Dynamic Gratings Research Articles

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

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

Non-steady-state photo-EMF in β-Ga2O3 crystals at λ = 457 nm

The non-steady-state photoelectromotive force is excited in a monoclinic gallium oxide crystal at wavelength λ = 457 nm. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved without application of any electric field to the sample. The dependencies of the signal amplitude on the frequency of phase modulation, light intensity, spatial frequency and light polarization are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal's photoconductivity, responsivity and diffusion length of electrons are estimated for the chosen light wavelength and compared with the ones for other wide-bandgap crystals.

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.

Realizing Dynamic Diffraction Gratings Based on Light-Direct Writing of Responsive 2D Ordered Patterns

A dynamic diffraction grating (DDG) can realize on-demand in-situ regulation of light diffraction and is highly necessary in the next-generation optical devices; however, its fabrication remains challenging because of difficulties in generating a responsive pattern in the bulk material or on the surface. In this study, a facile and general approach to construct DDG is developed based on the near-infrared radiation (NIR)-driven responsive 2D ordered surface pattern, which is fabricated by light-direct writing of wrinkles on a thin film of a photosensitive supramolecular polymer network comprised of a copolymer containing amino groups, anthracene carboxylic acid (ANA), and carbon nanotubes. The resulting surface pattern, which exhibits a height dependence on the NIR irradiation, can be used as the DDG whose optical diffraction can be regulated in-situ by NIR. Furthermore, because of the presence of a reversible ANA photodimer and hydrogen bonding in the supramolecular network, the patterned surface can be erased and rewritten by UV light with different wavelengths and self-healed by NIR irradiation. The results demonstrate a simple strategy for DDGs that will have a broad application in the fields of optics, electronics, and intelligent sensing.

Distributed Birefringence Measurement of a Polarization-Maintaining Fiber With an Extended Range Based on an Enhanced Brillouin Dynamic Grating

A distributed birefringence measurement of a polarization-maintaining fiber (PMF) can be realized by using a Brillouin dynamic grating (BDG). However, the pump depletion decreases the intensity of the BDG along the PMF and limits the measurement range to a few hundred meters. We demonstrate a distributed birefringence measurement of the PMF with an extended range to 3030 m based on an enhanced BDG with an 18.5-ns pump-1 pulse, frequency-upshifted continuous-wave pump-2, and 4-ns read pulse (spatial resolution: 40 cm). The enhanced BDG is promising to characterize the design of a fiber coil to improve the fiber optic gyroscope performance.

Implementation of stimuli with millisecond timing accuracy in online experiments.

Online experiments are growing in popularity. This study aimed to determine the timing accuracy of web technologies and investigate whether they can be used to support high temporal precision psychology experiments. A dynamic sinusoidal grating and flashes were produced by setInterval, CSS3, and requestAnimationFrame (hereafter, rAF) technologies. They were run at normal or real-time priority processing in Chrome, Firefox, Edge, and Internet Explorer on Windows, macOS, and Linux. Timing accuracies were compared with that of Psychtoolbox which was chosen as gold standard. It was found that rAF with real-time priority had the best timing accuracy compared to the other web technologies and had a similar timing accuracy as Psychtoolbox in traditional experiments in most cases. However, rAF exhibited poor timing accuracy on Linux. Therefore, rAF can be used as technical basis for accuracy of millisecond timing sequences in online experiments, thereby benefiting the psychology field.

Two-wave mixing in erbium-doped-fibers with spectral-hole burning at 77K

Experimental results on effective two-wave mixing (TWM) via dynamic population gratings in saturable erbium doped fibers (EDFs) at liquid nitrogen temperature (77 K) are reported. It is shown that formation of the spectral holes (SHs) by the recording coherent waves in the wavelength region 1500–1540 nm results in a complex dynamics of the transient TWM response. In addition to conventional slow, even-type response due to amplitude grating formation via spectrally uniform EDF absorption saturation we also observed a dominating contribution due to SH formation. In particular, at the recording wavelength 1527 nm the relaxation time of the SH contribution to the TWM process was significantly shorter (< 4.7 ms) and with the effective recording power ∼4 times lower than that for conventional EDF population grating.

Distributed measurement of Brillouin dynamic grating spectrum localized by an intensity- modulated correlation-domain technique.

Higher spatial resolution of the reflection spectrum measurement of Brillouin dynamic grating (BDG) was achieved by controlling phonon power distribution. We experimentally demonstrate the improvement effect of the light-source intensity-modulation method, proposed recently in a correlation-domain technique, and successfully detected an 8-cm cooled section in a 100-m-long polarization-maintaining fiber. Our method can improve the spatial resolution of BDG measurements, leading to high resolution discriminative and distributed fiber sensing of temperature and strain.

Birefringence-induced frequency shift error estimation for Brillouin dynamic grating reflected spectra

To investigate the error in temperature and strain simultaneous measurement based on Brillouin scattering and Brillouin dynamic grating (BDG), the error estimation for birefringence-induced frequency shift in BDG reflected spectra is systematically investigated. In view of the validity of characterization of the BDG reflected spectra by the coupled-wave equations and estimation of birefringence-induced frequency shift by Gaussian model, the BDG reflected spectra with different values of frequency sweep span, signal-to-noise ratio (SNR), linewidth and number of sweeps are numerically generated based on solving of the coupled-wave equations. The birefringence-induced frequency shift is estimated by fitting the Gaussian model to the numerically generated spectra. The relationship between the error in the birefringence-induced frequency shift and the above factors is obtained by the least-squares fit. The results reveal that the birefringence-induced frequency shift error decreases exponentially with increasing SNR. The error is approximately proportional to linewidth. The error decreases as a power of the number of sweeps and the exponent is −0.5. Too narrow or wide frequency sweep span corresponds to a large error. The frequency sweep span is suggested to be set to one linewidth. An estimation formula for birefringence-induced frequency shift error in BDG reflected spectra is proposed based on a large number of generated spectra.

Suppression of Fluctuant Sidelobes Around Chaotic Brillouin Dynamic Grating Based on Optical Switch

Chaotic Brillouin Dynamic Grating (BDG) can be individually generated and stably maintained due to the thumbtack-correlation characteristics of chaotic laser, which has its particular advantages over the conventional BDGs. However, the fluctuant sidelobes around chaotic BDG induced by the inherent characteristic of chaotic laser have an adverse effect on the signal-to-noise ratio (SNR) of chaotic BDG system. In this paper, we propose an optical switch scheme to suppress the fluctuant sidelobes around the chaotic BDG to optimize the system performance, which is performed by numerical simulation. By comparing the characteristics of the chaotic BDG with and without the optical switch scheme, it has been verified that the optical switch scheme is an effective suppression method of the fluctuant sidelobes around chaotic BDG. Moreover, the optical switch scheme has a very large parameter space for feedback intensity and injection current to suppress the fluctuant sidelobes around the chaotic BDG. And the reflection characteristics of chaotic BDG based on optical switch are also effectively improved.

Distributed Salinity Sensor With a Polyimide-Coated Photonic Crystal Fiber Based on Brillouin Dynamic Grating

The distributed measurement of salinity is experimentally demonstrated for the first time, to the best of our knowledge, by Brillouin dynamic grating (BDG) with a polyimide-coated (PI-coated) polarization-maintaining photonic crystal fiber as the active sensing component. The position-related salinity change can be measured by mapping the birefringence change through the BDG, where the salinity altering can induce swelling or shrinking actions of the PI coating through absorbing or releasing water. To reveal the systematic characterizations of the proposed sensor, four PI-coated sensors with different coating thicknesses of 5 μm, 8 μm, 15 μm and 20 μm were fabricated as demos, achieving salinity sensitivities of 30.1 MHz/(mol/L) for 5 μm, 75.4 MHz/(mol/L) for 8 μm, 119.2 MHz/(mol/L) for 15 μm, and 139.6 MHz/(mol/L) for 20 μm, respectively. The proposed sensor shows a linear and reproducible response to the salinity change. And its measurement uncertainty is less than 10 MHz, yielding a salinity accuracy of 0.072 mol/L. The temperature crosstalk can be compensated through measuring the temperature-induced Brillouin frequency shift changes using Brillouin optical time-domain analysis.

Poly Methacrylic Acid Sodium Salt (PMANa)/Polyurethane (PU) Latex-Polyelectrolyte Colloid Systems Enabling One-Pot Fabrication of Nonperiodic Structured Mechanoresponsive Smart Windows.

Mechanoresponsive smart windows can modulate optical transparency by mechanical actuation, showing the advantages of low cost, energy efficiency, and chemical stability. To date, mechanoresponsive smart windows are mainly built on periodic nano- or microstructures such as arrays, wrinkles, 3D photonic crystals, and polymeric nanocomposites. However, the production of periodic structures requires multiple and complicated processes, which restricts large-scale manufacturing of mechanoresponsive smart windows for practical application. Here, we report one-pot fabrication of poly methacrylic acid sodium salt (PMANa)/polyurethane (PU) mechanoresponsive smart windows with a nonperiodic naturally occurring microsized structure. The obtained smart windows display a high contrast ratio and stable transparency switching behavior under 80% tensile strain over 1000 cycles and have a large breaking elongation of 820%. It offers a promising platform for designing and fabricating multifunctional optical devices including anticounterfeiting labels and dynamical light gratings.

Observation on temperature and strain dependency of Brillouin dynamic grating in a few-mode fiber with a ring-cavity configuration.

We experimentally conduct Brillouin dynamic grating (BDG) operation using a 1-km-long four-mode fiber. By employing a simplified ring-cavity configuration with single-end pumping, the BDG is effectively generated in $ {{\rm LP}_{01}} $LP01 mode within a range of 250 m, and three higher-order modes, namely, $ {{\rm LP}_{11b}} $LP11b, $ {{\rm LP}_{21a}} $LP21a, and $ {{\rm LP}_{02}} $LP02, are chosen as probes to analyze the BDG with a spatial resolution of 1 m. To the best of our knowledge, this is the first time to characterize the responses of BDG frequency to temperature and strain for different modes in a conventional few-mode fiber. By employing the pump-probe pair of $ {{\rm LP}_{01}}{{\rm - LP}_{02}} $LP01-LP02 mode, the highest temperature and strain sensitivities of 3.21 MHz/°C and $ - 0.0384\;{\rm MHz}/{\unicode{x00B5}}{\unicode{x03B5}} $-0.0384MHz/µε have been achieved. Also, the performance of simultaneously distributed temperature and strain sensing based on BDG is evaluated.

Regulating surface wrinkles using light.

Regulating existing micro and nano wrinkle structures into desired configurations is urgently necessary yet remains challenging, especially modulating wrinkle direction and location on demand. In this work, we propose a novel light-controlled strategy for surface wrinkles, which can dynamically and precisely regulate all basic characteristics of wrinkles, including wavelength, amplitude, direction and location (λ, A, θ and Lc), and arbitrarily tune wrinkle topographies in two dimensions (2D). By considering the bidirectional Poisson's effect and soft boundary conditions, a modified theoretical model depicting the relation between stress distributions and the basic characteristics was developed to reveal the mechanical mechanism of the regulation strategy. Furthermore, the resulting 2D ordered wrinkles can be used as a dynamic optical grating and a smart template to reversibly regulate the morphology of various functional materials. This study will pave the way for wrinkle regulation and guide fabrication technology for functional wrinkled surfaces.

The Mechanism of Formation of Nanostructures of Hexagonal Symmetry on Surfaces of Metals by a Sequence of Doubled Ultrashort Pulses of Radiation of Orthogonal Polarization

Recent experimental results on formation of ordered surface nanostructures of hexagonal symmetry on metals under the action of a series of scanned doubled collinear femtosecond pulses of orthogonal polarization and variable time delay are analyzed. It is demonstrated that the appearance of structures is related to the formation of a dynamic grating by the first pulse and excitation of two pairs of surface plasmon–polaritons by the second pulse in directions symmetric with respect to its vector of polarization and forming an angle of π/3 with each other. This leads to the formation of corresponding dynamic gratings and fourfold degeneracy of the first grating. Three dynamic gratings symmetric with respect to the surface normal form a residual surface relief of hexagonal symmetry that can be observed experimentally.

Distributed cladding mode fiber-optic sensor

The analysis of substances outside the cladding has challenged the optical fiber sensors community for decades. A common solution relies on the propagation of light in cladding modes. However, the coupling of light to/from these modes is typically based on permanent gratings in specific locations, which restrict the sensors to point measurements only. In this work, we present dynamic, random-access coupling of light between core and cladding modes of standard fibers, in arbitrarily located short sections. Coupling is based on the stimulation of Brillouin dynamic gratings by two coded pump waves and intermodal scattering of a third optical probe wave. All waves are launched and collected in the core mode. No permanent gratings are inscribed. Distributed sensing of surrounding media is demonstrated with 2 m range and 8 cm resolution. Measurements distinguish between water and ethanol outside the fiber. The measurement accuracy of the local index outside the cladding is 0.004–0.0004 refractive index units.

Механизм формирования наноструктур гексагональной симметрии на поверхности металлов последовательностью сдвоенных ультракоротких импульсов излучения ортогональной поляризации

An experimental results causing the ordered hexagonal symmetry surface nanostructures formation on metals under the sequence of scanned doubled collinear femtosecond pulses of orthogonal polarization and variable time-delay have been analized. It was shown that the structures production caused by the dynamic grating formation by the first pulse, excitation of two paires of surface plasmon polaritons in symmetric to the polarization vector directions of second pulse directions with 60 degrees angle between formed respective dynamic gratings and forth-fold first grating degeneration. So three gratings of three-flold symmetry about surface normal creates experimentally observable resonant surface relief of hexagonal symmetry.

Dynamic Quasi-Distributed Ultraweak Fiber Bragg Grating Array Sensing Enabled by Depth-Resolved Dual-Comb Spectroscopy

Distributed sensing plays an important role in many industrial applications. Sensor arrays consisting of identical ultraweak fiber Bragg gratings can be applied to such scenarios by monitoring the optical spectral response of the sensors. Yet, the spatial resolution, the sampling speed, and the supported number of sensors in the array are mostly determined by the interrogation scheme. While dual-optical frequency comb techniques had been successfully demonstrated as a high-resolution spectroscopic technique in metrology, it had been beyond the reach of industrial sensing applications because of its high complexity and costs. It is proposed and demonstrated here that, enabled by a simple fiber-optic, dual-comb spectroscopy setup consisting of a fiber laser without frequency stabilization, interrogation of an ultraweak fiber sensor array can be realized. High spatial and spectral resolution as well as kilohertz interrogation speed is achieved in both static and dynamic measurements. It is expected that our depth-resolved dual-comb scheme can measure hundreds of sensors response in less than 1 ms. It is also flexible to deal with targets closely spaced or farther apart from each other. This shows the potential of applying dual-comb spectroscopy to more and more industrial application areas.

Light and Microwaves in Laser Frequency Combs: An Interplay of Spatiotemporal Phenomena

Interference of laser beams in a suitable medium creates dynamic optical gratings, which can serve for a wide variety of applications, ranging from real-time holography to ultrasound generation. Typically, the interference occurs in a sample material that is separated from the laser sources. Here, we explore a new aspect of laser-induced dynamic gratings: microwave generation occurring inside the cavity of semiconductor laser frequency comb generators, such as quantum cascade lasers and interband cascade lasers. The interplay between laser light and microwaves in these devices reveals intracavity spatiotemporal phenomena that are of great importance for the understanding of their physics and for their operation to be efficient. Grating effects related to laser locking dynamics, structured cavities, and microwave propagation are demonstrated. Applications in science and technology based on these phenomena, including the realization of novel hybrid electronic–photonic devices, will also be presented.