Simple distributed optical fiber sensor based on Brillouin amplification of microwave photonic signals

The Journal of China Universities of Posts and Telecommunications September 2009, 16(Suppl.): 24–28 www.buptjournal.cn/xben

Simple distributed optical fiber sensor based on Brillouin amplification of microwave photonic signals SUN Xiao-qiang ( ) XU Kun, PEI Yin-qing, FU Song-nian Key Laboratory of Information Photonics and Optical Communications, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China

Abstract

A simple distributed fiber sensor is proposed by Brillouin amplification and depletion of microwave photonic signals. This technique utilizes optical carrier suppression (OCS) modulation to generate two sidebands acting as the pump waves. After Fresnel reflection at the cleaved end of the fiber under test (FUT), the pump waves are converted to probe waves. By sweeping the modulated microwave frequency, the stimulated Brillouin scattering (SBS) spectra and Brillouin frequency shift (BFS) can be precisely characterized. As a result, the accurate measurement of the Brillouin frequency shift makes good determination of the distributed temperature or stain along the FUT, and the position along the FUT can be resolved and controlled by the delay of the microwave pulses. Furthermore, our proposed sensing system is implemented with a single-ended configuration which has shown many advantages such as immunity of the laser frequency fluctuation and compact setup without additional pump laser source and filters. Keywords distributed fiber sensor, stimulated Brillouin scattering, Fresnel reflections

1

Introduction

Distributed fiber sensors based on Brillouin backscattering have attracted tremendous interests in the measurement of distributed temperature or/and strain applied along an optical fiber because BFS has a linear dependence on both temperature and strain [1–7]. Most Brillouin-based sensing technologies can be classified into two types: spontaneous scattering and stimulated scattering. A Brillouin optical time-domain reflectometry (BOTDR) [1,3] belongs to the former, in which one light beam is injected into only one end of an FUT, and such single-ended configuration is highly desirable in remote fiber sensors. However, this technique suffers small amplitude of the signal due to the use of spontaneous Brillouin scattering. Furthermore, the conventional BOTDR system has a trade-off between the spatial resolution and BFS resolution, which usually results in the low spatial resolution in practical applications. In the meantime, Brillouin optical time-domain analysis (BOTDA) [2,4] and Brillouin optical correlation domain analysis (BOCDA) [5–7] are two Received date: 29-06-2009 Corresponding author: SUN Xiao-qiang, E-mail: [email protected] DOI: 10.1016/S1005-8885(08)60363-5

typical analysis systems of the latter, in which two light beams are injected into both ends of the FUT. In this technique, a loop configuration is required for the counterpropagation of the pump and probe waves to induce SBS, which can provide much larger signals than the former technique. As a matter of fact, considering the wide range of application, the fiber sensors with single-ended configuration and large signals are more favorable due to the high flexibility and reliability. In an early work of BOTDA systems [4], an optical pulse and a microwave modulated optical sideband are used to induce the SBS effect and realize a single-ended configuration by locating a reflector at the far end of an FUT. In this paper, we present a very simple BOTDA system with single-ended configuration using Brillouin amplification and depletion of microwave photonic signals. The technique is based on OCS modulation and Fresnel reflection in optical fibers. The first-order sidebands generated by OCS technique are used as the pump signal, and introduced to the FUT. The backward Fresnel reflections at the cleaved end of the FUT act as the probe signal, and experience the amplification and depletion process caused by the upper and lower sideband of the pump signal, respectively. By sweeping the microwave frequency and monitoring the power of the probe signals,

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SUN Xiao-qiang, et al. / Simple distributed optical fiber sensor based on Brillouin amplification of…

BFS can be obtained without additional pump laser source. Through controlling the time delay between the pump and probe signals, the positions along the sensing fiber can be addressed. The proposed single-ended configuration is very compact and stable to obtain the measurement of distributed temperature or strain applied along the optical fiber.

2 Operation principle The schematic diagram of the proposed scheme is shown in Fig. 1. A continuous wave (CW) light from a distributed feedback laser diode (DFB-LD) is injected into a MachZehnder modulator (MZM). The MZM is driven by a pulsegenerated microwave signal from a microwave source controlled by a pulse generator. The MZM is properly biased at the transmission null point to generate OCS signal. Then the generated first-order sidebands are amplified by an Erbium-doped fiber amplifier (EDFA), and introduced to the subsequent fiber under test as the pump, where they interact with the probe generated by the Fresnel reflection. The two sidebands of pump signal stimulate the SBS effect in the FUT, and hence the probe sidebands from Fresnel reflection experience a Brillouin gain and loss simultaneously, as described schematically in Fig. 2. Finally, the back-reflected two sidebands of the probe signal appear at the port 3 of the circulator, and both powers of the two sidebands are detected by a power meter. And the power value is processed by a data acquisition, which is trigged by the pulse generator.

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Fig. 2 presents the Brillouin amplification and depletion process of the back-reflected probe signals. The initial microwave signal is modulated on the MZM, which is biased at the transmission null point. And after OCS modulation, the optical field of the pump wave at the output of the MZM is given by Epump A1 exp j> 2S(v0  f m )t @  A1 exp j> 2S(v0  f m )t @

^

`

^

`

(1) where A-1 and A+1 are the complex amplitudes of the lower and upper sideband (in frequency domain), respectively; Ȟ0 and fm are frequency of the optical carrier and microwave signal, respectively. The two sidebands as the pump signal propagate through the FUT, and generate the Fresnel back-reflection signal at the cleaved end of the FUT acting as the probe signal. By sweeping the microwave frequency, the frequency spacing between the upper pump wave at Ȟ0+fm and the lower probe wave at Ȟ0–fm is 2fm. When the microwave frequency fm is close to the half BFS fB /2, the first lower probe sideband lies in the Brillouin gain spectrum (BGS) generated by the incident upper pump sideband, and is amplified through SBS process. Similarly, the first upper probe sideband lies in the Brillouin loss spectrum (BLS) generated by the incident lower pump sideband, and is depleted through SBS process. Therefore, at the output port 3 of the circulator, due to the amplification and depletion process of SBS effect, the intensity of two probe sidebands at the power meter are given by [8] I 1 I B1 exp ª g B (v) I A1Leff  D L º (2) ¬ ¼ I 1 I B1 exp ª  g B (v) I A1Leff  D L º (3) ¬ ¼

where gB (v) is the Brillouin gain spectrum which has a Lorentzian profile;

I A1

and

I A1

are the pump

intensity of lower and upper sideband at the near end A of the FUT; I B1 and I B1 are the probe intensity of Fig. 1 Schematic diagram of the distributed optical fiber sensor

Fig. 2 Operation principle of the Brillouin amplification and depletion process

lower and upper sideband at the far open end B of the FUT, respectively; Leff [1  exp(D L)]/ D represents usual effective interaction length for nonlinear effects; D is the fiber loss coefficient, and L denotes the total fiber length. Generally, only 4% power of input signal is reflected back from the air-silica interface due to the Fresnel reflection. As a result, the power of the incident pump sidebands is much larger than that of the reflected probe ones in our scheme. Therefore, the amplification and depletion of pump sidebands caused by probe signals is very weak [8]. Considering that the two

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sidebands of pump signal and probe signal initially have the same magnitude, I A1 I A1 I PA , I B1 I B1 ISB , the total detected optical intensity at the power meter turns out to be I out I 1  I 1 2 ISB exp  D L cosh g B (v) I PA Leff (4)



resolution is determined by the narrowest microwave pulse width, which should offer a sufficient gain for a BFS measurement.



As can be seen in Eq. (4), the dependence of the measured output intensity on the modulation frequency fm must fit a hyperbolic cosine of a Lorenzian curve, and the BGS can be deduced from the measured optical power at the power meter. Therefore, we can characterize the BGS by simply sweeping fm around fB/2, and recording the probe intensity at the power meter. The obtained accurate fB can give the details of the distributed temperature or strain applied along the FUT according to the linear relationship between them. During the SBS process, the relevant amplification length corresponds to the traveling length Leff where the pump pulse is present, i.e. Leff vgW m / 2 , Vg is the group velocity of the probe and pump

optical signal in FUT, Wm is the pulse width of the microwave signal. Hence, the overall SBS gain is given by § g B PPAW mvg · § g PA L · ¸ GB exp ¨ B P eff ¸ exp ¨ (5) ¨ Aeff ¸ ¨ 2 Aeff ¸ © ¹ © ¹ where gB§5×10–11 m/W is the effective Brillouin gain coefficient in optical fibers, and Aeff §50 Pm2 is the effective core area. A minimum contrast corresponds to a minimum GB that depends only on the pump power PP and IJm. Since the linewidth of BGS and optical source is very narrow, the resolution of the measurement can be excellent. Moreover, the use of only one laser eliminates the effects of any frequency drift and consequently improves the resolution on the sensing temperature and strain determination. In order to resolve the position in the FUT and realize the distributed sensing of temperature or strain, we utilize a pulse generator to control the microwave source. As shown in Fig. 3, at time T0 and for a short time IJm, a microwave signal at fm is modulated on the MZM to generate a pulse with two first-order sidebands, which is converted to probe signals after Fresnel reflection at the cleaved end. At time T1, the same microwave pulse is launched into the FUT as pump waves, and counter-propagates with the backward Fresnel reflection of the former microwave photonic pulse. The digital pulse delay generator is used to adjust the delay ǻT= T1–T0 between the first and second optical pulse, so they can cross at a definite location. Every position along the sensing fiber, as shown in Fig. 3, can be addressed in this way, which is given by Leff vgW m / 2 . The spatial resolution ǻz is simply given by 'z

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vgW m . For a given pump power, the ultimate spatial

Fig. 3 Schematic diagram of the determined position along FUT and OCS modulation of the sequence microwave signals

3

Experiment and results

The dependence of BFS on temperature or strain has already been extensively studied in Refs. [4–5]. Due to the limitation of the experiment conditions for distributed fiber sensing, we only have an experiment to demonstrate the BGS characterization in our setup, and a continuous microwave signal is used in the experiment. In the experiment, a continuous wave light from a DFB-LD with a linewidth of less than 300 kHz is launched to a MZM (Avanex PowerBit F-10), which is driven by a microwave signal from a microwave signal generator (Anritsu 68347C). The MZM is properly biased at the transmission null point to generate OCS signal with upper and lower sideband. Then the generated first-order sidebands are amplified by an EDFA, and introduced to the subsequent FUT, where they interact with the signal generated by the Fresnel reflection at the cleaved end of FUT. The SBS effect is stimulated, and the lower and upper sideband of the probe signal are amplified and depleted, respectively. Finally, the back-reflected two sidebands of the probe signal appear at the port 3 of the circulator, and both of them are detected by the power meter. Firstly, in order to demonstrate the interaction of the pump and probe sidebands due to the SBS effect, we use an optical spectrum analyzer (ANDO AQ6317B) to measure the optical spectrum of the output probe signal. By setting the microwave frequency at three values close to half BFS, we get the spectra of the probe signal before the power meter, as shown in Fig. 4. When microwave frequency is set at 4.423 GHz or 6.423 GHz,

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SUN Xiao-qiang, et al. / Simple distributed optical fiber sensor based on Brillouin amplification of…

there is no interaction between the pump and probe signal. However, when microwave frequency becomes 5.423 GHz, the upper sideband is depleted and the lower sideband is amplified due to the SBS effect. And the whole optical power of the two sidebands reaches a peak at 5.423 GHz, which can give a determination of BFS. We can also observe that the second-order lower sideband at frequency Ȟ0 – 2fm is amplified by the suppressed optical carrier at Ȟ0 because of their frequency spacing falling within the BGS. However, this induces interference to our measurement can be neglected due to the large carrier suppressed ratio of more than 20 dB.

microwave signal generator with a step of 1 MHz. The resolution of SBS spectral can be further improved in order to obtain more detailed measurement results.

Fig. 5 Normarized optical intensity in a 20-km SMF

Fig. 4 Optical spectra of the backward reflected sidebands with different microwave signals applied on the MZM

Next, we use our proposed approach for characterizing different fibers including a 20 km standard single mode fiber (SSMF) and a 2.7 km highly nonlinear fiber (HNLF). For the SSMF, when the pump power is 1.38 mW, by sweeping the modulation frequency and monitoring the optical power value of the power meter, we observe that the dependence of the measured optical intensity on the modulation frequency fits a hyperbolic cosine of a Lorenzian curve, as shown in Fig. 5, from which we can obtain the BGS according to Eq. (4). By this method, BGS of 20 km SSMF and 2.7 km HNLF is measured at several pump power levels, when the wavelength of DFB-LD is 1 552.84 nm, as shown in Fig. 6. It can be observed that the measured gain spectra agree well with theoretical lorentzian profile. Apart from the detailed characterization of gain spectral profile, the measurements also provide precise information on BFS, which is 10.846 GHz for the 20 km SSMF and 9.262 GHz for the 2.7 km HNLF at room temperature. Furthermore, the 3 dB Brillouin linewidth of SSMF and HNLF are found to be 36 MHz and 44 MHz, respectively. Experimental results verify that the proposed measurement approach can be used to present details on the BGS, which is useful for the sensing of temperature or strain. The above measurements are carried out by sweeping the

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(a) Characterization of SBS loss spectra in a 20 km SSMF

(b) Characterization of SBS loss spectra in a 2.7 km HNLF Fig. 6 Characterization of SBS loss spectra in a 20 km SSMF and a 2.7 km HNLF with respect to the pump power

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We also present a simulation of the spatial resolution and overall SBS gain, as shown in Fig. 7, which can demonstrate the trade-off between the spatial and BFS resolution. For the same pump power, with decreasing of the microwave pulse width, the spatial resolution ǻz can be reduced, while the overall optical gain induced by SBS effect will fall down, resulting in deterioration of the BFS resolution. Ref. [4] presents the details on the relationship between BFS and temperature/strain for the SBS effect induced by optical pulses, which is not demonstrated in our experiment due to the limitation of the experiment conditions.

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system is implemented with a single-ended configuration which has shown many advantages such as immunity of the laser frequency fluctuation and compact setup without additional pump laser source and filters. Acknowledgements This work was supported by the Hi-Tech Research and Development Program of China (2007AA01Z264), the National Natural Science Foundation of China (60702006, 60736002, 60837004), the NCET(0600-93), PCSIRT (IRT0609), MOST International Cooperation Program (2008DFA11670), the 111 Project (B07005), and the project funded by State Key Lab of AOCSN, China.

References

Fig. 7 Trade-off between spatial resolution and the SBS gain dependent on the microwave pulse width

4

Conclusions

We have proposed a very simple BOTDA system for distributed fiber sensor with single-ended configuration using Brillouin amplification and depletion of microwave photonic signals. This technique is based on OCS modulation and Fresnel reflection in optical fibers. The position along the FUT can be resolved and controlled by the delay of the microwave pulses. The accurate measurement of the BFS makes good determination of the distributed temperature or stain along the FUT. Furthermore, our proposed sensing

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