so that it deviates from the amount of frequency shift due to cross-phase modulation that occurs when approaching. The difference in the amount of frequency shift due to cross phase modulation that occurs in some cases is reduced.

 The frequency shift amount due to cross-phase modulation that occurs when approaching and the sign of frequency shift amount due to cross-phase modulation that occurs when leaving are opposite signs and cancel each other out, so the frequency shift amount due to accumulated cross-phase modulation is small, Transmission waveform distortion can be reduced.

 The loss of signal light is prevented by providing a gain corresponding to the loss in the entire transmission path by the EDFA 4c.

 The action of the two fibers is the same as in the first embodiment.

As a fourth embodiment, FIG. 5 shows a configuration example in which a concentrated Raman amplifier is used in an optical terminal device.
Here, an example of an optical transmission terminal station will be described.

 The signal light emitted from multiple optical transmitters (E / O) 1A that output multiple optical signals of different wavelengths propagates through the 3-1 fiber 3d whose mode field diameter is relatively small and is required. Next, after passing through the wavelength synthesizer 4a1 that combines the signal light and the excitation light, the signal light power is adjusted by the variable attenuator 5b through the optical isolator 5a. After that, it enters a multiplexer 1B that multiplexes a plurality of optical signals.

 The WDM signal light from the multiplexer 1B is amplified to the required signal light power through the 3-2 fiber 3e having a relatively small mode field diameter, and then passes through the wavelength synthesizer 4a2. After being amplified to the signal light power, the signal light power is adjusted by the variable attenuator 5b through the optical isolator 5a and propagated through the dispersion compensator 5c.

 After that, the signal light emitted from the dispersion compensator 5c propagates through the 3-1 fiber 3d having a relatively small mode field diameter arranged in the next stage and is amplified to the required signal light power. Then, after passing through the wavelength synthesizer 4a1 that combines the signal light and the excitation light, the signal light power is adjusted by the variable attenuator 5b through the optical isolator 5a, and the dispersion compensator 5c is adjusted. Propagate.

 Then, the signal light emitted from the dispersion compensator 5c propagated through the 3-2 fiber 3e having a relatively small mode field diameter arranged in the next stage and was amplified to the required signal light power. Then, after passing through the wavelength synthesizer 4a2 that combines the signal light and the excitation light, the light is incident on the transmission line through the optical isolator 5a.

Here, the chromatic dispersion per unit length of the 3-1 fiber 3d is positive with respect to those signal light wavelengths, and the chromatic dispersion per unit length of the 3-2 fiber 3e is set to these signal light wavelengths. On the other hand, by making it negative, the accumulated amount of chromatic dispersion can be reduced, and transmission waveform distortion due to chromatic dispersion can be reduced.
Here, the sign of the chromatic dispersion per unit length of the 3-1 fiber 3d and the 3-2 fiber 3e is not limited to this, and the sign may be reversed.

 As a fourth embodiment, a configuration example of a concentrated Raman amplifier is shown in FIG.

 Wavelength multiplexed signal light having two or more different wavelengths emitted from the preceding optical amplifier or fiber propagates through the 3-1 fiber 3d having a relatively small mode field diameter and is amplified to the required signal light power. Then, the signal light and the pump light are passed through the wavelength synthesizer 4a1 and transmitted through the 3-2 fiber 3e having a relatively small mode field diameter to the required signal light power. After amplification, the light passes through the wavelength synthesizer 4a2 and enters the optical amplifier or fiber in the next stage.

The excitation light applied to the Raman amplification emitted from the excitation light source 4b1 is incident on the 3-1 fiber 3d via the wavelength synthesizer 4a1 that combines the signal light and the excitation light, and other excitation light The excitation light applied to the Raman amplification emitted from the light source 4b2 is incident on the third-second fiber 3c via the wavelength synthesizer 4a2 that combines the signal light and the excitation light.
Here, the excitation light sources 4b1 and 4b2 may be one wave, or may be light that combines multiple wavelengths.

 By using a Raman amplifier characterized in that the pumping light propagates in the direction opposite to the propagation direction of the signal light in the optical fiber, the OSNR of the signal light is improved and the transmission waveform distortion due to the fiber nonlinear effect is reduced. Can be relaxed.

 In FIG. 6, the wavelength synthesizer 4a1 and the excitation light source 4b1 may be removed from this configuration.

 The 3-1 fiber 3d and the 3-2 fiber 3e do not serve as a transmission line, are inherent in the optical amplifier, and chromatic dispersion per unit length is not zero with respect to their signal light wavelengths. The transmission waveform distortion due to the fiber nonlinear effect can be alleviated.

 Further, by making the dispersion slopes of the 3-1 fiber 3d and the 3-2 fiber 3e substantially zero with respect to these signal light wavelengths, transmission waveform distortion due to the chromatic dispersion can be reduced.

 In particular, a fiber having a zero dispersion slope is called a dispersion flat fiber.

 An example of the refractive index distribution that composes the fiber is shown in "Dispersion flat fiber with W-type refractive index distribution", Akasaka et al., 1998 IEICE General Conference.

 Furthermore, the mode field diameters of the 3-1 fiber 3d and the 3-2 fiber 3e for these signal light wavelengths are relatively smaller than the mode field diameters of the optical fibers used in the transmission line. The length of the -1 fiber 3d and the 3-2 fiber 3e is sufficiently smaller than the relay interval of the transmission line. By reducing the absolute value of the chromatic dispersion, the fiber nonlinear effect and wavelength Transmission waveform distortion due to dispersion can be reduced.

 Then, the chromatic dispersion per unit length of the 3-1 fiber 3d is positive with respect to those signal light wavelengths, and the chromatic dispersion per unit length of the 3-2 fiber 3e is with respect to those signal light wavelengths. Therefore, the cumulative amount of chromatic dispersion can be reduced.

 Here, the sign of the chromatic dispersion per unit length of the 3-1 fiber 3d and the 3-2 fiber 3e is not limited to this, and the sign may be reversed.

 As a fifth embodiment, FIG. 7 shows a configuration example of a concentrated Raman amplifier.

 Wavelength multiplexed signal light having two or more different wavelengths emitted from the preceding optical amplifier or fiber propagates through the 3-1 fiber 3d having a relatively small mode field diameter and is amplified to the required signal light power. After that, the signal light and the excitation light are then emitted through a wavelength synthesizer 4a1.

 The emitted signal light passes through the optical component 5, for example, an optical filter, and then is transmitted to the 3-2 fiber 3e having a relatively small mode field diameter and amplified to the required signal light power. The light passes through the wavelength synthesizer 4a2 and enters the optical amplifier or fiber in the next stage.

 The excitation light applied to the Raman amplification emitted from the excitation light source 4b1 is incident on the 3-1 fiber 3d via the wavelength synthesizer 4a1 that combines the signal light and the excitation light, and other excitation light The excitation light applied to the Raman amplification emitted from the light source 4b2 is incident on the third-second fiber 3c via the wavelength synthesizer 4a2 that combines the signal light and the excitation light.

 Here, the excitation light sources 4b1 and 4b2 may be one wave, or may be light that combines multiple wavelengths.

 By using a Raman amplifier characterized in that the pumping light propagates in the direction opposite to the propagation direction of the signal light in the optical fiber, the OSNR of the signal light is improved and the transmission waveform distortion due to the fiber nonlinear effect is reduced. Can be relaxed.

 The 3-1 fiber 3d and the 3-2 fiber 3e do not serve as a transmission line, are inherent in the optical amplifier, and chromatic dispersion per unit length is not zero with respect to their signal light wavelengths. The transmission waveform distortion due to the fiber nonlinear effect can be alleviated.

 Further, by making the dispersion slopes of the 3-1 fiber 3d and the 3-2 fiber 3e substantially zero with respect to these signal light wavelengths, transmission waveform distortion due to the chromatic dispersion can be reduced.

 Furthermore, the mode field diameters of the 3-1 fiber 3d and the 3-2 fiber 3e for these signal light wavelengths are relatively smaller than the mode field diameters of the optical fibers used in the transmission line. The length of the -1 fiber 3d and the 3-2 fiber 3e is sufficiently smaller than the relay interval of the transmission line. By reducing the absolute value of the chromatic dispersion, the fiber nonlinear effect and wavelength Transmission waveform distortion due to dispersion can be reduced.

Then, the chromatic dispersion per unit length of the 3-1 fiber 3d is positive with respect to those signal light wavelengths, and the chromatic dispersion per unit length of the 3-2 fiber 3e is with respect to those signal light wavelengths. Therefore, the cumulative amount of chromatic dispersion can be reduced.
Here, the sign of the chromatic dispersion per unit length of the 3-1 fiber 3d and the 3-2 fiber 3e is not limited to this, and the sign may be reversed.

FIG. 8 shows an example of the monitor control signal as the sixth example.
In the case of the conventional EDFA, the response signal can be optically modulated by modulating the pump light and changing the EDFA gain.

 Here, the response signal can be optically modulated using the Raman effect in the optical fiber that is the transmission path by modulating the pumping light.

 Wavelength multiplexed signal light having two or more different wavelengths emitted from the optical amplifier or fiber of the previous stage propagates through the transmission line fiber 3 and is amplified to the required signal light power. The light passes through the wavelength synthesizer 4a that combines the pumping light, passes through the optical coupler 4d that supplies the monitor signal to the light receiving element 4c, and is propagated to the transmission line fiber 3 in the next stage.

 The pumping light applied to the Raman amplification emitted from the pumping light source 4b is incident on the transmission line fiber 3 via the wavelength synthesizer 4a that combines the signal light and the pumping light.

 Here, the excitation light source 4b may be a single wave, or may be light that combines multiple wavelengths.

 Further, the monitor signal is demultiplexed by the optical coupler 4d, then received by the light receiving element 4c, and converted into an electric signal.

 The converted electrical signal is processed by the control circuit 4e.

 Thereafter, the control circuit 4e supplies a control signal to the excitation light source 4b.

 Here, the flow of the signal light, the monitor signal, and the excitation light is the same in the uplink and the downlink.

 By following the above configuration, a response signal can be sent to the uplink and downlink by the control circuit.

 As a seventh embodiment, an optical loopback embodiment is shown in FIG.

 First, the case of the uplink will be described.

 Wavelength multiplexed signal light having two or more different wavelengths emitted from the optical amplifier or fiber of the previous stage propagates through the transmission line fiber 3 and is amplified to the required signal light power. It passes through the wavelength synthesizer 4a that combines the pump light, passes through the optical coupler 6c that supplies the monitor signal to the optical loopback path 6a, is propagated to the transmission line fiber 3 in the next stage, and then passes to the next optical amplification repeater. Incident.

 The pumping light applied to the Raman amplification emitted from the pumping light source 4b is incident on the transmission line fiber 3 via the wavelength synthesizer 4a that combines the signal light and the pumping light.

 Here, the excitation light source 4b may be a single wave, or may be light that combines multiple wavelengths.

 Furthermore, the monitor signal is demultiplexed by the optical coupler 6c, passes through the optical loopback path 6a, enters the transmission line fiber 3 via the downlink optical coupler 6c, and returns to the terminal station that transmitted the monitor signal. .

 The terminal station can observe the monitor signal and grasp the state of the repeater.

 In the case of a downlink, the optical loopback path 6b is applied without using the optical loopback path 6a, but the other configurations are the same.

 FIG. 10 and FIG. 11 show other examples of the optical loopback related to the seventh embodiment.

The flow of signal light, excitation light, and monitor light and their characteristics are the same as described in FIG.
(Supplementary note 1) A transmission path for transmitting signal light includes a first fiber having positive dispersion with respect to the signal light wavelength, a negative dispersion with respect to the signal light wavelength, and a mode with respect to the first fiber. A second fiber having a smaller field diameter and a third fiber having a smaller mode field diameter than the second fiber, and the signal light propagation direction is propagated in the order of the first, second, and third fibers; An optical amplification transmission system, wherein pump light is input from the signal light output side of a third fiber and Raman amplification is performed on the transmission line.
(Supplementary note 2) The optical amplification transmission system according to supplementary note 1, wherein the second fiber equalizes a transmission loss in the second fiber and a Raman amplification gain generated in the second fiber.
(Supplementary note 3) The optical amplification transmission system according to supplementary note 1, wherein the third fiber performs amplification to compensate for the loss of the entire transmission line.
(Supplementary note 4) The optical amplification transmission system according to supplementary note 1, wherein the total amount of the second fiber and the third fiber has a dispersion amount that corrects the dispersion value of the first fiber to almost zero.
(Supplementary note 5) The optical amplification transmission system according to supplementary note 1, wherein the third fiber is provided in an optical repeater.
(Appendix 6) A transmission path for transmitting signal light includes a first fiber having positive dispersion with respect to the signal light wavelength, and a mode field with respect to the first fiber having negative dispersion with respect to the signal light wavelength. The optical fiber is propagated in the order of the first and second fibers in the order of the first and second fibers, and pump light is input from the signal light output side of the second fiber, and Raman is transmitted through the transmission line. An optical amplification transmission system characterized in that amplification is performed to equalize a transmission loss in the second fiber and a Raman amplification gain generated in the second fiber.
(Supplementary note 7) The optical amplification transmission system according to supplementary note 6, wherein the output of the second fiber is optically amplified to a predetermined value by an optical amplifier.
(Appendix 8) A Raman amplification medium that amplifies the signal light by Raman amplification, a pump light for amplifying the Raman amplification medium, and a multiplexer that combines the signal light and the pump light, An optical amplification transmission system characterized in that the Raman amplification medium has a non-zero chromatic dispersion per fiber length and a zero dispersion slope with respect to a signal light wavelength.
(Supplementary note 9) A first optical amplifier that positively spreads the chromatic dispersion per fiber length with respect to the signal light wavelength and Raman-amplifies the signal light with pump light, and chromatic dispersion per fiber length with respect to the signal light wavelength And a second optical amplifier that is negative and Raman-amplifies the signal light with pumping light.

1 is an optical transmission station
1A is an optical transmitter
1B is a multiplexer
1C is a post-amplifier
2 is an optical receiver station
2A is an optical receiver
2B is a duplexer
2C is a preamplifier
3 is optical fiber
3a is positive dispersion fiber
3b is negative dispersion fiber
3c, 3d and 3e are concentrated Raman amplification fibers
4a, 4a1 and 4a2 are wavelength combiners that combine signal light and pump light.
4b, 4b1 and 4b2 are excitation light sources for Raman amplification
4c is the light receiving element for monitor signal
4d is monitor signal demultiplexing optical coupler
4e is the excitation light source control circuit
5 is an optical component, for example, an optical filter
5a is an optical isolator
5b is a variable attenuator
5c is a dispersion compensator
6a and 6b are optical loopback paths
6c is an optical coupler that demultiplexes the monitor signal into the optical loopback path.

Claims (1)

A transmission path for transmitting signal light includes a first optical fiber having positive dispersion with respect to the signal light wavelength, and negative dispersion with respect to the signal light wavelength, and a mode field with respect to the first optical fiber. A second optical fiber having a small diameter, and a third optical fiber having a mode field diameter smaller than that of the second optical fiber, the first optical fiber, the second optical fiber, the second optical fiber, The length is shortened in the order of 3 optical fibers,
The propagation direction of the signal light is propagated in the order of the first optical fiber, the second optical fiber, and the third optical fiber, and the excitation light is input from the signal light output side of the third optical fiber, rows that have Raman amplification in the transmission path,
The optical amplification transmission system characterized in that the second optical fiber equalizes the transmission loss in the second optical fiber and the Raman amplification gain generated in the second optical fiber .

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