Optical active fiber sensing technique based on the lasing wavelength demodulation for monitoring the human respiration and pulse

Sensors and Actuators A 296 (2019) 45–51

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Optical active fiber sensing technique based on the lasing wavelength demodulation for monitoring the human respiration and pulse Ke Li, Li Xia ∗ , Heng Yi, Shiyu Li, Ying Wu, Yiming Song School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China

a r t i c l e

i n f o

Article history: Received 25 January 2019 Received in revised form 10 June 2019 Accepted 23 June 2019 Available online 5 July 2019 Keywords: Optical active fiber sensing Wavelength demodulation Respiration and pulse monitoring

a b s t r a c t A novel optical active fiber sensing technique based on the lasing wavelength demodulation for monitoring the human pulse and respiration is firstly proposed and experimentally demonstrated in this work. A section of no core fiber (NCF) with optimal length is selected to act as a sensing head, as well as a filter in a ring cavity to construct a kind of fiber laser. The respiration rate (RR) and pulse rate (PR) signals are obtained by measuring the shifts of lasing wavelength that caused by respiration-induced body tremor and pulse vibration. The experimental results show that the corresponding RR and PR parameters can be sensitively measured. The suitable wavelet transform is adopted to acquire the respiration and pulse signals in detail. Our proposed technique has the advantages of high sensitivity, compact sensing head, stable response, and capability to simultaneously achieve the information of multiple parameters related to the human health monitoring. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Sensing for human health related parameters, such as respiration rate (RR) and pulse rate (PR), is increasingly investigated in various fields to meet the needs of modern clinical medicine and health monitoring [1]. The detection of RR and PR can be normally realized by mechanical stress, liquid crystal, electronic film, hydraulic sensor, radar detection, optical methods and nanotechnology [2–8]. Compared to other techniques, optical fiber sensing technique offers many advantages including immunity to electromagnetic interference, non-conducting, high sensitivity and small size with light weight. It is capable of real-time monitoring, which can provide good medical reference value. There are three main types of fiber sensors for human health monitoring at present: intensity-modulated sensors, fiber Bragg grating (FBG)-based sensors and modal configuration sensors. In 2010, ˙ proposed a RR monitoring device under magnetic resZyczkowski onance imaging environment to detect the evaporated humidity from the mouth and/or nose. The technique was based on the intensity modulation of reflected light, which would accompany with loss related deviation [9]. In 2012, Carmo designed a wavelengthmodulated FBG sensor tied up in wearable garments for heart rate measurement [10]. In 2017, Manujło presented a concept of human

∗ Corresponding author. E-mail address: [email protected] (L. Xia). https://doi.org/10.1016/j.sna.2019.06.045 0924-4247/© 2019 Elsevier B.V. All rights reserved.

breath detection by means of FBG based temperature sensor, in this presented approach, temperature of exhaled air is directly measured by FBG, and its Bragg wavelength changes allow for determine the RR [11]. However these FBGs need the special and expensive UV exposure setup to fabricate. In 2012, Mathew demonstrated a miniature optical breathing sensor based on an Agarose infiltrated photonic crystal fiber interferometer. The sensor detects the variation in relative humidity that occurs between inhaled and exhaled breath, and can determine the ˇ RR and the breathing status during respiration [12]. Sprager and Zazula adopted a kind of Michelson interferometer consisting of two optical paths (arms), i.e., the sensing and reference fibers in 2013. Minute lung- and/or heart-induced changes in the sensing fiber length cause changes in the phases of reflected light. But the determination of RR and heart rate could be easily disturbed due to the instability of mirrored ends at both fibers [13]. Recently, J. Li reported a photonic sensor based on a hybrid plasmonic microfiber knot resonator. This technique for sensing RR and PR is composed of a microfiber knot resonator situated on a smooth gold film, which can excite hybrid plasmonic modes [14]. However, this optical fiber structure is hard to be fabricated and maintained for the adoption of the special processed gold film. In this work, the optical active fiber sensing technique using a kind of fiber laser with a section of NCF for monitoring human respiration and pulse is proposed. The output lasing wavelength will shift as the disturbances are applied on the NCF. By demodulating the output wavelength shift, the signals of RR and PR can

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ticular, a self-imaging peak can work as a filter which can be used for wavelength selection in fiber laser [17]. According to the theory of multi-mode interference, the peak wavelength 0 of the MMI filter response can be expressed as the following equation [18]: 0 = P( Fig. 1. Schematic of SNS fiber sensing head.

be obtained independently or simultaneously. The technique can offer a high sensitive and stable response with a compact and easy fabricated sensing head.

2 nNCF DNCF

L

), P = 0, 1, 2,

(1)

where nNCF and DNCF are the effective refractive index (RI) and mode field diameter of the NCF, respectively. L is the NCF length and P is the self-image number. According to the transformation of f Eq. (1), we can get:



L=P

2 nNCF DNCF

0



, P = 0, 1, 2,

(2)

2. Operation principle and theoretical analysis The NCF used in this experiment is a kind of optical fiber with only solid cladding. The refractive index of the NCF cladding and surrounding air is 1.4446 and 1.0003 respectively. The diameter of NCF is 125 ␮m. The proposed sensing head consists of a single mode-NCF-single mode (SNS) fiber structure, as shown in Fig. 1. When the light is launched into the lead-in single mode fiber (SMF), higher order mode will be excited in NCF because of mode field mismatch. Since there is no coating in the NCF used, it is more sensitive to the changes of surrounding than normal multimode fiber (MMF) [15]. The environmental parameter variations will induce changes of the parameters of NCF such as effective refractive index and length, which result in the changes of the light field in the fiber and lead to the peak shift of received spectrum through multimode interference (MMI). Therefore, the SNS fiber structure can be applied for the sensing RR and PR by measuring the peak shift. The simulation results through beam propagation method (BPM) are shown in Fig. 2. Due to the self-imaging effect, the input light field periodically recurs in propagation direction [16]. The high order modes transmitting in NCF will interfere with each other causing the energy redistributed, as illustrated in Fig.2(a). The energy will gather to a peak when the modes phase difference is 2␲ in NCF. The normalized intensity variation along the NCF is shown in Fig. 2(b). With the light field gathering at the end of the NCF, the self-imaging peak can be found in the transmitted optical spectrum. From Fig. 2(b), it is noticed that the re-imaging length within the NCF is 14.8 mm at a wavelength of 1527.335 nm. In par-

When the emission wavelength of EDFA is stable at a specific power, the excitation wavelength in the fiber ring laser cavity is determined. At this time, we choose the first self-image peak as the sensing light wave. That is, the value of P is 1, and we can deduce that L is about 14.8 mm. This explains why 1527.335 nm is chosen in this experiment as the input wavelength in the previous numerical simulation stage. When a perturbation such as a respiration or pulse vibration appears, it will cause a dynamic strains variation on the NCF, and the fiber length will vary subtly with the mechanical strain. Thus it will cause a fluctuation in the received spectrum of the SNS structure. According to Eq. (1), RR and PR can be obtained by detecting wavelength shift when respiration and pulse vibration act on the SNS. 3. Experimental results and discussions The structure of the whole ring cavity laser is shown in Fig. 3. In the configuration, the light is emitted from an Erbium Doped Fiber Amplifier (EDFA, Amonics). The isolator (ISO) is used to obtain unidirectional propagation. The light is divided by a 99:1 optical coupler (OC) into two parts. The 99% one still transmits in the laser ring loop, and the 1% part is launched into an optical fiber demodulator (OFD, GS-YB-FBG-I. OFD is a high-speed Optical fiber ¨ wavelength demodulating device based on Diffraction object Grat¨ ¨ ¨ Conversion Arraywith scanning wavelength ingand Photoelectric in C/C + L band and sampling frequency in 200/2000/8000 Hz) for

Fig. 2. (a) Intensity distribution of the calculated field, (b) normalized optical intensity along the NCF.

K. Li et al. / Sensors and Actuators A 296 (2019) 45–51

Fig. 3. Schematic diagram of the sensing system.

the wavelength monitoring with the resolution of 1 pm. The SNS structure can be used as a filter in the tunable fiber laser. The transmission spectra measured by an optical spectrum analyzer (OSA) is shown in the Fig. 4. The lasing wavelength and the 3 dB bandwidth are 1527.3335 nm and 0.02 nm, respectively. When the perturbation, such as variation of strains induced by respiration or pulse, is applied on the SNS fiber structure, it will cause the dynamic variation of lasing wavelength. Then, in order to test the effect of strain applied to SNS fiber structure, a tunable fiber laser system was built in this project, and the micro-strain verification experiment was carried out. The specific diagram is shown in Fig.5. The SNS optical fiber sensor is fixed at both ends of the strain adjustment platform as shown in Fig. 5. The adjustment precision of spiral alignment is 10 ␮m. The finetuning experimental platform makes the sensing structure in the exact straightening state, thus avoiding the influence of the deformation of the SNS structure in the relaxation transition state. The screw microneedle on the adjusting frame is adjusted to exert axial stress on the SNS sensing structure. Because the deformation of the human cardiopulmonary system is relatively weak, and the original structure itself is relatively fragile, so the applied stress cannot be too large in actual operation. Five strain points were measured with 10 ␮m as step unit. Each adjustment corresponds to 100 ␮␧. The laser spectra of each adjustment are recorded. The corresponding laser spectra under different strains were shown in Fig. 6.

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The experimental results of stress and wavelength shift are recorded in Table 1. As shown in Fig. 7, the data in Table 1 are plotted as a linear graph. Through the axial strain experiment of SNS fiber structure, it is found that the structure is very sensitive to the axial strain. The original data is shown by the red-blue dot line, and the linear fitting is blue dotted line. It can be seen that the fitting effect is very good, R2 = 0.9995, and the stress sensitivity is about 0.634 pm/␮␧. Fig. 8 is the photograph of the respiration and pulse measurement system in the actual test. In this experiment, the NCF length is about 15 mm, which roughly corresponds to the first self-imaging length of MMI. The room temperature is kept stable at 26◦ , and the subject of this experiment is a 28-year-old healthy graduate student. The surface body temperature of the subject is constant at 36.7 ◦ . For the measurement of the RR, the SNS structure is placed on the subject’s chest with a medical tape when the subjects are lying on their backs in a soft reclining chair. The data is obtained by OFD in three minutes for more comprehensive information. After data processing, it can be seen in Fig. 9(a) that obvious respiration waveform is obtained. Though wavelet transform, the time domain signal becomes smooth and clear. The first 20 s of the denoised signal are shown in Fig. 9(b). By applying Fourier transform on the data shown in Fig. 9(b), the spectrum of the respiration waveform is calculated and presented in Fig. 9(c). As can be seen, the frequency of respiration is 0.288 Hz corresponding to about 17 respiration per minute (rpm). The real respiration number is also counted in one minute by a clock. 17 rpm are completely consistent with the real test situation. To obtain the PR, we adjust the incline angle of the soft reclining chair to let the subject sit obliquely, and put the sensing head on the wrist radial artery with a medical tape. In this test, in order to minimize the disturbance, we ask subject to control his breath temporarily to reduce body shaking. Fig. 10(a) shows the realtime pulse trace. After the wavelet denoising, the time-dependent changes of the pulse signals are distinctly shown in Fig. 10(b). The frequency and normalized amplitude of the pulses are visibly exhibited in Fig. 10(c). It can be seen from Fig. 10 that the monitored pulse frequency of subject is 1.257 Hz corresponding to circa 75 heart beats per minute (bpm). 75 bpm are in agreement with the real test results of 74 bpm. An iconic radial artery pulse waveform is illustrated in Fig. 11 (which is the enlarged part of 7–9s detail in Fig. 10(b)). It contains three main peaks marked in red dotted circles, including early sys-

Fig. 4. Lasing wavelength shift induced by the axis strain.

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Fig. 5. Strain test device diagram.

Fig. 6. Wavelength shift spectrum of corresponding axial strains spectrum.

Table 1 Strain variation and corresponding wavelength shift. Strain(␮␧)

0

100

200

300

400

500

Wavelength(nm) Variation(pm)

1527.334 0

1527.399 65

1527.463 129

1527.531 197

1527.596 262

1527.661 327

Fig. 7. Relation between axial strain and wavelength variation.

tolic peak (peak a), late systolic peak (peak b), and diastolic peak (peak c). The outline of waveform in Fig. 8 is known to be the result of the blood pressure from the left ventricle contracting and reverberation from the lower torso. Significant clinical parameters can

be extracted from these peaks. This image proves that our sensing technique can clearly detect the details of vital pulse. Furthermore, our sensing technique can be used to detect RR and PR simultaneously. As mentioned before, the above results

K. Li et al. / Sensors and Actuators A 296 (2019) 45–51

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Fig. 8. The respiration and pulse measurement system.

Fig. 11. Enlarged part of the waveform from Fig. 9 (a), showing the specific features vital for cardiovascular health monitoring and diagnosis.

(Fig. 10) were recorded with the subjects controlling their breath. Indeed, when the subject exhibits a normal respiration rate, the breath will cause muscular contractions and relaxations, which can

induce body tremor. In this case, the line shape will significantly differ, as shown in Fig. 12(a). An envelope waveform lasting for around 3–4 s is observed, and four human pulse peaks can be seen

Fig. 9. (a) Detection of the respiration, (b) the respiration waveform denoised by the wavelet, (c) the frequency spectrum of the pulse.

Fig. 10. (a) Detection of the pulse, (b) the pulse waveform denoised by wavelet, (c) the frequency spectrum of the pulse.

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Fig. 12. (a) Simultaneous detection of pulse and respiration, (b) the wavelet denoised waveform of the pulse and respiration, (c) the frequency spectrum of the pulse and respiration.

in the envelope, as shown in the red circle marker in Fig. 12(b). Fig. 12(c) shows the specific frequency spectrum of this case. The red circle markers (f1 and f2 ) in Fig. 12(c) represent the respiration frequency of 0.308 Hz (18 rpm) and the pulse frequency of 1.249 Hz (75 bpm), respectively. And it is a elementary knowledge in the field of medicine that the ratio of RR to PR is approximately 1:4 in a healthy human population. This measurement results show that our sensing technique has promising prospects for simultaneously monitoring RR and PR according to the common sense.

[5]

[6]

[7]

[8]

4. Conclusions A novel fiber pulse and respiration sensing technique based on the self-imaging effect has been proposed and experimentally demonstrated. With numerical simulation, the NCF part with suitable length is adopted to compose the sensing head. Based on the monitoring of the lasing wavelength, a highly sensitive and simultaneous detection of the RR and PR in real time is achieved. By adopting the wavelet denoising, the details of the respiration and pulse are obtained. The experimental results demonstrate that our pulse and respiration sensing technique can clearly detect clinical and physiological wrist pulse and respiration signals. It is prospected that our technique may pave a new road to the smart integration of various attachments in the human health monitoring applications. Acknowledgments This work is supported by the project of the National Natural Science Foundation of China (No. 61675078).

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Biographies Ke Li received the B.Eng degree from Changchun University of Science and Technology, Changchun, China, in 2014. Now he is working on the M. Eng degree in Huazhong University of Science and Technology. His recent research interests are the designing and testing of fiber optical sensors and fiber optical sensing systems. Li Xia is the professor of Huazhong University of Science and Technology, Wuhan, China. He received the B.Sc., M.Sc., and Ph.D. degrees in Electronic Engineering from Tsinghua University, China, from 1994 to 2004. His research fields include the design

K. Li et al. / Sensors and Actuators A 296 (2019) 45–51 and fabrication of passive fiber-based devices, optic sensing applications, optical surface plasma resonance techniques, etc. Heng Yi received the B.Eng degree from Huazhong University of Science and Technology, Wuhan, China, in 2016. Now he is working on the M.Eng degree in Huazhong University of Science and Technology. His recent research interests are the designing and testing of fiber optical sensors and fiber optical sensing systems. Shiyu Li received the B.Eng. degree from Huazhong University of Science and Technology, Wuhan, China, in 2018. Now he is working on the Ph.D. degree in Huazhong University of Science and Technology. His recent research interests are the designing and testing of fiber optical sensors and microfluidic chips.

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Ying Wu received the B. Eng degree from Huazhong University of Science and Technology, Wuhan, China, in 2016. Now he is working on the Ph.D. degree in Huazhong University of Science and Technology. His recent research interests are the modulation and demodulation of optical signals and the designing and testing of fiber optical sensing systems. Yiming Song received the B.Eng degree from Huazhong University of Science and Technology, Wuhan, China, in 2017. Now he is working on the M.Eng degree in Huazhong University of Science and Technology. His recent research interests are the analysis of propagation of optical signal and the designing and testing of fiber optical sensing systems.