Patent Description:
A phenomenon in which light interacts with sound waves present in a material to be scattered at a frequency different from that of incident light is called Brillouin scattering, and here, a difference between these frequencies is called Brillouin frequency shift. An optical fiber BOCDA sensor is a distribution measuring sensor that acquires the Brillouin frequency shift ΔνB at all positions along a length of a sensing fiber. A brief description of a principle of detecting the Brillouin frequency shift is as follows. Pumping light (or pump light) and probe light are controlled to be incident respectively from both ends of an optical fiber to be measured (or measurement target optical fiber), and an optical frequency is adjusted so that the optical frequency difference Δν between the pumping and probe light matches the Brillouin frequency shift value vB of the measurement target optical fiber. In this case, the pumping light is energy-converted into probe light due to stimulated Brillouin scattering and the probe light is Brillouin light amplified within the measurement target optical fiber. An optical signal of the amplified probe light may be converted into an electric signal by a photo detector so as to be measured.

Such a Brillouin frequency shift value is significantly affected by a material through which light travels, i.e., a material of an optical fiber and changes according to a strain applied to the optical fiber. When a strain of the optical fiber due to external stress is Δε and a temperature change is ΔT, a change amount ΔνB of the Brillouin frequency shift value is expressed as Equation <NUM>. In the following equation, a strain conversion factor Cε and a temperature conversion factor CT are known values, but it is more preferable to accurately check and use these factors according to actual application conditions in order to increase accuracy. When a standard single-mode optical fiber is used as a sensing fiber, the strain conversion factor Cε is about <NUM>/µε and the temperature conversion factor CT is about <NUM>/oC, but exact values should be checked and used according to actual application conditions.

When the Brillouin frequency shift is acquired, a strain or a temperature distributed in the sensing fiber may be measured using Equation <NUM>, and a sensor made based on this principle is referred to as an optical fiber BOCDA sensor. In order to obtain the Brillouin frequency shift distributed in such a sensing fiber, a Brillouin gain spectrum should be acquired from all positions of the sensing fiber, and the optical fiber BOCDA sensor of the related art uses a phase code frequency control method that scans a frequency during phase code modulation.

<FIG> is a schematic view of a phase code frequency control type optical fiber BOCDA sensor of the related art. As shown in <FIG>, light emitted from one light source is modulated by a phase modulator to have a difference of <NUM> degrees in a pseudo random bit sequence (PRBS) code pattern. The phase-modulated light is branched by a 1x2 optical fiber coupler and used as pumping light and probe light. The probe light is modulated in optical frequency near the Brillouin frequency by an electro-optic modulator and then incident to one end of the sensing fiber. Meanwhile, the pumping light passes through an optical circulator and then is incident to the other end of the sensing fiber. As these two lights travel through the sensing fiber, there is a time during which phases of the two lights always match at a first encountering bit of a phase code but do not match at a position of another bit. When the time during which the phases match is equal to or greater than <NUM> ns of a phonon lifetime of an acoustic wave, Brillouin scattering amplification occurs, thereby obtaining a Brillouin frequency. However, the time during which the phases match is shorter than the phonon lifetime of the acoustic wave at different positions of the two lights, so that Brillouin scattering amplification does not occur, and thus Brillouin gain cannot be obtained.

Meanwhile, the bit position where the two lights first meet in the sensing fiber does not change even if the frequency of the PRBS code is changed. However, the position is changed at a second or greater correlation bit position when the frequency of the phase code is changed. Therefore, in order to obtain a Brillouin amplified signal at a certain position in the sensing fiber, a delay fiber needs to be provided so that a detection section may be set at a second or greater correlation position. Accordingly, the Brillouin scattered amplified light obtained by a photo receiver converts a signal generated only at the correlation bit of the PRBS code into an electric signal. In a computer, a Brillouin gain spectrum is obtained by converting an electrical signal into a digital signal using an analog-to-digital converter (ADC) and a Brillouin frequency is obtained to obtain a temperature or strain according to Equation <NUM>.

<FIG> shows an embodiment of driving a phase code frequency-control type optical fiber BOCDA sensor of the related art, which is a configuration example realized by more concretely implementing the device shown in the schematic view of <FIG>. In the embodiment of <FIG>, the entire system is controlled by a data acquisition program of a computer. This program adjusts an operation of a pulse pattern generator (PPG), a signal generated by the PPG is input to a phase modulator (PM), and the PM modulates a phase of light output from a distributed feedback laser diode (DFB LD). In the embodiment of <FIG>, the light output from LD may have a peak wavelength of <NUM> and an intensity of <NUM> mW. The PPG generates an electrical signal of a pseudo random bit sequence (PRBS) pattern, and a symbol duration of one bit of the signal is set to be much shorter than the phonon lifetime of an acoustic wave generated in stimulated Brillouin scattering (SBS). A phase of one bit of the PRBS is modulated to one of <NUM> and π, and probabilities of occurrence of both states are the same. The modulated optical signal is divided by a <NUM> dB optical fiber coupler to generate probe light and pumping light. The probe light and the pumping light, which are phase-modulated and continuously oscillate, travel in opposite directions of a closed loop and then meet exactly at a midpoint of the loop and interact to generate a correlation peak. If the two lights meet again over the entire length of the phase code, a correlation peak is generated again at that position.

<FIG> is a view illustrating a change in a correlation peak position in an optical fiber based on phase code frequency modulation when the phase code frequency control type optical fiber BOCDA sensor of the related art is driven. As shown in <FIG>, if a length of one phase code bit is T time, a distance width of a correlation peak created when the probe light and the pumping light meet is calculated as Δd = (<NUM>/<NUM>) νgT (here, νg is a group speed of light in the optical fiber). If a loop length of the optical fiber is longer than a total length of the PRBS bits, the correlation peak is formed periodically as shown in <FIG> along a loop of the optical fiber.

Here, a correlation peak formed exactly at a middle position after passing through a <NUM> dB optical fiber coupler, which is a branch point of the optical signal, is called a zero-order correlation peak. In this case, there is no path difference between the two traveling lights. If two lights are intended to meet at a deviating position, rather than at exactly the middle in the sensing fiber, that is, if an order of a first formed correlation peak is intended not to be a zero order, an intentional path difference should be introduced while the probe light and the pumping light travel. To this end, a delay fiber (refer to <FIG> and <FIG>) for a path difference is inserted at a position close to the probe light. A position of the correlation peak thusly formed on the loop depends on a length of the delay fiber. The distance Δdn between the correlation peaks may be expressed as Δdn = NΔd = (<NUM>/<NUM>)NνgT, where N is the length of the PRBS code. In an experiment, <NUM><NUM>-<NUM> bits were used as the length. The length of the sensing fiber to be used for measurement is made shorter than Δdn so that only one correlation peak may be formed in the closed loop. When the PRBS frequency is changed, a time width of one bit changes from T to T', so that the length of the entire PRBS changes and the position of the correlation peak also shifts. The degree of this position shift is greater as the order of the correlation peak increases. The shift of the peak must be less than or equal to Δd, which is the distance width of the peak, to ensure a distance resolution of the optical fiber to be measured. This explanation may be expressed as an equation as follows.

Here, DL and PL are a distance between the length of the delay fiber and the correlation peak. Since a time width of one bit is the reciprocal of a modulation bit rate (BR), a change rate of the BR for moving a measurement position of the optical fiber is ΔBR = BR - BR'. Equation <NUM> may be obtained by simplifying Equation <NUM> using the equation for the BR change rate.

Equation <NUM> refers to an interval of the BR at which the measurement position in the sensing fiber should be changed. This interval is proportional to the modulation frequency and the distance width of one bit and inversely proportional to the length of the delay fiber.

A paper such as "Random-access distributed fiber sensing" (Avi Zadok etc., Laser Photonics Rev. <NUM>, No. <NUM>, L1-L5, <NUM>) discloses a sensor configuration including a delay fiber like the phase code frequency modulation type optical fiber BOCDA sensor of the related art as described above. However, the technique of the related art has several problems such as a problem in that it is difficult to modulate a phase code frequency, a problem in that a correlation peak width changes according to phase code frequency modulation, a problem in that a delay fiber must be installed, and a problem in that it is not easy to determine where the correlation peak is to be located in a sensing fiber, and the like.

An object of the present invention is to provide an optical fiber BOCDA sensor using phase code modulation of pumping light and probe light having a time difference capable of improving detection performance and detection accuracy by improving spatial resolution, while simplifying a control design and device configuration compared with the related art, by controlling a correlation peak position using two phase codes.

In one general aspect, the invention relates to anoptical fiber BOCDA sensor (<NUM>) for measuring a strain and a temperature at a certain position on a sensing fiber (<NUM>) using Brillouin frequency shift, comprising: a probe light phase modulator (<NUM>) and a pumping light phase modulator (<NUM>), which are adapted to be independently controlled, on a probe light optical fiber line (<NUM>) and a pumping light optical fiber line (<NUM>), respectively, so that a time difference is formed in a phase code pattern created in each of the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>), thereby adjusting a correlation peak position of pumping light and probe light on the sensing fiber (<NUM>), wherein the optical fiber BOCDA sensor (<NUM>) is configured to adjust the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) to have the same form but have only a time difference, wherein the optical fiber BOCDA sensor (<NUM>) is adapted to adjust a time difference of the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) so that the correlation peak position corresponds to a position of the sensing fiber (<NUM>) on which measurement is to be performed, wherein the optical fiber BOCDA sensor (<NUM>) is adapted to adjust bit widths of the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) according to a length of a section of a position on the sensing fiber (<NUM>) on which measurement is to be performed, wherein the optical fiber BOCDA sensor (<NUM>) is adapted to perform control to subtract a Brillouin spectrum obtained using a phase code pattern in which a bit width of a correlation peak position has a second size smaller than a predetermined first size from a Brillouin spectrum obtained using a phase code pattern in which a bit width of a correlation peak position has the first size, and wherein the maximum value of the bit width is the value of the bit width when the total sum of the Brillouin gains at the non-correlation position N-<NUM> bits is equal to the Brillouin gain at one bit of the correlation position I<NUM> and wherein the minimum value of the bit width is the value of the bit width when the Brillouin gain I<NUM> occurring at the correlation position is equal to the Brillouin gain Ic0 occurring in the entire length of the sensing fiber.

As a more specific configuration, the optical fiber BOCDA sensor <NUM> may include: a light source <NUM>; an optical fiber coupler <NUM> adapted to divide light traveling from the light source <NUM> through an optical fiber into lights traveling to the probe light optical fiber line <NUM> and the pumping light optical fiber line <NUM>, respectively; the sensing fiber <NUM> having one end connected to an end of the probe light optical fiber line <NUM> and the other end connected to the pumping light optical fiber line <NUM> and adapted to cause amplification to Brillouin scattered light scattered to a rear of pumping light if there is a difference as large as a Brillouin frequency between frequencies of probe light and pumping light; the probe light phase modulator <NUM> provided on the probe light optical fiber line <NUM> and adapted to modulate a phase of the probe light to a predetermined phase code pattern; a probe light electro-optic modulator <NUM> provided on the probe light optical fiber line <NUM> and adapted to adjust probe light traveling from the probe light phase modulator <NUM> to have frequency modulation near the Brillouin frequency of the sensing fiber <NUM>; an optical fiber isolator <NUM> provided on the probe light optical fiber line <NUM>, adapted to cause probe light traveling from the probe light electro-optic modulator <NUM> to travel toward the sensing fiber <NUM>, and adapted to block light traveling from the sensing fiber <NUM>; the pumping light phase modulator <NUM> provided on the pumping light optical fiber line <NUM> and adapted to modulate a phase of pumping light to a phase code pattern having a time difference from a phase code pattern used in the probe light phase modulator <NUM>; an optical fiber circulator <NUM> provided on the pumping light optical fiber line <NUM> and adapted to cause probe light traveling from the pumping light phase modulator <NUM> to travel toward the sensing fiber <NUM>; a photo receiver <NUM> adapted to acquire Brillouin scattered light traveling from the optical fiber circulator <NUM>; and a controller <NUM> adapted to control the phase code patterns respectively generated in the probe light phase modulator <NUM> and the pumping light phase modulator <NUM>.

Here, the optical fiber BOCDA sensor <NUM> may further include: a probe light optical fiber amplifier <NUM> provided at a front of the optical fiber isolator <NUM> on the probe light optical fiber line <NUM> and adapted to amplify probe light traveling to the optical fiber isolator <NUM>.

In addition, the optical fiber BOCDA sensor <NUM> may further include: a polarization scrambler <NUM> provided between the probe light electro-optic modulator <NUM> and the optical isolator <NUM> on the probe light optical fiber line <NUM> and adapted to remove an influence of polarization of the probe light traveling from the probe light electro-optic modulator <NUM>.

In addition, the optical fiber BOCDA sensor <NUM> may further include: a lock-in amplifier <NUM> provided at a rear of the photo receiver <NUM> and adapted to amplify Brillouin scattered light received by the photo receiver <NUM>; and a pumping light electro-optic modulator <NUM> provided on the pumping light optical fiber line <NUM> and adapted to modulate pumping light traveling from the pumping light phase modulator <NUM> into a sine wave to drive the lock-in amplifier <NUM>.

In addition, the optical fiber BOCDA sensor <NUM> may further include: a pumping light optical fiber amplifier <NUM> provided at a front of the optical fiber circulator <NUM> on the pumping light optical fiber line <NUM> and adapted to amplify pumping light traveling to the optical fiber circulator <NUM>.

In addition, the controller <NUM> may include: a pulse pattern generator <NUM> connected to the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> and adapted to generate and amplify the phase code patterns respectively used at the probe light phase modulator <NUM> and the pumping light phase modulator <NUM>; and an RF signal synthesizer <NUM> connected to the probe light electro-optic modulator <NUM> and adapted to drive the probe light electro-optic modulator <NUM> by generating an electric signal near the Brillouin frequency of the sensing fiber <NUM>.

In addition, the controller <NUM> may include a chopping module <NUM> connected to the pumping light electro-optic modulator <NUM> and the lock-in amplifier <NUM> and adapted to control light at regular time intervals.

According to the present invention, the process of modulating an optical frequency to control a correlation peak position in the phase code frequency modulation type optical fiber BOCDA sensor of the related art is excluded, and instead, a process of using control of two phase codes having a time difference to control the correlation peak position is used, thereby realizing more simplified control design and device configuration compared to the related art. Accordingly, various problems of the phase code frequency modulation method of the related art described above may be fundamentally eliminated.

More specifically, the related art has a problem in that it is difficult to modulate the phase code frequency and a problem in that the correlation peak width varies according to the phase code frequency modulation. However, in the present invention, phase modulators which may be controlled separately from each other are used, and thus control design may be made much easier. In addition, the related art has a problem in that a delay fiber must be installed, but in the present invention, there is no need to install the delay fiber, thus simplifying device configuration. In addition, the related art has a problem in that it is not easy to determine where the correlation peak position is located in the sensing fiber, but in the present invention, the correlation peak position may be allocated as desired, and thus control design is made much easier.

In addition, according to the present invention, since the correlation peak position may be arbitrarily controlled as desired, spatial resolution may be improved and detection performance and detection accuracy may be fundamentally improved.

Hereinafter, an optical fiber Brillouin correlation domain analysis (BOCDA) sensor using phase code modulation of a pumping light and a probe light having a time difference according to the present invention having the configuration described above will be described in detail with reference to the accompanying drawings.

As described above, in the related art, an optical frequency is modulated to control a correlation peak position, and in this process, there were problems that obscure various control designs and device configurations. The present invention discloses an optical fiber BOCDA sensor that replaces the process of modulating an optical frequency with a new method using two phase codes to control the shortcomings of the optical fiber BOCDA sensor of the related art, i.e., the correlation peak position.

<FIG> is a schematic view of an optical fiber BOCDA sensor using two phase codes having a time difference of the present invention. The optical fiber BOCDA sensor <NUM> of the present invention basically measures a strain and a temperature at a certain position on a sensing fiber <NUM> using a Brillouin frequency shift and includes a probe light optical fiber line <NUM> and a pumping light phase modulator <NUM>, which can be independently controlled, at a probe light optical fiber line <NUM> and a pumping light optical fiber line <NUM>, respectively. Here, in the optical fiber BOCDA sensor <NUM> of the present invention, a correlation peak position of pumping light and probe light can be adjusted by forming a time difference between phase code patterns created by the probe light phase modulator <NUM> and the pumping light phase modulator <NUM>. Specifically, in the optical fiber BOCDA sensor <NUM> of the present invention, the phase code patterns created by the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> have the same shape but have only a time difference.

The optical fiber BOCDA sensor of the related art shown in <FIG> with the optical fiber BOCDA sensor <NUM> of the present invention shown in <FIG> may be intuitively compared as follows. In the optical fiber BOCDA sensor of the related art, the delay fiber is provided on the probe light optical fiber line so that lights having the same phase code pattern are incident on the probe light optical fiber line and the pumping light optical fiber line and a Brillouin amplified signal is obtained at a certain position on the sensing fiber. In contrast, the optical fiber BOCDA sensor <NUM> of the present invention includes the probe light phase modulator <NUM> and the pumping light phase modulator <NUM>, which can be independently controlled, at the probe light optical fiber line <NUM> and the pumping light optical fiber line <NUM>, respectively. That is, in the present invention, in order to generate a time delay between phase codes of the probe light and the pumping light, it is only necessary to properly set each phase code pattern so that a time delay is formed between lights produced by the phase modulators <NUM> and <NUM>. That is, in the present invention, there is no need to provide the delay fiber as a requisite in the optical fiber BOCDA sensor of the related art.

Here, as described above, in the optical fiber BOCDA sensor of the related art, since the certain relationship is formed among the modulation frequency, the rate of change of the modulation frequency, the length of the delay fiber, and the distance between the correlation peaks, where the correlation peak position is to be located in the sensing fiber should be previously determined and the length of the delay fiber should be determined in consideration of the location of the correlation peak position, causing difficulties in control design and device configuration. However, in the present invention, since the delay fiber itself is eliminated from the configuration, the difficulties in the control design and device configuration related to the delay fiber are fundamentally eliminated. In addition, since the phase modulators <NUM> and <NUM> are controlled independently of each other, one phase modulator may be set to perform modulation with a certain phase code pattern and the other phase modulator may be set to perform modulation with the same phase code pattern. It is also freely possible for the phase code patterns to be set to have the same shape and have a time delay as desired. Of course, in determining the time delay value, the time delay value may be freely set as desired without any particular limitation, and as a result, it is possible to dramatically facilitate control design and device configuration compared to the related art.

A basic configuration and operation principle of the optical fiber BOCDA sensor <NUM> of the present invention will be described in detail with reference to <FIG>. The optical fiber BOCDA sensor <NUM> of the present invention may basically include a light source <NUM>, an optical fiber coupler <NUM>, a probe light optical fiber line <NUM>, a pumping light optical fiber line <NUM>, and a sensing fiber <NUM>, and may further include a probe light phase modulator <NUM>, a probe light optical electro-optic modulator <NUM>, an optical fiber isolator <NUM>, a pumping light phase modulator <NUM>, an optical fiber circulator <NUM>, a photo receiver <NUM>, and a controller <NUM>. Each part will be described briefly.

First, the light source <NUM>, the optical fiber coupler <NUM>, and the sensing fiber <NUM>, which are basic components, will be briefly described.

The light source <NUM> outputs light for an overall operation of the sensor. Here, in order to improve operation efficiency, it is preferable that the light output from the light source <NUM> has good coherence, and as a specific example, the light source <NUM> may include a single distributed feedback (DFB) laser diode.

The optical fiber coupler <NUM> is configured in the form of a 1x2 optical fiber coupler and serves to divide light traveling through the optical fiber from the light source <NUM> to travel to each of probe light optical fiber line <NUM> and the pumping light optical fiber line <NUM>.

The sensing fiber <NUM> is a part in which a detection signal to be acquired is generated. One end of the sensing fiber <NUM> is connected to an end of the probe light optical fiber line <NUM>, and the other end thereof is connected to the pumping light optical fiber line <NUM>. Accordingly, probe light and pumping light travel from both ends of the sensing fiber <NUM>, and two lights meet each other in the sensing fiber <NUM>. Here, if there is a difference by a Brillouin frequency between the frequencies of the probe light and the pumping light, the Brillouin scattered light scattered to a rear of the pumping light is amplified. As described above, the frequency shift value of the Brillouin scattered light is directly related to a strain and a temperature at a certain position on the sensing fiber <NUM>. Accordingly, it is possible to ultimately measure a strain and a temperature change at a certain position on the sensing fiber <NUM> by acquiring the Brillouin scattered light and measuring the Brillouin frequency shift.

Next, the probe light phase modulator <NUM>, the probe light electro-optic modulator <NUM>, and the optical fiber isolator <NUM> arranged on the probe light optical fiber line <NUM> will be briefly described.

The probe light phase modulator <NUM> is provided on the probe light optical fiber line <NUM> and serves to modulate a phase of the probe light with a predetermined phase code pattern. The phase code pattern may be freely set appropriately as desired by a user.

The probe light electro-optic modulator <NUM> is provided on the probe light optical fiber line <NUM> and serves to adjust probe light traveling from the probe light phase modulator <NUM> to have frequency modulation near a Brillouin frequency of the sensing fiber <NUM>.

The optical fiber isolator <NUM> is provided on the probe light optical fiber line <NUM> and serves to cause the probe light traveling from the probe light electro-optic modulator <NUM> to travel toward the sensing fiber <NUM>. In addition, the optical fiber isolator <NUM> also serves to block light traveling from the sensing fiber <NUM>.

Next, each of the pumping light phase modulator <NUM> and the optical fiber circulator <NUM> arranged on the pumping optical fiber line <NUM> will be briefly described.

The pumping light phase modulator <NUM> is provided on the pumping light optical fiber line <NUM> and serves to modulate a phase of pumping light like the probe light phase modulator <NUM>. Here, a phase code pattern used in the pumping light phase modulator <NUM> in the present invention is a pattern having a time difference from a phase code pattern used in the probe light phase modulator <NUM>.

The optical fiber circulator <NUM> is provided on the pumping optical fiber line <NUM> and causes probe light traveling from the pumping light phase modulator <NUM> toward the sensing fiber <NUM>, like the optical fiber isolator <NUM>.

Finally, each of the photo receiver <NUM> and the controller <NUM> for acquiring and controlling light will be briefly described.

The photo receiver <NUM> serves to acquire Brillouin scattered light traveling from the optical fiber circulator <NUM>. The photo receiver <NUM> is connected to the controller <NUM> and transmits the acquired optical signal data to the controller <NUM> so that signal analysis may be performed.

The controller <NUM> most basically serves to control the phase code patterns generated in the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> and serves to control and analyze various other devices.

The overall operation of the optical fiber BOCDA sensor <NUM> of the present invention configured as described above will be described as follows.

Light emitted from the light source <NUM> is divided in the optical fiber coupler <NUM> and used as probe light and pumping light. These two lights are phase-modulated into PRBS code patterns by the phase modulators <NUM> and <NUM>, respectively, and there is a time difference between the two PRBS code patterns. The probe light is adjusted to have a frequency modulation near the Brillouin frequency by the probe light electro-optic modulator <NUM> and is then input to one end of the sensing fiber <NUM>. Meanwhile, the pumping light passes through the optical fiber circulator <NUM> and is input to the other end of the sensing fiber <NUM>. As such, when the pumping light and the probe light meet in the sensing fiber <NUM>, if there is a difference in frequency between the frequencies of the two lights by the Brillouin frequency of the sensing fiber <NUM>, amplification occurs in the light scattered to the rear of the pumping light. The frequency of the Brillouin scattered light which is amplified and travels to the rear of the pumping light is proportional to a strain or a temperature of the sensing fiber <NUM>.

Here, since the pumping light and the probe light are phase-modulated with PRBS codes before being incident on the sensing fiber <NUM>, phases thereof always match at a position by a bit width where two codes first meet in the sensing fiber <NUM> and there is a time during which phases thereof do not match at another position of the PRBS codes. Thus, the Brillouin amplification, in which the back scattered light is increased, occurs only at a bit position in which the phases of the PRBS code patterns of the pumping light and the probe light in the sensing fiber <NUM> match.

For this reason, the photo receiver <NUM> acquiring the Brillouin scattered light acquires a Brillouin scattered light gain signal which is made only at a position by one bit width of the PRBS code. In this way, the probe light modulation frequency under the condition that the Brillouin scattering gain signal is maximally obtained is the Brillouin frequency at the corresponding position of the sensing fiber <NUM>, and thus a strain or a temperature may be obtained by Equation <NUM>. Meanwhile, in order to obtain the Brillouin frequency at another position of the sensing fiber <NUM>, the time difference between the PRBS codes, which are phase modulation patterns of the pumping light and the probe light, may need to be changed.

<FIG> shows an embodiment of driving the optical fiber BOCDA sensor using two phase codes having a time difference of the present invention. In the embodiment of <FIG>, the optical fiber BOCDA sensor using two phase codes having a time difference of the present invention may further include a probe light optical fiber amplifier <NUM>, a polarization scrambler <NUM>, a pumping light electro-optic modulator <NUM>, a pumping light optical fiber amplifier <NUM>, and a lock-in amplifier <NUM> in addition to the components shown in <FIG>. In addition, the controller <NUM> may include a pulse pattern generator <NUM>, an RF signal synthesizer <NUM>, and a chopping module <NUM>. Each part is briefly described as follows.

First, each of the components further included in the optical fiber BOCDA sensor <NUM> will be briefly described.

The probe light optical fiber amplifier <NUM> and the pumping light optical fiber amplifier <NUM> serve to amplify pumping light and probe light entering the sensing fiber <NUM>, respectively. Specifically, the probe light optical fiber amplifier <NUM> is provided in front of the optical fiber isolator <NUM> on the probe light optical fiber line <NUM> and amplifies probe light traveling to the optical fiber isolator <NUM>. In addition, the pumping optical fiber amplifier <NUM> is provided in front of the optical fiber circulator <NUM> on the pumping light optical fiber line <NUM> and amplifies pumping light traveling to the optical fiber circulator <NUM>.

The polarization scrambler (PS) <NUM> is provided between the probe light electro-optic modulator <NUM> and the optical isolator <NUM> on the probe light optical fiber line <NUM> and serves to remove an influence of polarization in the probe light traveling from the probe light electro-optic modulator <NUM>.

The pumping light electro-optic modulator <NUM> and the lock-in amplifier <NUM> should be provided together. First, the lock-in amplifier <NUM> is provided at the rear of the photo receiver <NUM> and serves to amplify the Brillouin scattered light received by the photo receiver <NUM>. In addition, the pumping light electro-optic modulator <NUM> is provided on the pumping optical fiber line <NUM> and modulates pumping light traveling from the pumping light phase modulator <NUM> into a sine wave to drive the lock-in amplifier <NUM>.

Next, each of the pulse pattern generator (PPG) <NUM>, the RF signal synthesizer <NUM>, and the chopping module <NUM> included in the controller <NUM> will be briefly described.

The pulse pattern generator <NUM> is connected to the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> and serves to generate phase code patterns respectively used in the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> and apply the generated phase code patterns. The patterns generated by the pulse pattern generator <NUM> may be freely adjusted by the user.

The RF signal synthesizer <NUM> is connected to the probe light electro-optic modulator <NUM> and serves to generate an electric signal near the Brillouin frequency of the sensing fiber <NUM> to drive the probe light electro-optic modulator <NUM>.

The chopping module <NUM>, which serves to control light at regular time intervals, is connected to the pumping light electro-optic modulator <NUM> and the lock-in amplifier <NUM> as illustrated. As will be described in detail later, the pumping light electro-optic modulator <NUM> and the lock-in amplifier <NUM> may be eliminated according to a control method of the optical fiber BOCDA sensor <NUM> of the present invention. In this case, the chopping module <NUM> may also be eliminated.

A specific operation of the optical fiber BOCDA sensor <NUM> of the present invention configured as described above will be described as follows.

As in the embodiment of <FIG>, light emitted from the light source <NUM> formed of a high-power DFB laser diode is divided by a 3dB optical fiber coupler <NUM> and used as probe light and pumping light. These two lights are phase-modulated by the phase modulators <NUM> and <NUM>, respectively, and in the embodiment of <FIG>, the pulse pattern generator <NUM> controlled by a computer and having two channels is connected to each of the phase modulators <NUM> and <NUM> to adjust PRBS code patterns respectively generated by the phase modulators <NUM> and <NUM>. A phase of one bit of a PRBS is one of <NUM> and π, and a probability of occurrence of two phase bits is almost the same within one code.

<FIG> shows an embodiment of adjusting a position of occurrence of a correlation peak in an optical fiber based on a change in time difference between the phases of pumping light and probe light when the optical fiber BOCDA sensor using two phase codes having a time difference of the present invention is driven. The two PRBS code patterns in the above description are completely the same but have only a time difference, and the time difference between the two patterns may be arbitrarily controlled by a computer. Accordingly, the position where the phases of the phase code-modulated pumping light and probe light match each other, that is, the position of the occurrence of the correlation peak, may be adjusted to a certain position as shown in <FIG>. In other words, the correlation peak region may be obtained by causing a phase of a bit width of one very short phase code to always match at a certain position on the sensing fiber <NUM> using the probe light phase modulator <NUM> and the pumping light phase modulator <NUM>, and here, a Brillouin gain may be obtained. A distance width Δd of the correlation peak may be obtained by a formula Δd = (<NUM>/<NUM>)νgT, where νg is a group speed of light traveling inside the optical fiber and T is a time width of one bit of the PRBS signal. A symbol duration of the one bit is generally set much shorter than <NUM> ns, a minimum time to cause stimulated Brillouin scattering (SBS). That is, it is possible to operate with a spatial resolution shorter than <NUM>.

Meanwhile, in order to find a Brillouin frequency νB, a frequency difference between the pumping light and the probe light must be controlled. In the present invention, the frequency difference is controlled by modulating the frequency of the probe light by the probe light electro-optic modulator <NUM>. Specifically, the probe light is made into an electric signal near the Brillouin frequency using the RF signal synthesizer <NUM>, the electric signal is amplified by an amplifier (an amplifier provided between the RF signal synthesizer <NUM> and the probe light electro-optic modulator <NUM> in <FIG>), and the frequency of the probe light is modulated by driving the probe light electro-optic modulator <NUM>. The probe light modulated into the Brillouin frequency band is amplified by the probe light optical fiber amplifier <NUM>. It is preferable to randomly adjust a polarization direction by a polarization scrambler <NUM> to remove the influence of polarization inside the sensing fiber <NUM> with respect to the probe light, and the probe light completed in several modulations and adjustment passes through the optical fiber isolator <NUM> and is incident on the sensing fiber <NUM>. Meanwhile, the pumping light is on-off modulated by the pumping light electro-optic modulator <NUM> and is used as a signal for triggering the lock-in amplifier <NUM>. Here, the chopping module <NUM> is used to smoothly perform such on-off modulation. Thereafter, the pumping light is amplified by the pumping light optical fiber amplifier <NUM>, passes through the optical fiber circulator <NUM>, and is incident on the sensing fiber <NUM>.

In this way, the pumping light incident and traveling into the sensing fiber <NUM> causes Brillouin amplification at a peak position of correlation with the probe light and then scatters backward and returns. The Brillouin scattered light is converted into an electrical signal, while passing through the optical circulator <NUM> and the photo receiver <NUM>. This signal is amplified by the lock-in amplifier <NUM> and converted into a digital signal by a data acquisition program of the controller <NUM> and stored. The data acquisition and control program operated by the controller <NUM> controls the pulse pattern generator <NUM>, the RF signal synthesizer <NUM>, and the chopping module <NUM> and acquires a signal of the lock-in amplifier <NUM> connected to the photo receiver <NUM>.

An actual experiment was performed using embodiment of driving the optical fiber BOCDA sensor <NUM> of the present invention, and detailed operating conditions in this case are as follows. A frequency of the phase code of the PRBS pattern was set to <NUM> and a spatial resolution was set to <NUM>. In addition, a length of the PRBS pattern was set to <NUM><NUM>-<NUM> = <NUM>, so that a maximum length of the sensing fiber that may be used was <NUM> × <NUM> = <NUM>,<NUM>. Meanwhile, the probe light electro-optic modulator <NUM> for Brillouin frequency detection was set to scan a <NUM> range from <NUM> to <NUM>. In addition, a time of one period of the Brillouin frequency detection was set to <NUM> msec, and data was acquired at a speed of <NUM> at the time of frequency scanning, so that <NUM> Brillouin gain spectra were acquired to obtain an average value and a maximum value to obtain the Brillouin frequency.

<FIG> shows a configuration embodiment of a sensing fiber used in an experiment for driving an optical fiber BOCDA sensor using two phase codes having a time difference of the present invention. In the embodiment of <FIG>, the sensing fiber <NUM> was made by fusion splicing a single mode optical fiber and a dispersion shift fiber (DSF), and the Brillouin frequency for each optical fiber was detected to confirm a possibility of measuring strain. In the case of single-mode optical fiber, four types of optical fibers with different Brillouin frequencies were fusion-spliced together.

<FIG> shows a Brillouin gain spectrum acquired using the sensing fiber of <FIG>. <FIG> shows a Brillouin gain spectrum three-dimensionally, and <FIG> shows a Brillouin gain spectrum two-dimensionally by representing a height value in <FIG> in color. As shown in <FIG>, a Brillouin gain distribution in which a gain peak can be definitely determined at each position was obtained. In particular, in the sensing fiber of <FIG>, the dispersion shift fibers (DSFs) are connected to <NUM> parts that are each <NUM>, and here, it can be seen that the gain spectrum of this part is very clearly divided in the 2D spectrum. Meanwhile, there is a part where the gain does not exist, which is the rest of the phase code where the optical fiber does not exist. That is, a phase code that scans <NUM>,<NUM> is used, and a part from <NUM> to <NUM> is a part without an optical fiber.

<FIG> shows a Brillouin frequency obtained from the Brillouin gain spectrum of <FIG>. That is, a frequency of the part having the maximum gain value is obtained from the Brillouin gain spectrum of <FIG> and is determined as the Brillouin frequency as shown in <FIG>, which is a value corresponding to a strain or a temperature. In the graph of <FIG>, a region where the sensing fiber <NUM> is actually present is a part corresponding to a length of a total <NUM>, starting from <NUM> to <NUM> on the x axis. When comparing from the left of the sensing fiber <NUM> of <FIG>, a single mode optical fiber having a Brillouin frequency of <NUM> by a length of <NUM> is present, and thereafter, a single mode optical fiber having another Brillouin frequency of <NUM> by <NUM> is present, a DSF having a Brillouin frequency of <NUM> is connected by <NUM>, and two more DSFs are further connected. Thereafter, a <NUM> long single-mode optical fiber is connected and the Brillouin frequency is <NUM>, which is slightly smaller than the previous one. Thereafter, an <NUM> long single-mode optical fiber has a Brillouin frequency of <NUM>. That is, it can be seen from the graph of <FIG> that the optical fiber BOCDA sensor <NUM> according to the present invention successfully achieves a spatial resolution of <NUM>, a Brillouin frequency resolution is <NUM>, and a measurement error is <NUM>.

As described above, the essence of the present invention is to form two identical phase code patterns having a time difference in the end and to control a correlation peak position of pumping light and probe light using the time difference. Therefore, the process of making the time difference between the two phase code patterns is important. The easiest method, as shown in <FIG>, is to create one phase code and to shift the number of bits of the code by a desired position for measurement of the sensing fiber. Another method is a method of controlling a correlation position by continuously generating two phase codes of the same pattern with a time difference as shown in <FIG>.

Here, the optical fiber BOCDA sensor <NUM> of the present invention may adjust a time difference of phase code patterns respectively created by the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> so that a correlation peak position corresponds to a position where measurement is to be performed on the sensing fiber <NUM>. Specifically, in the present invention, since a time difference between two phase codes is created in a program and input to the pulse pattern generator <NUM> so as to be used, the correlation peak position may be allocated by the program at any position desired to be measured on the sensing fiber <NUM>. Brillouin sensor systems of the related art mostly measure while scanning the entire position of the sensing fiber, but in the case of using the optical fiber BOCDA sensor <NUM> of the present invention, measurement may be easily controlled to be performed only at a corresponding position of the sensing fiber by simply selecting only the time difference between the two phase codes.

The optical fiber BOCDA sensor <NUM> of the present invention adjusts bit widths of phase code patterns respectively created in the probe light phase modulator <NUM> and the pumping light phase modulator <NUM> according to a length of a section of a position to be measured on the sensing fiber <NUM>. It is also possible to operate by adjusting the bit widths to correspond to a case where the length of the section to be measured is short or long.

<FIG> and <FIG> illustrate an embodiment of adjusting a bit width according to a length of a section in which a correlation peak occurs when there are three measurement sections on the sensing fiber. First, in <FIG>, a case where measurement sections (i.e., sensing parts) #<NUM>, #<NUM>, and #<NUM> are present on the sensing fiber is considered. In this case, if a correlation peak position occurs at a <NUM> bit position, the Brillouin gain spectrum corresponding to one bit width at the measurement section #<NUM> position is obtained, and a strain or a temperature is obtained by Equation <NUM> when a Brillouin frequency giving a maximum value is determined from this spectrum. The length of measurement section #<NUM> is as long as <NUM> bits wide, so if the peak position is made at <NUM>, <NUM>, <NUM> bits, <NUM> strains or temperatures may be obtained at the corresponding position. In addition, since the length of the measurement section #<NUM> has <NUM> bit widths, two strains or temperatures are measured by causing the peak positions to occur at <NUM> and <NUM> bit positions. Meanwhile, as shown in <FIG>, if the bit width of a pseudo-random bit sequence is almost matched to the length of each measurement section of the sensing fiber and a time difference is set and operated so that the correlation peak occurs at the corresponding position, measurement may need to be performed only once at each measurement position.

Meanwhile, when the length of the sensing fiber <NUM> is long, measurement is performed by increasing the bit width of the correlation peak position to increase the Brillouin gain, and if the bit width is increased in this manner, the spatial resolution is lost by that much. Here, the spatial resolution may be improved by acquiring two Brillouin gains having a difference in bit width of the correlation position and acquiring a signal difference. That is, if the Brillouin spectrum is obtained using a slightly larger bit width and a slightly smaller bit width of the correlation peak position and then a difference thereof is calculated, a value corresponding to the improved spatial resolution corresponding to the difference between the bits at the correlation peak position may be obtained. This method has the possibility to perform measurement without using the pumping light electro-optic modulator <NUM> and the lock-in amplifier <NUM>.

In summary, the optical fiber BOCDA sensor <NUM> may perform control to subtract the Brillouin spectrum obtained using a phase code pattern in which a bit width at the correlation peak position has a second size smaller than a predetermined first size from the Brillouin spectrum obtained using a phase code pattern in which a bit width at the correlation peak position has the first size to improve spatial resolution. This will be described with a detailed example as follows.

<FIG> shows an embodiment in which two Brillouin gains having a difference in bit width of a correlation position are obtained to obtain a signal difference. In order to obtain information of a bit width corresponding to a difference by subtracting the Brillouin gain spectrum obtained by operating with a slightly smaller bit width from the Brillouin gain spectrum obtained by operating with a large bit width where a start position of the correlation peak position matches, the correlation peak position is made to have a difference by a half of a total bit width to be measured and subtracted from a large bit width, and only a bit width of the remaining part is measured. In the example of <FIG>, a Brillouin gain spectrum of <NUM>-bit part is obtained with a large bit width, and the correlation peak position is made to have a difference by <NUM>/<NUM> bits and a Brillouin gain spectrum is obtained by operating with a small bit width and subtracted.

When the bit width of the phase code pattern increases, the Brillouin gain increases. However, it cannot be increased indefinitely. The reason for this is that Brillouin gain occurs if a time during which phases are the same is longer than an acoustic wave phonon lifetime even in a portion other than the correlation peak position. Therefore, there is a value that allows the bit width of the phase code pattern to be set to the maximum. This value will eventually be a bit width under the condition that the difference between the Brillouin gain value occurring at the correlation peak position and the Brillouin gain value occurring at other positions cannot be distinguished from each other. However, the Brillouin gain at positions other than the correlation peak position will eventually be all parts of the entire length of the sensing fiber excluding the correlation peak position.

Then, when the length of the sensing fiber is L, if the bit width of the phase code is l, the number of bits N = L/l over the entire length of the sensing fiber. Here, since the correlation peak position is one, the total number of bits of a non-correlation position will be N-<NUM>. Increasing the phase code bit width increases the Brillouin gain at the correlation position and at the same time increases the Brillouin gain at N-<NUM> bits at the non-correlation position. When the bit width is increased in this way, if the Brillouin gain at one bit of the correlation position is I<NUM>, gains at the correlation positions are indistinguishable if the total sum of the Brillouin gains at the non-correlation position N-<NUM> bits is equal to I<NUM>. Therefore, the phase code bit width cannot be increased any more. That is, the bit width here will be a maximum bit width of the sensing fiber in use. If the Brillouin gain at one bit of the non-correlation position is In, In x (N-<NUM>) = I<NUM>, and thus the bit width in this case becomes the maximum value.

Meanwhile, a minimum value of the bit width of the phase code pattern may be determined as follows. In the case of reducing the phase code pattern, if a phase angle of the phase code pattern is not completely controlled up to <NUM> degrees, some Brillouin gain Ic0 occurs in all phase code patterns. Therefore, the Brillouin gain I<NUM> occurring at the correlation peak position of the phase code pattern should be greater than Ic0 occurring in the entire length of the sensing fiber. That is, when the condition of I<NUM> = Ic0 is made, the bit width of the phase code pattern may be minimized. However, if the phase modulation is properly performed and the Brillouin gain at the non-correlation position is smaller than basic signal noise, a minimum bit width at this time becomes a bit width when the Brillouin gain at the correlation position is equal to the signal noise.

Claim 1:
An optical fiber BOCDA sensor (<NUM>) for measuring a strain and a temperature at a certain position on a sensing fiber (<NUM>) using Brillouin frequency shift, comprising:
a probe light phase modulator (<NUM>) and a pumping light phase modulator (<NUM>), which are adapted to be independently controlled, on a probe light optical fiber line (<NUM>) and a pumping light optical fiber line (<NUM>), respectively, so that a time difference is formed in a phase code pattern created in each of the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>), thereby adjusting a correlation peak position of pumping light and probe light on the sensing fiber (<NUM>), wherein
the optical fiber BOCDA sensor (<NUM>) is configured to adjust the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) to have the same form but have only a time difference, wherein
the optical fiber BOCDA sensor (<NUM>) is adapted to adjust a time difference of the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) so that the correlation peak position corresponds to a position of the sensing fiber (<NUM>) on which measurement is to be performed, wherein
the optical fiber BOCDA sensor (<NUM>) is adapted to adjust bit widths of the phase code patterns respectively created in the probe light phase modulator (<NUM>) and the pumping light phase modulator (<NUM>) according to a length of a section of a position on the sensing fiber (<NUM>) on which measurement is to be performed, wherein
the optical fiber BOCDA sensor (<NUM>) is adapted to perform control to subtract a Brillouin spectrum obtained using a phase code pattern in which a bit width of a correlation peak position has a second size smaller than a predetermined first size from a Brillouin spectrum obtained using a phase code pattern in which a bit width of a correlation peak position has the first size, and
wherein the maximum value of the bit width is the value of the bit width when the total sum of the Brillouin gains at the non-correlation position N-<NUM> bits is equal to the Brillouin gain at one bit of the correlation position I<NUM> and
wherein the minimum value of the bit width is the value of the bit width when the Brillouin gain I<NUM> occurring at the correlation position is equal to the Brillouin gain Ic0 occurring in the entire length of the sensing fiber.