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Anal. Chem. 1993, 65, 3493-3496
Photoacoustic Spectrometry Using a Fiber-optic Pressure Sensor Yuji Kawabata, Ken-ichi Yasunaga, Totaro Imasaka,' and Nobuhiko Ishibashi Faculty of Engineering, Kyushu University, Hakozaki, fikuoka 812, Japan
A photoacoustic sensor is constructed by using an optical fiber. The signal is measured as a light transmission loss caused by a microbendingeffect. The pressure sensor, consisting of a coiled optical fiber, is installed in a sample cell for absorption measurement by photoacoustic spectroscopy (PAS). A frequency-doubled NdYAG laser is introduced into the sample cell, and a photoacoustic signal is measured as an intensity decrease of a He-Ne laser beam passed through an optical fiber. The detection limit for a chelate compound of iron1L4,7-diphenyl-l,10-phenanthrolinedisulfonic acid is 1.2 X lo* M. Performanceof the fiberoptic PAS system is compared with a conventional PAS system using a piezoelectric transducer. Potential advantages of the present fiber-optic PAS system are also discussed.
INTRODUCTION Laser-induced photoacoustic spectroscopy (LI-PAS) is a sensitive analytical means in absorption spectrometry. A piezoelectric transducer (PZT) is often used for detection of the photoacoustic signal, since the PZT is highly sensitive to pressure change induced by light absorption in the condensed phase.' The lowest pressure directly detected by a PZT is reported to be lo-' Pa2and is improved by combination with a high-sensitivity detection apparatus such as lock-in amplifier. However, the output signal is interfered with by an electric noise around the PZT, due to ita extremely high impedance. Thus, an electric shield around a photoacoustic cell is essential for sensitivedetection of small light absorption. It causes some difficulty in ita application to remote sensing. Recently, fiber-opticpressure sensors have been developed based on interferometric2 and microbending effects.3.4 When an optical fiber is pressurized, the optical path length (interferometriceffect) and the transmission loss (microbending effect) increase. The lowest static pressure detectable by the sensor based on the microbending effect is calculated to be 1.3 X lo-' Pa4 when the detection bandwidth is 1Hz, and it is comparable to that of PZT. The static pressure and the photoacoustic pressure may cause a different perturbation to the optical fiber. But the high sensitivity to the static pressure indicates that the fiber-optic pressure sensor would also be advantageous for the measurement of the photoacoustic pressure. Furthermore, the pressure sensor based on interferometry is more sensitive, the lowest detectable
* Author to whom correspondence should be addressed.
(1) Oda, S.; Sawada, T. Anal. Chem. 1981,53,471-474. (2) Bucaro, J. A.; Dardy, H. D.; Carome, E. F. Acoust. Soc. Am. 1977, 62,1302-1304. (3) Fields, J. N.; Cole, J. N. Appl. Opt. 1980, 19, 3265-3267. (4) Fields,J. N.; Asawa,C. K.;Ramer, 0.G.; Barnoski,M. J. J. Acoust. SOC.Am. 1980,67,816-818. 0003-2700/93/03653493$04.00/0
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Figure 1. Experimental apparatus for measurement of frequency response of the fiber-optic pressure sensor.
pressure being calculated to be 7.6 X lo-' Pa2a t 250 kHz with a 1-Hz bandwidth. However, the optical path length is sensitive to ambient conditions, so the experimental conditions should be controlled in the interferometric system especially for pressure measurements in a low-frequency range.2 The optical modulation caused by the pressure change is not interfered with by the electricnoise, since the pathlength of the optical fiber and the intensity of light transmitted through the optical fiber are not affected by the electromagnetic field around the optical fiber. In this study, a fiber-optic pressure sensor is constructed based on the microbending effect, since it is potentially insensitive to the environmental change. This device is used as a detector in LI-PAS, and performance is compared with the conventional PAS system.
EXPERIMENTAL SECTION Reagents. Iron(I1) diammonium sulfate hexahydrate was obtained from Katayama Chemicals. The chelate reagent 4,7diphenyl-1,lO-phenanthrolinedisulfonicacid, disodium salt, was purchased from Wako Pure Chemicals. The iron(I1) salt (2.0 mg) and the chelate reagent (80 mg) were dissolved in distilled and deionized water (0.5 L), and the concentration of the iron(II)-chelate was adjusted to 1X 106 M. This stock solution was diluted stepwise with distilled and deionized water. An absorbance of the sample solution at 532 nm was confirmed to be proportional to a concentration of the iron(I1)-chelate by spectrometric measurement (Shimadzu, UV-200s). Sensitivity of the Fiber-optic Pressure Sensor. There are many types of optical fibers at present, and these optical fibers are made of various materials such as plastic or quartz. According to refs 4-7, an optical fiber which has a quartz core and a plastic cladding is considered to have a large sensitivityto pressure change. Such optical fiber was purchased from Furukawa Electric (new PCF). This optical fiber was a multimode step-index type. The core, cladding, and jacket diameters were 200, 230, and 500 pm, and made of quartz, fluoroacrylic resin, and fluororesin, respectively. The numerical aperture (NA) of the optical fiber was 0.4. The experimental apparatus for evaluation of the frequency response of the fiber-opticpressure ~~
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(5) Boechat, A. A. P.; Su, D.; Hall,D. R.; Jones, J. D. C. Appl. Opt. 1991,30,321-327. (6) Gardner, W . B. Bell Syst. Tech. J. 1976,64, 457-465. (7) Gloge, D. Bell Syst. Tech. J . 1976, 54, 245-261.
0 1993 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23,DECEMBER 1, 1993 lnl
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sensor is shown in Figure 1. The optical fiber is wound 10 times in a diameter of 22 mm and is fixed by an epoxy resin adhesive (Ciba-Geigy, Araldite). This coil is used as a transducer for measurement of the acoustic wave. This assemblyis dipped into water near a PZT (PZT 1, Tokin, N-21; piezoelectric constant, 2.5 X 10-2 V.m.N-l; thickness, 20 mm; resonance frequency, 70 kHz). A sinusoidal waveform, whose amplitude is i2.5 V, is applied to PZT 1 by an electric oscillator installed in a lock-in amplifier (NF Circuit Design Block, 5600). The pressure change generated by PZT 1is calculated to be 9.8 kPa by the equation given by the manufacturer in the specification sheet by use of the parameters applied voltage and piezoelectric constant. A He-Ne laser (Uniphase, 1103P, 633 nm, 2 mW) is focused into the distal end of the optical fiber by an objective lens (Newport, M-20X,NA, 0.4). The laser beam propagated in the optical fiber is passed through a pinhole (diameter, 1 mm), and its output power is detected by a photodiode (Hamamatau Photonics, S7808BQ). The alternative component is measured by the lock-in amplifier,and the frequency response of the sensor is investigated by changing the modulation frequency. The pressure generated by PZT 1is also measured by another PZT (PZT 2, Tokin, N-5; piezoelectric constant, 2.3 X le2 V-mN-1; thickness, 2 nm; resonance frequency, 960 kHz) to evaluate the actual pressure applied to the coiled optical fiber. Fiber-optic PAS. The experimental apparatus for fiberoptic LI-PAS is shown in Figure 2. The optical fiber is wound 40 times in an inner diameter of 16 mm, and this coil is installed in the sample The optical pathlength is 10cm. A frequencydoubled Nd:YAG laser beam (Quantal,YG581,532 nm, 50 mJ/ pulse; repetition rate, 20 Hz; pulse width, 10 ns) is introduced into the cell. The He-Ne laser is focused into the distal end of the optical fiber by the objective lens. The laser beam propagated in the optical fiber is passed through the pinhole (diameter, 1 mm), and its output power is measured by the photodiode. This output signal is fed to a homemade high-pass filter, whose cutoff frequency is 100 Hz, in order to reduce noise signal with low frequencies. The output signal is succeedinglyfed to an amplifier (NF Circuit Design Block, LI-75A, X100) and a band-pass filter (NF Circuit Design Block, 5600; band-pass frequency, 1kHz). The transient photoacoustic signal is stored in the memory of a digitizer (Autonics,5201). The frequency-doubledNd:YAG laser beam is split into two parts by a beam splitter. A part is detected by a photodiode (Leonics, D-1), which is used to trigger the digitizer. The transient photoacousticsignal is accumulated 1024 times by a signal averager (Autonics, F601) and is recorded by a chart recorder (Hitachi, 056).
RESULTS AND DISCUSSION Sensitivity of t h e Fiber-opticPressure Sensor. The pressure induced by PZT 1 was measured by the pressure sensor constructed from the coiled optical fiber. The microbending effect depends on the launching condition of the (8)Kawabata,Y.;Kamikubo, T.; Imaeaka,T.;Iehibashi,N. Anal. Chern. 1983,55, 1419-1420.
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Frequency I kHz Figure 3. Frequency response of the fiber-optlc presswe sensor. laser beam into the optical fiber.' However, the microbending effect was not so sensitive to the launch angle of the beam in this experiment since the optical fiber was wound tightly and the mode transfer in the optical fiber easily occurred. Therefore, the laser beam was focused into the optical fiber by the objective lens, which had the same NA value as the optical fiber. The laser beam is emitted conically from the optical fiber, and the microbending effect also depends on the position where the light intensity is measured in the emitted beam. A pinhole (diameter, 1 mm) was placed in front of the photodiode, and the sensitivity of the fiber-optic pressure sensor was observed by changing the position of the photodiode along the radial direction in the emitted beam. The light intensity at the center of the emitted beam was not sensitive to the pressure change. However, the higher sensitivity to pressure was obtained when the photodiode was located near the edge of the emitted beam. The light near the edge of the beam consisted of the transmitted light with high-order modes, whose intensities were quite sensitive to the microbending effect. Therefore, the light intensity near the edge of the emitted beam was observed for optical measurement ofthe pressure change. The optical modulation caused by pressure change partially arose from the modemode interference in the optical fiber. However, the light intensity at any position in the emitted beam was decreased by the applied pressure. Thus the optical modulation was dominantly due to the mode transfer caused by the microbending effect. The induced pressure depended on the modulation frequency, since PZT 1had a resonance frequency of 70 kHz. The actual pressure applied to the fiber-optic pressure sensor was calculated from the output signal of PZT 2 used as a detector. It had a resonance frequency of 960 kHz, which was far from a modulation frequency of 0.01-100 kHz used in this study. Therefore, PZT 2 was used under nonresonance conditions, and the frequency response was assumed to be flat. The output signal from PZT 2 was 4.3400 mV in this frequency range, and the calculated pressure from the piezoelectric constant and the thickness of PZT 2 was 9.3 X 10-.8' .7 kPa. The transmission loss caused by the microbending effect was divided by the calculated pressure at a specified frequency, in order to correct the frequency dependenceof the induced pressure. The frequency response of the sensor, normalized by the above procedure, is shown in Figure 3. No appreciable signal is observed below 2 kHz; however, the sensitivity increases rapidly above 20 kHz and has a maximum at around 40-60 kHz. The large fluctuation in the sensitivity is due to unstable mode distribution of light in the short optical fiber.3 It is well-known that distortion of the optical fiber along the fiber axis is dominant at low modulation frequencies.' On the other hand, distortion of the optical fiber across the fiber axis becomes stronger at
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
high modulation frequencies. The microbending effect is associated with distortion across the fiber axis, so that the optical pressure sensor is more sensitive when operated at high modulation frequencies; however, the sensitivity has an optimum value at around 60 kHz. Two possibilities are considered for an explanation of this phenomenon. First, the resonancefrequencyof the photoacousticcell is calculated to be 90 kHz.Q Second, the resonance frequency of PZT 1is located at 70 kHz and incomplete correction of the frequency response may be responsible for inducing a peak at this frequency. At an optimum modulation frequency of 54 kHz, the transmission loss (AT) was 0.038. The pressure at this modulation frequency was calculated to be 5.7 kPa from the output signal of PZT 2. Since the surface area of the coiled optical fiber was 1.1X 103m2, the applied force was calculated to be 6.3 N. Then, dT/dF was calculated from eq 1 to be
6 X 10-9 N-1. The dT/dF value for the straight optical fiber against the static force was also measured according to ref 4, and it was 0.2 N-1. Thus the dT/dFvalue for the coiled optical fiber against the acoustic wave is much smaller than that of the straight optical fiber against the static force. The discrepancy may come from the different perturbation applied to the optical fiber. The distortion of the optical fiber occurs strongly when the optical fiber is pressed by the static force. Thus, a relatively large dTldFvalue is observed for the static force. On the other hand, the dynamic force originates in the acoustic experiments, and this insthtaneous force causes the weak distortion of the optical fiber. The transmission loss is less sensitive to the dynamic force compared to the static force when the same force is applied to the optical fiber. Therefore, the dTldF value is considered to be much smaller in the acoustic experiments. This discrepancy may also come from distortion of the optical fiber in the coil fabrication process; a part of the guided modes easily transferable to the radiation mode is already dispersedoutside of the optical fiber. Thus, the coiled optical fiber is more resistive to pressure change. The epoxy resin adhesive used for fixing the coiled optical fiber might also decrease the sensitivity, since the optical fiber is partly isolated from the pressure induced. Fiber-optic PAS. The fiber-optic PAS cell was filled with the sample solution prepared at a concentration of 1 X lo” M, and the frequency-doubled NdYAG laser was introduced into the cell. When a transient photoacoustic signal was measured directly without using the band-pass filter, a wavy signal (90 kHz) was delayed 70 ms from the laser excitation. This wavy signal was confirmedto originate from the PAS signal of the sample, since no such signal appeared for the blank solution. This frequency agreed quite well with the resonance frequency of the photoacoustic cell. However, it was difficult to measure the signal sensitively, since a strong background signal appeared and this wavy signal could not be synchronized to accumulate it due to a time jitter. Then, the transient signal was passed through the band-pass filter and was accumulated 1024 times. The transient signal measured for the sample and blank solutions are shown in panels a and b of Figure 4, respectively. A wavy transient signal (700-1600 Hz) is observed in both the experimentalresults. This frequencycorrespondsto the passthrough frequency of the band-pass filter. The blank signal appeared, even when the exciting laser was interrupted before the sample cell. Thus, this interference signal comes from (9) Dewey, C. F. Opt. Eng. 1974,13, 483.
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the exciting laser; the discharge of the flashlamp in the N d YAG laser induces serious electrical noise in the electronic instruments through an ac power line and through the atmosphere. This interference is not an intrinsic characteristic of the fiber-optic pressure sensor. The electrical noise can be easily reduced by a noise-reduction circuit in the ac power line and shielding of the electronic instruments. The photoacoustic signal occurring from light absorption was obtained by subtracting the background signal in this study. The peak height in the residual signal was used as the photoacoustic signal. An analytical curve was constructed by measuring the sampleprepared at different concentrations. A straight analytical curve was obtained in the 0-1 X 10-6 M range for the iron(I1)-chelate. The detection limit was 1.2 X 1o-B M,which corresponded to an absorbance of 0.28. The poor sensitivity was ascribed to the low dT/dFvalue (6 X 10-9 N-1) for the coiled optical fiber. If the dT/dF value would be improved to 0.2 N-1, as obtained for a straight optical fiber, the lowest detectable pressure could be improved to 1.3 X 10-4 Pa+ which was comparable to that of conventional PZT. If the pressure sensor were constructed only from the coiled optical fiber without using any materials such as adhesive, the optical fiber would be strained efficiently by the photoacoustic pressure. Furthermore, the optical fiber was strongly compressedby photoacoustic pressure when the outside of the coiled optical fiber was covered by a hard material. These result in a substantial increase of the photoacoustic signal. The poor sensitivity of the system compared to ordinary LI-PAS was also ascribed to the difference in the detection method of the photoacoustic signal. A part of the beam emitted from the optical fiber was detected for the sensitive measurement of the photoacoustic pressure in this study. If the whole part near the edge of the emitted beam were collected and ita output power measured, the signal-to-noise ratio of the observed photoacoustic signal might be sufficiently improved. Furthermore,the background noise in the photoacoustic signal was reduced by accumulation of the signal in this study. If a high-sensitivity detection apparatus such as a boxcar integrator or a lock-in amplifier could be applied to this fiber-optic PAS system, the background noise could be further reduced. This results in improvement of the signal-to-noise ratio and a lowering of the detection limit of absorbance. The sensitivity of the fiberoptic pressure sensor was rather poor at around 1kHz, since it was far from the resonance frequency. These facts imply that the sensitivity of the present PAS system will be improved by modification of the sample cell and the excitation scheme. The photoacoustic signal was also ascertained to be proportional to the pulse energy of the exciting laser in the 0-65-mJ range. Thus, sensitivity will be much improved with an
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increase of the pulse energy of the exciting laser, similar to the conventional PAS system. In this study, the pressure change induced by light absorption is simply measured by the transmission loss of the He-Ne laser beam passed through an optical fiber. A frequency-doubled Nd:YAG laser can be transmitted to the sample cell through another optical fiber. Thus, a complete fiber-opticPAS system is readily constructed in this approach. Such a system is potentially resistiveto an electricinterference noise from the environment because the optical signal from the PAS cell is not affected by the electrical noise. The optical PAS system may be useful for remote sensing of chemical
species by absorption spectrometry in process control or practical trace analysis.
ACKNOWLEDGMENT This research is supported by Grants-in-Aids for Scientific Research from the Ministry of Education, Science and Culture, Japan. RECEIVEDfor review April 21, 1993. Accepted September 1, 1993.' @
Abstract published in Advance ACS Abstracts, October 15, 1993.
