In this Letter, we propose the use of optical pulse coding techniques for long-range distributed sensors based on Brillouin optical time-domain analysis (BOTDA). Compared to conventional BOTDA sensors, optical coding provides a significant sensing-range enhancement, allowing for temperature and strain measurements with 1 m spatial resolution over 50 km of standard single-mode fiber, with an accuracy of 2.2°C/44 , respectively. © 2010 Optical Society of America OCIS codes: 060.2370, 060.4370, 280.4788, 290.5900. In recent years, distributed optical fiber sensors based on stimulated Brillouin scattering (SBS) have attracted a great interest owing to their unique ability to carry out high-performance strain and temperature measurements over long distances [1,2]. In the time-domain approach, the so-called Brillouin optical time-domain analysis (BOTDA) [1-3], a pulsed pump beam and a counterpropagating continuouswave (CW) probe beam, at different frequencies, interact through the intercession of an acoustic wave. Power transfer between both optical beams takes place at any position along the fiber when the frequency offset between them is within the local Brillouin gain spectrum (BGS). The frequency showing maximum gain is called Brillouin frequency shift (BFS) and depends linearly on strain and temperature, allowing us to perform distributed sensing [1]. The best performance reported so far for long-range BOTDA sensors results in 2 m/5 m spatial resolution over 40 km/51 km single-mode fiber [2,3], where pump depletion and modulation instability are the main factors limiting the sensing range [1,4]. In this Letter, we propose, for what we believe to be the first time, the use of optical pulse coding in a BOTDA sensor. We demonstrate that this technique effectively enhances the signal dynamic range, resulting in an extension of the sensing distance in BOTDA-based systems and providing the best performance reported so far, to our knowledge, temperature and strain sensing with 1 m spatial resolution over 50 km of standard single-mode fiber with an accuracy of 2.2°C/44 at the far end of the fiber.In BOTDA sensors, the BGS is reconstructed along the fiber by sweeping the frequency offset ͑⌬͒ between the two counterpropagating optical signals around the BFS. Thus, intensity variations of the probe signal ⌬I CW are measured at the near end of the fiber ͑z =0͒ as a function of time t and ⌬ and can be expressed as [1] ͑1͒where I CWL is the input probe intensity at the far end of the fiber ͑z = L͒, ␣ is the fiber loss, L is the fiber length, v g is the group velocity, ⌬z is the spatial resolution related to the pump pulse duration, and g B ͑ , ⌬͒ and I P ͑ , ⌬͒ are the BGS and the pump intensity at position z = .From Eq. (1) we clearly notice that the CWintensity contrast ͑⌬I CW ͒ mainly depends on the spatial resolution and the pump intensity. Thus, when short spatial resolution is required, the energy transferred to the probe is actually small, reducing ⌬I CW . This feature leads to measurements with low signalto-noise r...

In this paper we perform an optimization of Brillouin optical time-domain analysis (BOTDA) sensors for achieving high resolution over long sensing ranges using distributed Raman amplification. By employing an optimized first-order bi-directional Raman amplification scheme and combining high-power fiber-Raman lasers and Fabry-Pérot lasers with low relative-intensity-noise (RIN), we demonstrate distributed sensing over 120 km of standard single-mode fiber with 2 meter spatial resolution and with a strain/temperature accuracy of 45με/2.1°C respectively. ©2011 Optical Society of America

Abstract:A theoretical and experimental analysis of the impact of pulse modulation format on Brillouin optical time-domain analysis (BOTDA) sensors using pulse coding techniques has been carried out. Pulse coding with conventional non-return-to-zero (NRZ) modulation format is shown to induce significant distortions in the measured Brillouin gain spectrum (BGS), especially in proximity of abrupt changes in the fiber gain spectra. Such an effect, as confirmed by the theoretical analysis, is due to acoustic wave pre-excitation and non-uniform gain which depends on the bit patterns defined by the different codewords. A successful use of pulse coding techniques then requires to suitably optimize the employed modulation format in order to avoid spurious oscillations causing severe penalties in the attained accuracy. Coding technique with return-to-zero (RZ) modulation format is analyzed under different duty-cycle conditions for a 25 km-long sensing scheme, showing that low duty-cycle values are able to effectively suppress the induced distortions in the BGS and allow for spatially-accurate, high-resolution strain and temperature measurements being able to fully exploit the provided coding gain (~7.2 dB along 25 km distance) with unaltered spatial resolution (1 meter). Although Simplex coding is used in our analysis, the validity of the results is general and can be directly applied to any intensity-modulation coding scheme. ©2010 Optical Society of America

Abstract:A theoretical and experimental analysis of optical pulse coding techniques applied to distributed optical fiber temperature sensors based on spontaneous Brillouin scattering using the Landau-Placzek ratio (LPR) scheme is presented, quantifying in particular the impact of Simplex coding on stimulated Brillouin and Raman power thresholds. The signal-to-noise ratio (SNR) enhancement and temperature resolution improvement provided by coding are also characterized. Experimental results confirm that, differently from Raman-based sensors, pulse coding affects the stimulated Brillouin threshold, resulting in lower optimal input power levels; these features allow one to achieve high sensing performance avoiding the use of high peak power pulses.

In this Letter, we combine the use of optical preamplification at the receiver and optical pulse coding techniques with an optimized modulation format to effectively extend the sensing range of Brillouin optical time-domain analysis (BOTDA) sensors. Combining a return-to-zero modulation format with 25% duty cycle and linear gain preamplification allows for temperature and strain measurements over 120 km of standard single-mode fiber with 3 m spatial resolution and an rms strain-temperature accuracy of 3:1°C=60 με respectively. In such a scheme, two counterpropagating optical waves interact with an acoustic wave in an optical fiber, leading to power transfer between the two optical signals. The most commonly used method (the so-called Brillouin-gain configuration [2]) employs a cw probe signal that is down-shifted in frequency with respect to an optical pulsed pump. Thus, when tuning the frequency shift between these two optical beams within the fiber Brillouin gain spectrum (BGS) range, the cw probe wave results are locally amplified by stimulated Brillouin scattering (SBS). Since the maximum amplification occurs at the fiber Brillouin frequency shift (BFS) value, which is linearly dependent on local strain and temperature, such two physical quantities can be measured along the sensing fiber as a function of the distance. The spatial information is obtained by a time-domain analysis, allowing for a spatial resolution that is proportional to the pump pulse width. The main limitations in long-range BOTDA sensors arise from the maximum allowed input pump power before onset of fiber nonlinear effects inducing measured BGS distortions, which finally lead to temperature and strain measurement errors [1,2]. Recently, the use of optical pulse coding techniques has been proposed for BOTDA sensors with meter-scale spatial resolution [2], allowing for an improved signal-to-noise ratio (SNR) and an extended sensing range, without incurring nonlinear effects.In this Letter, we propose the use of optical preamplification at the receiver combined with an optimized simplex pulse coding configuration employing return-to-zero (RZ) modulation format to further enhance the dynamic range of BOTDA sensors with meter-scale spatial resolution. We demonstrate, for the first time to our knowledge, a BOTDA sensor with an enhanced dynamic range of ∼27 dB, achieving 120 km sensing distance, with 3 m spatial resolution and a temperature/strain resolution of 3:1°C=60 με at the far fiber-end.In BOTDA sensors, the BFS parameter is obtained by fitting the measured BGS, which is determined by sweeping the frequency difference of the two interacting optical waves and detecting the SBS-induced intensity variations of the cw probe signal ΔI CW at fiber input (z ¼ 0). where I CWL is the input probe intensity at the far end of the fiber (z ¼ L), α is the fiber loss, L is the fiber length, v g is the group velocity, Δ z is the pulse interaction length determining the spatial resolution, and g B ðξ; ΔνÞ and I P ðξ; ΔνÞ are the BGS and the pump intensity...