Analysis of pulse modulation format in coded BOTDA sensors

Abstract

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

Rating the performance of a Brillouin distributed fiber sensor

“…Thanks to advanced and ingenious techniques the spatial resolution has been improved by at least 2 orders of magnitude from meter to centimeter range [1][2][3][4][5][6][7] , while recent efforts have been carried out to extend the distance range from some 30 km to 120 km and more by using remote amplification [8][9][10][11] or smart pump pulse coding [12][13][14][15][16] . Among the numerous publications in the field it remains difficult to fairly evaluate the real progresses that are reported without an objective metric based on a rationale.…”

Time/frequency coding for Brillouin distributed sensors

“…The spatial resolution was set to 2 m, corresponding to pump pulse duration of 20 ns (the DDS is able to deliver a distinct frequency at a 4 ns rate), in a RZ format: the zero segment duration was set to 60 ns, leaving enough time to the acoustic wave to sufficiently damp 4 . The code length was L=151 (prime number), each trace being averaged 4 times, corresponding to a total theoretical SNR enhancement of about 200 (including time-averaging and the coding gain).…”

“…Simplex code was first employed to increase the distance range up to 50 km with a spatial resolution of one meter 3 . Non return to zero (NRZ) coding causes strong distortions on the local Brillouin gain spectra, due to the pre-excitation of the acoustic wave leading to a distorted response 4 . In this paper, we introduce a unique and original coding scheme named "time/frequency code", specially configured and optimized for BOTDA sensing.…”

Long-range BOTDA sensing using optical pulse coding and single source bi-directional distributed Raman amplification

“…when the Brillouin pump can be considered undepleted and the interaction among pulses is negligible. Actually, to minimize the Brillouin interaction among pulses, and hence, possible distortions in the measured BGS, pulses with RZ modulation format have to be used [6]. In addition, bi-directional distributed Raman amplification can also be employed to further enhance the sensing performance; thus two CW Raman pumps can be coupled into both fiber ends, leading to distributed amplification of both Brillouin pump and probe lights [4].…”

“…In this paper we combine the use of optimized pulse coding with return-to-zero (RZ) modulation format [6] and bidirectional distributed Raman amplification [4]. We employ only one high-RIN fiber Raman laser, whose light is shared for both co-and counter-propagating Raman pumping, allowing us to evaluate the impact of optimized pulse coding under the effects of high noise conditions resulting from the high-RIN Raman pumps.…”