Fabrication Of Gratings Research Articles

Optically tunable diffraction efficiency by photo-mobile holographic composite polymer material

We report on the fabrication and optical characterization of one-dimensional transmission holographic volume phase gratings written on a novel holographic composite mixture. The mixture is derived from a recently developed photo-mobile polymeric composite material. The transmission gratings recorded on the photo-mobile film are constituted by polymeric walls and low viscous inter-walls material. Using an external pumping laser operating at λ = 457.9 nm the transmission efficiency linearly increases with the laser power and, in our range of power values, the phenomenon is completely reversible. At highest power level a 30% diffraction efficiency is achieved. The increase of the diffraction efficiency is related to the increase of the refractive index contrast of the grating mixture. In particular, under illumination, the low viscous material escapes from the irradiated area. This feature could explain the experimentally observed changes of the refractive index contrast.

Laser induced tunable Ge2Sb2Te5 phase-change gratings

Periodic photonic nano- and microstructures are routinely used for light manipulation at the nanoscale. However, their fabrication process is demanding in terms of time, cost and facilities. Here we demonstrate a rapid laser-assisted method for fabrication of gratings in Ge2Sb2Te5 (GST) thin films, based on the formation of laser induced periodic surface structures (LIPSS). LIPSS formation mechanisms dependent on the wavelength of the operating laser, lead to high flexibility of the process, producing gratings with tunable period and orientation with respect to the initial laser polarization. The phase-change properties of GST, on the other hand, allows to fabricate phase gratings with strong modulation of refractive index, which are rewritable in nature.

Line Position-Dependent Effect in Line-by-Line Inscribed Fiber Bragg Gratings.

Line-by-line direct writing by femtosecond laser has been proved to be a simple and effective method for the fabrication of low-loss fiber Bragg gratings (FBGs), and is more flexible compared with the traditional ultraviolet exposure method. In this paper, the line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs have been studied, to the best of our knowledge, for the first time. Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling, in which the core-guided fundamental mode would couple to the cladding-guided LP0n and LP1n simultaneously. By aligning the line positions across the core region, the first apodized line-by-line FBG was achieved. This work enriches the theories of line-by-line FBGs and provides an inscription guidance to meet different application requirements.

Wavelength and sensitivity tunable long period gratings fabricated in fluid-cladding microfibers

We report the fabrication of long period gratings in fluid-cladding microfibers by directly focusing a femtosecond laser beam on the microfibers surface to induce periodical modification a long one side of the microfibers. A long period grating is fabricated in a water-cladding microfiber with a diameter of ~ 5 μm, which demonstrates a resonant attenuation of 28.53 dB at wavelength of 1588.1 nm with 10 pitches. When water cladding is changed to be refractive index oil of n= 1.33 and alcohol solution with concentration of 5%, the resonance wavelength shifts to 1575.1 nm with resonant attenuation of 24.91 dB and 1594.1 nm with resonant attenuation of 35.9 dB, respectively. The long period grating demonstrates different temperature sensitivities of–0.524 nm/°C,–0.767 nm/°C and–1.316 nm/°C for water, alcohol solution and refractive index oil cladding microfibers, respectively, which means the alterable liquid cladding allows the availability of tunable wavelength and sensitivity. The fluid-cladding protects the microfibers from external disturbance and contamination and allows more flexibility in controlling the transmission property and sensing characteristics of long period gratings, which can be used as fiber devices and sensors for chemical, biological, and environmental applications.

Fabrication of x-ray absorption gratings via deep x-ray lithography using a conventional x-ray tube

Background: X-ray grating interferometry is an emerging imaging technique that strongly relies on fine grating structures. A common method to fabricate compatible gratings is deep x-ray lithography (DXRL). Aim: To develop a method to fabricate grating structures by DXRL, which does not require a synchrotron source. Approach: The synchrotron source is replaced by a conventional x-ray tube. The fabrication process is adapted for the divergent beam by cylindrically bending mask and substrate. Results: A 10-μm period absorption grating with 80-μm-thick gold lamellae is successfully fabricated from an intermediate 110-μm high structured resist. This grating is characterized and implemented in a preclinical Talbot-Lau interferometer designed for medical thorax imaging. Conclusion: This approach can overcome the strong dependence on synchrotron facilities for the fabrication of gratings for x-ray grating interferometry. As x-ray tubes are more widely available, this is a cost-efficient and scalable alternative suitable for industrial production.

Fabrication of uniform diffraction gratings using laser interference lithography for simultaneous measurement of refractive index

sThe refractive index is one of the important optical properties. Many researchers have studied various methods of measuring refractive index. In this paper, we can easily measure the refractive index of liquids by using diffraction gratings fabricated by flat-top laser interference lithography. Using the diffraction grating for the measurement of the refractive index has the advantage of a wide measurement range. To fabricate the diffraction gratings, we suggest a special laser manufacturing method named flat-top laser interference lithography. The reason which we have chosen Flat-top laser interference lithography is that it can fabricate uniformly and large area micro/nanostructures on the substrate, using a 360 nm continuous-wave(C.W) laser. This optical system is an experiment in which micro/nanopatterning is performed by converting the Gaussian beam shape into a Flat-top shape and causing laser interference. We fabricated transmission diffraction grating on a glass wafer by flat-top laser interference lithography. The transmission diffraction grating is set to measure the refractive index of several liquids. When the laser is normally incident to the transmission diffraction grating, diffraction occurs then we set the beam-profiler, which can sense several optical properties, at 1st diffraction order. Depending on the type of liquid, 1st diffraction order may be displaced. We can easily measure this displacement and calculate the refractive index of liquids including water, benzene, and ethyl alcogol each value of 1.310, 1.501, and 1.3677. The calculated refractive index corresponds to the true values.

Deposition of conformal thin film coatings on sawtooth substrates using ion bombardment

When a nominally flat surface is bombarded with a broad ion beam at oblique incidence, nanoscale ripples often develop on the surface. For high angles of incidence, surfaces typically develop into a terraced form at the late stages of their time evolution. In the present work, this process is exploited to prevent unwanted smoothing of ordered terraced substrates during the deposition of thin films. A Si surface prepatterned with a 500 nm pitch binary grating structure was bombarded at oblique incidence by a low energy Xe+ ion beam to establish an ordered terraced topography. Subsequently, Si/SiO2 bilayers were deposited on the surface, and further oblique incidence Xe+ bombardment was performed following the deposition of each Si layer to re-establish the ordered terraced topography. Self-organized processes, such as in the present work, that only require exposure of a surface to a plasma or ion source have the potential to provide a simple and inexpensive route for fabricating large-area nanostructured surfaces. The presented procedure has potential applications in the fabrication of multilayer blazed gratings for use in the extreme ultraviolet or soft x-ray regimes.

Tunable Phase Gratings by Wrinkling of Plasma-Oxidized PDMS: Gradient Skins and Multiaxial Patterns

Wrinkling instabilities in polymeric bilayers have been exploited as optical phase gratings with tunable performance. Here, we report strain modulated 1D and 2D phase gratings fabricated by the ubiquitous process of plasma-oxidation of polydimethylsiloxane (PDMS). While surface oxidation provides a remarkably facile glassy skin formation approach, minimizing delamination and debonding, it inherently results in a gradient conversion profile emanating from the top film interface. We examine and quantitatively model the consequences of this gradient layer on the optical properties of the resulting strain-tunable phase gratings. Diffraction efficiencies up to 48% are demonstrated. We then develop and validate a surface reconstruction methodology based on the diffraction pattern of our sinusoidal gratings and our model, which we extend to the high deformation regimes and to 2D gratings, obtained by superposition of two wrinkling generations, where both amplitudes and wavelengths can be independently tuned. Overall, this approach provides a rapid, robust and predictive framework for the design and fabrication of tunable, single, and multiaxial surface gratings.

Self-assembly of wedge-shaped PDMS substrate for the fabrication of variable line-space gratings

A low-cost and high-efficiency method for fabricating variable line-space gratings is reported. Specifically, oxygen plasma was used to process the stretched wedge polydimethylsiloxane substrate. After slowly releasing the stress, a gradient grating with a continuous change in line spacing was formed via self-assembly. The period of the self-assembly grating was 2.96 to 3.25 μm, from thick to thin, when the sample was stretched transversely. In longitudinal stretching, the period was 3.71 to 5.20 μm from thick to thin. The period variation range of the gratings obtained by the two types of stretching was 15% and 45%, respectively. Diffraction tests of the two samples showed that the diffraction spots were evident. The diffraction angles varied in the range of 11.31 deg to 12.41 deg and 7.12 deg to 9.93 deg. The gratings prepared using this method can compensate for the shortcomings of the traditional process and have a wide range of application prospects.

Arc-Induced Long-Period Fiber Gratings at INESC TEC. Part I: Fabrication, Characterization and Mechanisms of Formation.

In this work, we reviewed the most important achievements of INESC TEC related to the fabrication of long-period fiber gratings using the electric arc technique. We focused on the fabrication setup, the type of fiber used, and the effect of the fabrication parameters on the gratings’ transmission spectra. The theory was presented, as well as a discussion on the mechanisms responsible for the formation of the gratings, supported by the measurement of the temperature reached by the fiber during an electric arc discharge.

Two-step maskless fabrication of compound fork-shaped gratings in nanomultilayer structures based on chalcogenide glasses.

We demonstrate an easy two-step maskless method for the fabrication of so-called compound fork-shaped gratings in nanomultilayer structures (NMLSs) based on chalcogenide glasses. Such elements allow one to generate different configurations of optical vortex (OV) beams with given topological charges (TCs). The used NMLS is particularly interesting because it enables one to realize the direct one-step writing of the surface relief without selective etching. We show that it is possible to control TCs of the generated OV beam arrays, their locations, and intensity. The proposed approach can easily be adapted to the fabrication of compound diffractive gratings performing the desired one- and two-dimensional generation and splitting of other structured laser beams.

A method to increase line-density of grating based on PDMS stretching and PUA replication process

Grating plays an important role in many fields, so it is of great significance to develop a low-cost and rapid grating fabricating technology. In this paper, the stretching deformation process of PDMS is firstly studied by the COMSOL finite element simulation, and the simulation result proves theoretically that the line-density of PDMS grating could be effectively increased by stretching. Then, we propose a novel method to increase the line-density of grating based on PDMS stretching and PUA replication. The relationship between the elongation and uniform region, line-density increase ratio, groove depth reduction is discussed. The experimental results are consistent with the simulation result. The grating diffraction experiment also proves the effectiveness of the method. Because the process can be repeated several times, the line-density of the grating can be further increased. The experimental results show that the grating period decreased from 10 μm to 5 μm after stretching and replication three times. Besides, the process is also applied to the fabrication of high-density grating. Although this process will lead to some loss of grating contour, compared with other grating fabricating technologies, this process has the advantages of low-cost, simplicity, and rapid fabrication. Therefore, it has good application value in the field of grating fabrication.

Fabrication of X-ray Gratings for Interferometric Imaging by Conformal Seedless Gold Electroplating.

We present a method to produce small pitch gratings for X-ray interferometric imaging applications, allowing the phase sensitivity to be increased and/or the length of the laboratory setup to be minimized. The method is based on fabrication of high aspect ratio silicon microstructures using deep reactive ion etching (Bosch technique) of dense grating arrays and followed by conformal electroplating of Au. We demonstrated that low resistivity Si substrates (<0.01 Ohm·cm) enable the metal seeding layer deposition step to be avoided, which is normally required to initiate the electroplating process. Etching conditions were optimized to realize Si recess structures with a slight bottom tapering, which ensured the void-free Au filling of the trenches. Vapor HF was used to remove the native oxide layer from the Si grating surface prior to electroplating in the cyanide-based Au electrolyte. Fabrication of Au gratings with pitch in the range 1.2–3.0 µm was successfully realized. A substantial improved aspect ratio of 45:1 for a pitch size of 1.2 µm was achieved with respect to the prior art on 4-inch wafer-based technology. The fabricated Au gratings were tested with X-ray interferometers in Talbot–Laue configuration with measured visibility of 13% at an X-ray design energy of 26 keV.

8-inch-diameter field of view for X-ray differential phase-contrast imaging

The X-ray differential phase-contrast imaging (DPCI) based on Talbot–Lau​ interferometers (TLIs) is a promising imaging technique for non-destructive testing. The compact imaging configurations that have a large field-of-view (FoV) require the use of bent phase gratings and large-area analyzer gratings. The cascaded TLIs reduce the aspect ratio requirement of large-area analyzer gratings, which facilitates the fabrication of these gratings. However, the bent silicon-based phase gratings are difficult to obtain directly because of the brittleness of silicon wafers. In this work, we present a method to prepare 5-inch cylindrically bent silicon-based phase gratings and 8-inch analyzer gratings. Moreover, we also present their applications in the cascaded TLIs. The mechanical stress that originates from thermal oxidation enables us to cylindrically bend silicon-based phase gratings. Furthermore, as compared to the use of plane phase gratings, the use of cylindrically bent phase gratings allows Moiré fringes to appear in the entire 8-inch FoV. Finally, Moiré fringes and multi-contrast images of the samples (water, liquid soap filled in heat-shrink tubes) demonstrate the effectiveness of the fabrication of the bent silicon-based phase gratings and imaging in the entire 8-inch FoV. The proposed configuration is applicable for attaining multi-contrast images with 8-inch FoV at a low cost.

Optimization of displacement Talbot lithography for fabrication of uniform high aspect ratio gratings

Displacement Talbot lithography can rapidly pattern periodic nanostructures with high depth of focus over large area. Imperfections in the phase mask profile and the stage movement inaccuracies during the exposure cause linewidth variation in every second line of binary gratings. While this beating is barely visible in patterned photoresist, it leads to substantial depth variation when transferred into high aspect ratio silicon structures, because of micro-loading in deep reactive ion etching. A proper scan range compensated the defect, and a beating-free grating with pitch size of 1 μm and aspect ratio of 54:1 is demonstrated.

Ultrafast Laser Processing of Optical Fibers for Sensing Applications.

A review of recent progress in the use of infrared femtosecond lasers to fabricate optical fiber sensors that incorporate fiber Bragg gratings (FBG) and random fiber gratings (RFG) is presented. The important advancements in femtosecond laser writing based on the phase mask technique now allow through-the-coating (TTC) fabrication of Bragg gratings in ultra-thin fiber filaments, tilted fiber Bragg gratings, and 1000 °C-resistant fiber Bragg gratings with very strong cladding modes. As an example, through-the-coating femtosecond laser writing is used to manufacture distributed fiber Bragg grating sensor arrays for oil pipeline leak detection. The plane-by-plane femtosecond laser writing technique used for the inscription of random fiber gratings is also reviewed and novel applications of the resultant devices in distributed temperature sensing, fiber lasers and fiber laser sensors are discussed.

Fabrication of PDMS-based volume Bragg gratings by stitching of femtosecond laser filament

This work demonstrates the fabrication of poly-(dimethylsiloxane) (PDMS)-based volume Bragg gratings (VBGs) by stitching of femtosecond laser filament. The refractive index change induced by femtosecond laser pulses is low (orders of 10−4 to 10−3) in PDMS. Therefore, the VBG thickness is the key parameter to fabricate high-efficiency VBGs in PDMS. With the intention of increasing diffraction efficiency, this paper presents fabrication of multi-layered PDMS-based VBGs. We produce VBGs with 10 μm line pitch using a femtosecond pulse laser (100 fs pulse width, 1 kHz repetition frequency). The modified zone thickness can be controlled by arranging the relative displacement of filament at different depths and the distances between layers. High-efficiency VBGs were created by depth multiplexing of 350 μm long filamentary modification in PDMS. Diffraction efficiency of 87.2% was realized by forming 0.63 mm grating thickness.

Purely Gain-Coupled Distributed-Feedback Bragg Semiconductor Laser Diode Emitting at 770 nm

The transition lines of Mg, K, Fe, Ni, and other atoms lie near 770 nm, therefore, this spectral region is important for helioseismology, solar atmospheric studies, the pumping of atomic clocks, and laser gyroscopes. However, there is little research on distributed-feedback (DFB) semiconductor lasing at 770 nm. In addition, the traditional DFB semiconductor laser requires secondary epitaxy or precision grating preparation technologies. In this study, we demonstrate an easily manufactured, gain-coupled DFB semiconductor laser emitting at 770 nm. Only micrometer scale periodic current injection windows were used, instead of nanoscale grating fabrication or secondary epitaxy. The periodically injected current assures the device maintains single longitudinal mode working in the unetched Fabry–Perot cavity under gain coupled mechanism. The maximum continuous-wave output power reached was 116.3 mW at 20 °C, the maximum side-mode-suppression ratio (SMSR) was 33.25 dB, and the 3 dB linewidth was 1.78 pm.

Fabrication and Application of Polymer Optical Fiber Gratings: A Review

Fabrication and Application of Polymer Optical Fiber Gratings: A Review

大角带宽高衍射效率体全息光栅的设计和制备

大角带宽高衍射效率体全息光栅的设计和制备