Ultra-Miniature Fiber-Optic Medical Pressure Sensor System Using ...

Ultra-Miniature Fiber-Optic Medical Pressure Sensor System
Using White Light Interferometry
Kentaro Totsu 1 , Yoichi Haga 2 , Min Maung Lwin 3 and Masayoshi Esashi 4
1 Graduate School of Engineering, Tohoku University
2 Tohoku University Biomedical Engineering Research Organization (TUBERO)
3 Myotoku Ltd.
4 New Industry Creation Hatchery Center (NICHe), Tohoku University
Abstract
This paper describes the development of a fiber-optic Fabry-Perot interferometric pressure sensor of 125 µm in diameter
and detecting systems for medical use. A Fabry-Perot cavity is formed at optical fiber end. A deformation of the diaphragm of
the Fabry-Perot cavity induced by pressure varies the cavity length. White light interferometry is used to avoid error and noise
caused by bending of the optical fiber and a fluctuation of the light source. Two detection systems for monitoring the cavity
length of the sensor are developed. One is detecting the temporal fringe peak by using PZT scanner. The other is detecting the
modulated spectrum directly by using a commercialized high-speed spectrometer. A pressure change has been monitored in
real-time by using the developed detection systems. Animal experiments using a goat have been carried out and dynamic
pressure changes in internal pressure of heart and aorta have been successfully monitored.
Keywords: Optical fiber, Fabry-Perot interferometer, Spectrometer, Catheter, Guide wire
1 INTRODUCTION
In medical examination and treatment, pressure
measurement is an essential for monitoring the condition of
a human body. Furthermore, a spread of minimally invasive
therapy (e.g., interventional radiology) requires monitoring
pressure at very narrow space. Miniaturization of pressure
sensor has been done for biomedical applications [1-5]. The
pressure sensors especially utilize piezoresistives and optical
fibers. In particular, fiber-optic pressure sensors have
advantages of not only high potential of miniaturization but
also applicability to use in such electromagnetically harsh
environment as in an operating room in a hospital. Although
small fiber-optic sensors have been commercialized, still it
is difficult to meet the demands because of the size and the
shape. To realize the smallest and fine shape, a pressure
sensor of 125 µm in diameter was developed in our research
[6]. The Fabry-Perot interferometer sensor of which cavity
length was changed by pressure was formed at the tip of the
optical fiber. However the pressure sensor was small enough
to install in interventional tools (e.g., catheter, guide wire), a
bending of the optical fiber and a fluctuation of the laser
diode used as the light source affected the sensor output. To
reduce the effects of the bending and the fluctuation to the
sensor output, a real-time measurement system which used
spectrum modulation shifted by the distance change of the
mirrors of the sensor cavity was reported [7]. In this paper,
we describe an ultra-miniature fiber-optic pressure sensor
system focused on a development of the detection system
and experiments on animals for in vivo blood pressure
monitoring.
2 SENSOR SYSTEM
The structure of the fiber-optic pressure sensor system
is shown in Fig. 1. The sensing element is composed of a
thin silicon dioxide diaphragm with a mesa, an aluminum
mirror on the mesa and a spacer fabricated by
micromachining. The element is bonded to the end of a
multimode optical fiber of which outer cladding diameter
and the inner core diameter are 125 µm and 50 µm
respectively. A half-mirror is formed at the fiber end by
vacuum evaporation of chromium. The ring-shaped spacer
made of photo-sensitive polyimide is formed by spin coating
and patterning. The aluminum mirror and the chromium
half-mirror spaced by the cavity constitute a Fabry-Perot
interferometer (FPI) at the fiber end. The deformation of the
diaphragm induced by the pressure varies the cavity length.
White light interferometry is utilized to detect the
deformation of the diaphragm. Low-coherence light from a
white light source passes through the optical fiber to the
sensor, and then the light is modulated at the FPI of the
sensor. The light reflected by the sensor passes through the
optical fiber again and is detected by an optical and an
electrical processing unit. White light interferometry can
reduce the influence of bending loss of the optical fiber and
fluctuation of the light source on the output of the sensor
system.

350 °C. After bonding, the silicon part supporting the
sensing element is etched by XeF 2 .
The fabricated pressure sensor of 125 µm in diameter is
shown in Fig. 3. The deformable thin silicon dioxide
diaphragm is observed at the end of the optical fiber.
4 MEASUREMENTS
Two systems for detecting the cavity length of the
sensor interferometer were developed. One is detecting the
temporal fringe peak by using PZT scanner for changing the
cavity length of the detector interferometer. The other is
detecting the spectrum directly by using a commercialized
high-speed spectrometer.
Figure 1. Schematic of sensor system.
3 SENSOR FABRICATION
First, the sensing element is fabricated by silicon
micromachining, followed by bonding to an optical fiber end
coated by thin chromium layer as the half-mirror. Fig. 2
shows the fabrication process of the sensor structure.
(a) Silicon dioxide film is deposited on both sides of a 200
µm thick silicon wafer by plasma CVD using TEOS
(tetraethoxysilane) source. The film thickness of the top
side is 2.3 µm and that of the bottom side is 1.2 µm.
These silicon dioxide films are patterned for the mesa (top
side) which keeps the aluminum reflecting mirror flat and
the etching mask (bottom side) used in the latter silicon
deep RIE etching process (e).
(b) Silicon dioxide film (0.7 µm thick) is deposited by
atmospheric pressure CVD.
(c) Aluminum is evaporated in vacuum and patterned by
lift-off process for the reflecting mirror.
(d) Photo-sensitive polyimide is spin-coated and patterned
for the bonding layer and the spacer for the sensor
interferometer.
(e) The silicon wafer is etched through by silicon deep RIE.
Then the sensing element is released from the wafer.
(f) The sensing element is bonded to the optical fiber end
coated by thin chromium for the half-mirror. The bonding
process is carried out in a glass micro capillary as a guide
[6]. The inner diameter of the capillary is 127 µm. The
silicon column with the sensing element and a micro glass
ball of 120 µm in diameter are inserted into the capillary.
The optical fiber is inserted into the capillary so as to face
the sensing element on the silicon column. Another
optical fiber is inserted from the other side and presses the
polyimide layer of the sensing element on the fiber end.
After fixing the optical fibers, the capillary is heated up to
Figure 2. Schematic of process for micromachined
sensor structure.
Figure 3. SEM photograph of fabricated pressure sensor
of 125 µm in diameter.

4.1 PZT Scanner-type Detection System
Figure 4 shows the PZT scanner-type detection system
for the temporal fringe measurement. The system consists of
a Fabry-Perot interferometer mounted on the stacked PZT
(TOKIN, AE0203D16) to scan one of the mirrors of the
interferometer, the photo diode (HAMAMATSU
PHOTONICS, S5973) to detect the transmitted light from
the interferometer, the microcontroller (RENESAS
TECHNOLOGY, SH7045), the A/D converter (ANALOG
DEVICES, AD9220) and the D/A converter
(BURR-BROWN, DAC7644). The microcontroller transfers
scanning signal to the PZT via the D/A converter and the
amplifier for driving the PZT. When the PZT scans the
cavity length, fringe patterns are observed as shown in Fig. 5.
The normalized intensity I of the fringe patterns can be
White light source
Sensor
Output signal [mV]
100
80
60
40
20
0
0 100 200 300 400 500
Pressure [mmHg]
Figure 6. The output signal of PZT scanner-type
detection system as a function of pressure.
Photo diode
PZT Half-mirror
PZT
PZT drive
amplifier
I-V
converter
Amp.
δ/2
Optical fiber
D/A
converter
A/D
converter
Photo diode signal
100 mV
60 V
Coupler
Micro
controller
D/A
converter
2 ms
PZT scanning signal
Output
Figure 4. Schematic of PZT scanner-type detection
system.
Figure 5. Output fringe patterns captured by photo diode
when PZT scans cavity length of detector interferometer.
The peak of center fringe is obtained where cavity
length of sensor interferometer is equal to that of
detector interferometer.
δ s
/2
expressed as
I
( δ )
( δ −δ
)
⎡
2
⎛
⎤
⎡
⎢
2 ⎞
⎜ s ⎥
2π
= exp − ⎟ ⎢
⎢
cos
s
⎥
⎣
⎝ λcC
⎠
⎦
⎣ λc
⎤
( δ −δ
) ⎥⎦
(1)
where δ is the optical difference in the detector
interferometer, δ s is the optical difference in the sensor
interferometer, C is the coherence length of the light source,
λ c is the central wavelength of the light source [8]. When the
cavity length of the detector is equal to that of the sensor, the
center fringe with the highest intensity is obtained. The
intensity data captured by the A/D converter is transferred to
the microcontroller. The microcontroller identifies the
central fringe and converts the cavity length of the sensor to
the corresponding pressure value. The acquired pressure data
can be obtained via the D/A converter.
The detection system continuously captures the central
fringe. The maximum operating frequency is 1 kHz.
A pressure measurement using the PZT scanner-type
detection system is presented in Fig. 6. A least-squares fit of
these data yields the sensitivity of –0.17 mV/mmHg with the
correlation coefficient having a value of 0.9997 and the
resolution of 10 mmHg for pressures ranging from 0 to 450
mmHg.
4.2 Spectrometer-type Detection System
Figure 7 shows the spectrometer-type detection system,
which consists of the spectrometer and the personal
computer (PC). The sensor cavity length d corresponding to
applied pressure is determined by the spectrum of the
reflection light from the sensor. The cavity length d can be
obtained by using
λ λ
d =
1 2
2 n ( λ2
− λ1
)
(2)

Intensity [a.u.]
White light source
Spectrometer
Optical fiber
Coupler
Personal computer
Sensor
Figure 7. Schematic of spectrometer-type detection
system.
1400
1200
1000
800
600
400
200
0
400 450 500 550 600 650 700 750 800
Wavelength [nm]
400
300200
100
0
-100 [mmHg]
Figure 8. Reflection spectrum of developed pressure
sensor for different applied pressure.
where n represents the refractive index of the material of
the sensor cavity, λ 1 and λ 2 represent adjacent peaks in the
reflection spectrum [9]. A miniature fiber-optic high-speed
spectrometer (OCEAN OPTICS, USB2000) is used for the
sensor system. Fig. 8 shows the reflection spectrum of the
sensor for different pressures. Spectrum modulation was
obtained and the peak shifts were clearly monitored. The
cavity length calculated by the PC using the spectrum data,
as a function of applied pressure is shown in Fig. 9. A
least-squares fit of these data yields the sensitivity of –0.25
nm/mmHg with the correlation coefficient having a value
of 0.9993 and the resolution of 4 mmHg for pressures
ranging from –100 to 400 mmHg. The total rate of
sampling at the spectrometer, data transfer from the
spectrometer to PC, calculation and data display is about
70 Hz. Averaging of the acquired cavity lengths calculated
using more than one pair of peaks and interpolation
between sampling data of pixels of the spectrometer
contribute high resolution and low noise measurement.
The spectrometer-type detection system does not
contain mechanical moving parts. Therefore, this system is
more stable and reliable than the PZT scanner-type
detection system.
5 EXPERIMENTS ON ANIMALS
Experiments using animals have been carried out. The
developed spectrometer-type detection system including
the ultra-miniature fiber-optic pressure sensor was applied
for monitoring pressure in a left ventricle, left atrium, right
atrium and blood pressure in aorta of a goat. The sensor
was set in an injection needle of which outer diameter was
about 0.6 mm. The pressures are successfully monitored
and the waveform of the dynamic pressure change is
clearly observed as shown in Fig. 10.
Owing to the low sampling frequency of the sensor
1740
Cavity Length [nm]
1720
1700
1680
1660
1640
1620
1600
-100 0 100 200 300 400
Pressure [mmHg]
Figure 9. Calculated sensor cavity length as a function
of pressure.
Figure 10. Continuous display of blood pressure in aorta
of goat.

system, disturbed waveforms of pressure have been
observed.
The pressure sensor is small enough for variable
applications but a temperature change affects the sensor
output. The problem is caused by expansion of the air in the
sensor cavity. Therefore, careful calibration is required
before a pressure measurement. To avoid the thermal drift, a
packaging process using soldering to keep the sensor cavity
in vacuum has been proposed and the development is going
on [7]. Such absolute sensor is necessary for a long term
monitoring.
Considering clinical uses, we have to examine not only
fundamental sensor characteristics (e.g., thermal drift) but
the packaging and the biocompatibility as well. We continue
experiments on animals to test the structure and the
materials of the sensor.
6 CONCLUSIONS
We have developed an ultra-miniature fiber-optic
pressure sensor system. The diameter of the sensor is 125
µm. A Fabry-Perot cavity is formed at the optical fiber end
and a deformation of the diaphragm induced by pressure
varies the cavity length. The sensing element is bonded to
the optical fiber end utilizing a ring-shaped polyimide layer.
White light interferometry is adopted to reduce error and
noise caused by a bending of the optical fiber and a
fluctuation of the light source. Two measurement systems
for detecting the cavity length of the sensor have been
developed. One is detecting the temporal fringe peak by
using PZT scanner for changing the cavity length of the
detector interferometer. The other is directly detecting the
modulated spectrum of the reflection light from the sensor
interferometer using a commercialized high-speed
spectrometer. By tracking the shift of the peak wavelengths
of the spectrum, the cavity length is calculated and the
measured pressure is continuously displayed. Pressure is
successfully monitored in a heart and an artery of a goat in
real-time using the developed spectrometer-type detection
system. To realize higher sampling frequency, development
of a microcontroller-based detecting system is going on now.
In a preliminary test, sampling frequency of 500 Hz has
been achieved.
Because of the ultra-miniature sensor head, this sensor
can be installed in a catheter and a guide wire. Hence, the
developed sensor system can be utilized to monitor a local
pressure, which is required for safe and precision
interventional procedures.
Engineering Based on Bio-nanotechnology”.
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7 ACKNOWLEDGEMENTS
This work was supported in part by a Health and Labour
Science Research Grants (Research on Advanced Medical
Technology) from the Ministry of Health, Labour and
Welfare of Japan. The authors acknowledge the support of
Tohoku University 21COE Program “Future Medical