Optical microphone with fiber Bragg grating and ... - SPIE Digital Library

Optical microphone with fiber Bragg grating and signal processing techniques Daniele Tosi*a, Massimo Oliveroa, Guido Perronea Dept. of Electronics, Politecnico di Torino, c.so Duca degli Abruzzi 24, Torino, ITA 10129

a

ABSTRACT In this paper, we discuss the realization of an optical microphone array using fiber Bragg gratings as sensing elements. The wavelength shift induced by acoustic waves perturbing the sensing Bragg grating is transduced into an intensity modulation. The interrogation unit is based on a fixed-wavelength laser source and - as receiver - a photodetector with proper amplification; the system has been implemented using devices for standard optical communications, achieving a low-cost interrogator. One of the advantages of the proposed approach is that no voltage-to-strain calibration is required for tracking dynamic shifts. The optical sensor is complemented by signal processing tools, including a data-dependent frequency estimator and adaptive filters, in order to improve the frequency-domain analysis and mitigate the effects of disturbances. Feasibility and performances of the optical system have been tested measuring the output of a loudspeaker. With this configuration, the sensor is capable of correctly detecting sounds up to 3 kHz, with a frequency response that exhibits a top sensitivity within the range 200-500 Hz; single-frequency input sounds inducing an axial strain higher than ~10nε are correctly detected. The repeatability range is ~0.1%. The sensor has also been applied for the detection of pulsed stimuli generated from a metronome. Keywords: Fiber optic sensors, Fiber Bragg gratings, Vibro-acoustic sensor, Optical instrumentation, Optical microphone, Multisensor system, Adaptive signal processing, Frequency estimation.

1. INTRODUCTION Over the last few years, the usage of fiber Bragg gratings (FBG) for sensing applications has received an increased interest from several research groups, as they combine advantageous properties typical of optical fibers, such as safety, immunity to electromagnetic interferences, low invasiveness and remote sensing capability, with the specific characteristics of this technology, such as in-fiber integration, high sensitivity and possibility to easily realize multiplexed sensor networks [1-2]. For these reasons, FBG are emerging as a successful replacement of standard electrical and mechanical instrumentation, which is inapplicable in rugged or radiation-prone environments. One of the target applications is the detection of acoustic waves through optical sensors, in order to provide an alternative instrument capable of working under the effect of strong electromagnetic fields, e.g. for monitoring medical parameters of a patient during a Computerized Tomography (CT) scan. Some implementations of FBG microphones, targeted for medical applications, have been documented in literature [3-5], using principles of operation based on matched-filters demodulation or spectral scanning with a tunable laser. The possibility of detecting acoustic signals through optical hydrophones is also a key issue in underwater sensing [6-7]. In this paper we discuss the realization of a fiber Bragg grating microphone, derived from the approach initially proposed by Morey [8] and subsequently extended by Wilson et al. [9]. This configuration, whose principle of operation is based on the intensity modulation, does not involve any scanning or moving element that limits the overall sampling frequency and increases the costs and mechanical instability, and can be implemented using off-the-shelf components developed for optical communications. In order to improve the performances of the interrogation unit, the optical sensor has been complemented by signal processing techniques: adaptive filters are used to mitigate the effects of disturbances arising both from the sensor itself and from external interferences, while a suitable frequency estimator permits dealing successfully also with weak and pulsed signals embedded into noise. *daniele.tosi @polito.it; phone 39 011 2736-311; fax 39 011 564-4099

Eighth International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, edited by Enrico Primo Tomasini, Proc. of SPIE Vol. 7098, 70981E, (2008) 0277-786X/08/$18 doi: 10.1117/12.803184

Proc. of SPIE Vol. 7098 70981E-1

2. PRINCIPLE OF OPERATION A fiber Bragg grating is a periodic modulation of the refractive index within the core of a photosensitive optical fiber, which yields to a frequency-selective response, as depicted in Fig 1 [10]. The FBG behaves as a stop-band filter, in which the central wavelength is λB0 = 2neff Λ

(1)

where neff is the effective refractive index of the fiber and Λ is the period of the index modulation. The peak wavelength, and consequently the whole spectrum, linearly shifts upon application of stresses either due to mechanical forces or temperature variation, according to the following relation [11]:

λ B (ε , ∆T ) = λ B 0 + k ε ε + k T ∆T

(2)

where ∆T is the temperature variation with respect to the reference value, ε is the axial strain applied to the FBG, and kT and kε are the temperature and strain coefficients respectively. Of course (2) is valid for stress intensity not exceeding the range in which the grating spectral response is not distorted. In our application the axial strain is applied to the FBG by an acoustic excitation, yielding a wavelength shift proportional to the pressure [12]. On the other hand, since the sensor is targeted to the detection of dynamic phenomena, temperature static fluctuations can be neglected. Transmitted

Entering spectnim

sp e ctru in

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Fig. 1. Wavelength-selective behavior of a FBG.

FBG

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Photodetector Fig. 2. Principle of operation of the interrogator. The light emitted by a narrow-linewidth laser is filtered through the FBG, yielding an output power proportional to the FBG transmission coefficient. When the FBG is strained, its spectrum shifts and the laser intercepts a different spectral slice, changing the output voltage.

The FBG is interrogated by means of a fixed-wavelength narrow-linewidth laser source, exploiting the spectral shift induced by the axial strain to produce an intensity modulation proportional to the spectral shift and therefore to the


applied stress, as schematized in Fig. 2. The measured output power is approximately proportional to the FBG transmission coefficient evaluated at the laser wavelength, but an alternative scheme based on the reflected power is also possible. In both cases, with these arrangements, the unambiguous range suitable for interrogation is approximately half of the FBG full bandwidth; however it is important to note that the response is non-linear and thus a preliminary characterization is necessary in order to find the calibration curve.

3. EXPERIMENTAL SETUP The actual experimental setup of the FBG interrogator is and extension of the principle shown in Fig. 2 and is depicted in Fig. 3. The light source is a laser diode for telecom applications, packaged in a convenient butterfly case, with an emission wavelength close to 1560 nm and an output power of 140µW @ I = 180 mA. The laser is controlled by a ILX laser controller that includes a current source and temperature stabilization by a thermo-electric cooler (TEC). The light source is followed by a high quality isolator (> 30dB), in order to reduce the self-mixing (SM) effect due to the power reflected by the FBG [13]. However, despite the isolation, the SM is not completely removed, and introduces a slow and chaotic fluctuation perturbing the optical signal that are removed/attenuated thanks to the employment of adaptive filters, as will be further detailed in the next section. All the FBG employed in the experiments have been fabricated in our lab, with the phase mask technique [14], using a photosensitive fiber with coefficients kε = 1.10 pm/µε and kT = 10.2 pm/°C showing a Bragg wavelength close to 1560 nm using a phase mask with pitch 2Λ = 1071.46 nm. In order to protect the FBGs and, at the same time, preserve their strain sensitivity to sound pressure, the FBGs have been embedded in a suitable package, consisting in a thin double layer of polyimide tape with glue for glass. The chosen FBG is linked to the source and the receiver by means of two fiber spans of about 5m, in order to simulate a remote sensing application. The optoelectronic receiver is implemented with a PIN photodiode for low bit-rate communications, followed by a transimpedance amplifier and a low-noise voltage amplifier.

FBG array

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Fig. 3. Schematic of the optical microphone, arranged for multiple FBG interrogation.


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This interrogation system can be extended to a multiplexed structure, by splitting the input signal into multiple channels and replicating the receiver; however this extension requires the employment of wavelength-matched FBG. In any case this feature is limited to a small amount of sensing points, since the signal-to-noise ratio (SNR) decreases with the splitting ratio. The output voltages of the N channels are read by a NI-6036 data acquisition (DAQ) card with sampling rate fS. The maximum sampling frequency of the DAQ card is 200 kS/s, but it is unfeasible in a multiple channel configuration since the sampling intervals are not equally distributed if the DAQ is forced to switch between different channels at the highest speed. A LabVIEW 8.2 interface has been developed in order to collect the received data, store them in a buffer and perform the further elaboration routines detailed in the next section. In our experiments, the optical sensor has been applied without any calibration, supposing a linear relationship between voltage and strain. This is correct since the strain induced by input sounds is typically very low ( -5.52

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Fig. 7. Application of the RLS adaptive filter to a reference measurement of a 370 Hz tone, with a training phase of 20s. A RLS filter has been applied using 5 (a), 20 (b) and 60 (c) taps, obtaining the resulting plots (a-c) of the measured and filtered signals during the training sequence. Then, the resulting filters have been applied to the 370 Hz tone, estimating the spectrum of the signals with a 500-tap MVE (d-f). GPSD has been computed with the following settings: fa = 300Hz, fb = 500Hz, f-∆ = 360 Hz, f+∆ = 380 Hz.

5. RESULTS The optical microphone has been validated in a laboratory framework, using purposely poor setups in order to assess the effectiveness of the proposed approach. The sensing FBG have been pre-strained and fixed on a loudspeaker controlled by an arbitrary waveform generator; alternatively, the FBG have been applied to the output of a notebook PC, as shown in Fig. 8.

b)

Fig. 8. Experimental setup for sound detection: a) the packaged FBG is strained and locked to the edges of a loudspeaker with scotch-tape, without contact between the FBG and the membrane; b) the FBG is fixed to the poor-quality speaker of a notebook PC with scotch-tape.


5.1 Frequency response We measured the spectral response of the optical microphone, arranged in the configuration of Fig. 8a, obtaining the response curve plotted in Fig. 9. The maximum sensitivity occurs at a low frequency range (50-480 Hz), with a peak response at 250 Hz. The range 600-950 Hz exhibits a good response, approximately 20 dB lower than the previous region. The FBG sensor is capable of detecting intense sounds up to 3 kHz, with a sensitivity 50 dB lower than the top response.

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The measured spectral sensitivity depends on the transfer functions of the loudspeaker and the FBG package, on the energy coupling between the FBG and the speaker, on the propagation of the acoustic waves from the membrane to the FBG cover and on the spectral response of the optoelectronic circuit. However, further measurements show that the trend of Fig. 9 is qualitatively replicated in all the typical configurations.

Fig. 9. Spectral response of the optical microphone, evaluated by changing the resonance frequency of the loudspeaker (with a fixed driving voltage) and recording the correspondent PSD peak level.

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Fig. 10. Measure of the repeatability range of the FBG sensor before and after the RLS equalization, evaluated from 10 different measures of a 370 Hz pure tone generated through a loudspeaker.


5.2 Repeatability Figure 10 plots the repeatability of the optical sensor, evaluated from 10 different measurements of a sinusoidal tone of 370 Hz at the same acoustic power; the signal length is 0.4s, with sampling rate fS = 5kS/s. The PSD is evaluated on a frequency span of 0.01 Hz applying the MVE with 500 taps. The frequency pitch estimate lies within a range of 0.46 Hz, corresponding to a relative interval of 0.12%. The application of the RLS adaptive filter does not improve the repeatability of the peak frequency estimate, since the low frequency selectivity intrinsic in the Capon algorithm does not permit an improvement of the peak estimation sharpness when dealing with narrow spectra. 5.3 Detection of short pulses In order to simulate a medical context, in which the optical sensor is expected to detect pulsed stimuli such as the heartbeat, we probed the capability of the system to react to periodic beats. For this experiment the FBG has been fixed as shown in Figure 8b and an online-metronome [18] has been used to generate acoustic pulses via PC and the beat rate has been chosen to closely match a typical heartbeat signal. The results of this test are shown as time-plot in Fig. 11: the sensor clearly detects beats, reproducing the selected rate without distortion. 0.5 0 -0.5 2

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Time (s) Fig. 11. Measurement of the vibro-acoustic excitation provided by a metronome, with 60, 92, 112 and 144 beat/min respectively.


5.4 Voice detection The good sensitivity exhibited in the low-frequency range 300-500 Hz, in which the principal part of the spectrum of human voice is concentrated, opens up to the application of the optical microphone for voice detection. We performed a preliminary test, using a recorder speech as input sound played by the PC speaker and reconstructing the voice from the FBG sensor. The achieved results show that the sound measured by the sensor, although distorted, maintains the clear intelligibility of the speech. A better result is obtained if the sensor output is normalized by the spectral sensitivity shown in Fig. 9, in order to remove the dependence on the transfer function of the acousto-optical system.

6. CONCLUSIONS In this paper we described the realization of an optical microphone based on fiber Bragg gratings; the dynamic wavelength shift induced by sound axial pressure is transduced into an intensity modulation and then detected with an optoelectronic circuit. This arrangement, applied to detection of weak and dynamic phenomena, permits selecting the optimal working point to maximize the sensitivity, and avoiding precalibration. Moreover, the application of advanced signal processing routines has demonstrated to be effective in mitigating disturbances and increasing performances and robustness of the interrogator. The proposed sensor is mainly oriented to medical and biomedical target applications, with the purpose of providing an alternative instrument capable of working in rugged environment. Compactness, immunity to electromagnetic disturbances and compatibility are expected to make this sensor a useful tool for measurement of voice, breath and heartbeat of patients during exams where more common electrical sensors are not allowed, such as during Computerized Tomography scans. Preliminary tests, involving sound generation from loudspeakers and contact detection, highlight encouraging results and a good potential, which can be raised by adaptive noise canceling techniques.

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