4. State-of-the-art of BOTDA

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 limitations are reviewed.

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

#### 4.1 Strategies to avoid modulation instability

#### 4.1.1 Noise filtering

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

mitigation of MI. In comparison with other techniques, this technique is simple and requires no further change in the setup. However, DSFs of long length are much more costly than SMFs of the same length and DSFs usually have a higher attenuation.

4.1.3 Orthogonal polarized pump pulses

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complementary Brillouin interactions are already ensured.

4.2 Strategies to avoid nonlocal effects

effective solutions for the NLE are reviewed.

**-100 -50 0 50 100**

**Frequency detuning (GHz)**

polarized dual-pump regime with 10 GHz frequency detuning [67].

4.2.1 Dual probe

**(a)**

**Power in a.u. dB scale**

Figure 11.

71

Though the successful mitigation, according to a recent investigation [64], there are still pump power limitations. As shown in Figure 10(b), for a pump pulse with a power of around 1 W, the traces are distorted mainly by forward Raman scattering.

Since the single pump pulse power is limited by MI, a novel technique based on multiwavelength pumps was proposed to break the power limitation [65]. However, it has soon been identified that four wave mixing (FWM) between different spectral lines [65, 66] limits this approach. Thus, an orthogonal polarized dual-pump technique was proposed [67]. In this technique, two pump pulses with orthogonal SOP in different frequencies are launched into the FUT and interact with their corresponding probe waves. As shown in Figure 11, the orthogonal pumps effectively mitigate the MI in comparison with a single pump regime and successfully avoid the FWM interaction in comparison with the parallel polarized dual-pump regime both in time and frequency domain. It is worth to note that no additional polarization scrambler is required for orthogonal pulses, since two

In an early proposal to avoid NLE, a general postprocessing algorithm was used for the BFS profile reconstruction [45]. Based on the BFS distribution that matches the measured data with a minimization algorithm in multidimensions, the parameters of the unknown BFS profile can be derived. However, the experimental realization complicates the setup and increases the processing time [68]. Another idea for the mitigation of NLE in general is to use pulses for both pump and probe wave [69]. However, despite the effective mitigation of the NLE, the shortened interaction length increases the measurement time significantly. In this subsection, other

One of the origins of NLE is the pump pulse depletion, which can be further separated in first- and second-order NLE. First-order NLEs are mainly caused by

> **Single pump Parallel pumps Orthogonal pumps**

**Amplitude**

(a) Simulated MI gain spectrum and (b) experimental time traces with single pump and parallel/orthogonal

 **(mV)** **(b)**

**0 5 10 15 20 25**

**Single pump Parallel pumps Orthogonal pumps**

**Distance (km)**

Figure 9.

Experimental results of the (a) Brillouin gain and (b) maximum distance (b) with an increasing input pump power with and without ASE noise filtering [62].

important factor. Therefore, a narrow band-pass optical filter after the pulse amplification might mitigate the MI [62]. Conventionally, the Brillouin gain is proportional to the input pump power, as indicated in Figure 9(a). However, this is not the case if MI takes place. When the pump is depleted, the Brillouin gain decreases correspondingly. As indicated by the red line in Figure 9(a), the application of the filter enables a slight increase of the Brillouin gain when the pump power is beyond the MI threshold, indicating a lower pump depletion in comparison to the case without the 1 GHz band-pass filter, and a mitigation of MI. Besides, the sensing range is also extended due to the lower pump depletion. Figure 9(b) illustrates experimental results for the distance that the pump pulse reaches when its amplitude is depleted by 50%. Since the MI extends over a band of 60 GHz [62], it is believed that a filter wider than this bandwidth has a negligible contribution on the MI mitigation, while an enhanced mitigation is expected with a much narrower filter.

#### 4.1.2 Dispersion-shifted fiber

Since the origin of MI is the interplay between the Kerr effect and anomalous dispersion, the MI can be eliminated in a dispersion-shifted fiber (DSF) with normal dispersion D = 1.4 ps/(nmkm) at 1550 nm [63]. As discussed in Section 3.5.3, the MI threshold in a 25 km SMF is estimated to be around 135 mW. However, as shown in Figure 10(a), in DSF with the same length, no obvious trace distortion is observed at a pump pulse power of 400 mW, indicating a much higher MI threshold. An exponential fitting, which implies a pure fiber attenuation, further confirms the full

Figure 10. (a) BOTDA trace in 25 km DSF with 400 mW pump power [63]; (b) distortions due to forward Raman scattering [64].

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

mitigation of MI. In comparison with other techniques, this technique is simple and requires no further change in the setup. However, DSFs of long length are much more costly than SMFs of the same length and DSFs usually have a higher attenuation.

Though the successful mitigation, according to a recent investigation [64], there are still pump power limitations. As shown in Figure 10(b), for a pump pulse with a power of around 1 W, the traces are distorted mainly by forward Raman scattering.

#### 4.1.3 Orthogonal polarized pump pulses

Since the single pump pulse power is limited by MI, a novel technique based on multiwavelength pumps was proposed to break the power limitation [65]. However, it has soon been identified that four wave mixing (FWM) between different spectral lines [65, 66] limits this approach. Thus, an orthogonal polarized dual-pump technique was proposed [67]. In this technique, two pump pulses with orthogonal SOP in different frequencies are launched into the FUT and interact with their corresponding probe waves. As shown in Figure 11, the orthogonal pumps effectively mitigate the MI in comparison with a single pump regime and successfully avoid the FWM interaction in comparison with the parallel polarized dual-pump regime both in time and frequency domain. It is worth to note that no additional polarization scrambler is required for orthogonal pulses, since two complementary Brillouin interactions are already ensured.

## 4.2 Strategies to avoid nonlocal effects

In an early proposal to avoid NLE, a general postprocessing algorithm was used for the BFS profile reconstruction [45]. Based on the BFS distribution that matches the measured data with a minimization algorithm in multidimensions, the parameters of the unknown BFS profile can be derived. However, the experimental realization complicates the setup and increases the processing time [68]. Another idea for the mitigation of NLE in general is to use pulses for both pump and probe wave [69]. However, despite the effective mitigation of the NLE, the shortened interaction length increases the measurement time significantly. In this subsection, other effective solutions for the NLE are reviewed.

#### 4.2.1 Dual probe

important factor. Therefore, a narrow band-pass optical filter after the pulse amplification might mitigate the MI [62]. Conventionally, the Brillouin gain is proportional to the input pump power, as indicated in Figure 9(a). However, this is not the case if MI takes place. When the pump is depleted, the Brillouin gain decreases correspondingly. As indicated by the red line in Figure 9(a), the application of the filter enables a slight increase of the Brillouin gain when the pump power is beyond the MI threshold, indicating a lower pump depletion in comparison to the case without the 1 GHz band-pass filter, and a mitigation of MI. Besides, the sensing range is also extended due to the lower pump depletion. Figure 9(b) illustrates experimental results for the distance that the pump pulse reaches when its amplitude is depleted by 50%. Since the MI extends over a band of 60 GHz [62], it is believed that a filter wider than this bandwidth has a negligible contribution on the MI mitigation, while

Experimental results of the (a) Brillouin gain and (b) maximum distance (b) with an increasing input pump

**(b)**

**Distance**

 **(km)**

**Without filter**

**0 500 1000 1500 2000**

**0 2 4 6 8 10 12 14**

**Pp=0.398 W Pp=0.664 W Pp=1 W Pp=1.3 W Pp=2.16 W**

**Distance (km)**

**With filter**

**Pump power (mW)**

Since the origin of MI is the interplay between the Kerr effect and anomalous dispersion, the MI can be eliminated in a dispersion-shifted fiber (DSF) with normal dispersion D = 1.4 ps/(nmkm) at 1550 nm [63]. As discussed in Section 3.5.3, the MI threshold in a 25 km SMF is estimated to be around 135 mW. However, as shown in Figure 10(a), in DSF with the same length, no obvious trace distortion is observed at a pump pulse power of 400 mW, indicating a much higher MI threshold. An exponential fitting, which implies a pure fiber attenuation, further confirms the full

> **1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4**

**Brillouin gain (a.u.)**

(a) BOTDA trace in 25 km DSF with 400 mW pump power [63]; (b) distortions due to forward Raman

an enhanced mitigation is expected with a much narrower filter.

**0 5 10 15 20 25**

**Brillouin loss Exponential fitting (a) (b)**

**Distance (km)**

**0 100 200 300 400 500 600**

**With filter**

Fiber Optic Sensing - Principle, Measurement and Applications

**Pump power (mW)**

**curve**

**Theoretical**

**Without filter**

power with and without ASE noise filtering [62].

**0.0**

**0.2**

**0.4**

**Brillouin**

Figure 9.

 **gain (%)**

**0.6**

**0.8**

**(a)**

4.1.2 Dispersion-shifted fiber

**0.00 0.02 0.04 0.06 0.08 0.10 0.12**

**Brillouin loss (V)**

Figure 10.

70

scattering [64].

One of the origins of NLE is the pump pulse depletion, which can be further separated in first- and second-order NLE. First-order NLEs are mainly caused by

#### Figure 11.

(a) Simulated MI gain spectrum and (b) experimental time traces with single pump and parallel/orthogonal polarized dual-pump regime with 10 GHz frequency detuning [67].

Figure 12.

Schematic explanation of (a) the Second-order NLE due to the distortion of the pump spectrum [70] and (b) the strategy to avoid it using an FM probe wave [73].

interactions can take place between the probe wave and the leading as well as the trailing pedestals [75], as schematically depicted in Figure 13(a). Therefore, the net effect of the presence of the pulse pedestal is that the probe wave will be additionally amplified. However, this amplification is not useful for sensing. In case of a strong probe wave, not only the pulse but also the pedestals (especially the trailing pedestal) will deplete, which lead to severe distortions on the BOTDA traces (see

(a) The schematic description of the interaction between pulse and probe wave [75]; (b) the distorted BOTDA

Under a very low ER, the interactions between probe wave and pedestals may dominate the final BGS detection due to a long interaction length and leads to an error on the BFS estimation (see Figure 14(a) and (b)) [77]. For a conventional MZM, the ER is only 20 dB but can be enhanced up to 30 dB with special designs. Higher ERs of more than 40 dB can be achieved with switching type semiconductor

In a DPP-BOTDA system, two consecutive measurements are carried out with the same pulse peak power but slightly different durations, T and T þ ΔT. Usually, T is longer than the phonon excitation time, indicating a full excitation of the acoustic wave in both measurements and ΔT is short so as to achieve a high spatial resolution [78]. Since both pump pulses give the same amplification in the time slot T and only the longer pulse contributes amplification in the extra time ΔT, the subtraction of the two measured Brillouin amplified probe signals yields to the BGS

**10.80 10.85 10.90**

**Frequency (GHz)**

**0.20 0.25 0.30 0.35 0.40 0.45 0.50**

**Optical power (mW)**

The simulation of the BGS with pump pulse of (a) ER = 40 dB and (b) ER = 26 dB at the hot spots in different

temperatures (different BFSs). The uniform BFS of the rest of the fiber is ν<sup>B</sup><sup>0</sup> = 10.835 GHz [77].

optical amplifiers (SOA) and a 60 dB ER with RF switches [75, 76].

**(a) (b)**

traces under different pulse ER due to the pump pedestal depletion [76].

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

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

4.3 Enhancement of the spatial resolution

**10.80 10.85 10.90**

**Frequency (GHz)**

4.3.1 Differential pulse width pair (DPP)

**<sup>B</sup>=10.855 GHz <sup>B</sup>=10.865 GHz <sup>B</sup>=10.875 GHz <sup>B</sup>=10.885 GHz**

Figure 13(b)) [75, 76].

Figure 13.

in the time slot ΔT.

**0.20**

**0.22**

**Optical power (mW)**

Figure 14.

73

**0.24**

**0.26**

pump depletion due to a high probe power. Second-order NLEs are based on linear distortions of the pump pulse spectrum. Second-order NLEs can even happen when first-order NLEs are completely mitigated [70].

The most popular solution for first-order NLE mitigation is the dual-probe sideband regime. In contrast to a conventional BOTDA system, both probe wave sidebands generated by the MZM are involved in the SBS interaction. The pump pulse depletion due to the energy transfer to the lower frequency sideband is compensated by an energy transfer from the upper frequency sideband to the pump. A theoretical analysis reveals that in the dual-probe sideband regime, the probe power limit rises from 14 to 3 dBm [60, 71]. Furthermore, since the energy transfer from the high power (pump) to the low power (probe) is more efficient than the reverse process, slightly unbalanced dual-probe sidebands mitigate NLE better than balanced ones [60, 72].

Even with dual probe regime, second-order NLEs still exist [70]. This kind of NLE has its origin not in the depleted pump, but in the frequency-dependent distortion of the pump pulse spectrum that affects the interaction in the gain and loss configuration differently (see Figure 12(a)). However, a total mitigation of first- and second-order NLEs can be achieved with dual-probe sidebands [61] with additional FM in saw-tooth shape [73]. A similar performance enhancement can also be achieved with sinusoidal or a triangular shape [61, 74]. If the FM is synchronized to the pump pulses (see Figure 12(b)), a series of pulses at a specific fiber position interacts always with the same probe frequency. In turn, each single pulse experiences SBS interactions with probe waves that have different frequency detunings as it propagates along the fiber. Since the pulse depletion is accumulated along the fiber, a decreased distortion upon the pulse spectrum can be achieved by interacting with different frequencies within the BGS, indicating a mitigation of the second-order NLE. Together with the mitigation of the first-order NLE by dualprobe sidebands regime, the probe power limit has been successfully pushed to a record of 8 dBm [61], reaching the Brillouin threshold of SMF in 20 km range.

#### 4.2.2 Higher pulse extinction ratio

Another origin of NLE is the imperfect pulsing of the pump wave. Due to the limited extinction ratio (ER) of common pulsing methods, there is a residual power leakage. Since the pulse pedestals have the same frequency as the pump, SBS

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

#### Figure 13.

pump depletion due to a high probe power. Second-order NLEs are based on linear distortions of the pump pulse spectrum. Second-order NLEs can even happen when

Schematic explanation of (a) the Second-order NLE due to the distortion of the pump spectrum [70] and (b)

The most popular solution for first-order NLE mitigation is the dual-probe sideband regime. In contrast to a conventional BOTDA system, both probe wave sidebands generated by the MZM are involved in the SBS interaction. The pump pulse depletion due to the energy transfer to the lower frequency sideband is compensated by an energy transfer from the upper frequency sideband to the pump. A theoretical analysis reveals that in the dual-probe sideband regime, the probe power limit rises from 14 to 3 dBm [60, 71]. Furthermore, since the energy transfer from the high power (pump) to the low power (probe) is more efficient than the reverse process, slightly unbalanced dual-probe sidebands miti-

Even with dual probe regime, second-order NLEs still exist [70]. This kind of NLE has its origin not in the depleted pump, but in the frequency-dependent distortion of the pump pulse spectrum that affects the interaction in the gain and loss configuration differently (see Figure 12(a)). However, a total mitigation of first- and second-order NLEs can be achieved with dual-probe sidebands [61] with additional FM in saw-tooth shape [73]. A similar performance enhancement can also be achieved with sinusoidal or a triangular shape [61, 74]. If the FM is synchronized to the pump pulses (see Figure 12(b)), a series of pulses at a specific fiber position interacts always with the same probe frequency. In turn, each single pulse experiences SBS interactions with probe waves that have different frequency detunings as it propagates along the fiber. Since the pulse depletion is accumulated along the fiber, a decreased distortion upon the pulse spectrum can be achieved by interacting with different frequencies within the BGS, indicating a mitigation of the second-order NLE. Together with the mitigation of the first-order NLE by dualprobe sidebands regime, the probe power limit has been successfully pushed to a record of 8 dBm [61], reaching the Brillouin threshold of SMF in 20 km range.

Another origin of NLE is the imperfect pulsing of the pump wave. Due to the limited extinction ratio (ER) of common pulsing methods, there is a residual power leakage. Since the pulse pedestals have the same frequency as the pump, SBS

first-order NLEs are completely mitigated [70].

Fiber Optic Sensing - Principle, Measurement and Applications

the strategy to avoid it using an FM probe wave [73].

Figure 12.

gate NLE better than balanced ones [60, 72].

4.2.2 Higher pulse extinction ratio

72

(a) The schematic description of the interaction between pulse and probe wave [75]; (b) the distorted BOTDA traces under different pulse ER due to the pump pedestal depletion [76].

interactions can take place between the probe wave and the leading as well as the trailing pedestals [75], as schematically depicted in Figure 13(a). Therefore, the net effect of the presence of the pulse pedestal is that the probe wave will be additionally amplified. However, this amplification is not useful for sensing. In case of a strong probe wave, not only the pulse but also the pedestals (especially the trailing pedestal) will deplete, which lead to severe distortions on the BOTDA traces (see Figure 13(b)) [75, 76].

Under a very low ER, the interactions between probe wave and pedestals may dominate the final BGS detection due to a long interaction length and leads to an error on the BFS estimation (see Figure 14(a) and (b)) [77]. For a conventional MZM, the ER is only 20 dB but can be enhanced up to 30 dB with special designs. Higher ERs of more than 40 dB can be achieved with switching type semiconductor optical amplifiers (SOA) and a 60 dB ER with RF switches [75, 76].

### 4.3 Enhancement of the spatial resolution

#### 4.3.1 Differential pulse width pair (DPP)

In a DPP-BOTDA system, two consecutive measurements are carried out with the same pulse peak power but slightly different durations, T and T þ ΔT. Usually, T is longer than the phonon excitation time, indicating a full excitation of the acoustic wave in both measurements and ΔT is short so as to achieve a high spatial resolution [78]. Since both pump pulses give the same amplification in the time slot T and only the longer pulse contributes amplification in the extra time ΔT, the subtraction of the two measured Brillouin amplified probe signals yields to the BGS in the time slot ΔT.

#### Figure 14.

The simulation of the BGS with pump pulse of (a) ER = 40 dB and (b) ER = 26 dB at the hot spots in different temperatures (different BFSs). The uniform BFS of the rest of the fiber is ν<sup>B</sup><sup>0</sup> = 10.835 GHz [77].

pulse is characterized by a broadened spectrum with low amplitude and a narrow cap, whose width is determined by the high power pulse and PPP, respectively. A clear BFS shift between the PPP and total pulse case indicates an enhancement of the spatial resolution down to 10 cm. In comparison to DPP-BOTDA, the high spatial resolution by pre-excitation does not come at the expense of a decreased

Due to the fiber attenuation or other depletions, the sensing range of a conventional BOTDA system is usually limited by the low SNR at the far end of the fiber to only a few tens of kilometers [51]. Therefore, the key to extend the sensing range is

In this technique, the total pump power is spread over multiple-pump waves in different frequencies, with every single pump power still limited by MI [65]. The theoretical enhancement of the SNR could reach the number of pumps N. However, severe FWM occurs for too narrow pump frequency spacing, while the BGS linewidth from each pump may differ when they are too widely separated apart [65]. The solution for the latter is a postprocessing algorithm [81], while the solution to avoid the FWM is to shift the pump pulse propagation in time domain with a frequencyselective time shifter, which can be realized by N-consecutive FBGs separated by a certain length of fiber in the experiment. The schematic description of the frequencyselective time shifter is illustrated in Figure 17(a). After the time-shifted pump pulses have interacted with their corresponding probe waves, another consecutive FBG with a reversed sequence offers a reversed delay and combines the traces back in time domain so that they can be simultaneously detected. For a three pump system, an SNR improvement of 4.8 dB has been demonstrated (see Figure 17(b)) [65].

Another possibility to amplify the signal amplitude is the heterodyne detection. Provided that the Brillouin amplified probe wave at frequency ν<sup>s</sup> beats with an local oscillator at frequency νLO, the total electrical field can be expressed as [30]:

(a) Schematic explanation of the time shifter and recombiner; (b) SNR measured in the experiment for

standard BOTDA (single pulse), three pulses with and without time delay [65].

either to enhance the probe signal power or to eliminate the system noise.

SNR and therefore, it is more favorable for commercial use.

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4.4 Enhancement of the sensing range

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

4.4.1 Multi-frequency pump-probe interaction

4.4.2 Self-heterodyne detection

Figure 17.

75

#### Figure 15.

(a) Schematic description of the simultaneous DPP-BOTDA in the frequency domain and (b) Brillouin gain profile in the experiment. Along the fifth meter of the fiber, two 20 cm fiber sections were located 65 cm apart and manually stretched to have nonuniform strains [79].

Later on, the DPP-BOTDA technique has been developed into a simultaneous measurement by launching two pump pulses with slightly different durations in different frequencies [79]. Pump pulse 1 with duration T interacts with the probe wave via SBS loss, while simultaneously pump pulse 2 with duration T þ ΔT interacts with the probe wave via SBS gain (see Figure 15(a)). Therefore the subtraction of the BGS is automatically achieved at the detector with no postprocessing required. A 10 cm spatial resolution BOTDA system has been reported by using a 30 ns gain pump pulse and a 29 ns loss pump pulse (see Figure 15(b)) [79]. However, in comparison with the pre-excitation method, the subtraction of the traces adds noise to the data. Therefore, in order to achieve the required SNR, massive averaging must be applied.

#### 4.3.2 Pre-excitation

The reason for the spatial resolution limit of around 1 m is the excitation time of the phonon. A prepump excitation can solve this problem by shaping the pump pulse into two parts, that is, a long pedestal with low power (prepump pulse (PPP), part 1 in Figure 16(a)) for the phonon excitation, followed by a narrow high power pulse (part 2 in Figure 16(a)) [80]. In order to excite the phonon, the PPP is usually longer than 10 ns. To achieve high spatial resolutions, the high power pulse can be very short (�1 ns). Figure 16(b) shows experimental results when the PPP (12 ns duration), the high power pulse (1 ns duration), and the total pump pulse interrogate a 20 cm fiber section with strain individually. The resulting BGS of the total

#### Figure 16.

(a) Pulse shape for pre-excitation technique; (b) BGS of a fiber section with strain interrogated by the PPP only (red), high power pulse only (black), and total pulse (blue) [80].

pulse is characterized by a broadened spectrum with low amplitude and a narrow cap, whose width is determined by the high power pulse and PPP, respectively. A clear BFS shift between the PPP and total pulse case indicates an enhancement of the spatial resolution down to 10 cm. In comparison to DPP-BOTDA, the high spatial resolution by pre-excitation does not come at the expense of a decreased SNR and therefore, it is more favorable for commercial use.
