Brillouin expanded time-domain analysis based on dual optical frequency combs.

Brillouin Optical Time-Domain Analysis (BOTDA) is a widely-used distributed optical fiber sensing technology employing pulse-modulated pump waves for local information retrieval of the Brillouin gain or loss spectra. The spatial resolution of BOTDA systems is intrinsically linked to pulse duration, so high-resolution measurements demand high electronic bandwidths inversely proportional to the resolution. This paper introduces Brillouin Expanded Time-Domain Analysis (BETDA) as a modified BOTDA system, simultaneously achieving high spatial resolution and low detection bandwidth.

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).

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.

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 LP mode are sequentially used as the pump, while single polarization of the LP mode is used as the probe for intermodal BDG operation.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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/με and -57.

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.

High-repetition-rate distributed Brillouin sensor based on optical correlation-domain analysis with differential frequency modulation.

A kind of high-repetition-rate distributed Brillouin sensor is proposed and experimentally demonstrated based on optical correlation-domain analysis with differential frequency modulation, where the optical frequencies of the pump and the probe waves are modulated at slightly different RFs so that the temporal position of the measurement is continuously and repeatedly swept along a fiber under test. A distribution map of Brillouin frequency variation along a 100 m optical fiber is acquired at a repetition rate of 20 Hz with an accuracy of ±2.5 MHz and a spatial resolution of about 80 cm.

Simplified Brillouin optical time-domain sensor based on direct modulation of a laser diode.

We propose and experimentally demonstrate a novel kind of Brillouin optical time-domain sensor based on direct modulation of a laser diode (LD) which is free from the use of any microwave device. The Brillouin pump and the probe waves are alternately generated by the LD modulation, and an optical time-domain analysis adopted for distributed measurement. Maps of Brillouin frequency shift are obtained with a spatial resolution of 2 m and an accuracy of ±2 MHz in a 2 km optical fiber.

Observation of narrowband intrinsic spectra of Brillouin dynamic gratings.

We experimentally demonstrate that the reflection spectrum of a Brillouin dynamic grating in a polarization-maintaining fiber can be much narrower than the intrinsic linewidth of the stimulated Brillouin scattering, matching well with the theory of a fiber Bragg grating in terms of the linewidth and the reflectivity. A 3 dB bandwidth as narrow as 10.5 MHz is observed with the Brillouin dynamic grating generated in a 9 m uniform fiber.

High-resolution Brillouin optical time domain analysis based on Brillouin dynamic grating.

We propose and experimentally demonstrate a high-resolution distributed measurement of the Brillouin gain spectrum in a polarization-maintaining fiber (PMF) using Brillouin dynamic grating. In our approach, acoustic waves which contain the information of the local Brillouin frequency are generated by a long pump pulse in one polarization along the PMF and are read out by a short probe pulse with an orthogonal polarization at a different optical frequency from the pump. In experiments, a distributed strain measurement with a 10 cm spatial resolution is demonstrated in a 105 m PMF.