JP4777945B2 - Raman amplification optical fiber and Raman amplification optical transmission system - Google Patents

Description

  The present invention relates to an optical fiber for Raman amplification and a Raman amplification optical transmission system.

  In recent years, as an optical amplification technique in an optical communication system, a technique using Raman amplification characteristics, which is one of optical nonlinear phenomena in optical fibers, has been widely studied. The Raman amplification in the optical fiber can be realized in an arbitrary wavelength band by selecting the excitation wavelength (see, for example, Non-Patent Document 1), and the Raman amplification characteristic is the intensity of the excitation light and the amplification medium. It varies depending on optical fiber parameters, for example, the core additive material, the additive amount, the effective cross-sectional area, and the like (for example, see Non-Patent Document 2).

  The optical fiber parameters of the optical fiber are uniquely determined by the refractive index distribution of the optical fiber. For this reason, Non-Patent Document 3 reports the feasibility of high gain Raman amplification with a highly nonlinear optical fiber that improves the germanium addition concentration of the optical fiber core and reduces the effective area. Non-Patent Document 4 discloses a design condition of an optical fiber suitable for distributed Raman amplification using the optical fiber transmission line itself as an amplification medium in consideration of the dependency of Raman amplification characteristics on various optical fiber parameters. Yes.

  On the other hand, the optical nonlinear phenomenon in the optical fiber is proportional to the light intensity in the optical fiber, and is sequentially generated and accumulated in the propagation direction of the optical fiber. Accordingly, it is possible to control the optical nonlinearity in the optical fiber propagation direction. For this reason, for example, in Patent Document 1, by changing the relative refractive index difference and the core diameter of the core in the optical fiber propagation direction, the Brillouin shift frequency becomes non-uniform, reducing the accumulation of Brillouin scattering in the optical fiber propagation direction, A technique for improving the intensity of input light to the optical fiber is disclosed.

  Further, Non-Patent Document 5 discloses an effect of improving a signal-to-noise ratio (SNR) in the case where an optical transmission line is configured by a single mode optical fiber having a uniform refractive index distribution (hereinafter referred to as “SNR”). , Referred to as SNR improvement effect).

JP-A-10-96828 S. Namiki, Y. Emori, "Ultrabroad-Band Raman Amplifiers Pumped and Gain-Equalized by Wavelength-Division-Multiplexed High-Power Laser Diodes", IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL.7, NO.1, 2001 Year, p.3-16 STDavey, D, L, Williams, BJAinslie, WJMRothwell, B. Wakefield, "Optical gain spectrum of GeO2-SiO2 Raman fiber amplifiers", IEE PROCEEDINGS, Vol.136, NO.6, 1989, p.301- 306 T.Miyamoto, M.Tanaka, J.Kobayashi, T.Tsuzaki, M.Hirano, T.Okuno, M.Kakui, M.Shigematsu, "Highly Nonlinear Fiber-Based Lumped Fiber Raman Amplifier CWDM Transmission Systems", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.23, NO.11, 2005, p.3475-3483 C.Fukai, K.Nakajima, J.Zhou, K.Tajima, K.Kurokawa, I.Sankawa, "Optimum Optical Fiber Design for a DRA-Based DWDM Transmission System", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.23, NO. 3, 2005, p.1232-1238 R.J.Essiambre, P.Winzer, J.Bromage, C.H.Kim, "Design of Bidirectionally Pumped Fiber Amplifiers Generating Double Rayleigh Backscattering", IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.14, NO.7, 2002, p.914-916

  However, the conventional optical fiber has a problem that the Raman amplification characteristics and the SNR improvement effect in the Raman amplification optical transmission system using the optical fiber are not sufficiently improved.

  The present invention has been made in view of such a problem, and an object thereof is to provide Raman amplification light that can improve a signal-to-noise ratio (SNR) in an optical transmission system using Raman amplification. An object of the present invention is to provide a Raman amplification optical transmission system using the fiber and the Raman amplification optical fiber.

The Raman amplification optical fiber according to the first invention has a refractive index distribution that continuously changes in the propagation direction of signal light, has a zero dispersion wavelength in an arbitrary signal wavelength band, and is uniform in the propagation direction. An optical fiber having excellent chromatic dispersion characteristics, wherein the Raman gain factor continuously changes in the propagation direction, and the Raman gain factors at the incident end and the outgoing end of the optical fiber are different.

The optical fiber for Raman amplification according to the second invention is an optical fiber having a refractive index distribution that continuously changes in the propagation direction of the signal light, and having a negative chromatic dispersion characteristic in an arbitrary signal wavelength band, The Raman gain factor continuously changes in the propagation direction, and the Raman gain factors at the entrance end and the exit end of the optical fiber are different.

A Raman amplification optical transmission system according to a third aspect of the invention is a Raman amplification optical fiber according to the first or second aspect of the invention, an optical fiber having a uniform refractive index distribution in the propagation direction of signal light, and the Raman amplification. And a Raman pumping light source for injecting pumping light propagating in the opposite direction to the propagation direction of the signal light into the Raman amplification optical fiber. .

  According to the above-described Raman amplification optical fiber according to the present invention, it is possible to improve the SNR improvement effect in the Raman amplification optical transmission system by suitably controlling the Raman gain factor in the propagation direction of the signal light. Play.

  Further, in the Raman amplification optical fiber and the Raman amplification optical transmission system of the present invention, the zero dispersion wavelength characteristic in an arbitrary signal wavelength band is also satisfied at the same time, so that the high-speed optical transmission characteristic in the wavelength band is also suitable. Also play.

  Further, in the Raman amplification optical fiber and the Raman amplification optical transmission system of the present invention, since the negative chromatic dispersion characteristic in an arbitrary signal wavelength band is also satisfied at the same time, the accumulation in the single mode optical fiber in the use wavelength band is achieved. It is also possible to reduce dispersion and to achieve high speed optical transmission characteristics using the single mode optical fiber.

  Furthermore, in the optical fiber for Raman amplification according to the present invention, the refractive index distribution in the propagation direction is continuously controlled. Therefore, input due to Brillouin scattering, which is a problem in high input optical transmission systems and wavelength division multiplexing optical transmission systems. There is also an effect of reducing transmission characteristic deterioration due to light intensity limitation, four-wave mixing and the like.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a Raman amplification optical transmission system according to the present embodiment. As shown in FIG. 1, the Raman amplification optical transmission system of this embodiment has an optical transmission line 1 having a total length L. The optical transmission line 1 has a single-mode optical fiber F ₁ having a length L ₁ having a uniform refractive index distribution in the propagation direction of signal light, and the refractive index distribution continuously changing in the propagation direction of signal light. Thus, the optical fiber F ₂ for Raman amplification having a length L ₂ in which the Raman gain factor is continuously changed with respect to the propagation direction of the signal light.

Further, the Raman amplification optical fiber F ₂ includes a Raman excitation light source (Pump) 3, an erbium-doped fiber amplifier (EDFA) 4, and a light receiving unit 5 through a circulator (OC) 2 and the like. There are connected, excitation light from the Raman pump light source 3 is adapted to be incident through the circulator 2 to the Raman amplification optical fiber F _2, i.e. the signal light propagating direction in the opposite direction.

Here, the optical transmission line 1 having the entire length L is composed of only the single mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction, that is, the length of the single mode optical fiber F ₁ . Assuming that L ₁ = L and the length L _{2 of the} Raman amplification optical fiber F ₂ , the SNR improvement effect R _{SNR (F1)} associated with the presence or absence of the Raman pumping light from the Raman pumping light source 3 is not patented. Based on the description in Document 5, etc., it can be defined by the following equation (1).

Here, NF _{pas (F1)} , NF _{act (F1)} , and R _{NL (F1)} indicate the noise figure in the absence of Raman excitation light, the noise figure in the presence of Raman excitation light, and nonlinearity, respectively. These can be represented by the following formulas (2), (3), and (4), respectively.

_{Here, NF EDFA, hν, B e} , B s, and P _in denotes the noise figure of each erbium-doped fiber amplifier 4, the photon energy, electrical filter band, the optical filter band, and the incident signal power, the optical transmission line 1 does not depend on the optical fiber parameter used. _{Further, γ, L eff, G R} , T F, N ASE, and P _DRS each optical nonlinearity, the effective interaction length, Raman gain, the transmission loss, noise components of the spontaneous emission light, and double Rayleigh scattering noise Optical fiber parameters such as length L, effective area, nonlinear refractive index, Raman gain coefficient, and Rayleigh scattering coefficient of an optical fiber (here, single-mode optical fiber F ₁ ) used in the optical transmission line 1, It varies depending on the refractive index distribution of the optical fiber.

  Non-Patent Document 4 discloses a technique for describing an optical fiber parameter that changes depending on the refractive index distribution of the optical fiber as a function of the relative refractive index difference Δ of the optical fiber.

The relative refractive index difference Δ of the optical fiber is defined by the following equation (5) using the refractive index n _core of the core of the optical fiber and the refractive index n _SiO2 of pure quartz.

Therefore, by using the techniques disclosed in the equations (1) to (5) and Non-Patent Document 4 and Non-Patent Document 5, the refractive index distribution is uniform in the propagation direction, and the refraction It is possible to obtain the SNR improvement effect R _{SNR (F1)} associated with the presence or absence of Raman pumping light in the optical transmission line 1 using the single mode optical fiber F ₁ having an arbitrary optical fiber parameter uniquely determined from the rate distribution. it can.

On the other hand, the single-mode optical fiber F ₁ having a length L ₁ having a uniform refractive index distribution in the propagation direction of the signal light and the refractive index distribution changes with respect to the propagation direction of the signal light, that is, the propagation of the signal light. The SNR improvement effect associated with the presence or absence of the Raman pumping light in the optical transmission line 1 composed of the Raman amplification optical fiber F ₂ whose optical fiber parameters change with respect to the direction, as shown in FIG. It can be obtained by assuming that the optical fiber F ₂ is composed of n minute segments f ₁ , f ₂ , f ₃ ,..., F _n−1 , f _n .

As a result, the SNR improvement associated with the presence or absence of Raman pumping light in the optical transmission line 1 having the full length L, which is composed of the single mode optical fiber F ₁ and the Raman amplification optical fiber F ₂ of the present embodiment shown in FIG. The effects R _{SNR (F1 + F2)} are respectively the noise figure NF _{pas (F1 + F2)} in the absence of Raman excitation light, the noise figure NF _{act (F1 + F2)} in the presence of Raman excitation light, and the nonlinearity R _NL Evaluation can be made using the following formulas (6), (7), and (8) representing _{(F1 + F2)} .

Here, G _R (i) and T _F (i) are the Raman gain and transmission loss in the segment f _i, respectively, and the Raman gain G _R (m), transmission loss T _F (m) in the m th segment f _m , natural The noise component N _ASE of the emitted light and the double Rayleigh scattering noise P _DRS can be expressed by the following equations (9) to (12), respectively. Also, P (0) represents the incident signal power P _in.

Here, C _R , P _P , α _P , α _S , and R _F are a Raman gain factor, pump power, pump wavelength, loss coefficient at signal wavelength, and Rayleigh scattering coefficient, respectively.

In the above equation (6), by applying the optical fiber parameters in the single mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction, and obtaining the following equation (13) using the above equation, The prior art of the Raman amplification optical transmission system of this embodiment using the single mode optical fiber F ₁ and the Raman amplification optical fiber F ₂ , that is, the single mode optical fiber F ₁ having a uniform refractive index in the propagation direction. It is possible to obtain an SNR improvement effect with respect to the case of using only.

The Raman amplification optical fiber F ₂ is not limited to the configuration shown in FIG. 1, and may be incorporated in the light receiving unit 5 together with the circulator 2, the Raman excitation light source 3, and the erbium doped fiber amplifier 4. Absent. Further, the Raman amplification optical transmission system according to the present embodiment can be configured in a form excluding the erbium-doped fiber amplifier 4.

The Raman amplification optical fiber F ₂ has a zero dispersion wavelength in an arbitrary signal wavelength band and a uniform wavelength dispersion characteristic in the propagation direction of the signal light, or an average in an arbitrary signal wavelength band. Those having a zero dispersion wavelength and having a chromatic dispersion characteristic that changes in the propagation direction of the signal light, or those having a negative chromatic dispersion characteristic in an arbitrary signal wavelength band, and an incident end and an output end of the optical fiber It is preferable to apply a laser whose refractive index distribution is controlled so that the Raman gain factor in is different.

A Raman amplification optical fiber and a Raman amplification optical transmission system according to the first embodiment of the present invention will be described with reference to FIGS. This example is applied to the configuration described in the embodiment shown in FIG. 1 and described above. As the Raman amplification optical fiber F ₂ shown in FIG. 1, the zero dispersion wavelength λ ₀ is set in the wavelength 1550 nm band. It is an example using the optical fiber for Raman amplification which has.

  In this embodiment, as an example, the signal light wavelength and the excitation wavelength are 1550 nm and 1450 nm, respectively, and the total length L of the transmission line in the Raman amplification optical transmission system shown in FIG. 1 is 80 km.

FIG. 3 is a conceptual diagram showing the refractive index distribution in the cross-sectional direction of the Raman amplification optical fiber F ₂ having a zero dispersion wavelength λ ₀ in the wavelength 1550 nm band. Values 61, 62, and 63 shown in FIG. 3 indicate the refractive indexes of the first core portion, the second core portion, and the cladding portion, respectively. The cladding of the Raman amplification optical fiber F ₂ is made of pure quartz, and Δ ₁₁ , Δ ₁₂ , and a ₁₁ , a ₁₂ shown in FIG. 3 are the cladding, ie, the first core with respect to the refractive index of pure quartz. Part, the relative refractive index difference of the second core part, and the radius of the first core part and the second core part. In this embodiment, as an example, the power multiplier α of the refractive index distribution of the first core portion is 2, and the power multiplier α of the refractive index distribution of the second core portion is 1000. Further, the ratio R _a of the first core radius a ₁₁ to the second core radius a ₁₂ (R _a = a ₁₁ / a _{12 in} this embodiment) was set to 0.5.

FIG. 4 shows the relative refractive index difference Δ ₁₁ between the cladding portion and the first core portion when the zero dispersion wavelength λ ₀ is 1550 nm in the Raman amplification optical fiber F ₂ having the refractive index distribution shown in FIG. It is a graph which shows the relationship of the effective area _Aeff in wavelength 1550nm. From Figure 4, if the case where the relative refractive index difference delta ₁₁ of the cladding portion and the first core portion assuming 1.0%, the relative refractive index difference delta ₁₁ of the cladding portion and the first core portion, the clad portion and the second By changing the ratio RΔ of the relative refractive index difference Δ ₁₂ of the core from 0.05 to 0.2, the effective area A _eff is indicated by a broken line arrow while the zero dispersion wavelength λ ₀ is kept at 1550 nm. It can be seen that the change can be made in a region, that is, a region of approximately 31 μm ² to 46 μm ² .

When the ratio RΔ is assumed to be 0.05, the relative refractive index difference Δ ₁₁ between the cladding portion and the first core portion of the Raman amplification optical fiber F ₂ is varied from 0.7% to 1.3%. it makes it can be seen that it is possible to the zero-dispersion wavelength lambda ₀ while the 1550 nm, varying the effective area a _eff up to about 70μm ² ~22μm ^2.

FIG. 5 shows the relative refractive index difference Δ ₁₁ between the cladding part and the first core part and the relative refractive index difference Δ ₁₁ between the cladding part and the second core part in the Raman amplification optical fiber having the refractive index distribution shown in FIG. ₁₂ is a graph showing the relationship of the effective area A _eff at each zero dispersion wavelength λ _{0 with} respect to the relative refractive index difference Δ ₁₁ between the cladding part and the first core part when the ratio RΔ of ₁₂ is 0.05. In this embodiment, the zero dispersion wavelength λ ₀ is assumed to be 1550 nm. However, by using the Raman amplification optical fiber F ₂ having the refractive index distribution shown in FIG. 3, the Raman amplification light as shown in FIG. Within the scope of the cladding portion and the relative refractive index difference delta ₁₁ of the first core of the fiber F ₂ is 0.34% to 1.44%, changing the effective area a _eff from 94Myuemu ² to 22 .mu.m ^2, and zero It can be seen that the dispersion wavelength λ ₀ can be varied from 1350 nm to 1750 nm.

FIG. 6 shows the relative refractive index difference between the cladding portion and the first core portion when the relationship of the effective area A _{eff at} the wavelength of 1550 nm with respect to the relative refractive index difference Δ ₁₁ between the cladding portion and the first core portion shown in FIG. it is a graph showing the relationship between the Raman gain factor C _R with respect to delta _11. From FIG. 6, when the relative refractive index difference delta ₁₁ of the cladding portion and the first core portion assuming 1.0%, the relative refractive index difference delta ₁₁ of the cladding portion and the first core portion, the clad portion and the second core By changing the ratio RΔ of the relative refractive index difference Δ ₁₂ in the range from 0.05 to 0.2, the Raman gain factor C _R is indicated by the broken line arrow in FIG. 6 while the zero dispersion wavelength λ ₀ is kept at 1550 nm. It can be seen that it is possible to change the region shown, ie approximately from 1.0 WKm ⁻¹ to 1.5 WKm ⁻¹ .

When the ratio RΔ is assumed to be 0.05, the zero dispersion wavelength λ is changed by changing the relative refractive index difference Δ ₁₁ between the cladding portion and the first core portion of the optical fiber from 0.7 to 1.3%. ₀ while the 1550 nm, a Raman gain constant C _R generally it can be seen that it is possible to change from 0.6WKm ^-1 to 2.3WKm ^-1.

FIG. 7 shows the relationship between the clad part and the first core part when the relation of the effective area A _eff at each zero dispersion wavelength λ _{0 to} the relative refractive index difference Δ ₁₁ between the clad part and the first core part shown in FIG. it is a graph showing the relationship between the Raman gain factor C _R with respect to the relative refractive index difference delta _11. FIG. 7 shows that the Raman gain factor C _R is about 0.4 WKm ⁻¹ to 2 in the range where the relative refractive index difference Δ ₁₁ between the cladding portion and the first core portion of the optical fiber is 0.34 to 1.44%. It can be seen that it is possible to vary the zero dispersion wavelength λ ₀ from 1350 nm to 1750 nm while changing to 0.7 WKm ⁻¹ .

8 shows the SNR improvement effect R _{SNR (F1 + F2) with} respect to the pumping light power P _P in the Raman amplification optical transmission system according to the present embodiment, and the single mode in the optical transmission line 1 in the optical transmission system shown in FIG. It is a graph showing an example of SNR improvement effect R _{SNR (F1) at} the time of using only optical fiber F1. Incidentally, the zero-dispersion wavelength λ ₀ of the Raman amplification optical fiber F ₂ is 1550 nm, and the single mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction has the refractive index distribution shown in FIG. The relative refractive index difference Δ ₁₁ between the cladding and the first core and the Raman gain factor C _{R at a} wavelength of 1550 nm are 1.0% and 1 as an example with reference to the results of FIGS. .5 WKm- ¹ . Further, the ratio RΔ of the relative refractive index difference Δ ₁₁ between the cladding part and the first core part and the relative refractive index difference Δ ₁₂ between the cladding part and the second core part was set to 0.05.

In this embodiment, the Raman gain factor of the Raman amplification optical fiber F ₂ changes one-dimensionally with respect to the propagation direction of the signal light (in this embodiment, from the signal incident end side to the signal emitting end side). As a result of the change of the refractive index distribution with respect to the propagation direction of the signal light, the Raman gain is changed with respect to the Raman gain ratio C _R-F1 of the single-mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction. The rate of change between the Raman gain factor C _R-F2-in at the signal input end and the Raman gain factor C _R-F2-out at the signal output end of the optical fiber F ₂ for Raman amplification with continuously changing rate is expressed as R _CR As defined in the following equation (14), it was set to 0.5.

It was assumed that one half of the sum of C _R-F2-in and C _R-F2-out is equal to C _R-F1 . Further, the length L _{2 of} the Raman amplification optical fiber F ₂ , the incident signal light power P _in , and the transmission speed B were 20 km, −10 dBm, and 10 Gbit / s, respectively. FIG. 8 shows that the SNR improvement effect R _{SNR (F1)} when only the single mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction is used, and the Raman gain in the single mode optical fiber F ₁ and the propagation direction. The SNR improvement effect R _{SNR (F1 + F2) in} the case of using the Raman amplification optical fiber F _{2 in} which the rate is changed increases as the pumping light power P _P increases, and the pumping light powers are 170 mW and 180 mW, respectively. It can be seen that the SNR improvement effect is maximized at about 6.5 dB and 7.0 dB, respectively.

Therefore, when the Raman amplification optical fiber according to this embodiment is used as the difference between the maximum values of the respective SNR improvement effects R _{SNR (F1)} and R _{SNR (F1 + F2)} , the propagation direction is the same. The SNR improvement effect R _SNR when only an optical fiber having such a refractive index distribution is used is 0.5 dB.

That is, the single-mode optical fiber having a uniform refractive index distribution in the propagation direction by configuring the optical transmission line 1 with the single-mode optical fiber F ₁ and the Raman amplification optical fiber F _2. It can be seen that the SNR improvement effect in the Raman-amplified optical transmission system is further improved as compared with the case where only F ₁ is used.

9 shows an SNR improvement in the Raman amplification optical transmission system having the total length L (= L ₁ + L ₂ ) = 80 km shown in FIG. 1 having the Raman amplification optical fiber satisfying the design conditions shown in FIGS. and effects R _SNR, is a graph showing the relationship between the length L ₂ of the Raman amplification optical fiber F ₂ of the present embodiment. 3 solid lines in the figure shows the calculation results in a case where the Raman gain constant of the change rate R _CR is different. In FIG. 9, in any Raman gain factor change rate R _CR , good SNR is obtained when the length L _{2 of} the Raman amplification optical fiber F ₂ in which the Raman gain factor C _R changes in the propagation direction is in the vicinity of 30 km to 40 km. It can be seen that the improvement effect R _SNR can be obtained.

10 shows the SNR improvement effect R _SNR and the rate of change of the Raman gain factor in the Raman amplification optical transmission system shown in FIG. 1 having the Raman amplification optical fiber F ₂ that satisfies the design conditions shown in FIGS. is a graph showing the relationship between R _CR. The length L ₂ of the Raman amplification optical fiber F ₂ was 35km. From FIG. 10, the SNR improvement effect R _SNR becomes larger than 0 in the region where the change rate R _CR of the Raman gain factor is larger than 0, and the Raman amplification optical fiber F ₂ and the Raman amplification optical fiber F according to the present embodiment. _It can be seen that the SNR improvement effect can be obtained by the Raman amplification optical transmission system using ₂ .

As described above, according to the present embodiment, the Raman amplification optical transmission system is controlled by controlling the Raman gain factor of the Raman amplification optical fiber F ₂ and continuously changing the Raman gain factor in the propagation direction. The effect that the effect of improving the SNR can be obtained is obtained.

In addition, according to the Raman amplification optical fiber F ₂ and the Raman amplification optical transmission system according to the present embodiment, the zero dispersion wavelength characteristic in an arbitrary signal wavelength band is also satisfied at the same time. Therefore, the high-speed optical transmission characteristic in the wavelength band is also achieved. The effect of making it suitable is also show | played.

  Furthermore, in the optical fiber for Raman amplification of this embodiment, the refractive index distribution in the propagation direction is continuously controlled. Therefore, input due to Brillouin scattering, which is a problem in high-input optical transmission systems and wavelength division multiplexing optical transmission systems, is used. There is also an effect of reducing transmission characteristic deterioration due to light intensity limitation, four-wave mixing and the like.

The Raman amplifying optical fiber applied to the present invention is not limited to the Raman amplifying optical fiber F ₂ having a zero dispersion wavelength in the wavelength 1550 nm band of the present embodiment, but any refraction other than that shown in FIG. A rate distribution such as a segment type is applicable.

A Raman amplification optical fiber and a Raman amplification optical transmission system according to a second embodiment of the present invention will be described with reference to FIGS. This example is applied to the configuration described in the embodiment shown in FIG. 1 and described above, and Raman having negative wavelength dispersion in the wavelength 1550 nm band as the Raman amplification optical fiber F ₂ shown in FIG. This is an example using an amplification optical fiber.

In the Raman-amplified optical transmission system according to the present embodiment, a 1.3 μm-band zero-dispersion single-mode light is used as the single-mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction shown in FIG. Using the fiber, the 1.3 μm zero-dispersion single-mode optical fiber F _{1 has} a Raman gain factor C _R , loss factor α _S , and length L ₁ of 0.44 WKm ⁻¹ , 0.2 dB / km, and 80 km. The 1.3 μm-band zero-dispersion single mode optical fiber F _{1 had} a chromatic dispersion D and a dispersion slope S at a wavelength of 1550 nm of 17 ps / nm · km and 0.056 ps / nm · km, respectively.

FIG. 11 is a conceptual diagram showing the refractive index distribution in the cross-sectional direction of the Raman amplification optical fiber F ₂ having negative chromatic dispersion characteristics in the wavelength 1550 nm band used in this example. Values 71, 72, and 73 shown in FIG. 11 indicate the refractive indexes of the first core portion, the second core portion, and the cladding portion, respectively. The clad portion of the Raman amplification optical fiber F ₂ is made of pure quartz. In FIG. 11, Δ ₂₁ , Δ ₂₂ , and a ₂₁ , a ₂₂ are the clad portions, that is, the first core portion with respect to the refractive index of pure quartz, It represents the relative refractive index difference of the second core part, and the radii of the first core part and the second core part.

Here, the ratio of the first core radius a ₂₁ to the second core radius a ₂₂ is defined as R _a (R _a = a ₂₁ / a _{22 in} this embodiment). In this example, the relative refractive index difference Δ ₂₂ between the cladding and the second core was −0.5%.

FIG. 12 shows the cladding portion and the first core when the dispersion-to-slope ratio (RDS) at a wavelength of 1550 nm is 0.0033 nm ^{−1 in the} Raman amplification optical fiber F ₂ having the refractive index distribution shown in FIG. 6 is a graph showing the relationship between the relative refractive index difference Δ _{21 of the portion} and the second core radius a ₂₂ . Here, the dispersion-to-slope ratio (RDS) at an arbitrary wavelength λ is defined by the following equation (15) using the chromatic dispersion Dλ and the dispersion slope Sλ at the wavelength λ.

The dispersion-to-slope ratio (RDS) value used in FIG. 12 is 17 ps / representative value of the chromatic dispersion D and dispersion slope S of the 1.3 μm band zero-dispersion single mode optical fiber F _{1 at} a wavelength of 1550 nm. It was determined from nm · km and 0.056 ps / nm ² · km. By using the Raman amplification optical fiber F ₂ having the relationship between the relative refractive index difference Δ ₂₁ and the second core radius a ₂₂ shown in FIG. 12, the 1.3 μm band zero-dispersion single mode optical fiber in the wavelength 1550 nm band. It is possible to efficiently compensate for the accumulated dispersion in F ₁ .

FIG. 13 is a graph showing the relationship of chromatic dispersion D at a wavelength of 1550 nm with respect to the relative refractive index difference Δ ₂₁ between the cladding portion and the first core portion of the Raman amplification optical fiber F _{2 that} satisfies the design conditions shown in FIG. . As shown in FIG. 13, if the wavelength dispersion D of the Raman amplification optical fiber F _{2 at} the wavelength of 1550 nm is set to −200 ps / nm · km, the ratio _Ra is 0.32 to 0.00 as indicated by the broken line arrow in the figure. In the range of 20, by changing the relative refractive index difference Δ ₂₁ in the region of approximately 1.9% to 3.0%, the wavelength dispersion characteristic at the wavelength of 1550 nm of the Raman amplification optical fiber F ₂ can be maintained. It turns out that it is possible.

FIG. 14 is a graph showing the relationship of the effective area A _{eff at} the wavelength of 1550 nm with respect to the relative refractive index difference Δ ₂₁ between the cladding and the first core of the Raman amplification optical fiber F _{2 that} satisfies the design conditions shown in FIG. It is. 13 and 14, if the wavelength dispersion D of the Raman amplification optical fiber F _{2 at} the wavelength of 1550 nm is set to −200 ps / nm · km, the effective cross-sectional area of the Raman amplification optical fiber F ₂ is shown in FIG. region indicated by broken line arrows in, i.e., generally it is possible to vary from 23 .mu.m ² to 12 [mu] m ^2.

Figure 15 is a graph showing the design satisfies the Raman amplification optical fiber F ₂ of the cladding portion and the first core portion relative refractive index difference delta ₂₁ shown in FIG. 12, the relationship between the Raman gain factor C _R. 13 and 15, assuming that the chromatic dispersion D of the Raman amplification optical fiber F _{2 at} the wavelength of 1550 nm is −200 ps / nm · km, the Raman gain factor C _R in the propagation direction of the Raman amplification optical fiber F ₂ is It is possible to vary the region indicated by the dashed arrow in FIG. 15, that is, approximately 3.1 WKm ⁻¹ to 7.9 WKm ⁻¹ . Further, by setting the length L ₂ of the Raman amplification optical fiber F ₂ at this time to 6.8 km, the accumulation at the wavelength of 1550 nm in the 1.3 μm-band zero-dispersion single mode optical fiber F ₁ having the length of 80 km is performed. The variance can be made zero.

16 shows the SNR improvement effect R _SNR in the Raman amplification optical transmission system shown in FIG. 1 having the Raman amplification optical fiber F ₂ satisfying the design conditions shown in FIGS. 12 to 15, and the Raman amplification optical fiber F. rate of change of the _second Raman gain factor is a graph showing the relationship between R _CR.

Here, in this embodiment, the Raman gain factor of the Raman amplification optical fiber F ₂ is assumed to change one-dimensionally (in this embodiment, it increases from the signal incident end side toward the signal emitting end side). Further, in the case of the conventional technology, that is, when the optical transmission line is configured only by the single mode optical fiber F ₁ having a uniform refractive index distribution in the propagation direction (Raman gain rate change rate R _CR = 0), In order to compensate the accumulated dispersion at the wavelength of 1550 nm of the 1.3 μm-band zero-dispersion single mode optical fiber F ₁ with L ₁ = 80 km, the length L ₂ is used instead of the Raman amplification optical fiber F ₂ with the length L _2. The dispersion compensating fiber having a uniform refractive index profile in the propagation direction is connected.

The chromatic dispersion D, Raman gain factor C _R , and loss factor α _{S at} a wavelength of 1550 nm of the dispersion compensating fiber are, for example, −200 ps / nm · km, 5. 5 WKm ⁻¹ and 0.3 dB / km. The signal light wavelength λ _S and the pumping light wavelength λ _P were 1550 nm and 1450 nm, respectively, the incident signal light power P _in and the transmission speed B were −10 dBm and 10 Gbit / s, respectively.

FIG. 16 shows that the SNR improvement effect in the Raman amplification optical transmission system using the Raman amplification optical fiber F ₂ is improved in the region where the change rate R _CR of the Raman gain optical fiber F ₂ is larger than zero. I understand. In particular, it can be seen that an SNR improvement effect of 1 dB or more can be obtained in a region where the change rate R _CR is 0.48 or more.

As described above, according to this embodiment, the optical transmission line 1 a single mode optical fiber F ₁ and the Raman gain constant C _R to the propagation direction of the signal light continuously changes Raman amplification optical fiber was By configuring with F _2, it is possible to improve the SNR improvement effect in the Raman amplification optical transmission system. In addition, since the negative chromatic dispersion characteristics in any signal wavelength band are also satisfied at the same time, it is possible to reduce the accumulated dispersion in the single mode optical fiber in the used wavelength band, and the high speed using the single mode optical fiber. There is an effect that the optical transmission characteristics are also suitable.

  Further, in the Raman amplification optical fiber according to the present embodiment, the refractive index distribution in the propagation direction is continuously controlled, so that Brillouin scattering, which is a problem in high-input optical transmission systems and wavelength division multiplexing optical transmission systems, is used. In addition, there is an effect that the input light intensity limitation due to, and transmission characteristic deterioration due to four-wave mixing and the like are reduced.

  As described above, according to the present embodiment, a Raman amplification optical fiber that can improve a signal-to-noise ratio in an optical transmission system using Raman amplification, and a Raman amplification light using the Raman amplification optical fiber. A transmission system is obtained.

The Raman amplifying optical fiber applied to the present invention is not limited to the Raman amplifying optical fiber F ₂ having negative chromatic dispersion characteristics in the wavelength 1550 nm band of the present embodiment, but any one other than those shown in FIG. A refractive index distribution of, for example, a multiple clad type is applicable.

  In addition, the present invention is not limited to the above-described embodiments. For example, a Raman amplification optical fiber whose Raman gain factor continuously changes with respect to the propagation direction of the signal light is replaced with a Raman gain on the signal incident end side. Needless to say, various modifications can be made without departing from the spirit of the present invention, such as a configuration in which the Raman gain factor on the signal output end side becomes lower than the ratio.

  INDUSTRIAL APPLICABILITY The present invention is suitable for application to a Raman amplification optical fiber and a Raman amplification optical transmission system that improve the signal-to-noise ratio characteristics of signal light at the output end in optical signal communication using a single mode optical fiber. is there.

It is a figure which shows the structure of the Raman amplification optical transmission system using the optical fiber for Raman amplification which concerns on embodiment of this invention. It is explanatory drawing which shows the concept of the optical fiber parameter change in the propagation direction of the optical fiber for Raman amplification which concerns on embodiment of this invention. It is a conceptual diagram showing the refractive index distribution of the cross-sectional direction of the optical fiber for Raman amplification which has a zero dispersion wavelength characteristic in the wavelength 1550nm band which concerns on Example 1 of this invention. In the Raman amplification optical fiber according to the first embodiment of the present invention having the refractive index distribution shown in FIG. 3, the wavelength 1550 nm relative to the relative refractive index difference Δ ₁₁ between the cladding and the first core when the zero dispersion wavelength is 1550 nm. It is a graph which shows the relationship of effective area _Aeff in. In the Raman amplification optical fiber according to the first embodiment of the present invention having the refractive index distribution shown in FIG. 3, the relative refractive index difference Δ ₁₁ between the cladding part and the first core part, and the ratio between the cladding part and the second core part. It is a graph which shows the relationship of effective area _Aeff in each zero dispersion wavelength with respect to relative refractive index difference (DELTA) ₁₁ of a clad part and a 1st core part when ratio R (DELTA) of refractive index difference (DELTA) ₁₂ is 0.05. In the Raman amplification optical fiber according to the first embodiment of the present invention having the refractive index distribution shown in FIG. 3, Raman relative to the relative refractive index difference Δ ₁₁ between the cladding and the first core when the zero dispersion wavelength is 1550 nm. is a graph showing the relationship between the gain factor C _R. In the Raman amplification optical fiber according to Embodiment 1 of the present invention having a refractive index distribution shown in FIG. 3, a graph showing the relationship between the Raman gain factor C _R with respect to the cladding portion and the first core portion relative refractive index difference delta ₁₁ It is. In the Raman amplification optical transmission system according to a first embodiment of the present invention, the SNR improvement R _SNR with respect to the excitation light power P _P of the case of using the Raman amplification optical fiber of the present invention having a zero-dispersion wavelength characteristics in the wavelength 1550nm band It is a graph which shows an example of a calculation result. In the Raman amplification optical transmission system shown in FIG. 1 having the Raman amplification optical fiber according to the first embodiment of the present invention that satisfies the design conditions shown in FIGS. 4 and 6, the SNR improvement effect R _SNR and the Raman amplification is a graph showing the relationship between the length L ₂ of the optical fiber. In the Raman amplification optical transmission system shown in FIG. 1 having the Raman amplification optical fiber according to the first embodiment of the present invention that satisfies the design conditions shown in FIGS. 4 and 6, the SNR improvement effect R _SNR and the Raman amplification it is a graph showing the relationship between the change in R _CR Raman gain index of the optical fiber. It is a conceptual diagram showing the refractive index distribution of the cross-sectional direction of the optical fiber for Raman amplification which concerns on Example 2 of this invention which has a negative chromatic dispersion characteristic in a wavelength 1550nm band. In the Raman amplification optical fiber having the refractive index distribution shown in FIG. 11, a cladding portion and a first core portion that realize a dispersion-to-slope ratio (RDS) equivalent to a conventional 1.3 μm band zero-dispersion single mode optical fiber Is a graph showing the relationship between the relative refractive index difference Δ ₂₁ and the second core radius a ₂₂ . 12 is a graph showing the relationship of chromatic dispersion D at a wavelength of 1550 nm with respect to the relative refractive index difference Δ ₂₁ between the cladding part and the first core part in the Raman amplification optical fiber according to Example 2 of the present invention that satisfies the design conditions shown in FIG. It is. In the Raman amplification optical fiber according to the second embodiment of the present invention that satisfies the design conditions shown in FIG. 12, the relationship between the effective area A _{eff at} the wavelength of 1550 nm with respect to the relative refractive index difference Δ ₂₁ between the cladding and the first core is expressed as follows. It is a graph to show. In the Raman amplification optical fiber according to the second embodiment of the design satisfies the present invention shown in FIG. 12, a graph showing the relationship between the Raman gain factor C _R with respect to the cladding portion and the first core portion relative refractive index difference delta ₂₁ is there. In the Raman amplification optical transmission system shown in FIG. 1 having the Raman amplification optical fiber according to the second embodiment of the present invention that satisfies the design conditions shown in FIGS. 12 to 15, the SNR improvement effect R _SNR and the Raman amplification use is a graph showing the relationship between the change rate R _CR Raman gain index of the optical fiber.

Explanation of symbols

F1 single mode optical fiber F2 Raman amplification optical fiber 1 optical transmission line 2 circulator 3 Raman pumping light source (Pump)
4 Erbium-doped fiber amplifier (EDFA)
5 Light receiver

Claims (3)

An optical fiber having a refractive index distribution that continuously changes in the propagation direction of signal light , a zero dispersion wavelength in an arbitrary signal wavelength band, and a uniform wavelength dispersion characteristic in the propagation direction,
With Raman gain index varies continuously in the propagation direction, the Raman gain ratio different characteristics and be Lula Man amplification optical fiber at the output end and the incident end of the optical fiber. An optical fiber having a refractive index distribution that continuously changes in the propagation direction of signal light and having negative chromatic dispersion characteristics in an arbitrary signal wavelength band,
With Raman gain index varies continuously in the propagating direction, the Raman gain ratio different characteristics and be Lula Man amplification optical fiber at the output end and the incident end of the optical fiber. The Raman amplification optical fiber according to claim 1 or 2 ,
An optical fiber having a uniform refractive index distribution in the propagation direction of the signal light;
A Raman pumping light source disposed behind the Raman amplification optical fiber and injecting pumping light propagating in a direction opposite to the signal light propagation direction into the Raman amplification optical fiber; A characteristic Raman amplification optical transmission system.

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