Marcelo A. Soto

Modeling and evaluating the performance of Brillouin distributed optical fiber sensors

A thorough analysis of the key factors impacting on the performance of Brillouin distributed optical fiber sensors is presented. An analytical expression is derived to estimate the error on the determination of the Brillouin peak gain frequency, based for the first time on real experimental conditions. This expression is experimentally validated, and describes how this frequency uncertainty depends on measurement parameters, such as Brillouin gain linewidth, frequency scanning step and signal-to-noise ratio. Based on the model leading to this expression and considering the limitations imposed by nonlinear effects and pump depletion, a figure-of-merit is proposed to fairly compare the performance of Brillouin distributed sensing systems. This figure-of-merit offers to the research community and to potential users the possibility to evaluate with an objective metric the real performance gain resulting from any proposed configuration.

Simplex-coded BOTDA fiber sensor with 1 m spatial resolution over a 50 km range

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 degrees C/44 muepsilon, respectively.

Optical sinc-shaped Nyquist pulses of exceptional quality

Sinc-shaped Nyquist pulses possess a rectangular spectrum, enabling data to be encoded in a minimum spectral bandwidth and satisfying by essence the Nyquist criterion of zero inter-symbol interference (ISI). This property makes them very attractive for communication systems since data transmission rates can be maximized while the bandwidth usage is minimized. However, most of the pulse-shaping methods reported so far have remained rather complex and none has led to ideal sinc pulses. Here a method to produce sinc-shaped Nyquist pulses of very high quality is proposed based on the direct synthesis of a rectangular-shaped and phase-locked frequency comb. The method is highly flexible and can be easily integrated in communication systems, potentially offering a substantial increase in data transmission rates. Further, the high quality and wide tunability of the reported sinc-shaped pulses can also bring benefits to many other fields, such as microwave photonics, light storage and all-optical sampling.

Analysis of distributed temperature sensing based on Raman scattering using OTDR coding and discrete Raman amplification

The behaviour of distributed temperature sensors based on spontaneous Raman scattering and coded OTDR (optical time domain reflectometry) is studied both theoretically and experimentally; in particular a high performance scheme has been implemented using amplitude modulation according to Simplex coding, direct detection and additional use of lumped Raman amplification to further extend the sensing range. An efficient and cost-effective distributed temperature sensing system operating along 30 km of dispersion-shifted fibre with 17 m spatial resolution and 5 K temperature resolution is theoretically demonstrated and experimentally achieved using 255 bit Simplex coding and low-power commercially available laser diodes (80 mW CW power). Use of lumped Raman amplification to produce high-power coded pulses allows further 10 km distance enhancement, resulting in a total measurement range of 40 km.

Analysis of pulse modulation format in coded BOTDA sensors

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 (approximately 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.

Extending the Real Remoteness of Long-Range Brillouin Optical Time-Domain Fiber Analyzers

The real remoteness of a distributed optical fiber sensor based on Brillouin optical time-domain analysis is considerably extended in this paper using seeded second-order Raman amplification and optical pulse coding. The presented analysis and the experimental results demonstrate that a proper optimization of both methods combined with a well-equalized two-sideband probe wave provide a suitable solution to enhance the signal-to-noise ratio of the measurements when an ultra-long sensing fiber is used. In particular, the implemented system is based on an extended optical fiber length, in which half of the fiber is used for sensing purposes, and the other half is used to carry the optical signals to the most distant sensing point, providing also a long fiber for distributed Raman amplification. Power levels of all signals launched into the fiber are properly optimized in order to avoid nonlinear effects, pump depletion, and especially any power imbalance between the two sidebands of the probe wave. This last issue turns out to be extremely important in ultra-long Brillouin sensing to provide strong robustness of the system against pump depletion. This way, by employing a 240 km-long optical fiber-loop, sensing from the interrogation unit up to a 120 km remote position (i.e., corresponding to the real sensing distance away from the sensor unit) is experimentally demonstrated with a spatial resolution of 5 m. Furthermore, this implementation requires no powered element in the whole 240 km fiber loop, providing considerable advantages in situations where the sensing cable crosses large unmanned areas.

Optimization of long-range BOTDA sensors with high resolution using first-order bi-directional Raman amplification

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.

Long-range simplex-coded BOTDA sensor over 120km distance employing optical preamplification

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.

Analysis of optical pulse coding in spontaneous Brillouin-based distributed temperature sensors

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.

Intensifying the response of distributed optical fibre sensors using 2D and 3D image restoration

Distributed optical fibre sensors possess the unique capability of measuring the spatial and temporal map of environmental quantities that can be of great interest for several field applications. Although existing methods for performance enhancement have enabled important progresses in the field, they do not take full advantage of all information present in the measured data, still giving room for substantial improvement over the state-of-the-art. Here we propose and experimentally demonstrate an approach for performance enhancement that exploits the high level of similitude and redundancy contained on the multidimensional information measured by distributed fibre sensors. Exploiting conventional image and video processing, an unprecedented boost in signal-to-noise ratio and measurement contrast is experimentally demonstrated. The method can be applied to any white-noise-limited distributed fibre sensor and can remarkably provide a 100-fold improvement in the sensor performance with no hardware modification.