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Abstract— This paper demonstrates the viability of using

self-mixing interferometer technique with a customized electro-optic phase modulator to detect the electrocardiographic signals on surface skin. The signals were recorded under two different optical feedback levels where the self-mixing technique operated under strong optical feedback regime was preferred due to the clear advantage of stability and linear relationship between the input and output signal.

Index Terms—Self-Mixing, Electrocardiograph, electro-optic modulator, VCSEL

I. INTRODUCTION ELF-MIXING INTERFEROMETER (SMI) is an interferometric laser sensing technique established from perturbation of optical signals within the cavity of

semiconductor laser. SMI signal is acquired when a portion of emitted lights is reflected back from a target and re-injected into the laser cavity. The reflected light is then interferes with the light inside the cavity, hence produces variations to the lasing properties, such as threshold gain, emitted power and linewidth [1].

SMI highlights several advantages compared to the conventional interferometric techniques, such as its simplicity, compactness and physically self-aligned. This technique has been demonstrated for various basic sensing applications including absolute distance, displacement, vibration and velocity [2]. Using the advantage of optical sensing, the SMI technique has also been reported in several biomedical applications such as measuring blood flow [3], cochlea displacement [4] and cardiovascular diagnostics [5].

This paper demonstrates a proof-of-concept of another promising biomedical application using the SMI technique with a customized electro-optic (EO) phase modulator to detect electrocardiographic (ECG) signals on surface skin.

Problem with conventional ECG is it requires at a minimum a pair of electrodes which are made of conductive

Manuscript received July 31, 2012. This work was supported by the

School of Information Technology and Electrical Engineering, The University of Queensland Brisbane, QLD 4072, Australia.

A.A.A. Bakar is with the Dept. of Electrical, Electronic and Systems Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, MALAYSIA. (e-mail: ashrif@eng.ukm.my).

Yah Leng Lim, Stephen J. Wilson, Miguel Fuentes, Karl Bertling, Thierry Bosch and Aleksandar D. Raki are with the School of Information Technology and Electrical Engineering, The University of Queensland Brisbane, QLD 4072, Australia.

Thierry Bosch is with CNRS, LAAS, 7 avenue du colonel Roche, F-31400 Toulouse, France and Univ de Toulouse, LAAS, F-31400 Toulouse, France.

material connected at specific points on the patient’s body. This, however, limits the capability of the instrument to perform measurement in an environment where cause of electromagnetic interference (EMI) must be avoided. Classical case is where ECG is required coexisting with magnetic resonance imaging (MRI) which execution of MRI imaging sequences resulting to the distortion of the ECG measurement [6]. Hence, optical sensing solution for the same purpose of monitoring cardiac activity is anticipated where advantage of immune to RF interference can be attained.

However, one of the issues of the proposed technique is mainly due to the small amplitude of the biopotentials present from the cardiac activity (in millivolt range). Hence, the design of the sensing probe (EO phase modulator) must be extremely sensitive to detect the biopotential signals and flexible enough to provide a good contact with the skin.

SMI technique has the capability of measuring the vibration/displacement in nanometer resolution. Determining the myocardial signal using this technique is similar to the small displacement from the measurement point of view. Conventionally, the displacement can be measured by counting the peak of the SMI signal where it would give a minimum resolution of 2λ , where λ is the wavelength of the laser. However, for displacement well below 2λ , the measurement scheme becomes much more challenging as there are several sources of interference affect significantly to the quality of the SMI signal [7]. The difficulty also corresponds to which feedback regimes the technique is operated (either under weak or strong optical feedback).

A detailed experimental setup is presented in Section II, results and discussions are presented in Section III followed by conclusion.

II. EXPERIMENTAL SETUP Figure 1 illustrates the experimental setup of the proposed

system. The 850 nm proton implanted VCSEL (Litrax LX-VCS-850-T101) was biased by a constant current source using a customized laser diode driver (LD Driver) and was thermally stabilized using a high precision temperature controller (Thorlabs TED350) at 25 ± 0.001oC. The beam was launched into the fiber using a two lens system (comprising two identical lenses with the numerical apertures NA = 0.5 and the focal lengths f = 8.0 mm). In the collimated path of the beam (between the lenses) a neutral density (ND) filter was inserted in order to control the amount of reflected power entering the cavity.

Electrocardiographic signal detection using self-mixing interferometer technique with

customized electro-optic phase modulator Ahmad Ashrif A. Bakar, Yah Leng Lim, Stephen J. Wilson, Miguel Fuentes, Karl Bertling,

Thierry Bosch and Aleksandar D. Rakic

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3rd International Conference on Photonics 2012, Penang, 1-3 October 2012

978-1-4673-1463-3/12/$31.00 ©2012 IEEE 299

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Fig. 1. Schematic diagram of the SMI technique to detect simulated ECG signals using a customised EO modulator. VCSEL light is transmitted, propagated and focused into the polarization maintaining fibre (PMF) where the light is reflected off the mirror attached at the distal end of the EO modulator. An arbitrary function generator (FG) is used to emulate the ECG signals and intensity of light propagation is controlled by the Neutral Density (ND) filter. Finally, the SMI signal is detected as variation in the VCSEL terminal voltage. A programmable arbitrary function generator (FG) was utilized to provide synthesized ECG signal which is then fed to the RF input of the EO phase modulator. The SMI signal was detected as variation in the VCSEL terminal voltage and was amplified by a customized amplifier. Finally, the output signals were recorded using an oscilloscope (Tektronix TDS 2024).

The SMI signal is recorded when the light coupled into the fiber and passing through the fiber-modulator assembly is reflected from the mirror located at the distal end of the modulator back into the VCSEL cavity. The reflected signal is phase modulated by the electrical signal applied to the electrodes of the EO phase modulator (the RF input). The electrical signal induces change in the refractive index of the modulator waveguide. This change in refractive index alters the phase length of the fiber-modulator assembly and in turn creates optical path variation similar to the one that would have been created by a small displacement of the mirror at the distal end of the path.

Therefore, from the SMI technique point of view, the EO modulation of the path length is equivalent to the small mechanical displacement of the target mirror [8]. By using the signal of magnitude comparable to half-wave voltage of the EO phase modulator (Vπ of 2.5 V), we have obtained the SMI signal resembling the one usually associated with displacement measurements. Half-wave voltage is defined as the voltage required producing an electro-optic phase shift of 180o. For electrical signal amplitudes much smaller than Vπ a situation similar to that discussed in [9] arises. The amplitude of the ECG signal is expected to be relatively small, four orders of magnitude below Vπ of the EO modulator, creating a signal comparable to SMI displacement signal with an amplitude of λ/5000. As discussed in [9], in this small signal regime of operation, the obtained output SMI signal has a waveform resembling that of the input electrical signal (the ECG signal).

Note that the customized EO phase modulator used in this study was originally designed for high bandwidth optical communications, and was not optimized for the small signal input requirements of this project. Some recently reported EO modulators have Vπ commensurate with the EKG signal amplitudes allowing for systems with significantly increased sensitivity [10].

III. MEASUREMENT RESULTS

Preliminary measurements were performed with a sinusoidal input waveform (1 Hz) fed into the RF input of the EO phase modulator with the input signal level varying between 0.2 Vπ and Vπ. This voltage range is equivalent to a mechanical displacement of the target producing between one fifth of a fringe (λ/10 displacement) and one fringe (λ/2 displacement) [11].

Figure 2 shows the SMI output signals measured for two different attenuation levels, resulting in different optical feedback regimes. The output signals were captured in a single sweep without averaging or any subsequent filtering. In the strong feedback regime (no attenuator in the optical path) measurements were taken for three input signal amplitudes: Vπ, 0.3 Vπ and 0.2 Vπ resulting in SMI signals shown in Fig. 2 (a) (b) and (c), respectively. In this mode of operation, the SMI sensor behaves as a linear transducer replicating the shape of the input voltage signal in the output voltage signal.

The amplitude of the output voltage signal is directly proportional to that of the input signal leading to decreasing signal to noise ratio with the decreasing input signal amplitude.

The second set of results [shown in Fig. 2(d), (e) and (f)] were obtained with a 10 dB optical attenuator inserted in the path. Under the reduced optical feedback conditions the SMI output signal no longer directly replicates the input signal, but produces a symmetrical interferometric fringe pattern commonly associated with weak feedback. Under these conditions, the magnitude of the output signal is maintained as the input signal magnitude decreases. The reduction in the input signal amplitude results in the decreasing number of fringes and the change in the waveform.

While this regime of operation provides higher sensitivity in comparison with the strong feedback regime, it is necessary to lock the interferometer phase to less than half a fringe in order to maintain the stable operation of the system [9]. The stability of the interferometric waveform is affected by the phase drift and this effect becomes more pronounced as the magnitude of the input signal is reduced [8]. In spite of the reduced sensitivity, in this instance, the strong feedback operating regime was preferred due to its clear advantage of stability and the linear relationship between the input and the output signal. In the second experiment, we applied a digitally synthesized electrical signal (with additive Gaussian noise) resembling the ECG signal to the RF port of the EO phase modulator.

The signal exhibits typical morphology of the human ECG signal. The fundamental frequency of the signal was 1 Hz. The resultant SMI signals were acquired without averaging or filtering.

Figure 3(b) to (d) shows the output signals successfully reproducing the input ECG signal for the complete range of the input signal [Fig. 3(a)]. As the magnitude of the input signal decreases, all components of the ECG signal are reduced in amplitude but the output waveform still possesses a recognizable R wave.

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Fig. 2. SMI output signals measured under two levels of optical feedback. Signals (a), (b) and (c) were obtained with no attenuator in the optical path. Signals (d), (e) and (f) were obtained with 10 dB optical attenuator in the path leading to 20 dB round trip loss.

Depending on the desired application, purpose designed filters can be applied to this SMI signal to improve the waveform for either diagnostic or image triggering purposes.

Fig. 3. (a) The synthesized ECG input signal. Notation of P, Q, R S and T are the typical ECG tracing of the cardiac cycle. Recorded SMI output signals for the input signal peak-to-peak amplitude set at (b) Vπ , (c) 0.3 Vπ , and (d) 0.2 Vπ .

These results demonstrate the feasibility of detecting surface biopotentials using the proposed SMI technique. However, the smallest input voltage used (500 mV) is still two orders of magnitude higher than the typical peak-to-peak amplitude of the human ECG. There are several straight forward means by which the sensitivity of the technique can be improved. These include the different choice of the EO material with larger Pockels coefficient (including polymers), longer optical path lengths within the modulator and the reduced thickness of the waveguide, all leading to significant reduction in the required input voltage.

Adding post noise filtering to the output signal may significantly help to enhance the quality of the signals. In this paper, we also demonstrate that it is viable to measure small input signals of amplitude less than one fringe (λ/2). The shortcomings of this particular EO modulator are due principally to its intended use as a high-bandwidth communications device where Vπ is traded off for maximising frequency response. The low bandwidth requirements of this biopotentials application (<100 Hz) would permit a more generous Vπ (i.e. lesser value) which, in turn, will greatly enhance the sensitivity of the technique and extend the utility of this sensor to electroencephalographic (EEG) and electromyographic (EMG) measurements.

IV. CONCLUSION In summary, proof-of-concept of using the SMI technique

for monitoring the surface ECG signal using the custom-made EO phase modulator was demonstrated. The SMI setup has been verified with varying amplitudes of sinusoidal signal under two different types of optical feedback regimes. Strong optical feedback regime is chosen to be the optimum operation of this technique as it provides direct detection of myocardial signal although the signal suffers magnitude reduction and accumulated noise as mentioned in the text.

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