Download Few-Mode Fiber Based Sensor in Biomedical Application

Few-Mode Fiber Based Sensor in Biomedical Application
Jing Zhang
National Metrology Centre, Agency for Science, Technology and Research (A*STAR), Singapore
Email: [email protected]
ABSTRACT
A novel few-mode fiber based sensor for monitoring the vital signs of pulse (heart rate), and breathing rate (respiratory
rate) was developed. The sensor was applied in non-invasive measurement of pulse and breathing rates. The pulse,
breathing and even body movement affected the sensor’s output as the strain on the few-mode fiber changed with these
activities. This sensor has simple structure and easy to fabricate. Its signal is easy to monitor. It can be used in the
medical equipment in what situation non-invasive realtime monitoring and measurement of pulse rate, and
respiratory/body movement pattern of healthy subjects are required.
Keywords: few-mode optical fiber, fiber sensor, non-invasive measurement, vital signs
1. INTRODUCTION
Few-mode fibers have been a hot research area in the past few years. Mode-division multiplexed transmission in fewmode fibers has attracted great attention as it can increase the transmission capacity of a single optical fiber [1].
Meanwhile, with the development of these new fibers, new sensor technologies have been emerging. The higher orders
modes in a few-mode fiber are more sensitive to the bending, strain or stress, and temperature than the fundamental
mode in a single mode fiber. Kumar et al. studied a fiber-optic strain sensor based on interference between LP01-LP02
modes of a circularly symmetric few-mode fiber [2]. Fan et al. reported a novel fiber-optic bending sensor based upon
the propagation of LP21 mode [3]. By measuring LP21 mode specklegram rotation, which increments linearly with
bending angle by the stress-optic effect, the bending angle was obtained. Arnaoudov et al. reported single fiber cables or
stacked sequences of few- and multi-mode fibers could be used for perimeter sensor to detect changes it underwent
(fiber bending, torsion or movement) [4].
Meanwhile, fiber optics sensors find many applications in biomedical and medical areas. Lau et al. presented a system
enabling the non-invasive realtime monitoring and measurement of breathing rate and respiratory/body movement
pattern of healthy subjects by using microbend multimode fiber under deformation by mechanical force exerted during
breathing motion [5]. In this paper, we present the development of a few mode fiber based sensor for monitoring the
vital signs of pulse (heart rate), and breathing rate (respiratory rate).
2. THE METHOD
The basic sensing unit consists of a piece of few mode fiber spliced between two pieces of single mode fibers (Fig. 1).
We took the advantages of the higher order mode interfering with the fundamental mode in the few-mode fiber, and the
coupling between the higher order mode and the fundamental mode was subject to the stress/strain.
The fundamental mode and higher order modes are excited in the few-mode fiber by launching the light from a single
mode fiber with lateral offset. As a result of propagation constant difference, phase differences between these modes
accumulate in the few mode fiber. At the end of the few mode fiber, the second single mode fiber collects the light from
the different modes in few mode fiber offset. As the second single mode fiber only supports one mode, inter-modal
interferences happen.
SMF
FMF
SMF
Figure. 1 The basic FMF sensing unit.
Assuming there are two modes (LP01 and LP11) existing in the few mode fiber, at a given wavelength, the two modes
interfering in the second single mode fiber can be expressed as
E = E1 + E2
= sin(ωt + 2πL ∗ neff1 /λ) + sin(ωt + 2πL ∗ neff2 /λ)
= 2 sin (𝜔𝑡 + 2𝜋𝐿 ∗
𝑛𝑒𝑓𝑓1 +𝑛𝑒𝑓𝑓2
2𝜆
) cos(2𝜋𝐿 ∗
𝑛𝑒𝑓𝑓1 −𝑛𝑒𝑓𝑓2
2𝜆
)
(1)
where E1 and E2 are the interfering light waves from mode LP01 and LP11, respectively, ω is the frequency of the
propagating wave, λ is the wavelength L is the length of few mode fiber, and neff1 and neff2 are the effect refractive
index of the two guided modes. The intensity detected by a photodetector is
I ∝ cos(2πL ∗ (neff1 − neff2 )/λ).
(2)
The stress, bending and temperature distribution along the few mode fiber will affect the length (L) of the few mode
fiber and the effective indices (neff1 , neff2 ) of the modes. Stress and bending will cause the inter-modal coupling in the
few mode fiber, which causes the variation of the two modes’ optical powers and its phases (E1 , E2 ). All these changes
will contribute to the optical power change at the output after modes interference.
When inciting a wavelength and power stabilized laser into the sensing unit, the output optical power of the sensing unit
will be modulated by any change on the few mode fiber. Hence the few mode fiber can be used as a sensor to monitor
vital signs.
3. EXPERIMENTAL SETUP AND MEASUREMENTS
The length of the sensing unit should be optimized for the application. Long sensing unit will have more noise from
unwanted disturbance. Short sensing unit will have small sensing area. Few-mode fiber based sensor units were
designed to be about 5cm long for the application of measuring heart beat at the wrist. The first experimental setup
consisted of a sensing unit consisting of a piece of few-mode fiber spliced between normal single mode fibers (Fig. 2), a
wavelength and power stabilized laser source and an optical power monitor. The laser source worked at 1550nm
wavelength. The light source output power was about 0dBm. The few-mode fiber was a step-index fiber supporting two
modes at this wavelength. Large lateral offset were made at joints between few-mode fiber and the single mode fibers in
order to excite the higher order modes considerably. The larger lateral offset, the more insertion loss would be
introduced at the sensing unit.
After connecting the sensing unit, the output power level was about -10dBm. When there was no disturbance on the
sensing unit, the optical output power was stable and power fluctuation was less than 0.1dB (Fig. 3(a)). When wearing
the sensing unit on the wrist, the strain on the fiber changed, hence the coupling between the two modes in the fiber was
changed, causing the average optical output power level to be changed to about -20dBm. The optical output power
varied with the heart beat (~75 beats/minute), which had an amplitude of about 2 to 4dB (Fig. 3(b)).
FMF
Detector
Laser source
Signal
Processing
SMF
Figure. 2 The FMF based sensor for heart beat sensing on the wrist.
0
Intensity (dBm)
Quite condition
-20
(a)
0
Sensing heart beat
-20
-40
0
10
20
30
40
50
60
Time (s)
(b)
Figure. 3 (a) Reference signal under quite condition. (b) Measurement of heart beat by using one sensing unit touching with
the wrist.
One sensing unit will have small sensing area. In order to detect signals in a larger area, more sensing units can be
connected in series or parallel and distributed in a larger area. Series configuration (Fig. 4) was adopted in the
experiment. Three sensing units were cascaded in series and mounted on a chair pad as shown in Fig. 5 to sense the
vital signs of the person sitting on it. Each sensing unit was about 5cm long. The optical output power after the sensing
fiber was -19dBm. When a person sat on the chair pad, the strain on the fiber changed. Any movements, breathing, and
heart beating would affect the strain on the optical fiber. The strain on the fiber affected the optical power output from
the fiber. Hence, the person’s breathing, heart beating, and moving affected the optical output power of the sensing
fiber.
The sensing fiber output depended on the position of sitting. When the sensors happened to be a certain distance away
from the arteries, the dominant changes applied on the sensor was from breathing activity. The major optical power
variation in Fig. 6(a) was consistence with the normal breathing activity (13 breaths per minute), while the small ripple
in the major variation was consistent with the heart beating. Optical signals consisting more obvious heart beating
signal was obtained after slightly adjusting the sitting position and by controlling the breath activity (Fig. 6(b)). During
the time when breath was hold, the heart beat became dominant signal. The sharp and large signal caused by the body
movement was also observed in Fig. 6(b). By further adjusting the person’s position on the chair, clear signal with both
breath and heart beat were captured (Fig. 6(c)).
Laser source
Detector
Figure. 4 The fiber sensor in series configuration.
Signal
Processing
Signal monitoring
Laser source
Figure. 5 The fiber sensor embedded in the chair pad (cascaded configuration).
-20.2
Heart beat
Normal breath
-20.4
-20.6
(a)
Intensity (dBm)
-19.1
Heart beat
Body movement
-19.2
-19.3
Deep breath
-19.4
(b)
-15.6
Heart beat
-15.7
-15.8
Normal breath
-15.9
0
10
20
30
40
50
60
Time (s)
(c)
Figure. 6 Measurement of heart beat and breathing by using three sensing units embedded in chair pad.
The signal from the cascaded sensor units is much weaker than a single sensor, which implies that a parallel
configuration (Fig. 7) will have better signal to noise ratio. However, the cost of the sensor in parallel configuration will
be more than that in cascade configuration due to more detection channels.
Detector 1
Laser source
Detector 2
Splitter
Signal
Processing
Detector 3
Figure. 7 The parallel configuration for the fiber sensor.
4. CONCLUSION
A novel few-mode fiber based sensor for monitoring the vital signs of pulse (heart rate), and breathing rate (respiratory
rate) was developed. The sensor unit consisted of a piece of few-mode fiber spliced in between two pieces of normal
single mode fiber. One piece of single mode fiber was the light launching fiber. The other piece was the output fiber.
Due to the fact that the stress or strain would affect the coupling between the modes in the few-mode fiber, and more
than one modes existing in the few-mode fiber, the output optical power was subject to the stress or strain on the fewmode fiber and the interference between the higher order modes and the fundamental mode in the few-mode fiber. The
sensor unit was applied on the wrist near the artery, and obvious heart beat signal was obtained as the strain on the fewmode fiber changed with the pulses. Three sensor units were cascaded and embedded in a chair pad to enlarge the
sensing area. The pulse and breathing rate signals of the person sitting on the chair pad were obtained. Besides the pulse
and breathing signals, the body movement signal was also recorded. This sensor has simple structure and is easy to
fabricate. Its signal is easy to monitor and analyze. It can be used in the medical equipment in what situation noninvasive realtime monitoring and measurement of pulse rate, and respiratory/body movement pattern of healthy subjects
are required.
REFERENCES
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96km of few-mode fiber using coherent 6 x 6 MIMO processing,” J. Lightwave Technol. 30(4), 521–531
(2012).
[2] Kumar, A. ; Goel, N.K. ; Varshney, R.K., “Studies on a few-mode fiber-optic strain sensor based on LP01 LP02 mode interference”, Lightwave Technology, Journal of (Volume:19 , Issue: 3 ), pp.358 – 362, (2001).
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[4] Arnaoudov, R. ; Bock, W.J. ; Miletiev, R. ; Angelov, Y. ; Eftimov, T., “Performance evaluation of a few- and
multi-mode fiber-optic perimeter sensor”,
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