JP2013195225A - Device and method for measuring brillouin gain spectrum - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device and a method for measuring a Brillouin gain spectrum that do not require an expensive frequency shifter such as a SSB (Single Side Band) modulator and a delay fiber.SOLUTION: A method for measuring a Brillouin gain spectrum of the present invention comprises: a first light source (1) outputting probe light; a second light source (2) outputting pump light; a phase difference control unit (3) for changing a phase difference between the probe light and the pump light; a first light detector (17) receiving light containing Brillouin scattering light in a measured optical fiber (6); a second light detector (15) receiving multiplexed light composed of the probe light and the pump light; a frequency discriminator (21) outputting a voltage value according to a frequency of an output signal from the second light detector; and a subtractor (26) outputting a voltage according to a difference between a set reference voltage value and a voltage value output by the frequency discriminator. A central frequency of probe light or pump light is changed so as to reduce a voltage value output by the subtractor.

Description

  The present invention relates to a Brillouin gain spectrum measuring apparatus and method for measuring Brillouin scattered light of an optical fiber.

  In order to measure the temperature and strain distribution of an optical fiber, BOCDA (Brillouin Optical Correlation-Domain Analysis) for observing Brillouin scattered light of the optical fiber has been proposed (for example, see Patent Document 1). BOCDA receives pump light and probe light having a frequency difference such that a Brillouin scattering phenomenon occurs from opposite sides of the optical fiber, and observes Brillouin scattered light of the pump light and probe light generated in the optical fiber.

In BOCDA of Patent Document 1, a frequency difference corresponding to a Brillouin frequency shift (approximately 10 GHz) is given between the center frequencies of the pump light and the probe light incident on the optical fiber to be measured, and the frequency difference is changed by several GHz. In this way, the Brillouin gain spectrum is measured. For this reason, conventionally, the light from the light source is divided into two, LN intensity modulation is performed on one of the lights, and the sideband component is extracted by an optical filter to obtain frequency-shifted light.
However, a high-speed LN intensity modulator is expensive and has a large insertion loss. In some cases, an optical amplifier is required to compensate for the attenuation. Further, in order to suppress unnecessary frequency components and extract only the desired frequency shift light component, it is necessary to use a narrow line width optical filter and to precisely control the transmission spectrum in accordance with the change in the center frequency difference. An SSB (Single Side Band) type LN modulator that can generate only frequency-shifted light components can be used to eliminate the need for an optical filter, but such a modulator is more expensive and more inserted. There is a problem that the loss is large and a dedicated drive circuit is necessary.

Further, in Patent Document 1, a high-order (other than zero-order) correlation peak is used when sweeping the position where Brillouin scattered light is observed on the optical fiber to be measured. For this reason, conventionally, in order to provide a delay difference, an offset delay fiber is arranged in one of the optical paths of the probe light and the pump light, and the observation position is swept by changing the modulation frequency.
However, it is necessary to prepare a delay fiber having a sufficient length according to the length of the optical fiber to be measured. Since this delay fiber becomes longer according to the length of the optical fiber to be measured, there is a problem that the size of the apparatus is limited.

Japanese Patent No. 3667132

  In order to solve the above-described problems, an object of the present invention is to provide a Brillouin gain spectrum measurement apparatus and method that does not require an expensive optical frequency shifter such as an SSB modulator and a delay fiber.

  The Brillouin gain spectrum measuring apparatus according to the present invention includes a first light source (1) that outputs a first continuous wave light frequency-modulated at a predetermined modulation frequency, frequency-modulated at the predetermined modulation frequency, and the first light source (1). A second light source (2) that outputs a second continuous wave light having a predetermined center frequency difference from the continuous wave light, and a phase difference between the first continuous wave light and the second continuous wave light is changed. A phase difference control unit (3) for causing the first continuous wave light to branch, one of which is incident on one end of the optical fiber to be measured, and the second continuous wave light One of the second optical branching unit (13) for branching light and the second continuous wave light branched by the second optical branching unit is incident on the other end of the optical fiber to be measured, and The first continuous wave light output from the other end of the measurement optical fiber, and the A third light branching portion (16) for guiding scattered light of two continuous wave lights, a first photodetector (17) for receiving the light guided by the third light branching portion, and the first An optical multiplexing unit (14) for multiplexing the other of the first continuous wave light branched by the optical branching unit and the other of the second continuous wave light branched by the second optical branching unit; A second photodetector (15) that receives the multiplexed light from the optical multiplexing unit, and a frequency discriminator (21) that outputs a voltage value corresponding to the frequency of the output signal from the second photodetector. A subtractor (26) that outputs a voltage according to a difference between a set reference voltage value and a voltage value output from the frequency discriminator, and the first light source or the second light source includes: The center frequency is changed so that the voltage value output from the subtractor becomes smaller.

  The Brillouin gain spectrum measuring apparatus of the present invention includes an LPF (22) for smoothing the output signal from the frequency discriminator, and the subtractor is a difference between the reference voltage value and the smoothed voltage value of the LPF. A voltage corresponding to may be output.

  In the Brillouin gain spectrum measuring apparatus of the present invention, the reference voltage value is temporally changed in accordance with the phase difference between the first continuous wave light and the second continuous wave light that is changed by the phase difference control unit. You may further provide the correction | amendment part (10) to be made.

  In the Brillouin gain spectrum measuring method of the present invention, the first continuous wave light and the second continuous wave light having a predetermined center frequency difference frequency-modulated with a predetermined modulation frequency, the first continuous wave light and the An optical output procedure for outputting while changing the phase difference of the second continuous wave light, and one of the first continuous wave lights branched to one end of the optical fiber to be measured (6), and the second continuous wave One of the branched oscillation light is incident on the other end of the measured optical fiber, and the first continuous wave light and the second continuous wave light scattered from the other end of the measured optical fiber are scattered. A light receiving procedure for receiving a combined light obtained by combining the other of the first continuous wave light and the other of the second continuous wave light, and a set reference voltage value and the combined light. Voltage value according to the frequency of the received light signal Difference detecting the, as the voltage difference detected is smaller, has a control procedure for changing the center frequency of the first continuous oscillation light or the second continuous oscillation light, in this order.

  In the Brillouin gain spectrum measurement method of the present invention, in the control procedure, the voltage value corresponding to the frequency of the light reception signal of the combined light is smoothed, and the difference between the smoothed voltage value and the reference voltage value is detected. Also good.

  In the Brillouin gain spectrum measurement method of the present invention, in the control procedure, the reference voltage value may be changed with time in accordance with the phase difference between the first continuous wave light and the second continuous wave light. Good.

  According to the present invention, it is possible to provide a Brillouin gain spectrum measurement apparatus and method that does not require an expensive optical frequency shifter such as an SSB modulator and a delay fiber.

1 shows an example of a Brillouin gain spectrum measuring apparatus according to a first embodiment. An example of probe light and pump light is shown. An example of the position of the correlation peak where the Brillouin scattering phenomenon occurs is shown. An example of an output signal from the second photodetector with respect to the phase difference Δφ is shown. An example of the frequency-voltage conversion characteristic of a frequency discriminator is shown. An example of the Brillouin gain spectrum measuring apparatus which concerns on Embodiment 1 provided with the 2nd example of the control form of phase difference (DELTA) phi is shown. An example of the Brillouin gain spectrum measuring apparatus which concerns on Embodiment 2 is shown. An example of the Brillouin gain spectrum measuring apparatus according to the second embodiment when acquiring the phase difference Δφ from the phase difference control unit will be described. An example of the Brillouin gain spectrum measuring apparatus which concerns on Embodiment 2 provided with the 2nd example of the control form of phase difference (DELTA) phi is shown. An example of the Brillouin gain spectrum measuring apparatus according to the second embodiment in the case where the second example of the control form of the phase difference Δφ is provided and the phase difference Δφ is obtained from the phase difference control unit will be described. An example of a structure of a frequency discriminator is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 1 shows an example of a Brillouin gain spectrum measuring apparatus according to this embodiment. The Brillouin gain spectrum measuring apparatus according to the present embodiment includes a first light source 1, a second light source 2, a phase difference control unit 3, a first light branching unit 12, and a second light branching unit 13. The third optical branching unit 16, the first optical detector 17, the optical multiplexing unit 14, the second optical detector 15, the frequency discriminator 21, the LPF (Low Pass Filter) 22, and the offset A frequency setting unit 24, a reference voltage generation unit 23, and a subtractor 26 are provided. The first photodetector 17 and the second photodetector 15 are, for example, PD (Photo Diode). In addition, the Brillouin gain spectrum measurement method includes a light output procedure, a light receiving procedure, and a control procedure in this order.

In the light output procedure, the first continuous wave light and the second continuous wave light are output while changing the phase difference. Specifically, it operates as follows.
The first light source 1 outputs first continuous wave light (hereinafter, referred to as probe light) having a center frequency ν1 frequency-modulated with a predetermined modulation frequency. The second light source 2 outputs a second continuous wave light (hereinafter referred to as pump light) having a second continuous wave light having a center frequency ν2 that is frequency-modulated with a predetermined modulation frequency. The center frequency ν1 is lower than the center frequency ν2, and these differences correspond to the Brillouin frequency shift. Since frequency modulation and phase modulation are equivalent techniques, the term “frequency modulation” herein includes phase modulation techniques.

  In the present embodiment, the first light source 1 includes an LD 51 and an oscillator 52, and the second light source 2 includes an LD 53 and an oscillator 54. The LD 51 generates continuous wave light having a center frequency ν1. The oscillator 52 gives a signal for frequency modulation to the LD 51. As a result, the LD 51 outputs frequency-modulated probe light having a center frequency ν 1 in accordance with the signal from the oscillator 52. The LD 53 generates continuous wave light having a center frequency ν2. The oscillator 54 gives a signal for frequency modulation to the LD 53. As a result, the LD 53 outputs frequency-modulated pump light having ν2 as the center frequency in accordance with the signal from the oscillator 54. As described above, in the present embodiment, the first light source 1 and the second light source 2 are directly modulated to obtain frequency-modulated light having a desired center frequency difference. This eliminates the need for an expensive SSB modulator and its dedicated driver.

  FIG. 2 shows an example of probe light and pump light. The difference Δν between the center frequency ν1 and the center frequency ν2 is a frequency difference in which the Brillouin scattering phenomenon occurs, and is, for example, 10 GHz. The oscillator 52 and the oscillator 54 generate signals having the same frequency. The signal from the oscillator 52 and the signal from the oscillator 54 have a phase difference Δφ designated by the phase difference control unit 3. When pump light and probe light are incident from one end and the other end of the measured fiber, the correlation increases at the position where the phases of the two lights are synchronized in the optical fiber (correlation peak), and the frequency difference Δν and the bullian at that position The power moves from the pump light to the probe light via the acoustic wave in the optical fiber by the Brillouin scattering phenomenon according to the overlap of the gain spectrum. Here, when the phase difference control unit 3 changes the phase difference Δφ, the correlation peak position L between the pump light and the probe light can be moved from the broken line position to the solid line position as shown in FIG. it can. The phase may be changed by a driving signal from the oscillator 52, a driving signal from the oscillator 54, or both of them. As described above, in this embodiment, since the correlation peak position L is swept without using a delay fiber, a delay fiber is not required, and this allows downsizing of the apparatus.

  In the light receiving procedure, one of the branched probe lights is incident on one end of the measured optical fiber 6, the one of the branched pump light is incident on the other end of the measured optical fiber 6, and the other end of the measured optical fiber 6 is The scattered light of the output probe light and pump light is received using the first photodetector 17 and the combined light obtained by combining the other branched probe light and the other branched pump light is the second light. Light is received using the detector 15. Specifically, it operates as follows.

  The first light branching unit 12 branches the probe light. One of the probe lights is incident on one end of the optical fiber 6 to be measured, and the other of the probe lights is incident on the optical multiplexing unit 14. The second light branching unit 13 branches the pump light. One of the pump lights is incident on the third optical branching section 16, and the other of the pump lights is incident on the optical multiplexing section 14. The third optical branching unit 16 makes the pump light from the second optical branching unit 13 incident on the other end of the optical fiber to be measured 6, the probe light that has passed through the optical fiber to be measured 6, the probe light, and the pump light. Are output to the first photodetector 17. The first photodetector 17 receives light from the third light branching unit 16. Thereby, the power moved from the pump light to the probe light by the Brillouin scattering phenomenon at the position of the correlation peak between the probe light and the pump light can be observed. The Brillouin gain spectrum can be measured by changing the center frequency of the probe light or the pump light and changing the center frequency difference Δν in the Brillouin gain band so as to include at least the Brillouin frequency shift.

  The optical multiplexing unit 14 combines the probe light and the pump light. The second photodetector 15 receives the combined light. As a result, a beat signal obtained by combining the probe light and the pump light is output from the second photodetector 15.

  In the control procedure, the probe light is detected so that the difference between the set reference voltage value and the output voltage value from the frequency discriminator corresponding to the frequency of the light reception signal of the combined light is detected, and the detected voltage difference is reduced. Alternatively, the center frequency of the pump light is changed. Specifically, it operates as follows.

In measuring the Brillouin gain spectrum, the offset frequency setting unit 24 sets a frequency equal to the center frequency difference Δν to be observed. The reference voltage generator 23 generates a reference voltage having a value set by the offset frequency setting unit 24. The voltage value generated with respect to the frequency set by the reference voltage generator 23 is made equal to the frequency discriminator 21. The frequency discriminator 21 converts the frequency of the light reception signal output from the second photodetector 15 into a voltage value. The LPF 22 smoothes the output voltage value V _B from the frequency discriminator 21. The subtractor 26 outputs the difference between the voltage value from the LPF 22 in which the cutoff frequency is set lower than the modulation frequency of the frequency modulation and the reference voltage value V _S from the reference voltage generator 23. As a result, it is possible to detect a deviation between the center frequency difference Δν between the probe light and the pump light being output and a desired center frequency difference Δν.

  An example in which the frequency discriminator 21 is simply configured is shown in FIG. An HPF (High Pass Filter) 56 having a transmission characteristic in which the frequency of the beat signal output from the photodetector 15 corresponds to the slope portion of the filter characteristic, and a detector 57 for converting the power of the beat signal passing through the HPF 56 into a voltage value. And composed of As a result, a voltage value that changes in accordance with the frequency of the beat signal output from the photodetector 15 can be obtained. Since only the slope portion of the filter characteristics is used, the filter used here is not limited to HPF but may be LPF or the like. In addition, the fluctuation of the amplitude value of the beat signal output from the photodetector 15 also affects the voltage value output from the detector 57. This influence causes the power of the beat signal before being input to the HPF 56 to be separately detected. It can be removed by correcting the output voltage value from the detector 57 in advance by monitoring the signal.

  The second light source 2 receives the output signal from the subtractor 26 and changes the center frequency so that the output signal becomes small. As a result, the center frequency difference Δν between the probe light and the pump light can be stabilized at a desired frequency difference, and the measurement accuracy of the Brillouin gain spectrum can be increased. The light source for changing the center frequency is not limited to the second light source 2 but may be the first light source 1.

Here, in this embodiment, the phase difference Δφ between the probe light and the pump light is changed by the phase difference control unit 15. At this time, as shown in FIG. 4, the frequency of the beat signal output from the photodetector 15 is not constant depending on the phase difference Δφ and changes with time, and the manner of the change varies depending on the phase difference Δφ. Accordingly, the output voltage value V _B from the frequency discriminator 21 also varies with time as shown in FIG. Therefore, in the control procedure, the output voltage value V _B from the frequency discriminator 21 is smoothed. Thus, even when the phase difference Δφ between the probe light and the pump light is swept, it is possible to detect a deviation between the center frequency difference Δν between the probe light and the pump light and a desired set center frequency difference Δν.

  By executing the above procedure, the frequency difference Δν of the center frequency can be stabilized to a desired value, and the component when the difference between the center frequency ν1 and the center frequency ν2 is a specific value in the Brillouin gain spectrum. Can be observed. The offset frequency setting unit 19 sweeps the set frequency in the Brillouin gain band so as to include at least the Brillouin frequency shift. Thereby, the center frequency ν2 is swept and the measurement of the Brillouin gain spectrum is completed.

  As described above, the Brillouin gain spectrum measurement apparatus according to the present embodiment changes the frequency difference Δν by changing the voltage generated by the reference voltage generator 23 and measures the Brillouin gain spectrum. Further, the measurement position L is swept by changing the phase difference Δφ of the modulation signal. Also, by smoothing the output voltage value from the frequency discriminator due to the frequency change caused by the phase difference of the modulation signal and performing feedback control so that the difference from the reference voltage value is small, the frequency difference Δν of the center frequency To the desired frequency difference.

  In the present embodiment, feedback control is performed by feeding back to the center frequency ν2. However, feedback control may be performed by feeding back to the center frequency ν1.

  Further, the Brillouin gain spectrum measuring apparatus according to the present embodiment may include an oscillator and a phase shifter 55 as shown in FIG. 6 instead of the oscillator 2 and the oscillator 54. The oscillator 52 provides a signal for applying frequency modulation to both the LD 51 and the LD 53. The phase shifter 55 receives the phase difference Δφ designated from the phase difference control unit 3 and changes the phase of the drive signal from the oscillator 52 according to the phase difference Δφ. By adopting this configuration, it is possible to generate probe light and pump light with no deviation in modulation frequency.

(Embodiment 2)
FIG. 7 shows an example of a Brillouin gain spectrum measuring apparatus according to this embodiment. The Brillouin gain spectrum measurement apparatus according to the present embodiment includes a light source control unit 11 and a correction unit 25 instead of the LPF 22 described in the first embodiment. In this embodiment, in the control procedure, the difference between the reference voltage value V _S changed in accordance with the change in the phase difference Δφ and the output signal V _B from the frequency discriminator 21 is calculated.

The output signal waveform from the photodetector 15 with respect to the phase difference Δφ is obtained in advance. This may be actually obtained from the beat signal output from the light detector 15 or may be obtained by calculation as a combination of frequency modulated light having a phase difference. Then, when the phase difference Δφ is input from the light source control unit 11, the correction unit 25 uses the frequency value set by the offset frequency setting unit 24 as a signal output from the photodetector 15 when the phase difference Δφ. The time is changed in accordance with the phase difference Δφ so that the frequency changes with the same frequency change width in synchronization. The reference voltage generation unit 23 generates a reference voltage value having a DC voltage value component corresponding to the center frequency difference Δν input from the correction unit 25 and having a voltage value that changes with time according to the phase difference Δφ. The subtractor 26 calculates the difference between the reference voltage value V _S from the reference voltage generator 23 and the output signal V _B from the frequency discriminator 21. As a result, even when the phase difference Δφ between the probe light and the pump light is swept, the temporal change in the output voltage value of the frequency discriminator caused by the phase difference Δφ is canceled out. The deviation between the frequency difference Δν and the desired center frequency difference Δν can be detected correctly.

  In the present embodiment, correction for the phase difference Δφ is realized by correcting the frequency setting value from the offset frequency setting unit, but for example, the correction unit is not between the offset frequency setting unit and the reference voltage generation unit, It is also possible to arrange between the reference voltage generator and the subtractor so that the same correction is made directly on the reference voltage value from the reference voltage generator.

  The correcting unit 25 needs to acquire the phase difference Δφ between the probe light and the pump light. Therefore, the light source control unit 11 is provided in the present embodiment. The light source control unit 11 determines the phase difference Δφ according to the peak position L in the optical fiber to be measured for measuring the Brillouin gain spectrum, and outputs the phase difference Δφ to the phase difference control unit 3 and the correction unit 25. Note that the correction unit 25 may acquire the phase difference Δφ from the phase difference control unit 3 as illustrated in FIG. 8.

  The Brillouin gain spectrum measuring apparatus according to this embodiment may include an oscillator 52 and a phase shifter 55 as shown in FIGS. 9 and 10 instead of the oscillator 52 and the oscillator 54. The oscillator 52 provides a signal for applying frequency modulation to both the LD 51 and the LD 53. The phase shifter 55 receives the phase difference Δφ designated from the phase difference control unit 3 and changes the phase of the signal from the oscillator 52 according to the phase difference Δφ. By adopting this configuration, it is possible to generate probe light and pump light with no deviation in modulation frequency.

As described above, the Brillouin gain spectrum measuring apparatus according to the present embodiment changes the frequency difference Δν by changing the reference voltage value V _S and measures the Brillouin gain spectrum. Further, the measurement position L is swept by changing the phase difference Δφ of the modulation signal. A reference voltage value V _S is compared with the output signal V _B after a change in frequency generated by the phase difference of the modulation signal is given to the reference voltage generation unit 23 in advance, and feedback control is performed so that the difference becomes small. Thus, the frequency difference Δν of the center frequency is stabilized to a desired frequency difference.

  In the present embodiment, feedback control is performed by feeding back to the center frequency ν2. However, feedback control may be performed by feeding back to the center frequency ν1.

  The present invention can be applied to the information communication industry.

1: First light source 2: Second light source 3: Phase difference control unit 4: Offset frequency setting unit 6: Optical fiber to be measured 7: Oscillator 9: LPF
10: Correction unit 12: First optical branching unit 13: Second optical branching unit 14: Optical multiplexing unit 15: Second optical detection unit 16: Third optical branching unit 17: First optical detection unit 21 : Frequency discriminator 22: LPF
23: Reference voltage generator 24: Offset frequency setting unit 25: Correction unit 26: Subtractor 51, 53: LD
52, 54: Oscillator 55: Phase shifter 56: HPF
57: Detector

Claims (6)

A first light source (1) for outputting a first continuous wave light frequency-modulated at a predetermined modulation frequency;
A second light source (2) that outputs a second continuous wave light that is frequency-modulated at the predetermined modulation frequency and has a predetermined center frequency difference from the first continuous wave light;
A phase difference control unit (3) for changing a phase difference between the first continuous wave light and the second continuous wave light;
A first light branching section (12) for branching the first continuous wave light, one of which is incident on one end of the optical fiber to be measured;
A second light branching section (13) for branching the second continuous wave light;
One of the second continuous wave lights branched by the second light branching unit is incident on the other end of the optical fiber to be measured, and is output from the other end of the optical fiber to be measured. A third light branching section (16) for guiding continuous wave light and scattered light of the second continuous wave light;
A first photodetector (17) for receiving the light guided by the third optical branch;
An optical multiplexing unit (14) that multiplexes the other of the first continuous wave light branched by the first optical branching unit and the other of the second continuous wave light branched by the second optical branching unit. )When,
A second photodetector (15) for receiving the combined light from the optical combining unit;
A frequency discriminator (21) for outputting a voltage value corresponding to the frequency of the output signal from the second photodetector;
A subtractor (26) for outputting a voltage corresponding to a difference between a set reference voltage value and a voltage value output from the frequency discriminator,
The Brillouin gain spectrum measuring apparatus, wherein the first light source or the second light source changes a center frequency so that a voltage value output from the subtractor becomes small. Comprising an LPF (22) for smoothing the output signal from the frequency discriminator;
2. The Brillouin gain spectrum measurement apparatus according to claim 1, wherein the subtracter outputs a voltage corresponding to a difference between the reference voltage value and a smoothed voltage value of the LPF.   A correction unit (10) that temporally changes the reference voltage value according to a phase difference between the first continuous wave light and the second continuous wave light that are changed by the phase difference control unit; The Brillouin gain spectrum measuring apparatus according to claim 1. The first continuous wave light and the second continuous wave light having a predetermined center frequency difference frequency-modulated at a predetermined modulation frequency, and the phase difference between the first continuous wave light and the second continuous wave light. Light output procedure to output while changing,
One branching the first continuous wave light is incident on one end of the optical fiber to be measured (6), one branching the second continuous wave light is incident on the other end of the optical fiber to be measured, The scattered light of the first continuous wave light and the second continuous wave light output from the other end of the optical fiber to be measured is received, and the other of the first continuous wave light and the second are branched. A light receiving procedure for receiving the combined light obtained by combining the other of the continuous oscillation light and the other,
A difference between the set reference voltage value and a voltage value corresponding to the frequency of the light reception signal of the combined light is detected, and the first continuous wave light or the second second light is reduced so that the detected voltage difference becomes small. A control procedure for changing the center frequency of the continuous wave light;
The Brillouin gain spectrum measurement method which has these in order.   5. The control procedure according to claim 4, wherein in the control procedure, a voltage value corresponding to a frequency of a light reception signal of the combined light is smoothed, and a difference between the smoothed voltage value and the reference voltage value is detected. Brillouin gain spectrum measurement method.   5. The Brillouin according to claim 4, wherein, in the control procedure, the reference voltage value is temporally changed according to a phase difference between the first continuous wave light and the second continuous wave light. Gain spectrum measurement method.

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