Dynamic Gratings Research Articles

Photo-Induced Holographic Recording in an Optically Active Cholesteric Liquid Crystal Layer

Abstract A few microns thick layer of an optically active cholesteric liquid crystal is realized by using a photoisomerizable nematic component and a chiral dopant. It is shown that such a photosensitive optically active medium can be used as a holographic material for optical information and dynamic grating recording. The photo-induced gratings are written by exploiting the light-induced photoisomerization phase transition from an optically active chiral liquid crystal to an isotropic liquid, which results in the rotation of the light polarization plane from 90 to 0 degrees and corresponding to maximum, respectively, zero transmittance. The results highlight applications in the field of optical storage by the recording of static gratings, as well as in the feld of nonlinear beam-coupling via the holographic writing of dynamic gratings

F–P interferometric cavity formed by two types of fiber Bragg gratings

We proposed and demonstrated a new type of Fabry–Perot (F–P) interferometric cavity formed by a segment of bare fiber sandwiched by a dynamic grating in an Er3+ doped fiber and a fiber Bragg grating, in which the dynamic grating was formed in the section of Er3+ doped fiber by the forward-propagating probe field and its reflected component because of a high reflection fiber Bragg grating at the other end. An incoherent pump was applied to amplify the modulation amplitudes of the dynamic grating and therefore to control the output spectra of the F–P interferometric cavity. The transmitted spectra of the F–P interferometric cavity have been observed with different powers of the incoherent pump, which were accordance with the theoretical analyses. The scheme has potential applications in fiber-optic communication and fiber sensors.

Optimized chaotic Brillouin dynamic grating with filtered optical feedback

Chaotic Brillouin dynamic gratings (BDGs) have special advantages such as the creation of single, permanent and localized BDG. However, the periodic signals induced by conventional optical feedback (COF) in chaotic semiconductor lasers can lead to the generation of spurious BDGs, which will limit the application of chaotic BDGs. In this paper, filtered optical feedback (FOF) is proposed to eliminate spurious BDGs. By controlling the spectral width of the optical filter and its detuning from the laser frequency, semiconductor lasers with FOF operate in the suppression region of the time-delay signature, and chaotic outputs serving as pump waves are then utilized to generate the chaotic BDG in a polarization maintaining fiber. Through comparative analysis of the COF and FOF schemes, it has been demonstrated that spurious BDGs are effectively eliminated and that the reflection characterization of the chaotic BDG is improved. The influence of FOF on the reflection and gain spectra of the chaotic BDG is analyzed as well.

掺铒光纤动态光栅与光纤Bragg光栅Fabry-Perot腔特性研究

掺铒光纤动态光栅与光纤Bragg光栅Fabry-Perot腔特性研究

基于掺铒光纤中动态粒子数光栅的振动检测

基于掺铒光纤中动态粒子数光栅的振动检测

Liquid crystal light valve with a semiconductor substrate for dynamic holography in the infrared

Recording of dynamic holograms in the near infrared spectral range is reported for a liquid crystal light valve with a GaAs semi-insulating substrate. The interaction of weak signal and strong pump waves are studied. It is demonstrated that an out-of-phase local dynamic grating is recorded. Optimal amplitude and frequency of the applied voltage are found for maximum amplification of the weak beam. Step-like external phase modulation of the signal wave with amplitude π/2 is used to change the type of response from local to nonlocal and to achieve in such a way additional transient signal beam amplification. Study of the cells with different thicknesses of the liquid crystal layer at different grating spacings allows a seventeen-fold gain of the weak beam in the device with the thickest liquid crystal layer of 16μm at a grating period of 1000μm (the largest possible in our set-up). The amplitude of the refractive index modulation and nonlinear coupling constant n2 are estimated from the experimental results.

Open-cavity fiber laser with distributed feedback based on externally or self-induced dynamic gratings.

Dynamic population inversion gratings induced in an active medium by counter-propagating optical fields may have a reverse effect on writing laser radiation via feedback they provide. In this Letter we report, to the best of our knowledge, on the first demonstration of an open-cavity fiber laser in which the distributed feedback is provided by a dynamic grating "written" in a Yb-doped active fiber, either by an external source or self-induced via a weak (∼0.1%) reflection from an angle-cleaved fiber end. It has been shown that meters-long dynamic grating is formed with a narrow bandwidth (<50 MHz) and a relatively high-reflection coefficient (>7%) securing single-frequency operation, but the subsequent hole-burning effects accompanied by new grating formation lead to the switching from one longitudinal mode to another. providing a regular pulse-mode dynamics. As a result, periodically generated pulse trains cover a spectrum range of several terahertz delivering millions of cavity modes in sequent pulses.

Spectral dependence of two-wave mixing efficiency in saturable erbium-doped fibers

Detailed experimental data on the efficiency of two-wave mixing via population dynamic grating in erbium-doped fiber (EDF) with saturable optical absorption are presented. The measurements were performed for spectral region 1490–1560 nm of the EDF absorption/gain spectrum, both for absorption and for phase (refractive index) components of the dynamic grating. The reported data confirm the earlier results published for some particular recording wavelengths. In particular, the amplitude gratings recorded in EDF in the central part of this region (1520–1560 nm) are significantly less efficient (by more than 1 order of magnitude) than it can be estimated from measurements of a spatially uniform fiber absorption saturation. It is shown, however, that this effect cannot be explained by spatial migration of the Er3+ excited state. At the short-wavelength spectrum side around 1490 nm, the disagreement between the experimentally observed and the evaluated grating amplitude is minimum (∼0.3) and can, probably, be attributed to the polarization mismatch in the counterpropagating recording waves.

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 95m e-core TMF is performed with a 2m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085 MHz/με, +7.6 MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093 MHz/με, +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 με in strain and ±1.6°C in temperature.

Temperature-strain discrimination in distributed optical fiber sensing using phase-sensitive optical time-domain reflectometry.

A method based on coherent Rayleigh scattering distinctly evaluating temperature and strain is proposed and experimentally demonstrated for distributed optical fiber sensing. Combining conventional phase-sensitive optical time-domain domain reflectometry (ϕOTDR) and ϕOTDR-based birefringence measurements, independent distributed temperature and strain profiles are obtained along a polarization-maintaining fiber. A theoretical analysis, supported by experimental data, indicates that the proposed system for temperature-strain discrimination is intrinsically better conditioned than an equivalent existing approach that combines classical Brillouin sensing with Brillouin dynamic gratings. This is due to the higher sensitivity of coherent Rayleigh scatting compared to Brillouin scattering, thus offering better performance and lower temperature-strain uncertainties in the discrimination. Compared to the Brillouin-based approach, the ϕOTDR-based system here proposed requires access to only one fiber-end, and a much simpler experimental layout. Experimental results validate the full discrimination of temperature and strain along a 100 m-long elliptical-core polarization-maintaining fiber with measurement uncertainties of ~40 mK and ~0.5 με, respectively. These values agree very well with the theoretically expected measurand resolutions.

All-optical flip-flops based on dynamic Brillouin gratings in fibers.

A method to generate an all-optical flip-flop is proposed and experimentally demonstrated based on dynamic Brillouin gratings (DBGs) in polarization maintaining fibers. In a fiber with sufficiently uniform birefringence, this flip-flop can provide extremely long storage times and ultra-wide bandwidth. The experimental results demonstrate an all-optical flip-flop operation using phase-modulated pulses of 300ps and a 1m long DBG. This has led to a time-bandwidth product of ∼30, being in this proof-of-concept setup mainly limited by the relatively low bandwidth of the used pulses and the short fiber length.

Characterization of Brillouin dynamic grating based on chaotic laser

The Brillouin dynamic grating (BDG) based on chaotic laser has particular advantages over the conventional BDG, for example, the creation of single and permanent BDG. To gain insight into the chaotic BDG, we theoretically investigate the reflection and gain spectra characteristics of the chaotic BDG generated in the polarization maintaining fiber. We find that the reflection spectral width of the chaotic BDG is inversely proportional to the effective grating length and the variation in the gain spectral width is negligible with respect to the effective grating length. The widths of the reflection and gain spectra are not affected by the power of the chaotic pump wave. Besides, we analyze that the occurrence of the weak BDG in the generation process of the chaotic BDG leads to the side-lobe of the reflected pulse.

Theoretical evaluation of Brillouin dynamic grating length localized by optical correlation domain technique through reflection spectrum simulation

We derive formulae based on Fourier transformation to calculate the reflection spectrum of a Brillouin dynamic grating (BDG), which is localized along an optical fiber by an optical correlation domain technique. First, we calculate the typical reflection spectra of the BDG localized by the technique and confirm the validity of the formulae by showing coincidence with previous theoretical or experimental works. Next, we evaluate theoretically the spatial resolution in the BDG distributed measurement by the technique, through simulations considering different strained fiber lengths. It is confirmed theoretically, for the first time, that the resolution in the BDG measurement is worse than that for the Brillouin gain spectrum.

Phase-based, high spatial resolution and distributed, static and dynamic strain sensing using Brillouin dynamic gratings in optical fibers

A novel technique combining Brillouin phase-shift measurements with Brillouin dynamic gratings (BDGs) reflectometry in polarization-maintaining fibers is presented here for the first time. While a direct measurement of the optical phase in standard BDG setups is impractical due to non-local phase contributions, their detrimental effect is reduced by ~4 orders of magnitude through the coherent addition of Stokes and anti-Stokes reflections from two counter-propagating BDGs in the fiber. The technique advantageously combines the high-spatial-resolution of BDGs reflectometry with the increased tolerance to optical power fluctuations of phasorial measurements, to enhance the performance of fiber-optic strain sensors. We demonstrate a distributed measurement (20cm spatial-resolution) of both static and dynamic (5kHz of vibrations at a sampling rate of 1MHz) strain fields acting on the fiber, in good agreement with theory and (for the static case) with the results of commercial reflectometers.

Two-dimensional refractive index modulation by phased array transducers in acousto-optic deflectors

Acousto-optic deflectors are photonic devices that are used for scanning high-power laser beams in advanced microprocessing applications such as marking and direct writing. The operation of conventional deflectors mostly relies on one-dimensional sinusoidal variation of the refractive index in an acousto-optic medium. Sometimes static phased array transducers, such as step configuration or planar configuration transducer architecture, are used to tilt the index modulation planes for achieving higher performance and higher resolution than a single transducer AO device. However, the index can be modulated in two dimensions, and the modulation plane can be tilted arbitrarily by creating dynamic phase gratings in the medium using phased array transducers. This type of dynamic two-dimensional acousto-optic deflector can provide better performance using, for example, a large deflection angle and high diffraction efficiency. This paper utilizes an ultrasonic beam steering approach to study the two-dimensional strain-induced index modulation due to the photoelastic effect. The modulation is numerically simulated, and the effects of various parameters, such as the operating radiofrequency of the transducers, the ultrasonic beam steering angle, and different combinations of pressure on each element of the transducer array, are demonstrated.

Principle of Brillouin dynamic grating and its applications in optical fiber sensing

Brillouin dynamic grating (BDG) has been widely studied since it was proposed for the first time to achieve optical storage in 2007. In general, when two beams of pump light (their frequency difference equal to Brillouin frequency shift of the optical fiber) with the same polarization state are injected into the fiber, the coherent acoustic wave can be excited by the stimulated Brillouin scattering effect, forming a BDG. The BDG in an optical fiber has been widely used in optical fiber sensing, characterization of optical fibers, optical storage, all-optical signal processing, microwave photonics and high-precision spectral analysis due to the advantages of all-optical generation and flexible parameter control. In this paper, we analyze the principle of BDG generation and detection, and its applications in optical fiber sensing. The simultaneous measuring of strain and temperature is achieved within a spatial resolution of 20 cm through measuring Brillouin frequency shift and birefringence-induced frequency shift in a polarization-maintaining fiber. A high-sensitivity distributed transverse load sensor based on BDG with a measurement accuracy as high as 0.810-3 N/mm is proposed and demonstrated, whose principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization maintaining fiber. On the basis of the above research, a distributed measurement of hydrostatic pressure is demonstrated by using a 4-m photonics crystal fiber with a measurement error less than 0.03 MPa at a 20-cm spatial resolution, while the temperature cross-talk to the hydrostatic pressure sensing can be compensated for through measuring the temperature-induced Brillouin frequency shift changes by using Brillouin optical time-domain analysis. A system based on BDG in polarization maintaining fibers is reported to achieve a spatial resolution below one centimeter, while preserving the full accuracy on the determination of temperature and strain through measuring Brillouin frequency shift. Taking advantage of creating a long BDG in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a single-mode fiber, and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser, where a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m single-mode fiber.

Simultaneous measurement of strain and temperature using a polarization-maintaining photonic crystal fiber with stimulated Brillouin scattering

We demonstrate the simultaneous and distributed sensing of temperature and strain by combining the Brillouin dynamic grating (BDG) and Brillouin gain spectrum (BGS) using a polarization-maintaining photonic crystal fiber (PM-PCF). The characterization of the birefringence frequency shift of the PM-PCF shows that it remains almost unchanged within the temperature range of 5–80 °C, which allows a temperature-insensitive strain measurement using only the BDG. Combining the BDG and BGS, we demonstrate the simultaneous measurement of temperature and strain with a spatial resolution of 20 cm over a 5-m-long PM-PCF.

Coding-Enhanced Ultrafast and Distributed Brillouin Dynamic Gratings Sensing Using Coherent Detection

Probe coding can significantly improve the signal to noise ratio and measurement speed in Brillouin Dynamic Grating (BDG) fiber-optic distributed sensing, but only if coherent reception is employed. This is because both the amplitude and phase of the BDG impulse response vary along the fiber due to fiber nonuniformities, as well as the effect of the distributed measurand. As a result, the backreflections from individual code chips add coherently and, therefore, the total backscattered return cannot be decoded by a direct detection receiver (excluding some rare scenarios). Here, following a theoretical derivation of the BDG impulse response, a fully coherent BDG-based sensor is reported. Using a 64-chip Golay bipolar (phase) code, an eightfold sensitivity improvement (over single pulse interrogation) is demonstrated. With a code repetition rate of 1 MHz, basically determined by the length of the fiber, 750-Hz strain oscillations were measured with no averaging, with a spatial resolution of 20 cm.

Random access actuation of nanowire grid metamaterial

While metamaterials offer engineered static optical properties, future artificial media with dynamic random-access control over shape and position of meta-molecules will provide arbitrary control of light propagation. The simplest example of such a reconfigurable metamaterial is a nanowire grid metasurface with subwavelength wire spacing. Recently we demonstrated computationally that such a metadevice with individually controlled wire positions could be used as dynamic diffraction grating, beam steering module and tunable focusing element. Here we report on the nanomembrane realization of such a nanowire grid metasurface constructed from individually addressable plasmonic chevron nanowires with a 230 nm × 100 nm cross-section, which consist of gold and silicon nitride. The active structure of the metadevice consists of 15 nanowires each 18 μm long and is fabricated by a combination of electron beam lithography and ion beam milling. It is packaged as a microchip device where the nanowires can be individually actuated by control currents via differential thermal expansion.

Ultrafast creation and control of population density gratings via ultraslow polarization waves.

In the regime of resonant coherent light-matter interaction, light pulses may interact with each other indirectly via a polarization wave created by the other pulse. We show that such interaction allows fast creation and erasing of high-contrast dynamic population density gratings, as well as control of their period in a few-cycle regime. Our scheme uses counter-propagating optical pulses, which do not cross each other in the medium. The mechanism is able to work with pulse durations up to the single-cycle limit. Somewhat surprisingly, ultrafast grating wave vector control requires the generation of polarization waves with the phase velocity much smaller than that of light.