The method of Mach-Zehnder electro-optical modulation is applied to Intra-Body Communication (IBC), where the modeling and characterization of this type of IBC are discussed. The mathematical model of the electrostatic coupling IBC based on Mach-Zehnder electro-optical modulation is developed. The main characteristics of this IBC form have been simulated within the frequency range of 200 kHz-40MHz and compared to in-vivo measurements, with close agreements. Results show that the proposed method will help achieving good temperature characteristics, small size, and lower power consumption IBC system.
© 2012 OSA
Intra-body communication (IBC) is a technology using the human body as a transmission medium for electrical signals . Compared with short distance wireless communication, such as Zigbee and Bluetooth, it has the characteristics of high information security, low energy consumption, and high data transmission rate . Therefore, it is believed that IBC technology will offer significant advantages in Personal Area Network (PAN) [1,3],biomedical monitoring , and interaction between the humans and their environments [5,6].For instance, IBC technology can be used to build a biomedical monitoring network consisting of on-body sensors and implanted sensors, as shown in Fig. 1 , in which biomedical data collected from the different parts of the human body transmit within the human body, and eventually arrive at the cell phone or hospital by using a link sensor attached on the wrist. Therefore, the patients and their doctors can achieve the biomedical data of the human body almost anytime and anywhere.
The detection of signals transmitting within the human body is important for achieving reliable and high-speed IBC. Two related methods have been proposed, as shown in Fig. 2 . One is the electrical detection method shown in Fig. 2(a), in which a signal transmitting within the human body is directly detected by electrical sensors [1,7]. Given the comparatively low input impedance of electrical sensors, the signal transmission is susceptible to interference and the signal transmission rate is limited to 40 kbps . The other method is the electro-optical modulation, in which an electro-optical sensor is employed instead for detecting the signal transmitted within the human body [3,6,8], as shown in Fig. 2(b). The advantages of this method include 1) the extremely high input impedance of the electro-optical crystal (i.e. about 100 times higher than the electrical detection method), yielding significantly higher signal to noise ratios than obtained with the electrical detection method; 2) the ground electrode of the electro-optical sensor is electrically isolated from electronic circuits. Therefore, the influence of floating-ground potentials is eliminated . As a result, both the noise and the signal distortions are decreased greatly, while a signal transmission rate of 10 Mbps can be achieved .
Previous researches of the electro-optical modulation method used in IBC mainly focused on the sensor based on a bulk electro-optical modulator [3,6,8], which have several unsolved issues: 1) The theoretical model of this kind of IBC has not been discussed, therefore, it is difficult to determine the corresponding parameters of the electro-optical sensor used in IBC to date; 2) Given that this kind of electro-optical sensor has a phase delay caused by natural binary refraction, which is critically sensitive with temperature, its communication quality is greatly impacted by ambient temperature; 3) the phase delay of this kind of electro-optical sensor depends on the aspect ratio of the electro-optical crystal, which results in a comparatively big size of the IBC receiver. Therefore, it will limit the application of IBC. In this paper, an IBC method based on Mach-Zehnder electro-optical sensor is proposed, which has the advantages described as following: 1) good temperature characteristic. The optical intensity modulation of the Mach-Zehnder sensor is not influenced by the phase delay caused by natural binary refraction, which decreases the impact of ambient temperature; 2) small size. Due to a Mach-Zehnder electro-optical modulator is produced by using an optical waveguide and photo etching technologies with micron or nanometer resolution [9–11], it can be miniatured down to 100-200µm .Moreover, it can also be integrated with a laser diode (LD) and a photo detector (PD). Therefore, the miniaturization of a Mach-Zehnder sensor can be achieved, which will results in a compact size sensor; 3) low power consumption. The half-wave voltage of a Mach-Zehnder type sensor (generally less than 6V) is far lower than that of a bulk type sensor (generally more than 200V), which results in the comparatively lower power consumption. Moreover, a bulk type sensor generally needs a wave plate and a polarizing beam splitter, which yield additional optical losses. Therefore, compared with a bulk type sensor, the power consumption of the Mach-Zehnder type sensor is comparatively lower .
In this paper, the modeling and characterization of Mach-Zehnder electro-optical modulator for IBC have been investigated for the first time. We present a mathematical model of the electrostatic coupling IBC based on the Mach-Zehnder electro-optical modulator. Next, important characteristics of this IBC form have been simulated, and the corresponding in-vivo measurements were carried out and compared to the simulations. Results show that the proposed method will help to achieve good temperature characteristics, small size and lower power consumption IBC system.
2. Modeling of IBC based on a Mach-Zehnder electro-optical modulator
The electrostatic coupling IBC based on a Mach-Zehnder electro-optical modulator is illustrated in Fig. 3 , in which the signal is coupled into the human body through the signal electrode at the transmitting terminal, which is also coupled into the earth ground through the ground electrode. At the receiving terminal, the signal transmitted through the human body is coupled into a Mach-Zehnder electro-optical modulator through the signal electrode. The functions of the Mach-Zehnder electro-optical modulator can be described as follows. The refractive index of the arm of the Mach-Zehnder modulator changes with the voltage of the received signal which is applied onit. As a result, when the laser light from the laser diode (LD) passes through the arms of the Mach-Zehnder modulator, the phase of the optical wave changes in one arm relative to the other. Subsequently, the Mach-Zehnder modulator sums the optical waves from each arm, and the phase change is converted to amplitude change, which results in optical intensity modulation. Finally, the change in optical amplitude is converted into the corresponding change in electric signals by a photo detector (PD). Moreover, because the ground electrode is insulated from the PD and the corresponding circuit of the receiver based on the Mach-Zehnder modulator, as shown in Fig. 3, signal detection with low noise and low signal distortion can be achieved in this IBC form.
2.1. Circuit model
The circuit model of the electrostatic coupling IBC based on a Mach-Zehnder electro-optical modulator, illustrated in Fig. 3, was developed and reported in Fig. 4 . Considering that the impedances of the signal return paths (Cg1, Cg2 and Cg3) are far greater than that of the body channel, the human body can be modeled as a perfect conductor . Therefore, the human body is considered as a body node in the proposed circuit model. Meanwhile, compared with the modulator based on a bulk electro-optical crystal, which has one internal signal electrode and is generally modeled as a capacitance [3,6], the Mach-Zehnder electro-optical modulator has two internal signal electrodes corresponding to the two arms. The modulator itself can be modeled as two capacitances represented as Ce1and Ce2, respectively, in our circuit model. Additionally, some of the weak couplings (such as the coupling between the two ground electrodes) were ignored for simplifying the model of electrostatic coupling IBC, which may influence the precision of the model to some extent.
Based on the proposed circuit model and common practices, the relationship between the voltage (Vin)of the IBC transmitter and the measured voltage of the Mach-Zehnder modulator (Ve) can be shown to be given asFig. 4. Therefore, the signal attenuation (GH) caused by the human body can be represented as
2.2. Mathematical model
According to the circuit model shown in Fig. 4, the mathematical model of the electrostatic coupling IBC based on a Mach-Zehnder electro-optical modulator was next developed. In our investigation, in order to show the advantages of the IBC based on a Mach-Zehnder electro-optical modulator, the electro-optical modulation based on a bulk electro-optical crystal and the electro-optical modulation based on a Mach-Zehnder electro-optical modulator are discussed and compared. These two modulation methods are illustrated in Fig. 5 .
2.2.1. Modulation based on a bulk electro-optical crystal
As shown in Fig. 5(a), in the transverse electro-optical modulation of the IBC based on a bulk electro-optical crystal , the direction of the signal electric field (i.e. along the z-direction) applied on the x-cut electro-optical crystal is generally perpendicular to the light propagation direction, which in this case is along the y-direction. If the polarized directions of the polarizer and analyzer are orthogonal, and the angle between the z-direction and the polarized direction of the polarizer is π/4, the relationship between the input optical power(Pin) and the output optical power(Pout)of the electro-optical crystal can be expressed as 12]. Finally, the change in light signal is converted into the corresponding electric voltage (Vout) by the photo detector, which is given as
2.2.2. Modulation based on a Mach-Zehnder electro-optical modulator
As for the Mach-Zehnder electro-optical modulator shown in Fig. 5(b), if the electric field of the incident input light Ein equal Aexp(jωt), the electric fields in the two arms of the Mach-Zehnder electro-optical modulator can be described as Ea equal Eb equal (A/2)exp(jωt). Therefore, the electric field referring to the emergent light of the Mach-Zehnder modulator (Eout) can be expressed as Eq. (6), the relationship between the (Pin) and (Pout) of the Mach-Zehnder electro-optical modulator can be expressed as13]
2.2.3. Influence factors
18.104.22.168. The frequency response of the Mach-Zehnder electro-optical modulation
The Mach-Zehnder electro-optical modulation as a function of the signal temporal frequency is an important factor that influences the frequency characteristic of the IBC system that is based on it. However, because the frequency response of the Mach-Zehnder electro-optical modulation system consisting of an LD, an electro-optical modulator, and a PDis determined by several factors such as input optical power, operating wavelength, the insertion loss and crystal type of the modulator and parameters of the PD, it is generally challenging to achieve an accurate mathematical model for the frequency response of the Mach-Zehnder electro-optical modulation. As a result, the proposed mathematical model for the signal attenuation (GEO1) given in Eq. (8) that represents the Mach-Zehnder electro-optical modulation is typically taken to be a constant with signal frequency. In our investigation, the frequency response of the Mach-Zehnder electro-optical modulator was achieved by curve fitting of the measurements.
Our experimental setup for measuring the frequency response of the Mach-Zehnder electro-optical modulation was composed of a battery powered waveform generator (DSO8060, made by Qing Dao Hantek Electronic co., Ltd China), an LD, a Mach-Zehnder electro-optical modulator (10 Gb/sec intensity modulator, made by JDS Uniphase Corporation, USA), a PD and a digital oscilloscope(Agilent 54641A), in which the waveform generator served as an IBC transmitter, while the digital oscilloscope was used for measuring the output signal of the PD. Additionally, to avoid the influence of the AC power and the earth ground connected with the oscilloscope, a balun (a type of electrical transformer that converts balanced electrical signals relative to the earth ground to unbalanced signals, and the reverse) was used for separating the Mach-Zehnder electro-optical sensor from the digital oscilloscope  (see Fig. 6 ).
Firstly, the frequency response function of the signal attenuation (GEO2) associated with the Mach-Zehnder electro-optical modulation can be modeled using a piecewise fitting method as
Secondly, the frequency response function of the signal attenuation (Gbalun) associated with the balun may be modeled, also based on curve fitting, as
Temperature is an important influence factor of the IBC based on electro-optical modulation. Generally, the temperature effect on the electro-optical modulation mainly consists in a change in refractive index. As for a LiNbO3crystal, which has been widely used in electro-optical modulation, the temperature effect on its refraction index (no, ne) may be captured with the Sellmeier equations , given asEq. (4) that describes the relationship between Vout and Ve, the temperature characteristic of the electro-optical modulation based on a bulk electro-optical crystal can be determined using also Eq. (11). Similarly, the temperature characteristic of the Mach-Zehnder electro-optical modulator can also be described using Eq. (8) and (11).
2.2.4. The complete mathematical modelEq. (8).Based on the same parameters as our measurement setup and the same temperature, K = −0.41dB.
Finally, the complete mathematical model of the signal attenuation (GTotal) that refers to the electrostatic coupling IBC based on the Mach-Zehnder electro-optical modulation can be modeled and computed as
3.1. Frequency response
To verify the frequency characteristic of the proposed model represented as Eq. (13), in-vivo measurements of the electrostatic coupling IBC based on the Mach-Zehnder electro-optical modulation were carried out by using the experimental setup mentioned in section 22.214.171.124. Our first experiment was concerning the signal transmission path from the left arm to the right arm, as shown in Fig. 7 , in which a signal is coupled into the left arm of the human body by using an electrostatic coupling electrode including a signal electrode and a ground electrode, while it is received using a same electrode on the right arm. Also, in-vivo measurements of signal transmission paths from the arm to the torso and from the arm to the leg were next carried out. Meanwhile, the frequency response of the electrostatic coupling IBC based on the Mach-Zehnder electro-optical modulation was simulated according to the model of Eq. (13).
The effectiveness of Eq. (13)in modeling overall signal attenuation that is equivalent to the electrostatic coupling IBC based on the Mach-Zehnder electro-optical modulation is shown in Fig. 8 , in which both in-vivo measurements of the frequency response and the corresponding simulated results are compared. Results show that in simulation and the corresponding measurement, the attenuation decreases gradually in the 200 kHz-2 MHz window, keeps relatively stable in the 2 MHz-10 MHz window, and increases gradually in the 10 MHz-40 MHz window, which indicate similar trend in these frequency regions.
On the other hand, some potential advantages of the IBC based on the Mach-Zehnder electro-optical modulation can be found in other results. Figure 9 shows the in-vivo measurements of the frequency responses corresponding to the IBC based on the electrical sensor and the IBC based on Mach-Zehnder electro-optical modulation within the frequency range of 1MHz to 40 MHz. Results show that for the IBC based on the electrical sensor, all the signal attenuations corresponding to various paths (left arm-right arm, arm-torso and arm-leg) decrease from approximately 64.57dB to 47.16 dB in the range of1 MHz to 5 MHz, and have comparatively big variation (the maximum deviation is 15.96 dB) in the range of 5 MHz to 40 MHz. On the other hand, according to Fig. 9, the signal frequency has comparatively less effect on the signal attenuations in the case of the IBC based on the Mac-Zehnder electro-optical modulation. This phenomenon indicates that compared with the IBC based on an electrical sensor, the electrostatic coupling IBC based on Mach-Zehnder modulation has a comparatively steady frequency response, especially in the range of 2 MHz to 10 MHz (the maximum deviation of this range is only 0.82 dB).Therefore, if the signal frequency or the carrier frequency of IBC is set in this range, high quality of electrostatic coupling IBC will be expected.
3.2. Temperature characteristic
In our investigation, the advantage of the IBC based on the Mach-Zehnder electro-optical modulation in terms of temperature characteristic was also verified in terms of temperature characteristic by the comparison between the temperature characteristic of this kind of IBC and that of the IBC based on a bulk electro-optical sensor.
According to Eq. (4) and (11), the signal attenuation of the IBC based on a bulk electro-optical sensor can be estimated as a function of temperature. Figure 10shows the simulation results of the bulk electro-optical sensor used in IBC in the temperature range of 283.15 K-305.15 K, which shows that the signal attenuation varies greatly. Moreover, it also can be seen from Fig. 10 that the variation is periodical, and the maximum variation is up to approximately 55 dB, which indicates that ambient temperature affects the communication quality greatly in the IBC based on bulk electro-optical sensor.
Figure 11 shows the simulation results of the Mach-Zehnder electro-optical sensor used in IBC, the corresponding measurements and the errors between them, respectively. We can find that compared with Fig. 11, both the simulation results and the measurements shown in Fig. 11 have negligibly smaller variation. For instance, as the temperature increases from 283.15 K to 305.15 K, the increase in the simulation results is less than0.02 dB, while the measurements exhibit a periodic variation in the order of +/− 0.25 dB. Yet both are small, which indicates that the IBC based on Mach-Zehnder electro-optical modulation has good temperature characteristic.
In this paper, the Mach-Zehnder electro-optical modulation method is first used in IBC, while the characteristics of the electrostatic coupling IBC based on Mach-Zehnder electro-optical modulation are investigated. We have shown that the frequency response of the IBC based on Mach-Zehnder electro-optical modulation was modeled and shown to agree with the corresponding measurements in the frequency of 200 kHz to40 MHz. Also, we demonstrated that, compared with the IBC based on an electrical sensor, the electrostatic coupling IBC based on Mach-Zehnder electro-optical modulation has a steady frequency response in the range of 2 MHz to 10 MHz. Therefore, the stability of electrostatic coupling IBC is increased. Finally, compared with the IBC based on a bulk electro-optical sensor, the IBC based on Mach-Zehnder modulation has good temperature characteristic, which decreases the impact of ambient temperature and finally increases the communication quality of electrostatic coupling IBC. Moreover, the proposed method can also help to decrease the size and power consumption of the IBC system compared with the method based on a bulk electro-optical sensor.
The work was supported by the National Natural Science Foundation of China (60801050), the Excellent Talent Fund of Beijing, China (2011) and the Scientific and Technological Innovation Project of Beijing Institute of Technology (3040012241002). Jannick Rolland was supported by the NYSTAR Foundation (C050070).
References and links
1. T. G. Zimmerman, Personal Area Networks (PAN): Near-Field Intra-Body Communication, in Media Art and Science. Master Thesis: Massachusetts Institute of Technology (1995).
2. M. S. Wegmueller, M. Oberle, N. Felber, N. Kuster, and W. Fichtner, “Signal Transmission by Galvanic Coupling Through the Human Body,” IEEE Trans. Instrum. Meas. 59(4), 963–969 (2010). [CrossRef]
3. M. Shinagawa, M. Fukumoto, K. Ochiai, and H. Kyuragi, “A Near-Field-Sensing Transceiver for Intrabody Communication Based on the Electrooptic Eﬀect,” IEEE Trans. Instrum. Meas. 53(6), 1533–1538 (2004). [CrossRef]
4. M. S. Wegmueller, S. Huclova, J. Froehlich, M. Oberle, N. Felber, N. Kuster, and W. Fichtner, “Galvanic Coupling Enabling Wireless Implant Communications,” IEEE Trans. Instrum. Meas. 58(8), 2618–2625 (2009). [CrossRef]
5. Y. Song, Q. Hao, K. Zhang, M. Wang, Y. Chu, and B. Kang, “The Simulation Method of the Galvanic Coupling Intra-body Communication with Different Signal Transmission Paths,” IEEE Trans. Instrum. Meas. 60(4), 1257–1266 (2011). [CrossRef]
6. A. Sasaki, M. Shinagawa, and K. Ochiai, “Principles and Demonstration of Intrabody Communication with a Sensitive Electrooptic Sensor,” IEEE Trans. Instrum. Meas. 58(2), 457–466 (2009). [CrossRef]
7. K. Hachisuka, A. Nakata, T. Takeda, K. Shiba, K. Sasaki, H. Hosaka, and K. Itao, “Development of wearable intra-body communication devices,” Sens. Actuators A Phys. 105(1), 109–115 (2003). [CrossRef]
8. A. Sasaki and M. Shinagawa, “Principle and Application of a Sensitive Handy Electrooptic Probe for Sub-100-MHz Frequency Range Signal Measurements,” IEEE Trans. Instrum. Meas. 57(5), 1005–1013 (2008). [CrossRef]
9. C. E. Rogers 3rd, J. L. Carini, J. A. Pechkis, and P. L. Gould, “Characterization and compensation of the residual chirp in a Mach-Zehnder-type electro-optical intensity modulator,” Opt. Express 18(2), 1166–1176 (2010). [CrossRef] [PubMed]
10. J. M. Brosi, C. Koos, L. C. Andreani, M. Waldow, J. Leuthold, and W. Freude, “High-speed low-voltage electro-optic modulator with a polymer-infiltrated silicon photonic crystal waveguide,” Opt. Express 16(6), 4177–4191 (2008). [CrossRef] [PubMed]
11. J. William, M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Ultra-compact, low RF power, 10 Gb/s silicon Mach-Zehnder modulator,” Opt. Express 15, 17107–17113 (2007).
12. R. W. Boyd, “Electrooptic Modulators,” in Nonlinear Optics (Academic, Rochester, New York, 2008).
13. G. N. Lu and G. Sou, “A CMOS Op Amp using a regulated-cascade transimpedance building block for high-gain, low-voltage achievement,” in Proceedings of 1997 IEEE International Symposium on Circuits and Systems (Hong Kong,1997).165–168.
14. H. J. Zhu, R. Y. Xu, and J. Yuan, “High Speed Intra-Body Communication for Personal Health Care,” in Proceedings of 31st Annual International Conference of the IEEE EMBS Minneapolis (Minnesota, 2009), 709–712.
15. D. L. Zhang, Q. Z. Yang, P. R. Hua, H. L. Liu, Y. M. Cui, L. Sun, Y. H. Xu, and E. Y. B. Pun, “Sellmeier equation for doubly Er/Mg-doped congruent LiNbO3 crystals,” J. Opt. Soc. Am. B 26, 620–626 (2009).