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Addressing fiber Bragg grating sensors with wavelength-swept pulse fiber laser and analog electrical switch
Optics Communications 284 (2011) 1561–1564
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Addressing fiber Bragg grating sensors with wavelength-swept pulse fiber laser and analog electrical switch Ruo Ming Li a,⁎, You Long Yu b, P. Shum a a b
Network Technology Research Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637553, Singapore School of Instrument Science and Opto-electronic Engineering Hefei University of Technology, Hefei, 230009, China
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 23 November 2010 Accepted 23 November 2010 Keywords: Optical fibers Fiber Bragg grating sensor Fiber lasers
a b s t r a c t A pulse train with a wavelength dependent time sequence is generated in a fiber laser configuration, which contains a cascaded wavelength-division-multiplexing (WDM) fiber Bragg grating (FBG) array and a tunable F-P filter. By distributing pulses to corresponding channels with a 1 × N analog electrical switch, a novel FBG sensors interrogation technique with advantages of high signal-to-noise ratio (SNR) and high interrogation speed is experimentally demonstrated. Then, a FBG sensing system based on this interrogation technique and the mature unbalanced scanning Michelson interferometer (USMI) demodulation technique is realized. The system has shown a sensitivity of 1.610°/με, for the 1555 nm FBG, which agrees well with the theoretical value of 1.674°/με. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Fiber Bragg Grating (FBG) which can be fabricated by inscribing grating into a silica fiber is a wavelength-encoded element. It can be embedded into the materials of the infrastructure to provide the structure health monitoring by direct measurement of the strain, and temperature [1]. For the individual FBG sensor, the high cost of demodulation equipments is an obviously disadvantage. Multiplexing FBG sensors to a quasi-distributed multi-sensor network can provide the capability of monitoring plenty of sensors with shared demodulation equipments, thus this topic has attached considerable attentions during the past few years. For sensing network, the real time monitoring and the high signal-to-noise ratio (SNR) are two key issues. The timedivision-multiplexing (TDM) and wavelength-division-multiplexing (WDM) schemes are two commonly used multiplexing techniques. The typical TDM scheme, using broad band source (BBS) as the light source, can perform real-time monitoring. However, it must employ delay fiber, typically longer than 100 m to separate the FBG reflected signals. Usually, FBG sensors used in TDM scheme adopt uniform Bragg wavelength and low reflectivity. Due to the low reflectivity of FBG and the low power spectrum intensity of BBS, the TDM system generally suffers from a low SNR [2,3]. The WDM scheme often uses an optical filter to select signals from FBG reflected light. A higher SNR can be obtained with the help of wavelength-swept fiber laser [4,5], however, the wavelength tuning speed of the wavelength-swept fiber laser (WSFL), which bases on Erbium doped fiber, is limited by the
⁎ Corresponding author. E-mail address: [email protected] (R.M. Li). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.11.072
excited stated lifetime (T1) ,typical value 8 ms [6]. High wavelength tuning speed will cause the laser operating in the Q-switch state [7]. In the Q-switch state, laser output will change to pulse. Furthermore, in the situation of using Q-switch state WSFL to sweep the WDM FBG array, it is possible that the excited state population is exhausted when the transmission window of WSFL's filter overlaps with certain FBG's wavelength, thus some of sensors may be dropped. In this paper, a WSFL based pulse output wavelength-swept fiber laser (POWSFL) is built by taking a WDM FBG array as mirrors. The POWSFL can lase during the period when the filter's transmission window matches with the wavelength of the FBG reflected signals, thus a laser pulse train with a determined time–wavelength sequence is generated. Based on the time–wavelength sequence, a novel interrogation method with a SNR of 35 dB and high speed monitoring function is realized by allocating the pulse signals to corresponding channel with a micro control unit (MCU) and a 1 ×N analog electrical switch. 2. Experimental setup and operation principle The system is schematically shown in Fig. 1. A section of EDF is pumped by a laser diode, and the obtained amplified spontaneous emission (ASE) light is coupled into a cascaded WDM FBG sensor array. Then the reflected signal is coupled to a tunable F-P filter. Only the wavelength matched signal can pass the filter, and be amplified in the EDF. Finally, the amplified signal will be coupled back to FBG array forming a ring resonance cavity. The isolator is used to ensure unidirectional propagation and suppresses undesired reflections in the cavity. For the reflected ASE signal, the transmission window of filter is tuned as shown in Fig. 2. When the window overlaps with the reflected signal, the reflected signal can be amplified to lase. As a result of filter's
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Fig. 1. Schematic diagram of sensor system Gi, (i = 1, 2…n), fiber Bragged grating; ISi, (i = 1, 2), isolator; C1, 20:80 coupler; C2, 3 dB coupler; Mi, (i = 1, 2), mirror; EDF, erbium-doped fiber; BPF, band-pass filter; IMG, index matched glue; MCU: micro control unit; PZT-ceramic, piezoelectric ceramic; WDM, wavelength division multiplexer.
periodic sweeping, the POWSFL will lase at different FBGs' wavelength sequentially in a sweeping periodic and lase at the identical FBG's wavelength in each sweeping periodic. The emitted laser pulses will be coupled into the unbalanced scanning Michelson interferometer (USMI) for demodulation. After completing the optical to electrical (O-E) conversion, the MCU controlled 1 × N analog electrical switch, distributes pulses to each corresponding channel. As each channel is equipped with its signal processing unit, real-time addressing for FBG sensors array can be realized. As the EDF in POWSFL will only amplify the signal at the reflector's wavelength, the consumption of excited population is less than the continuously wavelength-swept laser under the same pumping strength and wavelength tuning speed. The less population consumption provides support for higher wavelength tuning speed. The deduction for the maximum tuning frequency is similar to the discussion of the ‘saturation limit’ in [7]. The condition for lasing during the time that the transmission window overlaps with the reflected FBG signals can be expressed as: τFP N Kti
ð1Þ
where τFP = ΔλFP /(fp × Δλ span) represents the period in which the centre wavelength of filter passes through the spectrum span equal to 3 dB bandwidth of the F-P filter (ΔλFP), fp is the filter's tuning frequency, and Δλspan is the sweeping filter covered spectrum span,
ti = nref Li /c, (i = 1,2,3,4) is the roundtrip time of the ith resonance cavity, and Li is the cavity length. K = lg(Pith /PS)/lg(G) is the number of round trips required for lasing from ASE, and G is the roundtrip net gain for small signals. PS = (ΔλFP /ΔλASE) × Ptotal is the power of signal light, ΔλASE is the spectrum bandwidth of the ASE, and Ptotal is the power of the ASE.Pith = φi × h × γi/Δt window is the average power of the laser pulse corresponding to ith FBG, h is the Planck constant, γi and φi are frequency and total number of the photon of the ith FBG in the period of 1/fp, and Δt window = ΔλFBG/(fp × Δλspan) is the time for centre wavelength of window passing through the 3 dB bandwidth of FBG spectrum (ΔλFBG). The power Pith is connected with the pump and decay rate. The maximum tuning frequency is limited by: fp b
ΔλFP : Kti × Δλspan
ð2Þ
It can be found that the maximum tuning frequency fp is related to several parameters. There is always a trade-off among those parameters in order to achieve higher wavelength sweeping frequency. For the demodulation with the USMI, the output signal of band pass filter (BPF) is taken as the under test signal of phase meter, while the driving saw-toothed signal of the PZT-ceramics (PZT: lead (Pb) zirconia (Zr) Titanate (Ti)), is set as the reference signal of phase
Fig. 2. Diagram of lasing from ASE.
R.M. Li et al. / Optics Communications 284 (2011) 1561–1564
meter. The measured strain of the FBGi (εxi ) can be expressed by the observed phase-shift Δϕ : εxi = −
λBi ΔΦ: 4πnLð1−Pe Þ
ð3Þ
Here the λBi is the central wavelength of the FBGi, L is the length difference between arms, and the Pe = 0.22 is the effective photoelectric constant. After calibration the phase shift corresponded to unit strain with a precise translation stage, according to (3), different strain applied axially on the different sensor elements can be obtained at the same time. It confirms that the sensor system can monitor the strain in the way of nearly real-time.
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lasing. As a result, the idle lasing (as shown in Fig. 3, marked with dash circle) can be observed. The idle lasing signal is blocked after O-E conversion, by the MCU controlled switch. The MCU's trigger signal (shown in Fig. 3 (b)) is a 30% duty-cycle square wave and its risingedge is phase synchronized with the falling-edge of the saw-toothed wave. The falling-edge of the square wave is used to trigger MCU to turn on the switch, and the rising-edge is used to close the switch. When the output power of pump LD is 55 mW, the POWSFL lases, and firstly emitted pulse in each sweeping period has the shortest wavelength. The O-E converted electronic pulse train is shown in Fig. 3 (c). Calculating from the 3 V amplitude and the 50 mW background of Fig. 3 (c), a 35 dB SNR can be obtained. The detector used in the experiment has a responsivity of 4 × 104 V/W at 1550 nm, so the amplitude 3 V corresponds to the power of the laser pulse 75 μw. Assuming that the reflectivity of FBG is 90%, the FBG reflected Δλ
To experimentally demonstrate the monitoring function, a four-FBG sensor system is presented, as shown in Fig. 1. The wavelengths of four cascaded gratings from left to right are λ 1 = 1557.80 nm, λ2 = 1549.88 nm, λ3 = 1551.94 nm, and λ4 = 1555.119 nm respectively. The 3 dB bandwidth of each FBG is 0.2 nm and its reflectivity is 90%. The fiber length between the circulator and the nearest FBG (G1) is 140.6 cm. From left to right, the spatial interval between the adjacent two sensors is 137.7 cm, 217.3 cm, and 158.7 cm, respectively. The ring-cavity is composed of a 980 nm pump laser diode, a 26.83 m EDF, a 980/1550 WDM coupler, a 16.56 m single mode fiber, a F-P filter with a 3-dB bandwidth of 66 pm and a free spectral range (FSR) of 56.7 nm, a coupler C1with a coupling ratio of 4:1, a circulator and an isolator (IS1), which has an isolation of 42 dB and an insertion loss of 0.14 dB. The isolation of IS2 is 36.5 dB, and the insertion loss is 0.34 dB. The total loss of the cavity is around 48.38 dB, including 22.75 dB absorption of EDF, and a 23 dB fiber-to-fiber loss of the F-P filter. The F-P filter's driving signal is a saw-toothed wave (as shown in Fig. 3 (a)) with a frequency of 1 KHz, a peak to peak voltage of 3.6 V and an offset voltage of 8 V. Limited by PZT-ceramic's operating frequency, the falling edge of (a) becomes gentle and there are some spurs in the falling edge. The spurs can be attributed to the resonance, as the frequency of the driving saw-toothed wave is close to the resonance frequency of the PZT-ceramic [8]. As the ideally steep falling edge of the driving saw-toothed wave is changed to gentle falling, the edge falling time, during which the filter goes through the full sweeping spectrum span, is long enough for
continuously ASE power Pr can be expressed as ΔλFBG × Ptotal , Then ASE with ΔλFBG = 0.2 nm, ΔλASE = 40 nm, and Ptotal = 4 mW, we can get Pr = 0.018 mW. After passing the filter which has a 23 dB loss, the remaining light power PASE is around 0.1 μw. the PASE corresponds to the output power when the laser cavity is disconnected between the C1 and EDF, and it is much lower than the power of laser pulse. That provides the evidence showing lasing. Finally, the switch is used to distribute signals to each corresponding channel, the typical outputs of one channel is shown in Fig. 3 (d). The length difference between arms of USMI is 3.2 mm (L), and the average length of the arms is 106 cm. To stretch the shorter arm, another PZT-ceramic is driven by a 40 Hz saw-toothed signal with Vp-p = 4.8 V and Voff = 5.6 V. To verify the system function, the static strain is applied to G4.The signal corresponding to G4 is shown in Fig. 4. The relationship between the phase-shift and the measured strain is shown in Fig. 5. The measured sensitivity of sensor is ~1.610°/μ ε, which agrees with theoretical value 1.674°/με well by taking Pe = 0.22 and λBi = 1555 nm into account for calculation. The quantity of sensors that the addressing technique can support is determined by the spectrum tuning span of the filter Δλspan and the spectrum interval between adjacent FBGs. Furthermore, the capability is also restricted by the spatial interval between the sensors and the tuning frequency fp, adopting half of the spectrum interval between the FBGs as the limitation. The system has a measurable range of −827 με to 827 με. From Eq. (3) we can know that the resolution of the strain measurement is determined by the length difference between arms of USMI (L = 3.2 mm), and the minimum detectable phase variation. The
Fig. 3. Interrogation related signals. (a)1, F-P filter's driving signal; (b) 2, MCU's trigger signal; (c) 3, output of detector; (d), output of the switch's channel 4.
Fig. 4. Signal corresponded to G4 (a) 1: Driving signal of PZT-ceramic in USMI; (b): outputs of the switch channel corresponded to G4 (c): trace (b) filtered with a 38~42 Hz band-pass filter.
3. Experimental result and discussion
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monitoring function. Cascaded WDM FBG sensor array and the sawtooth wave driving inter-ring F-P filter are used to realize a tunable ring fiber laser. The laser can generate pulse train with the wavelength dependent time-sequence. According to the time-sequence, the O-E converted pulse train is distributed by a 1 × N analog electrical switch, thus a novel real time addressing technique is demonstrated. The demodulation is performed by an unbalanced scanning Michelson interferometer, and a sensitivity of 1.610°/με has been achieved, which agrees well with our theoretic investigation.
Acknowledgement This work was supported by New Century Excellent Talents in University (NCET-04-0828), Ministry of Education, China. Fig. 5. Experimental data and theoretical prediction of phase shift Δϕ4with respect to ε4.
References minimum detectable phase variation is the greater value of the phase detector's resolution and minimum phase variation caused by lasing frequency changing, which is determined by longitudinal mode spacing. For our system, the minimum phase variation caused by lasing frequency is 0.04° and the phase detector's resolution is 0.01°. Thus the resolution of the strain measurement is 22 nε. 4. Conclusion We have proposed and experimentally demonstrated an active time-domain addressing architecture with high SNR and real-time
[1] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, Journal of Lightwave Technology 15 (Aug. 1997) 1442. [2] H.Y. Fu, H.L. Liu, X. Dong, H.Y. Tam, P.K.A. Wai, C. Lu, Electronics Letters 44 (May 2008) 618. [3] S.C. Liu, Y.L. Yu, J.T. Zhang, X.F. Chen, IEEE Photonics Technology Letters 19 (Sep-Oct 2007) 1493. [4] S.H. Yun, D.J. Richardson, B.Y. Kim, Optics Letters 23 (1998) 843. [5] Y.L. Yu, L. Lui, H. Tam, W. Chung, IEEE Photonics Technology Letters 13 (Jul 2001) 702. [6] W.T. Silfvast, Laser Fundamentals, 2nd ed, Cambridge University Press, 2004, p. 570. [7] R. Huber, M. Wojtkowski, K. Taira, J.G. Fujimoto, K. Hsu, Optics Express 13 (May 2005) 3513. [8] Z.J. Xiao Jia, Tian Shi, Piezoelectric & Acoustooptic 25 (2003) 203.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Addressing fiber Bragg grating sensors with wavelength-swept pulse fiber laser and analog electrical switch Ruo Ming Li a,⁎, You Long Yu b, P. Shum a a b
Network Technology Research Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637553, Singapore School of Instrument Science and Opto-electronic Engineering Hefei University of Technology, Hefei, 230009, China
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 23 November 2010 Accepted 23 November 2010 Keywords: Optical fibers Fiber Bragg grating sensor Fiber lasers
a b s t r a c t A pulse train with a wavelength dependent time sequence is generated in a fiber laser configuration, which contains a cascaded wavelength-division-multiplexing (WDM) fiber Bragg grating (FBG) array and a tunable F-P filter. By distributing pulses to corresponding channels with a 1 × N analog electrical switch, a novel FBG sensors interrogation technique with advantages of high signal-to-noise ratio (SNR) and high interrogation speed is experimentally demonstrated. Then, a FBG sensing system based on this interrogation technique and the mature unbalanced scanning Michelson interferometer (USMI) demodulation technique is realized. The system has shown a sensitivity of 1.610°/με, for the 1555 nm FBG, which agrees well with the theoretical value of 1.674°/με. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Fiber Bragg Grating (FBG) which can be fabricated by inscribing grating into a silica fiber is a wavelength-encoded element. It can be embedded into the materials of the infrastructure to provide the structure health monitoring by direct measurement of the strain, and temperature [1]. For the individual FBG sensor, the high cost of demodulation equipments is an obviously disadvantage. Multiplexing FBG sensors to a quasi-distributed multi-sensor network can provide the capability of monitoring plenty of sensors with shared demodulation equipments, thus this topic has attached considerable attentions during the past few years. For sensing network, the real time monitoring and the high signal-to-noise ratio (SNR) are two key issues. The timedivision-multiplexing (TDM) and wavelength-division-multiplexing (WDM) schemes are two commonly used multiplexing techniques. The typical TDM scheme, using broad band source (BBS) as the light source, can perform real-time monitoring. However, it must employ delay fiber, typically longer than 100 m to separate the FBG reflected signals. Usually, FBG sensors used in TDM scheme adopt uniform Bragg wavelength and low reflectivity. Due to the low reflectivity of FBG and the low power spectrum intensity of BBS, the TDM system generally suffers from a low SNR [2,3]. The WDM scheme often uses an optical filter to select signals from FBG reflected light. A higher SNR can be obtained with the help of wavelength-swept fiber laser [4,5], however, the wavelength tuning speed of the wavelength-swept fiber laser (WSFL), which bases on Erbium doped fiber, is limited by the
⁎ Corresponding author. E-mail address: [email protected] (R.M. Li). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.11.072
excited stated lifetime (T1) ,typical value 8 ms [6]. High wavelength tuning speed will cause the laser operating in the Q-switch state [7]. In the Q-switch state, laser output will change to pulse. Furthermore, in the situation of using Q-switch state WSFL to sweep the WDM FBG array, it is possible that the excited state population is exhausted when the transmission window of WSFL's filter overlaps with certain FBG's wavelength, thus some of sensors may be dropped. In this paper, a WSFL based pulse output wavelength-swept fiber laser (POWSFL) is built by taking a WDM FBG array as mirrors. The POWSFL can lase during the period when the filter's transmission window matches with the wavelength of the FBG reflected signals, thus a laser pulse train with a determined time–wavelength sequence is generated. Based on the time–wavelength sequence, a novel interrogation method with a SNR of 35 dB and high speed monitoring function is realized by allocating the pulse signals to corresponding channel with a micro control unit (MCU) and a 1 ×N analog electrical switch. 2. Experimental setup and operation principle The system is schematically shown in Fig. 1. A section of EDF is pumped by a laser diode, and the obtained amplified spontaneous emission (ASE) light is coupled into a cascaded WDM FBG sensor array. Then the reflected signal is coupled to a tunable F-P filter. Only the wavelength matched signal can pass the filter, and be amplified in the EDF. Finally, the amplified signal will be coupled back to FBG array forming a ring resonance cavity. The isolator is used to ensure unidirectional propagation and suppresses undesired reflections in the cavity. For the reflected ASE signal, the transmission window of filter is tuned as shown in Fig. 2. When the window overlaps with the reflected signal, the reflected signal can be amplified to lase. As a result of filter's
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Fig. 1. Schematic diagram of sensor system Gi, (i = 1, 2…n), fiber Bragged grating; ISi, (i = 1, 2), isolator; C1, 20:80 coupler; C2, 3 dB coupler; Mi, (i = 1, 2), mirror; EDF, erbium-doped fiber; BPF, band-pass filter; IMG, index matched glue; MCU: micro control unit; PZT-ceramic, piezoelectric ceramic; WDM, wavelength division multiplexer.
periodic sweeping, the POWSFL will lase at different FBGs' wavelength sequentially in a sweeping periodic and lase at the identical FBG's wavelength in each sweeping periodic. The emitted laser pulses will be coupled into the unbalanced scanning Michelson interferometer (USMI) for demodulation. After completing the optical to electrical (O-E) conversion, the MCU controlled 1 × N analog electrical switch, distributes pulses to each corresponding channel. As each channel is equipped with its signal processing unit, real-time addressing for FBG sensors array can be realized. As the EDF in POWSFL will only amplify the signal at the reflector's wavelength, the consumption of excited population is less than the continuously wavelength-swept laser under the same pumping strength and wavelength tuning speed. The less population consumption provides support for higher wavelength tuning speed. The deduction for the maximum tuning frequency is similar to the discussion of the ‘saturation limit’ in [7]. The condition for lasing during the time that the transmission window overlaps with the reflected FBG signals can be expressed as: τFP N Kti
ð1Þ
where τFP = ΔλFP /(fp × Δλ span) represents the period in which the centre wavelength of filter passes through the spectrum span equal to 3 dB bandwidth of the F-P filter (ΔλFP), fp is the filter's tuning frequency, and Δλspan is the sweeping filter covered spectrum span,
ti = nref Li /c, (i = 1,2,3,4) is the roundtrip time of the ith resonance cavity, and Li is the cavity length. K = lg(Pith /PS)/lg(G) is the number of round trips required for lasing from ASE, and G is the roundtrip net gain for small signals. PS = (ΔλFP /ΔλASE) × Ptotal is the power of signal light, ΔλASE is the spectrum bandwidth of the ASE, and Ptotal is the power of the ASE.Pith = φi × h × γi/Δt window is the average power of the laser pulse corresponding to ith FBG, h is the Planck constant, γi and φi are frequency and total number of the photon of the ith FBG in the period of 1/fp, and Δt window = ΔλFBG/(fp × Δλspan) is the time for centre wavelength of window passing through the 3 dB bandwidth of FBG spectrum (ΔλFBG). The power Pith is connected with the pump and decay rate. The maximum tuning frequency is limited by: fp b
ΔλFP : Kti × Δλspan
ð2Þ
It can be found that the maximum tuning frequency fp is related to several parameters. There is always a trade-off among those parameters in order to achieve higher wavelength sweeping frequency. For the demodulation with the USMI, the output signal of band pass filter (BPF) is taken as the under test signal of phase meter, while the driving saw-toothed signal of the PZT-ceramics (PZT: lead (Pb) zirconia (Zr) Titanate (Ti)), is set as the reference signal of phase
Fig. 2. Diagram of lasing from ASE.
R.M. Li et al. / Optics Communications 284 (2011) 1561–1564
meter. The measured strain of the FBGi (εxi ) can be expressed by the observed phase-shift Δϕ : εxi = −
λBi ΔΦ: 4πnLð1−Pe Þ
ð3Þ
Here the λBi is the central wavelength of the FBGi, L is the length difference between arms, and the Pe = 0.22 is the effective photoelectric constant. After calibration the phase shift corresponded to unit strain with a precise translation stage, according to (3), different strain applied axially on the different sensor elements can be obtained at the same time. It confirms that the sensor system can monitor the strain in the way of nearly real-time.
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lasing. As a result, the idle lasing (as shown in Fig. 3, marked with dash circle) can be observed. The idle lasing signal is blocked after O-E conversion, by the MCU controlled switch. The MCU's trigger signal (shown in Fig. 3 (b)) is a 30% duty-cycle square wave and its risingedge is phase synchronized with the falling-edge of the saw-toothed wave. The falling-edge of the square wave is used to trigger MCU to turn on the switch, and the rising-edge is used to close the switch. When the output power of pump LD is 55 mW, the POWSFL lases, and firstly emitted pulse in each sweeping period has the shortest wavelength. The O-E converted electronic pulse train is shown in Fig. 3 (c). Calculating from the 3 V amplitude and the 50 mW background of Fig. 3 (c), a 35 dB SNR can be obtained. The detector used in the experiment has a responsivity of 4 × 104 V/W at 1550 nm, so the amplitude 3 V corresponds to the power of the laser pulse 75 μw. Assuming that the reflectivity of FBG is 90%, the FBG reflected Δλ
To experimentally demonstrate the monitoring function, a four-FBG sensor system is presented, as shown in Fig. 1. The wavelengths of four cascaded gratings from left to right are λ 1 = 1557.80 nm, λ2 = 1549.88 nm, λ3 = 1551.94 nm, and λ4 = 1555.119 nm respectively. The 3 dB bandwidth of each FBG is 0.2 nm and its reflectivity is 90%. The fiber length between the circulator and the nearest FBG (G1) is 140.6 cm. From left to right, the spatial interval between the adjacent two sensors is 137.7 cm, 217.3 cm, and 158.7 cm, respectively. The ring-cavity is composed of a 980 nm pump laser diode, a 26.83 m EDF, a 980/1550 WDM coupler, a 16.56 m single mode fiber, a F-P filter with a 3-dB bandwidth of 66 pm and a free spectral range (FSR) of 56.7 nm, a coupler C1with a coupling ratio of 4:1, a circulator and an isolator (IS1), which has an isolation of 42 dB and an insertion loss of 0.14 dB. The isolation of IS2 is 36.5 dB, and the insertion loss is 0.34 dB. The total loss of the cavity is around 48.38 dB, including 22.75 dB absorption of EDF, and a 23 dB fiber-to-fiber loss of the F-P filter. The F-P filter's driving signal is a saw-toothed wave (as shown in Fig. 3 (a)) with a frequency of 1 KHz, a peak to peak voltage of 3.6 V and an offset voltage of 8 V. Limited by PZT-ceramic's operating frequency, the falling edge of (a) becomes gentle and there are some spurs in the falling edge. The spurs can be attributed to the resonance, as the frequency of the driving saw-toothed wave is close to the resonance frequency of the PZT-ceramic [8]. As the ideally steep falling edge of the driving saw-toothed wave is changed to gentle falling, the edge falling time, during which the filter goes through the full sweeping spectrum span, is long enough for
continuously ASE power Pr can be expressed as ΔλFBG × Ptotal , Then ASE with ΔλFBG = 0.2 nm, ΔλASE = 40 nm, and Ptotal = 4 mW, we can get Pr = 0.018 mW. After passing the filter which has a 23 dB loss, the remaining light power PASE is around 0.1 μw. the PASE corresponds to the output power when the laser cavity is disconnected between the C1 and EDF, and it is much lower than the power of laser pulse. That provides the evidence showing lasing. Finally, the switch is used to distribute signals to each corresponding channel, the typical outputs of one channel is shown in Fig. 3 (d). The length difference between arms of USMI is 3.2 mm (L), and the average length of the arms is 106 cm. To stretch the shorter arm, another PZT-ceramic is driven by a 40 Hz saw-toothed signal with Vp-p = 4.8 V and Voff = 5.6 V. To verify the system function, the static strain is applied to G4.The signal corresponding to G4 is shown in Fig. 4. The relationship between the phase-shift and the measured strain is shown in Fig. 5. The measured sensitivity of sensor is ~1.610°/μ ε, which agrees with theoretical value 1.674°/με well by taking Pe = 0.22 and λBi = 1555 nm into account for calculation. The quantity of sensors that the addressing technique can support is determined by the spectrum tuning span of the filter Δλspan and the spectrum interval between adjacent FBGs. Furthermore, the capability is also restricted by the spatial interval between the sensors and the tuning frequency fp, adopting half of the spectrum interval between the FBGs as the limitation. The system has a measurable range of −827 με to 827 με. From Eq. (3) we can know that the resolution of the strain measurement is determined by the length difference between arms of USMI (L = 3.2 mm), and the minimum detectable phase variation. The
Fig. 3. Interrogation related signals. (a)1, F-P filter's driving signal; (b) 2, MCU's trigger signal; (c) 3, output of detector; (d), output of the switch's channel 4.
Fig. 4. Signal corresponded to G4 (a) 1: Driving signal of PZT-ceramic in USMI; (b): outputs of the switch channel corresponded to G4 (c): trace (b) filtered with a 38~42 Hz band-pass filter.
3. Experimental result and discussion
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monitoring function. Cascaded WDM FBG sensor array and the sawtooth wave driving inter-ring F-P filter are used to realize a tunable ring fiber laser. The laser can generate pulse train with the wavelength dependent time-sequence. According to the time-sequence, the O-E converted pulse train is distributed by a 1 × N analog electrical switch, thus a novel real time addressing technique is demonstrated. The demodulation is performed by an unbalanced scanning Michelson interferometer, and a sensitivity of 1.610°/με has been achieved, which agrees well with our theoretic investigation.
Acknowledgement This work was supported by New Century Excellent Talents in University (NCET-04-0828), Ministry of Education, China. Fig. 5. Experimental data and theoretical prediction of phase shift Δϕ4with respect to ε4.
References minimum detectable phase variation is the greater value of the phase detector's resolution and minimum phase variation caused by lasing frequency changing, which is determined by longitudinal mode spacing. For our system, the minimum phase variation caused by lasing frequency is 0.04° and the phase detector's resolution is 0.01°. Thus the resolution of the strain measurement is 22 nε. 4. Conclusion We have proposed and experimentally demonstrated an active time-domain addressing architecture with high SNR and real-time
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