CN103337776A - All-optical fiber type self-mixing distance measuring system of laser - Google Patents

Abstract

The invention discloses an all-fiber laser self-mixing ranging system, which is characterized in that a tunable laser emits light to a measured object through the output end of the light collection and coupling system. The initial wavelength is λ, the tuning frequency is ν _m , and the tuning amplitude is The laser signal is Δλ, and receives the feedback optical signal from the scattering surface of the measured object through the output end of the light collection and coupling system to form a laser self-mixing signal; the photoelectric signal conversion unit receives the laser self-mixing signal and converts it into an electrical signal; the signal The processing unit uses the fluctuation frequency Δν _L of the laser self-mixing signal to obtain the distance between the scattering surface of the measured object and the output end surface of the tunable fiber laser. The invention has high measurement precision, stable working performance and wide application occasions, especially long-distance measurement.

Description

An all-fiber laser self-mixing ranging system

technical field

The invention relates to an optical detection system for the absolute distance of a target object, in particular to a self-mixing laser ranging system.

Background technique

The optical detection of the absolute distance of the target can be widely used in mining, electric power, water conservancy, communication, environment, construction, agriculture, forestry and many other fields. In the prior art, the time-of-flight ranging method based on pulsed laser has relatively low precision and low sensitivity; the heterodyne phase laser rangefinder system includes a reference arm and a measuring arm, and the structure is relatively complicated; tuning based on injection current The semiconductor laser self-mixing ranging technology has the advantages of compact structure, light weight, small size, high reliability, suitable for on-site measurement, etc., and has gradually been widely used.

The principle of laser self-mixing ranging is shown in Figure 4. The interference system consists of a tunable laser and an external reflective object. When the feedback light exists, the frequency and intensity of the laser itself can be modulated by changing the carrier density in the laser cavity to cause a change in the refractive index of the laser medium, forming self-mixing interference.

Let the length between the front end face 17 of the laser and the rear end face 18 of the laser be _L0 , the reflection coefficients of the front end face and the rear end face be _r1 and _r2 respectively, and the length of the external cavity between the front end face and the scattering end face 19 of the target object be L _ext , the reflection coefficient of the external cavity is r ₃ , the refractive index of the laser medium is n, the initial light field is E ₀ , and the light field after self-mixing interference is E(t), then:

$\mathrm{E.}\left(t\right)=r_1{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="HDA00003333907900011.png"\; state="\{\{state\}\}">r</figure-callout>}}_2\exp \{-\mathrm{<figure-callout\; id="4"\; label="j"\; filenames="HDA00003333907900011.png"\; state="\{\{state\}\}">j</figure-callout>}4\mathrm{\&pi;v}\frac{{\mathrm{nL}}_0}{c}+(g-\mathrm{\&gamma;})L_0\}{\mathrm{E.}}_0+$ (2)

${\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="r"\; filenames="HDA00003333907900011.png"\; state="\{\{state\}\}">r</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; \{</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; <figure-callout\; id="4"\; label="j"\; filenames="HDA00003333907900011.png"\; state="\{\{state\}\}">j</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;v</span>}\frac{{\mathrm{<span\; class="notranslate">\; nL</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; ext</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; g</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&gamma;</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \}</span>{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}$

In formula (2), g is the linear gain caused by unit length in the laser cavity, γ is the loss per unit length in the laser cavity, and ν is the oscillation frequency of the laser. Since the threshold gain of the laser is modulated by the feedback light, the laser output power is proportional to the laser threshold gain. Therefore, the output power of the laser is modulated by the feedback light, and the output optical power is expressed as formula (3):

I=I ₀ [1+mcos(2πντ _L )] (3)

为激光在内外腔传播一周的延迟时间，I₀为激光器没有外腔反馈时的激光强度。In formula (3), the modulation coefficient m is a constant proportional to the feedback strength when the working current is constant.

I ₀ is the laser intensity when the laser has no feedback from the external cavity.

In formula (3), the output intensity of the laser is related to the change of the length of the external cavity of the laser and the displacement of the feedback object. It is assumed that the optical frequency fluctuation of the laser caused by the added modulation is:

I(t)=I ₀ +ΔI(t) (4)

ν(t)=ν ₀ +Δν(t) (5)

In formula (4) and formula (5), I ₀ , ν ₀ represent the light intensity and light frequency of the laser without feedback and without modulation, and I(t), ν(t) represent the output laser light intensity of the laser under modulation respectively And light wave frequency, laser intensity and light wave frequency can be measured by optical power meter and spectrum analyzer. Δν(t) is the amount of frequency change with time under the action of the tuning unit, and its variation range is defined as Δν. Among them, ΔI(t) and Δν(t) are determined by the modulation characteristics of the laser.

I(t)=[I ₀ +ΔI(t)]{1+mcos[2πν(t)τ _L ]} (6)

Therefore, the distance between the scattering surface of the target object and the exit end surface of the laser is shown in formula (1):

It can be seen from formula (1) that the distance between the scattering surface of the target and the laser output end face is linearly related to the fluctuation frequency Δν _L of the self-mixing signal, and the measurement accuracy of the laser is mainly affected by the tuning range Δλ.

This kind of tuning the output wavelength of the semiconductor laser by injecting current to achieve the purpose of distance measurement mainly has the following problems:

1. Since the amplitude of the injection current of the semiconductor laser is limited by the working range of the injection current of the semiconductor, the tuning range of its output wavelength is limited. The tuning range of a typical current injection type F-P semiconductor laser is generally around 10nm. According to the principle of ranging, the tuning range Δλ is small, resulting in a decrease in measurement accuracy, and it is impossible to achieve high-precision ranging;

2. When the current injection type semiconductor laser is tuned, the working performance of the laser is unstable, and the output power fluctuates seriously, resulting in unstable output self-mixing signal and affecting the ranging effect;

3. The typical line width range of semiconductor lasers is 2nm, and they generally work in multiple longitudinal modes, resulting in poor monochromaticity and coherence of the laser beam, which affects its transmission and sensing characteristics, and the divergence angle is large when working at a long distance, which directly limits Applications and working distances of laser self-mixing ranging technology.

Contents of the invention

The purpose of the present invention is to avoid the shortcomings of the above-mentioned prior art and provide an all-fiber laser self-mixing distance measuring system with higher measurement accuracy, more stable working performance and wide application occasions, especially long-distance measurement.

The present invention adopts following technical scheme for solving technical problems:

The feature of the all-fiber laser self-mixing ranging system of the present invention is that a tunable fiber laser is used, and a photoelectric signal conversion unit with a photodetector, a signal processing unit and a signal processing unit located in the tunable fiber laser are arranged in cooperation with the tunable fiber laser. The light collection and coupling system at the front end of the tunable fiber laser; the tunable laser emits a laser signal with an initial wavelength of λ, a tuning frequency of ν _m and a tuning amplitude of Δλ to the measured object through the output end of the light collection and coupling system, And through the output end of the light collection and coupling system, the feedback optical signal from the scattering surface of the measured object is received to form a laser self-mixing signal; the photoelectric signal conversion unit uses a photodetector to receive the laser light formed in the tunable laser The self-mixing signal is converted into an electrical signal; the signal processing unit receives and processes the electrical signal from the photoelectric signal conversion unit, and uses the fluctuation frequency Δν _L of the laser self-mixing signal to obtain the scattering of the measured object by formula (1) The distance L _ext between the face and the output end face of the tunable fiber laser:

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; ext</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; .</span>$

The structural features of the all-fiber laser self-mixing ranging system of the present invention also lie in:

The structural form of setting the tunable fiber laser is: a resonant cavity with a linear cavity structure, and the resonant cavity is composed of a narrow-band reflector at the front end of the tunable fiber laser and a broadband reflector at the rear end. Composed of mirrors; the pump light output by the pump unit is connected to the resonant cavity by a wavelength division multiplexing device, and the active gain medium in the resonant cavity is excited by the pump light to generate laser light in the resonant cavity; The output end of the wavelength division multiplexing device is connected to the input end of the coupler; the first output end of the coupler is connected to the optical collection and coupling system, and the second output end of the coupler is connected to the photoelectric signal conversion A unit; the tuning unit is set as a tuning function device, and the tuning function of the tuning unit enables the tunable fiber laser to output a laser signal with a tuning frequency of ν _m and a tuning amplitude of Δλ from the first output end of the coupler.

The broadband reflector is set as a fiber loop reflector or a dielectric film fiber reflector, and the narrowband reflector is set as a fiber grating with a frequency selection function.

The structural characteristics of the all-fiber laser self-mixing ranging system of the present invention are also:

The structural form of the tunable fiber laser is set as follows: a resonant cavity with a ring cavity structure is used, and the pump light output by the pump unit is connected to the resonant cavity with a ring cavity structure by a wavelength division multiplexing device. The pump light excites the active gain medium in the resonant cavity, and generates laser light in the resonant cavity through a narrow-band mirror; the input end of the coupler is connected with the resonant cavity of the ring cavity structure, and the first output of the coupler The end is connected to the narrow-band reflector, the tuning unit is connected to the optical collection and coupling system, and the second output end of the coupler is connected to the photoelectric signal conversion unit; the tuning unit is set as a tuning function device, and the tuning function of the tuning unit makes all The tunable fiber laser outputs a laser signal with a tuning frequency of ν _m and a tuning amplitude of Δλ from the output end of the narrow-band mirror.

The narrow-band reflector is set as a fiber grating with a frequency selection function, and an isolator is added between the active fiber and the coupler, and the feedback light is isolated by the isolator to control the unidirectional transmission of the laser in the resonant cavity .

The light collection and coupling system is configured as a lens or a lens group, which is used to adjust the outgoing light intensity of the tunable fiber laser and the light intensity of the feedback optical signal from the measured object.

Compared with the prior art, the beneficial effects of the present invention are reflected in:

1. The moving range of the tuning wavelength of the tunable fiber laser used in the present invention can reach 100nm or even higher, which has a larger tuning range than the semiconductor laser tuned by injection current. Therefore, the accuracy of the system of the present invention is relatively higher.

2. The broadband tuning characteristics of the fiber laser can ensure the high-gain characteristics of the fiber laser tuning process with flat broadband and stable output power, so the stability and ranging effect of the all-fiber laser self-mixing ranging system of the present invention can be guaranteed.

3. The output laser linewidth of the fiber laser used in the present invention can reach 0.02nm. Compared with semiconductor lasers, it is a single longitudinal mode or few longitudinal mode output, and its good coherence characteristics, mode characteristics and small beam divergence angle can be fully applied It is suitable for long-distance sensing and measurement, and the coupling method of the all-fiber structure is simple and the structure is compact, which can meet the application requirements of special distance measurement occasions.

Description of drawings

Fig. 1 is a structural representation of the present invention;

Fig. 2 is a schematic structural diagram of a tunable fiber laser with a linear cavity structure in the present invention;

Fig. 3 is a structural schematic diagram of a tunable fiber laser with a ring cavity structure in the present invention;

Fig. 4 is the schematic diagram of self-mixing interference system;

Labels in the figure: 1 tunable fiber laser, 2 photoelectric signal conversion unit, 3 signal processing unit, 4 optical collection and coupling system, 5 measured object, 6 pump unit, 7 tuning unit, 8 broadband reflector, 9 narrowband reflection mirror, 10 wavelength division multiplexing device, 11 active gain medium, 12 coupler; 13 first output end; 14 second output end, 15 isolator, 16 laser output port, 17 laser front face, 18 laser rear face, 19 target object scattering end face.

Detailed ways

Referring to Fig. 1, the all-fiber laser self-mixing ranging system in this embodiment adopts a tunable fiber laser 1, and is equipped with a photoelectric signal conversion unit 2 and a signal processing unit 3 with a photodetector in cooperation with the tunable fiber laser 1 and the light collection and coupling system 4 located at the front end of the tunable fiber laser 1; the tunable laser 1 emits light to the measured object 5 through the output end of the light collection and coupling system 4. The initial wavelength is λ, the tuning frequency is ν _m , and the tuning amplitude is The laser signal is Δλ, and receives the feedback optical signal from the scattering surface of the measured object 5 through the output end of the light collection and coupling system 4 to form a laser self-mixing signal; The laser self-mixing signal formed in the laser 1 is converted into an electrical signal; the signal processing unit 3 receives and processes the electrical signal from the photoelectric signal conversion unit 2, using the fluctuation frequency Δν _L of the laser self-mixing signal, obtained by formula (1) The distance L _ext between the scattering surface of the measuring object 5 and the output end face of the tunable fiber laser 1:

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; ext</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 4</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In a specific implementation, the signal processing unit 3 is conventionally provided with amplification, filtering and demodulation parts; the light collection and coupling system 4 is provided as a lens or lens group, which is used to adjust the output light intensity of the tunable fiber laser 1 and the output light from the measured object 5 The light intensity of the feedback light signal.

As shown in Figure 2, the structural form of the tunable fiber laser 1 is: a resonant cavity with a linear cavity structure, and the resonant cavity is composed of a narrow-band reflector 9 and a rear end surface arranged at the front end of the tunable fiber laser 1. The broadband reflector 8 at the position is composed; the pump light output by the pump unit 6 is connected to the resonant cavity by the wavelength division multiplexing device 10, and the active gain medium 11 in the resonant cavity is excited by the pump light in the resonant cavity Generate laser light; the output end of the wavelength division multiplexing device 10 is connected to the input end of the coupler 12; the first output end 13 of the coupler 12 is connected to the light collection and coupling system 4, and the second output end 14 of the coupler 12 is connected to the photoelectric Signal conversion unit 2; the tuning unit 7 is set as a tuning function device, such as piezoelectric ceramics, etc., through the tuning function of the tuning unit 7, the tunable fiber laser 1 is output from the first output port 13 of the coupler 12. The tuning frequency is ν _m , Tune the laser signal with amplitude Δλ.

In this technical solution, the broadband reflector 8 can be set as a fiber loop mirror or a dielectric film fiber reflector, and the narrowband reflector 9 can be set as a fiber grating with a frequency selection function.

As shown in Figure 3, the structural form of the tunable fiber laser 1 is: a resonant cavity with a ring cavity structure is used, and the pump light output by the pump unit 6 is connected to the resonator of the ring cavity structure by a wavelength division multiplexing device 10. In the cavity, the active gain medium 11 in the cavity is excited by pump light, and laser light is generated in the cavity through a narrow-band mirror 9; the input end of the coupler 12 is connected to the cavity of the ring cavity structure, and the coupler The first output end 13 of 12 is connected with the narrow-band reflector 9, the tuning unit 7 is connected with the light collection and coupling system 4, and the second output end 14 of the coupler 12 is connected with the photoelectric signal conversion unit 2; the tuning unit 7 is set as a tuning function device, such as Piezoelectric ceramics, etc., through the tuning function of the tuning unit 7, the tunable fiber laser 1 outputs a laser signal with a tuning frequency of ν _m and a tuning amplitude of Δλ from the output terminal 16 of the narrow-band mirror 9 .

The narrowband reflector 9 is set as a fiber grating with frequency selection function. An isolator 15 is added between the active fiber 11 and the coupler 12. The isolator 15 isolates the feedback light to control the unidirectional transmission of the laser in the resonant cavity.

In the specific implementation, the fiber grating is wound on the piezoelectric ceramic driven by a periodic signal, and the piezoelectric ceramic causes the fiber grating to stretch or shrink, so that the grating constant changes periodically, and the periodicity of the output laser wavelength of the tunable laser is realized. Variety. A polarization controller can be added to the laser resonator to control the polarization state of the laser to eliminate the influence of fiber birefringence and ensure the stability of the output laser.

Periodic signals such as triangular wave signals act on piezoelectric ceramics, and the fiber grating strains under the action of piezoelectric ceramics, according to:

$\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; FBG</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; FBG</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; \&Delta;\&epsiv;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

In formula (8), λ _FBG is the center wavelength of grating reflection, Δλ _FBG is the change amount of grating reflection center wavelength, P is the effective elastic-optics coefficient, and Δε is the change amount of axial strain. When the applied force is pulling force, the grating constant increases, and the center wavelength of the grating reflection moves to the long-wave direction; otherwise, it moves to the short-wave direction, thereby realizing a wavelength-tunable output.

Claims (6)

1. a full fiber type laser is from the mixed ranging system, it is characterized in that adopting a tunable optical fiber laser (1), the light that is equipped with photosignal converting unit (2), the signal processing unit (3) with photodetector and the front end that is positioned at described tunable optical fiber laser (1) with described tunable optical fiber laser (1) is collected and coupled system (4); Described tunable laser (1) is collected by light and the output of coupled system (4) is that λ, tuned frequency are ν to the initial wavelength of testee (5) outgoing _m, tuning amplitude is the laser signal of Δ λ, and collect the light signal fed back that output with coupled system (4) receives from the scattering surface of testee (5) by light and form laser from mixed signal; Described photosignal converting unit (2) is to be received in the middle laser that forms of described tunable laser (1) from mixed signal and to be converted to the signal of telecommunication with photodetector; Described signal processing unit (3) receives and handles the signal of telecommunication from described photosignal converting unit (2), utilizes laser from the vibration frequency Δ ν of mixed signal _L, obtained the distance L between the output end face of the scattering surface of testee (5) and tunable optical fiber laser (1) by formula (1) _Ext:

$L_{\mathrm{ext}}=\frac{{\mathrm{\&Delta;v}}_L{\mathrm{\&lambda;}}^2}{{4v}_m\mathrm{\&Delta;\&lambda;}}---\left(1\right).$

2. full fiber type laser according to claim 1 is from the mixed ranging system, it is characterized in that: the version that described tunable optical fiber laser (1) is set is: adopt the resonant cavity of linear cavity structure, described resonant cavity is made up of the narrowband reflection mirror (9) of the front end face position that is arranged on tunable optical fiber laser (1) and the broadband mirrors (8) of rear end face position; By the pump light access resonant cavity of wavelength division multiplex device (10) with pump unit (6) output, excite the active gain medium (11) in the resonant cavity in described resonant cavity, to produce laser with described pump light; The output of described wavelength division multiplex device (10) connects the input of coupler (12); First output (13) with described coupler (12) connects described light collection and coupled system (4), with second output (14) the connection described photosignal converting unit (2) of described coupler (12); It is the tuber function device that tuned cell (7) is set, and it is ν from first output (13) the output tuned frequency of described coupler (12) that the tunning effect by described tuned cell (7) makes described tunable optical fiber laser (1) _m, tuning amplitude is the laser signal of Δ λ.

3. full fiber type laser according to claim 2 is from the mixed ranging system, it is characterized in that: described broadband mirrors (8) is set to fiber loop mirror speculum or deielectric-coating fiber reflector, the fiber grating that described narrowband reflection mirror (9) is set to have the frequency-selecting function.

4. full fiber type laser according to claim 1 is from the mixed ranging system, it is characterized in that: the version that described tunable optical fiber laser (1) is set is: the resonant cavity that adopts ring cavity structure, inserted in the resonant cavity of ring cavity structure by the pump light of wavelength division multiplex device (10) with pump unit (6) output, excite active gain medium (11) in the resonant cavity with described pump light, and in described resonant cavity, produce laser through narrowband reflection mirror (9); The input of coupler (12) is connected with the resonant cavity of described ring cavity structure, first output (13) of coupler (12) connects narrowband reflection mirror (9), tuned cell (7) connects described light to be collected and coupled system (4), and second output (14) of coupler (12) connects described photosignal converting unit (2); It is the tuber function device that tuned cell (7) is set, and it is ν from described narrowband reflection mirror (9) output (16) output tuned frequency that the tunning effect by described tuned cell (7) makes described tunable optical fiber laser (1) _m, tuning amplitude is the laser signal of Δ λ.

5. full fiber type laser according to claim 4 is from the mixed ranging system, it is characterized in that: the fiber grating that described narrowband reflection mirror (9) is set to have the frequency-selecting function, between Active Optical Fiber (11) and coupler (12), add an isolator (15), isolate the one-way transmission of feedback light control laser in described resonant cavity by described isolator (15).

6. full fiber type laser according to claim 1 is from the mixed ranging system, it is characterized in that described light is collected with coupled system (4) is set to lens or set of lenses, is used for regulating the output intensity of described tunable optical fiber laser (1) and from the light signal fed back light intensity of described testee (5).

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