1. Introduction

Tele-medicine or tele-healthcare has recently attracted a lot of attentions to monitor the elderly at home for significantly saving the medical resources. Usually, many electronic devices have been employed to detect the physiological or physical signals such as blood glucose concentration, blood pressure, gesture, and so on whereas only very few optical devices like oximeters [1–6] are used in the tele-healthcare systems. This is because the optical sensors are usually more bulky and expensive. However, the optical sensors could provide an alternative solution for high accuracy measurements based on interferences [7–13]. Conventionally, the Mach-Zehnder interferometer (MZI) has been extensively employed to play as optical sensors [14–18] based on two-beam interference. The standard MZI usually comprises two 3 dB couplers and two physically separated fibers. More recently, the monolithic abrupt-tapered Mach-Zehnder interferometer (AT-MZI) with integrated two 3 dB couplers and phase shifter in a single fiber has been proposed to make miniaturized fiber sensors [14–18] to detect the index variations from an extremely low liquid volume [14]. The abrupt-tapering method [19] by splicing two ellipsoidal heads or by heating and stretching optical fibers [20] is an efficient way to convert part of the core mode into cladding modes through an abrupt taper and the excited cladding modes can subsequently interfere with the core mode at the second abrupt taper next to the first one within a distance less than 180 μm [14]. Such kind of miniaturized fiber sensors could be promising to be used in a tiny or cramped space or for measurements of the specimen with a very limited volume. Though the AT-MZI had been employed for several kinds of sensing applications [14–18], it has not been proposed to serve as vibration sensors.

In this work, we demonstrate the vibration sensors by laterally introducing the acoustic waves, generated by an actuator, to the phase shifter (the span fiber between two abrupt tapers) of an AT-MZI to produce phase shift to change the resonant wavelengths. The un-tapered fiber between two abrupt tapers plays a role as a phase shifter to generate the optical path length difference (OPLD) and the advantages of using Er/Yb codoped fiber (EYDF) is to provide distinct OPLD between the core and cladding modes. This is because the rare earth ions are not easy to be diffused during flame heating and tapering, which makes the core boundary is not blurred [21]. In addition, the resonant dips can be all-optically tuned by changing the OPLD between the core mode and cladding modes [21]. On the other hand, the acoustic wave can also be guided to the phase shifter to perturb the phase relationship between core and cladding modes to move around the resonant dips. When a cw laser wavelength is intentionally selected at the steep edge of the certain spectral resonant dip, the cw light can thus be indirectly modulated into pulses by the external acoustic signals. Thus, the AT-MZI can be used as an acoustic-wave-sensitive vibration sensor. In contrast to the fiber vibration sensors using microcatilever [22], fiber Bragg gratings [23], and frequency beating from two longitudinal modes [24], the AT-MZI vibration sensor has a good contrast ratio between on and off state and it is robust, portable, and is capble of providing simultaneous measurements of pulse rate and elasticity. Moreover, it is also simpler and more cost-effective for the measurement systems. This sensor was also placed on the wrist of human body to detect the heart-beat pulses accurately. The pulsewidth and pulse shape including the rising and falling time and multi-peaks could be useful to reflect the internal characteristics of the blood vessels. This acoustic wave sensitive AT-MZI is simple, accurate, cost-effective, real-time and in situ monitoring and could be promising to be used in some harsh and cramped environments. It could also be useful to record and analyze the pulse manifestation such as deep pulse, smooth pulse, and stringy pulse, of human body for Chinese medicine in a more scientific way. Moreover, the measurement on the elasticity of a polymeric tube was also done in this work. The experimental results show that this AT-MZI could be highly promising for evaluating the elasticity of the tube like blood vessel, which could be an important precaution signal for monitoring of many diseases of human body.

2. Experimental set-up and characteristics of AT-MZI as a vibration sensor

To test the accuracy of the pulse rate measurements, the acoustic waves generated by an electrostrictive actuator (EA) device are introduced into the phase shifter of AT-MZI made of EYDF to move the resonant wavelengths around. An external cavity diode laser (ECDL) with a narrow linewidth is intentionally selected to be operating at around the sharp edge of a certain resonant wavelength. Thus, the acoustic wave can indirectly modulate the cw laser power into pulsed signals. The repetition rates as well as the pulse width are important variables for pulse rate and elastic coefficient measurements. Figure 1(a) shows the experimental set-up of the acoustic wave sensitive AT-MZI as a vibration sensor. The tip of the EA device fully attaches against the center of spanning fiber between two abrupt tapers, as shown in Fig. 1(b). The oscillation of actuator is controlled and triggered by the square wave signals from the function generator to make the phase shifter moving back and forth and to move the resonant dip wavelengths around. This AT-MZI is further employed to measure the pulse rate, shown in Fig. 1(c), and the elasticity of a polymeric tube, shown in Fig. 1(d). The use of EYDF for making AT-MZI is because the rare-earth ions are much more difficult, compared with germanium, to be diffused during the flame tapering process [21]. Accordingly, the core and cladding boundary is not blurred and thus a larger OPLD can be reached in a short device length. The absorption of EYDF is negligible due to the short device length. However, the EYDF in this work should avoid the pump laser light at 975 nm due to its huge absorption to change the inversion rates and the original resonant conditions [21]. In addition, the span length between two abrupt tapers is crucial to the free spectral range (FSR) and extinction ratios [25]. A longer span length can lead to a smaller FSR but poor extinction ratios. A smaller FSR can go with resonant dips with steeper rising and falling edges in spectra, which can give rise to a higher modulation depth and shorter switching time for real-time monitoring. Hence, there is a trade-off between a shorter device length and a shorter switching time. In this work, we choose the AT-MZI with a longer span length to achieve high sensitivity detections on the slow varying vibration signals. For vibration sensing, the diameters (D₁ and D₂) of the two abrupt tapers and the span length L (length of phase shifter) between them in the AT-MZI are 35.8 μm, 40.5 μm, and 25 mm, respectively. In the beginning, an EA device is operating at 160 V to make the phase shifter tightly stretched. When the tip of EA is subsequently operating at 10 V, the actuator moves to make the phase shifter return to a normal state and the OPLD between core and cladding modes changes accordingly. When the wavelength of the ECDL is tuned to be around the sharp edge of the resonant curves, the ECDL can be indirectly modulated by the EA. The wavelength shift of the AT-MZI is 0.475 nm when the EA changes its operating voltage from 160 V to 10 V, as shown in Fig. 2(a) under a 0.1 nm optical resolution (RES) of OSA. The EA is subsequently driven by a square wave electric signal to generate the acoustic wave to dynamically move around the resonant dips. The corresponding displacement of the actuator (from 160 V to 10 V) is 75 μm. Thus, the displacement sensitivity is 0.0063 nm/μm. It seems not to be very high. The wavelength shift can be further improved by using a shorter phase shifter since an effective displacement along the transverse direction at the two abrupt tapers can be significantly increased to change the excited cladding order. However, a shorter phase shifter will give rise to a flatter spectral curve, which will increase the switching time between the on state and off state. As a vibration sensor, as long as the amplitudes of the vibration signals are detectable by the AT-MZI, it is preferred to measure the vibration frequency than to obtain the good displacement sensitivity. The AT-MZI with sharper spectral curves here is advantageous for a shorter switching time, a better modulation depth, and a wider operating frequency bandwidth. Thus, displacement sensitivity of 0.0063 nm/μm is qualified to tell the vibration frequency. In Fig. 2(b), a cw ECDL at 1497.05 nm is intentionally tuned to fit one of the resonant dips to be attenuated when the EA is not triggered. Then the EA actuator is subsequently triggered under a frequency of 150Hz to move around the resonant dips of the AT-MZI. When the resonant dips red shifts by EA, the ECDL at 1497.05 nm is not significantly attenuated. When the EA returns to the original position, the ECDL at 1497.05 nm is substantially attenuated again. Consequently, the ECDL is externally modulated by the AT-MZI with a contrast ratio of 17.31 dB and the modulated laser output pulses are shown in Fig. 2(c) at a modulation speed of 150 Hz when a high speed InGaAs photodiode is used. The measured repetition rate and pulse width of the modulated laser is 147.1 Hz and 3.2 ms, respectively. The repeatability is good under a continuous test for more than 30 minutes when the AT-MZI is sealed and mounted on the glass texture placing on an optical desk. The detectable maximun vibration frequency is currently restricted by the frequency responses of the EA device below 150 Hz. It is promising to be much higher in the future works if an actuator like piezoelectric transducer with a higher operating frequency is employed.

 

Fig. 1 (a) Experimental set-up of the acoustic-wave-sensitive AT-MZI. FG: function generator, OSA: optical spectrum analyzer, EA: electrostrictive actuator, ECDL: external cavity diode laser, OSW: optical switch, OS: oscilloscope. (b) Microphotograph of the EA attaching against the phase shifter of the AT-MZI under a 1000x CCD microscope. (c) Photo of the micro AT-MZI attaching against the wrist. (d) Experimental set-up of the micro AT-MZI for evaluating the elasticity of a gas-inflated polymeric tube.

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Fig. 2 (a) Spectral responses of the resonant dips at around 1497.05 nm when the actuator is operated at different driving voltages. (b) The AT-MZI-modulated ECDL with a contrast ratio of 17.31 dB. (c) Laser pulse trains of the AT-MZI-modulated ECDL at 1497.05 nm wavelength.

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3 Pulse rate and elasticity measurements based on micro AT-MZI

Based on the above mentioned AT-MZI, the sensor with a more compact size is more stable and more accurate when it is used on human body. Thus, an AT-MZI with a shortened L of 5.5 mm and a D₁ and D₂ of 53.88 μm and 44.54 μm, respectively, is made and attached against the wrist of a student as shown in Fig. 1(c). This micro AT-MZI is fixed on a smooth plastic texture with a small window and through this window the phase shifter can contact with the skin. The pulse at the wrist is more obvious than other area and can give impulses to the phase shifter to change the OPLD. This micro AT-MZI with a shorter L can cover a smaller skin area to lead to higher accuracy of pulses. The pulses give perturbations to the phase shifter to modulate the ECDL at 1503.5 nm and the induced contrast ratio between on state and off state is 4.54 dB, shown in Fig. 3(a) . This ratio can be further improved by properly selecting the AT-MZI with sharper spectral curves. The measured pulse characteristics are shown in Fig. 3(b) where the repetition rate is 1.042 Hz. The modulation depth and pulsed width of the AT-MZI-modulated ECDL is 4.54 dB and 240 ms, respectively. The actual pulse rate counted by pressing the fingers on the wrist is the same with the results taken by the oscilloscope. The accuracy for pulse rate measurement is good since the AT-MZI is perturbed by the heart-beat pulses at the wrist under a rest condition. From Fig. 1(c), it is shown that an AT-MZI embedded in a wristband is promising to measure the pulse rates when one is in motion. This micro AT-MZI can be further miniaturized to a device length of less than 200 μm [1] and be employed in the telehealthcare systems as a pulse-rate counter in the future. In contrast to the commercial electronic pulse-rate counter, the fiber-optic micro AT-MZI cannot only measure the pulse rate but also reflect the health condition of the artery. It is very important to monitor the elasticity of the artery to give precaution signals to prevent the brain paralysis, myocardial infarction, heart diseases and the hardening of the arteries in advance. Conventionally, the working principle of measuring the elasticity of arteries is to block the arteries using an inflated air bag first. Then, the air is gradually vented while the recovery time is measured. Similarly, by attaching the micro AT-MZI, the third sample with L = 5 mm and (D₁, D₂) = (52.05 μm, 52.22 μm), against an 18-cm-long polymeric tube with a hollow core diameter of 3.1 mm and a side wall thickness of 245 μm, as shown in Fig. 1(d), the polymeric tube was inflated with hydrogen gas under the control by a precision mass flow controller at an air flow rate of 30 SCCM and 50 SCCM, respectively. The corresponding pulse characteristics including the rising and falling time from the inflated polymeric tube were also investigated to study the elasticity of the polymeric tube. The end of the polymeric tube was blocked and opened repeatedly. The blocking time is about two seconds for each time. The corresponding rising from the micro AT-MZI is around 2 seconds. However, the respective recovery (falling) time for 30 SCCM and 50 SCCM is about 0.06 and 0.094 seconds, as shown in Fig. 3(c). A short recovery time reflects a good elasticity of the tube, especially under a higher gas pressure. Clearly, the diameter expansion of the polymeric tube as well as the recovery time is linearly proportional to the inflated air pressure. The phenomena of linear responses show that the tube is still working well with good elasticity. If the nonlinear responses are found, the tube is under a fatigued condition, just follows the Hook’s law. Accordingly, the elastic coefficient of the arteries can be evaluated, especially for the arteries at the uneven area like the cervix. Based on Fig. 3(c), this micro AT-MZI is therefore effective for simultaneous measurements of pulse rate and elasticity by respectively counting the time period and measuring the slope of the curves. It is also highly promising to measure the elastic coefficient of arteries in human body as a topic of the future work. In the practical applications, an AT-MZI with an edge fitting to the laser wavelength from a laser diode can be made easily by precisely controlling the fabrication parameters such heating temperature and stretching speed of the steeping motors. The spectral characteristics of the AT-MZI are also environmentally stable when it is operating under the temperature of 50°C [26]. This micro AT-MZI is simple, cost-effective, and is capable of providing a non-invasive method of measurement. The repeatability for elasticity measurement is good since the AT-MZI is perturbed by the gas-inflated polymeric tube on an optical desk. In comparison with other existing techniques using microcantilevers, fiber Bragg gratings, and frequency beating [22–24], the performance of the AT-MZI for vibration, heart-beat pulse rate, and elasticity measurements are compact, portable, and integratable in the wristband when a diode laser and a photodiode are used for real applications in motion.

 

Fig. 3 (a) The externally modulated ECDL with a contrast ratio of 4.54 dB between on state and off state. (b) Laser pulse trains of the AT-MZI-modulated ECDL at 1503.5 nm. (c) Pulse characteristics of the inflated polymeric tube when the end of the tube is blocked and opened.

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4. Conclusion

We have demonstrated the simultaneous measurements of the heart-beat pulse rate of human body and the elasticity of a polymeric tube using an optical sensor based on the micro abrupt-tapered Mach-Zehnder interferometer. An acoustic wave generated from the pulsed blood pressure can change the OPLD between core and cladding modes to change the resonant dips. The wavelength shift is at least 0.475 nm to attenuate or not attenuate the external cavity diode lasers. The laser pulses show that the repetition rate can follow the pulse from human body very well. The elasticity of the polymeric tube can be evaluated based on the rising and falling time of the pulses, and similar measurement approach may be applicable for evaluating the elasticity of the arterial wall of blood vessel. The micro Mach-Zehnder interferometer is made by introducing two concatenated abrupt tapers in a single Er/Yb codoped fiber for high extinction ratio and compact size. An acoustic wave generated from an actuator is guided to the phase shifter of AT-MZI to change the OPLD between core and cladding modes. The sensing of the heart-beat pulses can also analyze the pulse manifestations to reflect the inner health condition of human body. This acoustic wave sensitive vibration sensor could also be promising for precision vibration sensing systems in a tiny or cramped space.