4.4.1 Multi-frequency pump-probe interaction

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

(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

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

**(b)**

**Power (a.u. in dB)**

(a) Pulse shape for pre-excitation technique; (b) BGS of a fiber section with strain interrogated by the PPP only

**10.3 10.5 10.7 10.9 11.1 11.3**

**Pre-pump pulse only High power pulse only Total pulse**

**Brillouin Frequency shift (GHz)**

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.

**0 2 4 6 8 10 12 14 16**

**2**

**1**

**Time (ns)**

(red), high power pulse only (black), and total pulse (blue) [80].

and manually stretched to have nonuniform strains [79].

Fiber Optic Sensing - Principle, Measurement and Applications

4.3.2 Pre-excitation

**Pump power (dBm)**

Figure 16.

74

**(a)**

Figure 15.

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].
