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Performance of dual-frequency ultrasound measurement based on DBR fiber laser hydrophone
Accepted Manuscript Title: Performance of Dual-frequency Ultrasound Measurement Based on DBR Fiber Laser Hydrophone Authors: Chengang Lyu, Shuai Zhang, Gan Fang, Jin Jie, Xugeng Zhang, Chuang Wu PII: DOI: Reference:
S0924-4247(17)30705-7 http://dx.doi.org/10.1016/j.sna.2017.09.017 SNA 10325
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
22-4-2017 28-8-2017 11-9-2017
Please cite this article as: Chengang Lyu, Shuai Zhang, Gan Fang, Jin Jie, Xugeng Zhang, Chuang Wu, Performance of Dual-frequency Ultrasound Measurement Based on DBR Fiber Laser Hydrophone, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.09.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Performance of Dual-frequency Ultrasound Measurement Based on DBR Fiber Laser Hydrophone Chengang Lyu 1,*, Shuai Zhang 1, Gan Fang 1, Jin Jie 1, Xugeng Zhang 1 and Chuang Wu 2 1 2
School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China; [email protected] Institute of Photonics Technology, Jinan University, Guangzhou 510632, China; [email protected]
* Correspondence: [email protected]; Tel.: +86-138-2055-3633
Highlights
We report a detailed investigation of potential of DBR fiber lasers as a sensor designed for dual-frequency ultrasound measurement.
We analyze in detail theories which demonstrate the ability of DBR laser to measure DFUS considering the effects of ultrasound far and near field.
We conduct a series of experiments compared with the theory to support the results.
Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different distances and different DFUS driving voltage.
ARTICLE INFO
ABSTRACT The ability of distributed Bragg reflector (DBR) fiber laser sensor in dual-frequency ultrasound (DFUS) measurement has been demonstrated. The influences of DFUS pressure on fiber grating laser sensor were theoretically analyzed, considering the effects of relative distance of the ultrasound field and the amplitude of the DFUS on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasound signals at 3 MHz and 5 MHz, and the test distance and driving voltage were set respectively according to the ultrasound frequency. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different distances (40 mm to 10 mm) and different DFUS driving voltage (5 V to 20 V). The results demonstrate that the DBR fiber grating laser acoustic sensor has a multi-frequency ultrasound recognizable ability, offering a potential for ultrasound medicine and biology
Keywords:Fiber measurements DBR Optical fiber sensors Dual-frequency ultrasound
1. Introduction DFUS at megahertz has been widely used in some sonochemistry or medical applications [1], due to the fact that it can steer the focus of ultrasonic energy to an arbitrary position by driving each element with a signal of the appropriate phase. What’s more, in detection and screening of medical osteoporosis, the introduction of 2.25 MHz to 5.0 MHz DFUS signal could reduce the error induced by soft tissues [2]. In ultrasound imaging, DFUS with large intervals at 2.25 MHz and 15 MHz could improve the resolution of the image [3]. In medical targeted therapy, 20 MHz and 30 MHz DFUS
signals can be more accurate and more efficient to complete the targeted drug delivery and release [4]. In high-intensity focused ultrasound (HIFU) treatment, the treatment efficiency could be improved by using the DFUS to expand the treatment area and increase the ablation rate [5]. Moreover, the impact of DFUS on the molecular weight distribution of corn meal hydrolysates and its mechanism were investigated in [6]. In [7], Kazuaki Ninomiya demonstrated the enhanced hydroxyl radical generation by combined use of DFUS and titanium dioxide nanoparticles as sonocatalyst. In [8], Bei Wang discussed the mechanism of DFUS assisted enzymolysis on the rapeseed protein by immobilized Alcalase. Therefore, how to characterize the performance of DFUS signals is important, and considerable efforts have been done in previous research [9]. Conventional dual-frequency acoustic sensors are mostly based on piezoelectric (PZT) materials as active elements [10,11]. In the literature [12], Axel Guiroy performed a nonlinear contrast agent imaging with a PZT-based transducer used for dual-frequency sensing detection in the experiment, but there were slightly obvious super-harmonic frequency responses in the spectrum, and the signal-to-noise ratio (SNR) of harmonic signal was not ideal. In the literature [13], K. Heath Martin described a new intravascular ultrasound imaging method using a piezoelectric transducer to visualize contrast flow in microvessels. Similarly, some bands with higher order cannot be recognized. The reason for the above results is that the acoustic sensors based on PZT materials working in a thickness mode have narrow frequency bandwidths [14], and the size of PZT element decreases while measurement of ultrasound frequency increases, resulting in a reduction of sensitivity. All these shortages limit PZT methods in wide-bandwidth high frequency ultrasound measurements. Furthermore, there are also some dual frequency transducers that cover huge frequency differences. For example, the design of [15] utilizes a dual-frequency (6.5 MHz/ 30MHz) transducer arrangement for exciting microbubbles at low frequency and detecting their broadband harmonics at high frequencies, minimizing detected tissue backscatter. And in [16], the dual frequency transducer prototype with a 2 MHz 1-3 composite transmitter and a 14 MHz receiver was fabricated and characterized. Similarly in [17], multiple dual frequency transducer with different transmission frequencies (6.5 and 5 MHz) was developed and evaluated and all transducer structures were constructed with the 30 MHz high frequency reception element in front of the low frequency transmission element. In recent research, optical fiber sensors have been explored as an alternative for acoustic detection [18]. With the small diameter of fiber (~125 μm), such optical system is particularly well suited for the detection of high-frequency medical ultrasound [19]. Recently the fiber optic hydrophone [20,21] with a dual polarization DBR fiber laser as a sensing element [22] shows attractive advantages in high frequency ultrasound measurement [23]. And in previous research, DBR fiber laser acoustic sensor has shown a flat response over a large range of ultrasound frequencies, which indicates it’s suitable for multi-frequency ultrasound measurement [24,25]. In this paper, we evaluated the performance of DFUS measurement based on DBR fiber laser sensor. We theoretically analyze the relationship between the DFUS pressure and the output of DBR sensor signals under the condition of different modulation depths. And then we carry out experiments which show the variation of DBR sensor outputs in ultrasonic far and near field. The results demonstrate the performance of DFUS response of DBR fiber laser sensor with different test distances and different ultrasonic amplitudes. 2. Device Principles and Theoretical Analysis DBR fiber laser operates in two orthogonal eigen-polarization modes due to the fiber birefringence caused by fiber fabrication and ultraviolet (UV) irradiation during fiber Bragg grating (FBG) inscription [26-28]. When the laser output was monitored with a photodetector (PD) and radio frequency (RF) spectrum analyzer [29], a polarization beat signal was generated by two orthogonal modes, which can be expressed as: 2 v b B (1) n0 0 where
v is the speed of light in vacuum, B nx ny is the modal birefringence, n0 nx ny is the average index of
the fiber [30], and 0 2n0 x y is the Bragg wavelength of the fiber grating. When the DBR fiber laser sensor is subjected to an acoustic field, the acoustic pressure breaks the near degeneracy by causing an unequal variation in the phase velocity of each eigenmode [31]. The electric field of the fiber laser output can be represented as follows: Ei Eb cos[bt (t)] Eb cos[bt k f (t )dt ]
(2)
where Ei is the electric field of the fiber laser output received by the photodetector, Eb is the amplitude of Ei , k f is the acoustic modulation sensitivity which depends on the strain tensor along x ' and y ' axes of the optical fiber, and
(t ) is the instantaneous frequency change of the beat frequency. In DFUS field, it can be written as: (t) 1 (t) 2 (t) U1 cos(1 t 1 ) U2 cos(2 t 2 ) (3)
where Ui , i , i , i 1, 2 are the amplitude, frequency and phase constant of ultrasound wave respectively. So Equation (2) can be written as: Ei Eb cos[b t k f
1 (t ) 2 (t )dt ]
Eb cos[b t k f
U1 cos(1t 1 ) U 2 cos(2t 2 )dt ] (4)
Eb cos b t M f 1sin 1t M f 2 sin 2t
where M U k / is the modulation index. In view of the distance between the transmitting surface of source transducer and the point on the acoustic axis where the DBR acoustic sensor is placed, there are at most four cases corresponding to the intensity of acoustic pressure, which are generally described as being divided into zones, the near field or Fresnel zone where interference effects occur, and the far field or Fraunhofer zone where interference effects are absent [32], as shown in Figure 1. The point at which the last maximum occurs is designated as the N point, shown in the 3rd zone of Figure 2, which separates the near and far field and is written as:
N R2 f / c
(5)
where R is radius of the transducer element size,
f is the ultrasound frequency, and c is ultrasound velocity in medium.
The on-axis ultrasound intensity response in the near field of transducers is characterized by a series of on-axis maxima and minima, as shown in the 4th zone in Figure 2. In the far field, the on-axis sound pressure drops to zero with increasing distance, as shown in the 1st and 2nd zone in Figure 2. The first case mostly happened in the 1st zone which is ultra-far ultrasound field, and DFUS pressure here is extralow. The whole ultrasound field could be seen as a low index modulation, which means M f 1sin(1t ) M f 2 sin(2t ) / 6 and the Equation (4) could be written as: 2
Ei Eb [cos(b t ) (M fi / 2) cos(b i )t (M fi / 2) cos(b i )t ] (6) i 1
In this case, DBR fiber sensor could give two obvious sidebands on each side of laser beat frequency, which indicates the DFUS signals. The second case is in 2nd zone where the DFUS pressure is relative-low and ultrasound field of each frequency could be seen as a separately low modulation, which means M f 1sin(1t ) / 6 and M f 2 sin(2t ) / 6 and the Equation (4) could be written as: 2 M fi cos b i t cos b i t Ei Eb {cos(b t ) 2 i 1
M f1 M f 2
[cos b 1 2 t cos b 1 2 t 4 cos b 1 2 t cos b 1 2 t ]}
(7)
In this case, in addition to DFUS signals, the sum and difference frequencies could also been obtained from the Equation (7). The third case is that the DBR fiber sensor was placed near the N point in 3rd zone where the ultrasound pressure of each dual-frequency is extra-high, and ultrasound field of each frequency could be seen as a strong modulation, which means M f 1sin(1t ) / 6 and M f 2 sin(2t ) / 6 . In order to facilitate the analysis, the Equation (4) are derived in complex numbers form: Ei Eb e jbt e
jM f 1 sin(1t ) jM f 2 sin(2t )
e
.
Based on Euler formula and trigonometric series with Bessel function, that is to say, e
jM f 1 sin(1t )
n
with the condition of M f 1sin(1t ) / 6 and e
jM f 2 sin(2t )
n
J n (m f 1 )e jn1t
J n (m f 2 )e jn2t with the condition of M f 2 sin(2t ) / 6 ,
there is
Ei Eb
n m
J n (m f 1 ) J m (m f 2 ) cos(b t n1t m2t ) (8)
The fourth case is in 4th zone where the ultrasound pressure is characterized by a series of maxima and minima in near field region, so one frequency ultrasound pressure is high and the other is low for a same plane, which means that
only one followed theoretical models of the Equation (6) or the Equation (7). Supposed here only M f 1sin(1t ) / 6 , the Equation (4) could be written as: Ei Eb
n
J n (m f 1 ){cos(b n1 )t
mf 2 2
[cos(b n1 2 )t
(9)
cos(b n1 2 )t ]}
In the third and fourth cases, there would be ultrasound signal frequencies, their harmonic generations, and their sum and difference frequencies mixing on both side of laser beat frequency. 3. Experiments and Results The experimental apparatus is shown in Figure 3. The DBR fiber laser was engraved in a short Er 3+-doped fiber (OFS LRL), which has peak absorption of 30 dB/m at 1530 nm and a mode field diameter of 5.2 μm. The FBGs were manufactured using phase-mask grating-writing technique with 193 nm ArF excimer laser. The lengths of the two FBGs are 22 mm with a 30 dB reflectivity and 8 mm with a 20 dB reflectivity, and are separated by a nominal cavity length of 10 mm. The pump light was emitted from a 980 nm laser diode with the maximum power of 450 mW, and propagated into the DBR fiber laser through a 980/1550 nm wavelength division multiplexer (WDM). The 1554.19 nm light wave signal was emitted from the low reflectivity FBG end, and then directed to the 1550 nm output port of WDM, as shown in Figure 3. The optical isolator (ISO) was used to reduce any unwanted reflection back to the fiber laser. By adjusting polarization states using polarization controller (PC) with a polarizer, two orthogonal polarization lasing modes could be tuned to the same direction, and then a beat signal of 462 MHz in radio frequency band was generated by photodetector (PD, Thorlab DET01CFC InGaAs Biased Detector). The received signal spectrum of DBR fiber sensor was finally recorded by an RF-spectrum analyzer (ESA, ROHDE&SCHWARZ FSV SIGNAL ANALYZER). The plane acoustic field was generated by two immersion transducers, which were driven at their central frequency output by a function generator. And the acoustic pressure is generated by 3 MHz ultrasound transducer whose nearfield distance is about 18 mm, and by 5 MHz ultrasound transducer whose near-field distance is about 21 mm. The whole device was placed inside a plastic tank filled with water. The DBR fiber laser fixed on a plastic frame was perpendicular to the ultrasound propagation direction, whose position could be adjusted along the acoustic axis, as shown in Figure 3. In experiments regarding the test distance of transducers, the DFUS response of DBR acoustic sensor with the driving voltage of 15 V can be measured on either a near-field or far-field range with appropriate implementation. In each measurement, every source transducer was set on respectively to make a separate detection in responding to single-frequency ultrasound field. And then, both two source transducers were open, and a DFUS field was generated and measured. When the DBR fiber laser acoustic sensor was placed at 40 mm from the surface of source transducers, which is ultra-far field to dual-frequency of 3 MHz and 5 MHz, Figure 4 shows the DFUS modulated spectra of DBR fiber sensor output, and the inserts of Figure 4 show its each single-frequency response. It can be seen from the inserts that 3 MHz and 5 MHz sidebands appear with a carrier-to-sideband ratio greater than 25 dB, due to the weak index modulation by low ultrasound pressure at far field. In such a condition, DFUS response of DBR sensor was indicated by two obvious sideband signals, which was mathematically described in the first case of previous section. When the DBR fiber laser acoustic sensor was moved closer to source transducers with a distance of 30 mm along the acoustic axis, the DBR sensor got a strong response to each ultrasound frequency field with about 15 dB carrier-tosideband ratio as the ultrasound pressure gradually became strong, shown in inserts of Figure 5. With the increasing pressure of ultrasound field, the sum and difference frequencies of 8 MHz and 2 MHz, generated by ultrasound firstorder sideband signals, have been obtained, as shown in Figure 5. Thereby the influences on ultrasound signals demodulation by emergence of unwanted signals have emerged, as expected from the Equation (7), the second case described in theoretical section. The DBR fiber laser acoustic sensor was moved closer to source transducers with a distance of 20 mm afterwards, which is around near field point N to both frequency ultrasound transducers. In this area, the ultrasound frequency responses of DBR were characterized with high modulations, which lead to high-complexity sideband signals including harmonic generations, their sum and difference frequencies and other unexpected signals with different frequencies, as shown in Figure 6, It can be seen that when the response of DBR fiber sensor to single frequency was characterized by harmonics, the responds of DBR fiber sensor to dual-frequency were submerged by the distortions generated by their harmonics and sum (or the difference) frequencies, which was described mathematically in third case of theoretical section. When the DBR fiber laser acoustic sensor was moved closer to source transducer with a distance of 10 mm, which is the near field to both 5 MHz and 3 MHz ultrasound transducers. In this area, the acoustic pressure intensity is unstable
with a series of maxima and minima. So in this experiment, the single-frequency response of DBR were characterized with a low modulation for 3 MHz transducer and a high modulation for 5 MHz transducer, as shown in inserts of Figure 7, which was described mathematically in the forth case of theoretical section. It can be seen in Figure 6 and Figure 7 that high pressure and unstable acoustic field could lead to high-complexity sideband signals, and demodulation of dual-frequency signals by DBR fiber sensor was more difficult in near field. It is even more serious when the DBR fiber sensor was placed at near field point N. In Figure 8, it demonstrated DFUS responses of DBR fiber laser sensor to 3 MHz and 5 MHz in different testing distances with the driving voltage of 15 V. It can be seen that DBR fiber laser could give a one-to-one frequency point to ultrasound signals in far field distance. But with measurement distance being shortened, the signals of ultrasonic responses became relatively complicated due to the measuring mechanism of DBR fiber laser sensor. In addition to the distance from the transducer, the sensing characteristics of the DBR acoustic sensor are also affected by the amplitude of the DFUS. In the far field region, the distance between the DBR acoustic sensor and the ultrasound transducer was fixed at 40 mm. The driving voltage of 3 MHz and 5 MHz dual-transducers was changed at 5 V, 10 V, 15 V and 20 V to adjust the ultrasound amplitude, respectively. As shown in Figure 9, the only obvious 3 MHz and 5 MHz sideband signals are highlighted in spectrum in far field region with the SNR of sidebands ranging from 11.5 dB to 23.5 dB when the driving voltage was switched from 5 V to 20 V, which seems to be an ideal measurement area. And the intensity of sidebands is also gradually decreasing with the reduction of the driving voltage. Similarly in Figure 10, the driving voltage of ultrasonic probe was increased from 5 V to 20 V when the test distance is 30 mm. As shown in Figure 10, the amplitude of sideband signals (2 MHz, 3 MHz, 5 MHz and 8 MHz) was greater with higher driving voltage. In particularly, it is obvious that the intensity of the 8 MHz sideband signal was increased from -100 dBm to nearly -80 dBm, which proved that the proposed DBR fiber acoustic laser had a good ultrasonic sensing performance, but the higher amplitude would make the spectrum response more complex. When the distance between the fiber laser and probe reaches 20mm, the sensing spectrum schematic of fiber laser against the changed driving voltage was shown in Figure 11. Under this circumstance, the amplitude of high-complexity sideband signals still continued to rise as the supplied voltage became stronger. And the 5 MHz signal was almost submerged in their harmonics, sum and difference frequencies when the driving voltage was 20 V. In near field region, the distance between the DBR acoustic sensor and the ultrasound transducer was fixed at 10 mm. The driving voltage of 3 MHz and 5 MHz dual-transducers was also changed at 5 V, 10 V, 15 V and 20 V as a comparison. As shown in Figure 12, the 3 MHz and 5 MHz sideband signals were submerged in unwanted harmonic signals, due to the complex distribution of near-field ultrasonic intensity. And it became more severe when the ultrasound amplitude increased. Therefore, the dual-frequency ultrasonic measurement spectrum diagram with lowamplitude signal is clearer and easier to identify the original frequency of the probe than that with high-amplitude signal. 4. Conclusions In this paper, the performance of DFUS measurement based on DBR fiber laser hydrophone was presented. The DBR fiber laser acoustic sensor demonstrated a unique multi-frequency sensing ability by means of beat frequency modulation, which makes it a potential candidate for DFUS sensors. What’s more, the amplitude of the DFUS has a significant impact on the magnitudes of the sideband signals. In summary, DBR fiber laser sensor exhibits the acoustic field dependence; it can simply achieve dual-frequency signals in far field without complex data analysis. And in near field ultrasound measurement, the dual-frequency signal responses present complexity, but it could be resolved by means of subsequent data analysis. The relevant studies are in progress. Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 61205075, No.11304122 and No. 61575143).
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Biographies Chengang Lyu received the B.Sc. degree in Optoelectronic Technology from Tianjin University, China in 2001 and the M.Sc. degree and Ph.D degree in physical electronics from Tianjin University, China in 2004 and 2007 respectively. He is currently an associate professor in the School of Electronic Information Engineering of Tianjin University, majoring in optical sensor and system and optical fiber sensor technology. Since 2011, Dr Lyu has been serving as a secretary of "the IOT and related professional teaching steering committee" and possessing an evaluation working for “National Natural Science Foundation of China”. Shuai Zhang received the B.Sc. degree in photoelectric information engineering from North University of China in 2015. He is pursuing the M.Sc. degree in Electronics and communication engineering from Tianjin University, China. His research interests include fiber optic sensing, wireless sensing system and the application of optical fiber in the field of Industrial Internet of Things. Gan Fang received the B.Sc. degree in school of information engineering from Nanchang University of China in 2015. He is pursuing the M.Sc. degree in Electronics and communication engineering from Tianjin University, China. His research direction is fiber optic sensing during graduate studies. He likes internet and apply himself to C++ research and development.
Figure 12. The amplitude sensing behavior of the DBR acoustic sensor in the near field region (test distance is 10 mm) with the transducer driving voltage of (a) 5 V, (b) 10 V, (c) 15 V and (d) 20 V.
S0924-4247(17)30705-7 http://dx.doi.org/10.1016/j.sna.2017.09.017 SNA 10325
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
22-4-2017 28-8-2017 11-9-2017
Please cite this article as: Chengang Lyu, Shuai Zhang, Gan Fang, Jin Jie, Xugeng Zhang, Chuang Wu, Performance of Dual-frequency Ultrasound Measurement Based on DBR Fiber Laser Hydrophone, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.09.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Performance of Dual-frequency Ultrasound Measurement Based on DBR Fiber Laser Hydrophone Chengang Lyu 1,*, Shuai Zhang 1, Gan Fang 1, Jin Jie 1, Xugeng Zhang 1 and Chuang Wu 2 1 2
School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China; [email protected] Institute of Photonics Technology, Jinan University, Guangzhou 510632, China; [email protected]
* Correspondence: [email protected]; Tel.: +86-138-2055-3633
Highlights
We report a detailed investigation of potential of DBR fiber lasers as a sensor designed for dual-frequency ultrasound measurement.
We analyze in detail theories which demonstrate the ability of DBR laser to measure DFUS considering the effects of ultrasound far and near field.
We conduct a series of experiments compared with the theory to support the results.
Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different distances and different DFUS driving voltage.
ARTICLE INFO
ABSTRACT The ability of distributed Bragg reflector (DBR) fiber laser sensor in dual-frequency ultrasound (DFUS) measurement has been demonstrated. The influences of DFUS pressure on fiber grating laser sensor were theoretically analyzed, considering the effects of relative distance of the ultrasound field and the amplitude of the DFUS on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasound signals at 3 MHz and 5 MHz, and the test distance and driving voltage were set respectively according to the ultrasound frequency. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different distances (40 mm to 10 mm) and different DFUS driving voltage (5 V to 20 V). The results demonstrate that the DBR fiber grating laser acoustic sensor has a multi-frequency ultrasound recognizable ability, offering a potential for ultrasound medicine and biology
Keywords:Fiber measurements DBR Optical fiber sensors Dual-frequency ultrasound
1. Introduction DFUS at megahertz has been widely used in some sonochemistry or medical applications [1], due to the fact that it can steer the focus of ultrasonic energy to an arbitrary position by driving each element with a signal of the appropriate phase. What’s more, in detection and screening of medical osteoporosis, the introduction of 2.25 MHz to 5.0 MHz DFUS signal could reduce the error induced by soft tissues [2]. In ultrasound imaging, DFUS with large intervals at 2.25 MHz and 15 MHz could improve the resolution of the image [3]. In medical targeted therapy, 20 MHz and 30 MHz DFUS
signals can be more accurate and more efficient to complete the targeted drug delivery and release [4]. In high-intensity focused ultrasound (HIFU) treatment, the treatment efficiency could be improved by using the DFUS to expand the treatment area and increase the ablation rate [5]. Moreover, the impact of DFUS on the molecular weight distribution of corn meal hydrolysates and its mechanism were investigated in [6]. In [7], Kazuaki Ninomiya demonstrated the enhanced hydroxyl radical generation by combined use of DFUS and titanium dioxide nanoparticles as sonocatalyst. In [8], Bei Wang discussed the mechanism of DFUS assisted enzymolysis on the rapeseed protein by immobilized Alcalase. Therefore, how to characterize the performance of DFUS signals is important, and considerable efforts have been done in previous research [9]. Conventional dual-frequency acoustic sensors are mostly based on piezoelectric (PZT) materials as active elements [10,11]. In the literature [12], Axel Guiroy performed a nonlinear contrast agent imaging with a PZT-based transducer used for dual-frequency sensing detection in the experiment, but there were slightly obvious super-harmonic frequency responses in the spectrum, and the signal-to-noise ratio (SNR) of harmonic signal was not ideal. In the literature [13], K. Heath Martin described a new intravascular ultrasound imaging method using a piezoelectric transducer to visualize contrast flow in microvessels. Similarly, some bands with higher order cannot be recognized. The reason for the above results is that the acoustic sensors based on PZT materials working in a thickness mode have narrow frequency bandwidths [14], and the size of PZT element decreases while measurement of ultrasound frequency increases, resulting in a reduction of sensitivity. All these shortages limit PZT methods in wide-bandwidth high frequency ultrasound measurements. Furthermore, there are also some dual frequency transducers that cover huge frequency differences. For example, the design of [15] utilizes a dual-frequency (6.5 MHz/ 30MHz) transducer arrangement for exciting microbubbles at low frequency and detecting their broadband harmonics at high frequencies, minimizing detected tissue backscatter. And in [16], the dual frequency transducer prototype with a 2 MHz 1-3 composite transmitter and a 14 MHz receiver was fabricated and characterized. Similarly in [17], multiple dual frequency transducer with different transmission frequencies (6.5 and 5 MHz) was developed and evaluated and all transducer structures were constructed with the 30 MHz high frequency reception element in front of the low frequency transmission element. In recent research, optical fiber sensors have been explored as an alternative for acoustic detection [18]. With the small diameter of fiber (~125 μm), such optical system is particularly well suited for the detection of high-frequency medical ultrasound [19]. Recently the fiber optic hydrophone [20,21] with a dual polarization DBR fiber laser as a sensing element [22] shows attractive advantages in high frequency ultrasound measurement [23]. And in previous research, DBR fiber laser acoustic sensor has shown a flat response over a large range of ultrasound frequencies, which indicates it’s suitable for multi-frequency ultrasound measurement [24,25]. In this paper, we evaluated the performance of DFUS measurement based on DBR fiber laser sensor. We theoretically analyze the relationship between the DFUS pressure and the output of DBR sensor signals under the condition of different modulation depths. And then we carry out experiments which show the variation of DBR sensor outputs in ultrasonic far and near field. The results demonstrate the performance of DFUS response of DBR fiber laser sensor with different test distances and different ultrasonic amplitudes. 2. Device Principles and Theoretical Analysis DBR fiber laser operates in two orthogonal eigen-polarization modes due to the fiber birefringence caused by fiber fabrication and ultraviolet (UV) irradiation during fiber Bragg grating (FBG) inscription [26-28]. When the laser output was monitored with a photodetector (PD) and radio frequency (RF) spectrum analyzer [29], a polarization beat signal was generated by two orthogonal modes, which can be expressed as: 2 v b B (1) n0 0 where
v is the speed of light in vacuum, B nx ny is the modal birefringence, n0 nx ny is the average index of
the fiber [30], and 0 2n0 x y is the Bragg wavelength of the fiber grating. When the DBR fiber laser sensor is subjected to an acoustic field, the acoustic pressure breaks the near degeneracy by causing an unequal variation in the phase velocity of each eigenmode [31]. The electric field of the fiber laser output can be represented as follows: Ei Eb cos[bt (t)] Eb cos[bt k f (t )dt ]
(2)
where Ei is the electric field of the fiber laser output received by the photodetector, Eb is the amplitude of Ei , k f is the acoustic modulation sensitivity which depends on the strain tensor along x ' and y ' axes of the optical fiber, and
(t ) is the instantaneous frequency change of the beat frequency. In DFUS field, it can be written as: (t) 1 (t) 2 (t) U1 cos(1 t 1 ) U2 cos(2 t 2 ) (3)
where Ui , i , i , i 1, 2 are the amplitude, frequency and phase constant of ultrasound wave respectively. So Equation (2) can be written as: Ei Eb cos[b t k f
1 (t ) 2 (t )dt ]
Eb cos[b t k f
U1 cos(1t 1 ) U 2 cos(2t 2 )dt ] (4)
Eb cos b t M f 1sin 1t M f 2 sin 2t
where M U k / is the modulation index. In view of the distance between the transmitting surface of source transducer and the point on the acoustic axis where the DBR acoustic sensor is placed, there are at most four cases corresponding to the intensity of acoustic pressure, which are generally described as being divided into zones, the near field or Fresnel zone where interference effects occur, and the far field or Fraunhofer zone where interference effects are absent [32], as shown in Figure 1. The point at which the last maximum occurs is designated as the N point, shown in the 3rd zone of Figure 2, which separates the near and far field and is written as:
N R2 f / c
(5)
where R is radius of the transducer element size,
f is the ultrasound frequency, and c is ultrasound velocity in medium.
The on-axis ultrasound intensity response in the near field of transducers is characterized by a series of on-axis maxima and minima, as shown in the 4th zone in Figure 2. In the far field, the on-axis sound pressure drops to zero with increasing distance, as shown in the 1st and 2nd zone in Figure 2. The first case mostly happened in the 1st zone which is ultra-far ultrasound field, and DFUS pressure here is extralow. The whole ultrasound field could be seen as a low index modulation, which means M f 1sin(1t ) M f 2 sin(2t ) / 6 and the Equation (4) could be written as: 2
Ei Eb [cos(b t ) (M fi / 2) cos(b i )t (M fi / 2) cos(b i )t ] (6) i 1
In this case, DBR fiber sensor could give two obvious sidebands on each side of laser beat frequency, which indicates the DFUS signals. The second case is in 2nd zone where the DFUS pressure is relative-low and ultrasound field of each frequency could be seen as a separately low modulation, which means M f 1sin(1t ) / 6 and M f 2 sin(2t ) / 6 and the Equation (4) could be written as: 2 M fi cos b i t cos b i t Ei Eb {cos(b t ) 2 i 1
M f1 M f 2
[cos b 1 2 t cos b 1 2 t 4 cos b 1 2 t cos b 1 2 t ]}
(7)
In this case, in addition to DFUS signals, the sum and difference frequencies could also been obtained from the Equation (7). The third case is that the DBR fiber sensor was placed near the N point in 3rd zone where the ultrasound pressure of each dual-frequency is extra-high, and ultrasound field of each frequency could be seen as a strong modulation, which means M f 1sin(1t ) / 6 and M f 2 sin(2t ) / 6 . In order to facilitate the analysis, the Equation (4) are derived in complex numbers form: Ei Eb e jbt e
jM f 1 sin(1t ) jM f 2 sin(2t )
e
.
Based on Euler formula and trigonometric series with Bessel function, that is to say, e
jM f 1 sin(1t )
n
with the condition of M f 1sin(1t ) / 6 and e
jM f 2 sin(2t )
n
J n (m f 1 )e jn1t
J n (m f 2 )e jn2t with the condition of M f 2 sin(2t ) / 6 ,
there is
Ei Eb
n m
J n (m f 1 ) J m (m f 2 ) cos(b t n1t m2t ) (8)
The fourth case is in 4th zone where the ultrasound pressure is characterized by a series of maxima and minima in near field region, so one frequency ultrasound pressure is high and the other is low for a same plane, which means that
only one followed theoretical models of the Equation (6) or the Equation (7). Supposed here only M f 1sin(1t ) / 6 , the Equation (4) could be written as: Ei Eb
n
J n (m f 1 ){cos(b n1 )t
mf 2 2
[cos(b n1 2 )t
(9)
cos(b n1 2 )t ]}
In the third and fourth cases, there would be ultrasound signal frequencies, their harmonic generations, and their sum and difference frequencies mixing on both side of laser beat frequency. 3. Experiments and Results The experimental apparatus is shown in Figure 3. The DBR fiber laser was engraved in a short Er 3+-doped fiber (OFS LRL), which has peak absorption of 30 dB/m at 1530 nm and a mode field diameter of 5.2 μm. The FBGs were manufactured using phase-mask grating-writing technique with 193 nm ArF excimer laser. The lengths of the two FBGs are 22 mm with a 30 dB reflectivity and 8 mm with a 20 dB reflectivity, and are separated by a nominal cavity length of 10 mm. The pump light was emitted from a 980 nm laser diode with the maximum power of 450 mW, and propagated into the DBR fiber laser through a 980/1550 nm wavelength division multiplexer (WDM). The 1554.19 nm light wave signal was emitted from the low reflectivity FBG end, and then directed to the 1550 nm output port of WDM, as shown in Figure 3. The optical isolator (ISO) was used to reduce any unwanted reflection back to the fiber laser. By adjusting polarization states using polarization controller (PC) with a polarizer, two orthogonal polarization lasing modes could be tuned to the same direction, and then a beat signal of 462 MHz in radio frequency band was generated by photodetector (PD, Thorlab DET01CFC InGaAs Biased Detector). The received signal spectrum of DBR fiber sensor was finally recorded by an RF-spectrum analyzer (ESA, ROHDE&SCHWARZ FSV SIGNAL ANALYZER). The plane acoustic field was generated by two immersion transducers, which were driven at their central frequency output by a function generator. And the acoustic pressure is generated by 3 MHz ultrasound transducer whose nearfield distance is about 18 mm, and by 5 MHz ultrasound transducer whose near-field distance is about 21 mm. The whole device was placed inside a plastic tank filled with water. The DBR fiber laser fixed on a plastic frame was perpendicular to the ultrasound propagation direction, whose position could be adjusted along the acoustic axis, as shown in Figure 3. In experiments regarding the test distance of transducers, the DFUS response of DBR acoustic sensor with the driving voltage of 15 V can be measured on either a near-field or far-field range with appropriate implementation. In each measurement, every source transducer was set on respectively to make a separate detection in responding to single-frequency ultrasound field. And then, both two source transducers were open, and a DFUS field was generated and measured. When the DBR fiber laser acoustic sensor was placed at 40 mm from the surface of source transducers, which is ultra-far field to dual-frequency of 3 MHz and 5 MHz, Figure 4 shows the DFUS modulated spectra of DBR fiber sensor output, and the inserts of Figure 4 show its each single-frequency response. It can be seen from the inserts that 3 MHz and 5 MHz sidebands appear with a carrier-to-sideband ratio greater than 25 dB, due to the weak index modulation by low ultrasound pressure at far field. In such a condition, DFUS response of DBR sensor was indicated by two obvious sideband signals, which was mathematically described in the first case of previous section. When the DBR fiber laser acoustic sensor was moved closer to source transducers with a distance of 30 mm along the acoustic axis, the DBR sensor got a strong response to each ultrasound frequency field with about 15 dB carrier-tosideband ratio as the ultrasound pressure gradually became strong, shown in inserts of Figure 5. With the increasing pressure of ultrasound field, the sum and difference frequencies of 8 MHz and 2 MHz, generated by ultrasound firstorder sideband signals, have been obtained, as shown in Figure 5. Thereby the influences on ultrasound signals demodulation by emergence of unwanted signals have emerged, as expected from the Equation (7), the second case described in theoretical section. The DBR fiber laser acoustic sensor was moved closer to source transducers with a distance of 20 mm afterwards, which is around near field point N to both frequency ultrasound transducers. In this area, the ultrasound frequency responses of DBR were characterized with high modulations, which lead to high-complexity sideband signals including harmonic generations, their sum and difference frequencies and other unexpected signals with different frequencies, as shown in Figure 6, It can be seen that when the response of DBR fiber sensor to single frequency was characterized by harmonics, the responds of DBR fiber sensor to dual-frequency were submerged by the distortions generated by their harmonics and sum (or the difference) frequencies, which was described mathematically in third case of theoretical section. When the DBR fiber laser acoustic sensor was moved closer to source transducer with a distance of 10 mm, which is the near field to both 5 MHz and 3 MHz ultrasound transducers. In this area, the acoustic pressure intensity is unstable
with a series of maxima and minima. So in this experiment, the single-frequency response of DBR were characterized with a low modulation for 3 MHz transducer and a high modulation for 5 MHz transducer, as shown in inserts of Figure 7, which was described mathematically in the forth case of theoretical section. It can be seen in Figure 6 and Figure 7 that high pressure and unstable acoustic field could lead to high-complexity sideband signals, and demodulation of dual-frequency signals by DBR fiber sensor was more difficult in near field. It is even more serious when the DBR fiber sensor was placed at near field point N. In Figure 8, it demonstrated DFUS responses of DBR fiber laser sensor to 3 MHz and 5 MHz in different testing distances with the driving voltage of 15 V. It can be seen that DBR fiber laser could give a one-to-one frequency point to ultrasound signals in far field distance. But with measurement distance being shortened, the signals of ultrasonic responses became relatively complicated due to the measuring mechanism of DBR fiber laser sensor. In addition to the distance from the transducer, the sensing characteristics of the DBR acoustic sensor are also affected by the amplitude of the DFUS. In the far field region, the distance between the DBR acoustic sensor and the ultrasound transducer was fixed at 40 mm. The driving voltage of 3 MHz and 5 MHz dual-transducers was changed at 5 V, 10 V, 15 V and 20 V to adjust the ultrasound amplitude, respectively. As shown in Figure 9, the only obvious 3 MHz and 5 MHz sideband signals are highlighted in spectrum in far field region with the SNR of sidebands ranging from 11.5 dB to 23.5 dB when the driving voltage was switched from 5 V to 20 V, which seems to be an ideal measurement area. And the intensity of sidebands is also gradually decreasing with the reduction of the driving voltage. Similarly in Figure 10, the driving voltage of ultrasonic probe was increased from 5 V to 20 V when the test distance is 30 mm. As shown in Figure 10, the amplitude of sideband signals (2 MHz, 3 MHz, 5 MHz and 8 MHz) was greater with higher driving voltage. In particularly, it is obvious that the intensity of the 8 MHz sideband signal was increased from -100 dBm to nearly -80 dBm, which proved that the proposed DBR fiber acoustic laser had a good ultrasonic sensing performance, but the higher amplitude would make the spectrum response more complex. When the distance between the fiber laser and probe reaches 20mm, the sensing spectrum schematic of fiber laser against the changed driving voltage was shown in Figure 11. Under this circumstance, the amplitude of high-complexity sideband signals still continued to rise as the supplied voltage became stronger. And the 5 MHz signal was almost submerged in their harmonics, sum and difference frequencies when the driving voltage was 20 V. In near field region, the distance between the DBR acoustic sensor and the ultrasound transducer was fixed at 10 mm. The driving voltage of 3 MHz and 5 MHz dual-transducers was also changed at 5 V, 10 V, 15 V and 20 V as a comparison. As shown in Figure 12, the 3 MHz and 5 MHz sideband signals were submerged in unwanted harmonic signals, due to the complex distribution of near-field ultrasonic intensity. And it became more severe when the ultrasound amplitude increased. Therefore, the dual-frequency ultrasonic measurement spectrum diagram with lowamplitude signal is clearer and easier to identify the original frequency of the probe than that with high-amplitude signal. 4. Conclusions In this paper, the performance of DFUS measurement based on DBR fiber laser hydrophone was presented. The DBR fiber laser acoustic sensor demonstrated a unique multi-frequency sensing ability by means of beat frequency modulation, which makes it a potential candidate for DFUS sensors. What’s more, the amplitude of the DFUS has a significant impact on the magnitudes of the sideband signals. In summary, DBR fiber laser sensor exhibits the acoustic field dependence; it can simply achieve dual-frequency signals in far field without complex data analysis. And in near field ultrasound measurement, the dual-frequency signal responses present complexity, but it could be resolved by means of subsequent data analysis. The relevant studies are in progress. Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 61205075, No.11304122 and No. 61575143).
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Biographies Chengang Lyu received the B.Sc. degree in Optoelectronic Technology from Tianjin University, China in 2001 and the M.Sc. degree and Ph.D degree in physical electronics from Tianjin University, China in 2004 and 2007 respectively. He is currently an associate professor in the School of Electronic Information Engineering of Tianjin University, majoring in optical sensor and system and optical fiber sensor technology. Since 2011, Dr Lyu has been serving as a secretary of "the IOT and related professional teaching steering committee" and possessing an evaluation working for “National Natural Science Foundation of China”. Shuai Zhang received the B.Sc. degree in photoelectric information engineering from North University of China in 2015. He is pursuing the M.Sc. degree in Electronics and communication engineering from Tianjin University, China. His research interests include fiber optic sensing, wireless sensing system and the application of optical fiber in the field of Industrial Internet of Things. Gan Fang received the B.Sc. degree in school of information engineering from Nanchang University of China in 2015. He is pursuing the M.Sc. degree in Electronics and communication engineering from Tianjin University, China. His research direction is fiber optic sensing during graduate studies. He likes internet and apply himself to C++ research and development.
Figure 12. The amplitude sensing behavior of the DBR acoustic sensor in the near field region (test distance is 10 mm) with the transducer driving voltage of (a) 5 V, (b) 10 V, (c) 15 V and (d) 20 V.