Time domain approach uses a light pulse as pump which interacts with a counter-propagating continuous wave (CW) in the Brillouin gain configuration. As addressed in Chapter 2, the process is called Brillouin gain when the CW signal is spectrally down-shifted with respect to the pump pulse signal and Brillouin loss when inversely the CW signal is spectrally up-shifted with respect to the pump pulse signal. At each instant during the pulse light propagation through the fibre, stimulated Brillouin scattering occurs only in the region where the two waves are overlapping. This detected temporal trace provides the spatial information along the fibre, since the pulse time of flight can be used to convert the temporal coordinate to a spatial coordinate. The Brillouin interaction occurs at a particular section of the fibre if the two counter-propagating wave’s frequencies match the local Brillouin frequency shift. Techniques based on spontaneous Brillouin scattering for Time Domain Reflectometry (BOTDR) [4] require only one access point to the test fibre to launch the pump wave, while those using SBS for Brillouin Analysis (BOTDA) must access both ends of the test fibre to launch the pump pulse and CW signal, [1-3]. However, the first and main advantage of BOTDA over BOTDR lies in the stimulated nature of the Brillouin interaction. Since BOTDR relies on spontaneous effect while BOTDA uses stimulation of the Brillouin intraction which enhances the signal, consequently, much higher SNR is achieved.
3.2.1 Brillouin optical time-domain reflectometry (BOTDR)
The BOTDR system was first developed by Kurashima in 1992 [21]. The principle of BOTDR is based on the OTDR technique but here the system uses Brillouin scattering instead of Rayleigh scattering. The Brillouin backscattered intensity from an intense pulse is recorded as a function of time. The optical wave is divided into two optical waves, one is modulated into a pulse and the other one is used as a reference light wave. The pulse power is amplified by an Erbium amplifier and launched into a test fibre. The Brillouin shift is determined by scanning, step by step, the frequency and by recording for each step the detected signal (using for instance a coherent and heterodyne receiver [22]). The frequency distribution of the Backscattered signal can then be reconstructed at each position by analyzing for each time step the amplitude as a function of the frequency and determining the peak value. With the coherent detection, the Brillouin backscattering signal (which is 100 times smaller than Rayleigh backscattering) does not suffer from noise fading thus no need of polarization scrambler or polarization diversity is needed. 1 m spatial resolution over a 10 km range was obtained with this system. However other improvements have been made and have increased the spatial resolution to 2 m over 30 km [23]. These performances can be improved through Raman amplification but require complex equipments [24]. Having the advantage of the one-end access system, we will see that the spatial resolution cannot be better than 1 m.
3.2.2 Brillouin optical time-domain analysis (BOTDA)
BOTDA was first proposed as a nondestructive attenuation measurement technique for optical fibres [5]. By clarifying the strain and temperature dependence of the Brillouin frequency shift [5-6], BOTDA has been used as strain and distributed strain and temperature measurement techniques.
Initially, two distinct lasers for generating pump and signal waves were used. This causes problems
of frequency drifts between the lasers. Niklès et al [25] suggested resolving the problem by using a microwave generator and a LiNbO3 electro-optic modulator (EOM) to generate pump and signal waves from a single light source, as shown in Figure 3-9. The EOM modulates the laser light at a frequency near the Brillouin frequency shift to generate a signal wave. The same EOM also produces a pump pulse by applying an electrical pulse to the EOM electrodes. The modulator is biased to operate in a suppressed carrier scheme, so that the lower modulation sideband can be used as a signal wave while the upper sideband is suppressed by an optical filtering. Nowadays the BOTDA system provides access to both fibre ends with the Nikles’s solution for better performances, since the pump pulse and the CW signal must counter-propagate in the sensing fibre.
The electrostriction that stimulates the acoustic wave is driven by the interferences between pump and signal, so that their states of polarization must be preferably aligned to create the maximum gain [26]. Orthogonal polarizations will result in a totally vanishing gain, and since the polarization normally varies randomly along an optical fibre [26], a non-zero gain can only be secured using polarization scrambling or a polarization-diversity scheme. This polarization dependence can also be favorably used to efficiently and rapidly measure the local birefringence properties along an optical fibre [27].
However the configuration shown in Figure 3-9 suffers from sensitivity to optical noise, as a result of the bidirectional propagation of optical waves showing the same frequency along the optical fibre:
(i) Interference between the 10% of the upper sideband transmitted by the Bragg grating filter and the same signal coming out from the fibre which creates reflexion over the fibre.
(ii) Interference between the modulation signal after the filter and the signal coming out from the fibre.
A sensor configuration was entirely revisited to reach a lower noise level for observing high contrast signals and a new configuration was proposed in Ref. [28] as depicted in Figure 3-10. This
Figure 3-9: Experimental set-up of the BOTDA technique using a single laser. Optical noise results from the bidirectional propagation of optical waves showing the same frequency along the optical fibre.
configuration avoids as much as possible the bidirectional propagations of waves showing the same optical frequency [29]. It also drastically reduces the optical noise resulting from the superposition of coherent waves with same frequency and showing a random fluctuation of the phase difference generated from spurious optical reflections and Rayleigh scattering in the system. The random phase difference generates an important noise intensity and is frequently observed in bidirectional fibre optics systems.
Using the configuration presented on the Figure 3-10 measurements were carried out over a 47 km sensing range with 7 m spatial resolution thanks to the massive noise reduction by more than 15 dB down to optical shot noise. To maintain a sufficient gain, the pump pulse must be made longer enough. For this system, the pulse width was set to 70 ns corresponding to a spatial resolution of 7 m.
The overall information was obtained by performing a frequency sweep of the signal wave (as shown in Figure 3-11(b), single traces were acquired with a 125 MHz detector and 256 averaging).
The scan in time/distance and frequency domain can be viewed as a 3D distribution in Figure 3-11(a) representing the spectral distribution of the Brillouin gain at any location along the fibre.
The resulting maximum Brillouin frequency shift over the fibre length of 47 km (accuracy of less than 1 MHz) is shown in Figure 3-11(c). The change of the curve shape at the 25 km position is due
Figure 3-10: The new BOTDA configuration for low optical noise. Using an optical circulator, light from the upper channel is extracted at the fibre output. This signal is boosted using an EDFA (Erbium doped amplifier) and filtered using a very narrowband fibre Bragg grating filter (<0.1 nm) to transmit only one sideband onto the detector. The filtering is crucial since it eliminates the unwanted modulator sideband that reduces the measurement contrast and any presence of the pump frequency due to the finite extinction ratio of the intensity modulator that would generate substantial optical noise at detection when combined with the Rayleigh light from the pump pulse.
Fibre
to the non uniformity doping concentration in the used single mode fibre. It must be pointed out that the range is limited by high power, which leads to non-linear effects, Raman scattering and/or modulation instability depending on the fibre type (fibres with normal or anormal dispersion).
Studies on this issue in Brillouin sensor based time domain will be carried out in the second part of this chapter.
BOTDA system has the same limitation of a spatial resolution of 1 m as BOTDR system, limited by the gain spectrum broadening due to the pump spectral broadening for short pulses. This 1 m spatial resolution can be secured up to a distance of 30 km and requires an average of less than 1000 to get performances identical to a BOTDR system.
(a)
(b) (c)
Figure 3-11: a) 3D distribution of the gain spectrum 47 km fibre length, obtained using 7 m spatial resolution with 256 time trace averaging. b) Trace of maximum Brillouin gain along the first 5 km of the fibre. c) Resulting Brillouin frequency shift of 47 km length fibre.
Brillouingain