KR20060102801A - Apparatus and method for compensation of the nonlinearity of an ofdr system - Google Patents

Abstract

The present invention relates to an optical frequency domain reflectometry (OFDR) system. An optical frequency domain reflectometry system according to the present invention includes a light source for outputting light that changes wavelength non-linearly with time, a light splitter for dividing light output from the light source, and a partial light split in the light splitter. A week interferometer for obtaining a main vein signal due to a path difference, an auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some other light divided by the optical splitter, a phase shifter for converting a phase of the auxiliary vein signal, and the auxiliary The nonlinear characteristic of the main vein signal is calculated using a phase calculator for calculating a phase value of the auxiliary vein signal from the pulse signal and the phase-converted auxiliary vein signal, and the phase value of the auxiliary vein signal calculated by the phase calculator. A compensator for compensating and a high speed Fourier transform of a vein signal whose nonlinear characteristics are compensated by the compensator, It includes FFT transformer section to find location.

OFDR, Hilbert transform, nonlinear, frequency shift, variable wavelength laser, Taylor theorem

Description

Apparatus and Method for Compensation of the Nonlinearity of an OFDR system}

1 is a diagram illustrating an apparatus for compensating for nonlinear frequency displacement of OFDR by a triggering method using a conventional auxiliary interferometer,

FIG. 2 is a diagram illustrating a system for reflecting optical frequency domain including a nonlinear frequency displacement compensation device according to the present invention; FIG.

3 is a signal waveform diagram of an auxiliary vein signal;

4 is a signal waveform diagram of the Hilbert transformed sub-macro signal;

5 is a waveform diagram of a phase of an auxiliary vein signal;

6 is a waveform diagram of nonlinear frequency displacement of a tunable laser;

7 is a signal waveform diagram of the jug signal output from the weekly interferometer,

8 is a signal waveform diagram of a main vein signal from which the nonlinear characteristics are removed;

9 is a diagram showing a fast Fourier transform result of a vein signal;

10 is a diagram showing the phase of the pulse signal and the frequency shift of the variable wavelength laser of the auxiliary interferometer obtained using a commercially available variable wavelength laser,

11 is a diagram showing an FFT conversion result (a) not compensating for a nonlinear frequency displacement of a variable wavelength laser and a FFT conversion result (b) after compensating for a nonlinear frequency displacement of a variable wavelength laser according to the present invention;

12 is a graph showing the phase and frequency shift of the auxiliary pulse signal obtained when using a distributed feedback laser diode (DFB-LD) as a light source,

13 shows the result of FFT conversion (a) that does not compensate for nonlinear frequency displacement of DFB-LD when using DFB-LD as a light source, and the result of FFT conversion after compensation for nonlinear frequency displacement of DFB-LD according to the present invention. It is a figure which shows the value b.

  <Brief description of symbols for the main parts of the drawings>

21; Variable wavelength laser (TLS) 22; Splitter

23; Weekly 24; Auxiliary Interferometer

25; Hilbert Converter 26; Phase calculator

27; Compensator 28; FFT Converter

The present invention relates to an optical frequency domain reflectometry (OFDR) system, and more particularly, to an apparatus and method for compensating for nonlinear frequency displacement of an optical frequency domain reflectometry system using Hilbert transformation. It is about.

Optical Frequency Domain Reflectometry (OFDR) systems measure the length, fault position, chromatic dispersion, polarization mode dispersion, and loss of a fiber using reflected light. It is a system. OFDR system is applied to optical communication and optical sensor field because of better resolution and wider dynamic range than optical time domain reflectometry (OTDR) system. Recently, optical low coherence reflectometry (OLCR) is used. New ways to replace optical coherence tomography (OCT) are being explored.

However, because OFDR scans the wavelength of laser with higher coherence than OLCR, the characteristics of the laser should be good. In case of non-uniform scanning, undesired waveforms may appear and degrade the accuracy. In general, the frequency of light output from a laser, which is a light source, varies linearly and not linearly with time, which is called nonlinear frequency displacement of OFDR. This nonlinear frequency shift lowers the resolution of the OFDR.

Therefore, in order to improve the resolution of the OFDR, the nonlinear frequency displacement of the OFDR should be compensated. As a method of compensating the nonlinear frequency displacement of the OFDR, a trigger method using an auxiliary interferometer has been conventionally used.

1 is a diagram illustrating an apparatus for compensating for nonlinear frequency displacement of an OFDR by a trigger method using a conventional auxiliary interferometer.

The conventional apparatus includes a tunable laser source (TLS) 11 for outputting light whose wavelength varies with time, an optical splitter 12 for dividing the light output from the tunable laser 11, The light from the light splitter 12 divides the light of one of the divided into two and interfering with each other to obtain a jug signal due to the path difference, and the light of the other of the light split from the light splitter 12 Compensating the nonlinear characteristics of the main vein signal by using an auxiliary interferometer 14 to obtain an auxiliary vein signal due to a path difference by interfering with each other, an electronic circuit 15 generating a trigger signal from the auxiliary vein signal, and a trigger signal. Compensator 16, and the FFT converter 17 for Fourier transform the compensated vein signal.

The light splitter 12 divides the light output from the variable wavelength laser 11 into 70:30, and outputs 70% to the interferometer 13 and 30% to the auxiliary interferometer 14. In the drawing, D1 and D2 are photo detectors for detecting the light returned from the interferometer and the subinterferometer, and the fiber under test (FUT) of the interferometer 13 is a test optical fiber for measuring with OFDR equipment, and Ref Is a reference arm and is necessary to cause interference. A polarization controller (PC) is mounted to improve visibility.

The light output from the variable wavelength laser 11 is split in the optical splitter 12 and input to the interferometer 13 and the auxiliary interferometer 14, respectively. The main vein signal (B) and the auxiliary vein signal (A) by the interference are obtained from the interferometer (13) and the auxiliary interferometer (14), and the photodetectors (D1, D2) are the main vein signal (B) and the auxiliary vein signal ( A) is detected. The electronic device 15 generates a trigger signal from the auxiliary vein signal A detected by the photodetector D2. This trigger signal is a signal whose period is varied by nonlinear wavelength displacement of light output from the variable wavelength laser 11.

The compensator 16 acquires the vein signal B detected by the photodetector D1 at the edge of the trigger signal, and arranges the obtained vein signal B at the same time interval. This makes it possible to compensate for the nonlinear characteristics included in the vein signal. The FFT converter 17 obtains a frequency component of the vein signal by performing fast Fourier transform on the vein signal whose nonlinear characteristic is compensated by the compensator 16.

Thus, the conventional device loses many components of the vein signal because it acquires the vein signal only at the edge of the trigger signal. To prevent this, the period of the trigger signal should be short. To do this, the difference between the two arms of the auxiliary interferometer should be large. However, if the length of the auxiliary interferometer becomes longer, phase noise caused by the variable wavelength laser is generated, and thus, the clean auxiliary vein signal cannot be obtained, which also adversely affects the trigger signal, and eventually the noise in the process of compensating the vein signal. This can cause unwanted results. For this reason, it is necessary to limit the path difference length of the auxiliary interferometer.

The problems of the triggering method using the auxiliary interferometer that compensates the nonlinear characteristics of the conventional OFDR are as follows.

First, in order to increase the frequency of the trigger signal, the path difference of the auxiliary interferometer should be large. However, if the path difference length is longer than the interference length, the signal will be distorted by phase noise, which causes instability of the whole system. The path difference length of the auxiliary interferometer shall not be longer than the interference length of the variable wavelength laser which is the light source. That is, because the path difference length of the auxiliary interferometer is limited, the frequency of the auxiliary vein signal and the trigger signal is limited, and thus there is a problem that a sufficiently clean auxiliary vein signal cannot be obtained.

Second, there is a need for an electronic device that generates a trigger signal by converting an auxiliary pulse signal into an electrical pulse. This electronic device has a problem that the configuration of the entire system is complicated.

Disclosure of the Invention The object of the present invention is to provide an apparatus and method for compensating for nonlinear frequency displacement of a vein signal by obtaining phase information of an auxiliary vein signal without using a trigger signal.

In order to achieve the above object, a nonlinear frequency displacement compensating apparatus of an optical frequency domain reflection measuring system according to the present invention includes a light source for outputting light whose wavelength changes nonlinearly with time, and a light source that is divided into light output from the light source. In an optical frequency domain reflectometry system having a daytime interferometer for obtaining a jug signal due to a path difference from some light, and an auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some other light divided from the light output from the light source. In the device for compensating for the nonlinear frequency shift included in the vein signal,

A phase converter for converting a phase of the auxiliary vein signal;

A phase calculator for calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal;

And a compensator for compensating for the nonlinear characteristics of the main vein signal using the phase value of the auxiliary vein signal calculated by the phase calculator.

In addition, the nonlinear frequency displacement compensation method of the optical frequency domain reflection measurement system according to the present invention is a path difference from a light source for outputting light whose wavelength changes non-linearly with time, and a partial light divided from the light output from the light source In the optical frequency domain reflectometry system having a daytime interferometer for obtaining a jug signal by means of, and an auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some other light split from the light output from the light source, In a method for compensating a nonlinear frequency shift,

A phase shifting step of converting a phase of the auxiliary vein signal;

Calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal;

And a compensation step of compensating for the nonlinear characteristics of the main vein signal using the phase value of the auxiliary vein signal calculated in the phase calculation step.

In addition, the optical frequency region reflection measurement system according to the present invention,

A light source for outputting light whose wavelength varies nonlinearly with time;

A light splitter for dividing light output from the light source;

A weekly interferometer for obtaining a jug signal due to a path difference from the partial light divided by the optical splitter;

An auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some other light split by the optical splitter;

A phase converter for converting a phase of the auxiliary vein signal;

A phase calculator for calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal;

A compensator for compensating for the nonlinear characteristics of the main vein signal using the phase value of the auxiliary vein signal calculated by the phase calculator;

It characterized in that the compensator includes a FFT converter to find the position of the cut plane of the optical fiber mounted on the interferometer by the fast Fourier transform the non-linear characteristics compensated in the vein signal.

Hereinafter, a nonlinear frequency displacement compensation device and method of an optical frequency domain reflection measurement system according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Variable frequency lasers used in OFDR systems output light whose frequency changes over time. In general, the variable frequency laser changes the frequency nonlinearly by various internal and external factors. The frequency variation of the variable frequency laser is expressed as v (t). This change in frequency over time affects the resolution of the OFDR system. Therefore, it is necessary to compensate for the nonlinear frequency displacement of this variable frequency laser.

The present invention discloses a method of obtaining phase information of a light source using an auxiliary vein signal obtained from an auxiliary interferometer and compensating for nonlinear frequency displacement of a variable frequency laser.

2 is a view showing a system for measuring the optical frequency domain reflection including a nonlinear frequency displacement compensation device according to the present invention.

The OFDR system of the present invention includes a nonlinear frequency displacement compensation device in a general OFDR system. The OFDR system of the present invention includes a tunable laser source (TLS) 21 for outputting light whose wavelength varies with time, an optical splitter 22 for dividing the light output from the tunable laser 21, and , The dichroism 23 which obtains a vein signal B due to a path difference by dividing one of the lights divided by the light splitter 22 into two and interfering with each other, and the other of the lights split by the light splitter 22. An auxiliary interferometer 24 for dividing one light into two and interfering with each other to obtain an auxiliary vein signal A due to a path difference, a Hilbert converter 25 for converting the auxiliary vein signal A into Hilbert and an auxiliary A phase calculator 26 for calculating the phase of the auxiliary pulse signal A from the pulse signal A and the Hilbert-converted signal, and the main signal B from the phase values calculated by the main signal B and the phase calculator 26. A compensator 27 for compensating for the nonlinear characteristics of the &lt; RTI ID = 0.0 &gt;), &lt; / RTI &gt; Include.

Here, the Hilbert transformer 25, the phase calculator 26, and the compensator 27 constitute a nonlinear frequency displacement compensation device according to the present invention. The Hilbert transformer 25 may be transformed into another phase shifter that converts the phase of the auxiliary pulse signal.

로 표현된다. 여기서, φ(t)는 시간의 함수인 위상신호이고, 이 위상신호를 미분하면 빛의 주파수변위

가 얻어진다.The electromagnetic field of light oscillating from the variable wavelength laser (TLS) 21 is generally

It is expressed as Here, φ (t) is a phase signal as a function of time, and when the phase signal is differentiated, frequency shift of light

Is obtained.

The light splitter 22 divides the light generated by the variable wavelength laser (TLS) 21 into 70:30, outputs 70% to the interferometer 23 and outputs 30% to the auxiliary interferometer 24. In the drawing, D1 and D2 are photo detectors for detecting the light returned from the interferometer 23 and the auxiliary interferometer 24, and the FUT (fiber under test) of the interferometer 23 is measured by OFDR equipment. Ref. Is a reference arm, which is necessary for causing interference. A polarization controller (PC) is mounted to improve visibility.

The light output from the variable wavelength laser 21 is divided by the optical splitter 22 and input to the interferometer 23 and the auxiliary interferometer 24, respectively. The main vein signal (B) and the auxiliary vein signal (A) due to interference are obtained from the interferometer 23 and the auxiliary interferometer 24, respectively, and the two photodetectors D1 and D2 are supplemented with the vein signal B and the auxiliary vein signal B. The pulse signal A is detected.

The auxiliary vein signal A obtained from the auxiliary interferometer 24 is expressed by Equation 1 below.

Where U ₀ and ξ ₀ are the voltage magnitude and phase constant of the auxiliary vein signal, respectively. Τ is a group delay caused by the path difference of the auxiliary interferometer. The signal waveform of this auxiliary vein signal is shown in FIG.

In the above Equation 1, if the delay time τ is short, φ (t) -φ (t-τ) = φ (t)-(φ (t) -τ (dφ / dt)) by Taylor's theorem. = τ (dφ / dt) = 2πv (t) τ. That is, the frequency shift v (t) of the variable wavelength laser can be known from the difference value of the phase of the auxiliary pulse signal. At this time, since the length of the auxiliary interferometer is constant and it is a known value, the delay time τ can be known in advance. Therefore, if the phase of the auxiliary vein signal is known, the frequency displacement of the variable wavelength laser used in the OFDR system by Taylor's theorem is explained. (t) can be found. Also, knowing the frequency shift v (t) of the variable wavelength laser can compensate for the nonlinearity of the vein signal.

In order to obtain the phase of the auxiliary vein signal, first, the auxiliary vein signal is Hilbert transformed by the Hilbert converter 25. The Hilbert transformer 25 changes the auxiliary pulse signal by 1 / 2π only without changing its magnitude. The Hilbert transformed auxiliary vein signal is shown in FIG. 4. In FIG. 4, the solid line is the original auxiliary vein signal and the dotted line is the Hilbert transformed auxiliary vein signal. The phase calculator 26 obtains the phase of the auxiliary vein signal by using the original auxiliary vein signal U _AU (t) and the Hilbert transformed auxiliary vein signal H (U _AU (t)). This is expressed as equation (2).

The phase of the auxiliary vein signal thus obtained is shown in FIG. 5.

Looking at the phase of the auxiliary vein signal, there is a phase difference of 2π at a specific time (T1, T2, T3, ...), and at this time, the phase is added retroactively by 2π to obtain the total phase shift. The nonlinear frequency shift v (t) of the tunable laser is obtained by applying Equation 3 with the obtained total phase shift.

Where c is the speed of light in a vacuum, n is the refractive index of the medium through which light passes, and ΔL is the path difference of the secondary interferometer. The nonlinear frequency shift of the tunable laser thus obtained is shown in FIG.

The compensator 27 compensates for the nonlinear characteristics included in the vein signal B by rearranging the vein signal B using the nonlinear frequency shift of the variable wavelength laser. 7 is a signal waveform diagram of the jug signal output from the weekly interferometer. This vein signal is a signal waveform obtained when two cutting planes exist in a test optical fiber.

The compensator 27 transforms the time axis of the vein signal into the frequency axis and removes the nonlinear characteristics included in the vein signal by using the nonlinear frequency displacement of the variable wavelength laser. 8 is a signal waveform diagram of a main vein signal from which a nonlinear characteristic is removed.

The FFT converter 28 converts the vein signal from which the nonlinear characteristic has been removed to a fast Fourier transform. The fast Fourier transform result of the vein signal is shown in FIG.

9, it can be seen that the FFT result of the vein signal changes abruptly at two delay times (τ). From these two delay times, it can be seen that there are two cut planes of the test optical fiber (FUT). The cut plane position can also be calculated from the times.

Fig. 10 shows the phase of the pulse signal of the auxiliary interferometer and the frequency shift of the variable wavelength laser generally obtained using a commercially available variable wavelength laser. As can be seen in FIG. 10, the phase of the auxiliary vein signal and the frequency of the variable wavelength laser vary nonlinearly with time, and the result of the derivative is represented by a small graph of FIG. 10. In other words, the frequency variable speed changes with time, which affects the resolution of the OFDR.

FIG. 11 is a diagram showing an FFT conversion result (a) not compensating for nonlinear frequency displacement of a variable wavelength laser and an FFT conversion result (b) after compensating for a nonlinear frequency displacement of a variable wavelength laser according to the present invention. The delay time is 20ns when the cutting plane position of the optical fiber is 4m and the delay time is 30ns when the cutting plane position of the optical fiber is 6m.

Fig. 11A shows that the FFT result of the main signal changes in a wide range of delay time? From 22ns to 32ns, and the changed range is about 10ns wide. In other words, the position of the cut surface of the optical fiber is located between about 4m to 6m. On the other hand, (b) of FIG. 11 shows that the FFT result of the vein signal changes abruptly when the delay time τ is about 25 ns. If this is interpreted, the position of the cut plane of the optical fiber is about 5 m. That is, it can be seen that the resolution of the OFDR system according to the present invention is much better than that of the conventional OFDR system.

FIG. 12 is a graph illustrating phase and frequency displacement of an auxiliary vein signal when a distributed feedback laser diode (DFB-LD) is used instead of a variable wavelength laser (TLS) as a light source. Unlike commercially available variable-wavelength lasers, the middle section (Region II) is linear, but both ends (Region I and Region III) are nonlinear.

13 shows the result of FFT conversion (a) that does not compensate for nonlinear frequency displacement of DFB-LD when using DFB-LD as a light source, and the result of FFT conversion after compensation for nonlinear frequency displacement of DFB-LD according to the present invention. It is a figure which shows the value b. Since the frequency shift of the DFB-LD is nonlinear at both ends as shown in FIG. 12, the conventional OFDR system generates unexpected noise as shown in FIG. 13A. However, when the nonlinearity is compensated for through the Hilbert transform as in the OFDR system of the present invention, the noise is removed as shown in FIG.

Although the technical spirit of the present invention has been described above with the accompanying drawings, it is intended to exemplarily describe the best embodiment of the present invention, but not to limit the present invention. In addition, it is obvious that any person skilled in the art may make various modifications and imitations without departing from the scope of the technical idea of the present invention.

As described above, according to the present invention, the frequency of the variable wavelength laser is obtained by obtaining the phase of the auxiliary vein signal irrespective of the path difference of the auxiliary interferometer, and the rearrangement of the vein signal allows the compensation of the nonlinear characteristics included in the vein signal. There is. In addition, since all the procedures for obtaining the phase of the auxiliary vein signal and compensating for the nonlinear characteristics of the vein signal are performed through a series of computer processes, the entire system is simplified.

Claims (16)

A light source that outputs light whose wavelength varies non-linearly with time, a daytime interferometer that obtains a vein signal due to a path difference from some light divided from the light output from the light source, and another light divided by the light output from the light source In an optical frequency domain reflectometry system having an auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some light, the apparatus for compensating nonlinear frequency displacement included in the vein signal A phase converter for converting a phase of the auxiliary vein signal; A phase calculator for calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal; And a compensator for compensating for the nonlinear characteristics of the main vein signal by using the phase value of the auxiliary vein signal calculated by the phase calculator. The nonlinear frequency displacement compensation device of claim 1, wherein the phase shifter is a Hilbert transform for converting the auxiliary vein signal into a Hilbert transform. 3. The nonlinear frequency displacement compensation device of claim 2, wherein the phase calculator calculates a phase value of the auxiliary vein signal using the following equation. [Equation]

Where U _AU (t) is the auxiliary vein signal, H (U _AU (t)) is the Hilbert transformed auxiliary vein signal, φ (t) is the phase value of the auxiliary vein signal. The method of claim 1, wherein the compensator obtains the nonlinear frequency shift of the light source from the phase value of the auxiliary vein signal, and applies the nonlinear frequency shift of the light source to the vein signal to compensate for the nonlinear characteristics of the vein signal. Nonlinear frequency displacement compensation device for optical frequency domain reflection measurement system. The nonlinear frequency displacement compensation device of claim 1, wherein the light source is a tunable laser source (TLS). The nonlinear frequency displacement compensation device of claim 1, wherein the light source is a distributed feedback laser diode (DFB-LD). A light source that outputs light whose wavelength varies non-linearly with time, a daytime interferometer that obtains a vein signal due to a path difference from some light divided from the light output from the light source, and another light divided by the light output from the light source In the optical frequency domain reflection measurement system having an auxiliary interferometer which obtains an auxiliary vein signal due to a path difference from some light, the method for compensating nonlinear frequency displacement included in the vein signal A phase shifting step of converting a phase of the auxiliary vein signal; Calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal; And a compensation step of compensating for the nonlinear characteristics of the main vein signal using the phase value of the auxiliary vein signal calculated in the phase calculation step. The nonlinear frequency displacement compensation method of claim 7, wherein the phase shifting step is a step of Hilbert transforming the auxiliary vein signal. The nonlinear frequency displacement compensation method of claim 8, wherein the phase calculating step calculates a phase value of the auxiliary vein signal using the following equation. [Equation]

Where U _AU (t) is the auxiliary vein signal, H (U _AU (t)) is the Hilbert transformed auxiliary vein signal, φ (t) is the phase value of the auxiliary vein signal. 8. The method of claim 7, wherein the compensating step obtains a nonlinear frequency shift of the light source from the phase value of the auxiliary vein signal and compensates the nonlinear characteristics of the vein signal by applying the nonlinear frequency shift of the light source to the vein signal. Nonlinear Frequency Displacement Compensation Method of Optical Frequency Domain Reflective Measurement System. A light source for outputting light whose wavelength varies nonlinearly with time; A light splitter for dividing light output from the light source; A weekly interferometer for obtaining a jug signal due to a path difference from the partial light divided by the optical splitter; An auxiliary interferometer for obtaining an auxiliary vein signal due to a path difference from some other light split by the optical splitter; A phase converter for converting a phase of the auxiliary vein signal; A phase calculator for calculating a phase value of the auxiliary vein signal from the auxiliary vein signal and the phase-converted auxiliary vein signal; A compensator for compensating for the nonlinear characteristics of the main vein signal using the phase value of the auxiliary vein signal calculated by the phase calculator; And a FFT transducer for finding the position of the cut surface of the optical fiber mounted on the interferometer by fast Fourier transforming the wort signal compensated for nonlinearity in the compensator. The optical frequency domain reflectometry of claim 11, wherein the phase shifter is a Hilbert transform for converting the auxiliary pulse signal to Hilbert. The optical frequency domain reflection measurement system of claim 12, wherein the phase calculator calculates a phase value of the auxiliary vein signal using the following equation. [Equation]

Where U _AU (t) is the auxiliary vein signal, H (U _AU (t)) is the Hilbert transformed auxiliary vein signal, φ (t) is the phase value of the auxiliary vein signal. 12. The method of claim 11, wherein the compensator obtains the nonlinear frequency shift of the light source from the phase value of the auxiliary vein signal, and applies the nonlinear frequency shift of the light source to the vein signal to compensate for the nonlinear characteristics of the vein signal. Optical wave reflectance measurement system. 12. The system of claim 11, wherein the light source is a tunable laser source (TLS). 12. The system of claim 11, wherein the light source is a distributed feedback laser diode (DFB-LD).

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