Fiber-optic ultrasound transmitter based on multimode interference in curved adhesive waveguide
HUIBO FAN,1,2,3 LIANG CHEN,1 AND XIAOYI BAO1,4
1
Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China
3
e-mail: hbfan@yzu.edu.cn
4
e-mail: xbao@uottawa.ca
2
Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX
Fiber-optic ultrasound transmitters can be used in
structural
health
monitoring
(SHM),
material
characterization, and biomedical imaging. However, the
bandwidth of current fiber-optic ultrasound transmitters
is tens of MHz which prevents the ultrasound image on
cells with resolution of µm level; for SHM, broadband
high sensitivity detection is essential for small defect
identification, and they are not available. Here, ultracompact in-fiber ultrasound transmitter is proposed by
using the multi-mode interference in the curved UVcured adhesive waveguide, which is fabricated between
two fiber endfaces over a sandwiched core-offset spliced
fiber segment. The curved adhesive waveguide can
improve the contrast of device reflection spectrum from
2.7 dB to 21.7 dB. The ultrasound is generated by optical
pulse excitation via photoabsorption and thermal
expansion of UV-cured adhesive, and then detected via
the multi-mode interference of the same waveguide over
hundreds of Hz to 306 MHz broad transverse acoustic
waves. This ultra-compact and cost-effective device
offers new opportunities to the advanced biomedical and
ultrasound-based applications. © 2019 Optical Society of
America
http://dx.doi.org/10.1364/OL.99.099999
Ultrasound transmitter is widely used in structural health
monitoring, nondestructive test (NDT) and biomedical imaging
[1-4]. Traditional piezoelectric transducer (PZT) with bulky
size and narrow frequency bandwidth is susceptible to
electromagnetic interference. Based on photoacoustic effect,
laser-ultrasound generation has shown great potential to offer
higher frequency in various applications [5,6]. In laserultrasound generation, an optical-absorption material could
convert pulsed optical energy into thermal energy with
thermal expansion, and then release to produce acoustic wave
between two optical excitations [7-9]. Especially, fiber-optic
ultrasound transmitters have been further studied based on
various photoacoustic materials [6,10-12], such as metal films
[13], gold nanocomposite or carbon nanotube mixing with
poly-dimethysiloxane (PDMS) [12,14], and epoxy and graphite
mixtures [15]. A typical fiber-optic ultrasound generator
consists of two reflectors at the fiber end face covered by above
mentioned photoacoustic materials, and heated by a pulse
laser. The fiber diameter determines the transverse acoustic
frequency. For SMF 28 with cladding diameter of 125 µm, it
gives tens of megahertz as a transducer [14,16].
An alternative approach is to generate the ultrasound wave
from side wall of the optical fibers [17,18]. For instance, fibers
with tiny core-offset dislocation could couple light from core
mode into cladding modes to enable the ultrasound generation
with bandwidth of around 30 MHz via the covered absorption
material with thickness of around 200 µm [18]. It is noting that
an ideal material for strong photoacoustic generation should
have large photoabsorption coefficient and large coefficient of
thermal expansion (CTE). Photoacoustic conversion materials
such as PDMS and epoxy resin have been widely used for
ultrasound generation, while little report on the other polymer
materials, such as UV-cured adhesive. Norland Optical
Adhesive 61 (NOA 61) with high photoabsorption coefficient
and CTE at the room temperature has a potential for
ultrasound generation [18,19]. Furthermore, a PZT ultrasound
sensor or a hydrophone is used to response the acoustic wave,
increasing the cost and complexity [18,20]. Laser-induced
ultrasound generation and integrated optical probe-sensing
measurement with high sensitivity are a potential for
ultrasound transmitter.
In this Letter, an ultra-compact fiber-based ultrasound
transmitter is fabricated, and characterized via a curved UVcured adhesive waveguide as an ultrasound generator. Multimode interference in the same adhesive waveguide between
two fiber endfaces over a sandwiched core-offset spliced fiber
segment acts as an ultrasound sensor. The curved waveguide
greatly increases high-order waveguide modes and enhances
multi-mode interference at one passage time of waveguide to
increase high frequency response, unlike normal Fabry-Perot
interferometers rely on multiple reflection to enhance quality
factor at cost of time response. Pulsed pump is used to excite
the adhesive waveguide, resulting in the transverse acoustic
wave detected by the continuous-wave probe light. Especially,
only adhesive is used for the ultrasound generation. Broad
bandwidth from hundreds of Hz to 306 MHz is generated and
detected thanks to gradually varied thickness of adhesive
waveguide along the fiber.
amplifier (EDFA) as a broadband source illuminates the device
and then reflection spectrum is obtained via an optical
circulator to characterize the device. Fig. 1(g) is the fiber device
photograph before covering UV-cured adhesive.
Fig. 1. (a)-(d) The fabrication process of curved UV-cured adhesive
waveguide between two fiber endfaces. (e) Side-view and crosssection diagram of curved adhesive waveguide. (f) A typical
photograph of UV-cured adhesive dripping process. (g) & (h) Side-view
photographs before and after covering UV-cured adhesive.
Fig. 1(a)-1(d) show the fabrication steps of proposed
ultrasound transmitter. First, three sections of SMF 28 are
core-offset spliced together with the lengths of d1 for region 1
and d2 for region 2. d1 and d2 should be comparable in order to
ensure a high contrast and uniform reflection spectrum.
Parameter h is the distance between the outer boundary of one
SMF and the center of another SMF, as shown in Fig. 1(e). In the
top-view, three SMF cores should be aligned in the same axis.
After that, a drop of UV-cured adhesive supported by a fiber
taper with diameter of about 35 µm is carefully dropped on the
top of region 1, as shown in the Fig. 1(c) and 1(f) that is a
typical dripping process photograph. Due to the adsorption
and surface tension of liquid, adhesive gradually moves to both
sides of region 1, finally forming a curved structure with
different thicknesses at different locations, as shown in Fig.
1(e) and 1(h) that is a typical sample photograph in the sideview. Then, UV light is used to cure the adhesive for about 20
minutes. Considering adhesive with refractive index of around
1.54 in the 1.55 µm band and mode confinement of circular
structure in the optical fiber cross section as shown in Fig. 1(e),
a curved adhesive waveguide is formed on the top of region 1.
For multi-mode interference and then being reflected at region
2 with cladding modes simultaneously, h is set to less than 20
µm to ensure the curved waveguide has the minimum
thickness of < 10-20 µm to generate high ultrasound
frequency. After the device fabrication, an Erbium doped fiber
Fig. 2. Optical field distributions when the adhesive thickness is 0 µm
(a) and 12 µm (b). (c) The energy evolution in the basic mode as light
passes through the adhesive waveguide. A defined optimized adhesive
thickness (OAT) corresponds to basic mode with most light propagates
in the adhesive waveguide. (d) The relationship between OAT and h.
To investigate the optical field distribution in the adhesive
waveguide between two fiber endfaces, a theoretical analysis is
carried via finite element beam propagation method.
Considering that the launch light, propagating in the region 1, is
primarily absorbed by the silica cladding of region 1 and
transmits into the region 2, two-dimensional analysis with the
uniform adhesive thickness is used for the simplicity. The
diameters and refractive indexes of single-mode fiber are set as
8 µm and 1.4521 for fiber core and 125 µm and 1.4469 for
fiber cladding [18]. d1 = d2 = 120 µm and h is set to 6 µm as an
example. In order to enhance the pumping efficiency for
ultrasound generation, the basic mode in SMF should deliver
power to adhesive waveguide with rare light diffusion and
power loss. Fig. 2(a) demonstrates the optical field distribution
(left) and the basic mode propagation (right) as launch light
passes through fiber device without covering adhesive,
showing that much light propagates into the fiber cladding of
region 1. In Fig. 2(b) with adhesive thickness of 12 µm, the
basic mode with most power propagates in the adhesive
waveguide with little light diffusion. Fig. 2(c) shows the energy
evolution in the basic mode as the adhesive thickness varies
from 0 to 24 µm. For the cases of adhesive thickness less than
12 µm, periodic optical oscillation is created in the region 1 due
to the interference between modes in the adhesive waveguide
and air. As the adhesive thickness increases gradually to 12 µm,
less light propagates into the air, resulting in the reduced
interference. Furthermore, much light in the single-mode fiber
would diffuse into air and silica cladding in the region 1 with
thin adhesive, resulting in the pump intensity decrease in
adhesive waveguide. As the adhesive thickness is larger than
12 µm, much power transfers into the high-order waveguide
modes, corresponding to the basic mode power drop in the
adhesive waveguide. Therefore, an optimized adhesive
thickness (OAT) is defined, corresponding to the basic mode
with most power propagating in the adhesive waveguide for
more efficient ultrasound generation. For h = 6 µm, the OAT is
12 µm, which would be varied as h is changed gradually, as
shown in Fig. 2(d). Furthermore, thicker waveguide would
induce lower transverse ultrasound frequency. Many highorder modes interference in the adhesive waveguide with
large thickness could enhance the ultrasound response with
high frequency. Therefore, ultrasound generation and
detection should consider an optimal waveguide thickness.
Fig. 3. (a) The reflectivity of fiber device before and after covering
adhesive with h = 11.8 µm. (b) Contrast comparison before and after
covering adhesive with 8 samples.
In experiment, the reflectivity of fiber device before and after
covering adhesive are first characterized with h = 11.8 µm, as
shown in Fig. 3(a). Before covering adhesive, the device
contrast is 2.7 dB with the maximal reflectivity of -15.1 dB,
while 21.7 dB and -27.8 dB after covering adhesive,
corresponding to the increased contrast by 19 dB to enhance
the dynamic range for ultrasound sensing. The reduced
reflectivity, due to the optical absorption of adhesive, could be
offset by increasing the input probe power. The curved
structure has the varied thickness ranging from maximum of
66 µm to minimum of 11.3 µm. We tested 8 devices under the
same fabrication condition, the spectrum contrast comparison
as shown in Fig. 3(b), revealing the increased contrasts for
most samples after covering adhesive. As the original contrast
gradually increases, it’s harder to obtain higher contrast due to
the irregularly curved structure and large optical absorption of
adhesive. In addition, all samples’ contrasts are larger than 8
dB. In the following experiment of ultrasound generation and
detection, the pump wavelength is set at the valley of reflection
spectrum to achieve the resonant optical absorption by the
adhesive waveguide, while the probe wavelength at the peak
for the maximal reflection signal, as shown in Fig. 3(a). As the
adhesive waveguide expands and shrinks because of pulse
pumping, the shift of reflection spectrum could be detected by
the probe light.
Fig. 4 shows the experimental setup for the ultrasound
generation and detection based on curved adhesive waveguide
device. By using a polarization controller and an electro-optic
modulator, a pulse laser is realized with a repetition rate of 100
kHz and duration time of 600 ns, and then amplified via an
EDFA. The reason of this duration time which is larger than
those values in many references is that optical absorption
coefficient of UV-cured adhesive is lower than those of optical
absorption materials such as graphite or carbon nanotube,
resulting in the completely thermal expansion with long time
in UV-cured adhesive [14,18]. 10% pump goes to oscilloscope
to monitor the peak power. The remaining pump laser and
probe laser are sent to the curved adhesive waveguide via a
coupler and a circulator. A filter is used to exclude the pump
light; only the probe light is detected by oscilloscope. The time
delay between the pump pulse by PD 2 and PD 1 is 54 ns,
which has been calibrated in advance in the following data. A
variable optical attenuator is used to avoid the optical power
saturation of PD 1. As pump light and probe light transmit to
the curved adhesive waveguide device, pulsed energy is
quickly absorbed by adhesive and then converted into the
thermal energy due to high photoabsorption coefficient of
adhesive, subsequently expanding via large thermal expansion
and shrinking when the pulse disappears, followed by the
acoustic wave generation. Due to the core-offset silica splicing
structure, only the transverse expanding and shrinking
direction is favoured for the adhesive waveguide, resulting in
dominated transverse acoustic wave with certain direction.
Therefore, several parameters of curved adhesive waveguide
could be changed and then detected by probe light, such as the
structure, optical path and effective refractive index, as shown
in Fig. 5.
Fig. 4. The experimental setup for ultrasound generation and detection.
PC: polarization controller; EOM: electro-optic modulator; AFG:
arbitrary function generator; EDFA: Erbium doped fiber amplifier.
VOA: variable optical attenuator; PD: photodetector.
Fig. 5(a) shows the time-domain traces of pump light with
calibrated peak power of 321 mW at wavelength of 1560.83
nm and the probe light with input power of 2.5 mW at
wavelength of 1555.85 nm. It clearly shows that at the time of
around 150 ns after the disappearance of pump power, an
ultra-short pulsed vibration is excited, corresponding to the
transverse acoustic wave generated from the contraction of
curved adhesive waveguide. Fig. 5(b) is the magnified plot of
probe light time-domain traces with calibrated pulsed peak
power of 113 mW and 321 mW, respectively. Higher pump
power gives larger vibration amplitude of adhesive waveguide.
Note trace with higher pump power has large delay time
relative to that with lower pump power due to the aging of
adhesive waveguide when the pump power is gradually
increased to observe the variation of time-domain traces.
Therefore, the broad ultrasound is created and detected based
on curved adhesive waveguide.
In summary, we have demonstrated an ultra-compact fiberbased curved adhesive waveguide device for ultrasound
generation and detection. Owing to the curved structure with
varied thickness and multi-mode interference effect,
ultrasound with broad band, ranging from hundreds of Hz to
306 MHz, is realized and detected. This novel ultrasound
transmitter offers new opportunities to the advanced
biomedical and NDT applications.
Funding. This research has been primarily supported by
the Natural Sciences and Engineering Research Council
(NSERC) of Canada (STPGP 506628, RGPIN-2015-06071), as
well as the Canada Research Chairs (CRC) Program
(950231352).
Acknowledgment. We thank Dr. Liang Zhang for helpful
discussion.
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Fig. 5. (a) The time-domain traces of pulsed pump light and probe
light, corresponding to the ultrasound generation. (b) The magnified
plot of probe light time-domain traces with different pulsed peak
power. (c) Frequency spectrum via fast Fourier transform of Fig. 5(b).
(d) The detailed diagram of curved adhesive waveguide device.
Frequency spectrum, ranging from hundreds of Hz to 306
MHz, is obtained from the probe time-domain trace via fast
Fourier transform, as shown in Fig. 5(c). The generated broad
ultrasound bandwidth is attributed to the gradually varied
thickness of curved adhesive waveguide, as shown in the
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