We experimentally demonstrate a linearly configured Brillouin optical correlation domain analysis (BOCDA) system enhanced by a differential measurement scheme. On-off control of the pump phase modulation with an intentional loss at the end of a fiber under test is applied for the acquisition of a Brillouin gain spectrum. This application leads to a four-fold enhancement of the spatial resolution and doubling of the measurement range in comparison with the former system under the same modulation parameters.
© 2014 Optical Society of America
Brillouin scattering based distributed fiber optic sensors for temperature and strain measurement have been widely studied as a tool for structural health monitoring, since they use the entire fiber as the sensing part, unlike the conventional sensors [1, 2]. There are several representative methods of the Brillouin sensors including Brillouin optical time domain analysis (BOTDA), Brillouin optical time domain reflectometry (BOTDR), and Brillouin optical correlation domain analysis (BOCDA) [3–5]. Pulse-based time-domain Brillouin sensors (BOTDR and BOTDA) share a common advantage of long measurement range up to 100 km: however, due to the nature of pulse-based operation, they generally show a limited spatial resolution (~1 m) and long measurement time (~several minutes). Meanwhile, the BOCDA system based on a synthesis of the optical coherence function can provide advantages of a higher spatial resolution (~mm order) and a higher sampling rate (~kHz) with random access of the sensing position in comparison with BOTDR and BOTDA. However, this comes at the cost of a limited measurement range (~several hundreds of meters) and complexity in the system configuration [5–7].
In the operation of BOCDA, a closed-loop configuration is commonly adopted for the counter-propagation of the pump and probe waves to induce stimulated Brillouin scattering (SBS), even though a single-end access to a fiber under test (FUT) is more desirable due to more flexibility in the deployment of the FUT. In 2008, a BOCDA system in a linear configuration was proposed by applying beat lock-in detection together with a narrowband optical filter . However, the beat lock-in detection requires two intensity modulators and an additional function generator with rather complicated signal processing for the acquisition of the Brillouin gain spectrum (BGS). Recently, enlargement of the measurement range in the linearly-configured BOCDA was demonstrated by applying a polarization maintaining fiber (PMF) as FUT and two polarization beam splitters for the separation of the pump and the probe . In 2010 a differential measurement scheme was applied to an ordinary BOCDA system where the BGS is acquired by on-off control of the phase modulation of the pump for the improvement of the spatial resolution .
In this paper, we newly apply the differential measurement scheme with an intentional loss at the end of FUT to the linearly-configured BOCDA system, resulting in a four-fold enhancement of the spatial resolution and double enlargement of the measurement range with a FUT of the conventional single-mode fiber (SMF) and a simpler configuration than those of previous works. The adoption of the differential measurement in our work additionally provides a new and important role to the linearly configured system that was overlooked in Ref.10: Conventional intensity-chop-based lock-in detection doesn’t work in the linear configuration due to large noise offset induced by the intensity variation of remaining pump wave even if suppressing the pump component with FBG. This problem is also described in Ref.8, where they apply the beat lock-in detection for solving this problem that requires an additional EOM and two function generators operated at different frequencies. Meanwhile, differential measurement scheme doesn’t suffer from the large noise offset since phase modulation doesn’t directly induce intensity variation of the pump.
In ordinary BOCDA systems, strong SBS between the pump and the probe takes place at a correlation peak that periodically appears along the FUT by sinusoidal frequency modulation of a light source. The spatial resolution Δz and the measurement range L are determined by the following equations :
On the contrary, the pump wave is phase-modulated at a fixed frequency (Ω) in the differential measurement scheme, and the phase modulation is periodically turned on and off (at fL) to construct the BGS by the difference between them . Figure 1 schematically shows the operation principle of the differential measurement.
The differential measurement provides two prominent advantages over intensity-chop-based lock-in detection: One is enhancement of the spatial resolution, which originates from the effective removal of the noise structure as already demonstrated in a previous work . The other point is that the system does not suffer from the noise coming from direct reflection of the pump, which generates a considerable amount of background noise in the lock-in detection based on the intensity-chop of the pump. This feature plays a key role in simplifying the structure of the currently proposed system in addition to the enhancement of the spatial resolution.
Figure 2 depicts the schematics of three different types of linear configurations of the BOCDA system for comparison. In the system based on the beat lock-in detection, two intensity modulators (IM’s) are used for the pump and the probe, respectively, as shown in Fig. 2(a), and the measurement range is reduced to half of the ordinary BOCDA system under the same modulation parameters. The decrease of the range is to avoid the ambiguity induced by simultaneous generation of two correlation peaks by the outgoing and the incoming pump waves in the FUT. In the BOCDA system based on a PMF, the reflection of the pump is removed by a PBS and an isolator located at the end of the FUT while the probe is fully reflected into an orthogonl polarization. This feature can enlarge the measurement range into that of an ordinary BOCDA system (L) by preventing the generation of one of the correlation peaks induced by the incoming pump wave.
In the proposed differential measurement, a phase modulator (PM) is applied to the pump, and a SMF with a cleaved end is used as a FUT. Assume that the length of FUT (L) is the interval between correlation positions. From Fig. 2(c), the detected output becomesFig. 2(a), we cannot discriminate the Brillouin gain signal from two correlation positions. For this reason, the measurement range becomes the half of the ordinary BOCDA system. However, for the case of R<<1 (Fresnel reflection 4%), the detected output signal can be expressed as11].
Therefore, double enlargement of the measurement range (L) is easily achieved in comparison to the case of the beat lock-in detection , without using any polarization-maintaining component such as PMF . Considering the four-fold possible enhancement in the spatial resolution , one can expect an overall improvement of about 8 times by differential measurement in terms of the effective number of points (range/resolution).
The experimental setup is shown in Fig. 3(a). As a light source, a distributed feedback laser diode (DFB-LD) was used, and a sinusoidal frequency modulation was applied to both the pump and probe waves, producing a correlation peak along a fiber under test (FUT). Here, the modulation amplitude Δf was about 5.18 GHz and the modulation frequency fm was set to about 1 MHz. These modulation parameters lead to a spatial resolution of about 20 cm and a measurement range of 100 m according to the above equations.
The output of the DFB-LD was divided into two beams by a 3-dB coupler. One of the beams was injected into the single side band modulator (SSBM) driven by the microwave sweep synthesizer (MSS) to generate and sweep the probe wave around the Brillouin frequency (10.3~11.3 GHz), by suppressing the carrier and the higher frequency component of the two first lower sidebands. And a polarization switch (PSW) was used for suppressing the polarization dependence of the stimulated Brillouin scattering (SBS). The other beam (pump wave) was phase-modulated at a fixed frequency (Ω) in the differential measurement scheme, and the phase modulation is periodically turned on and off to construct the BGS by the difference between them. Here, the on/off frequency fL was set to ~250 kHz. This pump light was launched into a 10 km delay fiber to control the order of the correlation peaks. The probe and the pump wave were combined by a 3-dB coupler and amplified by an Er-doped fiber amplifier (EDFA). The output of EDFA (~27 dBm) was launched into one end of the FUT through a circulator. The other end of the FUT was cleaved perpendicularly for the outgoing wave to be weakly reflected (~4%) due to Fresnel reflection. Thus, SBS was induced dominantly by the outgoing pump and the contribution from the incoming pump to the local BGS was negligible, as described in the working principle.
The optical output of the circulator was attenuated by a variable optical attenuator (VOA) and the remaining pump wave was filtered out by a fiber Bragg grating (FBG). An inset of Fig. 3(a) shows that the pump wave could be significantly suppressed by introduction of the FBG. Finally, the optical signal was received by a 125 MHz photo detector (PD), and processed by a lock-in amplifier (LIA) to obtain the BGS. The FUT was composed of a 100 m single-mode fiber (SMF), in which about 0.2% strain was applied to the three portions (at 20 m, 50 m, and 75 m) as shown in Fig. 3(b). The Brillouin gain spectrum (BGS) at the correlation position within the FUT was measured by sweeping the frequency ν of the microwave applied to the SSBM in the vicinity of the Brillouin frequency νB (~10.8 GHz). The sensing position was scanned with a 5 cm step by changing the fm from 1.02 MHz to 1.04 MHz for the distributed measurement.
The measurement results of the strain distribution of the FUT are shown in Fig. 4, where we can clearly observe the shifts of the Brillouin frequency at three strain-applied portions. The shifts of the Brillouin frequency were about 110 MHz, 100 MHz and 110 MHz, which agree well with the applied strain of 2.2 mε, 2.0 mε and 2.2 mε.
Figure 5(a) shows the 3D plots of the measurement results, where one can clearly see the narrower Brillouin gain spectra compared to those of ordinary BOCDA by using the differential measurement scheme . Figures 5(b)-(d) are zoomed views of distributed BGS at each section and they show the BGS at 75 m, 50 m, and 20 m, respectively. In Fig. 5(g), we can discriminate the only 5 cm strain applied segment, which shows that the spatial resolution of the experimental setup is less than 5 cm. [Note that the calculated spatial resolution of the system is about 20 cm from the Eq. (1).]
Next, to confirm the effect of the reflectivity of the end of the FUT, we measured a BGS by varying the reflectivity of the FUT end (R1, 60%; R2, 30%, R3, 20%). Figure 6 compares the measured BGS, where one can see the weak reflectivity of the FUT end can suppress BGS from the other correlation peak. This result matches well with our explanation given at Section 2. Therefore, we can solve the measurement range shortening to a half of the nominal value of BOCDA in linear configuration  by using Fresnel reflected end of the FUT.
In summary, we have successfully carried out a proof-of-concept demonstration of our proposed linear configuration system based on the differential measurement detection. The nominal spatial resolution and the measurement range are 20 cm and 50 m, respectively. However, the measured spatial resolution and the measurement range are 5 cm and 100 m, thanks to the phase modulation based differential measurement detection and the Fresnel reflection from the cleaved end of the FUT.
We have demonstrated a novel linear configured BOCDA system based on differential measurement scheme. In addition to the basic advantage of the single access to the FUT, four-fold enhancement of the spatial resolution and the double measurement range are obtained with simpler configuration. We believe that the proposed BOCDA system can provide more flexibility in the deployment of the FUT for large structure health monitoring application.
This work was partially supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology (2011-0016056).
References and links
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