ISSN 1054660X, Laser Physics, 2011, Vol. 21, No. 1, pp. 188–190. © Pleiades Publishing, Ltd., 2011.
Original Text © Astro, Ltd., 2011.
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1 1. INTRODUCTION
A longhaul or largescale fiber sensor system gen erally is composed of a fiber Bragg grating (FBG) based sensing system and a control center (CC) [1, 2]. The CC is responsible for providing the light source and discriminating among the backreflected sensing signals from the sensor system. The light source for FBG sensing applications is normally a broadband amplifiedspontaneousemission (ASE) source [3, 4]. However, when the ASE source is used, the optical power reflected from the FBGs is weak. The broad band ASE therefore limits the resolution and the capacity of a fiber sensor system.
Since fiber lasers are easily implemented, have a high output power and can be incorporated into a fiber system [5–16], they are excellent candidate laser sources for fiberoptic sensing applications. This work develops a novel fiber ring laser with signalaveraging functionality as a sensing CC for longhaul FBG sen sor systems. A signalaveraging module is introduced into the fiber laser to reduce the impact of noise on the system performance, increasing the SNR of the sys tem. The following section will experimentally dem onstrate the operation of the CC based on the pro posed fiber ring laser.
2. EXPERIMENTAL SETUP AND RESULTS As presented in Fig. 1, a sample longhaul FBG sensor array is connected to the proposed fiber ring laser to demonstrate experimentally the feasibility and effectiveness of the latter as a sensing CC. The sample passive fiber sensor array consists of a 30/60 km single mode fiber (SMF) and three FBG sensing elements. 1The article is published in the original.
The peak reflectivity of each FBG is around 94%, and the Bragg wavelengths of the FBGs λi (i = 1, 2, 3) are 1548.26, 1551.22, and 1554.62 nm, respectively. As shown in this figure, moreover, the fiber ring laser is constructed from a 2 × 2 30/70 coupler (C), an erbiumdoped fiber amplifier (EDFA), a tunable Fabry–Pérot filter (FPF), a photodetector (PD), a sampler, an analogtodigital converter (A/D) and a signal processor. Its lasing wavelength is determined by the tunable FPF. Such a fiber laser incorporates signal averaging functionality inside the CC, markedly improving the SNR performance of the sample longhaul FBG sensor array, as will be verified in the following.
Seventy percent of any lasing light is fed into the sensing FBG chain via the 2 × 2 fiber coupler to inter rogate the deployed FBG sensors. Figure 2 displays the output spectra at the 70% lasing port when the FPF is tuned with a voltage controller. To tune the voltage to within the above operating range, a lasing wavelength in the waveband 1523.78–1576.35 nm is chosen. The lasing peak power from this 70% output port is about 10.76 dBm. Figure 3 shows the stability of the laser with a central wavelength of 1552.48 nm, which is measured at the 70% lasing port within 15 min. From the measured power curve, the variation in the output laser power is (Pmax – Pmin)/ = 0.58%, where Pmax,
Pmin, and represent the maximum, minimum and average output laser powers, respectively.
The average wavelength variation is 0.04 nm. The stability of a longhaul fiber sensor system that is based on the proposed fiber laser scheme as its CC depends on the above two metrics and the adopted electronic signalprocessing method.
Pout
Pout
FIBER OPTICS
Tunable Erbiumdoped Fiber Ring Laser with SignalAveraging
Function for FiberOptic Sensing Applications
1S. T. Kuoa, P. C. Pengb, *, M. S. Kaoa, H. H. Lub, and J. F. Chenb
a Department of Electrical Engineering, National ChiaoTung University, Hsinchu, Taiwan, R.O.C. b Department of ElectroOptical Engineering, National Taipei University of Technology, Taipei, Taiwan, R.O.C.
*email: [email protected]
Received June 22, 2010; in final form, June 29, 2010; published online October 25, 2010
Abstract—This work proposes a novel erbiumdoped fiber ring laser as a sensing control center (CC) for
longhaul fiber Bragg grating (FBG) sensor systems. The tunable erbiumdoped fiber laser with signal aver aging functionality not only provides intense and stable laser light, but also suppresses the effect of noise on the system performance. A sample 30/60 km FBG sensor array is connected to the fiber ring laser to demon strate experimentally the feasibility and effectiveness of the laser as a CC. The experimental results indicate that the signal averaging operation inside the proposed setup increases the system signaltonoise ratio (SNR).
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TUNABLE ERBIUMDOPED FIBER RING LASER 189
The interrogation of the deployed FBG sensors depends on the scanning FPF operation. The back reflected sensing signals from FBGs λi (i = 1, 2, 3) are successively fed into the PD and converted to electri cal signals. Next, they are sampled, digitized and buff ered in that order. After the above process has been carried out 256 times, signalaveraging is performed by the signal processor and the average SNR for each FBG sensor is thus calculated. Figures 4a and 5a plot the output signals obtained without signalaveraging processing when 30 and 60 km FBG sensor arrays, respectively, are attached to the proposed fiber ring laser. The SNR of the system declines as the distance between the fiber laser scheme and the FBG sensors
increases. The signals that have undergone signal averaging processing, displayed in Figs. 4b and 5b, are not as rough as the output signals in Figs. 4a and 5a. Accordingly, the signalaveraging operation increases the SNR of each remote FBG sensor.
3. CONCLUSIONS
This work presents a tunable erbiumdoped fiber ring laser with signalaveraging functionality for long haul fiberoptic sensing applications. It significantly reduces the influence of noise on system performance. The operation and effectiveness of the proposed fiber laser as a sensing CC were experimentally demon Signalaveraging module Signal processor A/D Sampler PD 70% C 30% FPF Scanning waveform CC EDFA 30/60 km SMF FBG1 FBG2 λ1 λ2 λN λN – 1 λ3 FBGN FBGN – 1 FBG3
Fig. 1. Experimental setup for demonstrating feasibility of proposed fiber ring laser as a sensing CC. (C: 2 × 2 fiber coupler, EDFA:
erbiumdoped fiber amplifier, FPF: Fabry–Pérot filter, PD: photodetector, A/D: analogtodigital converter, SMF: singlemode fiber, FBG: fiber Bragg grating).
20 0 −20 −40 −60 1520 1530 1540 1550 1560 1570 1580 Wavelength, nm Power, dBm
Fig. 2. Output spectra from 70% lasing port within range
from 1523.78 to 1576.35 nm. 16 12 8 4 0 5 10 15 1560 1558 1556 1554 1552 1550 Time, min Wavelength, nm Power, dBm
Fig. 3. Laser stability in terms of output power variation
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KUO et al.
strated and characterized. The experimental results reveal that the signalaveraging operation inside the presented scheme increases the SNR of the system.
ACKNOWLEDGMENTS
This work was supported by the National Science Council of the Republic of China, Taiwan, under Contract NSC 982221E027007MY3 and NSC 982622E027038CC3.
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λ1 λ2 (a) λ3
λ1 λ2 (b) λ3
Voltage (1 mV/div)
Time (400 μs/div)
Fig. 5. Output signals (a) without and (b) with signalaver
aging processing when a 60 km FBG sensor array is attached to the proposed fiber laser.
λ1 λ2 λ3 λ1 λ2 λ3 Time (400 μs/div) Voltage (20 mV/div) (a) (b)
Fig. 4. Output signals (a) without and (b) with signalaver
aging processing when a 30 km FBG sensor array is attached to the proposed fiber laser.
