High‐sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering

The phase-sensitive optical time-domain reflectometry(φ-OTDR)is a good candidate for distributed dynamic strain sensing,due to its high sensitivity and fast measurement,which has already been widely used in intrusion monitoring,geophysical exploration,etc.For the frequency scanning basedφ-OTDR,the phase change manifests itself as a shift of the intensity distribution.The correlation between the reference and measured spectra is employed for relative strain demodulation,which has imposed the continuous measurement for the absolute strain demodulation.Fortunately,the Brillouin optical time domain analysis(BOTDA)allows for the absolute strain demodulation with only one measurement.In this work,the combination of theφ-OTDR and BOTDA has been proposed and demonstrated by using the same set of frequency-scanning optical pulses,and the frequency-agile technique is also introduced for fast measurements.A 9.9 Hz vibration with a strain range of 500 nεhas been measured under two different absolute strains(296.7μεand 554.8με)by integrating the Rayleigh and Brillouin information.The sub-micro strain vibration is demonstrated by theφ-OTDR signal with a high sensitivity of 6.8 nε,while the absolute strain is measured by the BOTDA signal with an accuracy of 5.4με.The proposed sensor allows for dynamic absolute strain measurements with a high sensitivity,thus opening a door for new possibilities which are yet to be explored.

High-speed PGC demodulation model and method with subnanometer displacement resolution in a fiber-optic micro-probe laser interferometer

As the key of embedded displacement measurement,a fiber-optic micro-probe laser interferometer(FMI)is of great interest in developing high-end equipment as well as precision metrology.However,conventional phasegenerated carrier(PGC)approaches are for low-speed scenes and local error analysis,usually neglecting the global precision analysis and dynamic effect of system parameters under high-speed measurement,thus hindering their broad applications.We present a high-speed PGC demodulation model and method to achieve subnanometer displacement measurement precision in FMI.This model includes a global equivalent resolution analysis and revelation of the demodulation error mechanism.Utilizing this model,the failure issues regarding the PGC demodulation method under high speed and large range are addressed.Furthermore,an ultra-precision PGC demodulation algorithm based on the combination of static and dynamic delay adaptive regulation is proposed to enable high-speed and large-range displacement measurement.In this paper,the proposed model and algorithm are validated through simulation and experimental tests.The results demonstrate a displacement resolution of 0.1 nm with a standard deviation of less than 0.5 nm when measuring at a high velocity of 1.5 m/s—nearly a tenfold increase of the latest study.

Large-range displacement measurement in narrow space scenarios:fiber microprobe sensor with subnanometer accuracy

The embedded ultra-precision displacement measurement is of great interest in developing high-end equipment as well as precision metrology.However,conventional interferometers only focus on measurement accuracy neglecting the sensor volume and requirement of embedded measurement,thus hindering their broad applications.Here we present a new sensing method for realizing large-range displacement measurement in narrow space scenarios based on the combination of a fiber microprobe interference-sensing model and precision phase-generated carrier.This is achieved by microprobe tilted-axis Gaussian optical field diffraction and high-order carrier demodulation to realize large-range displacement sensing.It is uncovered that the microprobe element misalignment and phase demodulation means play pivotal roles in the interference signal and the accuracy of large-range displacement sensing.The analysis shows that the proposed interference-sensing method can effectively reduce the nonlinearities.Experimental results illustrate that the measurement range extends from 0 to 700 mm.Furthermore,the maximum nonlinear error is reduced from tens of nanometers to 0.82 nm over the full range,allowing subnanometer accuracy for embedded measurements in the hundreds of millimeters range.

Gas sensing with 7-decade dynamic range by laser vector spectroscopy combining absorption and dispersion

Laser absorption spectroscopy(LAS) has been widely used for unambiguous detection and accurate quantification of gas species in a diverse range of fields. However, up-to-date LAS-based gas sensors still face challenges in applications where gas concentrations change in a wide range, since it is extremely difficult to balance spectral analysis strategies for different optical thicknesses. Here we present laser vector spectroscopy that combines absorption spectroscopy with dispersion spectroscopy, simultaneously taking advantage of the former's high sensitivity in the low-concentration region and the latter's high linearity in the high-concentration region. In the proof-of-concept demonstration of acetylene measurement, it achieves a linear dynamic range of 6 × 10^(7)(R^(2)> 0.9999), which surpasses all other state-of-the-art LAS techniques by more than an order of magnitude,with the capability of highly accurate quantification retained. The proposed laser spectroscopic method paves a novel way of developing large-dynamic-range gas sensors for environmental, medical, and industrial applications.

Single-shot BOTDA based on an optical chirp chain probe wave for distributed ultrafast measurement

Brillouin optical time-domain analysis(BOTDA)requires frequency mapping of the Brillouin spectrum to obtain environmental information(e.g.,temperature or strain)over the length of the sensing fiber,with the finite frequencysweeping time-limiting applications to only static or slowly varying strain or temperature environments.To solve this problem,we propose the use of an optical chirp chain probe wave to remove the requirement of frequency sweeping for the Brillouin spectrum,which enables distributed ultrafast strain measurement with a single pump pulse.The optical chirp chain is generated using a frequency-agile technique via a fast-frequency-changing microwave,which covers a larger frequency range around the Stokes frequency relative to the pump wave,so that a distributed Brillouin gain spectrum along the fiber is realized.Dynamic strain measurements for periodic mechanical vibration,mechanical shock,and a switch event are demonstrated at sampling rates of 25 kHz,2.5 MHz and 6.25 MHz,respectively.To the best of our knowledge,this is the first demonstration of distributed Brillouin strain sensing with a wide-dynamic range at a sampling rate of up to the MHz level.

High-Performance Distributed Brillouin Optical Fiber Sensing

This paper reviews the recent advances on the high-performance distributed Brillouin optical fiber sensing, which include the conventional distributed Brillouin optical fiber sensing based on backward stimulated Brillouin scattering and two other novel distributed sensing mechanisms based on Brillouin dynamic grating and forward stimulated Brillouin scattering, respectively. As for the conventional distributed Brillouin optical fiber sensing, the spatial resolution has been improved from meter to centimeter in the time-domain scheme and to millimeter in the correlation-domain scheme, respectively;the measurement time has been reduced from minute to millisecond and even to microsecond;the sensing range has reached more than 100 km. Brillouin dynamic grating can be used to measure the birefringence of a polarization-maintaining fiber, which has been explored to realize distributed measurement of temperature, strain, salinity, static pressure, and transverse pressure. More recently, forward stimulated Brillouin scattering has gained considerable interest because of its capacity to detect mechanical features of materials surrounding the optical fiber, and remarkable works using ingenious schemes have managed to realize distributed measurement, which opens a brand-new way to achieve position-resolved substance identification.

High-performance optical chirp chain BOTDA by using a pattern recognition algorithm and the differential pulse-width pair technique

Optical chirp chain Brillouin optical time-domain analysis(OCC-BOTDA) has the capabilities of fast measurement, high Brillouin threshold, and freedom from the nonlocal effect;at the same time, however, it also has problems introduced by transient stimulated Brillouin scattering. The influence of the transient interaction is reflected as the broadened asymmetric Brillouin spectrum, the ghost peak, and the frequency shift of the main peak. This introduces difficulty in computing the fiber Brillouin frequency shift with good measurement accuracy. Besides, the OCC modulation causes additional noise due to the uneven amplitude response for different frequency components. In this work, we propose a high-performance OCC-BOTDA using the principal component analysis(PCA) based pattern recognition algorithm and differential pulse-width pair(DPP) technique.After building the Brillouin spectrum database(i.e., all patterns), the fiber intrinsic Brillouin frequency shift can be recognized by the PCA algorithm from a nonstandard Brillouin spectrum profile, resulting in good measurement accuracy. Meanwhile, the DPP technique, subtracting between two Brillouin signals generated by two wide-width pump pulses, is utilized to reduce the OCC modulation noise and avoid the pulse self-phase modulation effect in long-range BOTDA sensing. In the experiment, a temperature measurement with 1.3 MHz measurement precision, 4 m spatial resolution, and 5 s measurement time is achieved over a 100 km single-mode fiber.

Hundred-Joule-level,nanosecond-pulse Nd:glass laser system with high spatiotemporal beam quality

A 100-J-level Nd:glass laser system in nanosecond-scale pulse width has been constructed to perform as a standard source of high-fluence-laser science experiments. The laser system, operating with typical pulse durations of 3–5 ns and beam diameter 60 mm, employs a sequence of successive rod amplifiers to achieve 100-J-level energy at 1053 nm at3 ns. The frequency conversion can provide energy of 50-J level at 351 nm. In addition to the high stability of the energy output, the most valuable of the laser system is the high spatiotemporal beam quality of the output, which contains the uniform square pulse waveform, the uniform flat-top spatial fluence distribution and the uniform flat-top wavefront.

Ultra-wide-dynamic-range gas sensing by optical pathlength multiplexed absorption spectroscopy

Laser absorption spectroscopy(LAS) has become the most widely used laser spectroscopic technique for gas detection due to its capability of accurate quantification and straightforward operation. However, since resolving weak absorption and averting over-absorption are always mutually exclusive, the dynamic range of the LAS-based gas sensor is limited and insufficient for many applications in fundamental study and industry. To overcome the limitation on the dynamic range, this article reports optical pathlength(OPL) multiplexed absorption spectroscopy using a gas cell having multiple internal reflections. It organically fuses together the transmission and reflection operation modes: the former directly uses the entire OPL of the gas cell, while the latter interrogates different internal short OPLs by optical interferometry, yielding >100-fold OPL variation. The achieved dynamic range is more than 6 orders of magnitude that surpasses other LAS techniques by 2–3 orders of magnitude. The proposed method promotes a novel way for the development of large-dynamic-range spectroscopic gas sensors for fundamental studies and industrial applications.

Non-destructive and distributed measurement of optical fiber diameter with nanometer resolution based on coherent forward stimulated Brillouin scattering

Precise control and measurement of the optical fiber diameter are vital for a range of fields,such as ultra-high sensitivity sensing and high-speed optical communication.Nowadays,the measurement of fiber diameter relies on point measurement schemes such as microscopes,which suffer from a tradeoff between the resolution and field of view.Handling the fiber can irreversibly damage the fiber samples,especially when multi-point measurements are required.To overcome these problems,we have explored a novel technique in which the mechanical properties of fibers are reflected by forward stimulated Brillouin scattering(FSBS),from which the diameters can be demodulated via the acoustic dispersion relation.The distributed FSBS spectra with narrow linewidths were recorded via the optimized optomechanical time-domain analysis system using coherent FSBS,thereby achieving a spatial resolution of 1 m over a fiber length of tens of meters.We successfully obtained the diameter distribution of unjacketed test fibers with diameters of 125μm and 80μm.The diameter accuracy was verified by high-quality scanning electron microscope images.We achieved a diameter resolution of 3.9 nm,virtually independent of the diameter range.To the best of our knowledge,this is the first demonstration of non-destructive and distributed fiber diameter monitoring with nanometer resolution.

Light people:Professor Perry Shum spoke about fiber-enabled industries

Editorial Fiber technologies have fundamentally reshaped the way we see,the way we sense,the way we communicate,and the way we live.They were so well developed that in some industries such as telecommunication,they were even taken for granted.For that,Light:Science&Applications invited Professor Pery Shum,a pioneer in fiber technologies and their industrialization,to speak about what chances fber technologies can bring to industries.