Permanent Gratings Research Articles

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.

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.

Recording of dynamic and permanent gratings in composite LC cells containing gold nano-island films

The nonlinear optical properties of composite liquid crystal (LC) cells containing gold nano-island film (AuNIF) deposited on one of the cell substrates are studied with the help of dynamic holographic techniques. A dynamic phase grating of a photoinduced change in the refractive index arises in a composite LC cell when the two-wave mixing of laser beams of a cw Nd:YAG laser is used. The nonlinear optical response is caused due to a surface-induced photorefractive effect observed under the simultaneous action of a periodic light interference pattern and a constant voltage. The result of this impact is appearance of an internal electric field (Eint) at the AuNIF-LC interface. This electric field defines the local reorientation of LC molecules. Our experimental studies prove that the Eint value can be controlled by the size and shape of Au nanoparticles in AuNIF. As a result, we obtained a record of either fast dynamic gratings or quasi-stationary gratings in the composite LC cells depending on the morphology of AuNIF.

Volatile and permanent optical gratings recorded in Bi2TeO5photorefractive crystal under high cw intensity

We report the recording of optical gratings on photorefractive $ {{\rm Bi}_2}{{\rm TeO}_5} $Bi2TeO5 crystals using $ \lambda = 532\,\,{\rm nm} $λ=532nm wavelength light. We studied the behavior of this material under high light intensity and found the presence of fast and slow gratings, both of photorefractive nature and exhibiting quite significant light intensity dependence for the $ 1 - 13\,\,{\rm kW}/{{\rm m}^2} $1-13kW/m2 range. A permanent grating was found after the complete erasure of fast and slow holograms recorded at room temperature. The experimental results show that the diffraction efficiency of the permanent grating increases with the recorded light intensity. The permanent grating performance as an optical Bragg filter was characterized by measuring the angular selectivity approximately 1.0 mrad. We also show that the diffraction efficiency of the permanent grating is quite dependent on the direction of light polarization.

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.

In Situ Supercontinuum Nanopatterning of Silicon Surface by Femtosecond Laser Superfilaments

We for the first time theoretically predict and experimentally demonstrate that selective, low-threshold excitation and interference between short-wavelength plasmons on silicon surfaces photoexcited by infrared femtosecond laser pulses give rise to appearance of permanent sub-diffraction surface gratings with their periods down to 100 nm, dramatically differing from near-wavelength surface gratings succeeding the common “laser photon-surface polariton” interference. Such selectivity was provided in this work by in situ white-light (supercontinuum) femtosecond source generated in a multi-filamentation regime in a thin water layer atop the silicon surface, supporting high-efficiency excitation and dynamic tracking of the transiently tunable plasmon resonance on silicon surface for its easy, robust and ultimate surface nanopatterning.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating. By microscope inspection under static displacement of the upper plate, surface gratings formed by the dye adsorption are revealed in both the front and rear windows of the cell, indicating that the diffraction amplification originates from a coherent superposition of the diffracted orders when the gratings are displaced half a period. The effect provides self-matched amplification of diffraction with a simple cell, a single photo-inscription stage, and elementary displacement steps.

Holographic memory performances of titanocene dispersed poly (methyl methacrylate) photopolymer with different preparation conditions

This work proposed a new kind of photosensitizer doped photopolymer and an optimized fabrication method. Holographic gratings are recorded in a glass-like photopolymer based on methyl methacrylate (MMA) as monomer, 2,2-azo-bisisobutyrolnitrile (AIBN) as thermo-initiator and a cationic initiator with high sensitivity and solubility, titanocene (Irgacure 784, BASF) (TI), as photo-initiator. In our fabrication, an optimized three-step thermal-polymerization method is investigated and depicted. 3 mm thick TI/PMMA photopolymers with different concentrations of AIBN and TI molecules are examined in detail. The photo-physical and photo-chemical processes inside the sample during continuous exposure are analyzed. In our photopolymers, permanent volume holographic gratings with diffraction efficiency approaching 74% and response time close to 20s, which makes this photopolymer available in volume holographic data storage.

Novel thermal annealing methodology for permanent tuning polymer optical fiber Bragg gratings to longer wavelengths.

The Bragg wavelength of a polymer optical fiber Bragg grating can be permanently shifted by utilizing the thermal annealing method. In all the reported fiber annealing cases, the authors were able to tune the Bragg wavelength only to shorter wavelengths, since the polymer fiber shrinks in length during the annealing process. This article demonstrates a novel thermal annealing methodology for permanently tuning polymer optical fiber Bragg gratings to any desirable spectral position, including longer wavelengths. Stretching the polymer optical fiber during the annealing process, the period of Bragg grating, which is directly related with the Bragg wavelength, can become permanently longer. The methodology presented in this article can be used to multiplex polymer optical fiber Bragg gratings at any desirable spectral position utilizing only one phase-mask for their photo-inscription, reducing thus their fabrication cost in an industrial setting.

Dynamic and Permanent Gratings in Suspensions of Absorbing Nanocrystals in Organic Solvent

Both dynamic and permanent optical gratings were recorded in the suspension of absorbing pigment nanocrystallites in organic solvent. The dynamic gratings were recorded due to the transfer of the nanocrystallites in the temperature gradient induced by light interference pattern, and the permanent gratings were formed due to light-induced adsorption of the nanocrystalites on the surface of the cell substrate. Full Text: PDF References B.Y. Zel'dovich, N. V Tabiryan, "Orientational optical nonlinearity of liquid crystals", Sov. Phys. Uspekhi 28, 1059 (1985). CrossRef K. Thyagarajan, P. Lallemand, "Determination of the thermal diffusion ratio in a binary mixture by forced Rayleigh scattering", Opt. Commun. 26, 54 (1978). CrossRef F. Bloisi, L. Vicari, P. Cavaliere, S. Martellucci, J. Quartieri, P. Mormile, G. Pierattini, A. Felice, "Soret effect in forced Rayleigh scattering", Appl. Phys. B 106, 103 (1987). CrossRef W. Köhler, "Thermodiffusion in polymer solutions as observed by forced Rayleigh scattering", Chem. Phys. 98, 660 (1993). CrossRef J. Zamir Anvari, R. Karimzadeh, N. Mansour, "Thermo-optic properties and nonlinear responses of copper nanoparticles in polysiloxane oil", J. Opt. 12, 035212 (2010). CrossRef N.M. Rouhollah Karimzadeh, "Investigation of the Soret effect in copper nanoparticle colloids by self-phase modulation and Z-scan", J. Mod. Opt. 60, 163 (2013). CrossRef N. Tabiryan, W. Luo, "Soret feedback in thermal diffusion of suspensions", Phys. Rev. E 57, 4431 (1998). CrossRef A. Eremin, R. Stannarius, S. Klein, J. Heuer, R.M. Richardson, "Switching of Electrically Responsive, Light-Sensitive Colloidal Suspensions of Anisotropic Pigment Particles", Adv. Funct. Mater. 21, 556 (2011). CrossRef I.C. Khoo, "Nonlinear optics of liquid crystalline materials", Phys. Rep. 471, 221 (2009). CrossRef H. Löwen, "Anisotropic self-diffusion in colloidal nematic phases", Phys. Rev. E 59, 1989 (1999). CrossRef S. Kredentser, A. Eremin, P. Davidson, V. Reshetnyak, R. Stannarius,Y. Reznikov, submitted to Eur. Phys. J. E (2015). CrossRef

Photo-induced refractive index and topographical surface gratings in functionalized nanocarbon solid film

We demonstrate that a single-walled carbon nanotube network noncovalently coupled with a pyrene-modified azo-benzene chromophore functions as a host matrix for a broad range of photo-orientation and photomechanical effects. The chromophore could be efficiently reoriented through repeated trans-cis-trans isomerization under linearly polarized 480 nm light, with Δn of 0.012 at 650 nm and fast characteristic rise-times of 0.12 s. Erasable phase diffraction gratings could also be written, with permanent surface relief gratings forming at sufficiently long irradiation times. In addition to demonstrating a mechanism for photo-manipulation of single-walled carbon nanotubes, these results show photo-orientation of chromophores in azo-functionalized single-walled carbon nanotube networks as a path towards the photosensitive tuning of the electrostatic environment of the nanotube.

Self Referencing Heterodyne Transient Grating Spectroscopy with Short Wavelength

Heterodyning by a phase stable reference electric field is a well known technique to amplify weak nonlinear signals. For short wavelength, the generation of a reference field in front of the sample is challenging because of a lack of suitable beamsplitters. Here, we use a permanent grating which matches the line spacing of the transient grating for the creation of a phase stable reference field. The relative phase among the two can be changed by a relative translation of the permanent and transient gratings in direction orthogonal to the grating lines. We demonstrate the technique for a transient grating on a VO2 thin film and observe constructive as well as destructive interference signals.

Interference and holography with femtosecond laser pulses of different colours

Interferometry and holography are two domains that are based on observation and recording of interference fringes from two light beams. While the aim of the first technique is to reveal and map the phase difference of two wave fronts, the main task of the second technique is to reconstruct one of the two recording waves via diffraction of the other wave from the recorded fringe pattern (hologram). To create fringes, mutually coherent waves from the same laser are commonly used. It is shown here that fringes can be observed and holograms can be recorded with ultrashort, sub-picosecond pulses even of different colour, generated in our experiment with two parametric amplifiers seeded, both by the same mode-locked Ti-sapphire laser. The appearance of permanent and transient gratings is confirmed by recording of an image-bearing hologram, by observation of two-beam coupling gain in a pump-probe experiment and by frequency conversion in Raman-Nath self-diffraction from a moving grating.

Time resolved measurements of gratings photo-induced by femtosecond pulses in a lead doped glass

We report on the formation of gratings photo-induced by femtosecond laser pulses in SF59 glass. Depending on the number of pulses used to excite the sample and the pump power density, transient or permanent gratings are induced. We demonstrate that the grating formation is not instantaneous and is produced by laser-induced defects. This results in a change of both the real and imaginary part of the index of refraction. A simple set-up that records the temporal evolution of both parameters during the laser excitation is also presented. It makes it possible to evaluate the weight of both contributions to the grating diffraction efficiency.

Advances in optical fiber Bragg grating sensor technologies

The authors review their recent advances in the development of optical fiber Bragg grating (FBG) sensor technologies. After a brief review of the fiber grating sensors, several newly developed FBG sensors are described. With the continuous development of fiber materials, microstructures and post-processing technologies, FBG sensors are still creative after the first demonstration of permanent gratings thirty years ago.

Effects of carbon nanotubes on electro-optical properties of dye-doped nematic liquid crystal

Electro-optical effects of planar nematic liquid crystal cells doped with dye and carbon nanotubes (CNTs) were investigated by utilizing the absorption property of the dye. Voltage-dependent transmittance and hysteresis measurements under applied ac and dc fields were presented. Cells doped with both Methyl Red and CNTs showed increased conductivity and capacitance, slower ac-rise switching time and dc-backflow on turn-off. The cells exhibited little hysteresis but those containing CNTs reduced residual dc by about 50%. First-order probe diffraction of permanent gratings revealed a correlation of ac-field-enhanced diffraction efficiency and Freedericksz threshold voltage.

Distributed feedback dye laser holographically induced in improved organic–inorganic photocurable nanocomposites

Distributed feedback (DFB) lasing in permanent volume transmission gratings formed in a laser dye-doped organic–inorganic nanocomposite has been investigated. DFB laser cavities were fabricated using one-step two-beam holographic exposure of Pyrromethene 567 (PM567) doped photopolymerizable acrylate monomers containing inorganic (LaPO4) nanoparticles. Compared to the formulation previously utilized, the material composition presented provides longer lifetime of the laser. Spectral and polarization properties, input–output and stability characteristics of the laser output have been investigated by varying the material composition and the patterning parameters. DFB lasing emission of the second and the third diffraction orders has been demonstrated. The spectral linewidth of ∼0.08 nm has been observed at a pump energy threshold of about 0.2 μJ/pulse for the second-order DFB lasing when pumped with 532 nm 500 ps laser pulses. Spectral tuning of the lasing output over ∼56 and ∼7 nm was obtained by varying the grating period and the content of inorganic nanoparticles in the polymer matrix, respectively.

Photo-induced gratings in thin color center layers on lithium fluoride

We study the recording of permanent Bragg gratings on surface-colored lithium fluoride (LiF) crystals by using the interference pattern of a continuous-wave UV argon-ion laser operating at 244 nm. Gratings with spatial periodicity ranging from 400 to 1000 nm are written by using a phase-mask interferometer and are stable for several months after the writing process. Absorption and photoluminescence spectra show the bleaching of primary F and F -aggregate laser-active color centers as a result of the process. Confocal microscopy is used to determine the pitch and the profile of the fluorescent gratings. The UV laser-induced optical bleaching in highly colored LiF ultrathin layers is responsible for the periodic spatial modulation of absorption and photoemission properties that characterize the gratings. In the colored surface layer, a reduction of as much as 50% of the initial color-center-induced refractive-index increase has been estimated in the bleached areas.

Mechanisms of Re-writable Hologram Recordings in NLC Cells

During investigations concerning application of LC cells as a media for dynamic holographic recordings, unwanted permanent recordings were observed on NLC cells with photosensitive layer (i.e., PVK – polivinylocarbazol). Applications of optically addressed NLC based cells with photosensitive layer as a re-writable media for holographic recording have been noticed. However, recent work has proved that in these cells optical addressing can produce permanent, quasi-permanent and erasable holographic gratings, the mechanism of these phenomena was not explained. In this paper, conditions of the optical recordings in NLC cells are presented. Some basic discussion regarding physical phenomena of permanency are discussed.

Electric-field enhancement and modulation of permanent grating diffraction in dye and carbon nanotube-doped liquid crystal

Permanent gratings were written, without the aid of external fields, in planar nematic liquid crystal cells doped with only Methyl Red (MR) as well as cells doped with both Methyl Red and single-wall carbon nanotubes (CNTs). Grating formation based on trans-cis photoisomerism of MR followed by surface adsorption of the cis-isomer was attempted. The diffraction efficiency was easily enhanced and modulated by application of ac fields covering wide ranges of frequency and voltage; thus, allowing for tuning the diffracted signals. Cells doped with MR and CNTs had a maximum relative diffraction efficiency of 67% while cells doped only with MR exhibited a maximum of 28%. The maximum diffraction efficiency appears to be associated with the Freedericksz threshold voltage. The permanent gratings are robust and have remained stable for over two years.