1. Introduction

Rayleigh scattering is widely exploited in distributed sensing with optical fibers in the time domain [1–4]. A weak backscattered signal is generated from the propagation of a short duration pulse along the fiber. It gives an indication of the intensity of the pulse at every fiber position, the pulse duration limiting the spatial resolution of the measurement. In Optical Time Domain Reflectometry (OTDR), the intensity-drop along the fiber and the echoes generated at discontinuities such as components, connectors and splices allow monitoring a fiber link for losses [5]. Distributed sensors based on the scattering of a coherent pulse [6] are capable of identifying disturbances such as vibrations [6,7], perimeter intrusion [8,9] and seismic events [10,11]. Such disturbances affect locally the relative phases of the backscattered light from different points within the pulse, and consequently their interference. Phase-OTDR systems are widely used as distributed acoustic sensors [7–19].

The backscattered light in a standard telecom fiber is very weak. Nevertheless, under strong optical amplification, Rayleigh backscattering has been used as the feedback needed for laser action [20–22]. A fiber laser based on the feedback from randomly distributed scattering centers has ill-defined modes and is generally described as a random fiber laser [20–22]. Random lasers based on Raman [23,24], Brillouin [25,26] and Rayleigh scattering in fibers [20–22] have been reported. They often incorporate amplifiers such as Erbium-doped fibers (EDFs), semiconductor optical amplifiers (SOAs) or both, to provide or complement the necessary gain needed for laser action [20–22,27–31].

The use of the distributed sensor fiber as part of a laser cavity has recently been demonstrated for Raman [32] and Brillouin [33] sensor systems. Since the sensor fiber is used in a reflection mode, i.e, operating as an open-ended fiber mirror, all instrumentation can be gathered on one side of the set-up and the sensor fiber deployed where needed. To the best of our knowledge, the operation of a phase-OTDR in a lasing mode has not been studied, where the backscattered light is not simply detected, but is recirculated in a laser cavity with synchronous amplification. A lasing distributed fiber sensor based on Rayleigh scattering has some interesting physics and it could be advantageous because of the potential improvement in signal-to-noise ratio (SNR) that could be expected from a strong laser output, when compared with the conventional configuration. In this letter, we describe the implementation of a lasing phase-OTDR based on Rayleigh scattering. Short pulse amplification takes place by gating a SOA in synchronism with the return of the backscattered light from a particular chosen section of the fiber being probed. We show that the linewidth of the laser self-narrows, allowing for sensor operation as a Φ-OTDR without the injection of a high-coherence laser source. In this proof-of-principle demonstration, we illustrate the use of the laser as a distributed vibration sensor with good SNR.

2. Experimental setup

The schematic of the lasing Φ-OTDR is shown in Fig. 1, in a hybrid configuration with a SOA and EDF as gain media [27]. The random optical feedback is produced by the Rayleigh backscattering from a 63-m piece of dispersion shifted fiber (DSF). Short pulses are generated by an unidirectional amplifying loop mirror containing a SOA, an isolator (Iso-1) and a 125-m spool of passive standard single mode fiber (SMF). Additional two-way optical amplification between the fiber loop and the distributed feedback sensing fiber is provided for by a piece of Erbium-doped fiber (EDF). The fiber loop acts as a time-gated amplifying mirror, the EDF provides two-way continuous wave (CW) gain and the DSF provides distributed feedback. The EDF is CW-pumped by a 980-nm laser diode. Isolator-2 ensures that undesired reflections after the sensing fiber are eliminated. The SOA was used for modulation, blocking light circulating in the cavity except when driven by 300-mA square current pulses (13-ns, 15-V @50Ω). Laser pulses were obtained by synchronizing the SOA current driver to the returning amplified Rayleigh backscattered light from a selected short section of the DSF. By tuning the SOA pulse rate in the range 1.050-MHz to 0.650-MHz, random feedback lasing was achieved from selected 2.6-m sections of the 63-m DSF. Adjustment of the repetition rate of the current pulses allows choosing the roundtrip time of the cavity and addressing any particular section of the fiber for optical feedback [27]. For the sensing proof-of-principle demonstration, a piezoelectric (PZT) element was attached to the DSF and driven at 5-kHz, imposing a vibration to the fiber at this frequency. Measurements were performed on an optical spectrum analyzer (OSA), oscilloscope (OSC), lock-in amplifier and electrical spectrum analyzer (ESA).

 

Fig. 1. Experimental setup of the lasing Φ-OTDR. The vibration from the PZT can be detected at any lasing section of the DSF. DC: Direct Current; CC: Current Controller; TC: Temperature Controller; SOA: Semiconductor Optical Amplifier; ISO: Isolator; SMF: Single Mode Fiber; OTDR: Optical Time-Domain Reflectometry; WDM; Wavelength Division Multiplex; PC: Polarization Controller; EDF: Erbium Doped Fiber; DSF: Dispersion Shifted Fiber; PZT: Piezoelectric; FC/APC: Angled Physical Contact Fiber Connector; OSA: Optical Spectrum Analyzer; ESA: Electrical Spectrum Analyzer; Lock-In: Lock-In Amplifier; OSC: Oscilloscope; PD: Photodiode Detector; Ref-In: Reference-In.

Download Full Size | PDF

3. Results and discussion

Lasing Φ-OTDR threshold is achieved at a pump power ∼205-mW into the EDF and properly adjusting the polarization controller (PC). The experimental results are shown in Fig. 2, where all measurements were performed on the laser output port of Fig. 1. The temporal behavior of the output is measured with the 125-MHz bandwidth photodiode detector (PD) and 3.5-GHz bandwidth oscilloscope (OSC). Figure 2(a) shows the temporal pulse train, where a uniform intensity can be observed. The repetition-rate of the train optical pulses is generated by the direct modulation of the SOA and matches a measured roundtrip time of 1.43-µs from a selected short section at the end of the DSF. Figure 2(b) shows a 13-ns duration square current pulse applied to the SOA and a 13-ns duration optical pulse detected, showed in Fig. 2(a). The electrical spectrum is measured with 125-MHz bandwidth photodiode and a 20-GHz bandwidth electrical spectrum analyzer (ESA). Figure 2(c) shows a series of tones repeating at 0.7-MHz interval, which are the result of the periodic gating of the lasing Φ-OTDR. The interval of the frequency-comb presented corresponds to the free spectral range (FSR) of the laser and matches the characteristic roundtrip time of the pulses shown in Fig. 2(a). The output optical spectra were analyzed with a 0.03-nm resolution optical spectrum analyzer (OSA). Figure 2(d) displays the optical spectra emitted by the laser just below (200 mW pump power) and well above threshold (365 mW pump power). In the latter case, the laser peak power stands 23-dB above the EDF amplified spontaneous emission (ASE) and the narrow lasing peak is measured at wavelength ∼1557-nm.

 

Fig. 2. Lasing Φ-OTDR experimental results: Laser output OSC temporal pulse train (a). Temporal square electrical pulse injection and optical pulse detection (b). Laser output ESA spectrum (c). Laser output OSA spectrum (d).

Download Full Size | PDF

In order to characterize the system’s operation parameters, OTDR profiles of different addressed section of the fiber were measured via the OTDR port in the loop. Figure 3 shows four OTDR profiles selecting different addressed sections of DSF. These sections are selected according to the repetition-rate of the direct modulation pulses in the SOA. The four examples illustrated in Fig. 3 clearly show the difference between regimes of ASE below threshold (205-mW pump power, top trace) and lasing Φ-OTDR above threshold (365-mW pump power, three bottom traces) for different addressed fiber sections. The smooth top trace of a conventional OTDR develops into a spiky speckle-like trace characteristic of the Φ-OTDR regime when the laser is above threshold (also seen in the zoomed-in Fig. 4(a)). This happens without the use of a narrow linewidth injection source, evidencing the strong spectral self-narrowing experienced by the laser above threshold. In contrast to the experiments reported in [27], here the source of Rayleigh backscattering is a passive fiber, not a rare-earth doped fiber. This excludes the possibility of a standing wave index-grating forming from unpumped ions [34–36]. The several peaks around the lasing peak arise from Φ-OTDR and appear due to the varying coherent superposition of Rayleigh backscattered light. As shown in Fig. 3, the single-ended passive dispersion compensating fiber can be used both to provide feedback for laser action and to perform distributed sensing at various locations. Hence, the laser can be directly used as a fast distributed sensor within the addressed section of the DSF, the laser intensity being interferometrically sensitive to external effects on the fiber, such as vibration or stress/strain, with the resolution given by half the modulation pulse-width as in a conventional Φ-OTDR. One advantage of the present set-up is that instead of a tiny, backscattered signal measurement, as is conventional in an OTDR, here it is the full output of the laser that provides the sensing signal. Hence, no averaging is needed, and a fast response is available.

 

Fig. 3. Lasing Φ-OTDR at different addressed section.

Download Full Size | PDF

 

Fig. 4. Lasing Φ-OTDR performance: Output optical power along a ∼30 m section of the DSF (a). Detection trace for vibrational wave along the full DSF (b). Inset in (b): Lasing Φ-OTDR resolution. Sensor signal at 5-kHz (c). Detection sensitivity for different vibration intensities at 5-kHz (d).

Download Full Size | PDF

Prior to measuring vibrations, in order to characterize the PZT operation parameters, the PZT was placed on an arm of a balanced fiber Mach-Zehnder interferometer (MZI) to produce frequency discrimination. The modulation frequency applied to the PZT was varied while maintaining a constant drive voltage. The PZT showed resonance effects at 5-kHz. Now, with the constant resonant frequency at 5-kHz to maximize the phase change, the voltage applied to the PZT was varied. The PZT presented low harmonic distortion up to a maximum applied voltage of 1.8-V_pp.

Because of the high coherence as in a conventional Φ-OTDR, the laser output is very sensitive to the phase within the addressed section and the output port shows large intensity variations as the SOA gating frequency is swept. Figure 4(a) shows the laser output power variation as the addressed section was displaced along a ∼30-m section within the DSF, showing the strong spatial fluctuations characteristic of a Φ-OTDR [11–16]. A 10-sec frequency sweep (1.050-MHz to 0.650-MHz) was carried out, so that the entire length of the DSF section was addressed while modulating the phase with the PZT attached to the DSF. Synchronous detection of the received signal was performed with a Lock-in amplifier, where static intensity fluctuations were eliminated. Only those fluctuations at 5-kHz were measured, and they appear with 1.37-m spatial resolution where the PZT is positioned on the DSF, at 15.8-m, as shown in Fig. 4(b) upon a sweep over the full DSF length. Hence, the phase signals can be used for directly measuring the external vibration on fiber.

The vibration frequency at 5-kHz with 1-V_pp applied to the PZT is measured, for investigating the system performance. Although solid-state based phase-modulating schemes may allow for testing a broader range of frequencies [37,38], the reflections may hinder their use here, and the PZT modulator is chosen. In the experiments, sinusoidal vibration signals are generated, and laser output detection is performed with a 10-MHz bandwidth photodiode with band-pass filtering in the range 1-10 kHz. Figure 4(c) shows the detection of the sensor signal with 1-V_pp applied (blue trace) and 2-V_pp applied (red trace). In the latter case, the phase modulation exceeds π-rad (red trace). Figure 4(d) shows the amplitude of the sensor signal measured for low voltages applied. In this regime the transfer function of the sensor has a linear phase dependence on the applied voltage, indicates that the lasing Φ-OTDR can be used as an addressable vibration sensor. Indeed, the phase disturbance introduced by the vibration on the backscattered intensity affects the sum of the optical fields of the two half sections encompassing the disturbance, hence the sinusoidal transfer function and linear dependence at low amplitudes.

4. Conclusions

In conclusion, a phase-OTDR system is demonstrated that operates above threshold as a fiber laser. Here, Rayleigh backscattered light in the sensing fiber is fed back into the optical amplifier(s), instead of the light reflected by a laser’s output mirror. It is found that it is possible to use a passive fiber to provide for the random distributed feedback mechanism needed to achieve laser action. Since passive fibers have very low loss compared with the Er-doped fibers used previously, this finding opens the way to address and interrogate much longer sensing fibers than demonstrated previously [27]. It was also found in this work that strong spectral self-narrowing takes place when the laser operates above threshold, and the system shifts from OTDR sensing to a Φ-OTDR mode of operation. The narrow linewidth of the laser regime is uncharacteristic of conventional random lasers, which are often associated with broadband emission and lack of speckle and self-interference [39]. Here, the laser output is phase-sensitive in the addressed section, and its use as a distributed vibration sensor is demonstrated. The experimental results show good signal-to-noise ratio. The sensor can measure the frequency and amplitude of perturbations along 63-m of the sensing fiber with a spatial resolution of 1.37-m. Based on these specifications, the proposed sensor should be able to quantify and track multiple small objects moving along the fiber.