Photocurrent sensor

Abstract

To provide a simple and highly accurate photocurrent sensor.SOLUTION: A photocurrent sensor comprises: a polarized light separation portion provided between a light source and an optical fiber; a reflection portion provided at the other end of the optical fiber to reflect measurement light transmitted through the polarized light separation portion and incident on the optical fiber and including an optical rotator for optically rotating incident polarized light by approximately 45 degrees; a sensor fiber provided between the polarized light separation portion and the reflection portion; a detector that converts the measurement light subjected to intensity modulation by reciprocating in the sensor fiber and passing through the polarized light separation portion again into an electric signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION An embodiment of the present invention relates to an optical current sensor. [Background technology]

[0002] With the advancement of power electronics equipment, there is a growing need for devices that can easily measure high-frequency currents. In conventional technology, various current sensors have been devised and put into practical use, such as those that measure voltage drops across resistors, current transformers, and those that use the Hall effect.

[0003] Furthermore, as a method for measuring current using light, an optical current sensor has been developed that uses the Faraday effect, which changes the polarization characteristics of light using a magnetic field.

[0004] Optical current sensors are known that utilize the Faraday effect of optical fibers and have various advantages, such as small size, flexibility, resistance to electromagnetic noise, long-distance signal transmission, and voltage resistance. For example, Patent Document 1 discloses, as an example of such a current measurement device, a reflective current measurement device that utilizes the Faraday effect, in which the polarization plane of light rotates due to the action of a magnetic field. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-271292 [Non-patent literature]

[0006] [Non-Patent Document 1] OITDA Standard: Fiber optic sensors - Polarimetric current measurement, Standard number: OITDA FS 01:2017 1st edition, published May 16, 2017 Summary of the Invention [Problem to be solved by the invention]

[0007] For example, the optical current sensor disclosed in Patent Document 1 has difficulty in achieving both accuracy and frequency band, and therefore does not meet the needs for current measurement.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a simple and highly accurate optical current sensor that achieves both accuracy and a wide frequency band. [Means for solving the problem]

[0009] The optical current sensor according to the embodiment includes a polarization separator provided between a light source and an optical fiber, a reflector provided at the other end of the optical fiber for reflecting measurement light that has passed through the polarization separator and entered the optical fiber and that has an optical rotation element for rotating the incident polarized light by approximately 45 degrees, a sensor fiber provided between the polarization separator and the reflector, and a detector for converting the measurement light that has been intensity-modulated by traveling back and forth through the sensor fiber and passing through the polarization separator again into an electrical signal. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the overall configuration of an optical current sensor according to an embodiment. [Figure 2] FIG. 2 is a diagram showing how the sensitivity of the optical current sensor according to this embodiment changes with temperature. [Figure 3] FIG. 3 is a diagram showing the angle of rotation of the Faraday mirror shown in FIG. 2 calculated by inverse calculation. [Figure 4] FIG. 4 is a diagram showing the relationship between sensitivity and temperature when the sum of the modulation intensities on the X and Y axes shown in FIG. 2 is taken as the output of the optical current sensor. [Figure 5] FIG. 5 is a diagram in which the sensitivity shown in FIG. 4 is corrected for temperature-dependent sensitivity changes of the optical current sensor using the temperatures found in FIG. [Figure 6]FIG. 6 is a diagram showing an example of the relationship between the ratio of the modulation degree of the X axis and the Y axis shown in FIG. 2 and the sensitivity of the optical current sensor. [Figure 7] FIG. 7 is a diagram showing an example of the structure of the Faraday mirror according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] The optical current sensor will be described in detail below with reference to the drawings. In the following embodiments, parts with the same numbers perform the same operation, and redundant description will be omitted. For example, when there are multiple identical or similar elements, a common reference number may be used to describe each element without distinguishing between them, or a subnumber may be used in addition to the common reference number to describe each element with distinction between them.

[0012] [Embodiment] First, the accuracy of the optical current sensor using the Faraday effect of an optical fiber will be explained. The accuracy of optical current sensors based on the Faraday effect is hindered by the existence of birefringence in the sensor fiber and the temperature dependence of the Verdet constant.

[0013] Here, birefringence is a phenomenon in which the refractive index differs between the X and Y axes of an optical fiber. When linearly polarized light is transmitted through an optical fiber that has birefringence, the emitted light becomes elliptically polarized. In the case of sensors that use the rotation of linearly polarized light due to the Faraday effect, the elliptical polarization of linearly polarized light can be a source of error in optical current sensors.

[0014] The Verdet constant is a coefficient that indicates how much linearly polarized light rotates in response to a magnetic field applied to an optical fiber. The Verdet constant changes with temperature, so corrections are made by measuring the temperature or by using an element with characteristics that are inverse to the temperature change of the Verdet constant.

[0015] Faraday mirrors are known as a method for reducing the effects of birefringence in optical fibers. By rotating the polarization by 90 degrees at the reflecting end of the optical fiber, light that passes through the X axis is folded back on the Y axis, so that both the light incident on the X axis and the light incident on the Y axis have passed through a medium with the same refractive index after going back and forth, thereby eliminating the effects of birefringence.

[0016] Furthermore, the Faraday mirror itself has birefringence, and the Verdet constant of the Faraday mirror is also temperature dependent, so this does not necessarily lead to improved accuracy.

[0017] Therefore, in this embodiment, a Faraday mirror (reflecting portion) is used in which the position of the Faraday rotator shown in Patent Document 1 is moved to the reflecting end of the optical fiber.

[0018] FIG. 1 is a diagram showing the overall configuration of an optical current sensor according to an embodiment. The optical current sensor includes a light source 1, a transmitting fiber 2, a coupler 3, a polarization splitter coupler 4, a sensor fiber 5, a receiving fiber 6a, a receiving fiber 6b, a detector 7a, a detector 7b, a signal processor 8, and a Faraday mirror 9.

[0019] The light source 1 emits light for measuring a measurement object (hereinafter referred to as measurement light) to the light transmitting fiber 2. The light source 1 may be a general light source such as a laser or an LED (Light Emitting Diode), for example.

[0020] The transmission optical fiber 2 is an optical fiber that guides the measurement light emitted by the light source 1 to the coupler 3. The transmission optical fiber 2 may be configured as a general optical fiber.

[0021] The coupler 3 is an element for splitting the incident measurement light into two, i.e., splitting the output light. For example, the coupler 3 in this embodiment splits the incident light (measurement light) into two, one of the outputs is guided to the polarization splitting coupler 4 and the other is discarded.

[0022] The polarization splitter coupler 4 is an element that splits the incident measurement light into orthogonal X and Y polarization components. In this embodiment, the polarization splitter coupler 4 guides only the measurement light in the X-axis direction out of the incident measurement light to the sensor fiber 5.

[0023] The sensor fiber 5 has a Faraday effect that changes the polarization characteristics of light by rotating linearly polarized light at an optical rotation angle in response to a magnetic field. For example, the sensor fiber 5 rotates the vibration direction of measurement light converted into linearly polarized light by the polarization separation coupler 4 at an optical rotation angle in response to the magnetic field based on the measurement target, and after making the measurement light go back and forth, it is output to the polarization separation coupler 4.

[0024] The polarization splitter coupler 4 converts the linearly polarized measurement light rotated by the sensor fiber 5 into measurement light with two-axis (X and Y axes) intensities according to the angle of rotation, and outputs the Y-axis light to the coupler 3 and the X-axis light to the receiving fiber 6b.

[0025] The light emitted from the polarization splitter coupler 4 and sent to the coupler 3 is partly branched off into the receiving fiber 6a.

[0026] The receiving optical fibers 6a and 6b guide the measurement light to the detectors 7a and 7b, respectively. The receiving optical fibers 6a and 6b may be configured with general optical fibers.

[0027] The detector 7a (first detector) and the detector 7b (second detector) have elements, such as photodiodes or phototransistors, that convert light into electrical signals. The detectors 7a and 7b output electrical signals to the signal processor 8 according to the intensities of the measurement light received from the receiving optical fibers 6a and 6b.

[0028] The signal processor 8 acquires the electrical signals output by the detectors 7a and 7b and outputs current information, which is information about the object to be measured, based on the electrical signals. The signal processor 8 is configured with one or more computers each including at least one or more processors and a storage unit. For example, the signal processor 8 outputs, as detection information, information about the magnitude of the current to be measured or the magnitude of the optical magnetic field generated by the current, which is the object to be measured, according to the strength of the electrical signals output by the detectors 7a and 7b. Here, the detection information may be output to an output device such as a display, or may be an external device such as a general computer equipped with a processor.

[0029] As in the above-described configuration, the method of receiving the Faraday rotation occurring in the sensor fiber 5 directly as an intensity signal is called an intensity modulation method. In general, the intensity modulation method is suitable for detecting AC signals.

[0030] As described above, when the measurement light is simply made to go back and forth through the sensor fiber 5, the X-axis signal received by the detector 7a is the cosine component of the Faraday rotation angle, and the Y-axis signal received by the detector 7b is the sine component of the Faraday rotation angle.

[0031] When the Faraday rotation angle is small, the change in the cosine component when the current changes is 0, and the change in the sine component is output depending only on its absolute value, regardless of whether the current is positive or negative. Therefore, with the above configuration, it is not possible to determine the polarity of the current, and it cannot be used for AC measurement.

[0032] Therefore, in this embodiment, this problem is solved by installing a Faraday mirror 9 at the reflecting end of the sensor fiber 5 as a reflecting section that reflects the measurement light and has an optical rotation element for rotating the incident polarized light by approximately 45 degrees.

[0033] In conventional optical current sensors, such as those disclosed in Patent Document 1 and Non-Patent Document 1, this problem is solved by rotating the light incident on the sensor fiber by 22.5° (45° round trip) and adding a bias to the Faraday rotation angle.

[0034] Another issue that must be overcome in optical current sensors is birefringence in the sensor fiber 5. A Faraday mirror is known as a method for reducing the effects of birefringence in the sensor fiber 5. By rotating the polarized light by 90 degrees at the reflecting end of the sensor fiber 5, the light that has passed along the X axis is folded back along the Y axis, so that both the light that has entered along the X axis and the light that has entered along the Y axis have passed through a medium with the same refractive index after making a round trip, thereby eliminating the effects of birefringence.

[0035] However, since the Faraday mirror 9 itself has birefringence and the Verdet constant of the Faraday mirror 9 also has temperature dependency, this does not necessarily lead to improved accuracy.

[0036] Therefore, in this embodiment, this is implemented by moving the Faraday mirror 9 to the reflecting end of the sensor fiber 5.

[0037] The Faraday mirror 9 in this embodiment is a function that is also necessary in conventional optical current sensors. Therefore, in this embodiment, the position of the conventional optical bias unit (Faraday mirror 9) is simply changed, and no additional elements are used. Therefore, the configuration of this embodiment can obtain the effects of the Faraday mirror 9 with a simple configuration similar to that of the conventional one.

[0038] In principle, the method of directly receiving Faraday rotation as an intensity signal requires only one channel, i.e., an electrical signal from either detector 7a or detector 7b. Therefore, this embodiment may also include only detector 7a or detector 7b. However, in this embodiment, by including detectors 7a and 7b, the signal processor 8 can correct for temperature changes in the sensitivity of the optical current sensor.

[0039] FIG. 2 is a diagram showing how the sensitivity of the optical current sensor according to this embodiment changes with temperature. The vertical axis in Figure 2 shows the change in light intensity and modulation rate of detectors 7a and 7b caused by Faraday rotation, divided by the current, and corresponds to the sensitivity of the optical current sensor. This current sensor is equipped with two detectors, one on the X axis and one on the Y axis. The black line plots the sensitivity obtained by detector 7a on the X axis, and the gray line plots the sensitivity obtained by detector 7b on the Y axis. Figure 2 also shows a line that sums the sensitivities (modulation rates) on the X and Y axes.

[0040] Since the Faraday mirror 9 rotates (rotates) the light by a predetermined angle (for example, approximately 45 degrees), the two should ideally match. However, as shown in Figure 2, the two do not match, and it can be seen that this ratio changes depending on the temperature.

[0041] FIG. 3 is a diagram showing the angle of rotation of the Faraday mirror 9 shown in FIG. 2, which is calculated by inverse calculation. As shown in Figure 3, it can be seen that the Faraday rotation angle changes with temperature. For example, signal processor 8 can determine the temperature from the change in the Faraday rotation angle by utilizing the fact that the Faraday rotation angle changes with temperature. That is, signal processor 8 can measure the temperature based on the difference between the first modulation factor of the intensity-modulated electrical signal detected by detector 7a (first detector) and the second modulation factor of the intensity-modulated electrical signal detected by detector 7b (second detector).

[0042] FIG. 4 is a diagram showing the relationship between sensitivity and temperature when the sum of the modulation intensities on the X and Y axes shown in FIG. 2 is taken as the output of the optical current sensor. FIG. 4 is an enlarged view of the line obtained by summing the sensitivity on the X and Y axes shown in FIG. 2. Normally, an optical current sensor outputs this value as a current signal, that is, it becomes the output from the signal processor 8. As shown in FIG. 4, the higher the temperature, the higher the sensitivity of the output, and it can be seen that a sensitivity change of about 1% occurs within the temperature range of FIG. 4. The reason for this temperature change is that the Verdet constant of the optical fiber is temperature dependent.

[0043] FIG. 5 is a diagram in which the sensitivity shown in FIG. 4 is corrected for temperature-dependent sensitivity changes of the optical current sensor using the temperatures found in FIG. As shown in Fig. 5, the signal processor 8 corrects the temperature-dependent sensitivity change, which is shown in Fig. 4. For example, the signal processor 8 corrects the temperature-dependent sensitivity change of the optical current sensor based on the difference between the first modulation factor of the intensity-modulated electrical signal detected by detector 7a (first detector) and the second modulation factor of the intensity-modulated electrical signal detected by detector 7b (second detector). As a result, the corrected sensitivity is kept constant with respect to temperature, as shown in Fig. 5.

[0044] 5 shows an example in which the temperature is first calculated and then correction is performed. However, the signal processor 8 of this embodiment can also correct the change in sensitivity due to temperature using only the modulation index, without calculating the temperature.

[0045] FIG. 6 is a diagram showing an example of the relationship between the ratio of the modulation degree of the X axis and the Y axis shown in FIG. 2 and the sensitivity of the optical current sensor. As shown in FIG. 6, there is a clear correlation between the modulation ratio and the sensitivity. Therefore, the signal processor 8 performs temperature correction based on the modulation ratio and the sensitivity. In this case, the signal processor 8 does not need to obtain temperature information from the optical current sensor. Furthermore, when temperature is used only to correct the sensitivity of the optical current sensor, the signal processor 8 does not need to measure the relationship between the sensitivity of the optical current sensor and the temperature in advance; it is sufficient to know only the relationship between the difference in modulation factor and the sensitivity of the optical current sensor. Therefore, the signal processor 8 has a memory function that stores coefficients for correcting the sensitivity of the current sensor. The signal processor 8 of this embodiment performs correction using this relationship and the coefficients stored in the memory function. For example, the signal processor 8 corrects the temperature-dependent sensitivity change of the optical current sensor based on the modulation ratio of the electrical signals detected by the first detector and the second detector and the coefficients.

[0046] In the optical current sensor according to this embodiment, temperature correction is performed using the modulation ratio, so there is a concern that the temperature may not be accurately determined when the modulation ratio itself is small, i.e., when the current to be measured is not flowing very fast. The signal processor 8 then updates this coefficient when the modulation ratio is equal to or greater than a predetermined value. For example, when the modulation ratio is low, the signal processor 8 can suspend updating this coefficient and use a previously acquired coefficient, i.e., a coefficient stored in the memory function, thereby preventing an increase in error.

[0047] Naturally, there is a concern that the error of the optical current sensor will increase when the current value is small. However, the current sensor standard allows a larger change in sensitivity when the current value is large than when the current is large. For this reason, implementing the method described in this embodiment is effective.

[0048] FIG. 7 is a diagram showing an example of the structure of the Faraday mirror 9 according to this embodiment. As shown in Figure 7, a Faraday mirror 9 is installed after the fiber, and a lens, Faraday element, and permanent magnet are installed to generate an optical rotation of 22.5 degrees in one direction, and reflection by the mirror generates an optical rotation of 45 degrees in both directions.

[0049] Furthermore, as shown in Figure 7, a permanent magnet is placed around the bulk Faraday element that actually generates the optical rotation to apply a magnetic field to the Faraday element. Therefore, the Faraday mirror 9 has the problem that its thermal time constant is inevitably long compared to the optical fiber that serves as the sensor.

[0050] For this reason, in this embodiment, the optical fiber is also coated to prevent water from entering the strand and maintain mechanical strength, as is the case with conventional optical fibers. In addition, a coating is applied to lengthen the thermal time constant, or the optical fiber is housed in a cylindrical container such as a stainless steel tube, thereby adjusting the response thermal time constant of the optical fiber to changes in ambient temperature. For example, adding a coating to the optical fiber can increase the thickness of the optical fiber itself and lengthen the thermal time constant. Any material that can be used to coat optical fibers, such as resin, can be used as the coating. Furthermore, any metal may be disposed on the side of the optical fiber to lengthen the thermal time constant. By aligning the time constants of the optical fiber and the Faraday mirror 9 in this way, the signal processor 8 can accurately correct the temperature-related changes in the sensitivity of the optical current sensor, even if the temperature suddenly changes.

[0051] (Effects of the embodiment) According to the embodiment described above, by installing the Faraday mirror 9 at the reflection end of the sensor fiber 5, it is possible to obtain the effect of the Faraday mirror 9 with the same configuration as the conventional one. Furthermore, the signal processor 8 can output highly accurate detection information that is not dependent on temperature by correcting the change in sensitivity of the optical current sensor due to temperature. Furthermore, the signal processor 8 can output highly accurate detection information (current information) that is not dependent on temperature by performing temperature correction based on the correlation between the modulation ratio and the sensitivity of the optical current sensor.

[0052] [Other embodiments] In the embodiment, an example in which temperature correction is performed by obtaining the temperature and an example in which temperature correction is performed using the modulation rate ratio have been described, but these temperature corrections may be combined. Also, in the embodiment, an example in which detector 7a receives measurement light for the X axis and detector 7b receives measurement light for the Y axis has been described, but detector 7a may receive measurement light for the Y axis and detector 7b may receive measurement light for the X axis.

[0053] In short, this invention is not limited to the above-described embodiments, and various modifications can be made in the implementation stage without departing from the spirit of the invention. Furthermore, the embodiments may be implemented in combination as appropriate as possible, and in such cases, the combined effects can be obtained. Furthermore, the above-described embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining the disclosed multiple constituent elements. [Explanation of symbols]

[0054] 1...Light source 2...Transmission fiber 3... Coupler 4...Polarization separation coupler 5...Sensor fiber 6a, 6b...receiving fiber 7a, 7b...Detector 8...Signal processor 9...Faraday mirror

Claims

1. a polarization splitter provided between the light source and the optical fiber; a reflecting unit provided at the other end of the optical fiber to reflect the measurement light that has passed through the polarization separating unit and entered the optical fiber, the reflecting unit including an optical rotator for rotating the incident polarized light by approximately 45 degrees; a sensor fiber provided between the polarization splitter and the reflector; a detector that converts the intensity-modulated measurement light, which has traveled back and forth through the sensor fiber and passed through the polarization splitter again, into an electric signal; An optical current sensor comprising:

2. the detector includes a first detector that detects measurement light in the X axis and a second detector that detects measurement light in the Y axis, the optical current sensor further includes a signal processor that measures a temperature based on a difference between a first modulation rate of the intensity-modulated electrical signal detected by the first detector and a second modulation rate of the intensity-modulated electrical signal detected by the second detector. The optical current sensor according to claim 1 .

3. the detector includes a first detector that detects measurement light in the X axis and a second detector that detects measurement light in the Y axis, the optical current sensor further includes a signal processor that corrects a change in sensitivity of the optical current sensor due to temperature based on a difference between a first modulation factor of the intensity-modulated electrical signal detected by the first detector and a second modulation factor of the intensity-modulated electrical signal detected by the second detector. The optical current sensor according to claim 1 .

4. the detector includes a first detector that detects measurement light in the X axis and a second detector that detects measurement light in the Y axis, the optical current sensor includes a memory function unit that stores a coefficient for correcting the sensitivity of the optical current sensor, and further includes a signal processor that corrects a change in sensitivity of the optical current sensor due to temperature based on a ratio of modulation degrees of the electrical signals detected by the first detector and the second detector and the coefficient. The optical current sensor according to claim 1 .

5. the signal processor updates the coefficients stored in the memory function unit when the ratio of the modulation degrees is equal to or greater than a predetermined modulation rate. The optical current sensor according to claim 4 .

6. the optical fiber and the reflecting section are designed so that the time constant of the optical fiber and the time constant of the reflecting section are the same; The optical current sensor according to claim 4 .

7. the optical fiber and the reflecting section are designed so that the time constant of the optical fiber and the time constant of the reflecting section are the same; The optical current sensor according to claim 1 .

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