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Abstract
We investigate the effect of a bidirectional erbium-doped fiber amplifier on the reduction of the differential phase noise in long-range distributed acoustic sensing by coherent detection. We succeeded in reducing the differential phase noise at the fiber position after the amplifier over a distance of 100 km by adjusting the gain of the bidirectional amplifier considering the amplified spontaneous emission. This will effectively improve the accuracy of strain measurement over long distances.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Distributed acoustic sensing (DAS), which enables highly sensitive vibration detection over a long length of optical fiber cables, has become a powerful tool in various application fields. For example, pipeline monitoring, submarine detection, earthquake observation, seismic and geological survey, and structural exploration are promising application fields of DAS [1–4]. DAS enables strain measurement along optical communication fibers of a kilometer to 100 km at intervals of a few meters. Generally, if the fiber length exceeds 50 km, the backscattering intensity decreases, and strain measurement becomes difficult. To extend the measurement range, noise was evaluated using fiber-enhanced backscattering [5]. Transmission loss increased as a drawback of enhancing backscattering, and the noise was not necessarily reduced at all positions along the fiber. The use of a micromachined fiber makes it possible to extend the sensing range to more than 150 km without large transmission loss [6]. For a phase-sensitive optical time-domain reflectometer (Φ-OTDR), several attempts have been made to recover the intensity of light using erbium-doped fiber amplifications (EDFA) or Raman amplifiers [7–16]. In these studies, the signal-to-noise ratio (SNR) was improved in nonquantitative strain measurements using amplifiers in response to changes in light intensity. This is referred to as direct detection and the accuracy in long-range measurements is inferior to that of coherent detection. For quantitative DAS, the use of distributed Raman amplifiers and remotely pumped EDFAs enables 100-km-range sensing [17,18]. Some amplifiers amplify incident light, and some amplify backscattered light. If the intensity of the incident light is too high, a nonlinear optical effect will cause stimulated Brillouin scattering (SBS), which will adversely affect the measurement quality [19,20]. Fading noise [21], which is a reflection from an unfavorable part of an optical fiber, and noise due to polarization [22] affect the measurement quality. For a coherent OTDR, on the other hand, there is an example of its use in high-precision temperature measurement with a resolution of 0.02 K on a 31 km fiber using a bidirectional EDFA [23].
Various methods have been proposed for DAS to quantitatively measure strain [24]. In some of the methods, dual pulses are introduced to detect changes in the relative phase difference in the fiber position. In other methods, a phase difference is provided with an optical local oscillator. The demonstration by the use of a linear frequency-sweep indicated the low phase noise up to 171 km in an ultra-low loss fiber without inline amplification [25]. The obtained phase difference may be subject to signal processing such as averaging or a wavelet transform for each trace to suppress noise [26]. A denoiser with a convolutional neural network in deep learning makes it possible to reach a measurement range of up to 100 km without the use of backscatter-enhanced fibers or amplification [27].
For seismic detection and traffic monitoring systems using DAS with existing fibers, it is necessary to expand the measurement range. In addition, these applications require a low noise level at each fiber position. The purpose of this study is to reduce the differential phase noise in long-range quantitative strain measurements with coherent detection in a situation using a normal communication cable. Bidirectional EDFAs are easy to implement, and their reliability is proven in the communication industry. We evaluated the performance improvement of the strain measurement over the entire optical fiber length by amplifying the signal light bidirectionally using a combination of two EDFAs.
2. Experiment
Figure 1 illustrates the experimental setup. A commercial DAS system (N5200A, AP Sensing) was used, which was a phase-based DAS with a laser wavelength of 1550 nm. N5200A measures a phase difference caused by a disturbance by the heterodyne coherent detection method. The amplification of the internal EDFA was constant and did not affect the measurements. The relationship between the obtained phase difference dφ and the strain ɛ is denoted as [28]:
Here, λ is the measurement wavelength, n is the group refractive index of the fiber, G is the gauge length used in the DAS system, and ξ is the scale factor considering the photoelastic effect (= 0.78). The obtained DAS data provide the phase difference that occurred within the gauge length. The phase difference is in the range of [-π π], and changes exceeding 2π at the measurement interval cannot be identified. The gauge length is defined differently depending on the measurement principle and affects the accuracy and spatial resolution. It is necessary to select the optimum gauge length in accordance with the characteristics of the strain distribution [29]. Figure 1(a) shows the basic configuration consisting of a 100-km fiber cable and the DAS system without an amplifier. The fiber was a single-mode fiber (AllWave, OFS), having a group refractive index of 1.468 and typical attenuation of less than 0.19 dB/km at 1550 nm. The 100 km fiber was sewn on four bobbins in series. The bobbins were connected to each other using SC/APC connectors. Figure 1(b) shows a diagram in which an EDFA (AMP-FL8013-CB-16, FiberLabs Inc.) is inserted at the 50 km point of the forward path to amplify the intensity of the incident light. The noise figure of EDFAs in use was < 5 dB. A narrowband tunable optical filter (WLTF-NM-S-1550-60/0.3-SM-0.9/1.0-SC/APC, WL Photonics) is also inserted as an optical band-pass filter (OBPF). The full width at half-maximum (FWHM) of the OBPF was 0.3 nm. The light backscattered from one of the 50 km fibers was bypassed using two circulators since the EDFA is equipped with an isolator. The insertion loss of the circulators was ≦ 0.8 dB. Figure 1(c) shows a diagram in which the EDFA was inserted at the 50 km point of the return path to amplify the intensity of backscattered light. Figure 1(d) shows a diagram in which two EDFAs were inserted at the 50 km point so that light traveling in both directions was amplified. This amplification system composed of two EDFAs is called a bidirectional amplifier in this paper.
The phase difference was measured with a gauge length of 5 m and a measurement frequency of 500 Hz. The evaluation was carried out by comparison with the root-mean-square (RMS) value of the change in the differential phase corresponding to the strain rate (hereinafter referred to as differential phase noise). In addition, to confirm the effect of the bidirectional amplifier, strain measurement was demonstrated using an optical fiber stretcher. A 10 Hz sinusoidal signal was applied to the optical fiber stretcher (PZ3-SMF2-APC-E, Optiphase). The loss of the inserted fiber stretcher is 0.5 dB.
Fig. 1. (a) Experimental setup of DAS system without amplifier, (b) and DAS systems with incident amplifier, (c) backscattering amplifier, and (d) bidirectional amplifier.
3. Result and discussion
The recovery of the intensity of backscattered light using amplifiers was confirmed using an OTDR. Figure 2 shows the intensities of the backscattered light for the four setups shown in Fig. 1. The backscattered light power is a relative value based on the 0 km position. The four sets of OTDR data were obtained without an amplifier, with the amplification of 8.0 dB at halfway (50 km) along the forward path, with the amplification of 8.0 dB in the return path, and with the amplification of 17.2 dB in both directions with the bidirectional amplifier. The gain is backscattering power not incident power.
Usually, in a distant position, the power of steady dark current and thermal noise becomes larger than that of the signal light and the shot noise associated therewith. In the case of amplifying the backscattered light, the optical noise due to the amplified spontaneous emission (ASE) affects the resultant signal. The optical noise related to ASE may include both beats with the signal light (signal-spontaneous beat noise) and beats generated between ASEs (spontaneous-spontaneous beat noise). Signal-spontaneous beat noise depends on time (distance), but spontaneous-spontaneous beat noise does not. In the case of amplifying the incident light, the backscattered light of the optical noise related to ASE obstructs the signal light. However, its effect is insignificant compared with the case of amplifying the backscattered light.
Figure 3(a) shows the RMS value of the differential phase noise without an amplifier and with the amplification of 8.0 dB in the forward path. The differential phase noise is calculated as using the moving average in the range of 100 m. All fiber bobbins were placed on a vibration isolation table, but they were slightly affected by environmental noise such as temperature drift. Since each fiber bobbin receives the same degree of environmental noise, the differential phase noise increases with distance. This increase is attributed to the decrease in the signal light intensity. The differential phase noise in the remote 50 km part was successfully reduced by amplifying the incident signal light with an EDFA. Figure 3(b) shows the differential phase noise when the gain of the forward-path EDFA was set to 11.0 dB. The differential phase noise decreased at around 50 km immediately after the EDFA. However, in the range after 75 km, the differential phase noise suddenly increased and saturated. It is considered that high amplification generated self-phase modulation (SPM) in nonlinear optical effects. SPM caused an anomalous dispersion, and the dispersion became greater with increasing distance from amplifiers. It interfered with coherent phase detection. As a result, no phase difference of the signal after 75 km could be obtained. The phenomenon that the differential phase noise in coherent detection suddenly increases can be understood as analogous to the threshold effect in the frequency modulation system of radio communication.
Fig. 3. Differential phase noise without and with the incident amplifier ((a) 8.0 dB and (b) 11.0 dB) at 50 km along the forward path.
Figure 4(a) shows the differential phase noise when the gain of the return-path EDFA was set to 8.0 dB. Although the differential phase noise in the remote 50 km part was reduced also in this case, the degree of reduction was smaller than in the case of amplification of the incident light. It is due to ASE generation in the DAS system. As demonstrated in Fig. 4(b), the gain of EDFA was increased to 11.0 dB. The differential phase noise in the remote 50 km part was suppressed, but the noise in the local 50 km part increased.
Fig. 4. Differential phase noise with and without the backscattering amplifier ((a)8.0 dB, (b)11.7 dB) at 50 km of the return path.
Figure 5 shows the differential phase noise with the bidirectional amplifier for the gain of 17.2 dB. It was confirmed that the differential phase noise in the remote 50 km part was significantly reduced compared with the case without an amplifier. We demonstrated the dependence of the differential phase noise on the gain of the return-path EDFA when the gain of the forward-path EDFA was fixed at 8.0 dB. At 17.2 dB, the noise reduction for the remote 50 km part by amplification and the increase in the noise for the local 50 km part owing to ASE were balanced.
Next, a fiber stretcher was used to investigate the effects of the bidirectional amplifiers on the practical measurements. The fiber stretcher was inserted at 75 km and driven by a sinusoidal voltage at 10 Hz. The applied strain was +/- 0.047 micro strain. Figure 6 shows the temporal waveform of the strain with and without the bidirectional amplifier. The waveforms were filtered with a high pass filter with 4 Hz cutoff.
Fig. 6. Time-varying phase modulation by fiber stretcher at 75 km with and without the bidirectional amplifier.
4. Conclusions
We tried to improve the long-range performance of the coherent DAS system by inserting EDFAs in the middle of the sensing fiber. We investigated the differential phase noise and successfully suppressed it at a distant fiber position after the bidirectional amplifier by adjusting its gain. From these results, we can expect that longer-range measurements beyond 100 km will be possible by introducing multiple bidirectional amplifiers. For such superlong-range measurements, it is necessary to adjust the gain of bidirectional amplifiers and the intervals between these amplifiers to suppress the noise generated by ASE and nonlinear optical effects. They should be determined by the optical fiber loss and the specifications of EDFA and OBPF.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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