3.5 Major issues and limitations of BOTDA

## 3.5.1 Polarization fading

whose inverse fast Fourier transformation approximates the pulse response of the FUT and represents the temperature and strain distribution. The high spatial resolution that BOFDA could achieve depends on the frequency sweep range, though, at

As illustrated in Figure 6(a), a typical conventional BOTDA setup mainly com-

The Brillouin amplified probe wave is detected by a fast photodiode (PD) whose minimum bandwidth should correspond to the inverse of the pump pulse duration, to avoid any trace distortions. Another narrow band-pass filter (FBG 2) blocks the residual reflected pump wave. The electrical signal from the PD, which represents the evolution of the probe power, is processed digitally with a large number of averaging by a signal processor, that is, an oscilloscope or a digitizer. The Brillouin gain, which is essential for BFS estimation, can be derived by dividing the output of the PD by the probe power before the pump pulse is launched into the fiber [51].

The BOTDA sensing performance is highly dependent on the estimation accuracy of the local BFS. According to recent investigations [52], system parameters such as the FWHM of the BGS ΔνB, the scanning frequency step δ, and the system noise σ, which is the inverse of the signal to noise ratio (SNR) of the normalized Brillouin gain, contribute significantly to an accurate estimation of a local BFS, as presented by the red curve in Figure 6(b). Provided that νBð Þz is the estimated BFS at distance z after a parabolic fitting of all experimental data above a given fraction η of the peak

value (see Figure 6(b) for η ¼ 0:5), the estimated error of the BFS is [52]:

(a) Conventional BOTDA setup. RF, radio frequency; MZM, Mach-Zehnder modulator; EDFA, erbiumdoped fiber amplifier; Pol.S., polarization scrambler; FUT, fiber under test; PD, photodiode; Cir, circulator; FBG, fiber Bragg grating; (b) typical measured local BGS (red) and BPS (black) with system parameters that

prises three parts. A highly coherent laser is split via a coupler into the probe (upper) and pump (lower) branch. A microwave signal, which can be scanned over the range of the BFS (�11 GHz � 150 MHz), is applied on a Mach-Zehnder modulator (MZM 1) in the probe branch and biased in the carrier suppression regime. The generated lower frequency sideband serves as the probe wave, while the upper one is blocked by a narrow band-pass filter (FBG 1). The probe power is controlled by an erbium-doped fiber amplifier (EDFA 1) and launched into one end of the FUT. In the pump branch, the pump pulse with a required duration is formed by another MZM, amplified by EDFA 2 and launched into the other end of the fiber via a circulator (Cir 1). In order to mitigate the influence of the SBS polarization effect [50], the polarization of either pump or probe wave is scrambled via a

polarization scrambler (Pol.S.) before launched into the fiber.

3.4 Evaluation of the BOTDA performance

contribute to an uncertainty of the BFS estimation.

Figure 6.

66

the cost of the measurement time.

Fiber Optic Sensing - Principle, Measurement and Applications

3.3 Basic setup of BOTDA

As discussed in Section 2.3, the Brillouin gain is highly dependent on the SOP of the pump and probe wave. Due to the weak birefringence of SMF, the SOP change of pump and probe leads to a highly nonuniform Brillouin gain along the fiber and consequently to a poor SNR for almost every fiber section, which is called polarization fading [50]. There are several solutions for polarization fading in a BOTDA system. The first proposed idea was to sequentially launch two orthogonal SOPs of the pump (or probe) wave and average the two measured results [4]. Another option is polarization scrambling, where the SOP of the pump and probe is varied rapidly so that the SOP is effectively randomized over time. It can be used in various scientific setups to cancel the errors caused by polarization-dependent effects. With a polarization scrambler and a digitizer, the traces can be averaged over thousands of pump pulses until a required high enough SNR is reached, though, at the cost of the BGS acquisition time.

#### 3.5.2 Limitations on pump pulse width

According to the round-trip relation, the Brillouin interaction in a conventional BOTDA system takes place in a fiber section with a length of cT=ð Þ 2n , which can be seen as the spatial resolution [54], where c is the light speed in vacuum, n is the fiber refractive index and, T is the pulse duration. In principle, the spatial resolution could be enhanced by using shorter pulses. However, two main factors limit the spatial resolution to only 1 m.

First of all, decreasing the pulse duration will shorten the Brillouin interaction length and hence lower the SBS gain and consequently the SNR. Furthermore, the pump power spectrum for a shorter pulse will be severely broadened. Therefore, the resulting effective BGS should be modified as the convolution of the pump spectrum density and the natural Brillouin linewidth [21], that is,

$$\mathbf{g}\_B^{\sharp \dagger}(\nu) = \mathbf{g}\_B(\nu) \otimes I\_p(\nu) \tag{10}$$

where Ipð Þν is the pump pulse power spectrum, which is significantly broadened to 1=T for pulses shorter than 20 ns [55]. The inherent broadening of the BGS leads to a decrease of the peak gain, which makes the estimation of the BGS peak more sensitive to the noise level. Moreover, it also indicates that the probe wave will be amplified only after the acoustic field is excited by the pump-probe interaction, which takes 10–30 ns. Due to the abovementioned reasons, the spatial resolution of a conventional BOTDA system is limited to around 1 m, which corresponds to a pulse width of 10 ns.

#### 3.5.3 Limitations on the pump wave power

In principle, the pump pulse power launched into the FUT should be high enough to compensate the fiber attenuation and hence generate an efficient Brillouin interaction. However, modulation instability (MI) in the optical fiber appears when the pulse power is beyond a certain threshold [56]. MI refers to the breakup of the balance between the anomalous dispersion and the Kerr effect, so that a train of soliton-like pulses rises from the noise as spectral sidebands symmetric to the pulse frequency [57, 58]. Several theoretical as well as experimental investigations have also demonstrated that there exists a periodical energy exchange between sidebands and pulse power along the fiber, that is, after a certain length of propagation, the pulse energy that has been spread to the sidebands will be transferred back to the pulse frequency and thus forms the Fermi-Pasta-Ulam recurrence [59].

Brillouin interaction at each fiber segment not only amplifies the probe wave but also slightly depletes the pump power intensity. Since for each time-resolved interval, the CW probe wave interacts with the pump pulse only once, and the impact of the depletion becomes much higher on the pulse than on the probe wave. Therefore, the pump spectrum density is no longer independent of the Brillouin interaction that the pump pulse has experienced before reaching the distant fiber segment with nonuniform BFS (see Figure 7(b)). NLE typically leads to an asymmetry of the BGS and hence to an error in the BFS estimation. It should be noticed that, due to the accumulative impact of the depletion on the pump pulse, the NLE is in general more

BFS as a function of the position along the fiber when the hot spot is placed at (a) the far end and (b) the near

**32 34 36 38 40 42 44**

**Ps = 0.70 mW**

**Ps = 1.10 mW**

**Ps = 1.75 mW**

**Ps = 2.87 mW**

**Ps = 3.48 mW**

**Position along the fiber (m)**

Another limitation on the probe power is the Brillouin threshold of the fiber. It is

As discussed in Section 3, due to the limitations on the pump pulse and probe power, the spatial resolution, and hence the sensing range and BFS measurement accuracy, a conventional BOTDA system is far from achieving an ideal performance. In order to compensate the SNR degradation, a higher averaging must be applied at the cost of measurement time. In this section, methods to break these

The techniques introduced in this section are categorized according to their enhanced sensor performances. It is worth to notice that the contribution of each technique may lead to enhancements in several performances, for example, a technique that overcomes the MI enables a pump power higher than the limit and thus

The origin of MI is system noise, in which especially the amplified spontaneous

emission (ASE) noise from the EDFA for pump pulse amplification plays an

also an extended sensing range due to the increased Brillouin gain.

4.1 Strategies to avoid modulation instability

theoretically the maximum power that the input probe wave can have with no depletion and no thermally induced SpBS from the probe wave [61]. Since the Brillouin threshold is usually higher than the probe power limit due to the NLE, a series of techniques have to be applied to solve the NLE and then push the probe

severe at the far end of the fiber (see Figure 8).

end of the fiber for different signal (probe) powers Ps [60].

**1015 1017 1019 1021 1023 1025 1027**

The State-of-the-Art of Brillouin Distributed Fiber Sensing

DOI: http://dx.doi.org/10.5772/intechopen.84684

**(a) (b)**

**Position along the fiber (m)**

**10.7380 10.7385 10.7390 10.7395 10.7400 10.7405**

**Brillouin frequency**

Figure 8.

 **shift (GHz)**

power limit toward the threshold of SBS.

4. State-of-the-art of BOTDA

limitations are reviewed.

4.1.1 Noise filtering

69

For a conventional BOTDA system, this periodical power exchange leads to the fluctuation of the pump pulse power during the propagation and therefore distortions of the traces. As illustrated in Figure 7(a), some of the fiber sections are not correctly interrogated due to the limited SNR. Furthermore, the critical power of the MI for a conventional BOTDA system with 25 km fiber is estimated to be as low as 135 mW [57]. Due to a rapid decrease of the Brillouin gain, MI severely limits the sensing range.

#### 3.5.4. Limitations on the probe wave power

In order to achieve a high SNR for a better BFS estimation, a high enough probe wave power should be launched into the FUT. However, due to nonlocal effects (NLEs), the probe wave power in a conventional BOTDA system, that is, single probe sideband system, is usually limited to only 14 dBm [60]. The NLE generally refers to the fact that the Brillouin interaction from a local fiber segment is influenced by the interaction of other segments and hence leads to an error in the BFS estimation of the local fiber segment. It should be noticed that not only a too high probe power, but other factors, such as a limited pump pulse extinction ratio (ER), would also lead to NLE.

The origin of the NLE due to the high probe wave can well be explained by Figure 7(b) and the DES of SBS. As pointed out by Eq. (2) in Section 2.1, the

#### Figure 7.

(a) Distorted Brillouin traces due to MI with different input pump powers and fitted curves [57]. (b) Schematic explanation of the NLE due to the pump depletion in the FUT with uniform BFS in a long distance but nonuniform BFS in a distant section. δν is the frequency shift of BGS between the long uniform sections and the distant fiber segment [60].

The State-of-the-Art of Brillouin Distributed Fiber Sensing DOI: http://dx.doi.org/10.5772/intechopen.84684

Figure 8.

a conventional BOTDA system is limited to around 1 m, which corresponds to a

frequency and thus forms the Fermi-Pasta-Ulam recurrence [59].

In principle, the pump pulse power launched into the FUT should be high enough to compensate the fiber attenuation and hence generate an efficient Brillouin interaction. However, modulation instability (MI) in the optical fiber appears when the pulse power is beyond a certain threshold [56]. MI refers to the breakup of the balance between the anomalous dispersion and the Kerr effect, so that a train of soliton-like pulses rises from the noise as spectral sidebands symmetric to the pulse frequency [57, 58]. Several theoretical as well as experimental investigations have also demonstrated that there exists a periodical energy exchange between sidebands and pulse power along the fiber, that is, after a certain length of propagation, the pulse energy that has been spread to the sidebands will be transferred back to the pulse

For a conventional BOTDA system, this periodical power exchange leads to the fluctuation of the pump pulse power during the propagation and therefore distortions of the traces. As illustrated in Figure 7(a), some of the fiber sections are not correctly interrogated due to the limited SNR. Furthermore, the critical power of the MI for a conventional BOTDA system with 25 km fiber is estimated to be as low as 135 mW [57]. Due to a rapid decrease of the Brillouin gain, MI severely limits the

In order to achieve a high SNR for a better BFS estimation, a high enough probe wave power should be launched into the FUT. However, due to nonlocal effects (NLEs), the probe wave power in a conventional BOTDA system, that is, single probe sideband system, is usually limited to only 14 dBm [60]. The NLE generally

refers to the fact that the Brillouin interaction from a local fiber segment is influenced by the interaction of other segments and hence leads to an error in the BFS estimation of the local fiber segment. It should be noticed that not only a too high probe power, but other factors, such as a limited pump pulse extinction ratio

The origin of the NLE due to the high probe wave can well be explained by Figure 7(b) and the DES of SBS. As pointed out by Eq. (2) in Section 2.1, the

*P ower*

(a) Distorted Brillouin traces due to MI with different input pump powers and fitted curves [57]. (b) Schematic explanation of the NLE due to the pump depletion in the FUT with uniform BFS in a long distance but nonuniform BFS in a distant section. δν is the frequency shift of BGS between the long uniform sections and

*Real BGS*

δν

<sup>ν</sup> <sup>ν</sup>*<sup>B</sup>*

*Measured BGS*

*Depleted pump power*

pulse width of 10 ns.

sensing range.

3.5.3 Limitations on the pump wave power

Fiber Optic Sensing - Principle, Measurement and Applications

3.5.4. Limitations on the probe wave power

(ER), would also lead to NLE.

the distant fiber segment [60].

**Brillouin gain (%)**

Figure 7.

68

**130 mW**

**0 5 10 15 20 25**

**Distance (km)**

**290 mW**

**(a)** (*b*)

**570 mW**

**Measurements Fitted curves**

BFS as a function of the position along the fiber when the hot spot is placed at (a) the far end and (b) the near end of the fiber for different signal (probe) powers Ps [60].

Brillouin interaction at each fiber segment not only amplifies the probe wave but also slightly depletes the pump power intensity. Since for each time-resolved interval, the CW probe wave interacts with the pump pulse only once, and the impact of the depletion becomes much higher on the pulse than on the probe wave. Therefore, the pump spectrum density is no longer independent of the Brillouin interaction that the pump pulse has experienced before reaching the distant fiber segment with nonuniform BFS (see Figure 7(b)). NLE typically leads to an asymmetry of the BGS and hence to an error in the BFS estimation. It should be noticed that, due to the accumulative impact of the depletion on the pump pulse, the NLE is in general more severe at the far end of the fiber (see Figure 8).

Another limitation on the probe power is the Brillouin threshold of the fiber. It is theoretically the maximum power that the input probe wave can have with no depletion and no thermally induced SpBS from the probe wave [61]. Since the Brillouin threshold is usually higher than the probe power limit due to the NLE, a series of techniques have to be applied to solve the NLE and then push the probe power limit toward the threshold of SBS.
