KR20230170460A - Active wavelength filter, distributed acoustic sensing apparatus using the same, and control method thereof - Google Patents

Abstract

According to one embodiment of the present disclosure, a polarization unit that receives unpolarized light from an optical input/output unit, separates it into two polarizations that are orthogonal to each other, and converts one of the two polarizations into the other polarization to output; A wavelength separator that separates the light output from the polarization unit according to wavelength and outputs the light, and a wavelength selector that selectively reflects one or more wavelengths among the light separated by wavelength by the wavelength separator, and the wavelength selector. Provides an active wavelength filter in which the light selectively reflected from passes through the wavelength separation unit and the polarization unit and is output to the optical input/output unit, and provides a distributed acoustic sensing device including an active wavelength filter and a control method thereof, and Even if the wavelength of the sensing light changes due to causes such as deterioration, distributed acoustic sensing can be performed normally by adjusting the passband of the active wavelength filter.

Description

Active wavelength filter, distributed acoustic sensing apparatus using the same, and control method thereof}

The present disclosure relates to an active wavelength filter, a distributed acoustic sensing device using the same, and a control method thereof.

Distributed acoustic sensing technology is used as a method to monitor the status of pipelines such as gas pipes and water pipes, roads, railroads, and facilities. Distributed acoustic sensing technology monitors the condition of facilities by injecting laser light into an optical fiber and measuring the backscattered light reflected at the point where force or sound is applied to the optical fiber. Distributed acoustic sensing technology is mainly used when the length of the facility to be monitored is long, such as several kilometers. Therefore, the intensity of the laser light input to the optical fiber must be strong. When laser light is amplified, noise is also amplified for various reasons, and noise lowers the signal-to-noise ratio and thus lowers sensing sensitivity. Additionally, if the elements that make up the distributed acoustic sensing device age, they may not exhibit desired performance.

KR 10-2330484 B1

The first aspect of the present disclosure is to provide an active wavelength filter capable of changing the center wavelength and bandwidth of the passband.

A second aspect of the present disclosure is to provide a distributed acoustic sensing device that can match the center wavelength of the pass band of the active wavelength filter to the center wavelength of the laser light source.

According to the third aspect of the present disclosure, a control method of a distributed acoustic sensing device is provided that automatically adjusts the center wavelength of the pass band of the active wavelength filter according to the change in the center wavelength of the laser light source.

The active wavelength filter according to the first aspect of the present disclosure receives unpolarized light from an optical input/output unit and separates it into two polarizations that are orthogonal to each other, and converts one of the two polarizations into the other polarization. A polarizing unit that converts and outputs light output from the polarizing unit, a wavelength separator that separates and outputs the light output from the polarizer according to wavelength, and a wavelength that selectively reflects one or more wavelengths among the light separated by wavelength by the wavelength separator. It may include a selection unit, and the light selectively reflected by the wavelength selection unit may pass through the wavelength separation unit and the polarization unit and be output to the optical input/output unit.

The polarization unit according to one embodiment includes a collimating lens that aligns the light incident from the optical input/output unit to be parallel, a beam displacer that separates the light passing through the collimating lens into two polarizations perpendicular to each other, and the beam. A half-wave retarder may be included to delay one of the two polarizations output from the displacer to match the other polarization.

The wavelength separator according to one embodiment may include a grating that reflects incident light in different directions depending on the wavelength.

The wavelength selection unit according to one embodiment includes an LC array that changes the arrangement according to a control signal at each pixel where the light separated according to the wavelength arrives from the wavelength separator to pass or block the light, and the LC array passes the light by the LC array. It may include a mirror that reflects the light.

A distributed acoustic sensing device including an active wavelength filter according to a second aspect of the present disclosure includes a light source that generates laser light, and a first optocoupler that splits the light output from the light source into sensing light and reference light. , an optical pulse modulator that outputs pulsed light by modulating the sensing light divided by the first optical coupler, an optical amplifier that amplifies the pulsed light, and filters the pulsed light amplified by the optical amplifier to pass the light in the pass band. An active wavelength filter capable of adjusting the center wavelength or width of the pass band, outputs the light input from the active wavelength filter connected to the first stage to the second stage, and backscattered light input from the optical fiber connected to the second stage is An optical rotator outputting to a third stage, a second optical coupler combining and outputting backscattered light received from the third stage of the optical rotator and reference light received from the first optical coupler, and outputting the combined light received from the second optical coupler. A photodetector that converts light into an electrical signal and outputs it, a signal processing unit that analyzes the electrical signal received from the photodetector to detect vibration applied to the optical fiber, and adjusts the center wavelength or width of the passband of the active wavelength filter. It can be included.

The signal processing unit according to one embodiment analyzes the electrical signal received from the photodetector and, if the intensity of the backscattered light is less than a reference value, adjusts the center wavelength of the passband of the active wavelength filter or adjusts the width of the passband. You can.

The signal processing unit according to one embodiment measures the intensity of the backscattered light while sequentially changing the center wavelength of the pass band of the active wavelength filter, and the center wavelength with the largest size of the backscattered light is the center of the active wavelength filter. The active wavelength filter can be controlled to set the wavelength.

The signal processing unit according to one embodiment measures the intensity of the backscattered light while changing the width of the passband for each center wavelength of the passband of the active wavelength filter, and the width of the passband at which the size of the backscattered light is greater than or equal to a reference value. The active wavelength filter can be controlled so that the passband width of the active wavelength filter is the width.

In a method of controlling a distributed acoustic sensing device including an active wavelength filter according to an embodiment according to a third aspect of the present disclosure, an optical pulse modulator modulates the light output by a light source into a pulse form and A first step in which an amplifier amplifies and generates pulsed light, a second step in which an active wavelength filter filters the pulsed light, the pulsed light passing through the active wavelength filter is input to an optical fiber, and is generated at an arbitrary point of the optical fiber. Backscattered light is input to an optical detector and converted into an electrical signal, a third step in which the signal processing unit analyzes the electrical signal to detect vibration applied to the optical fiber, and the signal processing unit changes the central wavelength or width of the pass band. A fourth step of controlling the active wavelength filter may be included as much as possible.

The fourth step according to one embodiment is a 4-1 step in which the signal processing unit analyzes the electrical signal received from the photodetector and compares the intensity of the backscattered light with a reference value, and the intensity of the backscattered light is greater than the reference value. If it is small, the signal processing unit may include a 4-2 step of adjusting the intensity of the backscattered light to be greater than the reference value by adjusting the center wavelength or width of the pass band of the active wavelength filter.

Step 4-2 according to one embodiment is a step in which the signal processing unit measures the intensity of the backscattered light while changing the center wavelength of the pass band of the active wavelength filter, and the center wavelength with the highest intensity of the backscattered light is the existing determining whether it matches the center wavelength of and determining whether the intensity of the largest backscattered light is greater than the reference value, and deciding to perform one of the center wavelength adjustment step, width adjustment step, center wavelength and width adjustment step, and the center It may include performing any one of a wavelength adjustment step, a width adjustment step, and a center wavelength and width adjustment step.

The center wavelength adjustment step according to one embodiment may be controlling the active wavelength filter so that the center wavelength with the greatest intensity of the backscattered light becomes the center wavelength of the active wavelength filter.

The width adjustment step according to one embodiment further passes the wavelength adjacent to the center wavelength of the active wavelength filter, so that the width of the pass band when the intensity of the backscattered light is greater than the reference value is adjusted to the width of the pass band of the active wavelength filter. The active wavelength filter may be controlled to achieve this.

The center wavelength and width adjustment step according to one embodiment determines the center wavelength with the largest size of the backscattered light as the center wavelength of the active wavelength filter, and selects a pixel corresponding to a wavelength adjacent to the center wavelength of the active wavelength filter. In addition, by controlling the pass, the active wavelength filter may be controlled so that the width of the pass band when the intensity of the backscattered light exceeds the reference value is the width of the pass band of the active wavelength filter.

The features and advantages of the present disclosure will become more apparent from the following detailed description based on the accompanying drawings.

Prior to this, terms or words used in this specification and claims should not be construed in their usual, dictionary meaning, and the inventor may appropriately define the concept of the term in order to explain his or her invention in the best way. It must be interpreted with meaning and concept consistent with the technical idea of the present invention based on the principle that it is.

According to the first aspect of the present disclosure, an active wavelength filter can change the center wavelength and bandwidth of the passband.

According to the second aspect of the present disclosure, the center wavelength of the pass band can be adjusted using an active wavelength filter, so that sensing sensitivity can be maintained even when the center wavelength of the laser light source of the distributed acoustic sensing device changes.

According to the third aspect of the present disclosure, the center wavelength of the passband of the active wavelength filter can be automatically adjusted according to the change in the center wavelength of the laser light source of the distributed acoustic sensing device.

Figure 1 is a diagram showing a distributed acoustic sensing device using an active wavelength filter according to an embodiment.
Figure 2 is a diagram showing an active wavelength filter according to one implementation.
Figure 3 is a graph showing the change in wavelength due to a light source or filter.
Figure 4 is a graph showing the pass band according to the type of filter.
Figure 5 is a graph comparing the output of the distributed photoacoustic sensing device according to the type of filter.
Figure 6 is a graph showing a change in the center wavelength of the passband of an active wavelength filter according to an implementation example.
Figure 7 is a graph showing a change in the width of the passband of an active wavelength filter according to one implementation.
Figure 8 is a flowchart showing the steps of a control method of a distributed acoustic sensing device including an active wavelength filter according to an embodiment.
Figure 9 is a flowchart showing steps for controlling an active wavelength filter according to one implementation.
Figure 10 is a flowchart showing the steps of controlling the size of backscattered light to be greater than a reference value according to one implementation.

The objects, advantages, and features of the present disclosure will become more apparent from the following detailed description and preferred embodiments taken in conjunction with the accompanying drawings, but the present disclosure is not necessarily limited thereto. Additionally, in describing the present disclosure, if it is determined that a detailed description of related known technology may unnecessarily obscure the gist of the present disclosure, the detailed description will be omitted.

In assigning reference numerals to components in the drawings, it should be noted that identical components are assigned the same reference numerals as much as possible even if they are shown in different drawings, and similar components are assigned similar reference numerals.

Terms used to describe one implementation of the present disclosure are not intended to limit the disclosure. It should be noted that singular expressions include plural expressions unless the context clearly dictates otherwise.

The drawings may be schematic or exaggerated to illustrate implementations.

In this document, expressions such as “have,” “may have,” “includes,” or “may include” refer to the existence of the corresponding feature (e.g., a numerical value, function, operation, or component such as a part). , and does not rule out the existence of additional features.

Hereinafter, an implementation example of the present disclosure will be described in detail with reference to the attached drawings.

FIG. 1 is a diagram showing a distributed acoustic sensing device 1 using an active wavelength filter 50 according to an embodiment. In Figure 1, electrical signals are shown as dotted lines, and optical signals are shown as solid lines.

The distributed acoustic sensing device 1 including an active wavelength filter 50 according to an embodiment of the present disclosure includes a light source (10), a first optical coupler (21), and an optical pulse modulator (Light pulse modulator (30), light amplifier (40), active wavelength filter (50), light circulator (60), optical fiber (70), second optical coupler (22) , may include a photo detector (80) and a signal processing unit (90).

The light source 10 can generate continuous output laser light. The light source 10 may include a laser oscillator. The light source 10 can generate highly coherent laser light with a linewidth of several kHz or less. The light source 10 may generate unpolarized laser light.

The first optical coupler 21 can split the light output from the light source 10 into sensing light and reference light. The sensing light is input into the optical fiber 70 and generates backscattered light by vibration applied to the optical fiber 70, and the reference light provides a standard for detecting vibration using the backscattered light. The first optical coupler 21 has a first end (21a) connected to the light source 10, a second end (21b) connected to the optical pulse modulator 30, and a third end (21c) connected to the second optical coupler. It can be connected to (22). The first optical coupler 21 receives the light output from the light source 10 as input to the first stage 21a, outputs 10% of the light as reference light to the third stage 21c, and outputs 90% of the light as sensing light. It can be output to the second stage (21b).

The optical pulse modulator 30 may output pulsed light by modulating the sensing light split by the first optical coupler 21. The optical pulse modulator 30 may include an acousto-optic modulator (AOM). The sensing light may be modulated into pulsed light while passing through the optical pulse modulator 30. The optical pulse modulator 30 can output pulsed light to the optical amplifier 40.

The optical amplifier 40 can amplify pulsed light. The optical amplifier 40 may include an erbium doped fiber amplifier (EDFA). The optical amplifier 40 can output amplified pulse light to the active wavelength filter 50. Amplified pulsed light may contain Amplified Spontaneous Emission (ASE) noise. Naturally amplified emission noise increases the background noise area of pulsed light, causing lower signal to noise ratio (SNR). Naturally amplified emission noise may be generated as pulsed light is amplified in the optical amplifier 40.

The active wavelength filter 50 can filter the pulse light amplified by the optical amplifier 40 and allow light in the pass band to pass. The active wavelength filter 50 can adjust the center wavelength of the pass band. The active Fazal filter can adjust the width of the passband. The center wavelength of the passband of the active wavelength filter 50 may be adjusted to correspond to the center wavelength of the light source 10. The width of the pass band can be adjusted so that the size of the sensing light is sufficiently large. The active wavelength filter 50 can remove noise and background noise generated from the optical amplifier 40. For example, the active wavelength filter 50 can remove naturally amplified emission noise. A detailed description of the active wavelength filter 50 will be described later. Pulse light from which noise has been removed by passing through the active wavelength filter 50 may be output to the optical rotator 60. Pulse light passing through the active wavelength filter 50 may have a narrow line width. Pulsed light with a narrow line width has high coherence and is therefore suitable for measurement using backscattered light.

The optical rotator 60 outputs the sensing light input from the active wavelength filter 50 connected to the first end 60a to the second end 60b, and inputs it from the optical fiber 70 connected to the second end 60b. The backscattered light can be output to the third stage 60c. The optical rotator 60 may include an optical fiber circulator (OC). The first end (60a) of the optical rotator 60 is connected to the active wavelength filter 50, the second end (60b) is connected to the optical fiber 70, and the third end (60c) is connected to the second optical coupler ( 22).

The second optical coupler 22 may output the combined backscattered light received from the third end 60c of the optical rotator 60 and the reference light received from the first optical coupler 21. The first end (22a) of the second optical coupler (22) is connected to the third end (21c) of the first optical coupler (21), and the second end (22b) is connected to the third end (21c) of the optical rotator (60). 60c) can be connected. The third end 22c and the fourth end 22d of the second optical coupler 22 may be connected to the photodetector 80. The second optical coupler 22 outputs 50% of the light combining the reference light input from the first stage 22a and the backscattered light input from the second stage 22b to the third stage 22c, and 50% of the light combined with the backscattered light input from the second stage 22b. It can be output in 4 stages (22d).

The photodetector 80 may convert the light received from the second optocoupler 22 into an electrical signal and output it. The photodetector 80 may include a balanced photodetector. The photodetector 80 may generate an electrical signal that reflects the interference pattern of backscattered light in the light received from the second optical coupler 22. The photodetector 80 may output an electrical signal containing pattern information of backscattered light to the signal processing unit 90.

The signal processing unit 90 may detect vibration applied to the optical fiber 70 by analyzing the electrical signal received from the photodetector 80. When vibration or sound is applied to a certain point of the optical fiber 70 through which pulsed light is input, backscattered light may occur in that part of the optical fiber 70. The signal processing unit 90 can analyze using Rayleigh backscattered light among backscattered light. The signal processing unit 90 can analyze the electrical signal and recognize at which point the vibration or sound is applied to the optical fiber 70.

The signal processing unit 90 can adjust the center wavelength or width of the pass band of the active wavelength filter 50. The signal processing unit 90 can output a control signal to the active wavelength filter 50, and the active wavelength filter 50 operates according to the control signal to move the center wavelength of the pass band, expand the width of the pass band, or You can narrow it down.

Passive wavelength filters such as WDM (Wavelength division multiplexing) or FBG (Fiber Bragg Grating) cannot adjust the center wavelength or width of the passband. Additionally, if a passive wavelength filter is used for a long period of time, it may deteriorate and its characteristics may change. A deteriorated passive wavelength filter may have a different center wavelength or width of the pass band. A deteriorated passive wavelength filter may cause noise to pass through the filter, increasing the signal-to-noise ratio. Alternatively, a portion of the pulsed light may not pass through the deteriorated passive wavelength filter, thereby reducing the intensity of the sensing light input to the optical fiber 70. Additionally, the wavelength of light emitted from the light source 10 may vary due to deterioration or other causes. If the wavelength of light emitted by the light source 10 changes, some light may be blocked by the deteriorated passive wavelength filter, thereby reducing the intensity of light input to the optical fiber 70. If the intensity of the sensing light input to the optical fiber 70 decreases, the sensitivity of the distributed acoustic sensing device 1 may decrease. As such, it is difficult for the passive wavelength filter to respond to wavelength changes due to deterioration of the elements constituting the distributed acoustic sensing device 1.

In contrast, the active wavelength filter 50 can respond by adjusting the center wavelength or width of the passband even if the wavelength of pulse light changes due to changes in the elements constituting the distributed acoustic sensing device 1, so distributed acoustic The performance of the sensing device 1 can be maintained.

The signal processing unit 90 may include a processor, memory, and bus. The signal processing unit 90 may further include a circuit that converts the electrical signal received from the photodetector 80 into digital data. A processor may include various types of semiconductor devices with information processing functions. Memory can store data and program code. The bus relays data transmission and reception between the processor and memory, and can relay data transmission and reception with external computer devices. The signal processing unit 90 may be implemented in such a way that program codes stored in memory are driven by a processor. The program code stored in the memory may be written to perform the function of detecting vibration by analyzing electrical signals and controlling the central wavelength or width of the pass band of the active wavelength filter 50. The program code stored in the memory may be written to perform a control method of a distributed acoustic sensing device including an active wavelength filter according to an embodiment of the present disclosure. The signal processing unit 90 may be included in a computer, PC, server, PLC, or other various information processing devices, or may be implemented as an independent information processing device.

Figure 2 is a diagram showing an active wavelength filter 50 according to one implementation.

The active wavelength filter 50 may include a wavelength separation unit 53 and a wavelength selection unit 54. The active flat wavelength filter may further include a polarization unit 52.

The polarization unit 52 receives unpolarized light from the optical input/output unit 51, separates it into two polarizations that are orthogonal to each other, and converts one of the two polarizations into the other polarization and outputs the light.

The polarization unit 52 is a collimating lens 52a that aligns the light incident from the optical input/output unit 51 in parallel, and a beam display that separates the light passing through the collimating lens 52a into two polarizations perpendicular to each other. It may include a half-wave retarder 52c that delays one of the two polarizations output from the beam displacer 52b and the beam displacer 52b to match the other polarization. The polarization unit 52 may further include a first lens 52d that allows polarized light to enter the grating 53a of the wavelength separation unit 53.

The optical input/output unit 51 may include an optical fiber 70. The input terminal of the optical input/output unit 51 may be connected to the optical amplifier 40. The amplified pulse light output from the optical amplifier 40 may be input to the input terminal of the optical input/output unit 51 and output to the collimating lens 52a. The collimating lens 52a can align pulse light in parallel and provide it to the beam displacer 52b.

The beam displacer 52b can separate unpolarized light into two polarized lights perpendicular to each other. The two polarizations are polarization perpendicular to the optical axis and polarization horizontal.

The half-wave retarder 52c may be disposed in a portion where one of the two polarized lights output by the beam displacer 52b is output. The half-wave retarder 52c is disposed in a portion where polarized light horizontal to the optical axis is output, and can change the polarized light horizontal to the optical axis into polarized light perpendicular to the optical axis and output it.

The first lens 52d may include an image lens. The first lens 52d can allow light that has passed through the beam displacer 52b and the half-wave retarder 52c to enter the grating 53a of the wavelength separator 53.

In summary, the light input from the input terminal of the input/output unit passes through the collimating lens 52a, is aligned in parallel, passes through the beam displacer 52b, and is separated into polarization perpendicular to the optical axis and polarization horizontal to the optical axis. The polarized light passes through the half-wave retarder 52c and is changed to polarization perpendicular to the optical axis, so that the polarized light perpendicular to the optical axis can be output to the wavelength separator 53 through the first lens 52d.

The wavelength separator 53 may separate the light output from the polarizer 52 according to the wavelength and output the light. The wavelength separation unit 53 may separate light according to wavelength and provide the light to the wavelength selection unit 54.

The wavelength separator 53 may include a grating (53a). The wavelength separator 53 may further include a second lens 53b.

The grating 53a may reflect incident light in different directions depending on the wavelength. Grating 53a may include a reflective diffraction grating. The grating 53a can split the input light by wavelength and output it. The light output from the polarizing unit 52 reaches the grating 53a and is reflected in different directions depending on the wavelength to reach the wavelength selection unit 54 through the second lens 53b. The light output from the polarization unit 52 has the same polarization and may be reflected in different directions depending on the wavelength by the grating 53a. Light reflected in different directions depending on the wavelength may pass through the second lens 53b, while light of the same wavelength may reach the same pixel of the LC array 54a. For example, three first wavelengths of light λ1 reach any pixel of the LC array 54a, three second wavelengths of light λ2 reach other pixels of the LC array 54a, and , three third wavelengths of light λ3 can reach another pixel of the LC array 54a.

The wavelength selection unit 54 may selectively reflect one or more wavelengths among the light separated by wavelength by the wavelength separation unit 53.

The wavelength selection unit 54 includes an LC array 54a whose arrangement is changed according to a control signal to pass or block the light at each part where the light separated according to the wavelength arrives from the wavelength separator 53, and an LC array ( It may include a mirror 54b that reflects the light passed by 54a).

The LC array (Liquid Crystal Array, 54a) can change the orientation of the liquid crystal according to a control signal applied from the signal processing unit 90. The LC array 54a can change the orientation of the liquid crystal on a per-pixel basis according to a control signal. Light of different wavelengths reaches each pixel of the LC array 54a. The smaller the pixel size, the more precisely the center wavelength and width of the passband of the active wavelength filter 50 can be adjusted. If a control signal is applied to an arbitrary pixel of the LC array 54a to change the orientation of the liquid crystal, light is blocked, and if the orientation of the liquid crystal is not changed, light can pass through. Alternatively, the LC array 54a may be configured to allow light to pass through when the orientation of the liquid crystal is changed, and to block light when the orientation of the liquid crystal is not changed. This specification explains how light is blocked by changing the orientation of the liquid crystal of the LC array 54a.

A control signal may be applied to each pixel of the LC array 54a to change the phase slope to adjust the intensity of reflected light for each wavelength. The LC array 54a can control the amount of light passing through any pixel by applying a control signal of desired intensity. For example, the LC array 54a allows the light of the first wavelength to pass through the pixel to which the light of the first wavelength has arrived without changing the orientation of the liquid crystal, and the pixel to which the light of the second wavelength has arrived leaves the liquid crystal. Light of the second wavelength is blocked by changing the orientation, and the pixel to which light of the third wavelength reaches can change the orientation of the liquid crystal to a desired degree to allow only a portion of the light of the third wavelength to pass through. That is, the LC array 54a can adjust whether light is reflected and the reflected intensity for each wavelength.

Mirror 54b may reflect light passing through LC array 54a. Mirror 54b may be placed side by side in LC array 54a. The mirror 54b may be made of a material that reflects the wavelength of laser light. The light passing through the LC array 54a may be reflected by the mirror 54b, pass through the LC array 54a again, and enter the wavelength separator 53.

In summary, the LC array 54a adjusts the orientation of the liquid crystal of the pixel to which light of each wavelength arrives according to the control signal from the signal processing unit 90 to pass, block, or partially pass the light, and the LC array 54a The light passing through may be reflected by the mirror 54b, pass through the LC array 54a again, and be output to the wavelength separator 53.

The light selectively reflected from the wavelength selection unit 54 heads to the optical input/output unit 51 through the same path as the incident path. The output terminal of the optical input/output unit 51 may be connected to the first terminal of the optical rotator 60. The wavelength selection unit 54 can adjust the wavelength at intervals of at least 0.05 nm or less. The active wavelength filter 50 can be applied to various application fields such as wavelength tunable filters, dispersion compensators, and multi-channel lasers.

Figure 3 is a graph showing the wavelength change due to the light source 10 or filter. Figure 3 shows the results of testing a passive wavelength filter (WDM filter (Wavelength Division Multiplexing filter) and FBG filter (Fiber Bragg Grating filter)) and an active wavelength filter 50 according to an embodiment of the present disclosure.

The performance of the light source 10 and filter of the distributed acoustic sensing device 1 may vary due to deterioration. If the light source 10 deteriorates, the central wavelength of the laser light may change. As the filter deteriorates, the center wavelength or width of the passband may change. In FIG. 3, the normal state represents the intensity of backscattered light according to location in a state in which the light source 10 or the filter is not deteriorated. The deterioration state of the light source 10 indicates the intensity of backscattered light depending on the position in a state in which the light source 10 is deteriorated and there is a change in the central wavelength of the laser light. The filter deterioration state indicates the intensity of backscattered light depending on the position in a state in which the passive wavelength filter is deteriorated and there is a change in the center wavelength or width of the pass band.

In normal conditions, when using the passive wavelength filter and the active wavelength filter 50, the intensity of backscattered light according to location is measured. This is because the central wavelength of the laser light output from the light source 10 matches the central wavelength of the filter, so the sensing light normally enters the optical fiber 70.

When using a passive wavelength filter (WDM filter and FBG filter) in a deteriorated light source, the intensity of backscattered light is measured to be very small regardless of location. The central wavelength of the laser light changes due to deterioration of the light source 10, and when the filter is in a normal state, the laser light does not pass through the passive wavelength filter. Therefore, most of the sensing light does not enter the optical fiber 70, and the backscattered light is measured at a low intensity.

On the other hand, when using the active wavelength filter 50 in a light source deterioration state, the intensity of backscattered light according to location is measured. Even if the central wavelength of the laser light changes due to deterioration of the light source 10, the active wavelength filter 50 adjusts the central wavelength or width of the pass band to the changed central wavelength of the laser light, so the laser light can pass through the passive wavelength filter. there is. Therefore, since the sensing light normally enters the optical fiber 70, backscattered light can be measured.

When the passive wavelength filters (WDM filter and FBG filter) are deteriorated in the filter deterioration state, the intensity of backscattered light is measured to be very small regardless of the position. If the central wavelength of the laser light output from the light source 10 does not change and the central wavelength or width of the pass band changes due to deterioration of the passive wavelength filter, the laser light does not pass through the passive wavelength filter. Therefore, most of the sensing light does not enter the optical fiber 70, and the backscattered light is measured as a weak intensity.

On the other hand, when using the active wavelength filter 50 in a filter deterioration state, the intensity of backscattered light according to location is measured. Even if the center wavelength or width of the pass band changes, the active wavelength filter 50 adjusts the center wavelength or width of the pass band again to match the center wavelength of the laser light, so that the laser light can pass through the passive wavelength filter. Therefore, since the sensing light normally enters the optical fiber 70, backscattered light can be measured.

In addition to deterioration, there may be cases where the central wavelength of the light source 10 does not exactly match the central wavelength of the filter due to various external factors such as temperature change or shock. When the active wavelength filter 50 is used as in the present disclosure, the center wavelength of the filter can be changed, so the distributed acoustic sensing device 1 can operate normally.

As described, by using the active wavelength filter 50 in the distributed acoustic sensing device 1 and adjusting the center wavelength and width of the pass band of the active wavelength filter 50, the elements constituting the distributed acoustic device are Even if deteriorated, the distributed acoustic device can perform its normal function.

Figure 4 is a graph showing the pass band according to the type of filter. Figure 4 shows the passband of a WDM filter (100 GHz DWDM filter), an FBG filter (Athermal packaged FBG filter), and an active wavelength filter 50 according to the present disclosure. Figure 4 shows the results of comparing wavelength filters using a broadband light source and an optical spectrum analyzer.

The width of the passband of the FBG filter is 0.25 nm, the width of the passband of the WDM filter is 0.69 nm, and the width of the passband of the active wavelength filter 50 is 0.075 nm. The width of the pass band of the active wavelength filter 50 is the narrowest. In other words, the narrow passband characteristic of the active wavelength filter 50 can effectively remove natural amplified emission noise from the distributed acoustic sensing device 1, thereby increasing the signal-to-noise ratio.

Figure 5 is a graph comparing the output of the distributed photoacoustic sensing device according to the type of filter. Figure 5 shows the intensity of backscattered light depending on location when using a passive wavelength filter (WDM filter and FBG filter) and an active wavelength filter 50. The signal level is the range from the lowest value to the maximum value of the intensity of the backscattered light, and the noise level is the range from the lowest value to the maximum value of the noise intensity.

In the case of the WDM filter, the signal level is 11,599, the noise level is 5,378, and the signal-to-noise ratio is 6.67 dB. In the case of the FBG filter, the signal level is 9,074, the noise level is 2,800, and the signal-to-noise ratio is 10.21 dB. In the case of the active wavelength filter 50 according to the present disclosure, the signal level is 14,401, the noise level is 3,811, and the signal-to-noise ratio is 11.55 dB. In comparison, since the active wavelength filter 50 has a narrow passband, it exhibits the best signal-to-noise ratio performance.

Figure 6 is a graph showing the change in the center wavelength of the pass band of the active wavelength filter 50.

The active wavelength filter 50 can change the center wavelength of the pass band. Figure 6 shows that the center wavelength of the passband changes from about 1550.3 nm to 1553.4 nm at intervals of about 0.18 nm. In this way, the active wavelength filter 50 can finely adjust the center wavelength to respond to even minute changes in the center wavelength. The active wavelength filter 50 is not limited to that shown in FIG. 6, and can change the center wavelength in a variety of ranges and change the center wavelength more finely.

If the central wavelength of the light source 10 changes due to various causes such as external factors or deterioration, the passive wavelength filter cannot pass the sensing light and the distributed acoustic sensing device 1 cannot operate normally, but the active wavelength filter 50 ) can maintain coherence even if the central wavelength of the light source 10 changes by adjusting the central wavelength.

FIG. 7 is a graph showing changes in the width of the pass band of the active wavelength filter 50.

In general, the smaller the line width of the sensing light, the higher the coherence and the lower the noise. However, in order for the backscattered light to have sufficient intensity, the intensity of the sensing light is also an important factor. The active wavelength filter 50 can adjust the width of the pass band so that the sensing light has sufficient intensity. When the intensity of the sensing light is not sufficient for various reasons, such as the intensity of the laser light output from the light source 10 or the intensity of the pulse light amplified by the optical amplifier 40, the active wavelength filter 50 passes through. By increasing the width of the band, it can be adjusted to allow sensing light of sufficient intensity to pass through.

The active wavelength filter 50 can only pass light of a wavelength that reaches a certain pixel. If the active wavelength filter 50 controls the pixel through which light with a wavelength of about 1551.9 nm arrives, it may have the same form as the first pass band PB1 in FIG. 7. If the active wavelength filter 50 controls the pixels through which light with a wavelength of 1551.7 nm and light with a wavelength of 1552.1 nm arrive to pass, it can have a form similar to the second pass band PB2 in FIG. 7. When light reaching a plurality of pixels of the active wavelength filter 50 passes through, the width of the pass band gradually widens, such as the third pass band (PB3), fourth pass band (PB4), and fifth pass band (PB5). You can.

FIG. 8 is a flowchart showing the steps of a control method of a distributed acoustic sensing device 1 including an active wavelength filter 50 according to an embodiment. Please refer also to Figure 1.

The control method of the distributed acoustic sensing device 1 including the active wavelength filter 50 is that the optical pulse modulator 30 modulates the light output by the light source 10 into a pulse form and the optical amplifier 40 modulates the light output by the light source 10. A first step (S1) of amplifying and generating pulse light, a second step (S2) of an active wavelength filter 50 filtering the pulse light, and the pulse light passing through the active wavelength filter 50 is transmitted through the optical fiber 70. ), and the backscattered light generated at an arbitrary point of the optical fiber 70 is input to the photodetector 80 and converted into an electrical signal, and the signal processing unit 90 analyzes the electrical signal to detect the optical fiber 70. A third step (S3) of detecting vibration applied to the signal processing unit 90 may include a fourth step (S4) of controlling the active wavelength filter 50 to change the center wavelength or width of the pass band. You can.

The first step (S1) is a process before inputting the sensing light to the active wavelength filter 50. In the first step (S1), the light source 10 generates laser light and provides it to the optical pulse modulator 30, and the optical pulse modulator 30 modulates the laser light into a pulse form and transmits the pulsed light to the optical amplifier 40. The optical amplifier 40 can amplify the pulsed light and output it to the active wavelength filter 50. In the first step (S1), part of the laser light is separated into the second optical coupler 22 as reference light by the first optical coupler 21 connected between the light source 10 and the optical pulse modulator 30, and the remainder is sensed. As light, it can be input to the optical pulse modulator 30.

In the second step (S2), the active wavelength filter 50 may pass light included in the pass band and block light outside the pass band to block naturally amplified emission noise.

The third step (S3) measures the backscattered light generated as the sensing light input to the optical fiber 70 passes through the optical fiber 70 to calculate the position at which vibration is applied to the optical fiber 70. The sensing light passing through the active wavelength filter 50 is input to the optical fiber 70 through the optical rotator 60, and the backscattered light input from the optical fiber 70 to the optical rotator 60 is output to the photo detector 80. Thus, the photodetector 80 can generate an electrical signal corresponding to the backscattered light. The signal processing unit 90 can analyze the electrical signal output from the photodetector 80 and calculate at which point the vibration was applied to the optical fiber 70.

In the fourth step (S4), the signal processing unit 90 automatically adjusts the center wavelength or width of the pass band of the active wavelength filter 50. The signal processing unit 90 analyzes the electrical signal received from the photodetector 80 and, if the intensity of the backscattered light is less than the reference value, adjusts the center wavelength or width of the passband of the active wavelength filter 50. can do. The signal processing unit 90 may control the active wavelength filter 50 independently of controlling the light source 10, the optical pulse modulator 30, and the amplifier to generate pulsed light. Alternatively, the signal processing unit 90 may perform the step of controlling the active wavelength filter 50 at predetermined times.

If the intensity of the backscattered light is measured to be lower than the reference value, the signal processing unit 90 can automatically adjust the central wavelength or width of the pass band of the active wavelength filter 50 and perform distributed acoustic sensing again normally.

Figure 9 is a flowchart showing steps for controlling an active wavelength filter according to one implementation.

The fourth step (S4) of controlling the active wavelength filter is the fourth-1 step (S41) in which the signal processing unit 90 analyzes the electrical signal received from the photodetector 80 and compares the intensity of the backscattered light with a reference value. , and when the intensity of the backscattered light is less than the reference value, the signal processing unit 90 adjusts the center wavelength or width of the pass band of the active wavelength filter 50 to adjust the intensity of the backscattered light to be greater than the reference value. It may include step S42.

In step 4-1 (S41), the reference value may be set as a value necessary for distributed acoustic sensing to operate normally. The signal processing unit compares the intensity of backscattered light with a reference value. If the intensity of backscattered light is greater than the reference value (Y), it is in a steady state (S4-3), so general distributed acoustic sensing can continue to be performed. If the intensity of the backscattered light is less than the reference value (N), it is not suitable for performing distributed acoustic sensing, so an operation to adjust the active wavelength filter 50 may be performed. This is because if the intensity of the backscattered light is less than the reference value, it is difficult to recognize the location of the vibration applied to the optical fiber 70.

In step 4-2 (S42), the signal processing unit 90 adjusts the active wavelength filter 50. The signal processing unit 90 increases the intensity of the pulse light passing through the active wavelength filter 50 by moving the center wavelength of the pass band of the active wavelength filter 50 to another wavelength or increasing the width of the pass band to increase the intensity of the backscattered light. increases.

Figure 10 is a flowchart showing the steps of controlling the size of backscattered light to be greater than a reference value according to one implementation.

The 4-2 step (S42) is a step (S421) in which the signal processing unit 90 measures the intensity of the backscattered light while changing the center wavelength of the pass band of the active wavelength filter 50, and the intensity of the backscattered light is the highest. Determine whether the center wavelength matches the existing center wavelength, determine whether the intensity of the large backscattered light is greater than the reference value, and perform one of the center wavelength adjustment step (S423a), width adjustment step (S423b), and center wavelength and width adjustment step. It may include a step of deciding to perform (S422), and a step (S423) of performing one of the center wavelength adjustment step (S423a), the width adjustment step (S423b), and the center wavelength and width adjustment step (S423c).

In step 4-2 (S42), the signal processing unit 90 measures the intensity of the backscattered light while sequentially changing the center wavelength of the pass band of the active wavelength filter 50, and selects the center wavelength with the largest size of the backscattered light. The active wavelength filter 50 can be controlled to become the center wavelength of the active wavelength filter 50. In step 4-2 (S42), the signal processing unit 90 measures the intensity of the backscattered light while changing the width of the passband for each center wavelength of the passband of the active wavelength filter 50, and sets the size of the backscattered light to a reference value. The active wavelength filter 50 can be controlled so that the width of the pass band becomes the width of the pass band of the active wavelength filter 50.

In the step (S421) of measuring the intensity of backscattered light while changing the center wavelength, the signal processing unit 90 measures the intensity of backscattered light while changing the center wavelength of the active wavelength filter 50 sequentially or in a predetermined order. and save it. At this time, the spacing of the center wavelength of the active wavelength filter 50 may vary depending on the smallest pixel size of the LC array 54a. If the size of the backscattered light is measured for each center wavelength of the pass band of the active wavelength filter 50, it is possible to know the center wavelength at which the size of the backscattered light is measured to be the largest.

In the step of deciding to perform one of the center wavelength adjustment step (S423a), the width adjustment step (S423b), and the center wavelength and width adjustment step (S422), the center wavelength with the greatest intensity of backscattered light is identical to the existing center wavelength. It may include a center wavelength matching test to determine whether the The center wavelength coincidence test can be performed to determine whether there is a need to change the center wavelength of the pass band of the active wavelength filter 50. The center wavelength coincidence test determines whether the existing center wavelength of the active wavelength filter 50 and the center wavelength with the greatest intensity of backscattered light measured in the step (S421) of measuring backscattered light while changing the center wavelength match. If the existing center wavelength and the center wavelength with the highest intensity of backscattered light match (Y), there is no need to change the center wavelength, but if they do not match (N), it is necessary to change the center wavelength.

The determining step (S422) may include a backscattered light intensity test to determine whether the largest backscattered light intensity is greater than a reference value. A backscattered light intensity test may be performed to determine whether there is a need to change the width of the passband of the active wavelength filter 50. The backscattered light intensity test determines whether the backscattered light with the greatest intensity among the backscattered lights measured in the step (S421) of measuring the backscattered light while changing the central wavelength is greater than the reference value. If the highest intensity backscattered light is greater than the reference value (Y), there is no need to change the width of the pass band, but if it is small (N), the width of the pass band needs to be widened.

In the determining step (S422), the signal processing unit 90 performs the center wavelength adjustment step (S423a) if the center wavelength matching test result is 'N' and the backscattered light intensity test is 'Y', and the center wavelength matching test is 'Y'. If Y' and the backscattered light intensity test is 'N', perform the width adjustment step (S423b), and if the center wavelength matching test is 'N' and the backscattered light intensity test is 'Y', perform the center wavelength and width adjustment step (S423b). You can decide to do it.

The signal processing unit 90 may perform one of the center wavelength adjustment step (S423a), the width adjustment step (S423b), and the center wavelength and width adjustment step (S423b), according to what is determined in the determining step (S422). S423).

The center wavelength adjustment step (S423a) controls the active wavelength filter 50 so that the center wavelength with the greatest intensity of backscattered light becomes the center wavelength of the active wavelength filter 50. The signal processing unit 90 controls the passage/blocking of the pixels of the LC array 54a of the active wavelength filter 50, so that the light is transmitted from the pixel where the light of the wavelength corresponding to the central wavelength with the highest intensity of backscattered light arrives. By controlling it to pass through, the center wavelength of the active wavelength filter 50 can be adjusted.

In the width adjustment step (S423b), the wavelength adjacent to the center wavelength of the active wavelength filter 50 is further passed, and the width of the pass band when the intensity of the backscattered light is greater than the reference value is adjusted to the width of the pass band of the active wavelength filter 50. The active wavelength filter 50 is controlled to have a wide width. The signal processing unit 90 controls the passage/blocking of pixels of the LC array 54a of the active wavelength filter 50, so that light passes through the pixel where the wavelength closest to the center wavelength arrives, and controls the light to pass through the pixel where the wavelength closest to the center wavelength arrives. When light of a certain wavelength passes, the backscattered light is further measured and compared with a reference value, and if the intensity of the backscattered light is greater than the reference value, the width of the passband can be determined to include the center wavelength and adjacent wavelengths. Even when passing more adjacent wavelengths, if the intensity of the backscattered light is less than the standard value, control it to pass more wavelengths adjacent to the adjacent wavelength, and measure the backscattered light more when the center wavelength, the adjacent wavelength, and the wavelength adjacent to the adjacent wavelength pass. It is compared with the reference value, and if the intensity of the backscattered light is greater than the reference value, the width of the passband can be determined to include the center wavelength, adjacent wavelengths, and wavelengths adjacent to the adjacent wavelength. As explained, adjacent wavelengths can be continued to be added until the intensity of the backscattered light is greater than the reference value.

In the center wavelength and width adjustment step (S423b), the center wavelength with the largest size of the backscattered light is determined as the center wavelength of the active wavelength filter 50, and the pixels corresponding to the wavelength adjacent to the center wavelength of the active wavelength filter 50 are selected. In addition, by controlling the pass, the active wavelength filter 50 is controlled so that the width of the pass band when the intensity of the backscattered light exceeds the reference value is the width of the pass band of the active wavelength filter 50. The signal processing unit 90 controls the on/off of the pixels of the LC array 54a of the active wavelength filter 50, passing the light reaching the pixel corresponding to the center wavelength with the largest size of backscattered light, and passing the light reaching the center wavelength. Control the light reaching the pixel corresponding to the wavelength adjacent to the wavelength to pass through more. When the light of the center wavelength and the adjacent wavelength passes, the backscattered light is further measured and compared with the reference value. If the intensity of the backscattered light is greater than the reference value, The width of the passband can be determined to include the center wavelength and adjacent wavelengths. Adjacent wavelengths can be continued to be added until the intensity of the backscattered light is greater than the reference value.

As described, the signal processing unit 90 adjusts the center wavelength and width of the pass band of the active wavelength filter 50 to filter the sensing light input to the optical fiber 70 so that backscattered light of sufficient intensity can be measured. You can. The signal processing unit 90 controls the active wavelength filter 50 so that the intensity of the sensing light input to the optical fiber 70 is sufficiently strong even when the characteristics of the light source 10 or the active wavelength filter 50 change due to various causes. Therefore, the distributed acoustic sensing device 1 can operate normally.

The present disclosure has been described in detail above through specific implementation examples. The implementation examples are for specifically explaining the present disclosure, and the present disclosure is not limited thereto. It will be clear that modifications and improvements can be made by those skilled in the art within the technical spirit of the present disclosure.

All simple modifications or changes to the present disclosure fall within the scope of the present disclosure, and the specific scope of protection of the present disclosure will be made clear by the appended claims.

This application was conducted as part of the open innovation R&D (20-A-T-002) project of Korea Water Resources Corporation (K-water).

1: Distributed acoustic sensing device using active wavelength filter
10: light source 21: first optocoupler
22: second optical coupler 30: optical pulse modulator
40: Optical amplifier 50: Active wavelength filter
60: optical rotator 70: optical fiber
80: photodetector 90: signal processing unit
51: optical input/output unit 52: polarization unit
52a: collimating lens 52b: beam displacer
52c: half-wave retarder 52d: first lens
53: Wavelength separation unit 53a: Grating
53b: second lens 54: wavelength selection unit
54a: LC array 54b: mirror

Claims (14)

A polarization unit that receives unpolarized light from the optical input/output unit, separates it into two polarized lights orthogonal to each other, and converts one of the two polarized lights into the other polarized light to output;
a wavelength separation unit that separates the light output from the polarization unit according to wavelength and outputs it; and
It includes a wavelength selection unit that selectively reflects one or more wavelengths among the light separated by wavelength by the wavelength separation unit,
An active wavelength filter, wherein the light selectively reflected from the wavelength selection unit passes through the wavelength separation unit and the polarization unit and is output to the optical input/output unit. In claim 1,
The polarization unit
a collimating lens that aligns the light incident from the optical input/output unit in parallel;
a beam displacer that separates the light passing through the collimating lens into two polarized lights perpendicular to each other; and
An active wavelength filter comprising a half-wave retarder that delays one of the two polarizations output from the beam displacer to match the other polarization. In claim 1,
The wavelength separator
An active wavelength filter that includes a grating that reflects incident light in different directions depending on the wavelength. In claim 1,
The wavelength selection unit
An LC array whose arrangement is changed according to a control signal at each pixel to which light separated according to wavelength arrives from the wavelength separator to pass or block the light; and
An active wavelength filter comprising a mirror that reflects light passed by the LC array. A light source that generates laser light;
a first optical coupler that splits the light output from the light source into sensing light and reference light;
an optical pulse modulator that outputs pulsed light by modulating the sensing light split by the first optical coupler;
an optical amplifier that amplifies the pulsed light;
an active wavelength filter that filters the pulsed light amplified by the optical amplifier, passes light in a pass band, and is capable of adjusting the center wavelength or width of the pass band;
an optical rotator that outputs light input from the active wavelength filter connected to the first stage to a second stage, and outputs backscattered light input from the optical fiber connected to the second stage to a third stage;
a second optical coupler that combines the backscattered light received from the third stage of the optical rotator and the reference light received from the first optical coupler and outputs a combination;
a photodetector that converts the light received from the second optical coupler into an electrical signal and outputs it;
A distributed sound system including an active wavelength filter, including a signal processing unit that analyzes the electrical signal received from the optical detector to detect vibration applied to the optical fiber and adjusts the center wavelength or width of the pass band of the active wavelength filter. Sensing device. In claim 5,
The signal processing unit
A distribution type including an active wavelength filter that analyzes the electrical signal received from the photodetector and adjusts the center wavelength of the passband or the width of the passband if the intensity of the backscattered light is less than the reference value. Acoustic sensing device. In claim 6,
The signal processing unit
Measure the intensity of the backscattered light while sequentially changing the center wavelength of the pass band of the active wavelength filter, and control the active wavelength filter so that the center wavelength with the largest size of the backscattered light becomes the center wavelength of the active wavelength filter. A distributed acoustic sensing device including an active wavelength filter. In claim 6,
The signal processing unit
The intensity of the backscattered light is measured while changing the width of the passband for each center wavelength of the passband of the active wavelength filter, and the width of the passband at which the size of the backscattered light is greater than a reference value is that of the passband of the active wavelength filter. A distributed acoustic sensing device including an active wavelength filter that controls the active wavelength filter to have a width. A first step in which an optical pulse modulator modulates the light output by the light source into a pulse form and an optical amplifier amplifies the light to generate pulsed light;
A second step in which an active wavelength filter filters the pulsed light;
Pulsed light passing through the active wavelength filter is input to the optical fiber, backscattered light generated at an arbitrary point of the optical fiber is input to the optical detector and converted into an electrical signal, and a signal processing unit analyzes the electrical signal and applies it to the optical fiber. A third step of detecting vibration; and
A method of controlling a distributed acoustic sensing device including an active wavelength filter, comprising a fourth step in which the signal processing unit controls the active wavelength filter to change the center wavelength or width of the pass band. In claim 9,
The fourth step is,
Step 4-1 in which the signal processing unit analyzes the electrical signal received from the photodetector and compares the intensity of the backscattered light with a reference value; and
When the intensity of the backscattered light is less than the reference value, the signal processing unit adjusts the central wavelength or width of the pass band of the active wavelength filter to adjust the intensity of the backscattered light to be greater than the reference value. A 4-2 step comprising: Control method of a distributed acoustic sensing device including an active wavelength filter. In claim 10,
Step 4-2 is
The signal processing unit measuring the intensity of the backscattered light while changing the center wavelength of the passband of the active wavelength filter;
Determine whether the center wavelength with the greatest intensity of the backscattered light matches the existing center wavelength, determine whether the intensity of the largest backscattered light is greater than the reference value, and perform a center wavelength adjustment step, a width adjustment step, and a center wavelength and width adjustment step. deciding to perform one of the following; and
A control method of a distributed acoustic sensing device including an active wavelength filter, comprising performing any one of a center wavelength adjustment step, a width adjustment step, and a center wavelength and width adjustment step. In claim 11,
The center wavelength adjustment step is
A method of controlling a distributed acoustic sensing device including an active wavelength filter, wherein the active wavelength filter is controlled such that the center wavelength with the greatest intensity of the backscattered light becomes the center wavelength of the active wavelength filter. In claim 11,
The width adjustment step is
Controlling the active wavelength filter to further pass wavelengths adjacent to the center wavelength of the active wavelength filter so that the width of the pass band when the intensity of backscattered light exceeds a reference value is the width of the pass band of the active wavelength filter. In, control method of a distributed acoustic sensing device including an active wavelength filter. In claim 11,
The center wavelength and width adjustment step is
The center wavelength with the largest size of the backscattered light is determined as the center wavelength of the active wavelength filter, and pixels corresponding to wavelengths adjacent to the center wavelength of the active wavelength filter are additionally controlled to pass, so that the intensity of the backscattered light is set to the reference value. A method of controlling a distributed acoustic sensing device including an active wavelength filter, wherein the active wavelength filter is controlled so that the width of the passband when the width becomes greater than or equal to the width of the passband of the active wavelength filter becomes the width of the passband of the active wavelength filter.

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