**1. Introduction**

In Japan, there is surge in demand for medical care of the elderly as their population continues to increase [1]. This is causing a serious concern especially considering the prevailing shortage of medical staff. Meanwhile, since the Tokyo Olympic Games will be held in 2020, there is a high need for self-healthcare management among healthy people. A simple home health system to monitor the vital signs in elderly people is becoming an absolute necessity, as there is increasing demand for their self-health management on a daily basis. Vital signs are fundamental indicators of human health. These indicators include heart rate, respiration rate, blood pressure, body temperature, level of consciousness.

In order to meet such needs, wearable sensors are being developed by manufacturers to monitor vital signs [2–4]. These sensors are glasses or wristwatch type, they have a characteristic that can measure vital signs continuously. Most of these sensors are of photoelectric pulse wave type measuring the changes in light absorption caused by hemoglobin in blood vessels. These sensors are compact, portable, and easy to install on a human body. However, there are a few issues with these sensors: moisture noise caused by perspiration, skin damage due to the probe pressure [5], and dependence of signal strength on probe mounting position [6]. In addition, people have psychologically stressful for people who do not use wrist watches or

eyeglasses from attaching these type wearable sensor. Many photoelectric pulse wave sensors can measure only the pulse rate and cannot measure blood pressure. The currently used measuring many sphygmomanometers are of stationary type and therefore cannot be carried by hand. Accordingly, they are not suitable for home use and continuous monitoring.

The FBG sensor is an optical fiber type highly accurate strain sensor. The FBG sensor has a feature that a plurality of sensors can be installed with one interrogator, the optical fiber length is 1 km or more. From these features, FBG sensors are used in building and civil engineering fields. Tam et al. have introduced FBG sensors in railway rail monitoring systems [7, 8]. There are research studies reporting measurement of vital signs using FBG sensors [9–11]. Furthermore, since the sensor part is an optical fiber, it can be introduced into a textile product [12]. Therefore, the FBG sensor is introduced into the wristband or the sleeve of the shirt, and the sensor can be installed on the living body simply by wearing the textile product.

The authors propose that the FBG sensor is installed to the pulsation point of the skin surface and the vital sign can be calculated from the measured signal. The vital signs such as pulse rate, respiratory rate, stress load, and blood pressure are calculated from the measured signal of FBG sensor. In this paper, the details of the strain signal measured at a pulsation point of a human body with the FBG sensor, method of calculation from the measured signal, and measurement accuracy for each vital sign are described.

## **2. FBG sensor system**

An FBG sensor system is composed of an interrogator part with a light source and a detector, and a sensor part with an optical fiber. The schematic and the specifications of the FBG sensor system used in this study are shown in **Figure 1** and **Table 1**, respectively. We used the FPG interrogator system, named PF25-S01 (Nagano Keiki Co., Ltd.) [13]. This interrogator is equipped with an ASE light source that emits near infrared light with a wavelength of 1525–1570 nm that passes through the core of the optical fiber.

In the FBG sensor, a diffraction grating is formed when the refractive index is periodically changed along the axis of the core of the optical fiber. The FBG sensor reflects only a specific wavelength corresponding to the interval period of the diffraction grating. The wavelength of the reflected light from the FBG is called Bragg wavelength that follows the Eq. (1),

$$
\lambda\_{\text{Bragg}} = \mathcal{Z} n\_{\text{eff}} \Lambda \tag{1}
$$

**45**

*Vital Sign Measurement Using FBG Sensor for New Wearable Sensor Development*

**Interrogator Optical fiber**

Wavelength (nm) 1525–1570 Cladding diameter

±0.1

In each detector, the detected light is photo-electrically converted into an electric

Size D × W × H (mm) 230 × 330 × 100 Material Silica glass (Core: Ge)

(μm)

145

Weight (kg) 4 Mode Single mode

Light source ASE Fiber diameter (μm) 250

Power (mW) 30 Core diameter (μm) 10.5 Sampling rate (kHz) 10 Detection range (nm) 1550 ± 0.5

signal that is further converted to a digital signal by an analog/digital converter. Subsequently, the phase angle is demodulated, and a wavelength shift (proportional to the displacement and distortion) is calculated. The method has an advantage in that the resolution of wavelength measurement is finer compared with other methods. By this method, the pressure of the FBG sensor part is detected by measuring

**3. Relationship between human body signal and FBG sensor signal**

Experiments were performed using a biological model (**Figure 2**) to study the signals measured by an FBG sensor attached to a living body. A piston was employed to simulate the heart, and a blood-mimicking fluid (manufactured by CIRS) was used to simulate the blood. The movement of the piston was controlled to set the flow rate of the pseudo blood passing through a 500-mm-long acrylic pipe (inner diameter 8 mm) that simulated a blood vessel. For a phantom biological model, Flow Phantom (Supertech, Inc., ATS 524), was used. A flow phantom, made of a rubber material, was used to simulate the resilience of a living body (artificial skin), and it was provided with a hole of 8 mm diameter located at a depth of 15 mm from the top surface. A pipe of 8 mm inner diameter, made of vinyl

the displacement in Bragg wavelength.

Detector InGaAs PIN PD

Wavelength resolution

*Specification of FBG sensor system.*

(pm)

**Table 1.**

*DOI: http://dx.doi.org/10.5772/intechopen.84186*

**Figure 1.** *FBG sensor system.*

where, *<sup>λ</sup>Bragg* is the Bragg wavelength, *neff* is the effective refractive index of the grating portion, and Λ is the grating interval. Since the effective refractive index is constant during a measurement, the Bragg wavelength changes accordingly as the lattice spacing changes. Therefore, when the Bragg wavelength varies, the lattice spacing changes due to the strain in the sensor part. Any distortion applied to the sensor section is detected based on this principle.

The Bragg wavelength reflected by the sensor portion passes through a circulator and is directed to a detection device that is a Mach-Zehnder interferometer. The optical path difference of the interferometer is approximately 5 mm. The homodyne detection method using the Mach-Zehnder interferometer detects the shift length of Bragg wavelength as interference phase shift [14, 15]. The Mach-Zehnder interferometer provides the light outputs through three detectors.

*Vital Sign Measurement Using FBG Sensor for New Wearable Sensor Development DOI: http://dx.doi.org/10.5772/intechopen.84186*

**Figure 1.**

*Fiber Optic Sensing - Principle, Measurement and Applications*

home use and continuous monitoring.

textile product.

sign are described.

**2. FBG sensor system**

through the core of the optical fiber.

wavelength that follows the Eq. (1),

sensor section is detected based on this principle.

ferometer provides the light outputs through three detectors.

eyeglasses from attaching these type wearable sensor. Many photoelectric pulse wave sensors can measure only the pulse rate and cannot measure blood pressure. The currently used measuring many sphygmomanometers are of stationary type and therefore cannot be carried by hand. Accordingly, they are not suitable for

sensor has a feature that a plurality of sensors can be installed with one interrogator, the optical fiber length is 1 km or more. From these features, FBG sensors are used in building and civil engineering fields. Tam et al. have introduced FBG sensors in railway rail monitoring systems [7, 8]. There are research studies reporting measurement of vital signs using FBG sensors [9–11]. Furthermore, since the sensor part is an optical fiber, it can be introduced into a textile product [12]. Therefore, the FBG sensor is introduced into the wristband or the sleeve of the shirt, and the sensor can be installed on the living body simply by wearing the

The FBG sensor is an optical fiber type highly accurate strain sensor. The FBG

The authors propose that the FBG sensor is installed to the pulsation point of the skin surface and the vital sign can be calculated from the measured signal. The vital signs such as pulse rate, respiratory rate, stress load, and blood pressure are calculated from the measured signal of FBG sensor. In this paper, the details of the strain signal measured at a pulsation point of a human body with the FBG sensor, method of calculation from the measured signal, and measurement accuracy for each vital

An FBG sensor system is composed of an interrogator part with a light source and a detector, and a sensor part with an optical fiber. The schematic and the specifications of the FBG sensor system used in this study are shown in **Figure 1** and **Table 1**, respectively. We used the FPG interrogator system, named PF25-S01 (Nagano Keiki Co., Ltd.) [13]. This interrogator is equipped with an ASE light source that emits near infrared light with a wavelength of 1525–1570 nm that passes

In the FBG sensor, a diffraction grating is formed when the refractive index is periodically changed along the axis of the core of the optical fiber. The FBG sensor reflects only a specific wavelength corresponding to the interval period of the diffraction grating. The wavelength of the reflected light from the FBG is called Bragg

*λBragg* = 2*neffΛ* (1)

where, *<sup>λ</sup>Bragg* is the Bragg wavelength, *neff* is the effective refractive index of the grating portion, and Λ is the grating interval. Since the effective refractive index is constant during a measurement, the Bragg wavelength changes accordingly as the lattice spacing changes. Therefore, when the Bragg wavelength varies, the lattice spacing changes due to the strain in the sensor part. Any distortion applied to the

The Bragg wavelength reflected by the sensor portion passes through a circulator

and is directed to a detection device that is a Mach-Zehnder interferometer. The optical path difference of the interferometer is approximately 5 mm. The homodyne detection method using the Mach-Zehnder interferometer detects the shift length of Bragg wavelength as interference phase shift [14, 15]. The Mach-Zehnder inter-

**44**

*FBG sensor system.*


#### **Table 1.**

*Specification of FBG sensor system.*

In each detector, the detected light is photo-electrically converted into an electric signal that is further converted to a digital signal by an analog/digital converter. Subsequently, the phase angle is demodulated, and a wavelength shift (proportional to the displacement and distortion) is calculated. The method has an advantage in that the resolution of wavelength measurement is finer compared with other methods. By this method, the pressure of the FBG sensor part is detected by measuring the displacement in Bragg wavelength.
