Optimizing multi-parameter distributed fiber sensors: a hybrid Rayleigh-Brillouin-Raman System approach.

An optimized single-end hybrid Rayleigh, Brillouin, and Raman distributed fiber sensing system has been developed for simultaneous measurement of multiple parameters. This system integrates 3-bit pulse coding for the Raman signal and the Brillouin amplification of the Rayleigh-backscattered signal, discriminating strain, temperature, and vibration using a single sensing fiber.

Recent Progress in Distributed Brillouin Sensors Based on Few-Mode Optical Fibers.

Brillouin scattering is a dominant inelastic scattering observed in optical fibers, where the energy and momentum transfer between photons and acoustic phonons takes place. Narrowband reflection (or gain and loss) spectra appear in the spontaneous (or stimulated) Brillouin scattering, and their linear dependence of the spectral shift on ambient temperature and strain variations is the operation principle of distributed Brillouin sensors, which have been developed for several decades. In few-mode optical fibers (FMF's) where higher-order spatial modes are guided in addition to the fundamental mode, two different optical modes can be coupled by the process of stimulated Brillouin scattering (SBS), as observed in the phenomena called intermodal SBS (two photons + one acoustic phonon) and intermodal Brillouin dynamic grating (four photons + one acoustic phonon; BDG). These intermodal scattering processes show unique reflection (or gain and loss) spectra depending on the spatial mode structure of FMF, which are useful not only for the direct measurement of polarization and modal birefringence in the fiber, but also for the measurement of environmental variables like strain, temperature, and pressure affecting the birefringence. In this paper, we present a technical review on recent development of distributed Brillouin sensors on the platform of FMF's.

Measurement error induced by the power-frequency delay of the light source in optical correlation-domain distributed Brillouin sensors.

Direct current modulation of a semiconductor laser is a key method adopted in most optical correlation-domain distributed Brillouin sensors such as Brillouin optical correlation-domain analysis (BOCDA) and reflectometry (BOCDR) for localizing the sensing position by synthesis of an optical coherence function. We report that distributed measurement of Brillouin frequency by the BOCDA or BOCDR system can suffer significant distortion caused by phase delay between output power and frequency variations of the modulated light source. We calculate characteristics of the distortion by numerical simulations, and compare the results with experimental data obtained by a BOCDA system for confirmation. Our results show that a maximum error of more than 30 MHz can occur in Brillouin frequency measured under ordinary operating conditions with MHz-order direct current modulation.

Optical time-domain reflectometry based on a Brillouin dynamic grating in an elliptical-core two-mode fiber.

Optical time-domain reflectometry based on a Brillouin dynamic grating (BDG-OTDR) in an elliptical-core two-mode fiber (e-core TMF) is proposed and experimentally demonstrated for the discriminative measurement of strain and temperature distributions. Acoustic gratings are generated by the spontaneous Brillouin scattering of a pump pulse, which is used to reflect a probe pulse. Two orthogonal polarizations of the LP01 mode are sequentially used as the pump, while single polarization of the LP11 mode is used as the probe for intermodal BDG operation. Distribution maps of intermodal and polarization birefringence are acquired by analyzing the two BDG spectra, which are used to separately measure the temperature and strain distributions. In experiments, distributed measurement of intermodal BDG spectra along a 95 m e-core TMF is performed with a 2 m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085 MHz/µÎµ, +7.6 MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093 MHz/µÎµ, +4.3 MHz/°C for the LP01x-LP11x mode, respectively. Distribution maps of the strain and temperature variation along the TMF are separately obtained by matrix calculation using the four coefficients with an error of ±105 µÎµ in strain and ±1.6°C in temperature.

Tailored pump compensation for Brillouin optical time-domain analysis with distributed Brillouin amplification.

A Brillouin optical time domain analysis (BOTDA) system utilizing tailored compensation for the propagation loss of the pump pulse is demonstrated for long-range and high-resolution distributed sensing. A continuous pump wave for distributed Brillouin amplification (DBA pump) of the pump pulse co-propagates with the probe wave, where gradual variation of the spectral width is additionally introduced to the DBA pump to obtain a uniform Brillouin gain along the position. In the experimental confirmation, a distributed strain measurement along a 51.2 km fiber under test is presented with a spatial resolution of 20 cm, in which the measurement error (σ) of less than 1.45 MHz and the near-constant Brillouin gain of the probe wave are maintained throughout the fiber.

Distributed measurement of hydrostatic pressure based on Brillouin dynamic grating in polarization maintaining fibers.

High-sensitivity distributed measurement of hydrostatic pressure is experimentally demonstrated by optical time-domain analysis of Brillouin dynamic grating (BDG) in polarization maintaining fibers (PMF's). The spectral shift of the BDG in four different types of PMF's are investigated under hydrostatic pressure variation from 14.5 psi (1 bar) to 884.7 psi (61 bar) with less than 2 m spatial resolution. The pressure sensitivity of BDG frequency is measured to be ‒1.69, + 0.65, + 0.78, and + 0.85 MHz/psi for a PM photonic crystal fiber (PM-PCF), two Bow-tie fibers, and a PANDA fiber, respectively, which is about 65 to 169 times larger than that of Brillouin frequency-based pressure sensing.

Broadband mode division multiplexer using all-fiber mode selective couplers.

We report an all-fiber mode division multiplexer formed with cascaded mode selective couplers with significantly broadened bandwidth potentially spanning S, C and L band. This was achieved by matching the effective refractive indices over a wide wavelength range for the few mode fiber and the single mode fiber used in the coupler. The multiplexer provides high coupling efficiency (>55% for the worst case) for the 4 spatial modes over the entire wavelength range of 1515-1590 nm. The all-fiber construction provides mechanical stability. Experimental results for the coupling efficiency and the mode extinction ratio for each spatial mode are presented along with the far field radiation patterns.

Ultrahigh-speed distributed Brillouin reflectometry.

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed.

Brillouin optical correlation domain analysis with more than 1 million effective sensing points based on differential measurement.

Large increase of effective sensing points in Brillouin optical correlation domain analysis (BOCDA) is achieved by simultaneously applying double modulation and optical time gate based on differential measurement scheme. The noise substructure of Brillouin gain spectrum induced by the double modulation is effectively suppressed by the differential measurement, leading to 2,000 times enlargement of the measurement range. Distributed strain and temperature sensing along a 10.5 km fiber with spatial resolution of less than 1 cm is experimentally demonstrated which corresponds to over 1 million effective sensing points.

Mapping of intermodal beat length distribution in an elliptical-core two-mode fiber based on Brillouin dynamic grating.

Distributed measurement and characterization of the intermodal beat length between LP(01) and LP(11) modes in an elliptical-core (e-core) two-mode fiber (TMF) are demonstrated by the analysis of Brillouin dynamic grating (BDG) spectra. The BDG is generated by the stimulated Brillouin scattering (SBS) of the LP(01) mode and probed by the LP(11) mode, with four different pairs of the spatial and polarization modes in the e-core TMF applied for the pump and the probe (LP(01)x-LP(11)y, LP(01)y-LP(11)y, LP(01)x-LP(11)x, and LP(01)y-LP(11)x). A mode selective coupler (MSC) is used for selective launch and retrieval of the LP(01) and the LP(11) modes in the BDG operation, and the local reflection spectra from the BDG are obtained by an optical time-domain analysis. A distribution map of the intermodal beat length is acquired for each pair of the pump-probe modes with a 1.5 m spatial resolution along a 75 m e-core TMF. Temperature- and strain-dependence of the BDG spectrum is also evaluated for each case.

Linearly configured BOCDA system using a differential measurement scheme.

We experimentally demonstrate a linearly configured Brillouin optical correlation domain analysis (BOCDA) system enhanced by a differential measurement scheme. On-off control of the pump phase modulation with an intentional loss at the end of a fiber under test is applied for the acquisition of a Brillouin gain spectrum. This application leads to a four-fold enhancement of the spatial resolution and doubling of the measurement range in comparison with the former system under the same modulation parameters.

Acousto-optic resonant coupling of three spatial modes in an optical fiber.

A fiber-optic analogue to an externally driven three-level quantum state is demonstrated by acousto-optic coupling of the spatial modes in a few-mode fiber. Under the condition analogous to electromagnetically induced transparency, a narrow-bandwidth transmission within an absorption band for the fundamental mode is demonstrated. The presented structure is an efficient converter between the fundamental mode and the higher-order modes that cannot be easily addressed by previous techniques, therefore can play a significant role in the next-generation space-division multiplexing communications as an arbitrarily mode-selectable router.

Characterization of stimulated Brillouin scattering in a few-mode fiber.

Characterization of stimulated Brillouin scattering (SBS) in a few-mode fiber (FMF) is experimentally demonstrated, where the Brillouin gain spectrum (BGS) of intramodal or intermodal SBS is analyzed for different pump-probe pairs of four LP modes--LP(01), LP(11), LP(21), and LP(02) modes--guided in the fiber. A mode-division multiplexer composed of concatenated mode-selective couplers is applied for selective launching of each LP mode, and a differential measurement scheme is adopted for the analysis of the BGS. The intermodal SBS is observed in each pump-probe pair of different modes, and the Brillouin gain is measured to be within 14%-45% of that of the SBS between the LP(01) modes. The SBS threshold of each LP mode in the FMF is also evaluated.

Intermodal stimulated Brillouin scattering in two-mode fibers.

Characterization of stimulated Brillouin scattering (SBS) in an elliptic-core two-mode fiber is experimentally demonstrated. The LP(01) or LP(11) mode is selectively launched to a fiber under test by a mode-selective coupler as a Brillouin pump, and the Brillouin gain spectra are analyzed for different mode pairs of the pump and probe waves. Intermodal SBS between counterpropagating LP(01) and LP(11) modes is observed, and the efficiency is measured to be about 58% of that of the SBS between the LP(01) modes.

High-sensitivity optical time-domain reflectometry based on Brillouin dynamic gratings in polarization maintaining fibers.

A high-sensitivity optical time-domain reflectometry based on Brillouin dynamic grating (BDG) is proposed and experimentally demonstrated in polarization-maintaining fibers, where a single-end access to a fiber under test is applied with co-propagation of pump and probe pulses for the operation of BDG. Distributed measurements of the BDG spectra are presented with 80 cm spatial resolution in 935 m range, showing strain and temperature sensitivities of 1.37 MHz/µÎµ and -57.48 MHz/°C, respectively.

Differential measurement scheme for Brillouin optical correlation domain analysis.

We newly propose and experimentally demonstrate a differential measurement scheme for Brillouin optical correlation domain analysis, where the difference between Brillouin gain spectra constructed by a normal and a phase-modulated Brillouin pump waves are analyzed to measure local Brillouin frequencies in optical fibers. In experiments, a five-fold enhancement in the spatial resolution is obtained compared to an ordinary BOCDA system under the same modulation parameters, as a result of the improved dynamic range by the suppression of background noises.

Effects of induced birefringence on Brillouin dynamic gratings in single-mode optical fibers.

The effects of linear or circular birefringence on the operation of Brillouin dynamic grating are experimentally investigated where the birefringence is induced by bending or twisting a single-mode optical fiber, respectively. Experimental results show that the reflection spectrum of the dynamic grating in a 5 m fiber is split by bend-induced linear birefringence with the separation frequency dependent on the bending diameter, matching well with the theoretical model. Meanwhile, the spectrum is almost immune to circular birefringence induced by twist with a rate of up to 24 turns/m.

Bidirectional measurement for Brillouin optical correlation domain analysis.

We propose and experimentally demonstrate a bidirectional measurement for Brillouin optical correlation domain analysis as a novel and simple way of the performance enhancement. Brillouin gain and loss spectra of two adjacent correlation peaks are simultaneously and independently analyzed by applying midpoint attenuation in a fiber under test, which doubles both the speed and the range of the distributed measurement.

Operation of Brillouin dynamic grating in single-mode optical fibers.

The first (to our knowledge) observation of Brillouin dynamic grating in conventional single-mode fibers is reported, and the characterization is demonstrated with respect to the external parameters for the grating generation. When a 100 m single-mode fiber is used, a reflectance of 8% with a spectral bandwidth as low as 2.4 MHz is achieved, which is less than 10% of ordinary Brillouin gain bandwidth.

Variable-frequency lock-in detection for the suppression of beat noise in Brillouin optical correlation domain analysis.

We propose and experimentally demonstrate a novel lock-in detection method to avoid a beat noise in Brillouin optical correlation domain analysis (BOCDA) which appears in the sweep of the sensing position and deteriorates the measurement accuracy by distorting the acquired Brillouin gain spectrum. In our analysis, the origin of the beat noise is shown to be the fluctuation of the Brillouin gain induced by the chopping of the intensity-modulated pump wave, and the optimal relation between the modulation and the lock-in frequencies is developed as an effective solution to circumvent the beat noise.