Time Division Multiplexing of Fiber Bragg Grating Sensors Using a Mode-Locked Fiber Laser Source

AbstractAn efficient multiplexing method for fiber Bragg grating sensors based on a broadband mode-locked laser source and interferometric interrogation is described. The system has the capability to interrogate several hundred fiber Bragg grating sensors on a single fiber, whilst achieving sub-microstrain resolution over bandwidths greater than 100 kHz. A demonstration system with 8 sensors is presented, which achieves strain resolutions less than 213 n/Hz1/2 up to 100 Hz.†

Keywords: Mode-locked laser, fiber Bragg grating sensor, time division multiplexing

IntroductionHigh-performance applications of fiber Bragg grating (FBG) strain sensors require multiplexing very large numbers of sensors onto a single fiber whilst achieving sub-microstrain resolution from sub-Hertz frequencies upwards. An interrogation approach based on an interferometric wavelength discriminator permits strain measurements approaching the nanostrain level over large bandwidths. Interferometric interrogation of fiber Bragg grating strain sensors has demonstrated strain resolutions of 6 nstrain/Hz1/2 at 1 Hz for a single sensor [1] and 4 nstrain/Hz1/2 at 0.1 Hz for a multiplexed array [2]. The latter multiplexing technique utilizes the spectral selectivity of the FBG sensors to distinguish between them (i.e. wavelength division multiplexing (WDM)). The number of sensors in this case is limited by the spectral width of the emission from the illuminating source and the required wavelength separation, and is typically less than 20 sensors. To increase the level of multiplexing, time division multiplexing (TDM) can be applied where sensors are arranged such that their signals arrive at the detector at different times. This requires producing switched broadband optical pulses; such as by directly switching a super-luminescent diode (SLD) [3] or by externally modulating an ELED [4]. In these cases, strain resolutions of 220 nstrain/Hz1/2 and 2 nstrain/Hz1/2 were achieved respectively. Limitations arise in both of these configurations due to the restricted pulse powers generated by SLDs and ELEDs. Also, multiple-stage optical modulators are required to achieve acceptable extinction ratios when operated with a broad-bandwidth input signal. Low extinction ratio pulse emission increases the crosstalk in multiplexed arrays and reduces the number of sensors that can be multiplexed. Another technique to interrogate multiplexed FBGs and achieve high resolution over bandwidths up to 2 kHz (extendable to 10 kHz), is based on a linear CCD spectrometer [5]. This technique has been demonstrated to achieve strain resolutions approaching the nanostrain level, although is currently limited to WDM only [6].

To achieve the combination of very high multiplexing gain, sub-microstrain resolution, low crosstalk and large bandwidth (many kHz) a configuration combining a mode-locked erbium doped fiber laser source (MLL) with interferometric interrogation is presented. One such MLL configuration can generate broadband (greater than 60nm), square optical pulses of duration 10 to 30 nanoseconds (tunable by pump power and fiber ring birefringence), peak pulse powers approaching 1 W, at a repetition rate of several hundred kiloHertz [7]. The spectral emission properties of this laser have been investigated further in [8].

In the proposed configuration, a serially multiplexed array of FBG sensors consists of low reflectivity FBGs (R<1%) each at a different wavelength. This sequence of FBGs is repeated periodically along a single fiber. A low cost method of fabricating multiplexed FBG arrays is based on single pulse grating fabrication on the draw tower [9]. Continued development of this technique has considerably improved the quality of the FBGs over the initial demonstrations and may make possible on-line fabrication of apodized FBGs at pre-determined wavelengths. Alternatively, other techniques based on strip and recoat [10] or by writing through the fiber coating [11] are currently available.

† Address: Naval Research Laboratory, Code 5674, 4555 Overlook Avenue SW, Washington DC 20375, email:geoff.cranch@nrl.navy.mil, tel: (202) 404 3800, fax: (202) 767 5792

G.A.Cranch†1, G.M.H.Flockhart1 and C.K.Kirkendall2

1SFA Inc, Largo, MD 207742Naval Research Laboratory, Washington DC, 20375

In the following work, the design and performance of an 8-element FBG array is reported. The strain resolution and crosstalk have been characterized and a discussion is given on how the system can be scaled up.

Experimental ArrangementThe experimental configuration is shown in fig. 1. The output from the MLL is launched into a fiber-optic Mach-Zehnder (MZI) interferometer with a 5.2 mm optical path imbalance, which is smaller than the coherence length of the light reflected from a sensor grating. The two outputs of the MZI are injected into each FBG array, which produces a return pulse from each FBG. Two FBG arrays, each with four FBGs separated by 91 m of optical fiber (SMF-28™), are thus interrogated simultaneously. The FBGs are written in SMF-28™ fiber and are 5 mm in length apodized by the Gaussian laser beam profile. The FBGs reflectivity ranges from 0.8% to 2.1% and have full-width half-maximums of 0.2 nm. FBGs are numbered 1 to 4 in FBG array 1 and 5 to 8 in FBG array 2. The center wavelength of FBG 1,2,5 and 6 are 1549.6nm and FBG 3,4,7 and 8 are 1546.6nm. Signals from FBGs at different wavelengths can be separated using standard WDM techniques and signals from FBGs at the same wavelength returns at different times. We have used a pseudo-heterodyne demodulation scheme [12] to linearize the interferometer fringe pattern.

Figure 1: Multiplexed FBG sensor system arrangement MLL – Mode Locked Laser, POL - Polarization controller PG – delay Pulse Generator, A/D – Analog to Digital converter,

CIRC – Circulator, PC – Personal Computer, PZT – fiber wrapped piezoelectric tube.

In the experimental arrangement we demonstrate the TDM aspect of this scheme only, since in terms of the interrogation approach, the WDM aspect is a straightforward extension of this scheme. The signals returning from each array thus consist of a temporal pulse train, where each pulse corresponds to an FBG and consists of a fringe pattern, the phase of which contains the strain information of interest from the FBG sensor. A tap of the direct output from the MLL is detected on a photodiode and is used to generate a trigger pulse for the A/D. This is done by frequency quadrupling the detected pulse frequency such that four equally spaced samples are taken between each MLL pulse. The fiber lengths between each FBG are set so that the pulse returns from each FBG occur at the A/D sample trigger. An analogue to digital converter (National Instruments PCI6110) takes a single sample of the fringe pattern produced by each FBG. Thus, the signals from all four FBG sensors are recorded by a single A/D. Once digitized, the fringe pattern can be processed to obtain the strain information. The individual FBG sensor signals are extracted during the signal processing. The launched pulse power density is 7 dBm at 1550nm in a 0.2 nm bandwidth. The combined loss of the MZI, circulators, FBG sensor and connectors results in –22 dBm return power per pulse at the detectors. High speed (5 MHz) PINFET receivers are used for detecting the return from each FBG array. The minimum power required at the detector is approximately –28 dBm per pulse, thus around 6 dB of optical power margin is available.The phase of the interferometer, , is related to the strain applied to the FBG sensor, , by the equation,

where is the fiber effective refractive index, is the interferometer fiber path imbalance, is the Bragg wavelength of the FBG and the factor of 0.78 includes the contribution to the Bragg wavelength shift from the stress-optic effect [13]. In the experimental system, =1.468 and =3.6 mm 0.2mm yielding, =16.7 mrad/.

The pulse returns from each FBG array measured by the detectors are shown in fig. 2(a). The length of fiber between each FBG results in equally spaced pulses. The variation in pulse heights is due to the different reflectivity of the FBG and some excess splice loss. Fig. 2(b) shows a typical pseudo-heterodyne carrier signal pair. The serrodyne frequency is 300 Hz and the amplitude is set to give the required 2 phase excursion of the interferometer. The signal-processing algorithm calculates the difference in strain between two corresponding FBG sensors (i.e. 1 and 5, 2 and 6, 3 and 7, 4 and 8). Thus the system is arranged to measure differential strain between two FBGs. This could be useful for conducting a temperature compensated measurement, if one FBG is placed close to its corresponding FBG but isolated from the applied strain. Alternatively, by also sampling the serrodyne signal, each FBG signal can be demodulated independently. Polarization fading can occur due to the high degree of polarization of the MLL emission. Reducing the net birefringence of the MZI prevents significant polarization fading from occurring.

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The strain resolution for each FBG pair is shown in fig. 3(a), measured over five averages. The strain resolution varies between 80 and 213 n/Hz1/2 ±25% up to 100 Hz limited by relative intensity noise from the MLL. The structure above 100 Hz is residual carrier and its harmonics. To characterize sensor-to-sensor crosstalk a tone is injected onto an individual sensor by attaching it to a PZT actuator. Fig. 3(b) shows the crosstalk between channels when a tone at 11 Hz is applied to FBG 1 (the large harmonic at 22Hz is due to nonlinearity in the PZT actuator driver). In this case the crosstalk is less than -43 dB.

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When a signal is applied to the other FBGs individually, the maximum crosstalk is -34 dB. Finally, fig. 4 shows the output of each FBG sensor pair when an 11 Hz signal of amplitude 6 is applied.

The number of sensors that can be multiplexed with time division multiplexing is limited by a number of factors. In the current setup, it is due to the sample rate and data throughput rate limitation of the PC. For FBGs spaced by 91m of optical fiber the required sample rate is 1.11 MHz. It would therefore be possible to reduce the spacing between FBGs at the same wavelength by increasing the sampling rate. Also, the spacing between consecutive FBGs can be reduced further by incorporating WDM with TDM. The spectral emission bandwidth of the MLL is around 60nm, thus accommodating 30 wavelength channels at a spacing of 2nm. Increasing the sample rate by an order of magnitude would permit greater than 1000 sensors on a single fiber to be interrogated. The sample rate for each FBG sensor is equal to the pulse repetition rate of the MLL, , which is 278 kHz. In the demonstration system, the pseudo-heterodyne carrier is 300 Hz, thus only 600 Hz of bandwidth or 0.2% of the digitized data is actually utilized. The available bandwidth can be more efficiently utilized with an interrogation approach based on heterodyne such as the one

demonstrated in [14], developed for interrogating multiplexed fiber-optic interferometric sensors. In this case the signal bandwidth can be up to or 139 kHz. Modifying the MLL design to increase will increase the sensor bandwidth further. This configuration has the potential for direct expansion to a large-scale system, with very high signal bandwidths available.

Conclusion

A novel multiplexing architecture for fiber Bragg grating sensors employing a mode-locked laser source with interferometric interrogation has been demonstrated. The performance of an 8-sensor prototype has been described. The system can potentially interrogate several hundred sensors multiplexed onto a single fiber, whilst achieving sub-microstrain resolution and bandwidths exceeding 100 kHz. High bandwidth, high resolution, multiplexed FBG sensors have applications in acoustic sensing for structural health monitoring and structural shape measurement.

Acknowledgement: The authors would like to acknowledge A.Bautista for loan of the high-speed receivers, C.G.Askins and E.J.Friebele for loan of the MLL and extremely useful discussions and S. Motley for assistance in fabricating the sensor array. This work was funded by the Office of Naval Research.

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