JP2005283466A - Vibration measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vibration measuring apparatus of high detecting sensitivity and with stable outputs, utilizing polarization modulation. <P>SOLUTION: Coherent light (laser beam) is fed into a waveguide 2 from a light source 1. The fed-into coherent light is bifurcated to enter into a first guided wave section 23 and a second guided wave section 24 and becomes the signal light and the reference light, respectively. Polarization of the signal light in the first guided wave section 23 is modulated responding to vibration imposed on the sensor section 231 of the first guided wave section 23 during its propagation. Polarization of the reference light in the second guided wave section 24 is revolved at high speed through a polarization turning device 3. The signal light passing through the first guided wave section 23 and the reference light passing through the second guided wave section 24 are multiplexed optically to feed into a photodetector 4. So-called homodyne detection is implemented in the photodetector 4. Output signals (intensity signals and phase signals) from the photodetector 4 represent modulation the signal light received from vibration. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a vibration measuring apparatus using coherent light propagating in a waveguide.

  It is well known that when stress is applied to an optical fiber due to vibration or the like, a change in the birefringence state occurs in the optical fiber, and the state of the polarization plane of light passing through the optical fiber changes. An optical fiber sensor vibrometer based on this principle has been put into practical use.

  The conventional method often used in optical fiber sensor vibrometers using polarization plane modulation is to divide the optical output from the modulated optical fiber into two orthogonal polarizations, and to compare the electric field strength ratio in each. From this change, the change in the phase angle of the polarization is calculated to obtain the modulation strength.

  However, in general, in an optical fiber, a change in polarization state occurs not only in the sensor unit but also in the entire optical path. For this reason, it is necessary to take measures such as using an expensive polarization-maintaining optical fiber in the entire optical transmission line.

  Further, even if such measures are taken, complete correspondence is impossible, and the polarization plane may rotate, and the phase angle of the polarization plane may coincide with the set azimuth angle of the detection element. At this time, there is a possibility that modulation (that is, vibration as a measurement target) cannot be detected.

  Further, in the conventional method, since light is directly converted into an electrical signal by a photodetector, there is a disadvantage that detection sensitivity is lower than that of optical heterodyne detection or optical homodyne detection.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a vibration measuring apparatus using polarization modulation that has high detection sensitivity and stable output.

  The vibration measuring apparatus according to the present invention includes a light source, a waveguide, a polarization plane rotator, and a photodetector. The light source is configured to inject coherent light into the waveguide. The waveguide includes a first waveguide that propagates part of the incident coherent light as signal light, and a second waveguide that propagates another part of the incident coherent light as reference light. It has. Further, the waveguide is configured to propagate the signal light and the reference light to the photodetector. The polarization plane rotator is configured to rotate any one of the polarization planes of the signal light and the reference light without substantially changing the ellipticity thereof. The photodetector is configured to detect a change in intensity and / or phase of the signal light based on the combined signal light and the reference light.

  The first waveguide unit may include a sensor unit that propagates the signal light. You may comprise the said sensor part with an optical fiber.

  The light source may be a light source using a laser.

  The polarization plane rotator may rotate the polarization plane at an angular frequency that is faster than the angular frequency in the vibration to be measured.

  The vibration measuring device may further include a low frequency pass filter. This low frequency pass filter is connected to the output side of the photodetector.

  The vibration measuring device may further include a phase detector. This phase detector is connected to the output side of the photodetector.

  The measurement device of the present invention is a homodyne detection method in which one coherent light is divided into signal light and reference light, and the signal light and reference light are combined again and detected by a photodetector. Compared with the method, there is an effect that the S / N ratio can be greatly improved and the detection sensitivity can be increased.

  In addition, there is an effect that a stable modulation output can be obtained by rotating the polarization plane of the reference light or the signal light at a high speed with respect to the frequency of the measurement signal.

(First embodiment)
Hereinafter, a vibration measuring apparatus according to a first embodiment of the present invention will be described with reference to FIG. This apparatus includes a light source 1, a waveguide 2, a polarization plane rotator 3, and a photodetector 4 as main components.

  In this embodiment, a laser device is used as the light source 1. The light source 1 is connected to the end of the waveguide 2. Thereby, the light source 1 can enter the coherent light into the waveguide 2. Here, the coherent light may be a wave whose phases are aligned to the extent necessary for target vibration measurement.

  The waveguide 2 includes an introduction part 21, a branch part 22, a first waveguide part 23, a second waveguide part 24, a coupling part 25, and a connection part 26.

  The introduction part 21, the first waveguide part 23, the second waveguide part 24, and the connection part 26 are all configured by optical fibers. The kind and structure of the optical fiber to be used are not particularly limited. However, if a single mode optical fiber is used, it is possible to improve the accuracy of vibration measurement as compared with a multimode optical fiber.

  The introduction unit 21 is connected to the light source 1 and propagates coherent light.

  The branching section 22 is connected to the introducing section 21 and branches the propagated coherent light into the first waveguide section 23 and the second waveguide section 24. As such a branch part 22, various commercially available duplexers can be used.

  The first waveguide unit 23 sends the branched coherent light to the photodetector 4 through the coupling unit 25. The coherent light introduced into the first waveguide unit 23 is referred to as signal light in this specification. Therefore, the first waveguide unit 23 is configured to propagate part of the coherent light as signal light.

  The optical fiber used for the first waveguide section 23 is preferably one that can receive vibration. However, as a transmission method of vibration, a method of transmitting directly from an object or a method of transmitting via some medium may be used.

  The first waveguide 23 is generally fixed to a vibration generation source or medium, but may not be fixed as long as vibration transmission is possible, and the fixing method is arbitrary.

  In this embodiment, a part of the first waveguide part 23 is a sensor part 231. That is, an arbitrary part of the first waveguide unit 23 constitutes the sensor unit 231. Thereby, in this embodiment, the sensor part 231 which propagates signal light is comprised by the optical fiber. The extending direction of the sensor unit 231 may be a straight line or an arc.

  Similar to the first waveguide section 23, the second waveguide section 24 sends the coherent light branched by the branch section 22 to the photodetector 4 via the coupling section 25. The coherent light introduced into the second waveguide unit 24 is referred to as reference light in this specification. Therefore, the second waveguide unit 24 is configured to propagate another part of the coherent light as the reference light.

  The coupling unit 25 is connected to the first waveguide unit 23 and the second waveguide unit 24, and combines the propagated signal light and reference light and sends them to the photodetector 4. As such a coupling part 25, various commercially available couplers can be used.

  The connection unit 26 transmits the light combined by the coupling unit 25 to the photodetector 4.

  In this embodiment, the polarization plane rotator 3 is attached to the second waveguide 24. The polarization plane rotator 3 is configured to rotate the polarization plane of the reference light that passes through the second waveguide section 24 without substantially changing its ellipticity. “Do not substantially change the ellipticity” means that the change in the ellipticity is a level that does not hinder the target vibration measurement. Of course, the polarization of the reference light may be either circularly polarized or elliptically polarized.

  A typical example of the polarization plane rotator 3 that does not substantially change the ellipticity is a system that rotates the plane of polarization by applying an AC magnetic field to the Faraday element. However, the method is not limited to this, and may be a method performed by a mechanical method, for example. In short, various systems can be used as long as the plane of polarization is rotated without substantially modulating the frequency of light.

  Theoretically, if the rotational angular frequency of the polarization plane in the polarization plane rotator 3 is theoretically faster than the angular frequency of vibration, direct detection is possible. The faster the rotational angular frequency of the polarization plane, the higher the upper limit of the frequency band of vibration that can be measured. However, vibration measurement is possible even if not necessarily fast by using means such as an appropriate filter and a phase detection circuit.

  The photodetector 4 receives the combined signal light and reference light (that is, mixed light), and detects changes in the intensity and phase of the signal light based on the mixed light. A detailed operation principle will be described in the operation of the first embodiment. As the photodetector 4 itself, it is only necessary to detect the time waveform (that is, the intensity change and the phase change) in the light intensity change, and various commercially available detectors (either digital conversion or analog conversion) can be used.

(Operation of the first embodiment)
First, the sensor unit 231 of the first waveguide 23 is attached to a measurement target. The attachment method may be any method as long as vibration can be transmitted to the sensor unit 231.

  Next, coherent light (laser light) is sent from the light source 1 to the introduction portion 21 of the waveguide 2. The sent coherent light is branched by the branching unit 22 and enters the first waveguide unit 23 and the second waveguide unit 24. The light entering the first waveguide 23 is used as signal light, and the light entering the second waveguide 24 is used as reference light. Therefore, at the time of branching, the signal light and the reference light are in phase.

  The polarization plane of the signal light in the first waveguide unit 23 is modulated according to vibration applied to the sensor unit 231 during propagation in the sensor unit 231.

  The polarization plane of the reference light in the second waveguide section 24 is rotated at high speed by the polarization plane rotator 3.

  The signal light that has passed through the first waveguide section 23 and the reference light that has passed through the second waveguide section 24 are combined by the coupling section 25 to become mixed light, which is sent to the photodetector 4. The photodetector 4 outputs an electrical signal corresponding to the mixed light. The electrical signal has a time waveform that substantially corresponds to the time waveform of the mixed light. This electrical signal may be an analog value or a digital value. In the photodetector 4, the modulation received by the signal light due to vibration can be extracted as an intensity signal and a phase signal in the electrical signal as an output by so-called homodyne detection.

  Here, the principle of vibration detection will be described. If the electric field intensity of the signal light is Is, the electric field intensity of the reference light is Ir, and the difference in phase angle between the signal light and the reference light is Apm (t), the detected current component i in homodyne detection by the photodetector 4 is become that way.

i = K {Iscos (ωt + A1 (t))}
× {Ircos (ωt + A2 (t))}
× cos (Apm (t))
= (1/2) K (Is × Ir)
{Cos (2ωt + A1 (t) + A2 (t)
+ Cos (A1 (t) -A2 (t))}
X cos (Apm (t)) --- (Formula 1)
In this equation, K is the current conversion coefficient of the photodetector, ω is the angular frequency of the light, A1 (t) is the phase angle of the signal light, and A2 (t) is the phase angle of the reference light.

  If the band of the laser light emitted from the light source 1 is from visible light to near infrared light, the frequency of the light is several hundred THz. On the other hand, the vibration frequency band actually measured is at most MHz or less. Thus, since the frequency ω of the laser beam is very large with respect to the vibration frequency, the maximum value Imax of the detected current can be written as follows.

Imax = (1/2) K (Ismax × Irmax)
X {1 + cos (A1 (t) -A2 (t))}
× cos (Apm (t)) ----------------- (Formula 2)
In Equation 2, the maximum values of Is and Ir are Ismax and Irmax, respectively.

  Considering that the plane of polarization rotates at a higher speed than the vibration frequency for measuring the reference light, Equation 2 is as follows.

Imax = (1/2) K (Ismax × Irmax)
X {1 + cos (A1 (t) -A2 (t))} ----- (Formula 3)
In Equation 3, the magnitude of Ismax changes corresponding to the period (or frequency) of vibration due to polarization plane modulation based on vibration applied to the signal light.

  On the other hand, Doppler frequency shift modulation and phase modulation occur due to the vibration to the waveguide 2, and the value of the term {1 + cos (A1 (t) −A2 (t))} also changes corresponding to the period of vibration. .

  Therefore, it is possible to measure the frequency of the vibration that has acted on the sensor unit 231 of the first waveguide unit 23 based on the magnitude and phase of Imax (that is, the output of the photodetector 4) expressed by Equation 3. In the above description, the upper limit of the current value is focused, but the same measurement is possible even when focusing on the lower limit (Imin).

  The measurement apparatus of the present embodiment is a homodyne detection method in which one coherent light is divided into signal light and reference light, and the signal light and reference light are combined again and detected by the photodetector 4. Compared with the direct detection method, there is an advantage that the S / N ratio can be greatly improved and the detection sensitivity can be increased.

  In addition, by rotating the polarization plane of the reference light or signal light at a high speed with respect to the frequency of the measurement signal, the modulation to the polarization plane can be detected stably without using a polarization-preserving fiber as a waveguide. There is also an advantage of being able to.

  In the first embodiment described above, the polarization plane rotator 3 is attached to the second waveguide 24, but it may be attached to the first waveguide 23 to rotate the polarization plane of the signal light. In this case as well, vibration can be measured by performing homodyne detection based on the same principle as described above.

  Furthermore, in the first embodiment, the branching portion 22 is used, but in short, any configuration may be used as long as the light can be branched into the first waveguide portion 23 and the second waveguide portion 24. For example, instead of using the branching device 22, the light may be demultiplexed using means such as a half mirror in the space. In this case, the space constitutes a part of the waveguide 2.

  Similarly, the coupling unit 25 may have any configuration as long as light can be multiplexed. Further, a configuration in which the signal light and the reference light are introduced into the photodetector 4 itself without using the coupling unit 25 and the optical detector 4 performs the multiplexing may be employed.

  In the present embodiment, the waveguide 2 is mainly composed of an optical fiber, but may be configured to propagate space as long as the above-described function can be exhibited. Further, a waveguide formed in the substrate may be used.

  Further, in the above embodiment, the output of the photodetector 4 is an electric signal, but this is for convenience of handling and is not essential. That is, the output of the photodetector 4 may be expressed by other physical quantities (for example, magnetism or light) as long as vibration detection is possible.

  In the above-described embodiment, the entire first waveguide 23 is configured by an optical fiber. However, only the sensor unit 231 may be an optical fiber, and other portions may be configured to propagate light in space. In this case, since the polarization plane rotation due to the vibration hardly occurs in the space propagation portion, there is an advantage that the vibration in the sensor unit 231 may be acquired with a high S / N ratio. In short, if an arbitrary part of the first waveguide 23 is a waveguide such as an optical fiber (which can transmit vibration), the sensor unit 231 can be used by using a part of the waveguide. The other part may be a waveguide, spatial propagation, or a combination thereof.

(Second Embodiment)
Next, a vibration measuring apparatus according to a second embodiment of the present invention will be described with reference to FIG. In this embodiment, a low-frequency pass filter 6 and an amplifier 7 are arranged after the photodetector 4.

  As shown in Expression 1, the output i of the photodetector 4 includes a high frequency component. This component theoretically does not affect the measurement of Imax, but in reality, it may have an adverse effect as noise. According to the apparatus of the second embodiment, since such a high-frequency component can be removed, there is an advantage that the influence of noise on vibration measurement data can be reduced. The output of the low frequency pass filter 6 is amplified by the amplifier 7.

  Other configurations and advantages of the second embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted.

(Third embodiment)
Next, a vibration measuring apparatus according to a third embodiment of the present invention will be described with reference to FIG. In this embodiment, a phase detector 7 is disposed in the subsequent stage of the photodetector 4 in place of the low-frequency pass filter 5.

  In Expression 2, Apm (t) is determined by the rotation angular velocity of the polarization plane rotator 3 and the polarization plane phase angle change angular velocity generated by the polarization plane modulation caused by vibration. Therefore, the phase detection circuit 7 compares and detects the rotational angular velocity of the polarization plane rotator 3 and Apm (t), and outputs the phase difference as an electrical signal. The magnitude of this electrical signal changes corresponding to the period of vibration received by the first waveguide 23. Thereby, the vibration which the 1st waveguide part 23 received can be measured. Other configurations and advantages of the third embodiment are the same as those of the first embodiment, and thus detailed description thereof is omitted.

  In the third embodiment, the phase detector 7 is disposed in place of the low-frequency pass filter 5, but a configuration in which both are used (for example, connected in series) may be employed.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention.

  For example, in the embodiment, the light from the light source 1 is branched into two, but it may be branched into three or more. In short, it is only necessary that the vibration (detection light) and the reference light are combined to detect the vibration.

It is explanatory drawing which shows schematic structure of the vibration measuring device which concerns on 1st Embodiment of this invention. It is explanatory drawing which shows schematic structure of the vibration measuring device which concerns on 2nd Embodiment of this invention. It is explanatory drawing which shows schematic structure of the vibration measuring device which concerns on 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Waveguide 21 Introduction part 22 Branch part 23 1st waveguide part 231 Sensor part 24 2nd waveguide part 25 Coupling part 26 Connection part 3 Polarization surface rotator 4 Optical detector 5 Low frequency pass filter 6 Amplifier 7 Phase Detector

Claims (6)

A light source, a waveguide, a polarization plane rotator, and a photodetector;
The light source is configured to enter coherent light into the waveguide,
The waveguide includes a first waveguide that propagates part of the incident coherent light as signal light, and a second waveguide that propagates another part of the incident coherent light as reference light. And the waveguide is configured to propagate the signal light and the reference light to the photodetector,
The polarization plane rotator is configured to rotate any polarization plane of the signal light and reference light without substantially changing its ellipticity,
The vibration detector is configured to detect a change in intensity and / or phase of the signal light based on the combined signal light and the reference light.   The vibration measuring apparatus according to claim 1, wherein the first waveguide unit includes a sensor unit that propagates the signal light, and the sensor unit includes an optical fiber.   The vibration measuring apparatus according to claim 1, wherein the light source is a light source using a laser.   The vibration measuring apparatus according to claim 1, wherein the polarization plane rotator rotates the polarization plane at an angular frequency faster than an angular frequency of vibration to be measured. .   The vibration measuring apparatus according to claim 1, further comprising a low-frequency pass filter, wherein the low-frequency pass filter is connected to an output side of the photodetector. The vibration measuring apparatus according to claim 1, further comprising a phase detector, wherein the phase detector is connected to an output side of the photodetector.

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