1 Introduction

Brillouin-scattering-based sensors have attracted much interest in recent years owing to their potential for distributed temperature or strain monitoring [1–4]. The advantage of these sensors lies in the fact that they can measure strain or temperature variations in fibers with a long measurement range up to 100 km and a decent spatial resolution of submeter [1–3]. The principle of Brillouin scattering for sensing applications resides in the temperature and strain dependence of the Brillouin frequency shift which can be measured using spontaneous or stimulated scattering. In particular, stimulated Brillouin scattering (SBS)-based systems have an advantage of relatively stronger signal over the spontaneous scattering-based sensors but require the counter-propagation of light waves through the fiber, since SBS amplification is possible only in the backward direction. In particular, Brillouin optical correlation domain analysis (BOCDA) has recently gained much interest due to its unique advantages such as random access of the sensing position, high-speed operation, and high spatial resolution, however, has a shortcoming of limited measurement range coming from the periodic nature of correlation position [4–8]. In the BOCDA, modulation parameters (amplitude and frequency) of a light source are chosen such that only a single correlation peak lies within the sensing fiber, so there is a trade-off between the spatial resolution and the measurement range.

In order to solve this trade-off problem, many approaches such as temporal gating scheme [9–11], double modulations [12, 13], double lock-in amplifiers [14], pulse correlation method [15], sensing fiber structure with different kinds of fiber [16, 17], and gain/loss measurement [18] have been proposed. In the temporal gating system, sinusoidal frequency-modulated pump and probe waves are additionally gated temporally to select only one localized peak among multiple correlation peaks, so that multiple correlation peaks can be utilized [9–11]. Consequently, the measurement range can be elongated by number of correlation peaks. In double-modulation scheme, two frequency modulations are simultaneously applied to the laser source, and thus, two different (denser/coarser) periodic correlation peaks are generated. By smaller frequency modulation, only one wanted correlation peak generated by higher frequency modulation is selected and unwanted correlation peaks are suppressed; therefore, the measurement range can be enlarged [12, 13]. In double-lock-in-amplifiers-based scheme [14], the lock-in detection is applied to both probe and pump waves at different lock-in frequencies to effectively remove the backward reflection of pump waves from the probe signal, and a single-sideband modulator is additionally used to suppress the other sideband component in the probe wave, which can make wider frequency modulation possible to enlarge the measurement range. In pulse correlation method [15], temporal gating based on the simultaneous pulse modulation of pump and probe waves is applied to permit correlation peaks to be synthesized only where pump and probe pulses are overlapped. In sensing fiber structure with different kinds of fibers [16, 17], we have demonstrated expansion of measurement range in BOCDA by cascading different fibers with different Brillouin frequency shifts. In this scheme, multiple correlation peaks within the FUT were used and each correlation peak could be discriminated due to different Brillouin frequency shifts. In [18], we demonstrated twice enlargement of measurement range by using a bidirectional measurement which utilizes two correlation peaks. In that scheme, the Brillouin gain and loss spectra of two correlation peaks are measured and analyzed by using an optical attenuator in the middle sensing fiber and two lock-in amplifiers to separate two correlation peaks, and also, beat lock-in detection was applied to measure the Brillouin gain and loss spectra simultaneously and independently. However, it is complicated due to the use of two intensity modulators and two lock-in amplifiers.

In this paper, we propose a novel method for the range enlargement of BOCDA by inducing selective SBS in two neighboring correlation positions while maintaining the spatial resolution. In the proposed scheme, the modulation parameters are controlled to allow two correlation positions within the sensing section, but SBS occurs alternatively at only one position under appropriate attenuation value and optical path combinations, thus enabling the measurement range to be twice. Compared with the previous work [18], this is simplified and cost-effective system with less optical components. In the experiment, we successfully achieved a 146-m measurement range (twice of ordinary range) with 10-cm spatial resolution.

2 Basic principle

In a typical BOCDA system, a sinusoidal frequency modulation is applied to the pump and probe waves, producing periodical correlation peaks along the test fiber [4]. The periodic correlation position z q is given by:

$$z_{q} = \frac{1}{2}(l + l_{{\text{d}}} ) - \frac{1}{{2f_{{\text{m}}} }}\frac{c}{n}q$$
(1)

where f m is the modulation frequency of the light source, c is the light speed in vacuum, n is the refractive index, q is an integer, l d and l are the lengths of delay fiber and test fiber, respectively [19]. By limiting the length of the fiber under test (FUT) shorter than the interval of the correlation peaks, we can locate only one non-zeroth correlation peak z q along the FUT sandwiched between the circulator and the optical isolator, as shown in Fig. 1; thus, SBS occurs only at this position. This correlation peak, i.e., measuring position, is scanned by sweeping the modulation frequency f m. Therefore, spatial range of measurement is limited to the interval of the correlation peaks d m :

$$d_{m} = \frac{{V_{\text{g}} }}{{2f_{\text{m}} }}$$
(2)

where V g is the group velocity of light [4]. Here, the spatial resolution Δz is given by:

$$\varDelta z = \frac{{V_{\text{g}} \varDelta \nu_{\text{B}} }}{{2\pi f_{\text{m}} \varDelta f}}$$
(3)

where Δv B is the Brillouin gain bandwidth and Δf is the amplitude of frequency modulation of the light source. Accordingly, to increase the measurement range, the modulation frequency f m must be lowered or the modulation amplitude Δf must be increased. However, when f m is lowered, the spatial resolution deteriorates, as shown in Eq. (3). Thus, a large Δf is required to extend the measurement range while maintaining the spatial resolution. However, Δf is practically limited to a few tens of gigahertz in most cases. Another method to extend the measurement range is to utilize multiple correlation peaks with proper time gating which requires accurate control of the time delay between the time gates [9].

Fig. 1
figure 1

Schematic diagram of the typical BOCDA system

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Figure 2 shows the schematic of the proposed BOCDA system, where two correlation peaks are located within the sensing section, but SBS occurs dominantly at only one of the positions under appropriate attenuation value and optical path combinations. We define two correlation peaks in the FUT as z q and z q−1 , which correspond to the measuring positions at each half section of FUT in Fig. 2a. Assuming the maximum intensities of the counter-propagating pump and the probe waves I 1 and I 0, respectively, the Brillouin gain signals (i.e., intensity variation of the probe wave) coming from two correlation positions (z = z q , z q−1 ) under small gain approximation are calculated as follows:

$$I(z_{q} ) = I_{\text{Probe}} (z_{q} ) \cdot [e^{{g_{\text{B}} I_{\text{Pump}} (z_{q} )\varDelta z}} - 1] \approx \alpha I_{0} g_{\text{B}} \varDelta z \cdot I_{1}$$
(4)
$$I(z_{q - 1} ) = I_{\text{Probe}} (z_{q - 1} ) \cdot [e^{{g_{\text{B}} I_{\text{Pump}} (z_{q - 1} )\varDelta z}} - 1] \approx I_{0} g_{\text{B}} \varDelta z \cdot \alpha I_{1}$$
(5)

where g B, Δz, and α are the Brillouin gain coefficient, the effective length of the correlation peak, and the attenuation coefficient, respectively. Therefore, the ratio of these two Brillouin signals at the photodiode is represented by

$$R = \frac{{\alpha I(z_{q - 1} )}}{{I(z_{q} )}} = \alpha$$
(6)
Fig. 2
figure 2

Schematic diagram of the proposed BOCDA system; SBS occurs only at the first (a) or second (b) section of FUT, depending on the status of optical switches

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Since the Brillouin gain spectrum (BGS) obtained in BOCDA is the sum of these two Brillouin signals from two correlation peaks (z = z q , z q−1 ), we can induce SBS dominantly at one specific correlation position in the FUT by controlling α, so selectively acquire a BGS from each correlation peak. To get a BGS from the other correlation peak (z = z q−1 ), we switch the optical paths of pump and probe beams, as shown in Fig. 2b. Then, BGS signal at z = z q−1 is dominant compared to the BGS at z = z q . In this way, we can measure the other half section of FUT; thus, twice enlargement of the measurement range is possible without deteriorating the spatial resolution.

3 Experiments

The proposed experimental setup of the BOCDA is shown in Fig. 3. A FUT consists of 146-m-long single-mode fiber (SMF) with an optical attenuator of 10 dB located at midpoint, which suppresses SBS at one of the two correlation positions. Both ends of the FUT are connected with a circulator. A sinusoidal frequency modulation is applied to the DFB-LD, and two correlation peaks are generated within the FUT. The modulation frequency f m was controlled from 1.4252 to 1.4457 MHz, depending on the measurement position in the FUT, and the modulation amplitude Δf was about 6.17 GHz. Therefore, the spatial resolution and measurement range were estimated to be about 10 cm from Eq. (3) and 73 m from Eq. (2), respectively. The modulated optical signal was divided into two beams by the optical coupler. One output of the coupler was used as the pump after propagating through 10-km delay fiber to control the order of correlation peaks and a high-power erbium-doped fiber amplifier (EDFA). The other output is injected into a single-sideband modulator (SSBM) that was driven by a microwave signal generator, so that a frequency-downshifted wave, serving as the probe light, was generated to be propagated in the opposite direction to the pump along the FUT. Additionally, a polarization switch was inserted after the SSBM for suppressing the polarization dependence of the Brillouin signal [20]. Note that the pump light is chopped by an intensity modulator for lock-in amplifier. The key point of the proposed setup is that an optical attenuator with 10-dB attenuation is inserted at the midpoint of FUT. As a result, SBS effectively occurs at only one of the two correlation positions due to the attenuation of pump wave. We can alternatively select the correlation position by simply changing the pump and probe waves using two optical switches, as shown in Fig. 2a, b. A 125-MHz photoreceiver (New Focus) was used as a detector, and the BGS was obtained through a lock-in amplifier.

Fig. 3
figure 3

Experimental setup for the range enlargement of BOCDA: SSB single-sideband modulator, EOM electro-optic modulator, PSW polarization switch, OSW optical switch, EDFA Er-doped fiber amplifier

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In the experimental setup, the FUT was composed of two 73-m SMFs, which are connected via an optical attenuator, as shown in Fig. 4a. The strain of about 0.18 % was applied to first 80-cm section 36 m away from the optical attenuator, and the strain of about 0.26 % was applied to second 80-cm section 7 m away from the optical attenuator (i.e., midpoint of the FUT). The BGS was measured every 3.5 cm along the FUT, sweeping Δv from 10.3 to 11.3 GHz. The Brillouin frequency (v B) was 10.87 GHz for SMF. Figure 4b, c shows the measured distribution of the BGS and the BFS along the FUT, respectively. In Fig. 4b, the left-hand-side and right-hand-side figures are the measurement results of the distributed BGS in the left and right sections of the FUT, respectively. The applied strain sections were clearly observed. The value of the BFS was about 90 and 130 MHz at the applied strain sections, respectively. This value of the BFS agrees well with the applied strain of 1,800 and 2,600 με, respectively. Thus, it is confirmed that the measurement range is successfully doubled by the proposed scheme with the selective generation of SBS in the FUT.

Fig. 4
figure 4

a Structure of the FUT, b 3D plots of the BGS near the applied strain sections, c measured Brillouin frequency shift as a function of position

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4 Conclusions

In this paper, we have proposed a novel method for the enlargement of the measurement range in the BOCDA system while maintaining the spatial resolution. In the proposed scheme, one of the two correlation peaks within the sensing section is alternatively selected by pump/probe light switching and appropriate pump attenuation. Using the proposed technique, we have successfully demonstrated doubling of measurement range. We expect the proposed scheme to be useful for long-range monitoring of civil structures by BOCDA systems.