JP2004240461A - Measuring device, light transmission system, and raman gain measurement method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable gain efficiency to be measured only by working in one edge of a transmission line. <P>SOLUTION: A measuring device is provided with: a step for applying distributed Raman amplification to an optical fiber transmission path and amplifying and transmitting signal light; an excited light adjusting step for adjusting output of excited light to the optical fiber transmission path; a measuring step for measuring transmission losses of an optical fiber in an outputted state and a suspended state of the excited light by test light according to an optical time domain reflectometry (OTDR); and a calculating step for calculating Raman gain of the optical fiber based on the difference between the transmission losses in the two states. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a measurement device, an optical transmission system, and a Raman gain measurement method.

  In recent years, with the development of low-loss optical fibers and wavelength bands using low-loss wavelength bands and the development of amplification techniques, long-distance transmission using optical fibers has been progressing. Further, in order to perform transmission at a lower cost and more efficiently, it is expected that low-loss repeaterless transmission will be realized in the future, and an optical fiber amplifier using the optical fiber transmission line itself as an amplification medium, such as EDFA, Application of a wider band optical amplification technology is also considered.

  In a communication system using such an optical fiber transmission line, development for commercialization of a technique called Distributed Raman Amplification (DRA) is progressing. Raman amplification means that when signal light and excitation light having a frequency higher than that of the signal light by about 13 THz are simultaneously input to an optical fiber made of silica glass, part of the energy of the excitation light is stimulated by the stimulated Raman scattering phenomenon in the silica glass. Is a phenomenon that shifts to the signal light, that is, the signal light is amplified. The gain due to Raman amplification is hereinafter referred to as Raman gain. Actually, the Raman gain has a wavelength dependence as shown in FIG. 5 with a peak at a wavelength 13 THz lower than the pump light. Hereinafter, this is called a Raman gain profile. FIG. 5 is an explanatory diagram showing a Raman gain profile.

  The distributed Raman amplification is a form in which pump light is input to an optical fiber for transmitting signal light, and a Raman amplification effect is obtained using the optical fiber transmission line itself as an amplification medium. In an optical fiber transmission system to which distributed Raman amplification is applied, the propagation loss in the transmission path is compensated by Raman amplification, so that the transmittable distance can be extended. << Raman gain efficiency >> In the optical transmission system corresponding to the long-distance transmission, it is necessary that the signal light which has received a certain loss from the transmitting station via the optical fiber transmission line keeps a desired input level on the receiving side. Conventionally, the receiving side measures the input level of the transmitted signal light, and sets and adjusts the power of the signal light of the transmitting station or the amplification factor in the relay transmission line.

  Hereinafter, a measurement method in a conventional optical transmission system will be described. Here, the gain efficiency of the optical fiber is used particularly for adjusting the input level of the signal light. The gain efficiency is used as a parameter indicating how much gain is obtained at the measurement point on the receiving side with respect to 1 W of the transmission light source power for each fiber. That is, when 1 W pump light is input to the amplification medium, the Raman gain (dB) received by the signal light propagating through the amplification medium is called Raman gain efficiency (dB / W). Raman gain efficiency varies from one fiber to another. The reason is that the Raman gain efficiency depends on the mode field diameter, the amount of GeO2 added, the absorption of water (OH), and the like, and these are different for each fiber type, manufacturer, manufacturing time, and lot. Because. Therefore, in order to control the gain received by the signal light in distributed Raman amplification in an optical fiber transmission system using an already installed optical fiber transmission line, the characteristics of the transmission line optical fiber and the loss characteristics in the relay station building are required. It is necessary to measure the on-site condition such as Raman gain efficiency.

  << Raman gain efficiency measuring method >> A conventional technique will be described below with reference to the drawings. Conventionally, the measurement of the Raman gain efficiency of distributed Raman amplification using the transmission line itself as an amplification medium has been performed with a configuration as shown in FIG. FIG. 2 is a configuration diagram of a conventional Raman gain and Raman gain efficiency measuring method example. Light that receives Raman gain by pumping light in measuring Raman gain efficiency is called test light. A test light source 120 is provided at one end of the transmission path, and a WDM (Wavelength Division Multiplex) coupler 114 is provided at the other end for multiplexing / demultiplexing the excitation light wavelength and the test light wavelength. The pump light source 140 is provided at the pump light wavelength band port of the WDM coupler 114, and the light receiver 130 for measuring the test light power is provided at the test light wavelength band port. As the light receiver 130, for example, an optical spectrum analyzer, an optical power meter, or the like can be applied. The test light is input to the transmission line, and the power of the test light (referred to as P1) detected by the light receiver while the output of the excitation light source is stopped is measured. Next, the test light power (referred to as P2) detected by the light receiver with the excitation light source output is measured. By subtracting P1 from P2 in decibel display, the Raman gain received by the test light is obtained. By dividing this gain by the pumping light output power, Raman gain efficiency is determined.

  When measuring the Raman gain efficiency, the conventional method requires that a measuring instrument, a light source, and an operator be arranged at both ends of the transmission line to perform the work. There is a need for a means for measuring the gain efficiency only by working at the end of the method. That is, when the above-described conventional method is used, for example, it is necessary to operate and operate the test light source 120 arranged at one end of the laid fiber ahead of the relay section 80 km or more and the light receiver 130 arranged at the other end in conjunction with each other. There is. In addition, since the maintenance person performing the measurement entered the relay station building and operated it manually, procedures such as adjusting the measurement timing, deploying personnel to the relay station building, and moving the measurement equipment were necessary. It was. In addition, there is a problem in that a measurable portion is restricted by an operation mode of each communication carrier, and a flexible measurement section cannot be set. Therefore, the present invention provides means for measuring the gain efficiency only by working at one end of the transmission path.

  In addition, when it is desired to obtain Raman gain efficiency at a desired wavelength for each of a plurality of types of pump wavelengths, test light beams having the same number of different wavelengths as the types of pump wavelengths are required, and there have been problems in terms of cost and versatility. . Therefore, the present invention provides means for obtaining Raman gain efficiency at desired wavelengths for a plurality of types of pump wavelengths using one test light wavelength.

  The present invention relates to a measuring apparatus, an optical transmission system, or a Raman gain measuring method applied to Raman gain of an optical fiber transmission line to which pump light is supplied.

  In the present invention, Raman gain efficiency can be measured based on the OTDR measurement method. The propagation loss from one end of the transmission path to the other end is measured by the OTDR method. FIG. 3 shows a state in which the pulse light input from the end A reaches the vicinity before the end B while being attenuated. FIG. 4 shows a state in which return light generated in the vicinity of the end B is attenuated and reaches the end A. The loss reduced by the input of the pump light is considered to be the Raman gain. As shown in FIGS. 3 and 4, this Raman gain is the same as that of the pump light until the test light reaches near the B-end. Raman gain received while propagating in the direction: FIG. 3 and Raman gain received while propagating in the opposite direction to the pump light until the return light reaches the A end: FIG. The propagation loss (dB) detected from the return light generated near the B-end of the transmission path is set to L1, and OTDR measurement is performed with the pump light output. Assuming that the detected propagation loss (dB) is L2, the magnitude of the Raman gain does not depend on the propagation directions of the test light (return light) and the pump light, and therefore the value obtained by subtracting L1 from L2 is 倍. By doing so, it is possible to obtain the Raman gain received by the test light in the transmission path. By dividing the Raman gain by the pump light output power, the Raman gain efficiency (dB / W) at the OTDR pulse light wavelength can be calculated.

  Further, it is possible to obtain Raman gain efficiency at a desired wavelength for each of a plurality of types of excitation wavelengths from measurement using one test light wavelength. Raman gain efficiencies for a plurality of types of pump wavelengths are measured using one test light wavelength, and are converted into Raman gain efficiencies at desired wavelengths at each pump wavelength using a Raman gain profile.

  By using the present invention, it is possible to measure the gain efficiency only by working at one end of the transmission path. As a result, the Raman gain efficiency in the transmission line can be obtained even when it is difficult to work at both ends of the transmission line at the same time because the relay stations at both ends of the transmission line are separated by several tens of kilometers.

  By using the present invention, Raman gain efficiency at desired wavelengths can be obtained for a plurality of types of pump wavelengths using one test light wavelength. As a result, when it is desired to obtain Raman gain efficiency at a desired wavelength for each of a plurality of types of pump wavelengths, it is not necessary to prepare test lights having the same number of different wavelengths as the types of pump wavelengths. Abundant Raman gain efficiency measurements are possible.

  The present invention relates to a measuring apparatus for measuring Raman gain of an optical fiber in which pump light is supplied to one end of an optical fiber, wherein the first return light power in a state where the pump light is output, and the pump light is stopped. The Raman gain occurring over the entire length of the optical fiber is determined based on the first and second return optical powers at points other than the other end of the optical fiber, where the ratio with the second return optical power in the state becomes constant. A measuring device characterized by performing measurement.

  Further, in the measuring device, it is desirable to adjust the power of the excitation light so that the ratio becomes constant.

  Further, in the measuring device, it is preferable that the return light is return light of test light input to one end of the optical fiber.

  The present invention relates to an optical transmission system having an optical fiber, an input means for inputting pumping light to the optical fiber, a first return light power in a state where the pumping light is output, and a state in which the pumping light is stopped. A Raman gain occurring over the entire length of the optical fiber is measured based on the first and second return optical powers at a point except for the other end of the optical fiber at which a ratio with the second return optical power becomes constant. And a measuring means at one end of the optical fiber.

  Further, in the optical transmission system, it is preferable that an adjusting means for adjusting the power of the pump light so that the ratio becomes constant is provided at one end of the optical fiber.

  Further, in the optical transmission system, it is preferable that the return light is return light of test light input to one end of the optical fiber.

  The present invention provides a step of inputting pump light, a step of measuring a first return light power in a state where the pump light is output, and a step of measuring a second return light power in a state where the pump light is stopped. Obtaining a ratio between the first return light power and the second return light power; and the first and second return at a point other than the other end of the optical fiber where the ratio becomes constant. Measuring the Raman gain generated over the entire length of the optical fiber based on the optical power, at one end of the optical fiber.

  Further, in the Raman gain measuring method, it is preferable that a step of adjusting the power of the pump light so that the ratio is constant is provided at one end of the optical fiber.

  Further, in the Raman gain measuring method, it is preferable that the return light is return light of test light input to one end of the optical fiber.

  FIG. 1 is a configuration diagram showing the configuration of the present embodiment. In the configuration shown in FIG. 1, a WDM coupler 114 for multiplexing / demultiplexing a pump light wavelength and a test light wavelength is connected to one end of the transmission line. An excitation light source 140 is provided at an excitation light wavelength port of the WDM coupler 114, and an optical wavelength filter 116, an optical attenuator 115, and an OTDR device 110 are connected to the test light wavelength port in this order.

  In the present invention, in order to measure the Raman gain efficiency, an OTDR (Optical Time-Domain Reflectometry: time domain light reflection measurement) device 110 is used.

  OTDR measures the distributional loss of an optical fiber transmission line by injecting pulsed light from one end of the optical fiber transmission line and time-divisionally measuring the return light due to backscattering occurring in the optical fiber transmission line. It is a technique to do. Since the detected pulse light power reciprocates between the incident end and the reflection point, if the amount of attenuation in the round trip is divided by half, the amount of attenuation is one-way.

  The causes of the return light include Fresnel reflection that occurs at the connection point or end point of the fiber and Rayleigh scattering that occurs continuously in the optical fiber. As shown in FIG. 1, the end of the transmission path to which the OTDR is connected is referred to as A end, and the other end is referred to as B end. FIGS. 6A and 6B show the return light power measured by the OTDR, and the horizontal axis represents the distance from the point A where the return light is generated. Hereinafter, such a graph is referred to as an OTDR graph. 6A and 6B, return light other than Rayleigh scattering does not occur in transmission paths other than the B-end. FIG. 6A shows a case where the B end is non-reflection-terminated by an isolator or the like, and the return light due to Rayleigh scattering becomes 0 at the end of the fiber. FIG. 6B shows a case where Fresnel reflection occurs at the B end due to discontinuity of the fiber, etc., where large reflection occurs at the end of the fiber, and the return light due to Rayleigh scattering becomes 0 at the end. I have. When the OTDR measurement is performed on the transmission path, the OTDR graph shows a characteristic as shown in FIG. Therefore, the distance from the end A to the end B can be known from such characteristics, and the loss of the pulsed light that has propagated from the end A to the vicinity immediately before the end B can be measured.

  In this embodiment, the spectrum width of the OTDR pulse light serving as the test light is particularly set to 1 nm. By setting the pass band of the optical filter for extracting the test light wavelength to 1 nm, amplified spontaneous emission (ASE) due to Raman amplification was removed, and saturation of the OTDR light receiver due to the ASE light was prevented. Further, the amount of the optical attenuator was adjusted so that the light receiver of the OTDR was not saturated and the return light generated near the B-end could be measured with high sensitivity.

  Next, the measurement procedure will be described. First, OTDR measurement is performed with the pump light output stopped, and a propagation loss (dB) (referred to as L1) detected from return light generated near the B-end of the transmission path is obtained. Next, OTDR measurement is performed in a state where the pumping light is output, and a propagation loss (dB) (referred to as L2) detected from the returning light generated near the B end of the transmission path is obtained. Raman gain is obtained by subtracting L1 from L2. By dividing the Raman gain by the pump light power, the Raman gain efficiency is obtained. However, it is necessary that the pumping light power is sufficiently attenuated at the point B so that the Raman gain does not occur, that is, the OTDR graph when the pumping light output is stopped and the pumping light output are parallel. By calculating the propagation loss at point B that satisfies such conditions, the Raman gain that occurs over the entire length of the transmission path is determined. Further, by dividing the Raman gain by the pumping light power, the Raman gain efficiency over the entire length of the transmission path can be obtained.

  If the OTDR graphs at the point B when the optical pumping light output is stopped and when the pumping light output is not parallel, the pumping light power may be reduced until the OTDR graphs become parallel. In this embodiment, an 80 km single mode fiber (SMF) is used as the transmission line fiber, 1463.8 nm is used as the pumping light wavelength, and 1562.2 nm whose frequency is 13 THz lower than the pumping light wavelength is used as the test light wavelength. The output power was 166 mW. As a result, as shown in the OTDR graph of FIG. 7, L1 = 17 dB and L2 = 12 dB, and L2-L1 = 5.0 dB was obtained as the Raman gain. This was divided by the pump power to obtain a Raman gain efficiency of 30.1 dB / W. FIG. 8 shows the relationship between the excitation wavelength, the test light wavelength, and the Raman gain profile of this embodiment. In FIG. 8, the test light wavelength is located at the peak of the Raman gain profile with respect to the pump light.

  Further, in the present embodiment, the spectrum width of the test light is set to 1 nm, but application of a spectrum width different from this is also effective. However, considering the case where the OTDR measurement becomes unstable when the spectrum width is too narrow, the spectrum width should be wide enough not to make the OTDR measurement unstable and sufficiently narrow with respect to the period of the change of the Raman gain profile. Condition.

  In this embodiment, a DFB (Distributed FeedBack) laser is used as the test light. However, it is also effective to use a wavelength-tunable laser that can have a spectrum width under the above conditions. In this embodiment, an optical wavelength filter having a pass band of 1 nm is applied. However, as long as the spectrum of the test light is not cut out and ASE light can be sufficiently removed, application of a different pass band is also effective.

  In this embodiment, a dielectric multilayer film (Di-electric) type is applied to the optical wavelength filter, but the application of another device having an optical filtering function is also effective. For example, it is effective to apply a grating type wavelength filter, a Fabry-Perot type wavelength filter, a Mach-Zehnder interferometer type wavelength filter, or the like.

  Further, in the embodiment, the Mach-Zehnder interferometer type is applied to the optical attenuator, but the application of another device having an optical attenuation function is also effective. For example, a dielectric multilayer (Di-electric) type optical attenuator, an LN type attenuator, and an LBO type attenuator can be applied.

  Further, a Fabry-Perot laser whose wavelength is narrowed by a fiber grating is used as an excitation light source, but a tunable laser capable of outputting sufficient power is also effective. .

  In this embodiment, the SMF is used as the transmission line fiber. However, a non-zero dispersion shifted fiber (NZDSF), a 1.55 μm dispersion shifted fiber (Dispersion Shifted Fiber: DSF), or the like is applied. Is also possible.

  Next, a second embodiment of the present invention will be described. In the first embodiment, Raman gain efficiency was obtained by using propagation loss detected from return light generated near the B-end of the transmission path. However, as shown in FIG. 9, in a transmission path having a large loss at the test light wavelength, the B end may not be detected due to noise in some cases. In such a case, it is also possible to obtain gain efficiency from return light generated at a point not buried in noise (point B 'in FIG. 9). However, the pumping light power is sufficiently attenuated at the point B ', and Raman gain does not occur. That is, it is necessary that the OTDR graphs when the pumping light output is stopped and the pumping light output are parallel. If the OTDR graphs at the point B 'when the pumping light output is stopped and when the pumping light output is not parallel, the pumping light power may be reduced until they become parallel.

  FIG. 9 shows an OTDR graph in a case where an SMF whose propagation loss from the A-end to the B-end at the test wavelength is 40 dB is measured using the OTDR that can measure the return light generated at the point of the propagation loss of 30 dB. In this embodiment, as in the first embodiment, 1463.8 nm is used as the excitation light wavelength, 1562.2 nm whose frequency is 13 THz lower than the excitation light wavelength is used as the test light wavelength, and the output power of the excitation light is 166 mW. And Assuming that the propagation loss up to the point B ′ when the pumping light is stopped is L′ 1 and the propagation loss up to the point B ′ when the pumping light is output is L′ 2, as shown in FIG. 30 dB, L2 = 25.2 dB, and 4.8 dB was obtained as the Raman gain. This Raman gain was divided by the pump light power to obtain a Raman gain efficiency of 28.9 dB / W.

  Although the SMF is used as the transmission line fiber in the present embodiment, a non-zero dispersion shifted fiber (NZDSF), a 1.55 μm dispersion shifted fiber (Dispersion Shifted Fiber: DSF), or the like can be applied. It is.

  A third embodiment of the present invention will be described. The first embodiment shows the Raman gain efficiency when the test light wavelength is 13 THz lower than the pump light frequency. However, the present invention is not limited to this, and the Raman gain efficiency can be measured at other wavelength intervals. For example, in the same SMF as the first embodiment, when the test light wavelength is 1550.0 nm, the pump light wavelength is 1463.8 nm, and the pump light output power is 166 mW, the Raman gain is 4.6 dB and the Raman gain efficiency is 27. 7 dB / W was obtained. FIG. 10 shows the relationship between the excitation wavelength, the test light source, and the Raman gain profile of the present embodiment.

  In this embodiment, the SMF is used as the transmission line fiber. However, a non-zero dispersion fiber (DSF), a 1.55 μm dispersion shifted fiber (Dispersion Shifted Fiber: DSF), or the like can be applied.

  Next, a fourth embodiment of the present invention will be described below. Using one test light wavelength, the Raman gain efficiency for a plurality of types of pump light wavelengths can be measured. For example, in the same SMF as the first embodiment, when the test light wavelength is 1550.0 nm, the pump light wavelength is 1463.8 nm, and the pump light output power is 166 mW, the Raman gain is 4.6 dB and the Raman gain efficiency is 27. 7 dB / W was obtained. Similarly, when the test wavelength was 1550.0 nm, the pump light wavelength was 1450.4 nm, and the pump light output power was 166 mW, a Raman gain of 5.1 dB and a Raman gain efficiency of 30.7 dB / W were obtained. Of course, the Raman gain efficiency for three or more pump light wavelengths can be measured using one test light wavelength, depending on how the test light wavelength is selected. FIG. 11 shows the relationship between the excitation wavelength, the test light source, and the Raman gain profile of the present embodiment.

  In this embodiment, the SMF is used as the transmission line fiber. However, a non-zero dispersion fiber (DSF), a 1.55 μm dispersion shifted fiber (Dispersion Shifted Fiber: DSF), or the like can be applied.

  A fifth embodiment of the present invention will be described below. As shown in FIG. 12, when the Raman gain profile for a certain pumping light wavelength is known, the Raman gain efficiency for another wavelength can be calculated from the Raman gain efficiency for the test light. FIG. 12 shows a Raman gain profile for an excitation light wavelength of 1463.8 nm in the SMF. The absolute value of the Raman gain profile changes depending on the pumping light output power, but the shape of the profile is specific to the type of fiber. The Raman gain profile in FIG. 12 shows a Raman gain of 2.5 dB at 1550 nm, but shows a Raman gain of 2 dB at 1540 nm. From the shape of the Raman gain profile, it can be seen that the Raman gain efficiency at 1540 nm is 2 ÷ 2.5 = 0.8 times 1550 nm. Therefore, when the pumping wavelength is 1463.8 nm in the same SSMF as in Example 1, the Raman gain efficiency at 1550 nm is 27.7 dB / W from the result of the second example. It can be calculated as 27.7 × 0.8 = 22.2 dB / W.

  In this embodiment, SMF is used as the transmission line fiber. However, non-zero dispersion fiber (DSF), 1.55 μm dispersion shifted fiber (Dispersion Shifted Fiber: DSF), and the like can be applied. is there.

A sixth embodiment of the present invention will be described below. Using one test light wavelength, it is possible to obtain Raman gain efficiency at a wavelength at a frequency 13 THz lower than the wavelength of each of the plurality of pump lights, that is, at the peak wavelength of the Raman gain profile. The peak wavelength of the Raman gain profile is hereinafter referred to as a Raman gain peak wavelength. The Raman gain efficiency at one test light wavelength for a plurality of pump wavelengths is measured by the method of the fourth embodiment. From the measurement results, the Raman gain efficiency at the Raman gain peak wavelength of each pump wavelength is calculated by the method of the fifth embodiment. Of course, the Raman gain efficiency obtained by the method of the present embodiment is not limited to the Raman gain peak wavelength of each pump wavelength. According to the method of the present embodiment, it is possible to obtain the Raman gain efficiency at an arbitrary wavelength within the known Raman gain profile.

FIG. 2 is a configuration diagram illustrating a method for measuring Raman gain and Raman gain efficiency according to the present invention. FIG. 9 is a configuration diagram of a conventional Raman gain and Raman gain efficiency measuring method example. FIG. 4 is an explanatory diagram of a transition of test light power in a transmission line showing the operation of the present invention. FIG. 7 is an explanatory diagram of a transition of the return light power in the transmission line showing the operation of the present invention. FIG. 4 is an explanatory diagram showing a Raman gain profile. FIG. 4 is a diagram illustrating characteristics of a transmission path end in an OTDR graph. (A) A case where the B end is non-reflection-terminated, and (b) a case where reflection occurs at the B end. FIG. 4 is an explanatory diagram showing a result (OTDR) obtained in the first example of the present invention. FIG. 3 is an explanatory diagram showing an excitation wavelength, a test light wavelength, and a gain profile wavelength arrangement according to the first embodiment of the present invention. FIG. 11 is an explanatory diagram showing a result (OTDR graph) obtained in the second example of the present invention. FIG. 9 is an explanatory diagram showing an excitation wavelength, a test light wavelength, and a gain profile wavelength arrangement according to a third embodiment of the present invention. FIG. 11 is an explanatory diagram showing an excitation wavelength, a test light wavelength, and a gain profile wavelength arrangement according to a fourth embodiment of the present invention. It is an explanatory view showing an excitation wavelength, a test light wavelength, and a gain profile of the fifth example of the present invention.

Explanation of reference numerals

10, 20 Relay station 11, 21 Amplifier 110 OTDR
111, 113 Optical amplifier 112 Optical fiber transmission line 114 WDM coupler 115 Optical attenuator 116 Wavelength filter 113 WDM coupler 120 Test light source 130 Optical receiver 140 Excitation light source

Claims (9)

In a measuring device for measuring the Raman gain of the optical fiber supplied with the excitation light at one end of the optical fiber,
The ratio of the first return light power in the state where the pumping light is output and the second return light power in the state where the pumping light is stopped becomes constant, and the second return light power at a point except for the other end of the optical fiber becomes constant. A measuring apparatus for measuring a Raman gain occurring over the entire length of the optical fiber based on first and second return light powers.   The measuring apparatus according to claim 1, wherein the power of the excitation light is adjusted so that the ratio is constant. The measuring apparatus according to claim 1, wherein the return light is return light of test light input to one end of the optical fiber. In an optical transmission system having an optical fiber,
Input means for inputting excitation light to the optical fiber,
The ratio of the first return light power in the state where the pumping light is output and the second return light power in the state where the pumping light is stopped becomes constant, and the second return light power at a point except for the other end of the optical fiber becomes constant. Measuring means for measuring Raman gain occurring over the entire length of the optical fiber based on the first and second return light powers;
At one end of the optical fiber.   The optical transmission system according to claim 4, further comprising an adjusting unit at one end of the optical fiber for adjusting the power of the pumping light so that the ratio is constant. The optical transmission system according to claim 4, wherein the return light is return light of test light input to one end of the optical fiber. Inputting excitation light;
Measuring a first return light power in a state where the pump light is output;
Measuring a second return light power in a state where the pump light is stopped;
Obtaining a ratio between the first return light power and the second return light power;
Measuring the Raman gain that occurs over the entire length of the optical fiber based on the first and second return optical powers at points other than the other end of the optical fiber, wherein the ratio is constant;
Is performed at one end of the optical fiber.   8. The Raman gain measuring method according to claim 7, further comprising a step of adjusting the power of the pump light so that the ratio becomes constant, at one end of the optical fiber. 9. The Raman gain measuring method according to claim 7, wherein the return light is return light of test light input to one end of the optical fiber.

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