Wavelength multiplexing of frequency-based self ...

Wavelength multiplexing of frequency-based self-referenced fiber optic intensity sensors Jose´ M. Baptista, MEMBER SPIE Instituto Superior de Engenharia do Porto Departamento de Electrotecnia Rua Dr. Antońio Bernardino de Almeida 431 4200-072 Porto, Portugal and INESC Porto Unidade de Optoelectrońica e Sistemas Electrońicos Rua do Campo Alegre 687 4169-007 Porto, Portugal Silvia Abad Universidad Pu´blica de Navarra Departamento de Ingenierıá Elećtrica y Elećtronica Campus de Arrosadıá s/n E31006 Pamplona, Spain Gaspar M. Rego Escola Superior de Tecnologia e Gestaõ Instituto Politećnico de Viana de Castelo Avenida do Atlaˆntico Apartado 574 4901-908 Viana do Castelo Portugal and INESC Porto Unidade de Optoelectrońica e Sistemas Electrońicos Rua do Campo Alegre 687 4169-007 Porto Portugal Luı´s A. Ferreira Francisco M. Arau´jo Multiwave Networks Portugal, Lda. R. Eng. Frederico Ulrich 2650 4470-605 Moreira da Maia Portugal Jose´ L. Santos, MEMBER SPIE Universidade do Porto Departamento de Fı´sica Faculdade de Cieˆncias Rua do Campo Alegre 687 4169-007 Porto, Portugal and INESC Porto Unidade de Optoelectrońica e Sistemas Electrońicos Rua do Campo Alegre 687 4169-007 Porto Portugal

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Opt. Eng. 43(3) 702–707 (March 2004)

0091-3286/2004/$15.00

© 2004 Society of Photo-Optical Instrumentation Engineers

Baptista et al.: Wavelength multiplexing . . .

Armindo S. Lage, MEMBER SPIE Universidade do Porto Departamento de Engenharia Electrotećnica e de Computadores Faculdade de Engenharia Rua Dr. Roberto Frias, s/n 4200-465 Porto, Portugal

Abstract. A new wavelength multiplexing configuration for selfreferenced fiber optic intensity sensors using fiber Bragg gratings and wavelength division multiplexing (WDM) couplers is investigated. First, the network multiplexing concept is characterized, and then the selfreferenced intensity sensor is presented, which is the basis of each individual sensor in the network. The implemented experimental setup of the multiplexing network is described, and results are presented considering the crosstalk, resolution, and power budget of the sensing multiplexing network. The characteristics and features of the configuration proposed are addressed. © 2004 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.1646177]

Subject terms: optical sensors; optical fiber sensors; optical fiber intensity sensors; optical sensors multiplexing network; optical fiber sensors multiplexing network; optical fiber intensity sensors multiplexing network. Paper 030258 received Jun. 3, 2003; revised manuscript received Aug. 7, 2003; accepted for publication Sep. 12, 2003.

1 Introduction Optical fiber intensity-modulated sensors are very attractive, since they are conceptually simple, reliable, small sized, and suitable for a wide range of applications at lower costs. However, to ensure accurate measurements, the implementation of a reference channel in the sensor is vital. Such a channel should provide insensitivity to source intensity fluctuations and to variable optical transmission losses in the fiber link, couplers, and connectors, which are often indistinguishable from transducer-caused effects.1,2 Previously, we reported a frequency-based approach having selfreferenced fiber optic intensity sensors that are sensitive only to the losses induced by the measurand in the sensing head, i.e., the sensor readout is independent of any other optical loss that can occur along the remaining optical system.3,4 In this work, we report a novel wavelength multiplexing network using fiber Bragg gratings and wavelengthdivision multiplexing 共WDM兲 couplers for wavelength discrimination to address multiple frequency-based selfreferenced fiber optic intensity sensors.5,6 The network multiplexing concept is characterized, and then a brief description of the sensor type deployed in the network and its exhibiting self-referencing properties are presented. The implemented multiplexing setup is described, with its performance being assessed by analyzing the measurement

characteristics of the sensing configuration, such as resolution, power budget, and crosstalk. 2

Network Multiplexing Concept

The proposed multiplexing concept is illustrated in Fig. 1. It is based on a reflective ladder topology, where the information of each sensor is carried by a specific wavelength, associated to a dedicated fiber Bragg grating.7 At the receiver end, the different wavelengths are discriminated by the use of appropriated WDM couplers.8 Moreover, the utilization of these devices allows the light returning from a specific sensor in the network to be directed to the corresponding detector without power loss, except losses arising from nonideal performance. Also, the use of appropriate WDMs as reflective ladder topology couplers would lower the attenuation of the optical signals in the network. This feature is important, considering that maximization of the power levels on detection is particularly relevant for the optimization of the performance of intensity-based sensors. 3 Sensor Concept The Michelson topology with optical feedback is presented in Fig. 2. Relative to the conventional Michelson configuration, it comprehends an extra mirror 共mirror 3兲 to provide optical feedback to the structure.

Fig. 1 Multiplexed sensing structure based on a reflective ladder topology with wavelength addressing of frequency-based self-referenced fiber optic intensity sensors (illustration of a four-sensors network). Optical Engineering, Vol. 43 No. 3, March 2004

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Fig. 2 Michelson configuration with optical feedback.

The frequency response of such a configuration, when the input optical power is modulated at a particular frequency, shows that for some frequencies, the amplitude of the output optical power waveform is maximum 共constructive interference frequencies f I ), while for other frequencies, it results in a minimum value for that amplitude 共offconstructive interference frequencies f OI ). The plot of these amplitudes versus the optical source electric modulating frequency is illustrated in Fig. 3 and gives the transfer function of this sensing configuration.9 The parameter g indicates the optical power attenuation factor externally induced in the fiber structure. For g⫽1, the interferometer is power balanced and there is no induced optical loss, while for g⫽0, all light is lost in the sensing arm of the interferometer. It can be seen that when the induced loss rises, in other words, when g gets closer to zero, the difference between peaks and valleys shortens until the frequency response becomes a horizontal line. On the other hand, when no external losses are induced in the fiber interferometer (g⫽1), the differences between peaks and valleys are at their maximum. The values of the valley and peak frequencies of the transfer function are inversely proportional to the fiber path imbalance of the Michelson configuration. In general, it is desirable to have a limited fiber length for these structures, from which comes the need of a larger modulation frequency, which is not demanding in terms of system bandwidth, considering that the sinusoidal modulation employed does not introduce harmonics higher than the fundamental. The ratio of the value of the transfer function at an offconstructive interference frequency to its value at a constructive interference frequency—R parameter—is defined

Fig. 3 Transfer function of the Michelson configuration with optical feedback. g is the induced optical power loss factor. 704

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as R⬅ V OI /V I , where V OI and V I are sine-wave voltage amplitudes proportional to those of the optical power sinewave waveforms that exit the system at an off-constructive interference frequency ( f OI ) and at a constructive frequency ( f I ). This parameter depends only on the optical losses inside the fiber sensing structure 共intrinsic and induced losses兲 not being influenced by optical power fluctuations that can occur outside the sensing head. Therefore, the modulation of the R parameter provides a selfreferencing scheme that makes the measurand readout independent of possible unwanted light intensity modulation along the optical system.3,4 4 Experimental Setup To demonstrate the described multiplexing concept, the experimental arrangement shown in Fig. 4 was implemented using a single-mode fiber 共Corning SMF28兲. Light from a Photonetics superluminescent erbium-doped fiber source was externally modulated by an AT&T electro-optic modulator 共model m2123c). To implement the Michelson topology of each sensor, two pairs of identical fiber Bragg gratings 共FBG兲 with distinct resonant wavelengths were used, permitting at the reception the independent wavelength identification of each individual sensor. The pair FBG1 and the pair FBG2 were placed in micrometer translation stages, and by applying strain, they were tuned to ␭ 1 ⫽1532 and to ␭ 2 ⫽1550 nm, respectively. Also, the FBG pairs were physically kept very close and temperature stabilized to reduce relative wavelength shifts and keep the operation in the optimal WDM region. In a practical system, similar athermal packaged fiber gratings should be used. The differences in FBG arising from wavelength detuning and spectral shaping have only impacts on power balance. Maximum optical power reflected at each sensor can be obtained by proper overlapping of the FBG spectral responses to maximize signal-tonoise ratio 共SNR兲. Then, attenuator A can be adjusted to optimize visibility. For both Michelson topologies, and to obtain optical feedback, a mirror was introduced. It was produced by deposition of silver in the fiber end of the nonused port of the Michelson coupler by utilization of an appropriated silvering technique 共the mirrors showed reflectivities close to 100%兲. In one of the arms of the Michelson topology of each sensor, a delay fiber roll of approximately 1768 m was inserted. In combination with the fiber core refractive index, this corresponds to a frequency for the first constructive interference peak for each sensor of 57.324 kHz 共not


Fig. 4 Implemented experimental setup.

considering the one associated with the case of the modulating frequency approaching zero兲. In principle, it is possible to adjust the fiber delays to match optimum frequencies for both sensors. However, even if this is not the case, frequency sweeping can be used to ensure optimized system performance. In the other arm of the Michelson topology of both sensors, a fiber attenuator was placed. Each fiber attenuator consisted in a long period fiber grating 共LPFG兲 centered at the wavelength of FBG1 and FBG2 , respectively. The LPFGs, placed in micrometer translation stages, were able to introduce attenuation when their physical position was varied from a bending to a stretching one. When the LPFGs were bent, they only introduced loss related to their nonideal behavior. When the LPFGs were stretched, their attenuation peaks were around 20 dB. The process revealed to be repeatable. The attenuators served two purposes. The first one was to balance the power in the interferometer arms previously to the measurements, and the second purpose was to introduce calibrated loss to simulate measurand action.10 At the reception, a WDM coupler was used to discriminate the two wavelengths associated with the two sensors, and the light exiting its output ports was detected. 5

Results

To observe the sensor transfer function, an electrical signal applied to the external modulator was swept between 250 Hz and 100 kHz. Figure 5 displays such a transfer function observed for both sensors, when the interferometers were power balanced 关Fig. 5共a兲兴, with some optical loss induced

by the attenuators 关Fig. 5共b兲兴 and when the attenuators applied maximum loss 关Fig. 5共c兲兴. These pictures can be compared with the curves in Fig. 3. Figure 5共a兲 exhibits clearly the first and second offconstructive interference frequency 共28.662 and 85.986 kHz, respectively兲, as well as the first constructive interference frequency 共57.324 kHz兲. To obtain the R parameter measurements, two sinusoidal electrical signals with different frequencies were superimposed on the external modulator: one centered in the constructive interference frequency 共57.324 kHz兲, and the other one centered in the off-constructive interference frequency 共28.662 kHz兲. At the detectors, two narrow bandpass filters tuned to those frequencies were used, determining the rms values of the corresponding output voltage signals, which are proportional to their amplitudes. The obtained experimental results are shown in Fig. 6 as well as the expected theoretical dependence, which were obtained directly from the theoretical formulas9 by substituting the specified characteristics for the components used to implement the experimental setup. Figure 7 shows the behavior of the sensors relatively to crosstalk. For those measurements, the output of one of the sensors was monitored, while the attenuator inserted in the other sensor was varied from the balancing point to the position corresponding to maximum attenuation. The results show a negligible crosstalk between the two sensors, which is a consequence of the spacing between the sensors’ associated Bragg wavelengths and the isolation provided by the WDM coupler (⬇20 dB). The resolution obtained for the sensors was found to be ⬇0.05 dB/ 冑Hz.

Fig. 5 Sensor transfer function observed for both sensors. Optical Engineering, Vol. 43 No. 3, March 2004

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show fairly good agreement with those predicted from the theory. It turns out that the system has negligible crosstalk between the two sensors, with the performance on this parameter of the multiplexing network being clearly dependent on the spacing between the sensors’ associated Bragg wavelengths and on the isolation between channels provided by the WDM coupler. The resolution obtained for the sensors was found to be ⬇0.05 dB/ 冑Hz. It should be emphasized that the sensing concept described in this work is particularly favorable in the minimization of system noise. This happens because what is monitored is the amplitude of two sine waves, i.e., the detection bandwidth can be made as narrow as practically feasible, with the consequent decrease of the system noise level. The power budget of the sensing network can be improved if shorter lengths of delay fiber are used, which implies working with higher frequencies. On the other hand, if the reflectivity of the FBGs is optimized, the power received by the detectors will increase correspondingly. Finally, a proper choice of the coupling coefficient of the couplers in the reflective ladder topology or their substitution by appropriated WDMs will have a strong impact on the optical power levels reaching the detector unit. All these factors are relevant for the consideration of networks supporting a larger number of sensors.

References

Fig. 6 Experimental and theoretical results for the R parameter measurements of the two sensors.

6 Conclusions In this work, a wavelength multiplexing concept is demonstrated for frequency-based self-referenced fiber optic intensity sensors relying on the utilization of Bragg gratings and WDM couplers. The experimental results obtained

Fig. 7 Experimental results relative to the crosstalk between the two sensors. 706


1. J. W. Berthold, ‘‘Sensors in industrial systems,’’ Chap. 13 in Optical Fiber Sensors: Applications, Analysis and Future Trends, J. P. Dakin and B. Culshaw, Eds., pp. 261–308, Artech House, London 共1997兲. 2. G. Murtaza and J. M. Senior, ‘‘Referenced intensity-based optical fibre sensors.’’ Intl. J. Optoelectron. 9共4兲, 339–348 共1994兲. 3. J. M. Baptista, J. L. Santos, and A. S. Lage, ‘‘Mach-Zehnder and Michelson topologies for self-referencing fiber optic intensity sensors,’’ Opt. Eng. 39共6兲, 1636 –1642 共2000兲. 4. J. M. Baptista, J. L. Santos, and A. S. Lage, ‘‘Self-referenced fiber optic intensity sensor based on a multiple beam Sagnac topology,’’ Opt. Commun. 181共4 – 6兲, 287–294 共2000兲. 5. A. D. Kersey, ‘‘Fiber optic sensor multiplexing techniques,’’ Chap. 15 in Fiber Optic Smart Structures, E. Udd, Ed., pp. 409– 444, John Wiley and Sons, New York 共1995兲. 6. J. D. C. Jones and R. McBride, ‘‘Multiplexing optical fiber sensors,’’ Chap. 4 in Optical Fiber Sensor Technology—Devices and Technology II, K. T. V. Grattan and B. T. Meggitt, Eds., pp. 117–166, Chapman and Hall, London 共1998兲. 7. K. O. Hill and G. Meltz, ‘‘Fibre Bragg grating technology fundamentals and overview,’’ J. Lightwave Technol. 15, 1263–1276 共1997兲. 8. D. C. Johnson and K. O. Hill, ‘‘Control of wavelength selectivity of power transfer in fused biconical monomode directional couplers,’’ Appl. Opt. 25共21兲, 3800–3803 共1986兲. 9. J. M. Baptista, J. L. Santos, and A. S. Lage, ‘‘Fiber optic intensity sensors based on a Michelson head configuration with selfreferentiation,’’ Proc. IEEE—Lasers Electro-Optics Soc. 13th Ann. Meeting 2, 460– 461 共2000兲. 10. S. W. James and R. P. Tatam, ‘‘Optical fibre long-period grating sensors: characteristics and application,’’ Meas. Sci. Technol. 14, 49– 61 共2003兲. Jose´ M. Baptista graduated in electrical and computer engineering (telecommunications and computers) from the University of Porto (1991). He received his MSc in physics of laser communications from the University of Essex, Colchester, England (1992), and the PhD in electrical and computer engineering from the University of Porto (2002). Currently he is teaching in the Polytechnic Institute of Porto and he is a researcher in the Optoelectronics and Electronics Systems Unit at Instituto de Engenharia de Sistemas e

Baptista et al.: Wavelength multiplexing . . . Computadores do Porto (INESC Porto). His research interests are in the areas of fiber optic sensors and optical communications. He is member of SPIE and IEEE.

in fiber optic sensors from 1992 to 2001. He is the author of numerous international communications, papers, and patents in the fields of fiber optic sensing and fiber optic communications.

Silvia Abad received the MSc degree in telecommunication engineering from the Universidad Pu´blica de Navarra (Spain) in 1998. She also received her PhD degree from the same university in 2002. In the autumn of 1999, she was a visiting researcher at Bell Labs (Murray Hill, New Jersey). In 2000 and 2001, she was also a visiting researcher at the optoelectronics unit of INESC Porto (Portugal). Her main research interests include fiber optic sensor networks, WDM networks and fiber amplifiers, and lasers.

Francisco M. Arau´jo graduated in applied physics (optics and electronics) in 1993 and received his PhD in the area of fiber Bragg gratings in 2000 from the University of Porto.

Gaspar M. Rego graduated in physics (optics and electronics) from the University of Porto (1992). He received the MSc degree in physics of laser communications from the University of Essex, Colchester, England (1993). Currently he is teaching at the Polytechnic Institute of Viana do Castelo, and is a researcher in the Optoelectronics and Electronics Systems Unit at INESC Porto. He is working toward his PhD in engineering sciences at the University of Porto. His research interests are in the area of fiber optic components. Luı´s A. Ferreira graduated in applied physics and in 1995 he obtained the MSc degree in optoelectronics and lasers, both from the University of Porto, Portugal. He received the PhD degree in physics from the same University in 2000. During the PhD, he also worked in the physics department of the University of North Carolina at Charlotte. Presently he is the leader of the Advanced Development Unit at MultiWave Networks Portugal. Previous positions included assistant professor in the physics department of the University of Porto, and senior researcher at the Optoelectronics and Electronic Systems Unit of INESC Porto, where he developed research

Jose´ L. Santos graduated in applied physics (optics and electronics) from the University of Porto (1983). He received the PhD from the same university in 1993 in the area of fiber optic sensors. He has the position of associate professor in the physics department of the University of Porto, being the manager of the Optoelectronics and Electronics Systems Unit at INESC Porto. He is member of OSA, SPIE, and the Planetary Society. Armindo S. Lage graduated in electrical and computer engineering (telecommunications and computers) from the University of Porto (1973). He received the PhD from the same university in the area of optics. He is an assistant professor in the Department of Electrical and Computer Engineering, a researcher of the Faculty of Engineering at the University of Porto, and also a researcher of Centro Computer Integrated Manufacturing (CIM) of Porto. Since 1979, his research interests have included optical metrology and signal processing. He is member of SPIE.


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