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We studied the impact of optical pulse coding techniques on Brillouin optical time-domain analysis (BOTDA) systems experimentally. The results show that complementary-coding-based schemes can provide significant enhanced signal-to-noise ratio (SNR) to the BOTDA system. Using complementary coding, the average time can be reduced significantly and so does the measurement time. The complementary-coded pulses with individual pulse duration of 0.1 ns allowed us to realize temperature sensing over 50 m of single-mode fiber with 1-cm spatial resolution.

Brillouin Optical Time Domain Analysis (BOTDA) is becoming a consolidated technique in applications requiring highresolution monitoring over extremely long distances. Extension of the measuring range has therefore become one of the main areas of research around BOTDA technology. To increase the sensing range, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. This has been achieved so far by applying techniques like pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that determine which is the best configuration to overcome current range limitations., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

Aiming at taking full advantage of bipolar codes, a method using a three-tone probe is proposed to alleviate the probe power limitation imposed by pump depletion in Golay-coded Brillouin distributed fiber sensors. Experimental results validate the technique, which reduces significantly the measurement distortions induced by the gain/loss unbalance resulting from pump depletion/amplification. The method supports a probe power increment of more than 12.5 dB, resulting in low-uncertainty measurements (< 0.9 MHz) at a real 100 km distance, using a 200 km-long fiber loop and 2 m spatial resolution. The method is evaluated with a record figure-of-merit of 380’000.

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results., European Commission, Ministerio de Economía y Competitividad, Universidad de Alcalá, Ministerio de Ciencia e Innovación, Comunidad de Madrid

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., Ministerio de Ciencia e Innovación, Comunidad de Madrid, European Commission

22nd International Conference on Optical Fiber Sensors (OFS2012), Beijing, China, October 14, 2012., In this paper we combine the use of optical pulse coding and seeded second-order Raman amplification to extend the sensing distance of Brillouin optical time-domain analysis (BOTDA) sensors. Using 255-bit Simplex coding, the power levels of the Raman pumps and the Brillouin pump and probe signals were adjusted in order to extend the real physical sensing distance of a BOTDA sensor up to 120 km away from the sensor interrogation unit, employing a 240-km long loop of standard single-mode fiber (SSMF) with no repeater. To the best of our knowledge, this is the first time that distributed measurements are carried out over such a long distance with no active device inserted into the entire sensing loop, constituting a considerable breakthrough in the field., Ministerio de Ciencia e Innovación, Comunidad de Madrid, European Commission, Swiss Commission for Technology and Innovation