Brillouin Dynamic Grating Research Articles

Observation of a Brillouin dynamic grating in silicon nitride waveguides

Brillouin enhanced four wave mixing in the form of a Brillouin dynamic grating (BDG) enables a uniquely tunable filter whose properties can be tuned by purely optical means. This makes the BDG a valuable tool in microwave photonics (MWP). BDGs have been studied extensively in fibers, but the only observation in an integrated platform required exotic materials. Unlocking BDG in a standard and mature platform will enable its integration into large-scale circuits. Here, we demonstrate the first observation of a BDG in a silicon nitride (Si3N4) waveguide. We also present a new and optimized design, which will enhance the BDG response of the waveguide, unlocking the path to large-scale integration into MWP circuits.

Distributed phase-matching measurement for dynamic strain and temperature sensing based on stimulated Brillouin scattering enhanced four-wave mixing

A Brillouin dynamic grating (BDG) can be used for distributed birefringence measurement in optical fibers, offering high sensitivity and spatial resolution for sensing applications. However, it is quite a challenge to simultaneously achieve dynamic measurements with both high accuracy and high spatial resolution. In this work, we propose a sensing mechanism to achieve distributed phase-matching measurement using a chirped pulse as a probe signal. In BDG reflection, the peak reflection corresponds to the highest four-wave mixing (FWM) conversion efficiency, and it requires the Brillouin frequency in the fast and slow axes to be equal, which is called the phase-matching condition. This condition changes at different fiber positions, which requires a range of frequency injection for the probe wave. The proposed method uses a chirped pulse as a probe wave to cover this frequency range associated with distributed birefringence inhomogeneity. This allows us to detect distributed phase matching for birefringence changes that are introduced by temperature and strain variations. Thanks to the single shot and direct time delay measurement capability, the acquisition rate in our system is only limited by the fiber length. Notably, unlike conventional BDG spectrum recovery-based systems, the spatial resolution here is determined by both the frequency chirping rate of the probe pulse and the birefringence profile of the fiber. In the experiments, an acquisition rate of 1 kHz (up to fiber length limits) and a spatial resolution of 10 cm using a 20 ns probe pulse width are achieved. The minimum detectable temperature and strain variation are 5.6 mK and 0.37 με along a 2 km long polarization-maintaining fiber (PMF).

High-Resolution Distributed Analysis on Brillouin Dynamic Grating by Differential Measurement and Matched Filtering

Improvement of spatial resolution in the distributed analysis on Brillouin dynamic grating (BDG) in a polarization-maintaining fiber (PMF) is demonstrated using a differential measurement (DM) scheme and a matched RF low-pass filter. The DM is applied to remove the reflection signals from uncorrelated positions, and a RF low pass filter optimized for a target spatial resolution is used to suppress the high-frequency common noise remaining in the BDG spectrum. In experimental confirmation distribution of the BDG spectra in a 1 km PMF is measured with a 5 cm spatial resolution, which is equal to the nominal spatial resolution determined by the synthesis of optical coherence function in the correlation-domain analysis system.

Opto-mechanical fiber sensing with optical and acoustic cladding modes

Optical fibers are an excellent sensor platform. However, the detection and analysis of media outside the cladding and coating of standard fibers represent a long-standing challenge: light that is guided in the single optical core mode does not reach these media. Cladding modes help work around this difficulty, as their transverse profiles span the entire cross-section of the fiber cladding and reach its outer boundary. In this tutorial, we introduce and discuss in detail two recent advances in optical fiber sensors that make use of cladding modes. Both concepts share optomechanics as a common underlying theme. First, we describe a spatially continuous distributed analysis using the optical cladding modes of the fiber. Light is coupled to these modes using Brillouin dynamic gratings, which are index perturbations associated with acoustic waves in the core that are stimulated by light. Unlike permanent gratings, which are routinely used to couple light with cladding modes, Brillouin dynamic gratings may be switched on and off at will and can be confined to short fiber sections at arbitrary locations in a random-access manner. Second, we present the extension of the cladding mode sensor concept to include acoustic rather than optical modes. The acoustic cladding modes may be stimulated and monitored by guided light in the single core mode, and their linewidths are modified by the elastic properties of surrounding media. The principles and analyses of both concepts are provided in detail, alongside examples of experimental setups and results.

High Spatial Resolution Sensing of Temperature and Strain Without Cross Sensitivity Based on Chaotic Brillouin Dynamic Grating

A distributed fiber-optic sensing system based on chaotic Brillouin dynamic grating (BDG) without cross-sensitivity of temperature and strain is proposed and experimentally demonstrated. The chaotic BDG is stimulated by interaction of two chaotic pump beams with a certain frequency difference along the slow axis of polarization-maintaining fiber (PMF) and detected by the continuous probe light along the fast axis. The generated chaotic BDG has the characteristic of single, permanent, and localized grating. The grating length is determined by the low coherence length of the chaotic laser, which ensures that the spatial resolution of the chaotic BDG sensing system can reach the centimeter scale. Two parameters, i.e., Brillouin frequency shift (BFS) and birefringence frequency shift (BireFS) are measured to achieve the temperature and strain simultaneous discrimination. In the experiment, a 2.3-cm spatial resolution non-cross-sensitivity measurement was demonstrated over a 100-m long PMF, where temperature and strain discriminative accuracy can reach 0.3 °C and 11 με, respectively.

Design of transverse pressure sensor based on Brillouin dynamic grating

Design of transverse pressure sensor based on Brillouin dynamic grating

Distributed chemical detection outside standard coated fibers using Brillouin optical time-domain analysis of cladding mode spectra

Propagation in the cladding modes of standard optical fibers enables the sensing of chemicals outside the fiber boundary, where light in the single core mode cannot reach. Coupling to the cladding modes typically relies on the inscription of permanent gratings, which restricts the operation of the sensors to point measurements only. In addition, most applications rely on bare, uncoated fibers, which are difficult to deploy outside the research laboratory. In this work, we report the spatially distributed analysis of cladding mode spectra in a standard, off-the-shelf coated fiber. The inscription of the gratings, removal of the coating, or other structural modifications are not required. Coupling is based on Brillouin dynamic gratings: Two optical pump fields stimulate an acoustic wave, which couples a probe field to a counter-propagating cladding mode. Spatial mapping is obtained through time-of-flight analysis: pulsed modulation of one pump wave and the monitoring of the output probe power as a function of time. All fields are launched and detected only in the core mode. The coupling spectrum is sensitive to local changes in the refractive index of the coating layer, to the third decimal point. The spatial resolution is one meter. The demonstrated range is a few meters, and is scalable to hundreds of meters. The technique is used to detect and monitor the local immersion of a fiber section in acetone. The results establish a practical method for spatially distributed fiber optic chemical sensors.

Distributed birefringence sensing at 10-9 accuracy over ultra-long PMF by optical frequency comb and distributed Brillouin amplifier.

Brillouin dynamic gratings (BDG) can measure the distributed birefringence of polarization-maintaining fibers (PMF), however, its sensing range is limited by both stimulated Brillouin scattering depletion and fiber losses in PMF, which are significantly higher than those in standard single-mode fibers. In this work, we theoretically and experimentally verify that BDG can be sustained over ultra-long distances when assisted by distributed Brillouin amplification, significantly extending the distributed birefringence measurement distance. Using an optical frequency comb pumped by a narrow linewidth laser to both generate and interrogate the amplified BDG, a birefringence measurement accuracy of 7.5 × 10-9 was achieved over 7 km sensing length, more than double the longest range reported. This opens a new opportunity to investigate small birefringence changes due to nonlinear optics effects and monitoring fiber network security from eavesdropping.

Brillouin dynamic grating erasure technique for fast all-optical signal processing.

Brillouin dynamic grating (BDG) is an attractive storage unit for all-optical signal storage and processing. However, the processing speed of the traditional "write-read" scheme is severely limited by the inter-process interference (IPI) due to the residual BDG. Here, we propose an all-optical "write-read-erase" scheme to avoid the IPI effect, which can effectively eliminate the residual BDG through an erase pulse. In a numerical simulation, for multi-processes to store a 7 × 7-bits Simplex code, each time, the residual BDGs from the former process are erased for the proposed scheme, and the power fluctuation of the retrieved waveform is suppressed within ±10%. In a preliminary experiment, residual BDG erase efficiencies up to 88.5% can be achieved by introducing erase pulses to neglect the IPI effect on the retrieved waveform. Without the IPI effect, all-optical signal processing will availably be speeded up, especially for short on-chip integrated circuits.

Distributed temperature–strain sensor based on inter-mode Kerr four-wave mixing of PMF: proposal and proof-of-concept

A novel concept of distributed temperature–strain sensors is proposed using inter-mode Kerr four-wave mixing of polarization-maintaining fiber (PMF), in which the dominated physical mechanism is basically different from reported Brillouin dynamic grating (BDG). The independent measurement for temperature and strain sensitivities (−0.431 MHz °C−1 and 0.067 MHz/10 με) was completed by detecting the change in birefringence under pump-Stokes frequency separation at 370 MHz. As a preliminary proof-of-concept, sensing with a spatial resolution of ∼5 m was demonstrated along ∼210 m PMF. Compared with BDG, the proposed sensor features potential of multiple parameters measurement including temperature–strain and liquid acoustic impedance.

Spectral analysis of Brillouin dynamic gratings in polarization-maintaining fibers.

Experiment observations show that the spectra of Brillouin dynamic gratings in polarization-maintaining fibers based on a polarization decoupled scheme are quite broad and usually have multiple peaks. In this paper, we measure the birefringence distribution along polarization-maintaining fibers with high spatial resolution using optical frequency-domain reflectometry. Based on birefringence measurements, both the simulation and experiment are carried out to determine the spectra of Brillouin dynamic gratings generated within the same polarization-maintaining fibers, indicating quantitatively that spectral broadening and irregularity are mainly due to phase mismatch caused by the non-uniformity of birefringence along polarization-maintaining fibers.

Experimental observation of chaotic Brillouin dynamic grating.

We experimentally observe the local Brillouin dynamic grating (BDG) based on a chaotic laser in a polarization-maintaining fiber for the first time, to the best of our knowledge. The grating length of the chaotic BDG can be adjusted by changing the optical spectral width of the chaotic laser. The characteristics of the reflection spectrum versus the grating length are further analyzed, which agrees with the theory of fiber Bragg grating. Temperature distributed measurements based on the chaotic BDG have been demonstrated with a spatial resolution of an order of centimeter.

Ultrahigh spectral resolution single passband microwave photonic filter.

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0-40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

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.

Design of High-sensitivity Hydrostatic Pressure Sensor Based on Brillouin Dynamic Grating

Design of High-sensitivity Hydrostatic Pressure Sensor Based on Brillouin Dynamic Grating

Slope-Assisted Brillouin-Based Distributed Fiber-Optic Sensing Techniques

Brillouin-based fiber-optic sensing has been regarded as a powerful distributed measurement tool for monitoring the conditions of modern large civil and geotechnical structures, since it provides continuous environmental information (e.g., temperature and strain) along the whole fiber used for sensing applications. In the past few decades, great research efforts were devoted to improve its performance in terms of measurement range, spatial resolution, measurement speed, sensitivity, and cost-effectiveness, of which the slope-assisted measurement scheme, achieved by exploiting the linear slope of the Brillouin gain spectrum (BGS), have paved the way for dynamic distributed fiber-optic sensing. In this article, slope-assisted Brillouin-based distributed fiber-optic sensing techniques demonstrated in the past few years will be reviewed, including the slope-assisted Brillouin optical time-domain analysis/reflectometry (SA-BOTDA/SA-BOTDR), the slope-assisted Brillouin dynamic grating (BDG) sensor, and the slope-assisted Brillouin optical correlation domain analysis/reflectometry (SA-BOCDA/SA-BOCDR). Avenues for future research and development of slope-assisted Brillouin-based fiber-optic sensors are also prospected.

Characteristics of Reflection Spectrum of Brillouin Dynamic Grating in Single Mode Fibers

Characteristics of Reflection Spectrum of Brillouin Dynamic Grating in Single Mode Fibers

布里渊动态光栅的研究进展

布里渊动态光栅的研究进展

Distributed Analysis on the Spatial Mode Structure in a PANDA-Type Few-Mode Fiber By Brillouin Dynamic Gratings

We present the experimental analysis on the spatial mode structure in a PANDA-type few-mode fiber (PANDA-FMF) by the intermodal operation of Brillouin dynamic grating (BDG). Polarization-maintaining mode selective couplers are newly developed to selectively launch different optical modes for the pump and probe waves, and the characterization of the intra- and intermodal Brillouin scatterings in the PANDA-FMF is demonstrated. Distributions of effective refractive index (ERI) differences between six guided modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sup> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sup> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11(o)</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sup> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11(o)</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sup> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11(e)</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sup> , and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11(e)</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sup> ) are measured by optical time-domain analysis of the intermodal BDG reflection spectra, and a unique ERI hierarchy among LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> modes with different lobe orientations and polarization directions is reported for the first time in the PANDA-FMF. The analysis on the origin of measured ERI hierarchy is also presented by numerical simulations on the effect of core ellipticity.

A novel technique in BDG sensors: combination of phase and frequency correlation techniques

This paper investigates a novel theoretical model based on Brillouin dynamic grating (BDG) sensors with ultrahigh spatial resolution over long sensing length. So far, frequency correlation or phase correlation technique is singly used to measure properties of the BDG sensors, although each of the techniques has its own drawbacks due to either low spatial resolution and short sensing length or processing complexities. We introduce a straightforward theoretical technique in BDG sensors based on the combination of phase and frequency correlations (CPFC) in a polarization-maintaining fiber. Correlation peak points in the CPFC technique are points where the correlation peaks in the phase correlation technique are matched with the correlation peaks in the frequency correlation technique. The spatial resolution in this novel technique is the same as the spatial resolution of the phase correlation technique. The peaks of the frequency correlation technique have no effect on the final spatial resolution of the CPFC technique and can only increase its sensing length. Long sensing length with no complexity and short calculation process are advantages of the CPFC technique. By simulation of this innovative theoretical technique, a 9 mm spatial resolution over 17.7 km sensing length has achieved, which to the best of our knowledge is the longest sensing length in millimeter range spatial resolution reported so far in BDG sensors.