4.4.2 Self-heterodyne detection

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]:

#### Figure 17.

(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].

Figure 18.

(a) BOTDA traces and (b) BFS standard deviation of consecutive measurements with (red) and without (black) self-heterodyne detection [30].

$$E\_T(\mathbf{t}, \nu) = E\_{\rm S0} \mathbf{g}\_{\rm SRS}(\nu\_t, \mathbf{z}) \exp\left\{ \mathbf{j} \cdot [2\pi\nu\_t \mathbf{t} + \phi\_{\rm SRS}(\nu\_t, \mathbf{z})] \right\} + E\_{\rm LO} \exp\left( \mathbf{j} \cdot 2\pi\nu\_{\rm LO} \mathbf{t} \right) \tag{11}$$

in Section 3.1. In Figure 19(b), the proposed technique is demonstrated with a perturbation frequency of 12.3 Hz. Obviously, this technique can only detect small temperature or strain distributions and is not applicable for a BFS nonuniformity

(a) Schematic explanation of the dynamic sensing with the working point set at the half value of the peak

**(a) (b)**

Brillouin gain and (b) dynamic strain measurement based on this technique [82].

**Strain ( )**

**0.0 0.2 0.4 0.6 0.8 1.0**

**Measurement Fitting**

**Time (s)**

Another idea to speed up the measurement time is to utilize multitone pumps [66]. As shown in Figure 20(a), multiple pumps with multiple frequencies are launched into the fiber aligning at different frequency detunings of the BGSs. Since the multi-SBS interactions happen simultaneously, the necessary frequency sweeping can be done in a single shot. Thus, the total measurement time including extra

However, there are still trade-offs and limitations in this technique. Undesired FWM occurs when the pump pulses are simultaneously launched. This can be solved by unequal spacing or a sequential launch of the pumps. The total number of pump tones, which determines the accuracy of the BGS reconstruction, is restricted by the intertone spacing Δf and the total tone span ftotal. The intertone spacing is usually larger than the total BGS width so as to avoid BGS overlaps. The total tone span should not exceed the BFS so that no pump lines are within the BGS of other pumps. Recently, a new improved method has been proposed to avoid these limitations by utilizing digital optical frequency combs (DOFC) as probe signals [83]. Since the probe wave power in the BOTDA system is usually low, multiple-probe waves suffer less from FWM than multiple pumps. The DOFC owns narrow frequency spacing, wide flat top, and total bandwidth, as depicted in Figure 20(b). According

averaging measurements for SNR improvement lasts only several seconds.

(a) Schematic explanation of the principle of the sweep-free multitone BOTDA [81]; (b) spectrum of

that exceeds half of the BGS linewidth.

**-100 -75 -50 -25 0 25 50 75 100**

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

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

**Frequency detuning (MHz)**

**t**

4.5.2 Sweep-free multi-tones

**0.0**

**Normalzied**

Figure 19.

Figure 20.

DOFC [83].

77

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

**0.2**

**0.4**

**t**

**0.6**

**0.8**

**1.0**

where Es<sup>0</sup> and ELO are the complex amplitude of the probe wave and local oscillator, gSBS and ϕSBS are the SBS gain and phase shift. Hence, the detected current at the PD can be written as:

$$I\_{\varepsilon}(t) = R\_{\varepsilon}P\_{T} = 2R\_{C}\sqrt{P\_{s0}\left[1 + \mathbf{g}\_{\text{SBS}}(\nu\_{s}, \mathbf{z})\right]^{2}P\_{LO}} \cdot \cos\left[2\pi f\_{IF}t + \phi\_{0} - \phi\_{\text{SBS}}(\nu\_{s}, \mathbf{z})\right] \tag{12}$$

where ϕ<sup>0</sup> and f IF are the phase and frequency difference between the probe wave and local oscillator, PLO and PS<sup>0</sup> are the optical power of the local oscillator and probe wave, Rc is the PD responsivity. As the detected current is dependent on the power of the local oscillator, the whole mechanism could be regarded as a signal amplification with a strong oscillator. As shown in Figure 18(a), the trace detected by self-heterodyne is cleaner and has an SNR enhancement of 10 dB [30]. Further investigations with five consecutive distributed measurements show that the self-heterodyne detection decreases also the BFS standard deviation (see Figure 18(b)), indicating a more accurate BFS estimation. In comparison with other techniques, self-heterodyne detection provides the most simple and feasible scheme for sensing range enhancement.

#### 4.5 Enhancement on measurement time

For a conventional BOTDA, it usually takes several minutes to finish a single measurement, which is impractical for dynamic strain sensing. One of the main factors that limit the measurement time is the sweeping of the probe frequency to scan the total BGS. Therefore, several techniques have been applied to solve this problem.

## 4.5.1 Slope-assisted BOTDA

One of the techniques for dynamic sensing is based on partially scanning the BGS [82]. In order to achieve the BGS profile in general, the technique requires a preliminary frequency scan without vibration. The probe scanning frequency is then set at half of the BGS linewidth, as shown in Figure 19(a). This 3 dB point has the steepest slope and widest linear range and is the most sensitive working point for tiny frequency shifts. The BGS is reconstructed according to the measured signal amplitude, and the strain values are obtained by the strain coefficient C<sup>ε</sup> mentioned The State-of-the-Art of Brillouin Distributed Fiber Sensing DOI: http://dx.doi.org/10.5772/intechopen.84684

Figure 19.

ETð Þ¼ t; ν ES<sup>0</sup> gSBS ν<sup>s</sup> ð Þ ; z exp j � 2πνst þ ϕSBS ν<sup>s</sup> f g ½ � ð Þ ; z þ ELO exp ð Þ j � 2πνLOt (11)

(a) BOTDA traces and (b) BFS standard deviation of consecutive measurements with (red) and without

where Es<sup>0</sup> and ELO are the complex amplitude of the probe wave and local oscillator, gSBS and ϕSBS are the SBS gain and phase shift. Hence, the detected current at the PD

PLO

**0**

**Standard deviation error (MHz)**

**2**

**4**

**6**

**8**

**10**

� cos 2π<sup>f</sup> IF<sup>t</sup> <sup>þ</sup> <sup>ϕ</sup><sup>0</sup> � <sup>ϕ</sup>SBS <sup>ν</sup><sup>s</sup> ð Þ ; <sup>z</sup> � �

**0 5 10 15 20**

**Conventional Self-heterodyne**

**Distance (km)**

(12)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ps<sup>0</sup> <sup>1</sup> <sup>þ</sup> gSBS <sup>ν</sup><sup>s</sup> ð Þ ; <sup>z</sup> � �<sup>2</sup>

where ϕ<sup>0</sup> and f IF are the phase and frequency difference between the probe wave and local oscillator, PLO and PS<sup>0</sup> are the optical power of the local oscillator and probe wave, Rc is the PD responsivity. As the detected current is dependent on the power of the local oscillator, the whole mechanism could be regarded as a signal amplification with a strong oscillator. As shown in Figure 18(a), the trace detected by self-heterodyne is cleaner and has an SNR enhancement of 10 dB [30]. Further investigations with five consecutive distributed measurements show that the self-heterodyne detection decreases also the BFS standard deviation (see Figure 18(b)), indicating a more accurate BFS estimation. In comparison with other techniques, self-heterodyne detection provides the most simple and feasible scheme for sensing range enhancement.

For a conventional BOTDA, it usually takes several minutes to finish a single measurement, which is impractical for dynamic strain sensing. One of the main factors that limit the measurement time is the sweeping of the probe frequency to scan the total BGS. Therefore, several techniques have been applied to solve this

One of the techniques for dynamic sensing is based on partially scanning the BGS [82]. In order to achieve the BGS profile in general, the technique requires a preliminary frequency scan without vibration. The probe scanning frequency is then set at half of the BGS linewidth, as shown in Figure 19(a). This 3 dB point has the steepest slope and widest linear range and is the most sensitive working point for tiny frequency shifts. The BGS is reconstructed according to the measured signal amplitude, and the strain values are obtained by the strain coefficient C<sup>ε</sup> mentioned

can be written as:

**-0.2 0.0 0.2 0.4 0.6 0.8 1.0**

**Relative amplitude**

Figure 18.

 **(a.u.)**

problem.

76

4.5.1 Slope-assisted BOTDA

IcðÞ¼ t RcPT ¼ 2RC

(black) self-heterodyne detection [30].

q

**-5 0 5 10 15 20 25**

**Conventional Self-heterodyne (a) (b)**

Fiber Optic Sensing - Principle, Measurement and Applications

**Distance (km)**

4.5 Enhancement on measurement time

(a) Schematic explanation of the dynamic sensing with the working point set at the half value of the peak Brillouin gain and (b) dynamic strain measurement based on this technique [82].

in Section 3.1. In Figure 19(b), the proposed technique is demonstrated with a perturbation frequency of 12.3 Hz. Obviously, this technique can only detect small temperature or strain distributions and is not applicable for a BFS nonuniformity that exceeds half of the BGS linewidth.

## 4.5.2 Sweep-free multi-tones

Another idea to speed up the measurement time is to utilize multitone pumps [66]. As shown in Figure 20(a), multiple pumps with multiple frequencies are launched into the fiber aligning at different frequency detunings of the BGSs. Since the multi-SBS interactions happen simultaneously, the necessary frequency sweeping can be done in a single shot. Thus, the total measurement time including extra averaging measurements for SNR improvement lasts only several seconds.

However, there are still trade-offs and limitations in this technique. Undesired FWM occurs when the pump pulses are simultaneously launched. This can be solved by unequal spacing or a sequential launch of the pumps. The total number of pump tones, which determines the accuracy of the BGS reconstruction, is restricted by the intertone spacing Δf and the total tone span ftotal. The intertone spacing is usually larger than the total BGS width so as to avoid BGS overlaps. The total tone span should not exceed the BFS so that no pump lines are within the BGS of other pumps.

Recently, a new improved method has been proposed to avoid these limitations by utilizing digital optical frequency combs (DOFC) as probe signals [83]. Since the probe wave power in the BOTDA system is usually low, multiple-probe waves suffer less from FWM than multiple pumps. The DOFC owns narrow frequency spacing, wide flat top, and total bandwidth, as depicted in Figure 20(b). According

Figure 20.

(a) Schematic explanation of the principle of the sweep-free multitone BOTDA [81]; (b) spectrum of DOFC [83].

"QUANOMET"). Jaffar Emad Kadum would like to acknowledge the financial support of Iraqi Ministry of Oil/State Company for Oil Projects (SCOP).

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

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

Author details

79

Braunschweig, Germany

Cheng Feng\*, Jaffar Emad Kadum and Thomas Schneider

\*Address all correspondence to: cheng.feng@ihf.tu-bs.de

provided the original work is properly cited.

Institute for High Frequency Technology, Technical University of Braunschweig,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Figure 21.

The linear fitting of (a) temperature and (b) strain measurement with conventional BOTDA and DOFC-BOTDA [83].

to the frequency difference between each line and the pump frequency, the pump pulse shapes the amplitude of each spectral comb line via the SBS interaction simultaneously. The total measurement time, including the necessary 100 acquisitions for the SNR improvement, results in only 10 ms [83]. Furthermore, the experimental results in Figure 21 confirm its equivalence to a conventional BOTDA regarding the temperature and strain measurement. However, the main disadvantage of this technique is the special requirement of the DOFC, which is generally not so simple to achieve.

## 5. Summary

In this chapter, the basics of SBS and its application for distributed sensing have been reviewed. The overview has started with an introduction of SBS together with its physical origin and applications due to inherent, striking advantages in a variety of fields such as slow light, optical and microwave photonic filters, and many more. Among all these exciting applications, distributed temperature and strain sensing is one of the most prominent.

The enhanced SNR and the moderate resolution are the superiority of distributed Brillouin sensors to the traditional distributed and point sensors in long-range sensing. However, conventional BOTDA sensors are limited by MI and NLE. The origins, as well as methods for the mitigation of MI and NLE, have been presented and discussed in detail. Thus, with these new methods, much longer sensing ranges became possible.

Besides the sensing range, methods to enhance the spatial resolution and the speed of the measurement have also been reviewed and discussed. Nowadays, distributed Brillouin fiber sensors can have a resolution in the centimeter range, or even below and act like thousands or millions of point sensors. At the same time, novel ideas such as multi-tone pumps have successfully shortened the measurement time in distributed SBS sensors from several minutes down to 10 ms. Due to the fruitful proof-of-concept results, some of the state-of-the-art techniques discussed in this chapter have already been applied in some BOTDA prototypes.

## Acknowledgements

Cheng Feng wishes to acknowledge the financial support from German Research Foundation (DFG SCHN 716/13-1) and Niedersächsisches Vorab (NL—4 Project

"QUANOMET"). Jaffar Emad Kadum would like to acknowledge the financial support of Iraqi Ministry of Oil/State Company for Oil Projects (SCOP).
