Sensing of ultrasonic fields based on polarization parametric indirect microscopic imaging Download: 623次
Inspection techniques based on ultrasonic waves were widely used in various applications, such as crack detection in industrial parts, medical ultrasound diagnosis, and the recently emerged photoacoustic imaging technology for biomedical tissues, due to strong penetration, good direction, and non-dangerous nature of ultrasonic waves[13" target="_self" style="display: inline;">–
As a consequence, more and more attention has been paid to the non-contact ultrasonic detection methods that enable fast scanning and remote measurement to overcome the difficulty of contact measurement, which include electrical and optical methods. Electrical non-contact transducers, such as a capacitance micromachined ultrasonic transducer (CMUT)[6,7], electromagnetic acoustic transducer (EMAT)[8], and air-coupled ultrasonic transducer (ACUT)[9], need to be placed at a short distance from the surface of the sample to detect ultrasonic waves. Although they have several advantages over the traditional piezoelectric transducer, they are limited by their short operation distance and low efficiency for many applications. Optical methods, including interferometric methods such as the Mach–Zehnder interferometer[10,11], full-field speckle interferometry[12], the low-coherence interferometer[13], the Fabry–Perot polymer film sensor[14], the polymer micro-ring resonator[1517" target="_self" style="display: inline;">–
In this Letter, we proposed a polarization microscopic imaging technique for ultrasonic field sensing based on the reported parametric indirect microscopic imaging (PIMI) system[30]. A model for theoretically describing the three-dimensional (3D) anisotropic photoelastic effect in solid was developed. The mechanism of polarization status variations of light passing through the stress and strain fields was analyzed. The PIMI parameters, which indicate the polarization status of light, were related to the dynamic anisotropic change of the stress and strain fields and, therefore, the ultrasonic field. Ultrasonic fields in an isotropic sample generated by a piezo transducer with resonant frequency of 5 MHz were imaged with this PIMI method. The parameters such as
As an ultrasonic wave propagates in a solid, the material particles are compressed and rarified depending on location and time, which results in a nonuniform field of strain in the medium. This strain field will result in an anisotropic distribution of the refractive index through the photoelastic effect, as shown in Fig.
Fig. 1. Schematic diagram of a change in polarization status of light through a solid sample in which an ultrasonic wave propagates. Under the influence of the ultrasonic wave (wavelength ) propagation, the solid is compressed and rarified depending on location and time, which produces nonuniform stress and strain fields. This leads to an anisotropic distribution of the refractive index and produces a phase difference of light through the 3D infinitesimal volume element. The phase difference can be detected by our PIMI system.
As an ultrasonic wave propagates in a homogeneous, isotropic solid material, the wave equation in displacement can be rewritten as follows[31,32]:
From this 3D displacement vector, one can derive the strain tensor at each point in the solid,
The refractive index at the corresponding point in the medium will be changed by the strain through the photoelastic effect. This is actually the change of size, shape, and orientation of the refractive index ellipsoid if the refractive index of the solid is expressed in terms of the refractive index ellipsoid. Since the strain in the solid is infinitesimally small, the anisotropic photoelastic relation between the strain and the refractive index is usually written as[33,34]
When the light wave propagates through the strain field induced by the ultrasonic wave, it will produce a phase difference
When light with a predetermined polarization status, e.g., linear polarization, passes through the medium, the phase difference produced by a certain strain field will lead to the variation of the indirect parameters, which represents the polarization status of the emerging beam. These parameters can be sensed and calculated by the PIMI system, as previously reported by the group[30,36,37]:
As illustrated in Fig.
Fig. 2. Experimental setup of ultrasonic field sensing. It consists of an ultrasonic excitation system and a PIMI system. The excitation system was employed to generate ultrasonic waves in the sample, and the PIMI system was used to image and characterize the ultrasonic field by extracting variations of optical properties of the sample with and without ultrasonic excitation.
A brief description of the PIMI imaging system is included here for the sake of completeness, and more details can be found in Ref. [30]. The system was built by adopting the basic optical microscopic path of an Olympus BX51M microscope and inserting a polarization modulation module with an angle precision of 0.05° in the beam path of illumination light. A Basler (piA2400–17gm) CCD with a pixel resolution of 3.45 μm was used for data acquisition of the optical intensity variation affected by the ultrasonic wave in the sample. The indirect optical images were taken with an illumination wavelength of 532 nm. With regard to the PIMI system, all of the polarization parameters, including the average of polarization intensities
As a sinusoidal voltage with amplitude of 20 V and frequency of 5 MHz is applied to PZT 1, an ultrasonic wave with the same frequency was generated in the sample, as indicated by the signal captured by PZT 2. In order to eliminate the effects of surface micro-morphology, defects, and scratches on the ultrasonic field images, the objective of PIMI system was focused on a plane at a depth of about 3 mm beneath the surface of the sample. The PIMI images were taken when the sample was with and without ultrasonic excitation, as shown in Fig.
Fig. 3. PIMI images under different ultrasonic conditions (a)–(c) without and (d)–(f) with ultrasonic excitation. (a) and (d) Average of polarization intensities , (b) and (e) polarization phase difference , (c) and (f) polarization angle of slow axis .
Figures
The PIMI images of Stokes parameters
Fig. 4. PIMI images of the Stokes parameters under different ultrasonic conditions (a)–(d) without and (e)–(h) with ultrasonic excitation. (a), (e) ; (b), (f) ; (c), (g) ; (d), (h) .
As can be seen from Fig.
In order to confirm the sensitivity of the parameter images in Figs.
Fig. 5. Difference between PIMI images without ultrasonic excitation and those with ultrasonic excitation. (a) , (b) , and (c) .
As described by the theoretical model, the ultrasonic wave induced a variation of strain (stress) fields, leading to the dynamic variation of the refractive index (refractive index ellipsoid) and, therefore, a change of polarization status of the light passing through the medium. The indirect parameter images recorded and calculated by the PIMI system then recover the information of the ultrasonic field, which was imposed on the polarization status of the emerging beam due to the photoelastic effect. The relation between the indirect parameter images and the ultrasonic field is well defined theoretically, which can be used to calculate the ultrasonic field quantitatively in further study. Figure
In order to confirm the sensitivity of PIMI images to the ultrasonic field, ultrasonic phases were varied from 0 to
Fig. 6. Image of under different ultrasonic phases. (a) Phase 0, (b) phase , (c) extracted intensity curves along the line in (a) and (b), (d) difference between (a) and Fig. 3(c) , (e) difference between (b) and Fig. 3(c) , (f) extracted intensity curves along the line in (d) and (e).
Figure
Fig. 7. PIMI images under different ultrasonic conditions (a)–(c) without and (d)–(f) with ultrasonic excitation: (a) and (d) , (b) and (e) , (c) and (f) . Difference between the PIMI images without ultrasonic excitation and those with ultrasonic excitation: (g) , (h) , and (i) .
The ultrasonic wavelength in the sample is approximately 1.19 mm if taking the wave velocity of 5950 m/s for compression wave in a quartz glass. Since the field of view (
As a proof of concept, the results above demonstrated that the PIMI system can sense the ultrasonic field by taking images of the indirect parameters, which represent the polarization status of the light affected by the refractive index variations in the sample. The relation between these parameters is well defined in theory, and the ultrasonic field can be further studied quantitatively with the proposed theoretical model. As shown in the experimental results, the image of parameter
In summary, by using the PIMI method, signals of ultrasonic waves transmitted in a sample have been clearly picked up in a non-contact way. The structural anisotropy associated with the stress and strain generated by the ultrasonic wave propagation can cause variation of the refractive index, which leads to the variations of the polarization status of the light passing through the medium. The PIMI method measures the indirect parameters, such as
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Yun Cao, Jichuan Xiong, Xuefeng Liu, Zhiying Xia, Weize Wang, N. P. Yadav, Weiping Liu. Sensing of ultrasonic fields based on polarization parametric indirect microscopic imaging[J]. Chinese Optics Letters, 2019, 17(4): 041702.