Grating Period Research Articles

Long-period fiber grating based on micro-holes-filled PDMS for temperature and pressure measurement

We use femtosecond laser pulses to evenly drill seven micro-holes along the fiber core in a single mode fiber, forming a long period fiber grating (LPFG) with a grating period of 520 μm, and each micro-hole has an aperture of about 15 μm. Then, LPFG is placed in a polydimethylsiloxane (PDMS) solution and fill each micro-holes with the PDMS solution. After filling PDMS, LPFG exhibits three distinct resonance peaks in the range of 1380 nm to 1560 nm, and the corresponding wavelengths are 1405.0 nm, 1469.6 nm, and 1517.8 nm, with intensities of −16.4 dB, −15.4 dB, and −14.6 dB, respectively. Due to the high thermal expansion coefficient, thermal optical coefficient, elastic optical coefficient and good elastic deformation of PDMS, the sensitivity of LPFG filled with PDMS to temperature and air pressure has been greatly improved. The experimental results show that after filling with PDMS, the sensitivity of LPFG to temperature increased from 22.5 pm/°C to −284.9 pm/°C, an increase of 12.7 times. And LPFG has increased its sensitivity to gas pressure from zero to −2.23 nm/MPa. In addition, LPFG also has good repeatability and stability, and a short response time. This method of using micro-holes to prepare LPFG and filling polymer materials in micro-holes to improve the mechanical strength and sensitivity of LPFG has certain novelty, and this sensor design scheme also has certain practical application value.

Interferometric Optical Pumping of an InGaN/GaN-Based Gain-Coupled Distributed Feedback Multi Quantum Well Laser

We describe an all-optical method to achieve—prior to further advanced processing steps—a perfect match of the relevant wavelength-sensitive parameters of an InGaN/GaN semiconductor distributed feedback laser. Instead of permanently etching and epitaxially over-growing a waveguide-based diffraction grating for the definition of an index-coupled distributed feedback laser, we suggest here—by employing a powerful ultraviolet pump laser—a non-permanent, photoinduced generation of an optical diffraction grating. The resulting complex refractive index modulation then forms a gain-coupled distributed feedback laser. Such an approach has the advantage of remaining flexible as long as possible—both in terms of the correct grating period and the ideal coupling constant. This flexibility is maintained until the definitive etch and the epitaxial over-growth of the diffraction grating are completed. Such devices can—like their dye laser counterparts in the early seventies—also be used as ultra-broadly tunable single-mode sources.

High-performance TM-pass polarizer based on anti-symmetric Bragg gratings.

We present an all-silicon transverse-magnetic-pass (TM-pass) polarizer based on anti-symmetric Bragg gratings. We obtain wide operation bandwidth and high polarization extinction ratio (PER) by maximizing the coupling between the forward TE0 mode and the backward TE1 mode through the reduction of the bridge element width. In the meantime, low insertion loss (IL) is acquired with long tapered structures and the exclusion of the center grating part. Experimental results indicate IL below 0.74 dB and PER over 40 dB covering the wavelength ranges of 1275-1360 nm and 1500-1523 nm, while the average IL within these ranges is as low as 0.27 dB. Additionally, simulation results suggest that the performance can be further improved by introducing chirp in the period of Bragg gratings, thus achieving IL < 0.11 dB and PER > 60 dB over a wide range of 280 nm (1290-1570 nm).

Bragg Grating Cavity Design Using Bragg Gratings

The aim of the experiment is to design Bragg Grating Cavity design (to show Fabry Perot effects) for High Quality Factor. The basic Bragg Grating design used in this project consists of two identical Bragg Gratings and a cavity between the two Gratings. Bragg devices are first simulated using Transfer Matrix Model (TMM) on MATLAB. Parameters like Number of Grating Periods, Cavity Length and Type of Grating are varied to note the effect on Quality Factor. 7 Bragg devices are then designed using KLayout, visualized using Lumerical Interconnect and compared to Matlab TMM model. Simulated results are compared to Measured results using MATLAB and the differences between measurement data and simulated data are discussed. Out of the several implemented designs, the rectangular grating type design with Grating periods of 195, cavity length of 54.6μm and corrugation width 0.045μm resulted in the highest experimental Quality Factor.

Surface-Guided Crystallization of Xanthine Derivatives for Optical Metamaterial Applications.

Numerous bio-organisms employ template-assisted crystallization of molecular solids to yield crystal morphologies with unique optical properties that are difficult to reproduce synthetically. Here, a facile procedure is presented to deposit bio-inspired birefringent crystals of xanthine derivatives on a template of single-crystal quartz. Crystalline sheets that are several millimeters in length, several hundred micrometers in width, and 300-600nm thick, are obtained. The crystal sheets are characterized with a well-defined orientation both in and out of the substrate plane, giving rise to high optical anisotropy in the plane parallel to the quartz surface, with a refractive index difference Δn ≈ 0.25 and a refractive index along the slow axis of n ≈ 1.7. It is further shown that patterning of the crystalline stripes with a tailored periodic grating leads to a thin organic polarization-dependent diffractive meta-surface, opening the door to the fabrication of various optical devices from a platform of small-molecule based organic dielectric crystals.

On-demand flat-top wideband OAM mode converter based on a cladding-etched helical fiber grating.

A new method enabling to provide an on-demand flat-top wideband orbital angular momentum (OAM) mode converter is proposed and experimentally demonstrated, which is based on utilization of a cladding-etched helical long-period fiber grating (CEHLPG). By appropriately selecting the grating period and precisely controlling the diameter of the CEHLPG in-situ, both the radial order and central wavelength of the flat-top band for the generated OAM mode can be flexibly tailored according to specific requirements. As typical examples, the first azimuthal order OAM modes with a flat-top bandwidth of 95 nm at -20 dB, a central operating wavelength of ∼1500 nm, and the radial-orders of 9, 8, 5, and 2, respectively, have been demonstrated consecutively. The proposed method provides an excellent flexibility and robustness in controlling both the radial order and the central wavelength of the resulting flat-top wideband OAM mode conversion, which may support a variety of practical optical vortex applications.

Investigation of mechanically induced long period grating by 3-D printed periodic grooved plates

Investigation of mechanically induced long period grating by 3-D printed periodic grooved plates

The Study on the Fundamental of Fiber Bragg Grating (FBG) Sensing Principle

Abstract: A single mode fibre core is laterally exposed to a periodic pattern of powerful UV laser light to create an optical sensor called an FBG. The exposure results in a long-lasting rise in the fiber's core's refractive index (ncore), producing fixed index modulation known as grating (Λ). A specific wavelength of input light, known as the Bragg wavelength or Bragg related to grating period, must be reflected by the grating inside the fibre optic core while transmitting all other wavelengths. FBG operates as intended by using the regular variations in the single mode fibre core's refractive index, a Bragg reflector can be constructed on an optical fibre. A specific wavelength of light will be reflected and all others will be transmitted as it passes through the FBG. A shift in the wavelength of the light reflected when the temperature or strain surrounding the grating changes is seen.

Periodical Ultra-Modulation of Broadened Laser Spectra in Dielectrics at Variable Ultrashort Laser Pulsewidths: Ultrafast Plasma, Plasmonic and Nanoscale Structural Effects

Self-phase modulation (SPM) broadening of prompt laser spectra was studied in a transmission mode in natural and synthetic diamonds at variable laser wavelengths (515 and 1030 nm), pulse energies and widths (0.3–12 ps, positively chirped pulses), providing their filamentary propagation. Besides the monotonous SPM broadening of the laser spectra versus pulse energy, which was more pronounced for the (sub)picosecond pulsewidths and more nitrogen-doped natural diamond with its intra-gap impurity states, periodical low-frequency modulation was observed in the spectra at the shorter laser pulsewidths, indicating dynamic Bragg filtering of the supercontinuum due to ultrafast plasma and nanoplasmonic effects. Damping of broadening and ultra-modulation for the longer picosecond pulsewidths was related to the thermalized electron-hole plasma regime established for the laser pulsewidths longer, than 2 ps. Unexpectedly, at higher pulse energies and corresponding longer, well-developed microfilaments, the number of low-intensity, low-frequency sideband spectral modulation features counterintuitively increases, thus indicating dynamic variation of the periods in the longitudinal plasma Bragg gratings along the filaments due to prompt secondary laser–plasmon interactions. The underlying sub- and/or near-wavelength longitudinal nanoscale Bragg gratings produced by femtosecond laser pulses in this regime could be visualized in less hard lithium niobate by atomic force microscopy cross-sectional analysis in the correlation with the corresponding sideband spectral components, supporting the anticipated Bragg filtering mechanism and envisioning the corresponding grating periods.

MetaVision: enabling independent wavefront control for diverse/oblique illumination angles

The potential of metasurface holography holds significant promise for revolutionary breakthroughs and groundbreaking advancements in imaging, chip-integrated AR/VR technology, and flat optical displays. Traditional diffractive systems, including metasurfaces, display fixed angular behavior due to the grating period defining incidence angles and diffraction limited response. To break this limit, we offer spin-encoded spatially multiplexed metaholograms designing technique facilitating efficient modulation of geometric phases. The proposed technique gives independent control over polarization states, permitting separate optical modifications for different oblique incident angles. Our suggested metasurface illustrates a multifunctional design method using traditional single-resonator geometry, effectively generating three high-fidelity far-field holographic images. Due to simple geometry and dense information multiplexing proposed approach holds potential for different applications, such as holographic optical elements (HOEs), enhanced optical storage, and anti-counterfeiting techniques.

Gradient Guided-Mode Resonance Biosensor with Smartphone Readout.

Integrating biosensors with smartphones is becoming an increasingly popular method for detecting various biomolecules and could replace expensive laboratory-based instruments. In this work, we demonstrate a novel smartphone-based biosensor system with a gradient grating period guided-mode resonance (GGP-GMR) sensor. The sensor comprises numerous gratings which each correspond to and block the light of a specific resonant wavelength. This results in a dark band, which is observed using a CCD underneath the GGP-GMR sensor. By monitoring the shift in the dark band, the concentration of a molecule in a sample can be determined. The sensor is illuminated by a light-emitting diode, and the light transmitted through the GGP-GMR sensor is directly captured by a smartphone, which then displays the results. Experiments were performed to validate the proposed smartphone biosensor and a limit of detection (LOD) of 1.50 × 10-3 RIU was achieved for sucrose solutions. Additionally, multiplexed detection was demonstrated for albumin and creatinine solutions at concentrations of 0-500 and 0-1 mg/mL, respectively; the corresponding LODs were 1.18 and 20.56 μg/mL.

Ni-MOF-74 film coated optical microfiber CO sensor cascaded with spiral fiber grating temperature compensator

Metal-organic framework (MOF) material is a good carbon monoxide (CO) storage material. This paper proposed a composite optical fiber structure based on spiral fiber grating (SFG) and tapered microfiber (TMF), which can realize the simultaneous sensing of CO concentration and temperature by coating different functional films. The refractive index (RI) of Ni-MOF-74 coated on tapered microfiber can be changed by absorbing CO molecules. The thermal expansion of Ultraviolet (UV) glue will change the period of the spiral fiber grating. Based on the above mechanism, the research shows that the CO sensitivity of the optical microfiber can reach 64.11 pm/ppm, and the temperature sensitivity is −30.1 pm/°C. The temperature sensitivity of the spiral fiber grating can reach 294 pm/°C, which can effectively compensate for the temperature sensitivity of the microfiber. This composite structure has a good application prospect in accurate CO gas concentration measurement.

Large-period plasmonic grating with sinusoidal profile for monitoring water quality with high sensitivity

Subwavelength-thick modulation of surface profile of a dielectric layer combined with a nanometer-thick metallic coating on it can be used for highly sensitive refractive index sensing using the concept of surface plasmon resonance. The performance efficiency of a 1D sinusoidal plasmonic grating of higher periodicity made of polymethylmethacrylate with gold coating on it is evaluated for monitoring water quality. The sinusoidal structure is fabricated by the cost-effective soft imprint lithography technique using a commercially available compact disk as master. Even though the master has a step profile, the solution-based technique aids in achieving the sinusoidal profile. The higher periodicity aids in attaining a higher sensitivity of 1533 nm/RIU at normal incidence and also provides a broad wavelength detection regime that can be controlled by the incidence angle. As the sinusoidal grating supports only the fundamental mode of the surface plasmon, it allows for more precise detection of any analyte. The optimized parameters for the structure are obtained through finite element method calculation.

Discovery of high-Q Fabry–Pérot supercavity modes

We report on a high-quality Fabry–Pérot supercavity mode observed in the terahertz frequency range. The experiment is carried out on a silicon chip with metallic gratings of equal period lithographically fabricated on both sides of the substrate. We show that the supercavity mode arises from interference between the Fabry–Pérot and substrate waveguide modes. As a result, Q factors as high as 880 are achieved at the terahertz frequency band. Possible applications of surface-enhanced electromagnetic field amplification are discussed and demonstrated experimentally.

Relationship between machining accuracy and optical properties of convex blazed grating in ultra-precision cutting.

For ultra-precision machining of convex blazed grating elements there are inevitable machining errors, surface defects, and surface roughness, all of which can have an impact on their diffraction efficiency. In this paper, we use PCGrate software based on the integration method to establish the machining error model, surface defect model, and surface roughness model of convex spherical blazed grating with a curvature radius of 41.104 mm, a substrate diameter of 14 mm, a grating density of 53.97 line/mm and a blazed angle of 3.86° as the basic specification. To investigate the effect of base curvature radius error, grating period error, blazed angle error, grating ridge and valley passivation radius, Poisson burr height, and blaze surface roughness on their -1 order diffraction efficiency in the 0.95-2.5 µm spectral range. The results show that when the curvature radius error of the spherical base is less than ±80µm, the influence on diffraction efficiency can be ignored. Among the three groups of grating microstructure parameters, the influence of blazed angle on grating diffraction efficiency is the largest, followed by a grating period, and the influence of grating apex angle is the smallest, among which when the error of blazed angle is less than ±0.1° and the error of grating period is less than ±0.1µm, the influence on diffraction efficiency can be ignored. The effect of the passivation radius of the grating valley on the diffraction efficiency is smaller than that of the passivation radius of the grating ridge, and the maximum reduction of diffraction efficiency is 0.096 and 0.144 when the grating ridge and valley passivation radius are 50nm∼650 nm, respectively. The diffraction efficiency decreases significantly in the wavelength range of 1.9-2.5 µm with the increase of Poisson burr height and blaze surface roughness, and its effect on the diffraction efficiency can be neglected when the Poisson burr height is less than 0.5 µm and the blaze surface roughness value is less than RMS 1 nm. The machining error, surface defect, and surface roughness models of the convex blazed grating are optimized to provide a comprehensive machining accuracy basis for ultra-precision cutting of convex grating components.

Electrical characterization of optical resonance effects in laterally-nanostructured organic photodetectors.

Optoelectronic devices based on organic semiconductor materials are on the rise for sensing applications due to their integrability with a variety of substrates - including flexible substrates for wearables. For sensing applications often narrowband absorption is desired with suppression of light at other wavelengths. Here, we investigate narrowband absorption enhancement of organic photodetectors (OPD) with an integrated lateral nanostructure. We show with finite-element simulations, that resonant excitation of low absorbing wavelength regimes allow for up to 3 times the absolute absorption at wavelengths on resonance compared to wavelengths off resonance. We present experimental results for CuPc/C60 OPDs fabricated on grating nanostructures with periods of 350 nm and 400 nm and a grating depth of 140 nm as well as a grating period of 370 nm and grating depths of 30 nm. Angle-resolved transmission spectra clearly show the optical resonance effects. In order to evaluate the electrical resonance effects a measurement system is introduced based on angular laser excitation. An angular resolution of 0.1° is achieved in the analysis of the OPD photocurrent response. Using the measurement setup an increase of the photocurrent by up to 50% is observed for the TE-resonance. It is demonstrated that the resonance wavelength is tuned simply by adjusting the grating period without changes in the layer thicknesses. This opens up new opportunities in realizing pixels of different wavelength response next to each other employing a single active stack design.

Application of the Thermal Diffusivity Standard for the Heat Transfer Parameter Control in Absorbing Materials

Metrological support creation and use of heat transfer etalons are important stages in the development of modern materials science. This is especially concerned to the emergence of new materials in the world with previously unattainable thermophysical parameters. The purpose of this work was to develop and experimentally verify the idea of joint application of the transient gratings method which is well-known in nonlinear optics and the single thermal diffusivity etalon of conventional type for the heat transfer metrological control in materials of a wide values range. The method proposed is based on thermal diffusivity etalon application as a source of calibrated optical signals that are excited in it by short laser pulses. Their lifetime is formed by the etalon thermal diffusivity and on the transient grating spatial period. The etalon linear graph of gratings lifetimes as a function of the gratings periods squared and grating lifetime of the material under study are used for the thermal diffusivity calculation. Thermal diffusivity of thin sub-surface layers of the samples under study – duraluminium, monocrystalline silicon and thermoelectric lead telluride film was measured. The results obtained are in close agreement with the reference values.

Wavelength sensitivity reconfigurable SPR photodetector with a blazed grating profile.

Surface plasmonic detectors based on one-dimensional half-wavelength gratings have attracted attention due to their wavelength- or polarization-specific photodetection. Although the effect of a grating period and a grating depth on the photoelectric conversion of 1D half-wavelength grating-based surface plasmon resonance (SPR) detectors has been discussed thoroughly in recent years, the effect of different grating profiles on device performance is still limited to the rectangular shape. In this article, we proposed a wavelength sensitivity reconfigurable photodetector enhanced by SPR with a blazed grating profile. The gold layer was fabricated on a silicon-based blazed grating to form a Schottky barrier and act as an SPR coupler. By measuring the photocurrent in the range of -58° to -48°of an incident angle, the peak shifts of a photocurrent signal waveform are found to depend on the wavelength over 800-1000 nm.

Simultaneous Efficient Excitation of Multiple Higher-Order OAM Modes Using a Single Long-Period Fiber Grating

In this paper, a method for the simultaneous generation of multiple higher-order orbital angular momentum (OAM) modes in a few-mode fiber (FMF) using a single long-period fiber grating (LPFG) is proposed and demonstrated. The high diffraction order of the grating is used to achieve the resonant coupling of multiple modes, and the coupling coefficients of different order modes are balanced by a preset-twist method. This solves the two coupling problems of over-coupling for lower-order modes and difficult excitation of higher-order modes faced by multi-order mode generation, and overcomes the challenge of balanced excitation of multiple modes with high coupling efficiency. In the experiment, the simultaneous generation and flexible combination of first-order, second-order and third-order OAM modes are successfully achieved in a six-mode fiber, in which the mode order, resonant wavelength and wavelength interval can be flexibly controlled. By fabricating with grating periods of 2300 μm, 782 μm and 990 μm, respectively, high conversion efficiency of first-order and second-order modes, first-order and third-order modes, second-order and third-order modes can be generated simultaneously. In addition, for the first time, the first-order, second-order and third-order modes are generated simultaneously at a grating period of 1630 μm with conversion efficiencies of 98.61%, 99.11% and 98.89%, respectively. The mode profiles and interference patterns of different order modes show that multiple higher-order OAM modes with insertion loss less than 1 dB and mode purity over 95% are successfully generated.

Investigation of angle-insensitive grating color filters at periods much smaller than the wavelength of incidence.

We designed and simulated one-dimensional (1D) and two-dimensional (2D) reflective grating color filters inside the aluminized polyethylene (PE) film. The filters have several advantages: high angle insensitivity (up to 45° for the 1D filter, 40° for the 2D filter), high reflectance at non-resonant wavelengths, deep resonance dips, and a large color gamut. Both structures are characterized by with their grating periods being much smaller than the wavelength of incidence. A grating modal analysis was utilized to reveal the physical mechanism behind such structures that exhibit angle-insensitive spectral responses which are favored in the fields of color display and packaging.