Fiber-Optic Vibroacoustic Sensors
The Development of Fiber-Optic Vibroacoustic Sensors Based on Novel Precision Methods of Low-Coherent Interferometry
Tech Area / Field
- INS-MEA/Measuring Instruments/Instrumentation
- PHY-OPL/Optics and Lasers/Physics
8 Project completed
Senior Project Manager
Tyurin I A
VNIIEF, Russia, N. Novgorod reg., Sarov
- Institute of Physics of Microstructures, Russia, N. Novgorod reg., N. Novgorod
- Universite de Franche-Comte, France, Besancon\nRiso National Laboratory/Optics and Fluid Dynamics Department, Denmark, Roskilde
Project summaryThe ISTC financial support is requested for the project aimed at development of the sensor vibroacoustic equipment having enhanced temperature durability and immunity to electromagnetic interference. This equipment can be used for nondestructive testing of technological equipment operating in extremely severe temperature and electromagnetic conditions. In the frames of the project new precision interference methods for detection of mechanical oscillations of bodies will be developed, the physical limitations for the sensitivity of fiber-optic low coherent interferometers recording acoustic signals will be studied, and the prototype fiber-optic vibroacoustic and acoustic emission sensors will be developed and tested in a “real world” conditions.
The relevance of the project is determined by the extreme importance of diagnostics problems and problems of state prediction for equipment of potentially dangerous industrial objects such as atomic power plants, chemical plants, etc. The development of vibroacoustic and acoustic emission sensors with enhanced thermal and electromagnetic stability will allow to move the diagnostics tools into the most critical and potentially dangerous elements of technological equipment, and thus to decrease the risk of accidents.
The key elements of any acoustic-emission or vibroacoustic system are sensors providing primary information on the oscillations of object. Today, the most of the used sensors are based on the piezoelectric effect. However, piezoelectric sensors do not fully meet all the demands. It refers, primarily, to temperature durability and immunity to electromagnetic interference.
This Project is aimed at the development of optic interference sensors for vibroacoustic and acoustic-emission diagnostics offering ultimate temperature operation and immunity to electromagnetic interference. The most interesting anticipated application areas for optical sensors are the diagnostics of objects at high temperatures (up to 1000 C), the diagnostics of the components of high-power electrical equipment, and diagnostics of the objects not allowing the mechanical contact, for instance, moving objects.
Optical interferometric methods enable detection of mechanical vibrations with amplitudes down to 10-3 10-4 A in the frequency range from fractures of Hz up to hundreds of MHz, so they are suitable for the in the most of typical tasks meet the most of demands of nondestructive testing. The main advantages offered by interference sensors are the following:
1. The optical sensor can be placed remotely from the object, without direct mechanical contact with it. This allows to weaken temperature restrictions and enables diagnostics of moving objects.
2. The probe light beam can be focused on the surface of the object into a spot of about a micron.
3. The optical sensor does not disturb the oscillations of the measured object.
4. The majority of optical interference sensors have a uniform frequency response over a broad frequency band.
The fiber optics makes interference sensors much more stable and noise immune allowing their application in noisy industrial environments. Tested object can be linked to the interferometer by an optical fiber with maximum length up to hundred meters thus the vulnerable precision parts of the system can always be placed at a “safe” distance from the tested equipment. Optical fiber offers high temperature durability, and it is absolutely insensitive to electromagnetic interference (contrary to an electrical “copper” communication line).
The recent years demonstrated the remarkable advancement in the development of novel interferometric measuring techniques based on fiber optics and advanced light sources. One of these techniques is the low coherent interferometry (LCI) employing broadband incoherent light sources. LCI is based on the well-known fact that the interferometric fringes cannot be observed if the optical delay exceeds the coherence length of light source. Keeping the accuracy of conventional laser interferometry, LCI enable absolute measurement of optical delay. Another important feature of LCI is that spurious reflections and scattering of light in the optical elements of the interferometer have does not affect the interferometer. First of allows to eliminating of extra “coherent” (or speckle) noise, and makes possible multi-element and distributed fiber-optic systems having no analogues in the traditional laser interferometry.
There are several LCI versions, which can be used for vibroacoustic and acoustic-emission diagnostics. The simplest LCI scheme is represented by the Michelson with the tested object in one arm. Michelson LCI can provide a good accuracy of displacement sensing even at extremely high light losses in the object arm (up to 120 dB). It allows their use for diagnostics of low-reflecting objects and of those with low-quality surface as well as at high absorption or scattering of light. However, the Michelson LCI is sensitive to external influences (temperature variations, mechanical effects, etc.) randomly altering the optical length and anisotropy of fiber arms. So the maintaining of operating point and balance of interferometer in industrial conditions can be a serious problem.
The more promising version for industrial vibroacoustic testing is a low coherent tandem interferometer (LCTI). LCTI is recognised as an important technique for the absolute remote measurement of quasistatic parameters, such as distance and refractive index. It can be expected that LCTI based systems are quite promising for vibroacoustic and acoustic emission diagnostics over the range from 1 Hz up to 100 kHz.
The Project assumes to employ LCTI as the basic tool for remote vibroacoustic diagnostics and also for distant measurement of temperature.
To detect displacements with the amplitude lower than 0.05 mcm, the Project provides the employment of extremely stable polarization interferometers as a reference interferometer in a tandem. In polarization interferometers, the optical delay is converted into the ellipticity of output light which can be accurately measured by well-developed polarimetric techniques. The polarization schemes are virtually free from temperature drifts and thus offer the actual sensitivity and precision near the shot-noise limit (about 10-5 A/Hz1/2 for 1 mW light power).
The polarization interferometer may be used also as an object (sensing) interferometer, or “polarization transducer” transforming deformations of surface into the changes of polarization of light. Particularly, polarization interferometer based on the Savart plate (or another polarization beamsplitter) may be employed as ultimately sensitive and stable differential non contact transducer of surface acoustic emission waves. In these scheme, the surface of object is illuminated by two different points by two beams produced by beamsplitter; the beams are separated by a distance exceeding the wavelength of surface acoustic wave. After the reflection from object the beams recombine in the beamsplitter. The deformation of surface produced by a surface acoustic wave results in difference in the optical paths of the beams, which is transformed into the change of polarization of output light.
Acoustic surface waves can be detected not only by “spatial differentiation” as described above but also by time differentiation when the positions of oscillating surface in different moments are compared. Such “time differentiation” can be realised in Sagnac fiber ring interferometers. They are expected to be more efficient than LCTI, in the high frequency part of the acoustic emission spectrum (100 kHz and higher). Ring interferometers offer a ultimate stability which is due to their self-compensation nature: interfering waves travel along the same path. The measured object closing the loop is placed into the ring asymmetrically, closely to the input coupler. The displacement of the reflecting surface of the object leads to the inequality of optical paths seen by counterrunning waves because they meet the object in different times.
To avoid problems with maintaining of operating point inherent to conventional Sagnac interferometers we intend to use polarization ring interferometer (PRI) proposed by one of the Project participants. The expected sensitivity of the interferometer is about 10-4 А/Hz1/2 at 1 mW source power, which is close to the ultimate shot-noise one.
Another ring interferometer scheme, which is of potential interest for vibroacoustic diagnostics is the low coherent resonance ring Doppler velocimeter. This scheme measures velocity of the object, consequently, its sensitivity increases in proportion to the frequency of oscillations. The low coherent resonance ring Doppler velocimeter (LCRRDV) is a double-pass scheme based of the ring resonator where the light is transmitted through the ring resonator twice: before and after the reflection from the tested object. The Doppler shift causes the decrease in total light power after two passes because of mismatch of “shifted” transmission lines of resonator in the reflected spectrum the transmission lines of the resonator. The employment of the high finesse ring resonator allows to multiply the sensitivity without enhancement of the length of the loop and, thus, to improve the “cost/sensitivity” ratio. The capability to measure linear velocities of objects using LCRRDV is experimentally demonstrated by the Project participants. The ultimate LCRRDV sensitivity is expected to be about 0.1 1 nm/(s Hz1/2).
The major anticipated result of the Project is the development of the prototype non-contact fiber-optic vibroacoustic sensors of new generation, offering high temperature stability, insensibility to electromagnetic interference and relatively low cost. These sensors will allow the non-destructive testing over the objects at temperatures up to 1000С, in the presence of intensive electromagnetic fields and the control of moving objects. The anticipated key specifications of the developed sensors are indicated in Table.
Differential polarization low-coherent (white light) interferometer
10-3 А 0.5 mcm
Polarization ring interferometer
10-4 А/ Hz 1/2
10-3 А 0.5 mcm
Ring resonance interferometer with the low-coherent (white light) source
10-4 А/ Hz 1/2
at f = 10 k Hz
10-4 А 50 nm, f = 10 k Hz
Besides that, high-precision temperature probes with the operating range from 0 to 1500 K and the error not exceeding 0.05 К, are to be developed.
The developed sensors are to be employed in the equipment control systems of NPP's and of other systems operating under severe temperature, radiation and electromagnetic conditions as well as in the control systems for industrial facilities containing explosion and fire hazardous and deleterious substances. Besides, the developed sensors can be employed in research, in particular, for studying the material failure (fracture).
The work under the project will allow the redirection of former nuclear-weapons scientists, specialists and technicians to peaceful activities meeting the main ISTC goal. The participation of the VNIIEF scientists and specialists earlier involved in defence programs will facilitate the conversion of VNIIEF science and technical potential from the military to peaceful purposes and the integration of VNIIEF scientists into the world’s scientific community. All the work results are to be published in the open press.
Foreign collaborators will cooperate with the Project executing staff in the following.
- Preparation of the Work Plan.
- Carrying out consultations.
- Information exchange and discussion of the results obtained.
- Recommendations on the international use of the Project results.
- Recommendations on application of the Project results in other fields.
The RFNC-VNIIEF scientists have gained wide experience in the development of sensor equipment for a variety of applications. These include, in particular, the area of arranging and carrying out measurements of elastic wave propagation, both in the laboratory and in the field, when performing underground tests of nuclear warheads at the Semipalatinsk and Novaya Zemlya test sites. The experience is also available in the field of developing and employing vibroacoustic and vibratory converters in the diagnostics systems for the equipment state of operating NPPs.
The team of RFNC-VNIIEF developers has participated in the successful performance of ISTC projects № 049, № 075 and № 738. Due to the ISTC support, a wide variety novel vibroacoustic (5 types) and vibration (3 types) sensors was developed for control systems of NPP equipment. The team has gained essential development-critical experience. The design documentation for the sensors developed under the ISTC projects has been approved by the RF Gosatomnadzor. The metrological certification of sensors was completed by the RF Gosstandart.
The suitability of the developed sensors was verified experimentally, in the conditions of full-scale NPP operation, at high pressures, temperatures, vibration loading and in intensive radiation fields.
The integrated efforts of scientists from the Russian Academy of Science (RAS) and of MINATOM specialists supported by ISTC will allow the development of special-purpose diagnostic equipment meeting state-of-the-art requirements.
The researches under the present project will result in a considerable progress in the area of sensor instrumentation. The direct benefit resulting from the project implementation will be the redirection of former nuclear-weapons specialists (tens of people) to non-military researches, the enhanced integration of Russian scientists from defence-related organizations into the international scientific community. The effort integration of highly-skilled Russian MINATOM specialists, earlier isolated, and of the scientists from the Russian Academy of Science as well as of the scientists from other countries should accelerate the development of monitoring systems for the facilities of the fuel and energy complex. The proposed activities will facilitate further expansion of collaboration among countries and will support the conversion of industrial and engineering and scientific potential from military to civil purposes.
The development of efficient control methods for the equipment of power and industrial facilities, posing the enhanced risk, will ensure enhanced safety for population and for operation of industrial facilities throughout the vast territories.
The Project is based on vast experience of VNIIEF in the field of development and operation of control equipment.
Technical approach and methodology are developed taking into account RFNC-VNIIEF serial production specification. It is anticipated to use VNIIEF’s experimental base and to involve highly qualified weapon specialists in development of nuclear material account and control systems.
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