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Applied Optics

Applied Optics

APPLICATIONS-CENTERED RESEARCH IN OPTICS

  • Editor: Joseph N. Mait
  • Vol. 51, Iss. 8 — Mar. 10, 2012
  • pp: 1061–1066
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Multiparameter measurement of absorbing liquid by time-resolved photoacoustics

Zuomin Zhao and Risto Myllylä  »View Author Affiliations


Applied Optics, Vol. 51, Issue 8, pp. 1061-1066 (2012)
http://dx.doi.org/10.1364/AO.51.001061


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Abstract

Measuring constituent concentrations of processing liquids provides highly useful data for industrial process control. Techniques that allow online measurement will greatly save resources and energy, making them highly attractive for enterprises. In this paper, we develop a technique based on time-resolved photoacoustics for simultaneously measuring the optical absorption coefficient, acoustic speed, and thermal-acoustic transformation coefficient of an absorbing liquid, using an experimental setup that merely employs a nanosecond pulsed laser with millijoule energy and a single piezoelectric transducer with a wide frequency bandwidth. As investigated samples, we use potassium chromate, glucose, and their mixing solutions. Experimental results show that the value of each parameter measured in a mixed solution is approximately equal to the sum value of the same parameter in the constituent solutions. This means that a simultaneous measurement of these parameters enables us to calculate two or three constituent concentrations in a mixed liquid, if the constituent substances differ clearly from one another in terms of their optical absorption, acoustic speed, or thermal-acoustic transformation properties.

© 2012 Optical Society of America

1. Introduction

Online determination of substances in industrial processing liquids can facilitate continuous processing, enable real-time information flow, save power energy, and improve material use, which are high on the list of priorities of industrial manufacture requiring concentration measurements and process control. Thanks to well-developed techniques and inexpensive devices, optical and ultrasound measurements have been successfully applied to these tasks. However, optical techniques usually measure optical parameters (absorption, scattering, or refractive index) and ultrasound determines acoustic speed or attenuation. Moreover, simultaneous monitoring of both optical and acoustic properties necessitates two or more sensing systems [1

1. M. Tormanen, J. Niemi, T. Lofqvist, and R. Myllyla, “Pulp consistency determined by a combination of optical and acoustical measurement techniques,” Meas. Sci. Technol. 17, 695–702 (2006). [CrossRef]

7

7. Y. Shen, Z. Lu, S. Spiers, H. MacKenzie, H. Ashton, J. Hannigan, S. Freeborn, and J. Lindberg, “Measurement of the optical absorption coefficient of a liquid by use of a time-resolved photoacoustic technique,” Appl. Opt. 39, 4007–4012 (2000). [CrossRef]

]. Naturally, the more parameters a technique can measure, the more information it reveals of the studied material. To that end, a hybrid technique was developed utilizing a single optical, electrical, and ultrasonic sensor to measure optical intensity, electrical impedance, and ultrasonic speed in order to determine ingredient concentrations in liquids [8

8. A. Kimoto and T. Kitajima, “An optical, electrical and ultrasonic layered single sensor for ingredient measurement in liquid,” Meas. Sci. Technol. 21, 035204 (2010).

]. This sensor comprises a pair of optical devices, a pair of electrodes, and a pair of ultrasonic devices.

We have previously proposed a hybrid technique utilizing a pair of laser-detectors to determine optical parameters and acoustic attenuation in turbid suspensions. This technique has been successfully applied to the measurement of fiber and fines consistencies in paper pulp [9

9. Z. Zhao and R. Myllylä, “Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattering light,” Appl. Opt. 44, 7845–7852 (2005). [CrossRef]

11

11. Z. Zhao, M. Törmänen, and R. Myllylä, “Backward-mode photoacoustic transducer for sensing optical scattering and ultrasonic attenuation: determining fraction consistency in pulp suspensions,” Meas. Sci. Technol. 21, 025105 (2010). [CrossRef]

]. In this study, we explore a new hybrid technique based on time-resolved photoacoustics (TR-PA), capable of measuring simultaneously the optical, acoustic, and thermal-acoustic transformation properties of an absorbing liquid. It is well known that the photoacoustic (PA) effect [12

12. A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).

] is the phenomenon of acoustic wave generation by a pulsed laser or a modulated light source. When a pulsed laser irradiates an absorbing material, the optical energy of the laser is absorbed by the absorbing material, where it transforms into heat energy, causing a thermal expansion of the illuminated region. Owing to inertial effects within the material, this type of modulated thermal expansion causes the illuminated region to extend and compress. As a result of this action, an acoustic wave is generated. The thus produced PA wave propagates out from the generation region (PA source) and can subsequently be detected by an ultrasonic transducer. Hence, there are three processes from PA generation to detection: optical energy absorption, thermal expansion, and acoustic propagation. With its custom-tailored design, our new apparatus, consisting only of a laser-detector pair, has the ability to simultaneously measure the optical absorption coefficient, thermal-acoustic transformation coefficient, and acoustic speed in an absorbing liquid. To the authors’ knowledge, no other publication exists that describes the simultaneous measurement of these three parameters.

The developed technique is particularly suitable for online measurement of constituent concentrations in liquid mixtures, as the three physical parameters (optical absorption, acoustic speed, and thermal-acoustic transformation) can be quickly and simultaneously determined by analyzing recorded time-resolved PA signals. No doubt, this technique will prove very useful for the chemical, food, and paper industries, as well as for environmental protection. For instance, it could be used in the pulping industry to determine lignin, sugar, and dissolved alkaline contents in cooking liquids. It may also be applied by the offshore oil industry to measure concentrations of dissolved oil, salts, and oil field chemicals in water.

2. Theoretical Aspects

When a pulsed laser beam irradiates a homogeneously absorbing medium, as shown in Fig. 1(a), the optical energy is absorbed in the illuminated region of the medium and then transformed into heat in the thermal de-exciting mechanism [12

12. A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).

]. Transient heat, produced by thermal expansion of the heated volume, will generate an initial stress. Since thermal diffusion from the heated volume can be ignored during nano- or microsecond laser pulses, the initial stress distribution equals that of the absorbed optical energy fulfilling the stress confinement condition [13

13. A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]

]. This stress is relaxed by generating an outward propagating acoustic wave. If the light beam diameter w is much larger than the optical penetration depth d (i.e., the thickness of the heated volume) in the absorbing medium, the PA source (i.e., the heated volume) is planar and the PA wave propagates along the depth direction of the medium. The acoustic pressure at the depth of z0 can be described by
p(t)E0αβv2Cpexp[α(z0vt)],(0<t<z0/v),
(1)
where E0 is the radiant exposure of the laser pulse at the medium surface, α the absorption coefficient, β the thermal expansion coefficient, Cp the specific heat, and v the acoustic speed in the medium.

Fig. 1. Principle of time-resolved photoacoustic measurement: (a) radiation and detection, (b) measurands of received signals.

Figure 1(b) shows a typical signal recorded by a wideband ultrasonic detector used to receive acoustic waves at depth z0. This signal can be described as S(t)=kRp(t), where k is a system constant and R is a function of detector response. Substituting Eq. (1) and doing the logarithm, the formula takes the form
logS(t)=αvt+(logSmαz0),
(2)
where Sm=kRαΓ is the amplitude of the signal which is not a function of time t, k is a system constant including the radiant exposure of the laser pulse at the medium surface, and Γ=βv2/Cp is the thermal-acoustic transformation coefficient (so-called Grüneisen coefficient). Equation (2) describes a linear relationship between the semilogarithm S and t, meaning that αv can be deduced from the linear slope. It is worth mentioning that αv is self-calibrated, as it is unaffected by the system constant, laser energy fluctuation and the response function of the detector. Acoustic speed v can be easily measured by z0/tm, where tm is the propagating time of the acoustic wave from the medium surface to the detector. Hence, the optical absorption coefficient α of the medium can be deduced after knowing v. To compute Γ from the signal amplitude Sm, requires prior determination of detector response and detecting system constant.

In general, the detector response R is a function of acoustic frequency. In the case of PA generation of a planar source, the center frequency of the generated PA wave can be calculated as facv/(2d)=αv/2 for an absorbing medium fulfilling the stress confinement condition [14

14. W. Sigrist, “Laser generation of acoustic waves in liquids and gases,” J. Appl. Phys. 60, R83–R121 (1986). [CrossRef]

]. The detector response can therefore be written as R(α), a function of optical absorption coefficient. In this study, very small quantities of an absorbing material (k2CrO4) were added into distilled water to make calibration solutions, allowing kR(α) of the detection system to be calibrated:
kR(α)=Sm,csαΓw.
(3)
In Eq. (3), Sm,cs is the signal amplitude produced in calibration solutions and Γw is the known Grüneisen coefficient of water. Finally, Γ of absorbing medium can be calculated by measuring signal amplitude, optical absorption coefficient, and calibration curve kR(α).

In summary, acoustic speed and optical absorption coefficient can be deduced directly from the time-of-flight of a time-resolved PA signal and its front profile. The thermal-acoustic transformation coefficient, on the other hand, can be calculated from signal amplitude, optical absorption coefficient, and the calibrated curve of the measurement system, determined by a suitable calibration liquid. If the transducer has a flat response in the studied acoustic frequency range, the relative value of the thermal-acoustic transformation coefficient can be deduced without calibration from optical absorption and signal amplitude.

3. Experimental Apparatus and Samples

Figure 2 shows the scheme of the experimental apparatus used for measuring liquid samples. A tunable laser (Opolette HE 355 II, OPOTEK Inc.) with a 5 ns pulse duration and a 355 nm pumping wavelength is used as the exciting source to produce PA waves. To satisfy the requirement of generating a planar PA source, optical lenses expand the laser beam to a diameter of 20 mm, before it passes through an optical aperture with a diameter of 15 mm. A rough glass slide is put in the optical path for further smoothing the optical energy distribution in the beam’s cross-section, before it enters a cuvette where the sample is placed. A plane of fused silica with a thickness of 5 mm functions as cuvette window to produce a limiting boundary condition for highly-efficient PA generation. Measuring 1 cm in thickness, the cuvette is capable of eliminating the effects of acoustic diffraction and attenuation. Aligned with the excited laser beam is a homemade detector, combining a piezoelectric PVDF transducer with a preamplifier. This detector is acoustically coupled to the back surface of the cuvette by a drop of silicon oil. The thickness of the PVDF foil is 52 µm, the gain of the preamplifier is 40 dB, and the 6 dB bandwidth is about 200 kHz—5 MHz. To prevent the laser beam from directly irradiating the detector surface when using a sample with low optical extinction, the back surface of the cuvette is covered by a reflecting layer of aluminum film with a thickness of 150 nm. A thermocouple is inserted into the cuvette for monitoring the temperature of the solution. In addition, a photodiode is used to monitor the energy fluctuation of the laser pulses. The output from the detector is connected to a two-channel digital card (PCI-1250, NI, sampling at 200MS/s) in a PC and, to compensate for the energy fluctuation effect of the laser pulses, the output is divided by the amplitude of the photodiode’s output. One hundred twenty-eight compensated signals are averaged by LabView software to decrease random noise in the detection channels and the amplitude and wave-shape of the averaged signals are recorded in the PC for postprocessing.

Fig. 2. Experimental scheme of time-resolved photoacoustic measurement.

As samples, this study used aqueous solutions of potassium chromate (K2CrO4) and mixtures of potassium chromate and glucose, because their properties are well known. Potassium chromate has a very high optical absorption coefficient at 355 nm (about 1000cm1 at a concentration of 35mg/cm3 [13

13. A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]

]). Moreover, potassium chromate solutions do not fluoresce and their optical properties are not altered by strong laser exposure. As a result, potassium chromate is suitable for the calibration of the PA measurement system and for the quantitative PA detection of absorption coefficients in solutions. Glucose, on the other hand, exhibits considerably weaker optical absorption characteristics at this wavelength. However, it will change the thermal parameters [15

15. H. MacKenzie, H. Ashton, S. Spiers, Y. Shen, S. Freeborn, J. Hannigan, J. Lindberg, and P. Rae, “Advances in photoacoustic noninvasive glucose testing,” Clinical Chem. 45, 1587–1595 (1999).

,16

16. Z. Zhao, “Pulsed photoacoustic techniques and glucose determination in human blood and tissue,” doctoral thesis (Acta Universitatis Ouluensis, Series C 169, 2002).

] and density of the solutions it is added to.

4. Experiments and Results

Before measurements, all liquid samples were prepared and put on the laboratory table at room temperature (22 °C) and the experimental apparatus was preheated for 30 minutes. The experiments included three steps. Potassium chromate solutions with concentrations from 0.02 wt. % to 0.1 wt. % were successively loaded into the cuvette. Then, to make glucose solutions, varying amounts of glucose (0.2 g, 0.4 g, 0.6 g, 0.8 g, and 1 g, respectively) were added into 10 ml of potassium chromate solutions with a concentration of 0.02%. Potassium chromate was used here as background absorber in the glucose solutions for PA generation, because glucose and distilled water have little optical absorption at the investigating wavelength—355 nm. Finally, five mixed solutions, each with a different concentration of potassium chromate and glucose, were measured. In every measurement, the signal amplitude was recorded ten times to calculate the standard deviation of the amplitude. The sample was then sucked out by a plastic pastette and the cuvette was washed twice using distilled water, before a new sample was loaded. When measured, the temperature difference, due to room temperature fluctuations during the experiment, was less than 0.3 °C in all samples.

Figure 3 shows a typical TR-PA signal recorded in a K2CrO4 solution. In agreement with Eq. (2), the inserted graph, drawn in the semilogarithm scale, illustrates good linearity at the signal front. As the K2CrO4 concentration increases, so does the recorded signal amplitude, while the arriving time of the amplitude remains almost unchanged. Figure 4 illustrates the measurement result. It demonstrates an optical absorption coefficient of 7.8±0.2cm1 and an acoustic speed of 1486+4m/s for a 0.02% K2CrO4 solution. Both of these values are nearly identical to those presented in [13

13. A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]

]. With increasing concentration, the absorption coefficient grows linearly with a huge slope rate, whereas acoustic speed is unaffected within the measurement error range. As the solutions were highly diluted, their Grüneisen coefficients should be identical and equal to that of water (Γw=0.12 at 22 °C). Based on this consideration, the kR(α) of the measurement system can be calculated for solutions with different concentrations (or optical absorption), as shown in Fig. 4. Doing a polynomial fit to these data allowed us to obtain a calibrated curve for computing the Grüneisen coefficient of the current measurement system:
kR(α)=0.008α4+0.086α30.3446α2+0.5791α+0.6558.

Fig. 3. Typical TR-PA signals recorded in K2CrO4 solutions with different concentrations (the inserted graph is shown in the semilogarithm axis).
Fig. 4. Experimental results of calibration solutions (standard deviations are smaller than the marks).

Figure 5 monitors how the three parameters changed with glucose concentration in aqueous solutions containing 0.02% K2CrO4. As seen, a 1% increase in glucose concentration increased the Grüneisen coefficient by about 3.4% and acoustic speed by 0.26%. These values are in good agreement with previously published data [7

7. Y. Shen, Z. Lu, S. Spiers, H. MacKenzie, H. Ashton, J. Hannigan, S. Freeborn, and J. Lindberg, “Measurement of the optical absorption coefficient of a liquid by use of a time-resolved photoacoustic technique,” Appl. Opt. 39, 4007–4012 (2000). [CrossRef]

,16

16. Z. Zhao, “Pulsed photoacoustic techniques and glucose determination in human blood and tissue,” doctoral thesis (Acta Universitatis Ouluensis, Series C 169, 2002).

]. The error bars for the relative change of the Grüneisen coefficient in response to concentration increase are mainly caused by fluctuations in laser pulse energy and the error in measuring the absorption coefficient. Yet another contributing factor is temperature; the Grüneisen coefficient of the solutions increased by 1.3% when room temperature increased by 0.3 °C [13

13. A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]

]. In addition, the absorption coefficient decreased about 0.72%, due to the diluting effect of glucose on potassium chromate.

Fig. 5. Relative change of the three parameters with glucose concentration (error bars for acoustic speed and absorption coefficient are smaller than the marks).

Figure 6 gives the experimental result of the three parameters obtained from five mixed solutions, consisting of k2CrO4 and glucose. Shown in square brackets under the horizontal axis are the used concentrations (for example, [0.091_9.1], indicating a mixed solution consisting of 0.091% of potassium chromate and 9.1% of glucose). It is easy to understand that the values of the three parameters increase with concentration and Table 1 illustrates the relative increase of each parameter. For comparison, the table also lists the results contributed by the sum of the K2CrO4 solution and glucose solution (calculated from the data presented in Figs. 4 and 5). It is evident that the result in Fig. 6 is practically identical to the sum of the results in Figs. 4 and 5.

Fig. 6. Measurement results of the three parameters obtained from mixed solutions containing K2CrO4 and glucose.

Table 1. Comparing Relative Changes in the Three Parameters Deduced from Fig. 6 and the Sum of Figs. 4 and 5

table-icon
View This Table

5. Discussion

Although the constituents of most processing liquids are known, determining the concentration of each constituent may require real-time measurements. This became our motivation to develop the multiparameter measurement technique presented in this study. Table 1 demonstrates that k2CrO4 and glucose concentrations in mixed solutions can be deduced by measuring three parameters, provided that the effects of K2CrO4 and glucose on these parameters are determined beforehand. In general, this technique is applicable to absorbing liquids consisting of three constituents in which no chemical reactions take place. The technique allows determining the concentration of each constituent, if they all contribute to optical absorption, acoustic speed, or thermal-acoustic transformation with a different weight factor.

Our experiment demonstrates that the absorption coefficient can be deduced with measurement error no larger than 3% (when measuring a sample three times), without calibration. It is worth mentioning that the result remains accurate, even if only a part of the signal’s front profile satisfies the exponential distribution. This offers the extra benefit that a high absorption coefficient can be correctly measured by a transducer whose frequency bandwidth does not cover the higher frequency components of the PA wave produced in the sample (of course, the deduced Grüneisen coefficient is not correct in this case). Dominating error sources in the absorption coefficient measurement are the following: 1) energy fluence in the cross-section of the laser beam is not strictly homogeneous, 2) the laser beam is probably not well collimated, and 3) the detector and the laser beam are not exactly aligned.

Acoustic speed was deduced by measuring the arrival time of a PA amplitude signal with a measuring error of up to 0.3%. Errors arise mainly from a slight difference in the transducer’s response to a wide range of PA frequencies and a possible change of acoustic coupling between the detector and cuvette wall when uploading and downloading samples during the measurement.

Finally, determining three parameters simultaneously in liquid necessitates using a purely absorbing liquid, because Eq. (1) only applies to purely absorbing materials. In the case of turbid samples, the TR-PA technique, as described above, has the capacity to measure the optical extinction coefficient (including the optical absorption and scattering coefficients) and acoustic speed, but fails at measuring the thermal-acoustic transformation coefficient, due to backscattering of the incident optical beam. However, if the laser is wavelength-tunable such that the optical absorption of the sample is much stronger at some wavelengths than optical scattering, the technique is still capable of correctly deducing the thermal-acoustic transformation coefficient. This is because the extinction coefficient approximately equals the absorption coefficient and backscattering can be ignored. Another benefit of applying a tunable laser is that it probably tunes the frequencies of the generated PA waves into the bandwidth of the detector. This not only expands the application range of the sample category, but also lowers detector requirements, since there is a trade-off between detector bandwidth and amplifying gain.

6. Conclusion

References

1.

M. Tormanen, J. Niemi, T. Lofqvist, and R. Myllyla, “Pulp consistency determined by a combination of optical and acoustical measurement techniques,” Meas. Sci. Technol. 17, 695–702 (2006). [CrossRef]

2.

Q. Zhu, D. Sullivan, B. Chance, and T. Dambro, “Combined ultrasound and near infrared diffused light imaging in a test object,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 46, 665–678 (1999).

3.

C. A. DiMarzio and T. W. Murray, “Medical imaging techniques combining light and ultrasound,” Subsurf. Sens. Technol. Appl. 4, 289–309 (2003).

4.

V. Cunningham and H. Lamela, “Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium,” Opt. Laser Technol. 42, 769–774 (2010). [CrossRef]

5.

J. Niemi, T. Lofqvist, and P. Gren, “On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques,” in Proceedings of IEEE Conference on Ultrasonic Symposium (IEEE, 2006), pp. 1797–1800.

6.

S. Y. Emelianova, S. R. Aglyamov, J. Shah, S. Sethuraman, W. G. Scott, R. Schmitt, Motamedi Massoud, A. Karpiouk, and A. A. Oraevsky, “Combined ultrasound, optoacoustic and elasticity imaging,” Proc. SPIE 5320, 101–112 (2004).

7.

Y. Shen, Z. Lu, S. Spiers, H. MacKenzie, H. Ashton, J. Hannigan, S. Freeborn, and J. Lindberg, “Measurement of the optical absorption coefficient of a liquid by use of a time-resolved photoacoustic technique,” Appl. Opt. 39, 4007–4012 (2000). [CrossRef]

8.

A. Kimoto and T. Kitajima, “An optical, electrical and ultrasonic layered single sensor for ingredient measurement in liquid,” Meas. Sci. Technol. 21, 035204 (2010).

9.

Z. Zhao and R. Myllylä, “Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattering light,” Appl. Opt. 44, 7845–7852 (2005). [CrossRef]

10.

Z. Zhao, M. Törmänen, and R. Myllylä, “A preliminary measurement of fibres and fines in pulp suspensions by the scattering photoacoustic technique,” Meas. Sci. Technol. 17, 128–134 (2006). [CrossRef]

11.

Z. Zhao, M. Törmänen, and R. Myllylä, “Backward-mode photoacoustic transducer for sensing optical scattering and ultrasonic attenuation: determining fraction consistency in pulp suspensions,” Meas. Sci. Technol. 21, 025105 (2010). [CrossRef]

12.

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).

13.

A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]

14.

W. Sigrist, “Laser generation of acoustic waves in liquids and gases,” J. Appl. Phys. 60, R83–R121 (1986). [CrossRef]

15.

H. MacKenzie, H. Ashton, S. Spiers, Y. Shen, S. Freeborn, J. Hannigan, J. Lindberg, and P. Rae, “Advances in photoacoustic noninvasive glucose testing,” Clinical Chem. 45, 1587–1595 (1999).

16.

Z. Zhao, “Pulsed photoacoustic techniques and glucose determination in human blood and tissue,” doctoral thesis (Acta Universitatis Ouluensis, Series C 169, 2002).

OCIS Codes
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(300.6250) Spectroscopy : Spectroscopy, condensed matter
(300.6430) Spectroscopy : Spectroscopy, photothermal

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: August 11, 2011
Revised Manuscript: November 5, 2011
Manuscript Accepted: November 8, 2011
Published: March 5, 2012

Virtual Issues
Vol. 7, Iss. 5 Virtual Journal for Biomedical Optics

Citation
Zuomin Zhao and Risto Myllylä, "Multiparameter measurement of absorbing liquid by time-resolved photoacoustics," Appl. Opt. 51, 1061-1066 (2012)
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-51-8-1061


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References

  1. M. Tormanen, J. Niemi, T. Lofqvist, and R. Myllyla, “Pulp consistency determined by a combination of optical and acoustical measurement techniques,” Meas. Sci. Technol. 17, 695–702 (2006). [CrossRef]
  2. Q. Zhu, D. Sullivan, B. Chance, and T. Dambro, “Combined ultrasound and near infrared diffused light imaging in a test object,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 46, 665–678 (1999).
  3. C. A. DiMarzio and T. W. Murray, “Medical imaging techniques combining light and ultrasound,” Subsurf. Sens. Technol. Appl. 4, 289–309 (2003).
  4. V. Cunningham and H. Lamela, “Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium,” Opt. Laser Technol. 42, 769–774 (2010). [CrossRef]
  5. J. Niemi, T. Lofqvist, and P. Gren, “On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques,” in Proceedings of IEEE Conference on Ultrasonic Symposium (IEEE, 2006), pp. 1797–1800.
  6. S. Y. Emelianova, S. R. Aglyamov, J. Shah, S. Sethuraman, W. G. Scott, R. Schmitt, Motamedi Massoud, A. Karpiouk, and A. A. Oraevsky, “Combined ultrasound, optoacoustic and elasticity imaging,” Proc. SPIE 5320, 101–112 (2004).
  7. Y. Shen, Z. Lu, S. Spiers, H. MacKenzie, H. Ashton, J. Hannigan, S. Freeborn, and J. Lindberg, “Measurement of the optical absorption coefficient of a liquid by use of a time-resolved photoacoustic technique,” Appl. Opt. 39, 4007–4012 (2000). [CrossRef]
  8. A. Kimoto and T. Kitajima, “An optical, electrical and ultrasonic layered single sensor for ingredient measurement in liquid,” Meas. Sci. Technol. 21, 035204 (2010).
  9. Z. Zhao and R. Myllylä, “Measuring the optical parameters of weakly absorbing, highly turbid suspensions by a new technique: photoacoustic detection of scattering light,” Appl. Opt. 44, 7845–7852 (2005). [CrossRef]
  10. Z. Zhao, M. Törmänen, and R. Myllylä, “A preliminary measurement of fibres and fines in pulp suspensions by the scattering photoacoustic technique,” Meas. Sci. Technol. 17, 128–134 (2006). [CrossRef]
  11. Z. Zhao, M. Törmänen, and R. Myllylä, “Backward-mode photoacoustic transducer for sensing optical scattering and ultrasonic attenuation: determining fraction consistency in pulp suspensions,” Meas. Sci. Technol. 21, 025105 (2010). [CrossRef]
  12. A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
  13. A. Oraevsky, S. Jacques, and F. Tittel, “Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress,” Appl. Opt. 36, 402–415 (1997). [CrossRef]
  14. W. Sigrist, “Laser generation of acoustic waves in liquids and gases,” J. Appl. Phys. 60, R83–R121 (1986). [CrossRef]
  15. H. MacKenzie, H. Ashton, S. Spiers, Y. Shen, S. Freeborn, J. Hannigan, J. Lindberg, and P. Rae, “Advances in photoacoustic noninvasive glucose testing,” Clinical Chem. 45, 1587–1595 (1999).
  16. Z. Zhao, “Pulsed photoacoustic techniques and glucose determination in human blood and tissue,” doctoral thesis (Acta Universitatis Ouluensis, Series C 169, 2002).

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