Sensors and Their Applications XI

K. T. V. Grattan

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315897493
• eBook ISBN: 9781351093491
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Condensed Matter

With research continuing to expand and develop, the marketplace for sensors and instrumentation remains one of the most significant for the United Kingdom, the European Union, and the economies of major developed nations. Sensors and Their Applications XI discusses novel research in the field of sensors and transducers, and provides valuable insight into new and topical applications of the technology. The book records the breadth and quality of the field and acts as a topical record of work in sensors and their applications. It will serve as an invaluable reference for physicists, engineers, and chemists working in this area of technology for many years to come.

Section 1

Applications of Sensors

Simultaneous Interrogation of Fibre Bragg Grating Sensors Using an Acoustooptic Tunable Filter

C. Boulet, D. J. Webb, M. Douay¹, P. Niay¹

Applied Optics Group, School of Physical Sciences, University of Kent, Canterbury, CT2 7NR, UK

¹Université des Sciences et Technologies de Lille, Bâtiment P5, Laboratoire PhLAM, 59655 Villeneuve d’Ascq, France

Abstract: Modulation of multiple RF driving signals allows an acoustooptic tunable filter to provide wavelength demultiplexing of fibre Bragg grating sensors. We demonstrated a signal/noise ratio of 44 dB and a crosstalk better than −39 dB.

1. Introduction

Interrogation systems for fibre Bragg grating (FBG) sensors generally perform two functions: high resolution monitoring of the FBG wavelength, and wavelength demultiplexing of the different FBGs in the system [1]. Occasionally these functions are combined, for example in the use of a scanning Fabry-Perot filter [2], though often as in that example this prevents signals being recovered from all gratings simultaneously, which severely limits the application of such systems in dynamic studies.

Acoustooptic tunable filters (AOTFs) have been used as both the interrogating and wavelength demultiplexing element in a sensor system [3]. More recently they have also been used to provide wavelength demultiplexing in a system [4] which incorporated interferometric wavelength shift detection [5] to recover the Bragg wavelength shifts. In that work the filter was sequentially tuned to each FBG. In this paper we present a significant improvement on that approach which allows simultaneous interrogation of the FBGs thus allowing the simultaneous recovery of signals from multiple gratings. As a proof of principle experiment two gratings are interrogated, but the approach is readily scaleable to more.

2. Mode of operation

In our arrangement, illustrated in figure 1, two RF signals are applied to the AOTF, which cause the device to transmit two narrow wavelength bands, λ₁ and λ₂, chosen to match the two gratings. In order to differentiate between the two signals at the detector, the two RF signals are amplitude modulated at different frequencies, f₁ and f₂. This in turn causes the light at λ₁ to be modulated at f₁ and the light at λ₂ at f₂. Demultiplexing is then accomplished by electronically filtering the detected signal at f₁ and f₂ as shown.

This procedure provides the wavelength demultiplexing, but it is still necessary to recover the measurand. This is done using interferometric wavelength shift detection: the system is illuminated by a broadband source through an unbalanced Mach-Zehnder interferometer. The interferometer is repetitively scanned over one free spectral range at a frequency f_C much lower than f₁ and f₂. In the absence of f₁ and f₂, the light returned from the FBGs is amplitude modulated at f_C and any measurand induced wavelength shift is transduced to a shift in the phase of that modulation. In our system the signals from the two gratings are obtained after the appropriate bandpass filter by rectifying the signal and using a lock-in amplifier as a phase meter.

Fig. 1. Experimental arrangement

3. Experimental

The broadband EDFA source delivers up to 20 mW light covering the spectral range 1530 to 1560 nm. This light then passes through an all-fibre Mach-Zehnder interferometer, which has a free spectral range set to 2 nm (1.2 mm optical path difference). One arm of the MZ contains a PZT driven by a triangular (serrodyne) waveform at 500 Hz. The AOTF can be driven by a RF signal ranging from 66 to 69 MHz giving a transmission window from 1520 to 1570 nm. The 5 mm long gratings were photo-written within standard fibre (H2 loaded Corning SMF28 fibre at 140 at) using phase masks. The Full Width Half Maximum of the reflection peaks were close to 0.1 nm and the reflectivities were about 95%.

To minimize crosstalk from our AOTF, which had a relatively large bandwidth of 4.5 nm, we used FBGs with Bragg wavelengths 1531 nm and 1550 nm. This required AOTF driving frequencies of 66.9 MHz and 68.0 MHz and these were in turn amplitude modulated at 30 kHz and 100 kHz.

4. Results

Figure 2 shows signals at different stages in the system. In figure 2a we see the signal just after the detector, which is a mix of the signals coming from both gratings. We are also able to see the “fly-back” of the ramp driving the PZT (designated by arrows). Figure 2b shows a signal after one of the bandpass filters. It shows a carrier at 100 kHz (corresponding to the grating at 1550 nm) and an envelope at 500 Hz. We are interested in the phase of the envelope which is obtained after rectification (figure 2c) using the lock-in amplifier.

The grating was periodically strained with an amplitude of 150 µε using a PZT at 15 Hz. This induces a 15 Hz phase modulation on the 500 Hz signal, which is then available at the output of the lock-in amplifier, as shown in figure 3.

Fig. 2. Signals at different stages of processing (see text for details)

In order to determine the noise limited resolution of the system, the lock-in amplifier was replaced with a spectrum analyser. The signal appears as a 500 Hz carrier with sidebands, due to the modulation, displaced by 15 Hz. The sideband signal to noise ratio was measured to be 48 dB with the spectrum analyser set to a bandwidth of 945 mHz. From this figure we deduce a noise limited strain resolution of 125 nɛ/√Hz.

The other channel was also examined using the spectrum analyser. As the grating corresponding to this channel was not modulated, we expect to see no sidebands and any signal at 15 Hz would be indicative of crosstalk. No signal was visible and hence based on the noise level, we place an upper limit on the crosstalk of – 39 dB.

The phase modulation induced by the Bragg grating creates sidebands of the 500 Hz carrier, the amplitudes of which are given by Bessel functions of the induced modulation [6]. By suitably choosing the free spectral range of the Mach-Zehnder interferometer, we can set the sensitivity and range of our sensor so that the first sideband is to a good approximation linearly related to the strain amplitude. Fig 4 shows the linearity.

Fig. 3. Recovered phase for 150 με, modulation at 15 Hz

Fig. 4. Linearity relation between sidebands amplitude and applied strain

5. Conclusion

We have confirmed that the technique proposed in this paper allows simultaneous interrogation of FBG sensors using an AOTF combined with interferometric wavelength shift detection. The system should be readily scaleable to more FBGs. At the moment, the frequency range of this approach is limited by the use of serrodyne modulation of the PZT element to generate the carrier. Resonances of the PZT excited by the flyback limit the carrier frequency to around 500 Hz. This problem may be overcome by the use of acoustooptic modulators or integrated optic phase modulator to generate the carrier. It should be possible to recover strain signals up to...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Invited Presentations
• Section 1: Applications of Sensors
• Section 2: Automotive Sensor Applications
• Section 3: Biosensors
• Section 4: Gas Sensing
• Section 5: Optical
• Section 6: Applications of Sensors
• Section 7: Water Quality Monitoring
• Section 8: Industrial Applications of Sensors
• Author Index
• Title Index