Time/frequency coding for Brillouin distributed sensors

Abstract

In this paper, we propose a novel coding for long range Brillouin Optical Time Analysis (BOTDA) distributed sensors based on a combination of time and frequency pulses, resulting in an additional coding gain of √2 with respect to traditional intensity-modulated codes. The generation of frequency-chirped pseudo-arbitrary pulses in return-to-zero (RZ) format with a Direct-Digital Synthesizer (DDS) is presented and the coding gain is experimentally verified, perfectly matching its theoretical value.

Increasing robustness of bipolar pulse coding in Brillouin distributed fiber sensors

“…While coding schemes based on incoherent pulse compression [19,20] have efficiently been used in high spatial resolution sensors, demonstrations in long-range BOTDA have been classically based on unipolar sequences, such as Simplex [9] or Golay [11] codes. Two kinds of pulse coding schemes have been recently proposed as a new breach in long range sensing: bipolar codes [12] and time/frequency or colored codes [13][14][15]. Both coding schemes offer improved SNR enhancement in comparison to traditionally-used unipolar coding schemes [9][10][11].…”

“…Besides enhancing BOTDA systems for long range [9][10][11][12][13][14][15], optical pulse coding has also be applied to high spatial resolution Brillouin sensors, based for instance in correlation-domain [19]. While coding schemes based on incoherent pulse compression [19,20] have efficiently been used in high spatial resolution sensors, demonstrations in long-range BOTDA have been classically based on unipolar sequences, such as Simplex [9] or Golay [11] codes.…”

“…On the probe wave side, the power of the CW probe is ultimately limited by the onset of amplified spontaneous Brillouin scattering, which also eventually sets a limit to the SNR on the measurements [5]. To overcome all those limitations to the performance of long-range Brillouin sensors, alternative solutions such as distributed Raman amplification [6][7][8] and optical pulse coding [9][10][11][12][13][14][15] have been proposed, providing a huge improvement in the sensor performance, especially when both techniques are combined in the same system [16,17]. The situation becomes even more critical and demanding in terms of SNR when the real remoteness of the sensor is extended using a linear fiber-loop configuration [17,18], in which half of the fiber is used for sensing while the second half, paired to the sensing segment, is used to convey the probe to the most remote sensing location, i.e.…”

Intensifying Brillouin distributed fibre sensors using image processing

“…The signal-to-noise ratio (SNR) of Brillouin optical timedomain analysers (BOTDA) has been substantially improved using advanced techniques, such as distributed Raman amplification 2 , optical pulse coding 3,4 or other kinds of signal processing 5 , especially when those methods are combined in a single system 6 . Among the different methods proposed in the literature, signal processing techniques, such as optical pulse coding 3,4 and wavelets 5 , have demonstrated to be very efficient tools to remove noise from a unidimensional array of data; and therefore, their use in BOTDA sensing requires processing time-domain traces at each scanned pump-probe frequency offset independently from each other. A 3D map of the Brillouin gain spectrum (BGS) versus distance can thus be obtained with an improved SNR after processing each time-domain trace [3][4][5] .…”

“…Among the different methods proposed in the literature, signal processing techniques, such as optical pulse coding 3,4 and wavelets 5 , have demonstrated to be very efficient tools to remove noise from a unidimensional array of data; and therefore, their use in BOTDA sensing requires processing time-domain traces at each scanned pump-probe frequency offset independently from each other. A 3D map of the Brillouin gain spectrum (BGS) versus distance can thus be obtained with an improved SNR after processing each time-domain trace [3][4][5] . Although methods such as time-frequency codes 4 take advantage of the double scanning (fibre position and pump-probe frequency detuning) required in a BOTDA sensor, the SNR enhancement provided by that method is basically given by the capability of the code to reduce noise in a unidimensional array of data.…”