Ultrasonics 54 (2014) 1047–1051

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Ultrasonics
journal homepage: www.elsevier.com/locate/ultras

Fiber-optic ultrasonic hydrophone using short Fabry–Perot cavity withmultilayer reﬂectors deposited on small stub
Kyung-Su Kim ⇑, Yosuke Mizuno, Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japana r t i c l e

i n f oArticle history:
Received 3 April 2013
Received in revised form 12 December 2013
Accepted 16 December 2013
Available online 21 December 2013
Keywords:
Ultrasonic hydrophone
Optical ﬁber sensor
Band-pass ﬁlter
Dielectric multilayer ﬁlm
Fabry–Perot resonatora b s t r a c t
A ﬁber-optic probe with dielectric multilayer ﬁlms deposited on a small stub is studied for mega-hertz
ultrasonic-wave detection in water. The small stub with a short Fabry–Perot cavity and distributed reﬂectors is attached on the ﬁber end. The structure is mechanically strong and withstands intense ultrasonicpressure. Ultrasonic waves at 1.56 MHz are successfully detected in water with a good signal-to-noise
ratio. The working principle and the characteristics are studied by comparing the ultrasonic sensitivity
with that of a conventional piezoelectric hydrophone. The distance response and directional response
are also investigated.
Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction
Mega-hertz ultrasonic-wave detection in liquids is of great signiﬁcance because of its broad application capabilities in both biomedical and industrial ﬁelds. Piezoelectric hydrophones based on
polyvinylidene diﬂuoride (PVDF) needles [1–5] have been commonly used, but they are fragile and sometimes broken by highsound pressure. Besides, the length of their output cables is limitedby the high electric impedance. As alternative sensors, ﬁber-optic
ultrasonic sensors have been developed with such advantages ashigh spatial resolution, durability against high sound pressure,and immunity to electromagnetic interference [6–23]. Their typicalexamples include ultrasonic sensors based on (1) reﬂection at theﬁber end [6–12], (2) ﬁber Bragg gratings (FBGs) [13–16], (3) ﬁber
Fabry–Perot (FFP) resonators with mirrors [17–20], and (4) FFP
resonators with distributed Bragg reﬂectors (DBRs) [21–23]. In
the ﬁrst example, the refractive-index modulation of surrounding
liquid due to ultrasound ﬁled is detected as the change in thereﬂection coefﬁcient at the ﬁber end surface [6–12]. This method
provides the information on the absolute sound pressure since
the relationship between the refractive index change and thesound pressure is already known quantitatively. This was successfully applied to the measurement of shock waves in water with its
characteristics, but suffered from its low sensitivity, though sometechniques including tapering have been developed to improve
the sensitivity. Ultrasonic sensors using FBGs, which act as the⇑ Corresponding author. Tel.: +81 459245052.
E-mail address: kskim@sonic.pi.titech.ac.jp (K.-S. Kim).
0041-624X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultras.2013.12.009

optical-band pass reﬂectors, have much higher sensitivity to ultrasonic ﬁeld, but the spatial resolution in the direction of ﬁber axis is
insufﬁcient because the FBGs need to be long enough to have asteep slope in the band-pass spectra [13–16]. Developing ultrasonic sensors using FFP resonators with mirrors is one of the effective methods to improve the spatial resolution, but they sufferfrom the complicated fabrication process and the fragile structure[17–20]. In the last example, the mirrors of the FFP sensors are replaced by multilayer reﬂectors to ensure the robustness of thesensing head exploiting their established deposition method [21–23]. This structure has been shown to have moderate sensitivity
with high spatial resolution as a ﬁber-optic ultrasonic sensor, however, it is hard to directly deposit multilayer reﬂectors at the end of
an optical ﬁber since the sensor head is extremely fragile.
In this study, the sensor based on ‘‘band-pass ﬁlter (BPF) on aﬁber end’’ (BOF) [24] is investigated as an ultrasonic probe. The
BOF is fabricated with a conventional deposition technique on aﬁrm small stub, which is connected to a ﬁber-optic cable. This
structure provides high mechanical durability and enables a lessexpensive production. The BOF has been studied as a temperature
sensor [24] and a sensor for external mechanical loading [25]. The
authors’ group has found a possibility to use the BOF as a refractive
index sensor [26]. We present a trial to apply the BOF sensor formega-hertz ultrasonic-wave detection in water. We experimentally show that ultrasonic waves at 1.56 MHz are successfully detected in water with a good signal-to-noise ratio (SNR). From theresults, we study the working principle and the characteristics bycomparing the ultrasonic sensitivity with that of a conventional

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hydrophone. We also measure its distance response and directional response.
2. Principle
2.1. Structure of BOF
The key part of our ﬁber-optic probe is known as the BOF, which
consists of the FFP resonators with DBRs. The BOF, however, iscomposed of dielectric multilayer ﬁlms (DMFs) deposited on asmall stub made of crystallized glass, which is connected to theoptical ﬁber end and ﬁxed with a sleeve as shown in Fig. 1. The
DMFs comprising SiO2 and TiO2 layers accumulated by ion-assisted
evaporation form an optical cavity, which serves as an optical ﬁlter.
The BOF has following three advantages: (1) DMFs deposited on
the small stub are much easier to manufacture compared withthose deposited directly on the ﬁber end, (2) it is suitable for mass
production with low cost, and (3) its sensor head is robust owing to
the lattice match of each layer, which enables high-frequency
ultrasonic-wave detection without damage. Fig. 2 represents thedetailed Fabry–Perot structure of the DMFs, which is composedof a cavity of 925-nm-thick SiO2 layer and DBRs on both sides. Each
DBR is composed of 4 pairs of 215-nm-thick SiO2 (n = 1.46) and
215-nm-thick TiO2 (n = 2.22) layers. The TiO2 top layer protecting
the whole structure is 450-nm thick.
2.2. Sensing principle
The typical reﬂection spectrum of the BOF is illustrated inFig. 3(a and b), where the sharp dip is observed at 1534.1 nm.
When broad-band light is injected into the BOF, it selectively eliminates a light component at a particular wavelength kc. If the stress
is applied to the BOF, kc shifts in proportion to its magnitude [25].
We have also found that kc is sensitive to the refractive index of themedia facing at the top layer of the BOF. The relation between theshift in kc and the refractive index has been clariﬁed [26]. Since
ultrasonic ﬁeld induces stress to the BOF as well as refractive-index
modulation of media (water) surrounding the BOF, the wavelength
kc is modulated in proportion to the magnitude of the ultrasonicﬁeld applied to the BOF. It is important to reveal which of thetwo contributions is the more prominent in using the BOFs to detect ultrasonic waves.
3. Experiments
3.1. Setup
Fig. 4 shows the experimental setup for ultrasonic-wave detection using the BOF-based ultrasonic hydrophone, which was originally developed for high-speed measurement of refractive index[27]. A tunable laser diode (TLD) of 1550-nm band was used as alight source and its output power was ﬁxed at 5 dBm. The output
light was injected into the BOF via an optical circulator. The reﬂected light from the BOF was converted to an electrical signal
using a photodiode (PD) with a bandwidth of over 5 MHz, andwas monitored employing an oscilloscope (OSC) with a sampling

Fig. 1. Structure of the BOF-based sensor.

Fig. 2. Detailed layer structure of the DMFs.

rate of 250 MS/s. By applying electrical burst waves to a diskshaped piezoelectric transducer (PZT) with a diameter of 25 mm,
ultrasonic burst waves at 1.56 MHz were generated. The numberof the burst was approximately 20. The end of the BOF-based ultrasonic hydrophone was ﬁxed above the center of the PZT. Since thewavelength of the TLD was set at the center of the slope in the BOF
response, the modulation in kc was converted into an alternative
voltage at the PD.

3.2. Sensitivity
Fig. 5(a) shows the measured alternating current (AC) component of the signal voltage Uac, when the optical wavelength was
1532.50 nm and the distance between the BOF end and the PZT
was 50 mm. The waveform was averaged for 64 shots. For
comparison, we also detected the same ultrasonic waves with aconventional needle hydrophone as shown in Fig. 5(b). The obtained waveforms were similar in shape and SNR, which indicatesthat the BOF-based sensor properly works as an ultrasonichydrophone.
Fig. 6(a) represents the optical wavelength dependence of thedirect current (DC) component of the signal voltage Udc and thatof the peak-to-peak value of Uac (Up–p). The DC component indicates the reﬂectivity of the BOF, while Up–p shows the amplitude of
the ultrasonic signal. As can be seen, if we differentiate the curve of
the obtained Udc with respect to optical wavelength, the shape of
Up–p can be obtained. The absolute value Up–p became the maximum at 1533.50 and 1535.75 nm; Uac at 1533.50 nm is plotted
as negative values and at 1535.73 nm as positive ones, because
the Uac waveforms at 1533.73 nm and 1535.50 nm are out of phasewith each other as shown in Fig. 6(b). The reverse of the phase iscaused by the difference in the sign of the slope of the Udc.
From the measurement using the piezoelectric needle hydrophone with known sensitivity, the acoustic pressure Pin was estimated to be 324 kPa (peak-to-peak value). To eliminate theeffects of the input optical power and the sensitivity of the PD on
the hydrophone characteristics, we deﬁne the sensitivity of the
BOF-based hydrophone as SBOF = |Up–p/Udc|/Pin. The sensitivity isplotted as a function of the optical wavelength as shown in
Fig. 7. The maximum sensitivity of 3.43  10 4 MPa 1 was obtained at 1534.2 nm for the 1.56-MHz ultrasonic ﬁeld.
The sensitivity to the refractive-index change of the surrounding media has already been veriﬁed in our previous study [26].
Using this value and the relationship between the sound pressure
and the refractive-index change [28], we can calculate the sensitivity to ultrasonic ﬁeld based on the refractive-index modulation ofwater facing the BOF. It was, however, calculated to be
3.95  10 5 MPa 1, which was almost 10 times as small as themeasured sensitivity (=3.43  10 4 MPa 1). Consequently, we concluded that the stress applied to the BOF structure is the major
principle of the ultrasonic sensitivity.

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Fig. 3. (a) Typical reﬂection spectrum of the BOF in air, and (b) its magniﬁed view around the dip.

Fig. 4. Schematic experimental setup for ultrasonic-wave measurement. OSC,
oscilloscope; PD, photodiode; PG, pulse generator; PZT, piezoelectric transducer;
TLD, tunable laser diode.

Fig. 7. Sensitivity of BOF-based ultrasonic hydrophone.

Fig. 5. Uac measured with (a) the BOF-based ultrasonic hydrophone, and (b) the conventional needle hydrophone.

Fig. 6. (a) Up–p and Udc as functions of optical wavelength, and (b) waveforms of Uac at 1533.50 and 1535.73 nm.

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Fig. 8. (a) Schematic setup, and (b) signal waveforms under different distances.

Fig. 9. (a) Relative time vs. distance; the slope indicates the reciprocal of the ultrasonic velocity in water, and (b) Up–p vs. distance.

Fig. 10. (a) Schematic setup, and (b) Up–p vs. angle h; comparison between experiment and theory.

3.3. Distance and directional response
Next, the dependence of the signal waveform on the distance Dfrom the PZT was investigated as depicted in Fig. 8(a) to conﬁrm
that the signal is ultrasonically generated. The optical wavelengthand power were ﬁxed at 1533.50 nm and at 5 dBm, respectively.

The measurement result is shown in Fig. 8(b) as D became longer
the signal waveform was delayed and its peak voltage was attenuated. Fig. 9(a) shows the relative time as a function of the distance
D. Since the time was determined using the ﬁrst clear peak observed in the signal waveform, the measurement error was of theorder of the acoustic wavelength, approximately ±0.2 ls (Refer also

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to Fig. 6). The curve was almost linear and its slope was the reciprocal of 1511 m/s, which agrees well with the acoustic velocity
in water at room temperature [29]. Fig. 9(b) shows the peak voltage dependence on D, where the voltage was attenuated as D became longer. The slight increase in the voltage at D  30 mm wasprobably caused by the nature of the sound distribution producedby the ﬁnite radiation surface. These results also show that the BOF
sensor can detect the ultrasonic signal correctly in the experiment.
Finally, we measured the dependence of the peak signal voltage
on the angle h with respect to the vertical axis to verify the directional response of the BOF as shown in Fig. 10(a). The BOF was rotated with an angular step of 15° from the vertical axis. The
wavelength was 1533.50 nm, and the optical power was 5 dBm.
The measured response is shown as solid squares in Fig. 10(b),which was almost symmetric with respect to the h = 0° axis. In
addition to a main lobe with a width of approximately 60°, twosidelobes were observed. This is similar to the theoretical far ﬁeld
response of the circular sensing aperture with a diameter of
1.27 mm (dotted line), which was almost the same as the BOF
sensing head with 1.35-mm diameter. Therefore, the directional
response of the BOF-based ultrasonic hydrophone seems to be
dominated by the diameter of the stub, though the optically sensitive part has a diameter of less than 10 lm, which is the mode ﬁeld
diameter of the single-mode ﬁber. This fact implies that the ultrasonic waves were detected as the deformation of the BOF instead of
the change in the refractive index of the surrounding media. The
diameter of the sensing stub is required to be reduced for high frequency detection.
4. Conclusion
We demonstrated a trial to apply the BOF-based sensor tomega-hertz ultrasonic-wave detection in water. Ultrasonic waves
at 1.56-MHz were experimentally detected with a good SNR. The
sensitivity of the sensor at this frequency was 3.43  10 4 MPa 1.
This value implied that the stress induced by ultrasonic waves tothe BOF has signiﬁcant contribution to the ultrasonic sensitivityof the BOF. We also experimentally investigated the distance
response and the directional response of the BOF. The frequency
responses in ultrasonic sensing using the BOF are left for futurestudies.
Acknowledgement
The authors are indebted to Dr. Onoda and Mr. Nakano, Watanabe Co., Ltd., Japan, for providing the BOF samples. This work waspartially supported by a Grant-in-Aid for Research Activity Start-up(24860029) from the Japan Society for the Promotion of Science(JSPS), and by research grants from the Hattori–Hokokai Foundation, the Mazda Foundation, and the JFE 21st Century Foundation.
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