JP2002072263A - Optical fiber transmission line and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber transmission line and an optical transmission system permitting to further lengthen the relaying distance. SOLUTION: The optical transmission system 1 is provided with an optical fiber transmission line 40 laid between a sending station 10 and a relay center 20, and moreover, an optical fiber transmission line 50 is laid between the relay center 20 and a receiving station 30. The optical transmission line 40 is connected from the sending station 10 to the relay center 20 sequentially via a 1st optical fiber 41, a 2nd optical fiber 42, and a 3rd optical fiber 43. Exciting light for Raman amplification is supplied from the relay center 20 to the 2nd optical fiber 42. An effective sectional area of the 1st optical fiber 41 is larger than that of the 2nd optical fiber 42, and a Raman gain of the 1st optical fiber 41 is smaller than that of the 2nd optical fiber 42, and each core area of the optical fibers 41, 42 is substantially made of pure quartz glass.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical fiber transmission line and an optical transmission system suitable for transmitting a large amount of information by signal light.

[0002]

2. Description of the Related Art An optical transmission system is capable of transmitting a large amount of information at a high speed over a long distance by transmitting signal light through an optical fiber transmission line. Various proposals have been made to increase the distance. For example, in Reference 1, "J.-P. Blondel, et al.," Network Applic
ation and System Demonstration of WDM Systems with
The optical transmission system described in "Very Large Spans", OFC'2000, PD31 (2000) is a wavelength division multiplexing (WDM: Wavelength Division) for wavelength multiplexing multi-wavelength signal light for optical transmission.
Multiplexing) transmission system, wherein an optical fiber amplifier (EDFA: Erbium Doped Fiber A) using an optical fiber doped with an Er element in an optical waveguide region as an optical amplification medium.
mplifier) and a Raman amplifier utilizing the Raman scattering phenomenon. Further, in the optical transmission system described in this document 1, an optical fiber transmission line composed of a low-loss optical fiber is laid in a relay section. An EDFA is provided in the transmitting station, the relay station, or the receiving station, and a means for supplying Raman amplification pump light to the optical fiber transmission line is also provided. By doing so, the relay section can be made longer.

[0003] Reference 2 "T. Naito, et al.," 1 Terabit / s
WDM Transmission over 10,000 km ", ECOC'98, pp.24-
25 (1998) ”and Reference 3“ K. Takashina, et al., “1 Tbit
/ s (100ch ・ 10Gbit / s) WDM Repeaterless Transmission o
ver 200km with Raman Amplifier ", OFC'2000, FC8 (20
The optical transmission system described in “00)” is also a WDM transmission system using both an EDFA and a Raman amplifier. Further, the optical fiber transmission line in the optical transmission system described in these documents has a positive dispersion optical fiber having a large effective area, a low loss, and a positive chromatic dispersion, and a negative chromatic dispersion in a subsequent stage. In addition, a negative dispersion optical fiber for dispersion compensation is connected, and these are laid in a relay section. Then, the transmitting station, the relay station, or the receiving station,
In addition to the provision of the DFA, a means for supplying Raman amplification pump light to the negative dispersion optical fiber is also provided. By doing so, the waveform deterioration of the signal light due to the non-linear optical phenomenon and the accumulated chromatic dispersion is suppressed, and the capacity is increased and the relay section is extended.

[0004]

By the way, compared to a long-distance optical transmission system (for example, a system connecting continents with a submarine optical cable), a medium-distance optical transmission system (for example, a submarine optical cable connects between the mainland and an island). Tying system)
In particular, there is a demand for further extension of the relay section. In other words, in a medium-distance optical transmission system connecting the mainland and the island,
In many cases, a transmitting station, a relay station, or a receiving station is provided only on the mainland or the island, and there is often no relay between the mainland and the island. Therefore, in such an optical transmission system, it is required to increase the length of the relay section according to the distance between the mainland and the island. However, the optical transmission systems described in any of the above-mentioned documents 1 to 3 have a limit in achieving a long relay section.

That is, in the optical transmission system described in Document 1, since the optical fiber transmission line is composed of only one type of optical fiber, from the viewpoint of the efficiency of Raman amplification of signal light in this optical fiber. The optical fiber preferably has a high additive concentration in the core region, or has a small effective area. However, when the additive concentration in the core region of the optical fiber is high, the transmission loss increases due to Rayleigh scattering caused by the additive. Also, if the effective area of the optical fiber is small, nonlinear optical phenomena are likely to occur and the waveform of the signal light is degraded, so that a high power signal light cannot be transmitted. Therefore, the optical transmission system described in Document 1 has a limit in increasing the length of the relay section.

[0006] In the optical transmission systems described in References 2 and 3, the optical fiber transmission line is composed of a positive dispersion optical fiber and a negative dispersion optical fiber. In general, a negative dispersion optical fiber has a high additive concentration in the core region,
Transmission loss is large due to Rayleigh scattering caused by the additive.
Therefore, the optical transmission systems described in References 2 and 3 also have a limit in increasing the length of the relay section.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical fiber transmission line and an optical transmission system capable of further extending a relay section. .

[0008]

An optical fiber transmission line according to the present invention includes at least a first optical fiber and a second optical fiber in a repeater section, and a second optical fiber is provided downstream of the first optical fiber. And a Raman amplification pumping light supply unit for supplying Raman amplification pumping light having a wavelength capable of Raman amplification of signal light propagating through the second optical fiber to the second optical fiber. The first optical fiber has an effective area larger than the effective area of the second optical fiber, the first optical fiber has a Raman gain smaller than the Raman gain of the second optical fiber, The core region of each fiber is substantially made of pure silica glass. Also, the optical transmission system according to the present invention transmits signal light in the order of the first and second optical fibers through the optical fiber transmission line according to the present invention including at least the first and second optical fibers in the relay section. It is characterized by doing.

According to the optical fiber transmission line of the present invention or the optical transmission system of the present invention, the signal light is the first light of the optical fiber transmission line having a relatively large effective area and a relatively small Raman gain. After first propagating through the fiber and reducing the power by propagating through the first optical fiber, it propagates through a second optical fiber having a relatively small effective area and a relatively large Raman gain. Therefore, even if the power of the signal light incident on the optical fiber transmission line is large, the occurrence of the nonlinear optical phenomenon in the optical fiber transmission line is suppressed. Also, the signal light whose power has been reduced by propagating through the first optical fiber is Raman-amplified when propagating through the second optical fiber, and the transmission loss in the second optical fiber is compensated. Furthermore, each of the first optical fiber and the second optical fiber has a small transmission loss because the core region is substantially made of pure silica glass.

Further, the optical fiber transmission line according to the present invention comprises:
The maximum value of the relative refractive index difference in the core region of each of the first and second optical fibers is in the range of -0.1% to + 0.1% based on the refractive index of pure silica glass. In this case, each of the first and second optical fibers has a small amount of addition even if the core region contains not pure silica glass but an additive, and the refractive index of the pure silica glass is small. It is preferable that the maximum value of the relative refractive index difference in the core region with respect to the index be within the above numerical range, since the transmission loss is sufficiently low.

Further, the optical fiber transmission line according to the present invention comprises:
A third optical fiber is further provided downstream of the second optical fiber, and the Raman amplification pumping light supply means supplies Raman amplification pumping light also to the third optical fiber. In this case, the signal light further propagates through the third optical fiber after propagating through the second optical fiber, and the signal light is also Raman-amplified when propagating through the third optical fiber.
At this time, it is preferable that the sum of the lengths of the first and second optical fibers is longer than the length of the third optical fiber. By doing so, the entire transmission loss of the optical fiber transmission line can be made sufficiently small, and the signal light can be sufficiently Raman amplified in the second optical fiber and the third optical fiber. .

[0012]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a configuration diagram of an optical transmission system 1 according to the present embodiment. The optical transmission system 1 shown in FIG.
It includes a transmitting station 10, a relay station 20, and a receiving station 30, an optical fiber transmission line 40 is laid between the transmitting station 10 and the relay station 20, and an optical fiber transmission line is provided between the relay station 20 and the receiving station 30. A fiber transmission line 50 is laid. For example, the transmitting station 10 and the relay station 20 are provided on the mainland, and the receiving station 30
Are provided on the island, the optical fiber transmission line 40 is provided on the seabed along the coast of the mainland, and the optical fiber transmission line 50 is provided on the seabed between the mainland and the island. Is a so-called festoon system.

The transmitting station 10 has a transmitter 11 and an EDF
A12 is provided. The transmitter 11 wavelength-multiplexes and outputs, for example, multi-wavelength signal light having a wavelength of 1.55 μm. The EDFA 12 collectively optically amplifies the multi-wavelength signal light output from the transmitter 11 and sends the optically amplified signal light to the optical fiber transmission line 40.

The optical fiber transmission line 40 has a first optical fiber 41, a second optical fiber 42, and a third optical fiber 43 connected in this order. EDF of transmitting station 10
The signal light transmitted from the A12 is transmitted to the first optical fiber 4
1, second optical fiber 42 and third optical fiber 43
Are sequentially transmitted to the relay station 20.

The relay station 20 is provided with an optical coupler 21, an excitation light source 22, a dispersion compensating optical fiber 23, and an EDFA 24. The optical coupler 21 outputs the Raman amplification pumping light output from the pumping light source 22 toward the third optical fiber 43 of the optical fiber transmission line 40, and also transmits the signal light that has reached the optical fiber transmission line 40. Is output to the dispersion compensating optical fiber 23. The pump light source 22 outputs pump light for Raman amplification having a wavelength capable of Raman-amplifying the signal light propagating through the second optical fiber 42 and the third optical fiber 43. When the signal light has a wavelength of 1.55 μm, the wavelength of the Raman amplification pump light is 1.4.
It is about 5 μm. The dispersion compensating optical fiber 23 compensates for the chromatic dispersion of the optical fiber transmission line 40 including the first optical fiber 41, the second optical fiber 42, and the third optical fiber 43. The EDFA 24 optically amplifies the signal light that has arrived after propagating through the dispersion compensating optical fiber 23,
The optically amplified signal light is transmitted to the optical fiber transmission line 50.

The optical fiber transmission line 50 has a first optical fiber 51, a second optical fiber 52, and a third optical fiber 53 connected in order. EDF of relay station 20
The signal light transmitted from the A24 is transmitted to the first optical fiber 5
1, second optical fiber 52 and third optical fiber 53
To the receiving station 30 in order.

The receiving station 30 is provided with an optical coupler 31, an excitation light source 32, a dispersion compensating optical fiber 33, and a receiver 34. The optical coupler 31 couples the Raman amplification pump light output from the pump light source 32 to the third optical fiber transmission line 50.
To the optical fiber 53, and the signal light that has reached the optical fiber transmission line 50 and arrived therefrom is output to the dispersion compensating optical fiber 33. The pumping light source 32 outputs Raman amplification pumping light having a wavelength capable of Raman-amplifying signal light propagating through the second optical fiber 52 and the third optical fiber 53. When the signal light is in the 1.55 μm band, the wavelength of the Raman amplification pump light is 1.45.
It is about μm. The dispersion compensating optical fiber 33 compensates for chromatic dispersion of the optical fiber transmission line 50 including the first optical fiber 51, the second optical fiber 52, and the third optical fiber 53. The receiver 34 demultiplexes the multi-wavelength signal light that has arrived after propagating through the dispersion compensation optical fiber 33 and receives the demultiplexed signal light of each wavelength.

Each of the optical fiber transmission lines 40 and 50 will be further described. Each of the first optical fiber 41 and the second optical fiber 42 of the optical fiber transmission line 40 and the first optical fiber 51 and the second optical fiber 52 of the optical fiber transmission line 50 have a core region substantially. It is made of pure quartz glass, and the element F is added to the cladding region. Each of such optical fibers 41, 42, 51, and 52 has a smaller transmission loss and is superior in hydrogen resistance and radiation resistance as compared with an optical fiber in which GeO ₂ is added to the core region. Further, even if these optical fibers 41, 42, 51 and 52 each contain an additive (eg, Ge, F, Cl, P, B, etc.) in the core region, the amount of the additive is very small. If the maximum value of the relative refractive index difference in the core region with respect to the refractive index of pure silica glass is in the range of −0.1% to + 0.1%,
This is preferable because the transmission loss is sufficiently low.

The third optical fiber 43 of the optical fiber transmission line 40 and the third optical fiber 5 of the optical fiber transmission line 50
Each of the core regions 3 may be substantially composed of pure quartz glass, but it is preferable that the core region contains an additive such as GeO ₂ . In the latter case,
The Raman gain of the optical fibers 43 and 53 is large.

The effective area of the first optical fiber 41 of the optical fiber transmission line 40 is represented by A ₄₁ , the Raman gain is represented by G _41, and the length is represented by L ₄₁ . The effective area of the second optical fiber 42 is represented by A ₄₂ , the Raman gain is represented by G _42, and the length is L
_{Expressed as 42} . The length of the third optical fiber ₄₃ is represented by L43. Then, there is a relation between these parameters: A ₄₁ > A ₄₂ (1a) G ₄₁ <G ₄₂ (1b) L ₄₁ + L ₄₂ > L ₄₃ (1c)

The effective area of the first optical fiber 51 of the optical fiber transmission line 50 is represented by A _51, and the Raman gain is represented by G
₅₁ and represents, represents a length of L _51. The effective area of the second optical fiber 52 is represented by A ₅₂ , the Raman gain is represented by G _52, and the length is represented by L ₅₂ . The length of the third optical fiber ₅₃ is represented by L53. Then, there is a relationship between these parameters: A ₅₁ > A ₅₂ (2a) G ₅₁ <G ₅₂ (2b) L ₅₁ + L ₅₂ > L ₅₃ (2c)

In the optical transmission system 1 according to the present embodiment,
The signal light output from the transmitter 11 of the transmitting station 10 is ED
The light is amplified by the FA 12 and transmitted to the optical fiber transmission line 40. The signal light output from the EDFA 12 propagates through the first optical fiber 41, the second optical fiber 42, and the third optical fiber 43 of the optical fiber transmission line 40 in order, and goes to the relay station 20. The second optical fiber 42 and the third optical fiber 43 are supplied with Raman amplification pumping light by the optical coupler 21 and the pumping light source 22 of the relay station 20, and propagate through these optical fibers 42 and 43. At this time, the signal light is Raman-amplified.

The signal light arriving at the relay station 20 propagates through a dispersion compensating optical fiber 23 via an optical coupler 21 and
The light is amplified by the FA 24 and transmitted to the optical fiber transmission line 50. The signal light output from the EDFA 24 propagates through the first optical fiber 51, the second optical fiber 52, and the third optical fiber 53 of the optical fiber transmission line 50 in order and goes to the receiving station 30. The second optical fiber 52 and the third optical fiber 53 are supplied with Raman amplification pumping light by the optical coupler 31 and the pumping light source 32 of the receiving station 30, and propagate through these optical fibers 52 and 53. At this time, the signal light is Raman-amplified. Then, the signal light that has reached the receiving station 30 propagates through the dispersion compensating optical fiber 33 via the optical coupler 31 and is thereafter received by the receiver 34.

In this embodiment, the optical fiber transmission line 40 satisfies the above relational expressions (1a) to (1c) and the optical fiber transmission line 5
Since 0 satisfies the above relational expressions (2a) to (2c) and the core regions of the optical fibers 41, 42, 51 and 52 are substantially made of pure silica glass, the following effects are obtained.

That is, the signal light transmitted from the transmitting station 10 first propagates through the first optical fiber 41 having a relatively large effective area in the optical fiber transmission line 40, and
After the power is reduced by propagating through the optical fiber 41, the light propagates through the second optical fiber 42 having a relatively small effective area. Therefore, even if the power of the signal light entering the optical fiber transmission line 40 from the transmission station 10 is large, the occurrence of the nonlinear optical phenomenon in the optical fiber transmission line 40 is suppressed. The same applies to the optical fiber transmission line 50.

The signal light transmitted from the transmitting station 10 first propagates through the first optical fiber 41 having a relatively small Raman gain in the optical fiber transmission line 40, and then propagates through the first optical fiber 41. Then, after the power is reduced, the light propagates through the second optical fiber 42 having a relatively large Raman gain and further propagates through the optical fiber 43. The second optical fiber 42 and the third optical fiber 43 are supplied with Raman amplification pump light from the relay station 20. Therefore, the signal light whose power has been reduced by propagating through the first optical fiber 41 is transmitted to the second optical fiber 42 and the third
Raman amplification when propagating through the optical fiber 43
The transmission loss in the optical fiber 42 and the third optical fiber 43 is compensated. The same applies to the optical fiber transmission line 50.

Each of the first optical fiber 41 and the second optical fiber 42 of the optical fiber transmission line 40 has a core region substantially made of pure silica glass (or a refractive index of pure silica glass as a reference). The maximum value of the relative refractive index difference in the core region is in the range of −0.1% to + 0.1%), and the F element is added to the cladding region, so that the transmission loss is small, and the resistance to hydrogen and the resistance to hydrogen are improved. Excellent radiation characteristics. The same applies to the optical fiber transmission line 50.

Further, since the third optical fiber 43 having a large Raman gain is provided at the subsequent stage of the second optical fiber 42 of the optical fiber transmission line 40, the relay station 20 is transmitted after propagating through the optical fiber transmission line 40. Can be further recovered. However, if the third optical fiber 43 is too long, the loss of the signal light propagating through the third optical fiber 43 increases, and the third optical fiber 43
, The loss of the Raman amplification pumping light is increased. Therefore, as shown in the above equation (1c), the sum of the lengths of the first optical fiber 41 and the second optical fiber 42 (L ₄₁ +
L ₄₂₎ is set so as to be longer than the length L ₄₃ of the third optical fiber 43. By doing so, the entire transmission loss of the optical fiber transmission line 40 can be sufficiently reduced, and the signal light is sufficiently Raman-amplified in the second optical fiber 42 and the third optical fiber 43. be able to. The same applies to the optical fiber transmission line 50.

As described above, the optical transmission system 1 and the optical fiber transmission lines 40 and 50 according to the present embodiment have a longer relay section between the transmitting station 10 and the relay station 20 than the conventional one. And the relay station 20
The relay section between the mobile station and the receiving station 30 can be made longer. Therefore, the optical transmission system 1 according to the present embodiment can be suitably used as, for example, a festoon system.

Next, examples of the optical fiber transmission lines 40 and 50 according to the present embodiment will be described. FIG. 2 is a table summarizing the specifications of the optical fiber transmission lines of the first to third embodiments. FIG. 3 is a table summarizing the specifications of the optical fiber transmission lines of the fourth and fifth embodiments. Each of the optical fiber transmission lines of the first to third embodiments does not include the third optical fiber, and is configured by connecting the first and second optical fibers. Each of the optical fiber transmission lines of the fourth and fifth embodiments includes a third optical fiber in which GeO ₂ is added to a core region, and is configured by connecting first to third optical fibers in order. In these figures, for each optical fiber, the relative refractive index difference of the core region with reference to the refractive index of pure silica glass, the effective area, and the Raman amplification at a wavelength of 1480 nm and a power of 20 dBm for an optical fiber having a length of 10 km. Raman gain and transmission loss when the pump light for use is incident are described. FIG. 2 also shows the total loss when the first and second optical fibers are connected at a length ratio of 1: 1. FIG. 3 also shows the overall loss when the first to third optical fibers are connected at a length ratio of 2: 2: 1.

In the first embodiment, the first optical fiber has a relative refractive index difference of 0% in the core region and an effective area of 105 μm.
m ² , the Raman gain is 0.7 dB, and the transmission loss is 0.160 dB / km. The second optical fiber is
The relative refractive index difference of the core region is 0%, and the effective area is 80%.
μm ² , the Raman gain is 1.0 dB, and the transmission loss is 0.175 dB / km. An optical fiber transmission line in which the first and second optical fibers are connected at a length ratio of 1: 1 has an average transmission loss of 0.168 d.
B / km.

In the second embodiment, the first optical fiber has a relative refractive index difference of + 0.06% in the core region, an effective area of 110 μm ² , a Raman gain of 0.6 dB,
The transmission loss is 0.161 dB / km. The second optical fiber has a relative refractive index difference of 0% in the core region, an effective area of 80 μm ² , a Raman gain of 1.0 dB, and a transmission loss of 0.175 dB / km. And
An optical fiber transmission line in which the first and second optical fibers are connected at a length ratio of 1: 1 has an average transmission loss of 0.168 dB / km as a whole.

In the third embodiment, the first optical fiber has a relative refractive index difference of + 0.06% in the core region, an effective area of 110 μm ² , a Raman gain of 0.6 dB,
The transmission loss is 0.161 dB / km. The second optical fiber has a relative refractive index difference of −0.05% in the core region,
The effective area is 80 μm ² and the Raman gain is 1.0 d
B and the transmission loss is 0.175 dB / km. An optical fiber transmission line in which the first and second optical fibers are connected at a length ratio of 1: 1 has an average transmission loss of 0.168 dB / km as a whole.

In the fourth embodiment, the first optical fiber has a relative refractive index difference of + 0.06% in the core region, an effective area of 110 μm ² , a Raman gain of 0.6 dB,
The transmission loss is 0.161 dB / km. The second optical fiber has a relative refractive index difference of 0% in the core region, an effective area of 80 μm ² , a Raman gain of 1.0 dB, and a transmission loss of 0.175 dB / km. The third optical fiber has a relative refractive index difference of 0.35% in the core region, an effective area of 80 μm ² , and a Raman gain of 1.
8 dB, and the transmission loss is 0.200 dB / km. The first to third optical fibers have a length ratio of 2: 2:
The optical fiber transmission line connected by 1 has an average transmission loss of 0.174 dB / km as a whole.

In the fifth embodiment, the first optical fiber has a relative refractive index difference of + 0.06% in the core region, an effective area of 110 μm ² , a Raman gain of 0.6 dB,
The transmission loss is 0.161 dB / km. The second optical fiber has a relative refractive index difference of −0.05% in the core region,
The effective area is 80 μm ² and the Raman gain is 1.0 d
B and the transmission loss is 0.175 dB / km. The third optical fiber has a relative refractive index difference of 0.8% in the core region, an effective area of 45 μm ² , a Raman gain of 3.5 dB, and a transmission loss of 0.210 dB / km. . The first to third optical fibers have a length ratio of 2:
An optical fiber transmission line connected at 2: 1 has an average transmission loss of 0.176 dB / km as a whole.

In any of the first to fifth embodiments, the effective area of the first optical fiber is larger than the effective area of the second optical fiber, and the Raman gain of the first optical fiber is smaller than that of the second optical fiber. The core region of each of the first and second optical fibers is substantially made of pure silica glass (or the relative refractive index difference is in the range of -0.1% to + 0.1%), The transmission loss of each of the first and second optical fibers is small. In all of Examples 1 to 5, the average transmission loss of the entire optical fiber transmission line is small. As in Embodiments 4 and 5, even in the case where a third optical fiber doped with GeO _{2 in} the core region is further provided in addition to the first and second optical fibers, the third optical fiber as a whole is Since the length ratio of the fiber is small, the average transmission loss of the entire optical fiber transmission line is also small.

[0038]

As described above in detail, according to the optical fiber transmission line according to the present invention or the optical transmission system according to the present invention, the signal light has a relatively large effective area in the optical fiber transmission line. After first propagating through a first optical fiber having a relatively small Raman gain and reducing the power by propagating through the first optical fiber, a second optical fiber having a relatively small effective area and a relatively large Raman gain is used. Propagates the optical fiber. Therefore, even if the power of the signal light incident on the optical fiber transmission line is large, the occurrence of the nonlinear optical phenomenon in the optical fiber transmission line is suppressed. The signal light whose power has been reduced by propagating through the first optical fiber is
Is propagated through the optical fiber, and the transmission loss in the second optical fiber is compensated. Furthermore, the first
Each of the optical fiber and the second optical fiber has a small transmission loss because the core region is substantially made of pure silica glass. As described above, the optical transmission system or the optical fiber transmission line according to the present invention can increase the length of the relay section between the relay station (or the transmission station) and the relay station (or the reception station) as compared with the conventional one. Can be planned, for example festoon
It can be suitably used as a system.

Further, the maximum value of the relative refractive index difference in the core region of each of the first and second optical fibers is in the range of -0.1% to + 0.1% based on the refractive index of pure silica glass. Is preferred. In this case, the first and second
Each optical fiber has a small amount of addition even if the core region contains additives instead of pure silica glass, and the relative refractive index in the core region is based on the refractive index of the pure silica glass. If the maximum value of the difference is within the above numerical range, the transmission loss is sufficiently low.

Further, a third optical fiber is further provided after the second optical fiber, and the Raman amplification pumping light supply means supplies the Raman amplification pumping light also to the third optical fiber. It is suitable. In this case, the signal light further propagates through the third optical fiber after propagating through the second optical fiber, and the signal light is also Raman-amplified when propagating through the third optical fiber. At this time, it is preferable that the sum of the lengths of the first and second optical fibers is longer than the length of the third optical fiber. By doing so, the entire transmission loss of the optical fiber transmission line can be made sufficiently small, and the signal light can be sufficiently Raman amplified in the second optical fiber and the third optical fiber. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical transmission system 1 according to the present embodiment.

FIG. 2 is a table summarizing the specifications of the optical fiber transmission lines of Examples 1 to 3;

FIG. 3 is a table summarizing the specifications of an optical fiber transmission line of each of Embodiments 4 and 5.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical transmission system, 10 ... Transmitting station, 11 ... Transmitter, 1
1 EDFA, 20 relay station, 21 optical coupler, 22
Excitation light source, 23: dispersion compensating optical fiber, 24: EDF
A, 30: receiving station, 31: optical coupler, 32: excitation light source,
33: dispersion compensating optical fiber, 34: receiver, 40: optical fiber transmission line, 41: first optical fiber, 42: second optical fiber, 43: third optical fiber, 50: optical fiber transmission line, 51: first optical fiber, 52: second optical fiber, 53: third optical fiber.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2K002 AA02 AB30 BA01 CA15 DA10 EA08 HA23 5F072 AB07 AB09 AK06 KK30 QQ07 RR01 YY17 5K002 AA06 CA13 FA01 FA02

Claims (5)

[Claims] 1. An optical fiber transmission line including at least a first optical fiber and a second optical fiber in a relay section, wherein the second optical fiber is provided at a stage subsequent to the first optical fiber, wherein the second optical fiber is provided. Raman amplification pumping light supply means for supplying, to the second optical fiber, Raman amplification pumping light having a wavelength capable of Raman-amplifying the signal light propagating through the optical fiber. The area is larger than the effective area of the second optical fiber, the Raman gain of the first optical fiber is smaller than the Raman gain of the second optical fiber,
An optical fiber transmission line, wherein each of the core regions of the first and second optical fibers is substantially made of pure silica glass. 2. A maximum relative refractive index difference in a core region of each of the first and second optical fibers based on a refractive index of pure silica glass in a range of −0.1% to + 0.1%. 2. The optical fiber transmission line according to claim 1, wherein: 3. A third optical fiber is further provided downstream of the second optical fiber, and the Raman amplification pumping light supply means supplies the Raman amplification pumping light to the third optical fiber. The optical fiber transmission line according to claim 1, wherein the optical fiber transmission line is supplied. 4. The optical fiber transmission line according to claim 3, wherein the sum of the lengths of the first and second optical fibers is longer than the length of the third optical fiber. 5. The optical fiber transmission line according to claim 1, wherein the relay section includes at least first and second optical fibers, and the signal light is transmitted in the order of the first and second optical fibers. Optical transmission system.