JP2005027355A - Optical repeater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve problems that a Raman amplifier is difficult to use on an EDFA band and an optical relay needs adjustments corresponding to systems. <P>SOLUTION: A plurality of semiconductor lasers with different central wavelengths are multiplexed for each wavelength and then coupled by a WDM coupler to configure the Raman amplifier. A Fabry-Perot type semiconductor is used and an external resonator such as a fiber grating is connected to it. The amplifier has a means for detecting output power of each exciting light or a means for detecting signal light output power of a wavelength in which gain by each exciting light source is made maximum, and a means for controlling a drive current of each exciting light source for keeping power at a constant value. When the amplifier is applied to a repeater, the amplifier has a means in which exciting light having shorter wavelength band than a signal wavelength band of the repeater by 100nm is made incident on DCF being a constituting element of the optical repeater. Further the amplifier has a means for monitoring a signal light between an amplifier 1 and an input of DCF and between an amplifier 2 and an output of DCF, and may also have a means for monitoring input light to the optical repeater according to circumstances. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a Raman amplifier that can be used for amplification of signal light in various optical communication systems and an optical repeater using the same, and is particularly suitable for amplification of wavelength division multiplexed light.

  Most of the optical amplifiers used in current optical fiber communication systems are rare earth doped fiber amplifiers. In particular, an Er-doped optical fiber amplifier (hereinafter referred to as EDFA) using a fiber doped with Er (erbium) is often used. However, the practical gain wavelength band of EDFA is about 1530 nm to 1610 nm. (Reference: Electron. Lett, vol.33, no.23, pp. 1967 -1968) Also, EDFA has wavelength dependence on gain, and when used for wavelength division multiplexing, gain depends on the wavelength of signal light. There is a difference. FIG. 23 shows an example of the gain wavelength dependence of EDFA. The gain change with respect to the wavelength is particularly large at 1540 nm or less and 1560 nm or more. Therefore, a gain flattening filter is used to obtain a constant gain (usually within a gain deviation of 1 dB) over the entire band including such a wavelength.

  The gain flattening filter is a filter designed so that the loss becomes large at a wavelength where the gain is large, and the loss profile has almost the same shape as the gain profile. However, in the EDFA, when the average gain changes as shown in FIG. 24, the gain profile also changes like curves a, b, and c. In this case, the loss profile of the optimum gain flattening filter also changes. Accordingly, when flattening is realized by a gain correction filter with a fixed loss profile, the flatness deteriorates when the gain of the EDFA changes.

  On the other hand, some optical amplifiers are called Raman amplifiers utilizing Raman scattering of optical fibers (reference document: Nonlinear Fiber optics, Academic Press). The Raman amplifier has a gain peak at a frequency about 13 THz lower than the frequency of the pumping light. In the following description, assuming that 1400 nm band excitation light is used, a frequency lower by about 13 THz is expressed as a wavelength longer by about 100 nm. FIG. 25 shows the wavelength dependence of gain when excitation light having a center wavelength of 1450 nm is used. At this time, the peak of gain is 1550 nm, and the bandwidth within 1 dB of gain deviation is about 20 nm. Since a Raman amplifier can amplify an arbitrary wavelength as long as an excitation light source can be prepared, use in a wavelength band that cannot be amplified by EDFA is mainly studied. On the other hand, Raman amplifiers are not used in the EDFA gain band. This is because the Raman amplifier requires a larger pumping light power in order to obtain a gain equivalent to that of the EDFA. In addition, if high-power excitation light is incident on the fiber to increase the gain, stimulated Brillouin scattering due to the excitation light occurs and noise increases, which makes it difficult to use the Raman amplifier. A technique for suppressing stimulated Brillouin scattering in an amplifier is disclosed (for example, see Patent Document 1).

  In addition, the Raman amplifier has a polarization dependence on the gain, and amplifies only the component of the polarization component included in the signal light that matches the polarization of the pumping light. Therefore, measures to reduce gain instability due to polarization dependence are required. For this purpose, a fiber that preserves the polarization plane is used as an amplifying fiber, or a pumping light source having a random polarization state is used. It is considered.

  In addition, the Raman amplifier is also required to expand the gain band. As this method, it is considered to use a plurality of pump lights having different wavelengths (reference documents: OFC98, PD-6). However, no efforts have been made to reduce the gain deviation to 1 dB or less.

  On the other hand, there is an optical repeater that simultaneously compensates for transmission loss and chromatic dispersion that occur in an optical fiber transmission line, which combines an Er-doped fiber amplifier (EDFA) with a dispersion compensating fiber (DCF) to compensate for chromatic dispersion. It is a configuration. FIG. 46 shows this conventional example, in which a dispersion compensating fiber A is sandwiched between two Er-doped fiber amplifiers B and C. The first Er-doped fiber amplifier B amplifies low-level signal light to a relatively high level and is characterized by excellent noise characteristics. The second Er-doped fiber amplifier C amplifies the optical signal attenuated in the dispersion compensating fiber A to a high level again, and is characterized by a high output level.

  By the way, when designing the optical repeater, it is necessary to appropriately set each of the repeater input level, repeater output level, and dispersion compensation amount (loss in the dispersion compensation fiber A). There is also a restriction item that there is an upper limit for the input light level of A. This is because if the input power to the dispersion compensating fiber A is increased, the influence of the nonlinear effect in the dispersion compensating fiber A becomes larger, and the deterioration of the transmission waveform becomes remarkable. The upper limit value of the input power to the dispersion compensating fiber A is determined by the self-phase modulation (SPM) effect during single-wave transmission and the cross-phase modulation (XPM) effect during WDM transmission. As described above, in the optical repeater, it is necessary to design a repeater having excellent gain flatness and noise characteristics under the conditions in consideration of some variation factors.

  FIG. 47 shows an optical level diagram inside the repeater. The gain G1 [dB] of the first Er-doped fiber amplifier B is set to the difference between the repeater input level Pin [dB] and the input upper limit value Pd [dB] to the dispersion compensating fiber A. The gain G2 [dB] of the second Er-doped fiber amplifier C is the loss Ld [dB] in the dispersion compensating fiber A, the repeater gain Gr [dB], and the gain G1 [1] of the first Er-doped fiber amplifier B. dB] and Gr + Ld-G1 [dB]. Since these design parameters differ from system to system, G1 [dB] and G2 [dB] differ from system to system. Therefore, the Er-doped fiber amplifiers B and C must be redesigned from system to system. The noise characteristic in such a system has a deep relationship with the loss Ld [dB] in the dispersion compensating fiber A, and it is known that the noise characteristic becomes worse as the loss increases. At present, the loss of the transmission line and the dispersion of the dispersion compensating fiber A are compensated by changing the gains of the Er-doped fiber amplifiers B and C, or separately provided with a variable attenuator. The former degrades the gain flatness, and the latter degrades the noise characteristics.

JP-A-2-12986

  Er-doped fiber amplifiers are widely used in optical fiber communications, but there are some problems with Er-doped fiber amplifiers. The Raman amplifier also has a typical semiconductor laser output of about 100 to 200 mW, and the gain obtained is relatively small, or the gain is sensitive to changes in the power and wavelength of the pumping light. When using a Perot type semiconductor laser, the noise due to the gain fluctuation caused by the mode hop becomes noticeable, or it is necessary to change the drive current of the pump laser when adjusting the gain magnitude. When the current is changed, the fluctuation of the center wavelength is about 15 nm at the maximum, and there is a problem that the wavelength dependency of the gain is greatly changed. In addition, such a shift of the center wavelength also leads to a change in coupling loss of the WDM coupler that multiplexes the pumping light, which is not preferable. Furthermore, the optical repeater also has problems such as the need to redesign the Er-doped fiber amplifiers B and C for each system. In addition, degradation of noise characteristics due to insertion of a dispersion compensating fiber is an unavoidable problem in the current system.

  An object of the present invention is to provide a Raman amplifier capable of obtaining a necessary gain, reducing the wavelength dependence of gain to such an extent that it is not necessary to use a gain flattening filter, and being usable in an EDFA band. It is also necessary to redesign the EDFA for each system by applying this Raman amplifier to an optical repeater composed of an Er-doped fiber amplifier (EDFA) and a dispersion compensating optical fiber (DCF). The present invention provides an optical repeater that can compensate for variations in transmission line loss and DCF loss without degrading the characteristics of the optical repeater. In addition, Raman amplification of DCF shows that noise characteristic degradation due to DCF insertion, which could not be avoided in the past, is reduced.

  As shown in FIG. 1 or FIG. 2 or FIG. 3, the Raman amplifier according to the present invention comprises excitation light generating means 1 for generating a plurality of excitation lights, and a plurality of excitation lights output from the excitation light generating means 1 and A Raman amplifier that combines a signal light propagated through an optical fiber 2 and gives a Raman gain to the signal light. The pumping light generating means 1 is a Fabry-Perot type, DFB type, DBR type semiconductor laser or MOPA 3 Each excitation light has a central wavelength different from each other, and the interval between the central wavelengths is 6 nm or more and 35 nm or less.

  The Raman amplifier according to the present invention is characterized in that a plurality of pump lights have a wavelength difference between a maximum wavelength and a minimum wavelength within 100 nm.

  In the Raman amplifier according to the present invention, as shown in FIG. 3, the pumping light generating means 1 is configured to propagate the pumping light of adjacent wavelengths to the optical fiber 2 in two different directions to bi-directionally pump the signal light. It is characterized by.

  In the Raman amplifier according to the present invention, as shown in FIG. 3, the pumping light generating means 1 divides a plurality of pumping lights into two groups and prevents the pumping lights of adjacent wavelengths from entering the same group. The pump light is combined in each group, and the two pump lights combined in the group are propagated to the optical fiber 2 in two different directions.

  As shown in FIG. 26 or FIG. 27, the Raman amplifier according to the present invention monitors the input light or the output light, controls the pump light power of the pump light generating means 1 based on the result, and outputs the output light power. An output light power control means 4 that maintains a predetermined value is provided.

  As shown in FIG. 4 or FIG. 5, the Raman amplifier according to the present invention monitors output light including signal light subjected to Raman gain, and controls each pump light power of the pump light generating means 1 based on the result. The output light power control means 4 for flattening the wavelength dependence of the amplifier output is provided.

  In the Raman amplifier according to the present invention, as shown in FIG. 4, the output light power control means 4 demultiplexes the monitor light branched from the output light into light having a wavelength obtained by adding about 100 nm to the wavelength of each pump light. The light of these wavelengths is monitored, and the power of each pumping light of the pumping light generating means 1 is controlled so that the power of each light of the wavelength is made uniform.

  In the Raman amplifier according to the present invention, as shown in FIG. 5, the output light power control means 4 further distributes the monitor light branched from the output light into the same number as the pump light, and from each of them, the wavelength of each pump light is reduced to about each. The wavelength light added with 100 nm is transmitted to monitor each wavelength light, and each excitation light power of the excitation light generating means 1 is controlled so that the power of each wavelength light is made uniform. .

  As shown in FIG. 28, the Raman amplifier according to the present invention monitors the input light power and the output light power, and controls each pump light power of the pump light generating means 1 so that the ratio between them is constant, Output light power control means 4 for maintaining the gain at a predetermined value is provided.

  In the Raman amplifier according to the present invention, as shown in FIG. 1 or FIG. 2 or FIG. 3, the excitation light generating means 1 has an external resonator 5 for stabilizing the wavelength such as a fiber grating on the output side of a Fabry-Perot type semiconductor laser 3. It is characterized by providing.

  As shown in FIG. 1 or FIG. 2 or FIG. 3, the Raman amplifier according to the present invention is a polarization combiner for combining the excitation light with the excitation light generating means 1 on the output side of the Fabry-Perot type semiconductor laser 3. 6 is provided.

  As shown in FIG. 1 or FIG. 2 or FIG. 3, the Raman amplifier according to the present invention is based on the principle that the excitation light generating means 1 is a multi-wavelength Fabry-Perot type, DFB type, DBR type semiconductor laser or MOPA using a Mach-Zehnder interferometer A planar lightwave circuit type wavelength multiplexer is provided.

  As shown in FIG. 6A or 6B, the Raman amplifier according to the present invention includes polarization plane rotation means 7 that rotates the polarization plane by 90 degrees, and is generated in the optical fiber 2 by the pumping light generation means 1. In addition, a plurality of pumping lights, the pumping lights generated by the polarization plane rotating means 7 and the pumping lights whose polarization planes are orthogonal to each other exist at the same time.

  The Raman amplifier according to the present invention is characterized in that the amplification optical fiber 2 has a nonlinear refractive index n2 of 3.5E-20 [m2 / W] or more.

  The Raman amplifier according to the present invention is characterized in that the amplification optical fiber 2 is present as a part of the transmission line.

  The Raman amplifier according to the present invention is configured such that the amplification fiber 2 is a transmission line, and the breakdown is configured by connecting SMF and a fiber having a dispersion of less than −20 ps / nm / km. It is a feature.

  The Raman amplifier according to the present invention has an amplification fiber 2 as a transmission line, and is composed of SMF and a fiber having a dispersion of less than -20 ps / nm / km. The pump light propagates from the fiber with dispersion of less than 20 ps / nm / km toward the SMF.

  The Raman amplifier according to the present invention is an optical fiber 2 for amplification that is independent from a transmission fiber for propagating signal light, and exists as a Raman amplification fiber that can be inserted into the transmission fiber. It is characterized by being.

  As shown in FIG. 7, the optical repeater according to the present invention is an optical repeater that is inserted into an optical fiber transmission line 8 and compensates for a loss in the optical fiber transmission line 8, and includes a Raman amplifier 9 according to the present invention. And the Raman amplifier 9 is configured to compensate for the loss in the optical fiber transmission line 8.

  The optical repeater according to the present invention uses the Raman amplification effect in the optical fiber transmission line 8 by making the residual pumping light of the Raman amplifier 9 enter the optical fiber transmission line 8 as shown in FIGS. It is characterized by.

  As shown in FIG. 8, the optical repeater according to the present invention is characterized by including a rare-earth doped fiber amplifier 10 at the front stage, the rear stage, or both the front and rear stages of the Raman amplifier 9.

  The optical repeater according to the present invention is characterized in that the residual pumping light of the Raman amplifier 9 is used as the pumping light of the rare earth-doped fiber amplifier 10 as shown in FIGS.

  As shown in FIG. 45, an optical repeater according to the present invention is an optical repeater that is inserted into an optical fiber transmission line 8 to compensate for chromatic dispersion in the optical fiber transmission line 8, and is a Raman amplifier according to the present invention. 9 is used to compensate for chromatic dispersion in the optical fiber transmission line 8 by using a dispersion compensating fiber for the optical fiber 2, and to compensate for part or all of the loss in the optical fiber transmission line 8 and the amplification optical fiber 2. It is characterized by this.

  The optical repeater according to the present invention is characterized in that the residual pumping light of the Raman amplifier 9 enters the optical fiber transmission line 8 and utilizes the Raman amplification effect in the optical fiber transmission line 8 as shown in FIGS. It is what.

  As shown in FIG. 8, the optical repeater according to the present invention is characterized by including a rare-earth doped fiber amplifier 10 at the front stage, the rear stage, or both the front and rear stages of the Raman amplifier 9.

  The optical repeater according to the present invention is characterized in that the residual pumping light of the Raman amplifier is used as the pumping light of the rare earth-doped fiber amplifier 10 as shown in FIGS.

  As shown in FIG. 9, the optical repeater according to the present invention compensates for fluctuations in the input level to the optical fiber 2 that is a dispersion compensating fiber and loss fluctuations in the optical fiber 2 by Raman amplification in the optical fiber 2. And a control means for maintaining the output from the optical fiber 2 at a predetermined value.

  As shown in FIG. 10, the optical repeater according to the present invention includes control means for compensating for loss or gain in the optical fiber 2 that is a dispersion compensating fiber by means of Raman amplification in the optical fiber 2 and keeping it constant. It is what.

  The optical repeater according to the present invention is characterized in that the gain of the rare-earth doped fiber amplifier 10 is kept constant and the gain of the repeater is adjusted by the gain of the Raman amplifier 9.

  The optical repeater according to the present invention is characterized in that the wavelength dependence of the gain of the rare earth doped fiber amplifier 10 is compensated by using the wavelength dependence of the gain of the Raman amplifier 9 as shown in FIG. It is.

  As shown in FIG. 8, the optical repeater according to the present invention is an optical repeater that is inserted into an optical fiber transmission line 8 and compensates for loss and chromatic dispersion in the optical fiber transmission line 8. A single Raman amplifier 9, and a rare-earth doped fiber amplifier 10 at the front stage, the rear stage, or both the front and rear stages of the Raman amplifier 9, and the optical fiber 2 for amplification of the Raman amplifier 9 is used for dispersion compensation. It is characterized by using a fiber.

  The optical repeater according to the present invention is characterized in that the residual pumping light of the Raman amplifier 9 is used as the pumping light of the rare earth-doped fiber amplifier 10 as shown in FIGS.

  As shown in FIG. 9, the optical repeater according to the present invention compensates for fluctuations in the input level to the optical fiber 2 for Raman amplification, which is a dispersion compensating fiber, and loss fluctuations in the optical fiber 2 by Raman amplification in the optical fiber 2. And the control means which keeps the output from the said optical fiber 2 to a predetermined value is provided.

  As shown in FIG. 10, the optical repeater according to the present invention has a control means for compensating for the loss or gain in the Raman amplification optical fiber 2 which is a dispersion compensating fiber by the Raman amplification in the fiber 2 and keeping it at a predetermined value. It is characterized by comprising.

  As shown in FIG. 11, the optical repeater according to the present invention is characterized in that the gain of the rare earth doped fiber amplifier 10 is kept constant and the gain of the repeater is adjusted by the gain of the Raman amplifier 9. is there.

  Next, the operation of the Raman amplifier of the present invention and the optical repeater using the Raman amplifier will be described.

  In the Raman amplifier according to the present invention, a relatively high gain can be obtained when a small and relatively high output Fabry-Perot type semiconductor laser 3 is used as the pumping light generating means 1 as shown in FIGS. In addition, since the Fabry-Perot type semiconductor laser 3 has a wide line width of the oscillation wavelength, the generation of stimulated Brillouin scattering by the excitation light can be almost eliminated. When a DBF-type or DBR-type semiconductor laser or MOPA is used, the gain shape does not change depending on the driving conditions because the fluctuation range of the oscillation wavelength is relatively small. Moreover, the generation of stimulated Brillouin scattering can be suppressed by applying modulation.

  Furthermore, the wavelength dependence of the gain can be reduced to such an extent that a gain flattening filter is not required by setting the interval between the central wavelengths of the excitation light to 6 nm or more and 35 nm or less. The reason why the central wavelength interval of the pumping light is 6 nm or more is that the oscillation bandwidth of the Fabry-Perot semiconductor laser 3 connected to the external resonator 5 having a narrow reflection bandwidth is about 3 nm as shown in FIG. This is because the WDM coupler 11 (FIGS. 1, 2, and 3) for multiplexing the pumping light can have some margin in the wavelength interval between the pumping lights in order to improve the multiplexing efficiency. The WDM coupler 11 is designed so that light of different wavelengths is incident from different ports and the incident light is coupled to one output port with almost no loss. For light, the loss increases with either input port. For example, in a WDM coupler 11, the width of the wavelength band in which this loss increases is 3 nm. Therefore, in order to prevent the band of the semiconductor laser 3 from being included in this band, as shown in FIG. 12, 6 nm obtained by adding 3 nm to the band width of the semiconductor laser 3 is the lower limit of the central wavelength interval of the pumping light. As appropriate. On the other hand, when the interval between the central wavelengths of the semiconductor laser 3 is set to 35 nm or more as shown in FIG. 13A, the gain valley is in the middle of the Raman gain band obtained by the pumping light of adjacent wavelengths as shown in FIG. And the flatness of the gain deteriorates. This is because, with respect to the Raman gain obtained by one pumping light, the gain is halved at a distance of 15 nm to 20 nm from the gain peak wavelength. Therefore, by setting the interval between the central wavelengths of the excitation light to 6 nm or more and 35 nm or less, the wavelength dependency of gain can be reduced to the extent that it is not necessary to use a gain flattening filter.

  Further, in the Raman amplifier according to the present invention, the difference between the maximum value and the minimum value of the center wavelength of the pump light is set to 100 nm or less, so that the wavelength overlap of the pump light and the signal light is prevented and the waveform of the signal light is deteriorated. Can be prevented. Since the waveform of the signal light will be degraded if the wavelengths of the pump light and the signal light are close, the wavelength of the pump light and the signal light must be chosen so that they do not overlap, but if the pump light is in the 1.4 μm band, If the difference between the maximum value and the minimum value of the center wavelength of the light is 100 nm or less, as shown in FIG. 14, the difference in wavelength between the center wavelength of the gain generated from one pump light and the pump light is about 100 nm. Duplication of wavelengths of light and signal light can be prevented.

Further, in the Raman amplifier according to the present invention, the pumping light of adjacent wavelengths is propagated in two different directions to the optical fiber 2 and the signal light is bidirectionally pumped. The wavelength characteristics required for the WDM coupler 11 can be improved. As shown in FIG. 15, the center wavelengths are all λ ₁ , λ ₂ , λ ₃ , λ _{4 and the} interval is 6 nm or more and 35 nm or less in all the pump lights combined in both directions. As seen, the center wavelengths are λ ₁ and λ ₃ , λ ₂ and λ ₄ , the wavelength interval is doubled, and the required characteristics of the WDM coupler 11 can be afforded.

  In the Raman amplifier according to the present invention, the input light or the output light to the Raman amplifier is monitored, and based on the result, each pump light power of the pump light generating means 1 is controlled to obtain the output light power of the Raman amplifier. Since the control means 4 that maintains the predetermined value is provided, a constant output can be obtained regardless of variations in the input signal power to the Raman amplifier and variations in the loss of the Raman amplification fiber.

  Further, the Raman amplifier according to the present invention includes the output optical power control means 4 for flattening the Raman gain, so that the gain can be flattened. In such a Raman amplifier, as shown in FIGS. 16 (a) and 16 (b), wavelength light having a wavelength obtained by adding about 100 nm to the wavelength of each pump light is monitored, and each pump light is adjusted so that the power of these wavelength lights is made uniform. Therefore, the gain can be flattened. In addition, a wavelength stabilization fiber grating (external resonator 5), which will be described later, is connected, so that a change in the center wavelength due to a change in the drive current can be suppressed. .

  In the Raman amplifier according to the present invention, the control means 4 for monitoring the input signal power and the output signal power, controlling the pumping light power so that the ratio between them is constant, and maintaining the gain of the Raman amplifier at a predetermined value. Therefore, a constant gain can be obtained regardless of fluctuations in input signal power to the Raman amplifier and variations in the loss of the Raman amplification fiber.

  In the Raman amplifier according to the present invention, since the external resonator 5 for stabilizing the wavelength such as a fiber grating is provided on the output side of the Fabry-Perot semiconductor laser 3, the gain due to the mode hop of the Fabry-Perot semiconductor laser 3 is provided. Noise due to fluctuations can be suppressed. In addition, when an external resonator 5 for wavelength stabilization is connected to the semiconductor laser 3, the bandwidth becomes narrower when viewed with respect to one pumping light source, but is multiplexed by the WDM coupler 11 (FIGS. 1, 2, and 3). In this case, since the wavelength interval can be narrowed, finally, higher-power and broadband excitation light can be obtained.

  Further, in the Raman amplifier according to the present invention, since the pumping light of the semiconductor laser 3 is used by combining the polarization for each wavelength, the polarization dependence of the gain is eliminated and at the same time the pumping light power incident on the optical fiber 2 is used. Can be increased. The gain in Raman amplification is obtained only for the component that matches the polarization of the pumping light. Therefore, when the pumping light is linearly polarized and the amplification fiber is not a polarization maintaining fiber, the signal light and the pumping light The gain fluctuates due to the fluctuation of the relative polarization, but combining the linearly polarized pump light source so that the planes of polarization are orthogonal crosses the polarization dependence of the gain and at the same time the pump light power incident on the fiber. Will increase.

  In the Raman amplifier according to the present invention, as a means for multiplexing a Fabry-Perot type, DFB type, DBR type semiconductor laser or MOPA of a plurality of wavelengths, a planar lightwave circuit type wavelength multiplexing based on a Mach-Zehnder interferometer is used. Therefore, even when a large number of Fabry-Perot semiconductor lasers with a plurality of wavelengths are combined, they can be combined with extremely low loss, and high-power pump light can be obtained.

  Further, the Raman amplifier according to the present invention includes a polarization plane rotating means 7 that rotates the plane of polarization by 90 degrees as shown in FIG. 6, and a plurality of pump lights generated by the pump light generating means 1 in the optical fiber 2 and those. In principle, a constant gain can be obtained regardless of the polarization plane of the signal light. Since the Raman amplification band depends on the pumping light band, combining the pumping lights having a plurality of wavelengths with the WDM coupler 11 broadens the pumping light incident on the optical fiber 2 for amplification, and as a result. As a result, the Raman gain is widened.

  Further, in the Raman amplifier according to the present invention, since the optical fiber 2 having a nonlinear refractive index n2 of 3.5 E -20 [m2 / W] or more is used, it is a result of research so far. Is obtained.

  In the Raman amplifier according to the present invention, since the optical fiber 2 exists as a part of the transmission fiber for propagating the signal light, the amplifier can be configured as it is in the transmission optical fiber.

  In the Raman amplifier according to the present invention, the optical fiber 2 exists as a Raman amplifying fiber inserted into the transmission fiber independent of the transmission fiber for propagating signal light. In addition, it is easy to use an optical fiber suitable for Raman amplification, a chromatic dispersion compensating fiber, and a lumped amplifier can be configured.

  In the optical repeater according to the present invention, the loss of the optical fiber transmission line 8 is compensated by using the Raman amplifier, so that an optical repeater having the function of the Raman amplifier according to the present invention can be obtained.

  Further, in the optical repeater according to the present invention, the residual pumping light of the Raman amplifier is incident on the optical fiber transmission line 8 and the Raman amplification effect in the optical fiber transmission line 8 is used to reduce the loss of the optical fiber transmission line 8. Some can be compensated.

  In the optical repeater according to the present invention, the rare-earth doped fiber amplifier 9 is provided at the front stage, the rear stage, or both the front and rear stages of the Raman amplifier, and the loss of the optical fiber transmission line 8 is compensated by the Raman amplifier 9 and the rare-earth doped fiber amplifier 10. Therefore, desired amplification characteristics suitable for various transmission systems can be obtained.

  In the optical repeater according to the present invention, the number of semiconductor lasers to be used can be reduced by using the residual pumping light of the Raman amplifier as the pumping light of the rare earth doped fiber amplifier 10.

  In the optical repeater according to the present invention, since the dispersion compensating fiber is used for the optical fiber 2 of the Raman amplifier 9, the Raman amplifier 9 compensates the chromatic dispersion of the optical fiber transmission line 8, and the optical fiber transmission line. 8 and part or all of the loss in the amplification fiber 2 can be compensated.

  In the optical repeater according to the present invention, the rare-earth doped fiber amplifier 10 is provided at the front stage, the rear stage, or both the front and rear stages of the Raman amplifier 9, and the loss of the optical fiber transmission line 8 is determined by the Raman amplifier 9 and the rare-earth doped fiber amplifier 10. Since the chromatic dispersion is compensated, desired amplification characteristics suitable for various transmission systems can be obtained.

  In the present invention, the combination of the Raman amplifier 9 and the rare-earth doped fiber amplifier 10 can provide a repeater that can be adapted to various systems. In the case of using DCF for the amplification fiber of the Raman amplifier 9 Will be described as an example. FIG. 17 shows an example of design parameters of a conventional optical repeater, and G1 and G2 are different for each system. Also, repeater input and DCF loss inevitably fluctuate due to variations in repeater spacing and DCF. This fluctuation is directly linked to the fluctuation of the gain of the EDFA, and the change of the gain leads to deterioration of the flatness. FIG. 18 schematically shows the relationship between the gain and flatness of the EDFA. Since the flatness is optimized by limiting the use band and the average gain, the average gain deviates from the optimized point. Then, the wavelength dependency of the gain changes and the flatness deteriorates. In order to avoid deterioration of flatness, the EDFA gain needs to be kept constant. Conventionally, a variable attenuator has been used as a means for compensating for fluctuations in input level and DCF loss. FIG. 19A shows an example in which the attenuation level of the variable attenuator is adjusted in accordance with the fluctuation of the input level, and the input level to the DCF is controlled to be constant. FIG. In this example, the attenuation is adjusted and the intermediate loss is controlled to be constant. In either case, the two EDFAs have a constant gain. However, this method is disadvantageous in terms of noise characteristics because unnecessary loss is added by the variable attenuator.

  In the present invention, the change in the design parameter of the repeater is compensated by the Raman amplification effect of the DCF, so that the gain of the EDFA is kept constant, the necessity of designing the EDFA for each system is eliminated, and the flatness and noise characteristics are also reduced. It is possible to compensate for variations in repeater spacing and DCF without sacrificing. FIG. 20 shows the design value of the EDFA when the DCF Raman amplification effect is applied to the repeater specification of FIG. By appropriately selecting the Raman gain of DCF, the characteristics of EDFA required for the three specifications can be made common. Further, as shown in FIGS. 21A and 21B, fluctuations in the input level and DCF loss can be compensated by changing the Raman gain without changing the gain of the EDFA. In either case, the Raman amplification gain is adjusted so that the output level of the DCF is constant while keeping the gain of the EDFA constant. Furthermore, compensating for the loss of the DCF itself by Raman amplification reduces the deterioration of noise characteristics due to DCF insertion that could not be avoided. FIG. 37 shows measured values of the noise figure degradation amount when a Raman amplifier using the same DCF as the noise figure degradation amount when DCF is inserted.

  In addition, since the optical repeater according to the present invention includes a Raman amplifier having a single pumping light wavelength, the operation range is narrower than that of an optical repeater including a Raman amplifier pumped at a plurality of wavelengths. Other than the simple bandwidth, the same characteristics as the optical repeater described so far can be obtained. 38 and 39 show measurement examples of an optical repeater using a Raman amplifier pumped at a single wavelength and an optical repeater using a Raman amplifier pumped at a plurality of wavelengths.

  In the Raman amplifier of the present invention, the gain flattening filter is selected by selecting the wavelength of the excitation light source so that the interval between the center wavelengths is 6 nm to 35 nm and the difference between the maximum value and the minimum value of the center wavelength is within 100 nm. An optical amplifier that can maintain the flatness even when the gain changes is small, and compensates for the loss and chromatic dispersion of the transmission line. It can be applied as a container. In a repeater configured in combination with EDFA, it is possible to suppress EDFA gain fluctuations due to repeater input fluctuations and DCF loss fluctuations, avoid deterioration of gain flatness, and adapt to various systems. it can.

(Embodiment 1 of Raman amplifier)
FIG. 1 shows a first embodiment of a Raman amplifier according to the present invention, in which a signal light input fiber 12, an amplification fiber (optical fiber) 2, a WDM coupler 13, a pumping light generating means 1, a monitor light distribution coupler 14, The monitor signal detection and LD control signal generation circuit 15, the signal light output fiber 16, and the polarization independent isolator 25 are configured. Here, the monitor light distribution coupler 14 and the monitor signal detection / LD control signal generation circuit 15 constitute the output light power control means 4.

The pumping light generating means 1 includes a Fabry-Perot type semiconductor laser 3 (3 ₁ , 3 ₂ , 3 ₃ , 3 ₄ ) and a wavelength grating fiber grating (external resonator) 5 (5 ₁ , 5 ₂ , 5 ₃ , 5 ₄ ), a polarization combiner (polarization combiner) 6 (6 ₁ , 6 ₂ ), and a WDM coupler 11. Here, the oscillation wavelengths of the semiconductor lasers 3 ₁ , 3 ₂ and the transmission wavelengths of the fiber gratings 5 ₁ , 5 ₂ are both the same wavelength λ ₁ , the central wavelengths of the semiconductor lasers 3 ₃ , 3 ₄ , the fiber gratings 5 ₃ , The transmission wavelengths of 5 ₄ are the same wavelength λ ₂ , and the oscillation wavelengths of the semiconductor lasers 3 ₁ , 3 ₂ , 3 ₃ and 3 ₄ are the functions of the wavelength stabilizing fiber gratings 5 ₁ , 5 ₂ , 5 ₃ and 5 ₄ . Thus, the center wavelength is stabilized at λ ₁ and λ ₂ . The wavelength interval between the wavelengths λ ₁ and λ ₂ is 6 nm or more and 35 nm or less.

The pumping light generated by the semiconductor lasers 3 ₁ , 3 ₂ , 3 ₃ , 3 ₄ is combined by the polarization combining coupler 6 for each of the wavelengths λ ₁ , λ ₂ , and output light from each polarization combining coupler 6. Are combined by the WDM coupler 11 to become the output light of the pumping light generating means 1. The semiconductor laser 3 and the polarization combining coupler 6 are connected by a polarization plane preserving fiber 17 so that two pump lights having different polarization planes can be obtained. The output light of the pumping light generating means 1 is coupled to the amplification fiber 2 by the WDM coupler 13, while the signal light (wavelength division multiplexed light) is incident on the amplification fiber 2 from the signal light input fiber 12, and the amplification fiber 2 is combined with the pumping light of the pumping light generating means 1 and Raman amplified, passes through the WDM coupler 13, a part of the monitor light branching coupler 14 is branched as a monitor signal, and the rest is a signal light output fiber. 16 is output. The monitor signal is monitored by a monitor signal detection and LD control signal generation circuit 15, which generates a signal for controlling the drive current of each semiconductor laser 3 so that the gain deviation within the signal wavelength band is reduced.

  The amplifying fiber 2 may be a special fiber suitable for Raman amplification, such as one having a nonlinear refractive index n2 of 3.5 E -20 [m2 / W] or more, or a signal input fiber 12 to which signal light is input. May be used as they are. Further, an RDF (Reverse Dispersion Fiber) having a dispersion of less than −20 ps / nm per km may be connected to the SMF and used as an amplification fiber that also serves as a transmission line. (In general, RDF has a dispersion of less than -20 ps / nm, so it is better to use the same length as SMF and double the length of SMF.) In such a case, the excitation light for Raman amplification is converted from RDF to SMF. It is convenient to adopt a configuration that propagates toward the In this Raman amplifier, the amplifying fiber 2 can be connected to and inserted into a transmission fiber (not shown) through which signal light is transmitted, and the amplifying fiber 2, pumping light generating means 1, WDM coupler 13, and monitor light branching A centralized Raman amplifier may be configured by incorporating a set of the coupler 14 and the monitor signal detection and LD control signal generation circuit 15.

FIG. 22 shows the measured output spectrum of the Raman amplifier of FIG. The excitation light wavelengths λ ₁ and λ ₂ used in this measurement were 1435 nm and 1465 nm, and the signal light was input at eight equal intervals between -20 dBm / ch 1540 nm and 1560 nm. The amplification fiber 2 is a dispersion compensating fiber of about 6 km, and the power of the pumping light is adjusted so as to compensate for the loss of the dispersion compensating fiber while keeping the interchannel deviation within 0.5 dB.

(Embodiment 2 of Raman amplifier)
FIG. 2 shows a second embodiment of the Raman amplifier according to the present invention, in which the excitation light from the excitation light generating means 1 travels through the amplification fiber 2 in the same direction as the signal light. More specifically, a WDM coupler 13 is provided on the front end side of the amplification fiber 2, and the pump light from the pump light generation means 1 passes through the WDM coupler 13 from the front end (input end) side of the amplification fiber 2 to the rear end ( The data is transmitted to the output end) side. Since this configuration is amplified before signal attenuation occurs, it is known that the noise characteristics of signal light are better than those of the configuration of the first embodiment. It is also known that the gain is small compared to the configuration of the first embodiment.

(Embodiment 3 of Raman amplifier)
FIG. 3 shows a third embodiment of the Raman amplifier of the present invention, which is configured so that the pumping light from the pumping light generating means 1 travels in both directions in the amplification fiber 2. Specifically, WDM couplers 13 are provided on the front end side and the rear end side of the amplification fiber 2, respectively, and the pump light from the pump light generating means 1 divided into two groups is amplified through the respective WDM couplers 13. The pumping light coupled to the optical fiber 2 and input to the WDM coupler 13 on the front end side proceeds to the rear end side of the amplification fiber 2, and the pumping light input to the WDM coupler 13 on the rear end side is amplified. It is made to advance to the front end side.

The central wavelengths of the semiconductor lasers 3 ₁ , 3 ₂ belonging to the first group A and the semiconductor lasers 3 ₅ , 3 ₆ belonging to the second group B of the excitation light generating means 1 are the same, The center wavelengths of the semiconductor lasers 3 ₃ and 3 ₄ belonging to the group A and the semiconductor lasers 3 ₇ and 3 ₈ belonging to the second group B are the same. Further, the fiber gratings 5 _{1 to} 5 ₈ are matched with the center wavelength of the semiconductor laser 3 to which each is connected.

(Embodiment 4 of Raman amplifier)
In the embodiment of FIG. 3, the center wavelengths of the semiconductor lasers 3 ₁ and 3 ₂ belonging to the first group A are λ ₁ , the center wavelengths of the semiconductor lasers 3 ₃ and 3 ₄ belonging to the group A are λ 3, and the second group The central wavelengths of the semiconductor lasers 3 ₅ and 3 ₆ belonging to B are λ ₂ , the central wavelengths of the semiconductor lasers 3 ₇ and 3 ₈ belonging to the group B are λ _4, and λ ₁ , λ ₂ , λ ₃ and λ ₄ are It can also be configured as adjacent wavelengths. Also in this case, the interval between the center wavelengths is 10 nm or more and 30 nm or less, and the difference between the maximum center wavelength λ ₄ and the minimum center wavelength λ ₁ is 100 nm or less. With such a configuration, it is possible to provide a margin in the wavelength interval of the pumping light multiplexed in the same group, and the performance required for the WDM coupler 4 can be reduced.

(Embodiment 5 of Raman amplifier)
FIG. 40 shows a fifth embodiment of the Raman amplifier according to the present invention, in which an appropriate one is selected from the Raman amplifiers 9 described in the above embodiments and connected in multiple stages. By appropriately selecting Raman amplifiers 9 having different characteristics in accordance with desired amplification characteristics and noise characteristics, characteristics that cannot be obtained by a single Raman amplifier 9 can be obtained.

  In each of the above embodiments, the output light power control means 4 can be configured as shown in FIG. 4 or FIG. 4 has a monitor optical branching coupler 14 shown in FIG. 1, 2 or 3, a wavelength demultiplexer 18, an optical / electric conversion means 19 such as a photodiode, and an LD control circuit 20. A signal detection and LD control signal generation circuit 15 is connected. The wavelength demultiplexer 18 demultiplexes the output light branched by the monitor light branching coupler 14 into a plurality of wavelength lights. In this case, the maximum amplification wavelength (wavelength obtained by adding 100 nm to the excitation light wavelength) by each excitation light. The light in the vicinity is demultiplexed. Specifically, if the excitation wavelengths are 1430 nm and 1460 nm, the light in the vicinity of 1530 nm and 1560 nm is demultiplexed. The optical / electric conversion means 19 converts the received wavelength light into an electric signal, and the output voltage is changed according to the level of the received light level. The LD control circuit 20 changes the drive current of the semiconductor laser 3 in accordance with the output voltage from the optical / electrical conversion means 19, calculates the output voltage from the optical / electrical conversion means 19, and processes each wavelength. The semiconductor laser 3 is controlled so that the optical power of the light is made uniform. That is, the output light power control means 4 functions to eliminate the wavelength dependence of the Raman gain and flatten the gain.

  The configuration of FIG. 5 includes the monitoring light branching coupler 14 shown in FIG. 1, 2 or 3, a branching coupler 21, a bandpass filter 22, an optical / electric conversion means 19 such as a photodiode, and an LD control circuit. 20 is connected to a monitor signal detection and LD control signal generation circuit 15. The branching coupler 21 branches the output light branched by the monitor light branching coupler 14 to the same number as the number of excitation lights. Each band-pass filter 22 has a different transmission center wavelength, and in this case, light near the maximum amplification wavelength (wavelength obtained by adding 100 nm to the excitation light wavelength) due to each excitation light is transmitted. Specifically, the excitation wavelength is 1430 nm. And 1460 nm, light having wavelengths near 1530 nm and 1560 nm is transmitted. The optical / electric conversion means 19 converts the received wavelength light into an electric signal, and the output voltage is changed according to the level of the received light level. The LD control circuit 20 changes the drive current of the semiconductor laser 3 in accordance with the output voltage from the optical / electrical conversion means 19, calculates the output voltage from the optical / electrical conversion means 19, and processes each wavelength. The semiconductor laser 3 is controlled so that the optical power of the light is made uniform. That is, the output light power control means 4 functions to eliminate the wavelength dependence of the Raman gain and flatten the gain. 4 and 5 show a configuration in which the output light is monitored and the excitation light generating means 1 is controlled as shown in FIG. 27. However, the input light is monitored and the excitation light generating means 1 is controlled as shown in FIG. Alternatively, as shown in FIG. 28, it is also possible to monitor the output light and the input light together to control the excitation light generating means 1.

In the Raman amplifier of each configuration described above, instead of combining the pumping light with the polarization combining coupler 6, as shown in FIGS. 6 (a) and 6 (b), a polarization plane rotating means for rotating the polarization plane of the pumping light by 90 degrees. 7 can be provided so that a plurality of pumping lights generated by the pumping light generating means 1 and pumping lights whose polarization planes are orthogonal to each other simultaneously exist in the amplification fiber 2. In FIG. 6A, the Faraday rotor 3 ₁ and the total reflection mirror 3 ₂ are provided at one end of the amplification fiber 2, the polarization plane of the excitation light propagated to the amplification fiber 2 is rotated by 90 degrees, and the amplification fiber is again formed. This is because it is set back to 2. In the figure, means for taking out the signal light propagated to the amplification fiber 2 and Raman-amplified from the fiber 2 is not shown. FIG. 6B shows a polarization plane in which a PBS 33 and a polarization-maintaining fiber 34 are provided at one end of the amplification fiber 2 and the excitation light output from one end of the amplification fiber 2 is connected by twisting the main axis by 90 degrees. The polarization plane is rotated 90 degrees by the holding fiber 34 and is input again to one end of the amplification fiber 2 through the PBS 33.

(Embodiment 1 of optical repeater)
FIG. 7 shows a first embodiment of an optical repeater constructed using the Raman amplifier of the present invention, and an optical repeater inserted in the optical fiber transmission line 8 to compensate for the loss in the optical fiber transmission line 8. It is an example. In this optical repeater, a rare earth doped fiber amplifier (hereinafter referred to as EDFA) 10 is connected after the Raman amplifier 9 as shown in FIGS. The signal light is input to the Raman amplifier 9 and amplified, and further input to the EDFA 10 to be amplified and output to the optical fiber transmission line 8. The gain adjustment may be adjusted on the Raman amplifier 9 side, on the EDFA 10 side, or on both, but the loss of the optical fiber transmission line 8 is compensated as a whole. In addition, the wavelength dependence of the gain of the EDFA 10 can be reduced by the wavelength dependence of the Raman amplifier 9 by properly combining the difference between the wavelength dependence of the gain of the EDFA 10 and the wavelength dependence of the Raman amplifier 9. is there.

(Embodiment 2 of optical repeater)
FIG. 8 shows a second embodiment of the optical repeater configured using the Raman amplifier of the present invention. In the optical repeater of FIG. 7, an EDFA 10 is provided in the preceding stage of the Raman amplifier 9.

(Embodiment 3 of optical repeater)
FIG. 9 shows a third embodiment of an optical repeater configured using the Raman amplifier of the present invention. A Raman amplifier 9 using a dispersion compensating fiber (DCF) as an amplification fiber 2 is provided between two EDFAs 10. It is a thing. Between the Raman amplifier 9 and the EDFA 10 at the subsequent stage, a branch coupler 23 for branching the output light from the Raman amplifier 9, and monitor signal detection and LD control for controlling the gain of the Raman amplifier 9 by monitoring the branch light. A signal generation circuit 24 is provided. The monitor signal detection and LD control signal generation circuit 24 is a control circuit that can maintain the output power of the Raman amplifier 9 at a predetermined value. In the case where the Raman amplifier 9 itself includes the output light power control means 4 shown in FIGS. 4 and 5, the output light power is controlled to be a predetermined value, and at the same time, the level deviation between a plurality of output signals. The power of the pumping light is controlled so that becomes smaller.

  In the optical repeater of FIG. 9, the output light level of the Raman amplifier 9, that is, the input light level to the second EDFA 10 is always kept constant without being affected by the loss of DCF or the output level of the first EDFA 10. It is. This ensures that the gain of the second EDFA 10 is kept constant when the output of the repeater is specified. Thereby, the gain flatness deterioration of the second EDFA 10 due to the DCF loss fluctuation or the like is avoided. Further, if the first EDFA 10 is controlled so that the gain is constant, the fluctuation of the input to the repeater is compensated by the fluctuation of the gain of the Raman amplifier 9. That is, the adjustment of the repeater gain is performed only by the gain of the Raman amplifier 9, and the flatness deterioration due to the gain fluctuation of the EDFA 10 can be completely avoided.

(Embodiment 4 of optical repeater)
FIG. 10 is an embodiment in which control means for monitoring the optical level and adjusting the gain of the Raman amplifier 9 is added between the first EDFA 10 and the Raman amplifier 9 in the embodiment of FIG. Using this, it is possible to control the pumping light so that the level difference between the input and output of the Raman amplifier 9 is kept constant, and this can compensate only for the loss variation of the DCF.

(Embodiment 5 of optical repeater)
FIG. 11 shows an example in which the gain flattening monitor mechanism provided in the Raman amplifier 9 is moved to the output terminal of the repeater and used as a monitor for flattening the gain of the entire repeater in the above embodiment. In this case, the first EDFA 10 and the second EDFA 10 may be either constant gain control or constant output control. The power of each pumping light is individually controlled so that the level deviation between output signals at the repeater output becomes small.

(Embodiment 6 of optical repeater)
The optical repeater of the present invention uses a dispersion compensating fiber for the amplification fiber 2 of the Raman amplifier having the configuration shown in FIGS. 1 to 3 to compensate for the chromatic dispersion of the optical fiber transmission line 8, and the optical fiber transmission line 8 and An optical repeater that compensates for some or all of the loss in the amplification fiber 2 can also be configured.

(Embodiment 7 of optical repeater)
In each embodiment of the optical repeater, an optical repeater including a Raman amplifier 9 using the pumping light generating means 1 as shown in FIGS. 41 to 44 can be configured.

(Embodiment 8 of optical repeater)
29 to 32, a WDM coupler 13 is inserted in the middle of the amplification fiber 2 of the Raman amplifier 9, and the residual pump light from the pump light generating means 1 propagated to the amplification fiber 2 is transferred to the Raman amplifier 9. The incident light enters the transmission line 8 through the WDM coupler 27 provided in the optical fiber transmission line 8 on the input side or the output side, and the Raman gain can be generated also in the optical fiber transmission line 8. 29 to 32, reference numeral 26 denotes an optical isolator.
(Embodiment 9 of optical repeater)

  As shown in FIGS. 33 to 36, when the optical repeater is composed of the Raman amplifier 9 and the EDFA 10, the WDM coupler 13 is inserted in the middle of the amplification fiber 2 of the Raman amplifier 9 and propagated to the amplification fiber 2. The residual excitation light from the excitation light generating means 1 is incident on the EDFA 10 and can be used as the excitation light / auxiliary excitation light of the EDFA 10. 33 to 36, reference numeral 26 denotes an optical isolator.

The block diagram which shows 1st Embodiment of the Raman amplifier of this invention. The block diagram which shows 2nd Embodiment of the Raman amplifier of this invention. The block diagram which shows 3rd Embodiment of the Raman amplifier of this invention. The block diagram which shows the 1st example of the output optical power control means in the Raman amplifier of this invention. The block diagram which shows the 2nd example of the output optical power control means in the Raman amplifier of this invention. The block diagram which shows the example from which the polarization plane rotation means in the Raman amplifier of this invention differs. The block diagram which shows 1st Embodiment of the optical repeater of this invention. The block diagram which shows 2nd Embodiment of the optical repeater of this invention. The block diagram which shows 3rd Embodiment of the optical repeater of this invention. The block diagram which shows 4th Embodiment of the optical repeater of this invention. The block diagram which shows 5th Embodiment of the optical repeater of this invention. Explanatory drawing which showed the reason for making wavelength interval of excitation light 6 nm or more. Explanatory drawing which showed the reason which makes wavelength interval of excitation light 35 nm or less. Explanatory drawing which showed the reason for making the difference of the maximum wavelength and minimum wavelength of excitation light into 100 nm or less. Explanatory drawing which showed the example of the wavelength arrangement | sequence of the excitation light in bidirectional | two-way excitation. Explanatory drawing which showed a mode that the pump gain power was controlled and the band gain was flattened while the pump gain power was made constant. Explanatory drawing which showed the characteristic relevant to the design of an optical repeater. Explanatory drawing which showed the relationship between the gain fluctuation of EDFA, and flatness deterioration. The explanatory view which showed the mode of compensation of the DCF loss variation by the variable attenuator while showing the mode of compensation of the input level variation by the variable attenuator. Explanatory diagram showing characteristics related to the design of an optical repeater using the DCF Raman amplification effect. Explanatory drawing which showed the mode of compensation of the input level fluctuation | variation by a Raman amplification effect, and the mode of compensation of the DCF loss fluctuation | variation by a Raman amplification effect. Explanatory drawing which showed the example from which the output spectrum by a Raman amplifier differs. Explanatory drawing which showed the wavelength dependence of the gain by EDFA. Explanatory drawing which showed the fluctuation | variation of the gain by EDFA. Explanatory drawing which showed the wavelength dependence of the gain by Raman amplification. The block diagram of the control method which monitors input light and controls output optical power. The block diagram of the control method which monitors output light and controls output light power. The block diagram of the control method which monitors input light and output light and controls output light power. The block diagram which showed the 1st example of the method of transmitting the residual excitation light of a Raman amplifier to an optical fiber transmission line, and obtaining a Raman gain. The block diagram which showed the 2nd example of the method of transmitting the residual excitation light of a Raman amplifier to an optical fiber transmission line, and obtaining a Raman gain. The block diagram which showed the 3rd example of the method of transmitting the residual excitation light of a Raman amplifier to an optical fiber transmission line, and obtaining a Raman gain. The block diagram which showed the 4th example of the method of transmitting the residual excitation light of a Raman amplifier to an optical fiber transmission line, and obtaining a Raman gain. The block diagram which showed the 1st example of the method of utilizing the residual excitation light of a Raman amplifier as excitation light of EDFA. The block diagram which showed the 2nd example of the method of utilizing the residual excitation light of a Raman amplifier as excitation light of EDFA. The block diagram which showed the 3rd example of the method of utilizing the residual excitation light of a Raman amplifier as excitation light of EDFA. The block diagram which showed the 4th example of the method of utilizing the residual excitation light of a Raman amplifier as excitation light of EDFA. Explanatory drawing which showed deterioration of the noise figure by the dispersion compensation fiber insertion. Explanatory drawing which showed the excitation wavelength number of the Raman amplifier, and the characteristic of the repeater. Explanatory drawing which showed the excitation wavelength number of the Raman amplifier, and the characteristic of the repeater. The block diagram of the optical repeater formed by connecting a plurality of Raman amplifiers in multiple stages. The block diagram which showed an example of the excitation light generation means which has a single excitation light source. The block diagram which showed the other example of the excitation light generation means which has a single excitation light source. The block diagram which showed an example of the excitation light generation means which has two excitation light sources. The block diagram which showed the other example of the excitation light generation means which has two excitation light sources. The block diagram of the Raman amplifier which uses the fiber for dispersion compensation as the fiber for amplification. The block diagram which showed an example of the conventional optical repeater. 46 is an explanatory diagram showing an optical level diagram in the optical repeater of FIG. 46. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pumping light generation means 2 Optical fiber 3 Fabry-Perot type semiconductor laser 4 Output light power control means 5 External resonator 6 Polarization synthesizer 7 Polarization plane rotation means 8 Optical fiber transmission line 9 Raman amplifier 10 Rare earth doped fiber amplifier

Claims (11)

A rare earth-doped fiber amplifier comprising an amplification optical fiber formed including an optical fiber doped with a rare earth element, and having a substantially fixed amplification gain characteristic;
An amplification optical fiber and excitation light generation means for supplying excitation light having one or more center wavelengths to the amplification optical fiber are provided, and the intensity of the excitation light output from the excitation light generation means is controlled. A Raman amplifier having a gain profile that can vary depending on
Control means for controlling the intensity of the output light by adjusting the gain profile of the Raman amplifier;
An optical repeater comprising:   2. The Raman amplifier compensates for input light intensity fluctuation and / or loss of input light in an amplification optical fiber constituting the Raman amplifier, and outputs light having a substantially constant intensity. The optical repeater described in 1.   3. The optical repeater according to claim 1, wherein a plurality of the rare earth-doped fiber amplifiers are arranged, and the Raman amplifier is arranged between the plurality of rare earth-doped fiber amplifiers. Excitation light generating means for forming the Raman amplifier is:
One or more excitation light sources;
An external resonator which is arranged corresponding to the excitation light source and holds the center wavelength at a substantially constant value regardless of the intensity fluctuation of the laser light emitted from the excitation light source;
The optical repeater according to any one of claims 1 to 3, further comprising:   The optical fiber for amplification in the Raman amplifier is formed including a dispersion compensating fiber, and has a function of compensating chromatic dispersion of input light. Optical repeater.   6. The residual light that does not contribute to Raman amplification among pumping light supplied to an optical fiber for amplification in the Raman amplifier is used as pumping light for the rare earth-doped fiber amplifier. The optical repeater as described in one.   Of the pumping light supplied to the amplification optical fiber in the Raman amplifier, residual light that does not contribute to Raman amplification is supplied to the optical transmission line that transmits the signal light, and Raman amplification is performed in the optical transmission line. The optical repeater according to any one of claims 1 to 5. The excitation light generating means includes
An excitation light source including a semiconductor laser that emits laser light having a predetermined center wavelength;
For the laser light emitted from the excitation light source, an external resonator that maintains the center wavelength at a substantially constant value regardless of changes in the drive current of the semiconductor laser;
Laser light intensity control means for adjusting the gain profile in the Raman amplifier by controlling the intensity of the laser light whose central wavelength is held at a substantially constant value by adjusting the drive current;
The optical repeater according to any one of claims 1 to 7, further comprising: The excitation light generation means includes a plurality of excitation light sources that each emit laser light having a predetermined center wavelength,
Each said excitation light source is formed so that the difference value of the center wavelength of the laser beam which has a different wavelength may be 6 nm or more and 35 nm or less between adjacent wavelengths. The optical repeater according to one. The excitation light generating means includes
An excitation light source that emits laser light having a predetermined center wavelength;
An external resonator disposed corresponding to the excitation light source and having a fiber grating;
The optical repeater according to any one of claims 1 to 9, further comprising: As the excitation light generating means,
An optical fiber for amplification, and first pumping light generating means for supplying pumping light having a plurality of center wavelengths to the optical fiber for amplification via a predetermined multiplexer;
The excitation light having one or more central wavelengths and having a single central wavelength located between adjacent central wavelengths in the excitation light supplied from the first excitation light generation means and supplied from the first excitation light generation means Second pumping light generating means for supplying pumping light traveling in a direction opposite to the light to the amplification optical fiber via a multiplexer different from the predetermined multiplexer;
The optical repeater according to any one of claims 1 to 10, further comprising:

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