Long-range and high-precision correlation optical time-domain reflectometry utilizing an all-fiber chaotic source

Feedback insensitivity in a self-chaotic microcavity laser

Insensitivity to external optical feedback is experimentally demonstrated in a self-chaotic deformed square microcavity laser for the first time, to the best of our knowledge. Both the optical and radio frequency (RF) spectra of the microlaser remain unaffected for external optical feedback with feedback strength as high as 9.9 dB. In addition, the autocorrelation function curve exhibits no time-delayed peaks. The insensitivity makes the self-chaotic microcavity laser promising for applications in feedback-insensitive optical sources.

Ultra-long chaotic FBG sensing with high-order random fiber lasing amplification

We propose and demonstrate an ultra-long chaotic fiber Bragg grating (FBG) sensing system based on wavelength-scanning correlation optical time-domain reflectometry (COTDR) assisted by sixth-order random fiber lasing amplification (RFLA). Cascaded random Raman fiber lasing generated in the long fiber span can provide up to sixth-order distributed Raman amplification for the chaotic probe light and its echo signal without ruining the chaotic behavior, which can significantly extend the sensing distance of COTDR. As a result, a 152-km-long wavelength-scanning COTDR is experimentally demonstrated to simultaneously realize FBG sensing and location with a spatial resolution as high as 6 cm, which is the longest COTDR to the best of our knowledge. Temperature sensing of the specific FBG is performed, and the temperature sensitivity of the proposed system is 0.25 dB/°C with a good linearity. The proposed chaotic FBG sensing system with high-order RFLA can act as a new platform for ultra-long, large-capacity FBG sensing, which has potential applications in overhead transmission powerline monitoring and structural health monitoring.

High-Sensitivity Fiber Fault Detection Method Using Feedback-Delay Signature of a Modulated Semiconductor Laser

We propose a high-sensitivity fiber fault detection method using the feedback-delay signature of a modulated semiconductor laser. The modulated laser is directed to a fiber fault and then receives the fault echo, which, in principle, forms an external cavity feedback laser. The fault location, i.e., the external cavity length, is measured by the feedback-delay signature appearing on the laser modulation response curve. The resonance effect between the modulation frequency and external cavity frequency significantly enhanced the laser sensitivity to feedback light and then led to highly sensitive fault detection. Numerical simulations based on laser rate equations predicted that −118.1 dB sensitivity to fault echo light can be obtained.

Long-distance distributed acoustic sensing utilizing negative frequency band

Wider bandwidth always means better overall performance for an information system. Naturally, this criterion can also be applied to phase-sensitive optical time domain reflectometry (Φ-OTDR), which is a typical distributed optical fiber sensing (DOFS) system. Thus, an indispensable way to enhance the performance of Φ-OTDR is to increase the available system bandwidth, which is usually limited by the electrical components. As a kind of frequency resources, the negative frequency band (NFB) has been used in communication systems based on coherent receivers and high-order modulation, but is still rarely used in DOFS. In this paper, we make a comprehensive study on how to utilize NFB in Φ-OTDR and thus double the available system bandwidth. Moreover, the related improvement of sensing performance is experimentally demonstrated. The positive and negative frequency multiplexing is utilized together with frequency division multiplexing to break the inherent trade-off between sensing distance and scan-rate. As a result, 21.6 kHz scan-rate is experimentally achieved on a 103 km fiber, with 97 pε/Hz strain resolution and 9.3 m spatial resolution. To the best of our knowledge, this is the best sensing performance in long distance Φ-OTDR > 100 km. The proposed scheme can also be applied to other DOFS systems with heterodyne-detection, opening up new possibilities for performance enhancement in DOFS systems.

Fault detection of few-mode fiber based on high-order mode with high fault detection sensitivity

We report on the fusion splice loss characteristics of each mode in few-mode fiber (FMF). The fusion fault loss characteristics of all modes of the six-mode step-index and graded-index fibers are measured, respectively. The high-order modes with high fault detection sensitivity are presented. The fault events with different loss amplitudes can be accurately characterized by high-order modes. On this basis, a fault detection and location method for FMF based on backscattered light of high-order modes is proposed and experimentally demonstrated. Rayleigh backscattering light of high-order modes is utilized to detect the faults of 7.2 km six-mode step-index fiber with three fusion splice points with different fusion quality, the detection results of each mode are compared. The high-order modes LP_21a, LP_21b, and LP₀₂ with high fault detection sensitivity are shown, three fault points located around 1.03, 3.1, and 3.2 km of the FMF are successfully detected. While the LP₀₁, LP_11a, and LP_11b modes can detect only one fault point, which is located around 3.19 km of the FMF.

Effect of digital acquisition on the complexity of chaos

The effect of data acquisition on the complexity of chaos using permutation entropy has been studied experimentally and numerically. The complexity of chaos is quantified using the normalized permutation entropy at the feedback round-trip time. Two "abnormal" variations of the complexity of chaos with bias current have been observed experimentally. The different vertical scales of the oscilloscope in data acquisition are attributed to this abnormal change. The method to remove the effect of data acquisition has also been proposed.

Phase-sensitive correlation optical time-domain reflectometer using quantum phase noise of laser light

We propose and experimentally demonstrate a simple approach to realize a phase-sensitive correlation optical time-domain reflectometer (OTDR) suitable for detection and localization of dynamic perturbations along a single-mode optical fiber. It is based on the quantum phase fluctuations of a coherent light emitted by a telecom DFB diode laser. Truly random probe signals are generated by an interferometer with the optical path difference exceeding the coherence length of the laser light. Speckle-like OTDR traces were obtained by calculating cross-correlation functions between the probe light and the light intensity signals returned back from the sensing fiber. Perturbations are detected and localized by monitoring time variations of correlation amplitude along the fiber length. Results of proof-of-concept experimental testing are presented using an array of ultra-low-reflectivity fiber Bragg gratings as weak reflectors.