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The pump wave transfers intense energy to the probe wave, called Brillouin gain; in contrast, the probe wave absorbs energy from the pump wave, called Brillouin loss. Brillouin gain [ 8 ] or that of the pump wave at PD2 i. Recently, a scheme based on a combination of Brillouin gain and loss was newly proposed to enhance the signal to noise ratio SNR of the BGS measurement [ 34 ].

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It can be realized by periodic switching of the pump and probe wave and detected at either PD1 or PD2. Besides, the simultaneous detection of PD1 and PD2 followed by a subtraction may work equally. The laser unit shown in Fig. Second, one laser is divided into two parts. One part is amplified by an EDFA working as pump wave; the second part serving as probe wave is modulated by an electro-optic intensity modulator EOM to generate two sidebands working as probe wave[ 25 ]. An optical filter is inserted before launching into the FUT or laid after PD1 so as to cut off the influence of the frequency-upshifted sideband.

Third, the second part can be also modulated by a single-sideband modulator [ 35 ] to get well-suppressed frequency-downshifted sideband directly serving as probe wave. There are also three schemes, depicted in Fig.

The first and simple scheme is related to the first scheme of the laser unit. The second and third schemes in Fig. For instance, a high-cost vector network analyzer provides a frequency-tuned RF signal to modulators and simultaneously detect the Brillouin signal [ 36 ]. It is notable that the use of LIA for detection unit requires an intensity-chopping of the pump wave by an additional EOM [ 10 ] or periodic switching of upshifted or downshifted sideband at the SSBM [ 34 ], which is advantageous for characterization of very weak BGS or a short-length FUT due to its high SNR and accuracy[ 12 , 34 , 37 ].

High accuracy pump-probe-based BGS characterization. The optical lights after a circulator include the following components:. Thanks to the lock-in detection, the component of P probe 0 is effectively cut off by the LIA since it has no relationship with the chopped pump power given by. If R denotes the reflectivity of pump power arising from both Rayleigh scattering and some reflection points, the reflected pump power can be expressed:. From it, one can deduce the signal-to-noise ratio SNR :.

In other words, an increase of probe power can drastically enhance SNR and also improve the system accuracy. As an example, Figure 7 b shows a characterized fundamental-order or higher-order resonance BGS in a w -shaped high-delta fiber with fluorine inner cladding F-HDF [ 37 ]. The measurement system has a high accuracy of 0. As mentioned above see Fig. Early in [ 38 ], it was first observed that the spectrum broadening and hole burning occurs in a SBS generator i.

The reason was thought as the waveguide interaction among different angular components of the pump and backscattered Stokes signals. Recently, it was found that pump depletion of pump-probe-based system configuration could induce a significant measurement error of the local Brillouin frequency shift in the far end of the probe Stokes wave [ 41 ].

Considering the steady-state condition and neglecting the transmission loss of the fiber, the coupling equations of Eq. Figure 8 a illustrates the normalized BGS at different positions of an arbitrary-length SMF, which are numerically calculated according to Eqs. We define the BGS saturation as the critical condition of spectral hole burning phenomenon. Consequently, the analytical solutions to Eqs. The critical condition of the spectral hole burning phenomenon can be theoretically expressed by:.

By numerically solving Eqs. It means that the pump depletion gets worse since much longer segments in the fiber suffer spectral hole burning.

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It is notable to address that the physical nature of the critical powers is essentially the same as that of the critical position because they can be also deduced by the contour i. The critical positions divide the fiber into two parts with and without the spectral hole burning phenomenon.

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Measured BGS in the middle c or at the end d of a m-long fiber. Figure 8 c and 8 d illustrates the measured BGS under different pump power for two different positions in the middle and at the far end, respectively of a m-long dispersion compensated fiber DCF.

Optical Fibers and RF: A Natural Combination

The probe power is fixed at 9. At the far end, the BGS [see Fig. While in the middle, the experimental result [see Fig.

The hollow in the BGS may introduce great errors to pump-probe-based Brillouin distributed sensors since it disables the peak-searching of the Brillouin frequency shift. The pump power leading to the BGS saturation is approximately characterized as the critical pump power for instance, It illustrates that the critical pump powers approximately measured for several probe powers open symbols have very similar trend as the theoretical analysis curves.

It is clear that the position of bigger z requires greater critical powers.

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This is because the pump depletion is weaker and the distortion of the BGS is less serious if the position is much closer to the fiber far end. Critical powers for two different positions in a m-long DCF. Solid and dashed curves, simulation; open symbols: experiment. The first report of Brillouin based distributed optical fiber sensors [ 43 ] was based on the same principle as that of optical time domain reflectometry OTDR or Raman based OTDR ROTDR technique as a non-destructive attenuation measurement technique for optical fibers.

Figure 10 illustrates the characterized strain or temperature dependence in a standard SMF under the experimental setup of Fig. Note that Eq. Naturally, the GeO 2 doping induces the reduction of the longitudinal acoustic velocity in GeO 2 -doped core V l 1 with respect to that in pure-silica cladding V l 2 i.

It provides a waveguide of longitudinal acoustic modes in the core region as schematically depicted in Fig. A recent study further proves that the acoustic modes sense better confinement than the optical modes in a GeO 2 -doped optical fiber [ 13 ]. The enhanced confinement results in the existence of multiple L 0 l acoustic modes in a single-mode optical fiber SMF [ 8 ], and also leads to that the first-order L 01 acoustic mode among all L 0 l modes is best confined in the core and even better confined than the fundamental LP 01 optical mode [ 13 ].

Furthermore, the enhanced confinement shows that the effective acoustic velocity of L 01 mode V a is close to V l 1 i. The normalized strain coefficients include three respective factors[ 49 ]:. Each three parts in the right sides of Eq. Strict experimental characterization on a series of optical fibers with different GeO 2 concentration is depicted in Fig. It specifies the previously reported values [ 25 , 50 , 51 ].

The normalized strain and temperature coefficients defined in Eq. The theoretical study further indicates that both the strain and temperature dependences in Eq. Besides the sensing of measurands see Eq. Two different mapping ways, as schematically illustrated in Fig. One is to repeat the localized BGS in scanned positions along the FUT; the other is to repeat the Brillouin interaction under different frequency offset. There are three different mapping or position-interrogation techniques, including time domain [ 33 , 52 - 56 ], frequency domain [ 57 , 58 ], and correlation domain [ 59 - 61 ].

Fiber-Optic Characteristics

Regarding the injection ways of optical fields, there are two opposite groups, i. Comparably, the analysis has much higher SNR than the reflectometry. Note that there is an additional method between analysis and reflectometry, called one-end analysis [ 55 , 62 , 63 ]. Its only difference from the traditional two-end analysis is the one-end injection and its SBS process occurs between the forward pump and the backward probe wave that is reflected at the far end of FUT.

For two-end or one-end analysis, named Brillouin optical time domain analysis BOTDA [ 33 , 52 , 54 ], one of pump and probe waves is pulsed in time and the other is continuous wave CW. Subsequently, they are successively interacted along the FUT during the time-of-flight of the pulsed wave. The required modification is to insert an optical pulse generator for example, an electro-optic intensity modulator driven by an electric pulse generator.

Both of them originate from the so-called synthesis of optical coherence function SOCF [ 65 , 66 ]. Thanks to the current-frequency transferring effect of DFB-LD [ 67 ], the optical frequencies of the light sources are simultaneously modulated also in a sinusoidal function.

Only at some particular locations called correlation peaks , the frequency offset is maintained as the constant frequency offset because of the in-phase condition so that the local SBS interaction or the beating of the local Stokes and oscillator is constructive. At other locations rather than correlation peaks, the frequency offset is always vibrating with time, which leads to a broadened and destructive SBS or SpBS. Consequently, the maximum measurement length or sensing range, L CD is decided by the distance between two neighboring correlation peaks [ 59 ]:.

Because of the difference of the physical pictures between time domain and correlation domain, their sensing performance is different. The detailed difference of other performances will be described in Section 4 and Section 5. As explained in Section 3 , all Brillouin based distributed optical fiber sensors interrogate the Brillouin frequency shift so as to deduce strain and temperature information based on Eq.

It naturally gives a physical challenge, i. Another way is to use two distributed sensing systems with two individual fibers [ 72 - 75 ]: one Raman-based or Rayleigh-based sensor is to monitor the temperature; the other Brillouin sensor to monitor the temperature and strain.

After distributed sensing measurements, the strain and temperature responses can be calculated by mathematics. However, the above practices make the entire sensing system complicated and the calculated responses of strain and temperate change with service time. Practical applications of Brillouin based distributed optical fiber sensors require a method to effectively discriminate them by use of two intrinsic parameters denoted by y 1 and y 2 in one sensing fiber.