Distributed optical fiber Brillouin sensors provide innovative solutions for the monitoring of temperature and strain in large structures. The effective range of these sensors is typically of the order of 20-30 km, which limits their use in certain applications in which the distance to monitor is larger. In this work, we have developed a new technique to significantly extend the measurement distance of a distributed Brillouin Optical Time-Domain Analysis (BOTDA) sensor. Distributed Raman Amplification in the sensing fiber provides the means to enhance the operating range of the setup. Three Raman pumping configurations are theoretically and experimentally investigated: co-propagating, counter-propagating and bidirectional propagation with respect to the Brillouin pump pulse. We show that some of the amplification schemes tested can extend the measurement range and improve the measurement quality over long distances.
We show that the spectral broadening of the pump pulse through self-phase modulation in a time-domain distributed Brillouin sensor has a considerably detrimental effect in the measurement, especially in the case of long distances and high-resolution pulses. Using 30 ns pump pulses with peak power of 276 mW, self-phase modulation leads to a doubling of the effective gain linewidth after some 20 km, which is equivalent to a contrast loss of 2 dB in the measurement. The impact is higher for shorter pulses (higher resolution). The theoretical modeling is fully confirmed by experimental results. © 2011 Optical Society of America OCIS codes: 290.5900, 190.2640, 060.2370 For at least two decades, Brillouin fiber sensors have attracted great interest in the fiber-sensing community for their temperature and strain-monitoring capability [1,2]. In time-domain-distributed Brillouin sensors, pulses are used to interrogate the local interaction in the fiber. The accuracy on the measurand is scaled by the spectral spreading of the effective gain, which, at its turn, is given by the convolution between the pulse spectrum and the natural Brillouin gain spectrum (BGS). According to standard time-bandwidth relations, the Gaussian pulse is presumably the best candidate for this interrogation when compared to other profiles (rectangular, triangular). However, we show here that this is not the best choice when addressing long ranges, because this pulse shape leads to a significant spectral broadening of the BGS along the fiber. An observed broadening of the BGS was suspected to be caused by self-phase modulation (SPM) in an early work by Lecoeuche et al. [3] and by Izumita et al.[4] in a coherent optical time-domain reflectometer system. SPM leads to small phase chirps during intensity transitions in the pump pulse (leading and trailing edges) that eventually become important in long fibers. The frequency broadening associated with this phase modulation leads to a reduced peak gain and uncertainties in the determination of the Brillouin shift ν B , but it leaves the temporal intensity distribution of the pump pulse unchanged, and hence the spatial resolution is preserved. Although the former [3][4][5] works showed a correct intuition addressing qualitatively the issue, either no theoretical model was given or the model was incomplete [5].We present here a quantitative model of the detrimental impact of SPM supported by a clear experimental demonstration. Two optical pulses with different temporal profiles were judiciously chosen (rectangular and Gaussian) showing the same FWHM and carrying the same energy, to evenly study and compare the SPM impact on their spectrum. Then we clearly experimentally demonstrate the spectral broadening of the BGS due to SPM in a Brillouin distributed sensor in various conditions in terms of pump pulse temporal profiles, power, and width. The results are compared with a theoretical model showing good agreement. SPM is a consequence of the nonlinear Kerr effect in the fiber that results in an intensity-...
This paper describes the design, characterization and calibration of a high power transfer standard for optical power measurements in optical fibres based on an integrating sphere radiometer. This radiometer, based on two detectors (Si and InGaAs), can measure powers between 100 nW and 10 W within the wavelength range of (400–1700) nm. The radiometer has been calibrated over the total spectral range of use against an electrically calibrated pyroelectric radiometer and different fibre laser diodes and ion lasers. The total uncertainty obtained is lower than ±1.5% for these wavelengths and power ranges (excluding the water absorption region).
The influence of chromatic dispersion on CW-pumped supercontinuum generation in km-long standard fibers is experimentally investigated. We perform our study by means of a tunable, high-power fiber ring laser pumping a dispersion-shifted fiber in the wavelength range of small and medium anomalous dispersion. Our results show that, at low input powers, chromatic dispersion plays a dominant role on nonlinear pump spectral broadening, giving rise to a broader spectrum when pumping just above the zero-dispersion wavelength of the fiber. At higher input powers, however, the width of the generated supercontinuum spectrum is mostly due to the Raman effect, hence more independent of the value of the chromatic dispersion coefficient. We show that, in this case, the optimum pumping wavelengths for supercontinuum generation are not so close to the zero-dispersion wavelength of the fiber as in the previous case. In these conditions, as the chromatic dispersion grows we can obtain square-shaped and high-power density spectra, which seem extremely promising for applications in optical coherence tomography.
We review some recent results on the application of distributed Raman amplification schemes, including ultralong lasers, to the extension of the operating range and contrast in Brillouin optical time domain analysis (BOTDA) distributed sensing systems.Brillouin optical time domain analysis (BOTDA) is a widely-used technique for the distributed measurement along optical fibers. The main interest of BOTDA lies in the possibility of using it to develop sensors for the continuous monitoring of temperature and/or strain in optical fibers [1][2][3][4][5]. Additionally, BOTDA has been used to perform power distribution measurements along optical fibers, enabling the measurement of fiber attenuation [6], chromatic dispersion [7] and parametric amplification [8]. The measurement range of BOTDA systems is typically shorter than 50 km [9] and generally limited to 20-30 km with a spatial resolution between 1-2 meters. The measurement range limitation is basically due to fiber attenuation. The losses in the fiber cause a drop of signal contrast with distance and a growth in the measurement uncertainty, as a simple result of the pump power reduction that critically scales the actual Brillouin gain. In these systems there is also a trade-off between resolution and measurement range: while it is desirable to have the highest possible resolution, this requires the use of short optical pulses, so the effective distance for amplification is reduced accordingly. This increases the difficulty in detection because the signal-to-noise ratio (SNR) is reduced. To some extent, one can compensate this by raising the pump power. However, the pump power cannot be increased indefinitely since other competing nonlinear effects (modulation instability and Raman) and also significant pump depletion start to take place. Thus, depending on system requirements, one has to choose between short pulses for increased resolution, or longer pulses for increased range. Several studies have been realized to extend the system resolution without impairing the measurement range. These rely on signal processing methods [10,11], distributed amplification along the fiber to overcome the fiber loss [12][13][14], or a combination of both [15]. In this talk we will review some of our recent results in the application of Raman-amplified schemes, including advanced higher-order amplification based on the ultralong laser architecture [16], to the improvement of the performance of BOTDA distributed sensors.
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