rus
Issue #6/2014
G. Buymistryuk, V. Nikolaev, M. Bazlov
Sensing Devices On The In-Fiber Optic Doppler Effect
Sensing Devices On The In-Fiber Optic Doppler Effect
The construction principles of a fiber optic Doppler vibro-acoustic sensor are discussed. Its technical characteristics and advantages over sensors on the basis of fiber optic interferometers and diffraction gratings are presented.
Теги: acoustooptical heterodyne doppler effect optical fiber optical fiber sensor vibro-acoustic structure monitoring акустооптический гетеродин виброакустический мониторинг конструкций волоконно-оптический датчик оптическое волокно эффект допплера
The principle of operation of the fiber optical Doppler sensor (FODS)
based on the newly discovered in-fiber optic Doppler effect and consists in the fact that the frequency of the light wave f0 laser radiation transmitted through the curved optical fiber length L moves (under the influence of acoustic pressure fiber moves or vibrates at a velocity of dL/dt) in the spiral fiber Doppler element shown in Fig. 1, on the value of fD:
,
where n – refractive index of the fiber; λ0 – wavelength of light in the optical fiber. In contrast to the interferometric vibro-acoustic sensors (Fabry-Perot, Mach-Zehnder, Michelson and Sagnac), which analyzes the change in the phase of the optical signal in the vibro-acoustic Doppler sensors analyze the change in frequency of the optical signal.
New phenomenal result of frequency analysis of the optical signal at the vibro-acoustic effect on the straight (Fig. 2a) and a curved optical fiber (Fig. 2b) is the appearance of Doppler frequency shift only in a curved fiber [1].
Doppler frequency shift in a spiral optical fiber element, caused by strains (movements) εх and εу is defined as:
,
where: Rа, Da – average radius and diameter of a fiber circular part; N – number of turns of fiber; nэ – equivalent refractive index.
A spiral FODS haven’t directional sensitivity. It is an important advantage compared to sensors based on Bragg diffraction gratings, which have spatially selective sensitivity – they do not receive vibrations perpendicular to the axis of the Bragg grating. However, with the need to make the corresponding diagram of directivity FODS (selective sensitivity), it can be made U-shaped or elongated-elliptical shape as shown in Fig.3. An example of a spiral FODS is shown in Fig.4.
For such configurations, as shown in Fig. 3 Doppler frequency is determined by the:
,
where: l – length of an elongated portion of the sensor.
The main drawback of interference fiber optic sensors is that a system is needed to control the phase of the signal in order to maintain optimum sensitivity.
Frequency output signal of FODS does not depend from temperature – the main source of instability and uncertainty in the sensors based on interference and diffraction.
The principle of operation of the frequency FODS under vibro-acoustic action – in contrast to the frequency fiber Bragg grating – a direct conversion rate of deformation in the Doppler frequency shift. In particular, a FODS measures the rate of deformation in x and y directions, however, its sensitivity is higher than that of a Bragg grating.
Functional diagram of an acoustic emission sensor based on in-fiber optic Doppler effect is shown in Fig. 5.
Doppler frequency shift fD (for example, 0.1 Hz – 1 MHz) is detected from the optical signal using an acousto-optical heterodine, shifting f0 by a constant modulation fM (for example, 80 MHz), the formation of the beats at a frequency fM + fD, optoelectronic conversion and photodetectors conversion frequency deviation fD in the output signal Uout by means of frequency detector [2].
FODS has a very high sensitivity of the vibro-acoustic (now achievable resolution deformation is 10–11 elongation fiber (~10–5 ppm) in an extremely wide frequency range from 0.1 Hz to 10 MHz by implementation of the principle of laser Doppler velocity measurement (LDV) micro-displacements of the optical fiber. Deformation (elongation – strain) fibers were measured by a precision LDV (resolution ≤ 10–7 μm/V), for example type “Vibroducer-1002” from company “Melectro”.
Evaluation of sensitivity was performed so that the voltage applied to the piezoelectric transducer (for example, generator G3–123) was changed to the corresponding test frequency, so that the output of the FD varies from 0 to Umax. In particular, the voltage Umax at 100 kHz corresponds to a deformation of fiber on the 100 microns. Then the power of the acoustic signal at the minimum recorded value of the FD at the exit, which corresponded to 10–5 microns. Thus, full dynamic range D at a frequency of 100 kHz was the magnitude of the voltage D100 kHz = 20 lg [102/10–5] = 140 dB. Similarly, at 1 kHz, the threshold strain is 10–6 microns, D1 kHz = 160 dB.
However, multi-component and precise mechanical design demodulator broadband acoustic signals in a traditional surround LDV with acousto-optic frequency shifter on a single crystal TeO2 – relatively expensive, has considerable size and contains a number of sources of instability signal, due to a combination of several optical and electrical changes.
In-fiber processing method on micro-taper
With the help of laser forming is performed micro-taper optical fiber, acousto-optic frequency shifter – a key element of AOH – sold on the in-fiber interactions resulting optical modes: between fundamental mode radiation LD with frequency fo and mode, modulated frequency fм, using a conical piezoelectric transducer 1 in micro-taper of the optical fiber, as shown in Fig. 6.
The interaction of the optical modes occurs in the area of the optical fiber 2 with a transverse dimension about of 10 microns [3].
The frequency of the output radiation into the optical fiber (3) f0 – fм were shifted to by value of frequency fм modulation, for example, 40 MHz.
Obvious advantages of such acousto-optical heterodine: compactness, absence of adjustable dimension optical input-output collimators of radiation, increased signal/noise ratio. Typical time response and Fourier spectrum of the Doppler FODS acoustic emission with internal exfoliating tubular composite structure shown in Fig.7.
Experimental studies of heat-resistant and radiation-hard acoustic emission for metal and composite structures for various industries now in progress and will continue intensively. They will be very interesting for researchers.
based on the newly discovered in-fiber optic Doppler effect and consists in the fact that the frequency of the light wave f0 laser radiation transmitted through the curved optical fiber length L moves (under the influence of acoustic pressure fiber moves or vibrates at a velocity of dL/dt) in the spiral fiber Doppler element shown in Fig. 1, on the value of fD:
,
where n – refractive index of the fiber; λ0 – wavelength of light in the optical fiber. In contrast to the interferometric vibro-acoustic sensors (Fabry-Perot, Mach-Zehnder, Michelson and Sagnac), which analyzes the change in the phase of the optical signal in the vibro-acoustic Doppler sensors analyze the change in frequency of the optical signal.
New phenomenal result of frequency analysis of the optical signal at the vibro-acoustic effect on the straight (Fig. 2a) and a curved optical fiber (Fig. 2b) is the appearance of Doppler frequency shift only in a curved fiber [1].
Doppler frequency shift in a spiral optical fiber element, caused by strains (movements) εх and εу is defined as:
,
where: Rа, Da – average radius and diameter of a fiber circular part; N – number of turns of fiber; nэ – equivalent refractive index.
A spiral FODS haven’t directional sensitivity. It is an important advantage compared to sensors based on Bragg diffraction gratings, which have spatially selective sensitivity – they do not receive vibrations perpendicular to the axis of the Bragg grating. However, with the need to make the corresponding diagram of directivity FODS (selective sensitivity), it can be made U-shaped or elongated-elliptical shape as shown in Fig.3. An example of a spiral FODS is shown in Fig.4.
For such configurations, as shown in Fig. 3 Doppler frequency is determined by the:
,
where: l – length of an elongated portion of the sensor.
The main drawback of interference fiber optic sensors is that a system is needed to control the phase of the signal in order to maintain optimum sensitivity.
Frequency output signal of FODS does not depend from temperature – the main source of instability and uncertainty in the sensors based on interference and diffraction.
The principle of operation of the frequency FODS under vibro-acoustic action – in contrast to the frequency fiber Bragg grating – a direct conversion rate of deformation in the Doppler frequency shift. In particular, a FODS measures the rate of deformation in x and y directions, however, its sensitivity is higher than that of a Bragg grating.
Functional diagram of an acoustic emission sensor based on in-fiber optic Doppler effect is shown in Fig. 5.
Doppler frequency shift fD (for example, 0.1 Hz – 1 MHz) is detected from the optical signal using an acousto-optical heterodine, shifting f0 by a constant modulation fM (for example, 80 MHz), the formation of the beats at a frequency fM + fD, optoelectronic conversion and photodetectors conversion frequency deviation fD in the output signal Uout by means of frequency detector [2].
FODS has a very high sensitivity of the vibro-acoustic (now achievable resolution deformation is 10–11 elongation fiber (~10–5 ppm) in an extremely wide frequency range from 0.1 Hz to 10 MHz by implementation of the principle of laser Doppler velocity measurement (LDV) micro-displacements of the optical fiber. Deformation (elongation – strain) fibers were measured by a precision LDV (resolution ≤ 10–7 μm/V), for example type “Vibroducer-1002” from company “Melectro”.
Evaluation of sensitivity was performed so that the voltage applied to the piezoelectric transducer (for example, generator G3–123) was changed to the corresponding test frequency, so that the output of the FD varies from 0 to Umax. In particular, the voltage Umax at 100 kHz corresponds to a deformation of fiber on the 100 microns. Then the power of the acoustic signal at the minimum recorded value of the FD at the exit, which corresponded to 10–5 microns. Thus, full dynamic range D at a frequency of 100 kHz was the magnitude of the voltage D100 kHz = 20 lg [102/10–5] = 140 dB. Similarly, at 1 kHz, the threshold strain is 10–6 microns, D1 kHz = 160 dB.
However, multi-component and precise mechanical design demodulator broadband acoustic signals in a traditional surround LDV with acousto-optic frequency shifter on a single crystal TeO2 – relatively expensive, has considerable size and contains a number of sources of instability signal, due to a combination of several optical and electrical changes.
In-fiber processing method on micro-taper
With the help of laser forming is performed micro-taper optical fiber, acousto-optic frequency shifter – a key element of AOH – sold on the in-fiber interactions resulting optical modes: between fundamental mode radiation LD with frequency fo and mode, modulated frequency fм, using a conical piezoelectric transducer 1 in micro-taper of the optical fiber, as shown in Fig. 6.
The interaction of the optical modes occurs in the area of the optical fiber 2 with a transverse dimension about of 10 microns [3].
The frequency of the output radiation into the optical fiber (3) f0 – fм were shifted to by value of frequency fм modulation, for example, 40 MHz.
Obvious advantages of such acousto-optical heterodine: compactness, absence of adjustable dimension optical input-output collimators of radiation, increased signal/noise ratio. Typical time response and Fourier spectrum of the Doppler FODS acoustic emission with internal exfoliating tubular composite structure shown in Fig.7.
Experimental studies of heat-resistant and radiation-hard acoustic emission for metal and composite structures for various industries now in progress and will continue intensively. They will be very interesting for researchers.
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