JP6754138B2 - Laser pressure / strain meter - Google Patents
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- JP6754138B2 JP6754138B2 JP2017081210A JP2017081210A JP6754138B2 JP 6754138 B2 JP6754138 B2 JP 6754138B2 JP 2017081210 A JP2017081210 A JP 2017081210A JP 2017081210 A JP2017081210 A JP 2017081210A JP 6754138 B2 JP6754138 B2 JP 6754138B2
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Description
本発明は、レーザ圧力・ひずみ計に関するものである。本発明は、特に、極めて高感度に津波や地殻変動量を計測するレーザ圧力・ひずみ計に関するものである。 The present invention relates to a laser pressure / strain meter. The present invention particularly relates to a laser pressure / strain meter that measures tsunami and crustal movements with extremely high sensitivity.
従来、津波計としては水晶振動式の水圧変化計が幅広く利用されている。この計測器は、水圧変化に伴う水晶振動子の振動周波数の変化を電気信号として検出する方式を採用しているため、海中で電気信号を絶縁するための絶縁処理を施さなければならない。このため、海中でこの計測器を長期間使用すると、次第に絶縁が劣化し、これが故障の要因となることがある。また、この計測器には電源が必要であり、その駆動用バッテリーで決まる計測器の寿命がある。 Conventionally, a crystal vibration type water pressure change meter has been widely used as a tsunami meter. Since this measuring instrument employs a method of detecting a change in the vibration frequency of the crystal oscillator due to a change in water pressure as an electric signal, it is necessary to perform an insulation treatment for insulating the electric signal in the sea. Therefore, if this measuring instrument is used in the sea for a long period of time, the insulation gradually deteriorates, which may cause a failure. In addition, this measuring instrument requires a power source, and has a life of the measuring instrument determined by its drive battery.
そこで、このような従来の津波計の手段に代わるものとして、レーザを利用した津波計が提案されている（例えば特許文献１または２参照）。図１０に、特許文献２に開示されたレーザ津波計の構成を示す。図１０において、１は高感度センサ部、２は低感度センサ部、３は凹面鏡、４は凹面鏡ホルダ、５は直角プリズム、６はコリメータ、７はシール部、８は円筒容器、９は光ファイバを示す。このレーザ津波計は、円筒容器８の内部に一対の凹面鏡３からなるファブリー・ペロー型光共振器を取り付けることにより、水圧による円筒容器の内径変化をその光共振器の共振周波数変化として計測する方式を採用している。本方式の特徴は、沖合に設置したレーザの出力光を、光ファイバ９を介して海底に配置したセンサ部へ供給する遠隔計測が可能であり、またセンサ部に電源が不要であることである。 Therefore, as an alternative to such a conventional tsunami meter means, a tsunami meter using a laser has been proposed (see, for example, Patent Document 1 or 2). FIG. 10 shows the configuration of the laser tsunami meter disclosed in Patent Document 2. In FIG. 10, 1 is a high-sensitivity sensor unit, 2 is a low-sensitivity sensor unit, 3 is a concave mirror, 4 is a concave mirror holder, 5 is a right-angle prism, 6 is a collimator, 7 is a seal portion, 8 is a cylindrical container, and 9 is an optical fiber. Is shown. This laser tsunami meter is a method of measuring a change in the inner diameter of a cylindrical container due to water pressure as a change in the resonance frequency of the optical resonator by attaching a Fabry-Perot type optical resonator composed of a pair of concave mirrors 3 inside the cylindrical container 8. Is adopted. The feature of this method is that it is possible to perform remote measurement by supplying the output light of the laser installed offshore to the sensor unit arranged on the seabed via the optical fiber 9, and the sensor unit does not require a power supply. ..
さらに、同レーザ計測技術を利用した他の計測応用として、円筒容器内部に複数の光共振器を取り付け、０°、１２０°、２４０°の三方向のひずみの変化を同時に検出する三成分ひずみ計が提案されている（例えば、特許文献３参照）。 Furthermore, as another measurement application using the same laser measurement technology, a three-component strain meter that simultaneously detects changes in strain in three directions of 0 °, 120 °, and 240 ° by mounting multiple optical resonators inside the cylindrical container. Has been proposed (see, for example, Patent Document 3).
なお、本発明者等により、光周波数の基準光源として、１．５５μｍ帯のアセチレン分子やシアン化水素分子の吸収線ピークに光周波数を安定化させた周波数安定化レーザが開発されており（例えば、非特許文献１，２参照）、周波数可変レーザとして、狭線幅な発振スペクトルが得られる１．５５μｍ帯エルビウムファイバレーザが開発されている（例えば、非特許文献３参照）。 In addition, the present inventors have developed a frequency-stabilized laser in which the optical frequency is stabilized at the absorption line peak of acetylene molecules and hydrogen cyanide molecules in the 1.55 μm band as a reference light source for the optical frequency (for example, non-standard light source). (See Patent Documents 1 and 2), as a frequency-variable laser, a 1.55 μm band erbium fiber laser capable of obtaining a narrow line width oscillation spectrum has been developed (see, for example, Non-Patent Document 3).
特許文献１乃至３に記載のレーザを利用した津波計ならびにひずみ計の分解能を高めるためには、高いフィネスを有する光共振器を円筒容器８の内部に取り付ける必要がある。そのため、凹面鏡３、直交プリズム５およびコリメータ６からなる光共振器部の作製に高精度な光軸調整や、軸ずれに対する長期信頼性が要求され、その結果、センサ部の構造が複雑になり、製造コストもかかるといった課題があった。また、光共振器長も実質上制限されるため高感度化にも制限があるという課題もあった。 In order to improve the resolution of the tsunami meter and the strain meter using the laser described in Patent Documents 1 to 3, it is necessary to mount an optical resonator having a high finesse inside the cylindrical container 8. Therefore, high-precision optical axis adjustment and long-term reliability against axis misalignment are required for manufacturing an optical resonator portion including a concave mirror 3, an orthogonal prism 5, and a collimator 6, and as a result, the structure of the sensor portion becomes complicated. There was a problem that the manufacturing cost was also high. In addition, since the length of the optical resonator is also substantially limited, there is also a problem that there is a limit to high sensitivity.
本発明は、このような課題に着目してなされたもので、光共振器として低損失かつ光軸調整が不要な長尺光ファイバリング共振器を利用することで、簡便かつ安価で超高感度なレーザ圧力・ひずみ計を提供することを目的とする。 The present invention has been made focusing on such a problem, and by using a long optical fiber ring resonator having low loss and no need for optical axis adjustment as an optical resonator, it is simple, inexpensive, and ultra-high sensitivity. An object of the present invention is to provide a laser pressure / strain meter.
かかる目的を達成するために、本発明に係るレーザ圧力・ひずみ計は、光ファイバリングをセンサ部に巻き付けて、光ファイバリング共振器を形成し、前記光ファイバリング共振器の共振周波数に、前記センサ部の外部に設置した波長可変レーザの発振周波数を負帰還させて安定化し、前記波長可変レーザの発振周波数により前記光ファイバリング共振器の共振周波数を検出するよう設けられ、外部の圧力および／またはひずみの変化に伴う前記センサ部の形状変化を、該光ファイバリング共振器の共振周波数変化から検出可能に構成されていることを特徴とする。またＱ値の高い光ファイバリング共振器にレーザ周波数をトラッキングするために、狭線幅のレーザ光源を用いることにより高感度化が実現できる。
In order to achieve such an object, in the laser pressure / strain meter according to the present invention, an optical fiber ring is wound around a sensor unit to form an optical fiber ring resonator, and the resonance frequency of the optical fiber ring resonator is set to the above. The oscillation frequency of the wavelength variable laser installed outside the sensor unit is negatively fed back to stabilize, and the resonance frequency of the optical fiber ring resonator is detected by the oscillation frequency of the wavelength variable laser. Alternatively, it is characterized in that the shape change of the sensor unit due to the change of strain can be detected from the change in the resonance frequency of the optical fiber ring resonator. Further, in order to track the laser frequency in the optical fiber ring resonator having a high Q value, high sensitivity can be realized by using a laser light source having a narrow line width.
本発明に係るレーザ圧力・ひずみ計は、前記センサ部に、圧力および／またはひずみの変化に対し高感度に形状が変化する高感度検出部と、前記高感度検出部よりも低感度に形状が変化する低感度検出部とを有し、前記高感度検出部および前記低感度検出部における形状変化量の差分に基づいて、前記圧力および／またはひずみの変化を高精度に検出可能に構成されていてもよい。 The laser pressure / strain meter according to the present invention has a high-sensitivity detection unit whose shape changes with high sensitivity to changes in pressure and / or strain, and a shape with lower sensitivity than the high-sensitivity detection unit. It has a low-sensitivity detection unit that changes, and is configured to be able to detect changes in pressure and / or strain with high accuracy based on the difference in the amount of shape change between the high-sensitivity detection unit and the low-sensitivity detection unit. You may.
また、前記センサ部に、それぞれ０°、１２０°、および２４０°の各方向にかかるひずみに対応して高感度に形状が変化する３つの検出部を有し、各検出部で検出した三成分のひずみの変化により地殻変動量を計測可能に構成されていてもよい。 Further, the sensor unit has three detection units whose shape changes with high sensitivity in response to strains applied in each of 0 °, 120 °, and 240 °, respectively, and the three components detected by each detection unit. It may be configured so that the amount of crustal movement can be measured by the change in the strain of.
本発明に係るレーザ圧力・ひずみ計は、波長可変レーザの発振周波数を光ファイバリング共振器の共振周波数に一致させるように安定化することより、レーザ発振周波数の変化によりひずみ量を直接検出することができる。このため遠隔地に配置した光ファイバリング共振器により高感度な圧力・ひずみ計測が可能となる。
The laser pressure / strain meter according to the present invention stabilizes the oscillation frequency of the tunable laser so as to match the resonance frequency of the optical fiber ring resonator, thereby directly detecting the amount of strain by changing the laser oscillation frequency. Can be done. Therefore, the optical fiber ring resonator placed in a remote place enables highly sensitive pressure / strain measurement.
本発明により、光ファイバをセンサ部に多数回巻きつけた簡便な構成のレーザ圧力・ひずみ計を実現できる。また、光ファイバリング共振器は光軸のずれの問題がないため長期信頼性にも優れている。さらに、光ファイバリング共振器はファイバを多数回巻き付けることにより長尺化し、それにより高いＱ値を有することから、圧力・ひずみ計測に高い分解能が得られる。以上のように本発明によると、光共振器として低損失かつ光軸調整が不要な長尺光ファイバリング共振器を利用することで、簡便かつ安価で超高感度な実用性の高いレーザ圧力・ひずみ計を提供できる。 According to the present invention, it is possible to realize a laser pressure / strain meter having a simple structure in which an optical fiber is wound around a sensor unit many times. In addition, the optical fiber ring resonator has excellent long-term reliability because there is no problem of misalignment of the optical axis. Further, since the optical fiber ring resonator is lengthened by winding the fiber many times and has a high Q value, high resolution can be obtained for pressure / strain measurement. As described above, according to the present invention, by using a long optical fiber ring resonator having low loss and no need for optical axis adjustment as an optical resonator, a simple, inexpensive, ultra-sensitive and highly practical laser pressure can be used. A strain gauge can be provided.
以下、図面に基づいて、本発明の実施の形態について説明する。
図１は、本発明の第１の実施形態のレーザ圧力・ひずみ計を示すブロック図である。図において、レーザ圧力・ひずみ計は、センサ部１０の外周に、例えば１０８程度の高いＱ値を有する光ファイバリング共振器１１を適度な強さで巻き付け、その共振周波数を、共振周波数検出部１２を用いてモニタする系からなる。センサ部１０として、例えば金属製の円筒あるいは球構造体を用いることができる。
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a laser pressure / strain meter according to the first embodiment of the present invention. In the figure, the laser pressure and strain gauge, the outer periphery of the sensor unit 10, wound optical fiber ring resonator 11 having, for example, high Q value of about 10 8 at moderate intensity, the resonance frequency, the resonance frequency detection unit It consists of a system to monitor using 12. As the sensor unit 10, for example, a metal cylinder or a spherical structure can be used.
図２に第１の実施形態の光ファイバリング共振器１１の構成例を示す。図２（ａ）は、偏波保持単一モードファイバ１３、偏波ビームスプリッタ１４および２分の１波長板１５をリング状に閉じた偏波保持型光ファイバリング共振器の構成例を示す。偏波ビームスプリッタ１４の偏波軸に偏波状態を合わせることで、外部からの光信号を共振器内へ無損失で結合できる利点がある。また、２分の１波長板１５により、共振器内への光電力の帰還率ｒ２とレーザ外部への出力率１−ｒ２とを任意に調整できる。ここで、ｒは、偏波ビームスプリッタ１４における光電界の反射係数である。 FIG. 2 shows a configuration example of the optical fiber ring resonator 11 of the first embodiment. FIG. 2A shows a configuration example of a polarization-holding optical fiber ring resonator in which the polarization-holding single-mode fiber 13, the polarization beam splitter 14, and the half-wave plate 15 are closed in a ring shape. By aligning the polarization state with the polarization axis of the polarization beam splitter 14, there is an advantage that an optical signal from the outside can be coupled into the resonator without loss. Further, the half-wave plate 15 can arbitrarily adjust the feedback rate r 2 of the optical power into the resonator and the output rate 1-r 2 to the outside of the laser. Here, r is the reflection coefficient of the optical electric field in the polarization beam splitter 14.
一方、図２（ｂ）は、標準単一モードファイバ１６と２つの光カプラ１７とをリング状に閉じた非偏波保持型光ファイバリング共振器の構成例を示す。この共振器は偏波無依存であるため、外部からの入力光信号の偏波状態の制御が不要であるといった利点がある。図２（ａ）および（ｂ）に示すいずれの共振器においても、共振器内に挿入する光デバイスとしてインライン型の市販光ファイバモジュールを用いることができる。 On the other hand, FIG. 2B shows a configuration example of a non-polarization holding type optical fiber ring resonator in which a standard single mode fiber 16 and two optical couplers 17 are closed in a ring shape. Since this resonator is polarization-independent, there is an advantage that it is not necessary to control the polarization state of the input optical signal from the outside. In any of the resonators shown in FIGS. 2 (a) and 2 (b), an in-line type commercially available optical fiber module can be used as an optical device to be inserted into the resonator.
つぎに、図２（ｂ）に示す非偏波保持型光ファイバリング共振器を例にとり、光ファイバリング共振器の共振の鋭さについて述べる。共振器への入力光の電界振幅をＡｉｎ、２つの光カプラ１７における光反射係数（共振器内へ帰還する光電界の振幅比率）をいずれもｒとすると、共振器から出力される光電界振幅Ａｏｕｔは次式で与えられる。
光周波数をｆ、共振器を構成する光ファイバ長をｌ、光ファイバの屈折率をｎ、光速をｃとすると、次式の関係が成り立つ。
光ファイバリング共振器の伝達関数は、式（１）を用いて次式で与えられる。
式（４）は、ｓｉｎ（δ／２）＝０の関係を満たすとき、すなわちｆ＝Ｎｃ／ｎｌ（Ｎは整数）の共振周波数において、伝達関数が最大値１を示すことを表している。例えば、センサ部における圧力が変化し、光ファイバリング共振器の長さがΔｌだけ変動した場合には、この伝達関数の共振周波数の変化Δｆ＝−ＮｃΔｌ／ｎｌ２をモニタすることにより、その圧力変化を計測することができる。この圧力計測における分解能は、伝達関数の半値全幅ｆＦＷＨＭに比例するため、計測感度を高めるためにはｆＦＷＨＭの小さい、すなわちＱ値の高い共振器を用いることが重要となる。 Equation (4) expresses that the transfer function shows a maximum value of 1 when the relation of sin (δ / 2) = 0 is satisfied, that is, at the resonance frequency of f = Nc / nl (N is an integer). For example, when the pressure in the sensor unit changes and the length of the optical fiber ring resonator fluctuates by Δl, the pressure is monitored by monitoring the change Δf = −NcΔl / nl 2 of the resonance frequency of this transfer function. Changes can be measured. Since the resolution in this pressure measurement is proportional to the full width at half maximum f FWHM of the transfer function, it is important to use a resonator having a small f FWHM , that is, a high Q value in order to increase the measurement sensitivity.
光ファイバリング共振器のｆＦＷＨＭは、式（２）を式（４）に代入することにより次式で与えられる。
式（５）は、ｆＦＷＨＭの値を下げるためには光カプラの帰還率Ｒを高め、共振器長ｌを長くすればよいことを示している。ここで、２つの光カプラによる共振器の挿入損失をＬ［ｄＢ］とすると、Ｌ＝−１０log（Ｒ２）の関係が成り立つ。一般の光デバイスには０．５ｄＢ前後の挿入損失があるため、Ｌを０に近づけることには限界がある。一方、共振器を構成する光ファイバは、０．２ｄＢ／ｋｍと低損失であるため、ｌを数ｍから数１００ｍのオーダーに長くすることで、ｆＦＷＨＭの値を容易に下げることができる。このことが光ファイバリング共振器の特長である。 Equation (5) shows that in order to reduce the value of fFWHM , the feedback rate R of the optical coupler should be increased and the resonator length l should be lengthened. Here, assuming that the insertion loss of the resonator due to the two optical couplers is L [dB], the relationship of L = -10log (R 2 ) is established. Since a general optical device has an insertion loss of about 0.5 dB, there is a limit to bringing L close to 0. On the other hand, since the optical fiber constituting the resonator has a low loss of 0.2 dB / km, the value of fFWHM can be easily lowered by lengthening l to the order of several meters to several hundreds of meters. This is a feature of the optical fiber ring resonator.
実測例として、試作した共振器長ｌが４．５ｍ、挿入損失Ｌが２ｄＢである非偏波保持型光ファイバリング共振器の伝達関数を図３に示す。周波数間隔ｃ／ｎｌが４５ＭＨｚで、半値全幅ｆＦＷＨＭが３．５ＭＨｚ（式（４）より算出される値３．３ＭＨｚと同等）の鋭い共振モードが観測されている様子を示している。本共振器は光ファイバで構成されているため、外乱の存在する環境下においても光軸の問題はなく、この鋭い伝達関数は長期に亘り保持される利点がある。 As an actual measurement example, FIG. 3 shows a transfer function of a prototype non-polarized optical fiber ring resonator having a resonator length l of 4.5 m and an insertion loss L of 2 dB. It shows that a sharp resonance mode is observed in which the frequency interval c / nl is 45 MHz and the full width at half maximum f FWHM is 3.5 MHz (equivalent to the value 3.3 MHz calculated from the equation (4)). Since this resonator is composed of an optical fiber, there is no problem with the optical axis even in an environment where disturbance exists, and this sharp transfer function has an advantage of being maintained for a long period of time.
一方、従来の２枚のミラーを対向させたファブリー・ペロー共振器を用いてこれと同等の伝達関数を得るためには、例えば、その共振器長を５０ｍｍとした場合、凹面鏡の光軸を高精度に調整し９９．７％もの反射率を達成しなければならない。また、ファブリー・ペロー共振器では、外乱による光学軸のずれの影響を受け易いため、あるいはレーザ鏡間での空気の対流や長期的な汚れなども無視できず、このような高反射率を有する共振器を長期に亘り保持する機構を実現することは極めて困難である。 On the other hand, in order to obtain a transfer function equivalent to this by using a conventional Fabry-Perot resonator in which two mirrors are opposed to each other, for example, when the resonator length is 50 mm, the optical axis of the concave mirror is raised. It must be adjusted to precision to achieve a reflectance of 99.7%. Further, the Fabry-Perot resonator has such a high reflectance because it is easily affected by the displacement of the optical axis due to disturbance, or air convection between laser mirrors and long-term contamination cannot be ignored. It is extremely difficult to realize a mechanism for holding the resonator for a long period of time.
図４に、第１の実施形態の共振周波数検出部１２の構成の一例を示す。光周波数の基準となる参照光源１８と、センサ部１０に巻き付けた光ファイバリング共振器１１の共振周波数に発振周波数をトラッキング（一致）させる周波数可変レーザ１９と、光ファイバリング共振器１１の中心周波数へのトラッキング制御に用いる負帰還制御回路２０と、これら２台のレーザの出力光を分岐／合波するための光カプラ２１と、２台のレーザ間のヘテロダインビート信号を検出するための光検出器２２と、そのビート信号の中心周波数を測定するための周波数カウンタ２３とからなる。参照光源１８の発振周波数ν０と周波数可変レーザ１９の発振周波数ν１との差分を、ヘテロダインビート信号（中心周波数ｆＢ＝ν１−ν０）より測定し、その中心周波数の時間に対する変動量ΔｆＢ＝Δν１をモニタすることで、センサ部１０に巻き付けた光ファイバリング共振器１１の共振周波数変化を計測できる。 FIG. 4 shows an example of the configuration of the resonance frequency detection unit 12 of the first embodiment. A reference light source 18 that serves as a reference for the optical frequency, a frequency-variable laser 19 that tracks (matches) the oscillation frequency with the resonance frequency of the optical fiber ring resonator 11 wound around the sensor unit 10, and a central frequency of the optical fiber ring resonator 11. The negative feedback control circuit 20 used for tracking control to the frequency, the optical coupler 21 for branching / combining the output light of these two lasers, and the optical detection for detecting the heterodyne beat signal between the two lasers. It includes a device 22 and a frequency counter 23 for measuring the center frequency of the beat signal. The difference between the oscillation frequency [nu 1 of the oscillation frequency [nu 0 and the frequency-tunable laser 19 of the reference light source 18, as measured from the heterodyne beat signal (center frequency f B = ν 1 -ν 0) , the amount of variation with respect to time of the center frequency By monitoring Δf B = Δν 1 , the resonance frequency change of the optical fiber ring resonator 11 wound around the sensor unit 10 can be measured.
例えば、このビート信号の検出に用いるレーザ光源の波長を光通信波長１．５５μｍ帯に設定することで、共振器周波数検出部１２とセンサ部１０との間を長距離の光ファイバ伝送路で結ぶことができる。その結果、共振器周波数検出部１２を平野部に配置し、そこから遠く離れた海底や火山地域にセンサ部１０を配置した、安全な遠隔計測が可能となる。また、複数個所にセンサ部１０を配置し、面状に多点計測を行う光ファイバ計測網を構築することも容易となる。そのため、参照光源１８としては、１．５５μｍ帯のアセチレン分子やシアン化水素分子の吸収線ピークに光周波数を安定化させた周波数安定化レーザが有効である（非特許文献１，２参照）。また、周波数可変レーザ１９としては、狭線幅な発振スペクトルが得られる１．５５μｍ帯エルビウムファイバレーザが有効である（非特許文献３参照）。 For example, by setting the wavelength of the laser light source used for detecting the beat signal to the optical communication wavelength 1.55 μm band, the resonator frequency detection unit 12 and the sensor unit 10 are connected by a long-distance optical fiber transmission line. be able to. As a result, safe remote measurement is possible by arranging the resonator frequency detection unit 12 in the plain portion and arranging the sensor unit 10 in the seabed or volcanic area far from the plain portion. Further, it is also easy to construct an optical fiber measurement network in which sensor units 10 are arranged at a plurality of places to perform multi-point measurement on a plane surface. Therefore, as the reference light source 18, a frequency-stabilized laser in which the optical frequency is stabilized at the absorption line peak of the 1.55 μm band acetylene molecule or hydrogen cyanide molecule is effective (see Non-Patent Documents 1 and 2). Further, as the frequency variable laser 19, a 1.55 μm band erbium fiber laser that can obtain an oscillation spectrum with a narrow line width is effective (see Non-Patent Document 3).
図５は、本発明の第２の実施形態を示すブロック図である。図において、外部の圧力変化に対する計測感度が異なる高感度センサ部２４と低感度センサ部２５とを設け、それぞれのセンサ部の外周に、例えば１０８程度の高いＱ値を有する光ファイバリング共振器１１を巻き付ける。そして、２台の周波数可変レーザ１９の発振周波数を、それぞれ高感度センサ部２４および低感度センサ部２５に巻き付けた光ファイバリング共振器１１の共振周波数にトラッキングし、光カプラ２１、光検出器２２および周波数カウンタ２３を用いて、両レーザの発振周波数の差を測定する。これにより、高感度センサ部２４および低感度センサ部２５に巻き付けた２つの光ファイバリング共振器１１の間の共振周波数差を検出する。 FIG. 5 is a block diagram showing a second embodiment of the present invention. In the figure, provided with high-sensitivity sensor unit 24 which measures different sensitivities to external pressure changes and a low-sensitivity sensor unit 25, the outer periphery of each of the sensor unit, for example, an optical fiber ring resonator having a high Q value of about 10 8 Wrap 11 around. Then, the oscillation frequencies of the two frequency-variable lasers 19 are tracked to the resonance frequencies of the optical fiber ring resonator 11 wound around the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25, respectively, and the optical coupler 21 and the optical detector 22 are tracked. And the frequency counter 23 is used to measure the difference between the oscillation frequencies of both lasers. As a result, the resonance frequency difference between the two optical fiber ring resonators 11 wound around the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25 is detected.
光ファイバリング共振器１１の共振周波数は、センサ外部の圧力変化の他、例えばセンサ部周辺の温度変化によっても変動してしまう。そのため、温度変化の影響を補償することが重要となる。そこで、図５では、圧力変化に対する感度の異なる２つのセンサを隣接して配置し、それらが共通の温度変化の影響を受けるようにすることで、前記共振周波数差を測定した際に、温度変化に伴う共振周波数変化分を相殺できる。一方、圧力変化に伴う共振周波数変化に対しては、両センサの感度が異なるため、その差分を検出することができる。ここで、光ファイバリング共振器１１の構成は、第１の実施形態と同様である。 The resonance frequency of the optical fiber ring resonator 11 fluctuates not only due to a pressure change outside the sensor but also due to, for example, a temperature change around the sensor unit. Therefore, it is important to compensate for the effects of temperature changes. Therefore, in FIG. 5, two sensors having different sensitivities to pressure changes are arranged adjacent to each other so that they are affected by a common temperature change, so that the temperature change when the resonance frequency difference is measured is obtained. The amount of change in resonance frequency that accompanies this can be offset. On the other hand, since the sensitivities of both sensors are different with respect to the change in resonance frequency due to the change in pressure, the difference can be detected. Here, the configuration of the optical fiber ring resonator 11 is the same as that of the first embodiment.
図６に、第２の実施形態におけるセンサ部の構成例を示す。図６（ａ）は、異なるヤング率を有する２つの円筒型金属材料を利用した充填型センサの構成例である。低ヤング率の金属部を高感度センサ部２４、高ヤング率の金属部を低感度センサ部２５として用いる。充填型センサは、複雑な加工が不要といった利点がある。 FIG. 6 shows a configuration example of the sensor unit according to the second embodiment. FIG. 6A is a configuration example of a filling type sensor using two cylindrical metal materials having different Young's modulus. A metal portion having a low Young's modulus is used as the high-sensitivity sensor unit 24, and a metal portion having a high Young's modulus is used as the low-sensitivity sensor unit 25. The filling type sensor has an advantage that complicated processing is not required.
一方、図６（ｂ）〜（ｆ）は、センサ内部が空洞である空洞型センサの構成例である。本センサは、同一の材料で高感度センサ部２４および低感度センサ部２５を構成できることより、両センサ部の熱膨張係数が完全に一致し、温度変化に伴う共振周波数変化をより高い精度で相殺できる利点がある。図６（ｂ）に示すセンサ部は、厚みの異なる２つの円筒構造体からなり、肉薄部を高感度センサ部２４、肉厚部を低感度センサ部２５として用いている。図６（ｃ）に示すセンサ部は、図６（ｂ）のものと同じ動作原理であるが、光ファイバリング共振器１１を巻き付ける向きが直交の関係にある。図６（ｄ）に示すセンサ部は、その一部に薄目の蛇腹構造を有し、特定の直径方向（蛇腹構造のある軸に対し直交の向き）の圧力変化に対し高い感度が得られるように工夫を施したものである。図６（ｅ）および（ｆ）は、それぞれ図６（ｃ）および（ｄ）のセンサを球構造にしたものである。 On the other hand, FIGS. 6 (b) to 6 (f) are configuration examples of a hollow type sensor in which the inside of the sensor is hollow. Since the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25 can be configured with the same material in this sensor, the thermal expansion coefficients of both sensor units are completely the same, and the resonance frequency change due to temperature change is canceled out with higher accuracy. There are advantages that can be done. The sensor unit shown in FIG. 6B is composed of two cylindrical structures having different thicknesses, and the thin portion is used as the high-sensitivity sensor portion 24 and the thick portion is used as the low-sensitivity sensor portion 25. The sensor unit shown in FIG. 6 (c) has the same operating principle as that of FIG. 6 (b), but the directions in which the optical fiber ring resonator 11 is wound are orthogonal to each other. The sensor unit shown in FIG. 6D has a thin bellows structure in a part thereof, so that high sensitivity to a pressure change in a specific diameter direction (direction orthogonal to an axis having the bellows structure) can be obtained. It is a device that has been devised. 6 (e) and 6 (f) are spherical structures of the sensors of FIGS. 6 (c) and 6 (d), respectively.
図７に、第２の実施形態による水圧計測の結果の一例を示す。センサ部として図６（ｃ）に示す空洞型センサを利用した。直径１２０ｍｍ、高さ２００ｍｍの円筒型アルミ材を加工し、高感度センサ部２４および低感度センサ部２５の外壁の厚みがそれぞれ３ｍｍおよび１０ｍｍである空洞構造を形成した。また、各センサ部の外周に光ファイバを巻き付けるための溝を施した。 FIG. 7 shows an example of the result of water pressure measurement according to the second embodiment. The hollow type sensor shown in FIG. 6C was used as the sensor unit. A cylindrical aluminum material having a diameter of 120 mm and a height of 200 mm was processed to form a hollow structure in which the outer walls of the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25 had thicknesses of 3 mm and 10 mm, respectively. Further, a groove for winding the optical fiber is provided on the outer circumference of each sensor unit.
計２台の光ファイバリング共振器１１としては、図２（ｂ）に示す非偏波保持型光ファイバリング共振器を利用した。２つの分岐比１：９９の光カプラモジュールおよび、反射光の影響を除くための光アイソレータモジュールをリング状に融着接続し、長さ４．５ｍのリング共振器を構成した。共振器の挿入損失は約２ｄＢであり、図３に示す伝達関数を有する。この光共振器を２つ作製し、それらを高感度センサ部２４および低感度センサ部２５に巻き付けた。また、共振周波数差を検出するための２台の周波数可変レーザ１９には、１．５５μｍで発振するエルビウム添加ファイバレーザを使用した。本レーザは、全長が４ｍの偏波保持ファイバリング共振器からなり、その共振器内に、３ｄＢ帯域が１．２ＧＨｚであるファイバブラッググレーティング狭帯域光フィルタを挿入することで、単一モード動作を得ている。 As a total of two optical fiber ring resonators 11, the non-polarized light holding type optical fiber ring resonator shown in FIG. 2B was used. Two optical coupler modules having a branching ratio of 1:99 and an optical isolator module for eliminating the influence of reflected light were fused and connected in a ring shape to form a ring resonator having a length of 4.5 m. The insertion loss of the resonator is about 2 dB and has the transfer function shown in FIG. Two of these optical resonators were manufactured and wound around the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25. Further, as the two variable frequency lasers 19 for detecting the resonance frequency difference, an erbium-added fiber laser oscillating at 1.55 μm was used. This laser consists of a polarization-maintaining fiber ring resonator with a total length of 4 m, and by inserting a fiber Bragg grating narrow-band optical filter with a 3 dB band of 1.2 GHz into the resonator, single-mode operation is performed. It has gained.
また、共振器を構成する光ファイバの一部を円筒型ピエゾ（ＰＺＴ）素子に巻き付け、ＰＺＴ素子への印加電圧によりレーザ周波数を可変にしている。レーザの線幅は５ｋＨｚと狭く、１００ｍＷ以上の高出力（スロープ効率３５％）が得られている。ＬＮ光位相変調器とバランスミキサからなる位相敏感検波系とを用いて、本ファイバレーザの発振周波数と、センサ部に巻き付けた光ファイバリング共振器１１の共振周波数とのずれ量を電圧信号として検出し、その検出電圧信号を、ＰＩ制御回路を介してレーザ共振器内のＰＺＴ素子へ帰還することで、両周波数の一致（トラッキング制御）を図っている。 Further, a part of the optical fiber constituting the resonator is wound around a cylindrical piezo (PZT) element, and the laser frequency is made variable by the voltage applied to the PZT element. The line width of the laser is as narrow as 5 kHz, and a high output of 100 mW or more (slope efficiency of 35%) is obtained. Using a phase-sensitive detection system consisting of an LN optical phase modulator and a balance mixer, the amount of deviation between the oscillation frequency of this fiber laser and the resonance frequency of the optical fiber ring resonator 11 wound around the sensor is detected as a voltage signal. Then, by feeding back the detected voltage signal to the PZT element in the laser oscillator via the PI control circuit, matching of both frequencies (tracking control) is achieved.
高感度センサ部２４および低感度センサ部２５にトラッキングした２台のファイバレーザ出力光のヘテロダインビート信号のＲＦスペクトルを、図７（ａ）に示す。狭線幅なファイバレーザを用いることで、ヘテロダインビート信号の短期的な揺らぎ幅を±１１０ｋＨｚの範囲に抑えることができている。つぎに、センサ部を水槽の中に沈め、その水位を１ｃｍずつ下げたとき（水位誤差２ｍｍ）のビート信号の中心周波数変化を測定した結果を、図７（ｂ）に示す。図より、ビート信号の中心周波数は、水位変化に対し−１２．２ＭＨｚ／ｃｍの傾きで変化していることがわかる。水位変化の測定において、図７（ａ）に示すようなビート信号の短期的な周波数揺らぎが測定誤差を与え、この誤差が測定分解能を決めている。この周波数揺らぎ幅が２２０ｋＨｚであることより、本試験における水位変化の測定分解能は、１８０μｍであるといえる。以上のように、第２の実施形態を用いて、１ｍｍ以下の極めて高い分解能を有する水圧計測が実現できている。 The RF spectra of the heterodyne beat signals of the two fiber laser output lights tracked by the high-sensitivity sensor unit 24 and the low-sensitivity sensor unit 25 are shown in FIG. 7A. By using a fiber laser with a narrow line width, the short-term fluctuation width of the heterodyne beat signal can be suppressed to the range of ± 110 kHz. Next, FIG. 7B shows the results of measuring the change in the center frequency of the beat signal when the sensor unit is submerged in the water tank and the water level is lowered by 1 cm (water level error 2 mm). From the figure, it can be seen that the center frequency of the beat signal changes with a slope of -12.2 MHz / cm with respect to the change in water level. In the measurement of the water level change, the short-term frequency fluctuation of the beat signal as shown in FIG. 7A gives a measurement error, and this error determines the measurement resolution. Since the frequency fluctuation width is 220 kHz, it can be said that the measurement resolution of the water level change in this test is 180 μm. As described above, the water pressure measurement having an extremely high resolution of 1 mm or less can be realized by using the second embodiment.
図８は、本発明の第３の実施形態を示すブロック図である。図において、０°、１２０°、２４０°の三方向のひずみの変化を計測するための３つのセンサ部（０°センサ部２６、１２０°センサ部２７、２４０°センサ部２８）に、それぞれ例えば１０８程度の高いＱ値を有する光ファイバリング共振器１１を巻き付け、各センサ部における共振周波数の変化を、計３台の共振周波数検出部１２でモニタする系からなる。この三成分ひずみ計の基本動作は、第１の実施形態と同様である。 FIG. 8 is a block diagram showing a third embodiment of the present invention. In the figure, for example, three sensor units (0 ° sensor unit 26, 120 ° sensor unit 27, 240 ° sensor unit 28) for measuring changes in strain in three directions of 0 °, 120 °, and 240 ° are used, for example. winding an optical fiber ring resonator 11 having a high Q value of about 10 8, a change in the resonant frequency of each sensor unit, consisting of a system for monitoring a total of three resonance frequency detection unit 12. The basic operation of this three-component strain gauge is the same as that of the first embodiment.
図９に、第３の実施形態におけるセンサ部の構成例を示す。図９（ａ），（ｂ）は円筒構造を有する円筒型センサ、図９（ｃ），（ｄ）は球構造を有する球型センサの構成例である。図９（ａ）と（ｃ）に示すセンサ部は、一部の外壁の厚みを薄くし、その部分に付与されるひずみに対する検出感度を高めている。一方、図９（ｂ）と（ｄ）に示すセンサ部は、一部の外壁を蛇腹構造にし、それと直交する方向のひずみに対する検出感度を高めている。いずれの構造においても、０°、１２０°、２４０°の三方向に対し高感度センサ部を設けている。 FIG. 9 shows a configuration example of the sensor unit according to the third embodiment. 9 (a) and 9 (b) are configuration examples of a cylindrical sensor having a cylindrical structure, and FIGS. 9 (c) and 9 (d) are configuration examples of a spherical sensor having a spherical structure. In the sensor unit shown in FIGS. 9A and 9C, the thickness of a part of the outer wall is reduced to increase the detection sensitivity to the strain applied to the part. On the other hand, in the sensor unit shown in FIGS. 9B and 9D, a part of the outer wall has a bellows structure, and the detection sensitivity to the strain in the direction orthogonal to the bellows structure is enhanced. In any structure, a high-sensitivity sensor unit is provided in three directions of 0 °, 120 °, and 240 °.
本発明に係るレーザ圧力・ひずみ計は、光ファイバを介した遠隔計測が可能であり、沖合から遠く離れた海底における水圧変化（津波）、地震および、地中のひずみの変化（地殻変動）を極めて高感度に計測できる。このため、早期にかつ安全に津波・火山などの情報が得られ、それらからの災害を小さいものにすることができる。 The laser pressure / strain meter according to the present invention can perform remote measurement via an optical fiber, and can detect changes in water pressure (tsunami), earthquakes, and changes in strain in the ground (crustal movement) on the seabed far from offshore. It can be measured with extremely high sensitivity. Therefore, information on tsunamis, volcanoes, etc. can be obtained early and safely, and disasters from them can be minimized.
１ 高感度センサ部
２ 低感度センサ部
３ 凹面鏡
４ 凹面鏡ホルダ
５ 直角プリズム
６ コリメータ
７ シール部
８ 円筒容器
９ 光ファイバ
１０ センサ部
１１ 光ファイバリング共振器
１２ 共振周波数検出部
１３ 偏波保持単一モードファイバ
１４ 偏波ビームスプリッタ
１５ ２分の１波長板
１６ 標準単一モードファイバ
１７ 光カプラ
１８ 参照光源
１９ 周波数可変レーザ
２０ 負帰還制御回路
２１ 光カプラ
２２ 光検出器
２３ 周波数カウンタ
２４ 高感度センサ部
２５ 低感度センサ部
２６ ０°センサ部
２７ １２０°センサ部
２８ ２４０°センサ部
1 High-sensitivity sensor part 2 Low-sensitivity sensor part 3 Concave mirror 4 Concave mirror holder 5 Right-angle prism 6 Collimator 7 Seal part 8 Cylindrical container 9 Optical fiber
10 Sensor unit 11 Optical fiber ring resonator 12 Resonant frequency detector 13 Polarization holding single mode fiber 14 Polarization beam splitter 15 1/2 wavelength plate 16 Standard single mode fiber 17 Optical coupler 18 Reference light source 19 Frequency variable laser 20 Negative feedback control circuit 21 Optical coupler 22 Optical detector 23 Frequency counter 24 High-sensitivity sensor section 25 Low-sensitivity sensor section 260 ° sensor section 27 120 ° sensor section 28 240 ° sensor section
Claims (3)
外部の圧力および／またはひずみの変化に伴う前記センサ部の形状変化を、前記光ファイバリング共振器の共振周波数変化から検出可能に構成されていることを
特徴とするレーザ圧力・ひずみ計。 The optical fiber ring is wound around the sensor unit to form an optical fiber ring resonator, and the oscillation frequency of the wavelength variable laser installed outside the sensor unit is negatively fed back to the resonance frequency of the optical fiber ring resonator to stabilize it. It is provided so as to detect the resonance frequency of the optical fiber ring resonator by the oscillation frequency of the wavelength variable laser.
A laser pressure / strain meter characterized in that a change in the shape of the sensor unit due to a change in external pressure and / or strain can be detected from a change in the resonance frequency of the optical fiber ring resonator.
The sensor unit has three detection units whose shape changes with high sensitivity in response to strains applied in each of 0 °, 120 °, and 240 °, respectively, and the strains of the three components detected by each detection unit. The laser pressure / strain meter according to claim 1, wherein the amount of crustal movement can be measured by the change of the above.
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| JP2560260B2 (en) * | 1994-10-05 | 1996-12-04 | 科学技術庁防災科学技術研究所長 | Laser strain gauge |
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