We demonstrate a novel distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). Both the phase and the amplitude of the Rayleigh scattering (RS) light can be demodulated in real-time. The technique is based on I/Q demodulation and homodyne detection using a 90° optical hybrid. The theoretical analysis is given, and as a proof of the concept, the dynamic strain sensing is experimentally demonstrated, with a sensing range of 12.566 km and a spatial resolution of 10 m.

Coherent Φ-OTDR based on I/Q demodulation and homodyne detection

Fiber-optic distributed sensing, employing the Brillouin effect, is already a commercially available measurement technique for the accurate estimation of the static strain/temperature fields along tens of kilometers with a spatial resolution of the order of a meter. Furthermore, relentless research efforts are paving the way to even much wider usability of the technique through recently achieved enhanced performance in each of its critical dimensions: measurement range has been extended to hundreds of kilometers; spatial resolution is of the order of a centimeter or less, signal to noise ratio has been significantly improved; fast dynamic events can be captured at kHz’s sampling rates; and a much better understanding of the underlying physics has been obtained, along with the formulation of figures of merit, and the preparation and early adoption of appropriate standards and guidelines. This paper describes the basics, as well as the state of the art, of the leading Brillouin interrogation methods, with emphasis on the significant progress made in the last 3 years. It also includes a short introduction to coding, which has proven instrumental in many of the recently obtained performance records.

[INVITED] State of the art of Brillouin fiber-optic distributed sensing

So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4ne resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours.

Single-shot distributed temperature and strain tracking using direct detection phase-sensitive OTDR with chirped pulses.

Extensive research on Brillouin- and Raman-based distributed optical fibre sensors over the past two decades has resulted in the commercialization of distributed sensors capable of measuring static and quasi-static phenomena such as temperature and strain. Recently, the focus has been shifted towards developing distributed sensors for measurement of dynamic phenomena such as dynamic strain and sound waves. This article reviews the current state of the art distributed optical fibre sensors capable of quantifying dynamic vibrations. The most important aspect of Rayleigh and Brillouin scattering processes which have been used for distributed dynamic measurement are studied. The principle of the sensing techniques used to measure dynamic perturbations are analyzed followed by a case study of the most recent advances in this field. It is shown that the Rayleigh-based sensors have longer sensing range and higher frequency range, but their spatial resolution is limited to 1 m. On the other hand, the Brillouin-based sensors have shown a higher spatial resolution, but relatively lower frequency and sensing ranges.

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Contributed Review: Distributed optical fibre dynamic strain sensing.

Distributed fiber-optic vibration sensors receive extensive investigation and play a significant role in the sensor panorama. Optical parameters such as light intensity, phase, polarization state, or light frequency will change when external vibration is applied on the sensing fiber. In this paper, various technologies of distributed fiber-optic vibration sensing are reviewed, from interferometric sensing technology, such as Sagnac, Mach-Zehnder, and Michelson, to backscattering-based sensing technology, such as phase-sensitive optical time domain reflectometer, polarization-optical time domain reflectometer, optical frequency domain reflectometer, as well as some combinations of interferometric and backscattering-based techniques. Their operation principles are presented and recent research efforts are also included. Finally, the applications of distributed fiber-optic vibration sensors are summarized, which mainly include structural health monitoring and perimeter security, etc. Overall, distributed fiber-optic vibration sensors possess the advantages of large-scale monitoring, good concealment, excellent flexibility, and immunity to electromagnetic interference, and thus show considerable potential for a variety of practical applications.

https://www.mdpi.com/1424-8220/16/8/1164/pdf

Distributed Fiber-Optic Sensors for Vibration Detection

Phase sensitive optical time domain reflectometry is suitable for vibration detection due to its high sensitivity. However, for conventional amplitude-measuring method, the strain-amplitude relation is nonlinear and the monotonic interval of strain-amplitude transfer function is very small (corresponding to less than $2\pi $ phase shift in light duration). Thus, the vibration detection is qualitative and it is difficult to distinguish different kinds of vibrations. In this letter, a quantitative vibration measurement by analyzing the differential phase in the interrogated fiber separation is proposed. A statistics calculating method is present to suppress the uncertainty in phase-measuring. Dynamic strain of 400 $\text{n}\varepsilon $ (peak-to-peak, corresponding to phase shift of 17 rad) was reproduced with high quality by proposed method.

The Development of an $\Phi $ -OTDR System for Quantitative Vibration Measurement

A two-bit all-optical digital comparator using single mode Fabry-Perot laser diodes (SMFP-LDs) at an input data rate of 10 Gbps is proposed and demonstrated. All-optical comparator is demonstrated using cascaded logic units which are based on injection locking, multi-input injection locking and supporting beam principles for suppressing the dominant mode of SMFP-LDs. Digital comparators are the key components for the decision making circuits, the integral part of the arithmetic and logical units of optical data processors. The output performance of the proposed all-optical comparator is verified with output waveform, rising-falling time, output eye diagram, and bit error rate (BER) at 10 Gbps input Non Return to Zero (NRZ) PRBS of 231-1 signal. The rising/falling time of about 47 ps, clear output waveforms, and clear output eye diagram with an extinction ratio of about 12 dB are obtained. A maximum power penalty of 1.3 dB is measured at a BER of 10-9.

Realization of All-Optical Digital Comparator Using Single Mode Fabry–Pérot Laser Diodes

A novel, simple, and universal all-optical NAND gate using single-mode Fabry-Perot laser diode (SMFP-LD) is demonstrated. The basic operating principle of the proposed NAND gate is the modulation of the dominant lasing mode of SMFP-LD by the injection of external beams. The dominant lasing mode gain is suppressed only when both input beams are high. As SMFP-LD is used to demonstrate the logic function, no additional beam is required as needed in the multimode Fabry-Perot laser diode (MMFP-LD) schemes. The operating principle is explained and the experimental results are presented at 10 Gb/s input data. The logic function is realized with a clear eye opening and an extinction ratio of 14.6 dB with a rising and falling time of 38.3 and 67.1 ps, respectively.

Demonstration of All-Optical nand Gate Using Single-Mode Fabry–Pérot Laser Diode

We propose a novel idea for the suppression of the dominant mode of the single-mode Fabry-Perot laser diode (SMFP-LD) to realize all-optical multi-logic functions and a digital adder. The basic principle of the proposed scheme is the power management of input beams to suppress the dominant mode of the SMFP-LD for multi-input injection locking. The proposed principle is explained and implemented to realize all-optical multi-logic functions and a digital adder at an input data rate of 10 Gbps. A clear eye opening with an extinction ratio of about 12 dB and a rising-falling time of less than 40 ps are observed at the outputs. The bit error rate (BER) performance is measured for all logic gates and half adder operation. We found there is no BER floor up to BER of 10−12 and the maximum power penalty of about 1.2 dB at a BER of 10−9.

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Realization of all-optical multi-logic functions and a digital adder with input beam power management for multi-input injection locking in a single-mode Fabry-Pérot laser diode.

A novel and simple controlled all-optical on/off switch using the gain modulation in a single-mode Fabry-Perot laser diode (SMFP-LD) is demonstrated The working principle of the proposed on/off switch is explained and the experimental results with 10 Gb/s data signal are presented The 10 Gb/s data signal is switched on and off with a 350 MHz control signal The extinction ratio of 156 dB is observed at the output port with a 2 dB power difference between the control signal and input signal We measured the rising and falling time of the switch output as 48 and 59 ps, respectively

Bikash Nakarmi

Papers

The Development of an $\Phi $ -OTDR System for Quantitative Vibration Measurement

Realization of All-Optical Digital Comparator Using Single Mode Fabry–Pérot Laser Diodes

Demonstration of All-Optical nand Gate Using Single-Mode Fabry–Pérot Laser Diode

Realization of all-optical multi-logic functions and a digital adder with input beam power management for multi-input injection locking in a single-mode Fabry-Pérot laser diode.

A Simple Controlled All-Optical on/off Switch Using Gain Modulation in Single Mode FP-LD