Distributed fiber temperature and strain sensor usingcoherent radio-frequency detection of spontaneous

Brillouin scattering

Jihong Geng,* Sean Staines, Mike Blake, and Shibin JiangNP Photonics, Inc., 9030 South Rita Road, Tucson, Arizona 85747, USA

*Corresponding author: geng@npphotonics.com

Received 7 March 2007; accepted 22 May 2007;posted 12 June 2007 (Doc. ID 80689); published 9 August 2007

A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency ��500 MHz� region with excellent long-term stability has been demonstrated for distributedmeasurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouinfiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shiftedlocal oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scatteringwith high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distrib-uted measurements of both temperature and strain over long fiber has been developed utilizing advanceddigital signal processing techniques. © 2007 Optical Society of America

OCIS codes: 060.2370, 120.0280.

1. Introduction

Sensor technology is one of the most important ele-ments in the country’s ongoing efforts in homelandand national security. Various conventional technol-ogies that make use of seismic, acoustic, infrared, andmagnetic sensors have been developed for securityapplication. Optical fiber sensor technology that usesoptical fiber as the sensor element, as one of the mostpromising technologies, offers a number of advan-tages over those conventional technologies. For ex-ample, optical fiber sensors are lightweight, compact,easily multiplexable, safe in hazardous environ-ments, capable of distributed sensing, and immune toEMI (electromagnetic interference) radiation, andthey resist chemical corrosion, require no electricalpower at the sensing point, and in most cases havethe potential to be produced at low cost.

Brillouin-based distributed fiber sensors have beenattracting intense interest for one and a half decades[1,2] due to their potential use for monitoring tem-perature and strain in oil pipe lines, power cables,large-scale structures, and catastrophic land slip-

page. Brillouin-based sensor technology has twodifferent regimes: the stimulated Brillouin-basedtechnique (i.e., Brillouin optical time domain analy-sis, or BOTDA) [3] and the spontaneous Brillouin-based technique (i.e., Brillouin optical time domainreflectometry, or BOTDR) [4]. Both regimes employlinear dependence of the Brillouin frequency shift(and Brillouin power as well) on temperature and�orstrain.

The spontaneous Brillouin-based BOTDR techniqueis more attractive because, unlike the counterpropa-gating approach in the stimulated Brillouin-basedBOTDA technique, it offers a single-end launch ap-proach as well as the potential capabilities of simul-taneous temperature and strain sensing if both theBrillouin frequency shift and the Brillouin backscat-tering power are determined simultaneously [4]. Inthe BOTDR technique, the spontaneous Brillouinscattering (SBS) signal can be measured by eitherdirect detection or coherent detection. Coherent de-tection is more favorable because it offers muchhigher sensitivity and dynamic range than direct de-tection. However, the difficulty in coherent detectionis that the frequency of the SBS signal is downshiftedby approximately 11 GHz from that of a 1.55 �mpump light. If a local oscillator (LO) in the coherent

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detection is the pump laser itself, the coherent beatsignal between the SBS and the LO is around11 GHz, which lies out of the bandwidth of most com-monly used conventional heterodyne receivers. Also,the 11 GHz beat signal must be detected and ana-lyzed by using expensive microwave technology.

There have been some efforts trying optically tofrequency shift a LO in order to set the SBS�LO beatfrequency within the bandwidth of a conventionalheterodyne receiver. These efforts include the use ofa mode-locked Brillouin fiber laser [5] and a high-speed electro-optic (EO) phase modulator [6]. Al-though the mode-locked Brillouin fiber laser providesa well-defined frequency-shifted light source with ex-cellent long-term frequency stability, the laser pulsewidth cannot be flexibly changed. Furthermore, therepetition time of the mode-locked pulses �3–11 �s� issmaller than the roundtrip time of light in sensingfiber of longer than 1 km. As a result, multiple pulseswould be launched into the sensing fiber if no selec-tive gate were used, which could result in cross talk ofBrillouin scattering signal of multiple pulses. The useof a high-speed EO phase modulator provides a verysimple alternative to generate a frequency-shiftedLO, but its performances are dramatically dependenton the performances of a microwave generator or syn-thesizer that drives the phase modulator. Also, high-order sideband components make the signal analysismore complex because of signal interferences be-tween those high-order components and Rayleigh�Brillouin backscattering light. The limited conversionefficiency of EO modulators from carrier to sidebandscan also significantly limit system sensitivity in thecoherent detection.

In this paper, we propose a novel configuration thatenables coherent detection of SBS in fiber in the radiofrequency (RF) region by using an actively stabilizedlow-noise CW single-frequency Brillouin fiber laseras a frequency-shifted LO. Although CW single-frequency Brillouin fiber lasers have been demon-strated for a long time [7,8], this is, to our knowledge,the first demonstration to implement a stable low-noise CW Brillouin fiber laser in a distributed fibertemperature and strain sensor. Recently, we havereported a highly stable CW Brillouin fiber laser withextremely narrow spectral linewidth ��100 Hz� andlow intensity noise and phase noise [9,10]. Here, wedemonstrate that the CW single-frequency Brillouinfiber laser is an ideal low-noise LO with well-definedfrequency shift and long-term frequency stability,which is an essential requirement for distributedBrillouin fiber sensor applications, for coherent de-tection of SBS in long sensing fiber.

2. Principle

A schematic diagram of our proposed system is illus-trated in Fig. 1. A single-frequency piezo-electrically-tuned Er-doped fiber laser at 1551 nm (Scorpion, NPPhotonics) with a maximum output power up to150 mW and a spectral linewidth of less than 10 kHzis used as a light source in the sensor system. Part ofthe laser beam is amplitude modulated by a modula-

tor to generate laser pulses before launching into longsensing fiber. Another part of the beam is split topump a CW Brillouin fiber ring laser, which is usedas a LO for coherent detection. Details of the Bril-louin fiber laser have been given elsewhere [9]. TheSBS of the pulsed light in sensing fiber, which carrieslocal information of temperature and strain along thesensing fiber, can be detected by coherent heterodynedetection.

Figure 2 shows a spectral illustration for our sys-tem configuration. Since both the LO (i.e., the low-noise CW Brillouin fiber laser) and the SBS insensing fiber are generated from a single fiber laser,there are two major advantages in this scheme. First,the LO frequency is naturally shifted to be very closeto that of the Stokes component of the SBS in thesensing fiber. This allows us to bring the coherentbeat signal, which contains distributed information oftemperature and strain, from expensive microwave�11 GHz� to the RF ��500 MHz� region. This offersthe capability of developing a low-cost real-time dis-tributed fiber temperature and strain sensor in longsensing fiber by using low-cost advanced digital tech-nology instead of expensive microwave technology.Second, the frequency difference between the LO andthe SBS in the sensing fiber is inherently stable, evenin the case that there exists slow frequency drift inthe Er-doped fiber laser. This provides excellentlong-term stability of frequency measurement in ourBrillouin distributed temperature and strain fibersensor.

The key component in the proposed Brillouin sen-sor system is the LO, i.e., the CW single-frequencyBrillouin fiber laser. Just like conventional coherentlidar application, the following two noise performancesof a LO are essential for the coherent Brillouindetection, i.e., low phase noise (or narrow spectrallinewidth) and low intensity noise. Besides the above-

Fig. 1. Diagram of the novel distributed fiber temperature andstrain sensor.

Fig. 2. Spectral illustration for LO (local oscillator), launchedpulsed laser, and SBS (spontaneous Brillouin scattering) in fiber.

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mentioned advantages of our heterodyne approach,the CW single-frequency Brillouin fiber laser itself isan ideal LO as well for coherent detection, in terms ofnoise performance only. Our experiment has showedthat the Brillouin fiber laser has an extremely narrowlinewidth (as narrow as �100 Hz) [9]. Furthermore,the Brillouin fiber laser also exhibits extremely lowintensity noise due to the intensity noise reductioneffect [10]. The experiments have indicated that theBrillouin laser’s intensity noise approaches the shot-noise limit at the RF region ��50 MHz� [9], where thebeat signal is detected in the proposed system byphoto mixing the LO with the SBS in the sensingfiber. The use of the shot-noise-limited LO will benefitnot only enhancing the signal-to-noise ratio (SNR) ofthe system for Brillouin frequency measurement, butalso improving accuracy for the power measurementof SBS that is required for performing simultaneousmeasurements of temperature and strain.

Practically, the SBS�LO beat frequency can be de-signed to be within the preferred frequency region�50 MHz � beat � 500 MHz� by using a specific typeof fiber (with different Brillouin frequency shift fromthat of the sensing fiber) to build the Brillouin ringcavity and by adjusting the temperature of the Bril-louin fiber ring cavity. As we did before [9], the Bril-louin fiber ring cavity is integrated in a temperatureprecisely-controlled package. Once a specific type offiber is selected for building the Brillouin ring cavity,the Brillouin frequency shift of the packaged Bril-louin fiber laser with respect to the Er-doped fiberlaser is dependent only on the temperature of the ringcavity, which can be considered to be a constant whenthe temperature is well controlled within ���0.01 °C.Thus, the measured RF beat frequency between theLO and the SBS is varied only with the Brillouinfrequency shift of the SBS in sensing fiber, whichcontains local information about temperature and�orstrain.

3. Results

The proposed Brillouin sensor system has been ex-perimentally demonstrated. Figure 3 shows a typicalfast Fourier transform (FFT) spectrum of the SBS�LO beat signal in fiber for 20 ns and 100 ns laserpulses, which is measured by a digital oscilloscopewith 5 averages. It is actually the spontaneous Bril-louin gain spectrum of the fiber. The bandwidth ismeasured to be about 20 MHz for 100 ns pulses and50 MHz for 20 ns pulses. Obviously, the spectralbandwidth for the 100 ns pulses corresponds to theBrillouin gain bandwidth of the sensing fiber becausethe pulse width is longer than the lifetime of acousticwave in fiber. But for the 20 ns pulses, the spectralbandwidth is broadened and the SNR is reduced be-cause the launched pulses are shorter than the life-time of acoustic wave in fiber. This feature revealsthe fact that better spatial resolution (with shorterpulses) of the sensor system has to suffer from thedegraded SNR, which further results in degradedmeasurement resolution and accuracy for tempera-ture and strain.

The SBS�LO beat frequency can be rapidly ex-tracted after we integrate the sensor system withadvanced digital technology, such as digital signalprocessing (DSP) and field-programmable gate array(FPGA) technology, which allows us to develop a real-time distributed fiber sensor.

Figure 4 shows the relative Brillouin frequencyshift along two fiber spools that are connected to-gether with FC�APC connector. The first spool thatwas 250 meter double-cladding (DC) single-mode fi-ber was placed in the lab at room temperature (RT),while the second spool with Corning SMF-28 fiberwas heated by an oven at temperatures of 32 °C,40 °C, and 48 °C on three different days, respectively.In these measurements, the launched laser was100 ns pulse and peak power of less than 100 mW,and the data were averaged 5000 times in less than 1second. The frequency resolution in the measure-ments with 100 ns pulses shows to be about 2 MHz;it can be better ��0.5 MHz� if more averages are

Fig. 3. Typical FFT (fast Fourier transform) spectrum of the Bril-louin�LO beat signal for launched laser pulses of 100 ns (uppertrace) and 20 ns (lower trace). The data were averaged 5 times.

Fig. 4. Relative Brillouin frequency shift (5000 averages taken in�1 second) for different fibers at different temperatures. The firstsection of double-cladding (DC) fiber was at room temperature(RT). The second section of SMF-28 fiber was placed in an oven.Three oven temperatures (32 °C, 40 °C, and 48 °C) were set up inthree different days.

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taken (65 k in 4 seconds). The different fiber spoolsexhibit different Brillouin frequency shift. From thefrequency shift measurements for SMF-28 fiber inthe oven, we can derive the temperature sensitivityfor 1.10 MHz�°C, which is in excellent agreementwith that in the literature [11]. The data for the DCfiber in Fig. 4 also exhibit an excellent measurementrepeatability using our system because the frequencydifference between the three measurements, whichwere taken in three different days, was almost neg-ligible for the 2 MHz frequency resolution.

Strain measurement has also been demonstrated.Figure 5 shows the relative Brillouin frequency shiftas a function of distance when some of the fiber wasin tension. The sharp peak at 12.36 km correspondsto the 20 meter fiber section that was under tension.The tension on the fiber was not calibrated; the strainin the figure was estimated using a strain sensitivityof 0.05 MHz��� [11]. It takes only 1 second for 1024averages over 1.5 km sensing fiber. The capability ofrapid data processing enables the sensor system to bepotentially useful for intrusion detection applica-tions. Spatial resolution in this measurement waslimited by the pulse width of 100 ns. Better spatialresolution can be obtained by using shorter laserpulses, but it suffers from degraded frequency reso-lution (or strain resolution) because of the lower SNR.

The system can measure not only the Brillouinfrequency shift but also the Brillouin scattered powerover long sensing fiber. Figure 6 shows the measuredrelative frequency shift and intensity of the Brillouinscattering signal along sensing fiber. The total 30 kmlength was made up of seven sections of fiber fusionspliced together, but maintained at different temper-atures. All of them were Corning single-mode fiber.Except for the section from 10.7 km to 21.2 km thatwas Corning SMF-28 fiber, all others were CorningSMF-28e fiber. Three sections of SMF-28e fiber wereplaced in three ovens, which are zoomed in for detailsin the insets: 200 m fiber starting at 10.55 km in anoven at 38.5 °C, 30 m at 25.65 km in an oven at

38.5 °C, and the final 4.4 km at 25.73 km in an ovenat 39.5 °C. The rest of the sections of fiber remainedat room temperature. The data were taken with 50 nslaunched pulses, which corresponds to 5 m spatialresolution. As demonstrated in Ref. [12], longer sens-ing range (up to 100 km) can easily be obtained by thecoherent detection. Due to fiber attenuation, how-ever, the longest sensing range of our current systemwas limited to 50 km. This limitation resulted fromthe 30 dB dynamic range of the digital data acquisi-tion system used in our current experiment (the ver-tical resolution of our analog-to-digital converter was10 bits). One can also see this limitation even in Fig.6 by the fact that the fluctuation of the measuredBrillouin frequency shift significantly increased atthe far end of the 30 km sensing fiber. We expect toeliminate the limitation after using a variable-gainRF amplifier to compensate fiber attenuation in longfiber.

It has been demonstrated that simultaneous dis-tributed measurements of temperature and strainare possible if both Brillouin frequency shift andpower of the backscattered Brillouin signal in fiberare determined [4]. This fact offers an opportunity ofusing our system for simultaneous distributed mea-surements of both temperature and strain along longfiber.

4. Conclusion

We have demonstrated a novel configuration for co-herent radio-frequency detection of spontaneous Bril-louin scattering for distributed measurements of bothtemperature and strain over long fiber. We use anactively stabilized CW single-frequency Brillouin fi-

Fig. 5. Relative Brillouin frequency shift (1024 averages taken in1 second) as a function of distance. The sharp peak at 12.36 kmcorresponds to the 20 meter fiber section that was under tension.

Fig. 6. Relative Brillouin frequency shift and intensity (in Logscale) of the scattered signal as a function of distance. Three sec-tions of fiber in ovens are zoomed in as shown in two insets: 200 mstarting at 10.55 km in an oven at 38.5 °C, 30 m at 25.65 km in anoven 38.5 °C, and 4.4 km at 25.73 km in an oven at 39.5 °C. Therest of the sections of fiber are at room temperature �24 °C�.

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ber laser as a well-defined, low-noise, frequency-shifted local oscillator for the coherent heterodynedetection. This technique offers an opportunity to useour system for simultaneous distributed measure-ments of both temperature and strain along long fiberwith excellent long-term stability and repeatability.

The authors acknowledge Dave Waymont of Way-mont Consulting Limited, UK, and X. Y. Bao of Uni-versity of Ottawa, Canada, for their fruitful supportand discussions.

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