JP7352962B2 - Brillouin frequency shift measurement device and Brillouin frequency shift measurement method - Google Patents

Description

The present invention relates to a Brillouin frequency shift measuring device and a Brillouin frequency shift measuring method.

Conventionally, many studies have been conducted on distributed sensors that utilize Brillouin scattering within optical fibers. In early research, a method using a single mode light source was proposed (Non-Patent Document 1). Further, the research conducted so far is summarized in Non-Patent Document 2. Recently, a method using an optical frequency comb has been proposed as a method that can detect the Brillouin gain spectrum without frequency sweeping (Non-Patent Documents 3 to 6). On the other hand, although this method uses a single mode light source, a method has been proposed that uses a region where changes in the Brillouin gain spectrum can be considered linear to measure temperature and strain from changes in the intensity of scattered light (non-linear). Patent Document 7). In addition, by modulating the single mode pump light, it becomes multi-mode, and by making the Brillouin gain spectra corresponding to each mode overlap, the Brillouin gain spectrum is shaped, widening the region where the Brillouin gain spectrum can be considered linear. have been proposed (Non-patent Documents 8 and 9).

T. Horiguchi, K. Shimizu, T. Kurashima, M. Tateda, and Y. Koyamada, “Development of a distributed sensing technique using Brillouin scattering,” J. Lightwave Technol. 13, 1296-1302 (1995). K. Y. Song, K. Hotate, W. Zou, and Z. He, “Applications of Brillouin dynamic grating to distributed fiber sensors,” J. Lightwave Technol. 35, 3268-3280 (2017). A. Voskoboinik, J.Wang, B. Shamee, S. R. Nuccio, L. Zhang, M Chitgarha, A. E. Willner, and M. Tur, “SBS-based fiber optical sensing using frequency-domain simultaneous tone interrogation,” J. Lightwave Technol 29, 1729-1735 (2011). C. Jin, L. Wang, Y. Chen, N. Guo, W. Chung, H. Au, Z. Li, H-Y. Tam, and C. Lu, “Single-measurement digital optical frequency comb based phase-detection Brillouin optical time domain analyzer,” Opt. Exp. 25, 9213-9224 (2017). Y. Tanaka and Y. Ozaki, “Brillouin frequency shift measurement with virtually controlled sensitivity,” Appl. Phys. Exp. 10, 062504 (2017). Y. Tanaka, Y. Ozaki, and Y. So, “Scanless Brillouin gain spectrum measurement based on multi-heterodyne detection,” in Tech. Digest of International Conf. on Optical Fiber Sensors 2018, TuE88 (2018). H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, “Slope-assisted Brillouin optical correlation-domain reflectometry: proof of concept,” Photon. Jour. 8, 6802807 (2016). Guangyao Yang, Xinyu Fan, Bin Wang, and Zuyuan He, "Enhancing strain dynamic range of slope-assisted BOTDA by manipulating Brillouin gain spectrum shape," Opt. Express 26, 32599-32607 (2018) J. Marinelarena, J. Urricelqui, and A. Loayssa, IEEE Photonics J. 9,6802710 (2017).

In the conventional method using a single mode light source, the method using the frequency sweep of the probe light inherently requires a long measurement time, and the frequency sweep of the light source requires complicated control. Furthermore, when detecting the frequency difference between the scattered light and the pump light (reference light), the scattered light is weak and has fluctuations in frequency. Therefore, it is necessary to collect and average a large amount of data, which also requires measurement time.

Although the conventional method of detecting the Brillouin gain spectrum all at once using an optical frequency comb eliminates the need for frequency sweeping, it complicates signal processing when reproducing the spectrum.

A method that uses a single mode light source and uses a region where the change in the Brillouin gain spectrum can be considered linear on the frequency axis, which is modified by light source modulation, etc., is effective for shortening the measurement time, but the intensity Preliminary calibration that associates changes with changes in temperature, strain, etc. becomes complicated. Further, a method of shaping the Brillouin gain spectrum by overlapping the Brillouin gain spectra created by each mode of the pump light is effective for shortening the measurement time, but it complicates modulation adjustment.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a Brillouin frequency shift measuring device etc. that can shorten measurement time and simplify signal processing. There is a particular thing.

(1) The present invention includes a branching section that branches light from a laser light source into two, an optical frequency shift section that shifts the frequency of one of the two branched lights, and an optical frequency shift section that shifts the frequency of one of the two branched lights. a first optical frequency comb generating section that generates a first optical frequency comb from the optical frequency comb; and a second optical frequency comb generating section that generates a second optical frequency comb having a frequency interval different from that of the first optical frequency comb from the other branched light. an optical frequency comb generator, a spectrum shaping unit that shapes the spectral intensity of the first optical frequency comb and/or the second optical frequency comb, and generates a probe light by pulsing the first optical frequency comb. a first optical modulator that pulses the second optical frequency comb to generate pump light; and a second optical modulator that pulses the second optical frequency comb to generate pump light; a photodetector that detects light emitted from the other end of the optical fiber when the pump light is input from the other end; The present invention relates to a Brillouin frequency shift measuring device including a control section for determining a Brillouin frequency shift in a Brillouin frequency shift.

Further, the present invention includes a branching step of branching light from a laser light source into two, an optical frequency shifting step of shifting the frequency of one of the two branched lights, and a step of shifting the frequency of one of the two branched lights. a first optical frequency comb generating step of generating one optical frequency comb; and a second optical frequency comb generating step of generating a second optical frequency comb having a frequency interval different from that of the first optical frequency comb from the other branched light. a comb generation step, a spectral shaping step of shaping the spectral intensity of the first optical frequency comb and/or the second optical frequency comb, and a step of pulsing the first optical frequency comb to generate probe light. a second optical modulation step of pulsing the second optical frequency comb to generate pump light; and a second optical modulation step of pulsing the second optical frequency comb to generate pump light, and inputting the probe light from one end side of the optical fiber to be measured, a light detection step of detecting light emitted from the other end of the optical fiber when the pump light is input from the end; and a control step for determining a Brillouin frequency shift.

According to the present invention, by using an optical frequency comb, measurement time can be shortened without deteriorating measurement accuracy. Further, in the present invention, the Brillouin frequency shift is determined from the intensity of the emitted light rather than extracting frequency information from the emitted light from the optical fiber, so signal processing can be simplified.

(2) Furthermore, in the Brillouin frequency shift measuring device and the Brillouin frequency shift measuring method according to the present invention, the spectrum shaping section (in the spectrum shaping step) adjusts the Brillouin frequency shift and the light intensity detected by the light detection section. The spectral intensities of the first optical frequency comb and/or the second optical frequency comb may be shaped so that the relationship of the above applies to a predetermined function.

FIG. 1 is a diagram showing the configuration of a Brillouin frequency shift measuring device according to the present embodiment. FIG. 3 is a diagram showing an example of probe light, pump light, Brillouin gain spectrum, and amplified probe light. FIG. 3 is a diagram showing the relationship between the Brillouin frequency shift and the intensity of amplified probe light. FIG. 7 is a diagram illustrating an example when a composite gain coefficient becomes a constant in a certain frequency range. The figure which shows the result calculated|required by the simulation of the relationship between Brillouin frequency shift and the intensity of amplified probe light when an arbitrary function is made into a straight line. FIG. 3 is a diagram showing an experimental system for a confirmation experiment to confirm the effect of the method of the present embodiment. The figure which shows the result of observing the change of the temperature distribution in the measured fiber when the end 5 m of the 30 m long measured fiber is heated. The figure which shows the result of changing the relative frequency difference of pump light and probe light, and measuring the intensity of amplified probe light.

This embodiment will be described below. Note that this embodiment described below does not unduly limit the content of the present invention described in the claims. Furthermore, not all of the configurations described in this embodiment are essential components of the present invention.

FIG. 1 is a diagram showing the configuration of a measuring device (Brillouin frequency shift measuring device) according to an embodiment of the present invention. The measuring device 1 is a device that measures the Brillouin frequency shift in the sensing area SA (measurement target) of the optical fiber 2, and includes a single wavelength laser light source 10, an optical splitter 11 (branching part), and an optical frequency shifter. 20 (optical frequency shift section), optical frequency comb generators 30 and 31 (first optical frequency comb generation section, second optical frequency comb generation section), wavelength filters 40 and 41 (spectrum shaping section), and optical modulation devices 50 and 51 (first optical modulation section, second optical modulation section), signal generator 52, optical circulator 60, photodetector 70 (photodetection section), and control device 80 (control section). include.

The optical splitter 11 branches the light (frequency ν ₀ ) from the laser light source 10 into two at a predetermined splitting ratio (for example, 1:1).

The optical frequency shifter 20 shifts the frequency of one of the two lights split by the optical splitter 11. In the example shown in FIG. 1, the frequency of the light incident on the optical frequency comb generator 31 of the two split lights is shifted. Let the amount of frequency shift by the optical frequency shifter 20 be ν _s .

The optical frequency comb generator 30 generates an optical frequency comb (first optical frequency comb) from one of the branched lights. An optical frequency comb is light with a comb-shaped spectrum consisting of components (modes) arranged at equal intervals on the frequency axis. The optical frequency comb generator 31 generates light whose frequency interval (frequency interval between adjacent modes in the spectrum) is slightly different from that of the first optical frequency comb from the other branched light (the light incident from the optical frequency shifter 20). A frequency comb (second optical frequency comb) is generated.

The wavelength filter 40 shapes the spectral intensity (intensity of each mode) of the first optical frequency comb. The wavelength filter 41 shapes the spectral intensity of the second optical frequency comb. In the example shown in FIG. 1, the wavelength filter 40 shapes the spectral intensity of the first optical frequency comb so that the intensities of each mode in the spectrum of the first optical frequency comb are different from each other, and the wavelength filter 41 shapes the spectral intensity of the second optical frequency comb. The spectral intensity of the second optical frequency comb is shaped so that the intensity of each mode in the spectrum of the second optical frequency comb is equal.

The optical modulator 50 generates probe light Pr by applying a time gate to the shaped first optical frequency comb at a constant timing (applying intensity modulation using a periodic gate pulse). The probe light Pr enters the sensing area SA of the optical fiber 2 from one end side. The optical modulator 51 applies a time gate to the shaped second optical frequency comb at a constant timing to generate a pulse, thereby generating pump light Pm. The signal generator 52 generates a signal (intensity modulation signal) for driving the optical modulators 50 and 51.

The optical circulator 60 outputs the pump light Pm input to the first port from the second port and guides it to the other end of the sensing area SA, and also outputs the pump light Pm from the other end of the sensing area SA to the second port. The light (amplified probe light AP) input to the probe is output from the third port and guided to the photodetector 70. The photodetector 70 detects the amplified probe light AP and outputs the detected light intensity as an electrical signal.

The control device 80 is an information processing device (for example, a personal computer) equipped with a processor (such as a CPU) and a storage unit (RAM, hard disk, etc.), and controls the intensity of the signal from the photodetector 70 (the intensity of the signal from the photodetector 70). Based on the detected light intensity), the Brillouin frequency shift (frequency shift of the Brillouin gain spectrum) in the optical fiber 2 (sensing area SA) is calculated. Further, the control device 80 outputs a control signal to the signal generator 52 to control the signal generator 52.

In the optical fiber 2, a refractive index distribution (moving diffraction grating) is generated by thermally induced acoustic waves. The pump light Pm incident on the other end of the optical fiber 2 (sensing area SA) is reflected thereby, and a stroke light whose frequency is downshifted is generated and propagates backward (toward the other end of the optical fiber). Furthermore, electrostriction occurs due to the beats of the stroke light and the pump light Pm, resulting in a refractive index distribution. This series of processes is repeated to generate a Brillouin Gain Spectrum (BGS). When the probe light Pr enters opposite the pump light Pm (to one end of the optical fiber), the probe light Pr is amplified by the Brillouin gain spectrum, and the amplified probe light AP is emitted from the other end of the optical fiber. .

FIG. 2 shows an example of the probe light Pr, the pump light Pm, the Brillouin gain spectrum (BGS), and the amplified probe light AP. The upward arrows in the figure indicate each mode of the optical frequency comb. Further, BGS _n (n=1 to 4) in the figure indicates the Brillouin gain spectrum generated by the n-th mode of the pump light Pm.

In the measuring device 1 of this embodiment, the frequency interval Δν _probe of the optical frequency comb that is the probe light Pr is made to be slightly different from the frequency interval Δν _pump of the optical frequency comb that is the pump light Pm. The larger the number of modes of the optical frequency comb, the smaller the difference between the frequency interval Δν _probe and the frequency interval Δν _pump . Furthermore, the frequency of the pump light Pm is shifted relative to the frequency of the probe light Pr. In the optical fiber 2, when the light intensity of the pump light Pm is above a certain level, a Brillouin gain spectrum occurs in a certain frequency range lower than the frequency of the pump light Pm, and in that frequency range, light propagating counter-propagating with the pump light Pm (Probe light P
r) amplify. For example, when the frequency of the pump light Pm has a wavelength of 1.5 μm, a Brillouin gain spectrum occurs at a frequency approximately 11 GHz lower. The frequency difference between the frequency of one mode of the pump light Pm (the center frequency of the spectrum peak) and the center frequency of the Brillouin gain spectrum generated by the mode is called a Brillouin frequency shift (BFS). As shown in FIG. 2, each mode of the probe light Pr is amplified by each Brillouin gain spectrum (BGS ₁ to BGS ₄ ) generated at a frequency lower than the frequency of each mode of the pump light Pm. Note that the frequency interval Δν _pump and Δν _probe of the optical frequency comb is set to be wider than the Brillouin frequency shift.

The center frequency of the Brillouin gain spectrum shifts in proportion to the heat and strain applied to the optical fiber 2. Therefore, by determining the Brillouin frequency shift, the temperature and strain of the optical fiber 2 can be measured. In the measuring device 1 of this embodiment, the intensity (total power) of the amplified probe light AP is detected by the photodetector 70. By appropriately setting the spectral intensities of the optical frequency comb that becomes the probe light Pr and the optical frequency comb that becomes the pump light Pm in advance and shaping them by the spectrum shaping section (wavelength filters 40 and 41), for example, as shown in FIG. , the detection intensity of the amplified probe light AP becomes proportional to the Brillouin frequency shift (BFS) over a wide frequency range. That is, the spectrum shaping section (wavelength filters 40, 41) determines whether the relationship between the Brillouin frequency shift and the light intensity detected by the photodetector 70 corresponds to a predetermined function (for example, a monotonically increasing or monotonically decreasing relationship). ) to shape the spectral intensity of the optical frequency comb. Thereby, the Brillouin frequency shift can be uniquely determined from the light intensity detected by the photodetector 70 by simple calculation.

Further, amplification of the probe light Pr occurs only at the point where the pump light Pm and the probe light Pr collide. The collision point of the pump light Pm, which is pulsed light, and the probe light Pr can be controlled by the gate timing (pulse emission timing) by the optical modulators 50 and 51. Therefore, by controlling the signal generator 52 and setting the collision point at an arbitrary position in the sensing area SA, it is possible to obtain the intensity of the amplified probe light AP at an arbitrary position. Then, by setting collision points at multiple positions in the sensing area SA, obtaining the intensity of the amplified probe light AP for each set collision point, and finding the Brillouin frequency shift for each set collision point, the sensing area The distribution of heat and strain in SA can be measured.

Next, an example of a method for shaping the spectral intensity of an optical frequency comb will be described. Regarding the pump light Pm consisting of N modes, the wavelength filter 41 equalizes the intensity of each mode. Further, assuming that the probe light Pr is also composed of N modes, the intensity of the k-th mode from the lowest frequency of the probe light Pr is defined as _pk . Here, the intensity P _ap (ν _BFS ) of the probe light AP amplified by the Brillouin gain spectrum is expressed by the following equation (1).

ここで、Ｇ_０（ν_ＢＦＳ,νｋ）は、ｋ番目のモードに対する利得係数であり、ブリルアン周波数シフトν_ＢＦＳと、ｋ番目のモードの強度ｐ_ｋの関数となっている。ここで、合成利得係数Ｇ_Ｓ（ν_ＢＦＳ）を、以下の式（２）で定義する。

Here, G ₀ (ν _BFS , νk) is a gain coefficient for the kth mode and is a function of the Brillouin frequency shift ν _BFS and the intensity p _k of the kth mode. Here, the composite gain coefficient G _S (ν _BFS ) is defined by the following equation (2).

合成利得係数Ｇ_Ｓ（ν_ＢＦＳ）は、増幅されたプローブ光ＡＰの強度Ｐ_ａｐ（ν_ＢＦＳ）のブリルアン周波数シフトν_ＢＦＳに対する比である。ｋ番目のモードに対する利得係数Ｇ_０（ν_ＢＦＳ,νｋ）が予め測定されており、且つプローブ光Ｐｒの強度が大きくなくプローブ光Ｐｒの影響を受けないとすれば、合成利得係数Ｇ_Ｓ（ν_ＢＦＳ）が一定の周波数範囲で定数になる（すなわち、ν_ＢＦＳとＰ_ａｐ（ν_ＢＦＳ）との関係が一定の周波数範囲で比例関係となる）ような各モードの強度Ｐ_ｋは、数値計算で容易に求めることができる。図４は、合成利得係数Ｇ_Ｓ（ν_ＢＦＳ）が一定の周波数範囲で定数になるときの例であり、図中点線の各曲線は、増幅されたプローブ光ＡＰの各モードの強度とブリルアン周波数シフト（ＢＦＳ）との関係を示し、図中実線の直線は、増幅されたプローブ光ＡＰの強度（全パワー）とブリルアン周波数シフトとの関係を示している。

The composite gain coefficient G _S (ν _BFS ) is the ratio of the intensity P _ap (ν _BFS ) of the amplified probe light AP to the Brillouin frequency shift ν _BFS . If the gain coefficient G ₀ (ν _BFS , νk) for the k-th mode has been measured in advance and the intensity of the probe light Pr is not large and is not affected by the probe light Pr, then the combined gain coefficient G _S (ν The strength P _k of each mode such that _BFS ) becomes constant in a certain frequency range (that is, the relationship between ν _BFS and P _ap (ν _BFS ) is proportional in a certain frequency range) can be calculated by numerical calculation. can be easily determined. FIG. 4 is an example when the composite gain coefficient G _S (ν _BFS ) becomes a constant in a certain frequency range, and each dotted curve in the figure shows the intensity of each mode of the amplified probe light AP and the Brillouin frequency. The solid straight line in the figure shows the relationship between the intensity (total power) of the amplified probe light AP and the Brillouin frequency shift.

Each Brillouin gain spectrum generated by the pump light Pm consisting of N modes is given by a Lorentz function, for example. At that time, the entire Brillouin gain spectrum G _BFS (v) is expressed by the following equation (3).

ここで、ｇ_ｋは、ポンプ光Ｐｍのｋ番目のモードによって生成したブリルアン利得スペクトルの最大値を決める定数であり、特に、ポンプ光Ｐｍの各モードの強度が等しいとき、ｇ_ｋ＝ｇ_０（一定）である。また、ν_{ｐｕｍｐ,ｋ}は、ポンプ光Ｐｍのｋ番目のモードの周波数であり、Δν_ｐｕｍｐは、ポンプ光Ｐｍのスペクトル間隔（隣り合うモードの周波数間隔）であり、Δνは、定数である。

Here, g _k is a constant that determines the maximum value of the Brillouin gain spectrum generated by the k-th mode of pump light Pm. In particular, when the intensity of each mode of pump light Pm is equal, g _k = g ₀ ( constant). Further, ν _pump,k is the frequency of the k-th mode of the pump light Pm, Δν _pump is the spectral interval (frequency interval between adjacent modes) of the pump light Pm, and Δν is a constant.

Further, the spectrum P _probe (ν) of the probe light Pr is expressed by the following equation (4).

ここで、ν_{ｐｒｏｂｅ,ｋ}は、プローブ光Ｐｒのｋ番目のモードの周波数であり、Δν_{ｐｒｏｂｅ}は、プローブ光Ｐｒのスペクトル間隔である。また、ｆは、プローブ光Ｐｒの各モードのスペクトル形状を与える関数であり、ｆ（ν_{ｐｒｏｂｅ},ｋ）＝１とみなしてよい。

Here, ν _probe,k is the frequency of the k-th mode of the probe light Pr, and Δν _probe is the spectral interval of the probe light Pr. Further, f is a function that gives the spectral shape of each mode of the probe light Pr, and may be regarded as f(ν _probe ,k)=1.

Here, it is assumed that the intensity of each mode of the pump light Pm is equal. Further, the spectral line widths of the pump light Pm and the probe light Pr are assumed to be sufficiently narrow (mathematically, they are approximately considered as a delta function sequence). The intensity of each mode of the probe light AP amplified by the Brillouin gain spectrum is proportional to p _k G _BFS (v _probe,k ). Therefore, the intensity P _ap (ν _BFS ) of the probe light AP amplified by the Brillouin gain spectrum is given by the following equation (5).

ここで、Ａは、比例係数である。Ｐ_ａｐ（ν_ＢＦＳ）は、ブリルアン周波数シフトν_ＢＦＳの関数となっており、プローブ光Ｐｒの各モードの強度ｐ_ｋを適切に設定することで、任意の関数に近似した関数を合成する（ν_ＢＦＳとＰ_ａｐ（ν_ＢＦＳ）との関係が所定の関数に当てはまるようにする）ことができる。なお、比例係数Ａは、事前に一定のポンプ光（単一波長）に対して、プローブ光（単一波長）の強度とブリルアン利得スペクトルにより増幅されるプローブ光の強度との関係を測定しておくことで求めることができる。

Here, A is a proportionality coefficient. P _ap (ν _BFS ) is a function of the Brillouin frequency shift ν _BFS , and by appropriately setting the intensity p _k of each mode of the probe light Pr, a function that approximates an arbitrary function can be synthesized (ν The relationship between _BFS and P _ap (v _BFS ) can be made to fit a predetermined function). Note that the proportionality coefficient A is determined by measuring the relationship between the intensity of the probe light (single wavelength) and the intensity of the probe light amplified by the Brillouin gain spectrum with respect to a fixed pump light (single wavelength) in advance. You can find it by setting it.

For example, if straight lines are synthesized as arbitrary functions, the intensity P _ap (ν _BFS ) of the probe light amplified by the Brillouin gain spectrum becomes an output proportional to the change in the Brillouin frequency shift ν _BFS due to the temperature and strain of the optical fiber. You can make it happen. When an arbitrary function is a straight line, an error function e(ν _BFS ) is defined by the following equation (6) for a desired straight line (P _ap (ν _BFS )=aν _BFS +b).

そして、ｅ（ν_ＢＦＳ）＝０とおき、この関数に対して最小二乗法によりプローブ光Ｐｒの各モードの強度ｐ_ｋを決定する。図５に、任意の関数を直線とした場合のブリルアン周波数シフトν_ＢＦＳと増幅されるプローブ光の強度Ｐ_ａｐ（ν_ＢＦＳ）との関係をシミュレーションにより求めた結果を示す。

Then, e(ν _BFS )=0 is set, and the intensity p _k of each mode of the probe light Pr is determined using the least squares method for this function. FIG. 5 shows the results obtained by simulation of the relationship between the Brillouin frequency shift ν _BFS and the intensity P _ap (ν _BFS ) of the amplified probe light when an arbitrary function is a straight line.

FIG. 6 is a diagram showing an experimental system for a confirmation experiment to confirm the effect of the method of this embodiment. In FIG. 6, LD is a single wavelength laser light source, SS is a spectrum shaper, IM is an optical intensity modulator, PS is a polarization scrambler, and BPF is an optical bandpass PD is a photodetector, LIA is a lock-in amplifier, and FUT is a fiber under test (corresponding to sensing area SA).

The wavelength of the single wavelength laser light source LD was 1553 nm. The upper optical path of the experimental system is the optical path that generates the probe light Pr, in which the light from the light source is deeply modulated at 12.02 GHz by the optical intensity modulator IM, and the frequency interval (Δν _probe ) is 12.02 GHz. An optical frequency comb (first optical frequency comb) consisting of five modes was generated. That is, this optical intensity modulator IM functions as a first optical frequency comb generator.

The lower optical path of the experimental system is the optical path that generates the pump light Pm, and the light from the light source is shallowly modulated at 10.86 GHz by the first optical intensity modulator IM, so that the original optical frequency on the frequency axis is Among the sidebands occurring on both sides of the spectrum, the spectrum on the high frequency side was cut out using an optical bandpass filter BPF. This was deeply modulated at 12.00 GHz by a second optical intensity modulator IM to generate an optical frequency comb (second optical frequency comb) consisting of five modes with a frequency interval (Δν _pump ) of 12.00 GHz. That is, the first optical intensity modulator IM, the optical bandpass filter BPF, and the second optical intensity modulator IM function as an optical frequency shifter and a second optical frequency comb generator.

Furthermore, in each of the upper and lower optical paths of the experimental system, the optical frequency combs were shaped by a spectrum shaper SS so that the intensity of the amplified probe light AP was proportional to the Brillouin frequency shift. Here, the spectrum shaper SS is one that splits the incident light, arranges the lights of different wavelengths in space, adjusts the intensity of each light with a variable attenuator, and then combines them again (for example, a waveguide A type of spectral shaper) was used. Note that as the spectrum shaper SS, one using a spatial liquid crystal light modulator can also be used. In addition, each optical frequency comb was intensity-modulated by a periodic gate pulse using an optical intensity modulator IM and pulsed to generate probe light Pr and pump light Pm. This gate pulse has a width of 3 m (15 ns) and a period of 30 m (150 ns) when converted into space, giving a spatial resolution of 3 m and a measurement range of 30 m.

The 5 m end portion (the part shown in (b) in the figure) of the 30 m long fiber under test FUT is heated, and the intensity of the amplified probe light AP at each measurement point (pulse collision point) of the fiber under test FUT is measured over time. A change in the Brillouin frequency shift at each measurement point was measured by acquiring the data in series, and a change in the temperature distribution in the fiber FUT to be measured was observed. The results are shown in FIG. The graph in FIG. 7 shows the results of measuring the change in temperature (Brillouin frequency shift (BFS)) from the end of the fiber under test FUT by 3 m. From the top, the results for the 0 to 3 m section, the results for the 3 to 6 m section, The results for the 6-30m section are shown. The Brillouin frequency shift changes more greatly in the section closer to the heated part of the fiber under test FUT, and the Brillouin frequency shift does not change in the section farther from the heated part. From this result, it was confirmed that changes in the temperature distribution of the optical fiber can be measured with high accuracy.

Furthermore, in this experimental system, the relative frequency difference (equivalent to Brillouin frequency shift) between the pump light Pm and the probe light Pr was varied, and the intensity of the amplified probe light AP was measured. The results are shown in Figure 8 (black dots in the figure). From this result, it was confirmed that over a wide frequency range of 100 MHz (corresponding to a temperature change of 100 degrees), the intensity of the amplified probe light AP changes linearly with respect to changes in the relative frequency difference.

According to the Brillouin frequency shift measurement device of this embodiment, by using an optical frequency comb, measurement time can be shortened without deteriorating measurement accuracy. Further, since the Brillouin frequency shift is determined from the intensity of the emitted light rather than extracting frequency information from the emitted light from the optical fiber, signal processing can be simplified. Further, since the spectral intensity (intensity of each mode) of the optical frequency comb can be shaped using a pre-designed filter, calibration is easy.

Note that the present invention is not limited to the above-described embodiments, and various modifications are possible. The present invention includes configurations that are substantially the same as those described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objectives and effects). Further, the present invention includes a configuration in which non-essential parts of the configuration described in the embodiments are replaced. Further, the present invention includes a configuration that has the same effects or a configuration that can achieve the same objective as the configuration described in the embodiment. Further, the present invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

For example, in the above embodiment, a case has been described in which the spectral intensity of the optical frequency comb that becomes the probe light and the spectral intensity of the optical frequency comb that becomes the pump light are shaped, but only the spectral intensity of the optical frequency comb that becomes the probe light Alternatively, only the spectral intensity of the optical frequency comb serving as the pump light may be shaped.

1... Measuring device (Brillouin frequency shift measuring device), 2... Optical fiber, 10... Laser light source, 11... Optical splitter, 20... Optical frequency shifter, 30, 31... Optical frequency comb generator, 40, 41... Wavelength filter , 50, 51... Optical modulator, 52... Signal generator, 60... Optical circulator, 70... Photodetector, 80... Control device

Claims (2)

A branching part that branches the light from the laser light source into two,
an optical frequency shifter that shifts the frequency of one of the two branched lights;
a first optical frequency comb generator that generates a first optical frequency comb from one of the branched lights;
a second optical frequency comb generator that generates a second optical frequency comb having a frequency interval different from that of the first optical frequency comb from the other branched light;
a spectrum shaping unit that shapes the spectral intensity of the first optical frequency comb and/or the second optical frequency comb;
a first optical modulator that pulses the first optical frequency comb to generate probe light;
a second optical modulator that pulses the second optical frequency comb to generate pump light;
a light detection unit that detects light emitted from the other end of the optical fiber when the probe light is input from one end of the optical fiber to be measured and the pump light is input from the other end of the optical fiber; and,
a control unit that determines a Brillouin frequency shift in the optical fiber based on the light intensity detected by the light detection unit ;
The spectrum shaping section is
Shaping the spectral intensity of the first optical frequency comb and/or the second optical frequency comb so that the relationship between the Brillouin frequency shift and the light intensity detected by the photodetector fits a predetermined function. , Brillouin frequency shift measurement device. a branching step of branching the light from the laser light source into two;
an optical frequency shifting step of shifting the frequency of one of the two branched lights;
a first optical frequency comb generation step of generating a first optical frequency comb from one of the branched lights;
a second optical frequency comb generation step of generating a second optical frequency comb having a different frequency interval from the first optical frequency comb from the other branched light;
a spectral shaping step of shaping the spectral intensity of the first optical frequency comb and/or the second optical frequency comb;
a first optical modulation step of pulsing the first optical frequency comb to generate probe light;
a second optical modulation step of pulsing the second optical frequency comb to generate pump light;
a photodetection step of detecting light emitted from the other end of the optical fiber when the probe light is input from one end of the optical fiber to be measured and the pump light is input from the other end of the optical fiber; and,
a control step of determining a Brillouin frequency shift in the optical fiber based on the light intensity detected in the light detection step ,
In the spectrum shaping step,
Shaping the spectral intensity of the first optical frequency comb and/or the second optical frequency comb so that the relationship between the Brillouin frequency shift and the light intensity detected in the photodetection step fits a predetermined function. , Brillouin frequency shift measurement method.

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