Refractive Index Modulation Amplitude Research Articles

Real-time distributed observing on regeneration rates of an apodized type I fiber Bragg grating

Regenerated fiber Bragg gratings (RFBGs) have garnered widespread attention owing to their fascinating high-temperature resistance. The flexible modulating refractive index of fiber Bragg gratings (FBGs) is conducive to boost the versatility of RFBG fabrication. Nevertheless, the impact of different refractive index modulation (RIM) amplitudes on regeneration rate remains ambiguous. Given this, an optical low-coherence reflectometry (OLCR) is employed for real-time monitoring of RIM during the regeneration process. Experimental results illustrate the regeneration rate remains consistent at different RIM amplitudes. Furthermore, a functional relationship between regeneration ratio and regeneration time is proposed. To enhance the OLCR signal intensity, a promising optimized strategy is raised by introducing a phase shift at the edges of FBG. This work deepens our understanding of the FBG regeneration process and immensely improves the manufacturing flexibility of RFBG.

Holographic nanodiamond–polymer composite grating with unprecedented slow-neutron refractive index modulation amplitude

We demonstrate exceptionally high slow-neutron diffraction efficiency (≈ 70% of transmitted intensity diffracted to the first order) from a holographic nanodiamond–polymer composite grating of only tens of micrometers thickness at the average neutron wavelength of 5.3 nm. By meticulous choice of materials for extreme refractive index modulation in a thin grating structure, we overcome typical wavelength and angular selectivity issues usually encountered when pursuing high diffraction efficiencies with a thick grating. This achievement paves the way for the implementation of flux-efficient diffractive elements, well-suited for polychromatic beams, and weakly collimated slow-neutron beams.

A novel photo-thermo-refractive glass with chlorine instead of bromine for holographic application

The well-known standard photo-thermo-refractive (PTR) glass has some limitations in holographic applications in the visible region due to light scattering on the large NaF nanocrystals formed during the photo-thermo-induced (PTI) crystallization process. The large size of the NaF nanocrystals can be reduced by forming more nucleation centers (silver nanoparticles). We suppose this can be achieved by introducing more silver into the batch composition of the glass. However, bromine does not permit high silver concentrations due to the spontaneous crystallization and high Mie-scattering. Therefore, we introduce a novel PTR glass by replacing bromine with chlorine. The novel glass enables us to increase the silver concentration and consequently increase the concentration of the formed nanoparticles. The optical properties, differential scanning calorimetry, and X-ray diffraction analyses were performed on the novel glasses. An experimental and theoretical investigation of the PTI crystallization process was carried out by monitoring the evolution of the surface plasmon resonance (SPR) absorption band. It was found that the UV exposure and heat treatment lead to precipitation of the NaF crystalline phase on a chlorine-containing dielectric shell that surrounds the silver nanoparticle. It was also found that using chlorine instead of bromine leads to a blue shift in the SPR absorption peak location.Volume Bragg gratings were successfully recorded in the novel PTR glasses. The maximum value of the refractive index modulation amplitude obtained for the novel glass is comparable with that for the standard PTR glass.

Femtosecond laser point-by-point Bragg grating inscription in BDK-doped step-index PMMA optical fibers.

In this paper, the inscription of 2-mm-long fiber Bragg gratings (FBGs) on benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a femtosecond laser and a point-by-point FBG inscription technique is reported. The highest reflectivity of approximately 99% is obtained with a pulse energy of 68.5 nJ, showing a large refractive index modulation amplitude of 7.2 × 10-4. Afterwards, grating stabilities at room and higher temperatures of up to 80°C are investigated.

Red-sensitive organic nanoparticle-polymer composite materials for volume holographic gratings with large refractive index modulation amplitudes

We demonstrate volume holographic recording at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean refractive index modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.

Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

Very high contrast volume holographic gratings recorded in photopolymerizable nanocomposite materials.

Volume holographic phase gratings possessing the saturated refractive index modulation amplitudes as large as 4.5×10-2 were recorded at a wavelength of 532 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. This prominent result was achieved by a combination of the HBP nanoparticles with triazine and aromatic ring units and an electron donor/acceptor photo-initiator system doped in an acrylate monomer blend with low viscosity. As a result, efficient mutual diffusion of HBP nanoparticles and monomer having their very large refractive index difference took place. Obtained results suggest a potentiality of our newly developed HBP-dispersed NPC gratings as efficient volume holographic optical elements for various photonic applications including wearable headsets for augmented and mixed reality.

Analytical analysis of and quasi- symmetry in thick diffraction gratings using coupled-wave theory

The concept of parity-time () symmetry in optics could potentially provide emerging solutions to exploit the benefits of optical gain in optical processing. With this regard, and quasi- symmetry were studied in thick optical diffraction gratings by derivation of explicit analytical expressions using the coupled-wave theory and verified by numerical calculations. The conditions to observe and quasi- symmetry were discussed in terms of the gain-loss ratio and the refractive index modulation amplitudes for both reflective and transmissive gratings. The results show that the criterion to attain quasi- symmetry for the reflective and transmissive gratings is in reverse in terms of the ratio of the modulation amplitude of the imaginary to that of the real part of the refractive index (). According to the model, a grating with fringes parallel to the surface experiences broken symmetry for a wide range of gain-loss ratios. Furthermore, by varying the gain-loss ratio or , some extrema could be observed in the normalized transmission power of the diffraction orders, while operating in the quasi- symmetry regime. The analytical solutions could predict and justify the positions of those extrema which could be advantageous to the design purposes.

A new hybrid FBG with a π-shift for temperature sensing in overhead high voltage transmission lines

Temperature monitoring in overhead high voltage transmission lines (OHV TLs) on continuous basis have become a subject of much interest and research in recent years. Since the transmission lines are subjected to various harsh meteorological and geographical conditions such as high temperature, this can cause disastrous effects on the lines such as flashovers, line breakings, and even tower collapsing. For this aim, a distributed on-line temperature monitoring system based on a new hybrid fiber Bragg grating (FBG) technology is proposed in this paper, since the development of the FBG sensing technology became prominent for the monitoring of temperature on the power lines for having several unique advantages compared to the traditional electronic sensors. The new proposed hybrid FBG consists of two apodized fiber Bragg gratings: one Nuttall and one Cos8 separated by a pi-phase shift. This showed an optimum remarkable performance in terms of the evaluation parameters including the peak reflectivity, full width at half maximum, side lobes analysis and ripple factor as well. Moreover, it has shown a stable operation over the increased temperature when subjected to temperature variation, under the variation of the controlling parameters such as the grating length and the refractive index modulation amplitude. The proposed hybrid is compared to the traditional uniform FBGs in terms of the evaluation parameters showing its superiority as a best temperature sensor in OHV TLs.

Distributed Measurement of Regeneration Ratios of an Apodized Type I Fiber Bragg Grating

Type I fiber Bragg grating (FBG) regeneration is studied in terms of refractive index modulation (RIM) and characterized using optical low-coherent reflectometry (OLCR) for the first time. Highly resolved spatial OLCR results are used to demonstrate a uniform RIM regeneration ratio along an apodized type I the FBG during the regeneration process. RIM regeneration ratios are hence proved to be similar for FBGs with different RIM amplitudes. A clearer understanding of FBG regeneration results allows the establishment of a novel RIM regeneration ratio method to quantify and accurately evaluate FBG regeneration.

Ag nanoparticle-enhanced alkyl radical generation in photopolymerization for holographic recording

Abstract We report a new Ag nanoparticle-dispersed polymer nanocomposite for volume holographic recording through acrylic photopolymerization. The initial grating buildup dynamics at the inhibition stage are measured at various Ag nanoparticle concentrations. The refractive index modulation amplitude as large as 0.0069 at 633 nm is seen at the optimum Ag nanoparticle concentration of 1 wt.% with respect to the monomer. Electron paramagnetic resonance measurements show that Ag nanoparticles influence both the generation of alkyl radicals and the scavenging of oxygen in free radical photopolymerization. This mechanism intrinsically determines the molecular weight of polymer being formed and, thereby, affects the refractive index modulation amplitude of the formed grating as a function of Ag nanoparticle concentrations. Moreover, we confirm that two-beam holographic exposure leads to a periodic assembly of dispersed Ag nanoparticles using a dark-field microscopy. Our results suggest a simple way to control the photopolymerization and, therefore, to tailor polymers for practical uses.

Studies on the refractive index modulation in tunnel-coupled quantum-confined active regions of AlGaAs/GaAs semiconductor heterostructures by the p–n junction electric field

Experimental studies of the influence of the parameters of the quantum-confined active regions of GaAs (QW)/ Al0.35Ga0.65As (barrier) semiconductor heterostructures, fabricated by the MOCVD technique, on the modulation of the refractive index and the optical absorption by the field of the reverse-biased p–n junction were carried out. It is shown that the use of thin quantum-confined active regions is mission-critical for obtaining a large modulation of the refractive index in the spectral region with low optical losses. It is shown that the use of structures with two thin tunnel-coupled GaAs quantum wells 3-nm-thick each compared to a single GaAs quantum well, can significantly increase the modulation amplitude of the refractive index and reduce internal optical losses at the operating wavelength. It is experimentally demonstrated that a higher value of the refractive index modulation amplitude in samples based on double-well system is provided by unipolar modulation of the absorption spectrum due to the effect of the transition from the ‘coupled quantum wells’ state at moderate electric fields to the ‘uncoupled quantum wells’ state at strong electric fields. It is demonstrated that there is an increase of the refractive index modulation with an increase in coupling of quantum wells for electrons by reducing the barrier thickness from 6.5 to 4 nm. For a double-well system with 3-nm-thick GaAs QW and 4-nm-thick Al0.35Ga0.65As barrier, a value of 0.000 148 for the modal refractive index change is achieved at internal optical loss α ≤ 1 cm–1, originated from residual absorption in the quantum-confined active region, and it provides a material refractive index modulation of 0.0181.

Ultra-sensitive quasi-distributed temperature sensor based on an apodized fiber Bragg grating.

This work targets a remarkable quasi-distributed temperature sensor based on an apodized fiber Bragg grating. To achieve this, the mathematical formula for a proposed apodization function is carried out and tested. Then, an optimization parametric process required to achieve the remarkable accuracy that is based on coupled mode theory (CMT) is done. A detailed investigation for the side lobe analysis, which is a primary judgment factor, especially in quasi-distributed configuration, is investigated. A comparison between elite selection of apodization profiles (extracted from related literatures) and the proposed modified-Nuttal profile is carried out covering reflectivity peak, full width half maximum (FWHM), and side lobe analysis. The optimization process concludes that the proposed modified-Nuttal profile with a length (L) of 15mm and refractive index modulation amplitude (Δn) of 1.4×10-4 is the optimum choice for single-stage and quasi-distributed temperature sensor networks. At previous values, the proposed profile achieves an acceptable reflectivity peak of 10-0.426 dB, acceptable FWHM of 0.0808nm, lowest side lobe maximum (SL max) of 7.037×10-12 dB, lowest side lobe average (SL avg) of 3.883×10-12 dB, and lowest side lobe suppression ratio (SLSR) of 1.875×10-11 dB. These optimized characteristics lead to an accurate single-stage sensor with a temperature sensitivity of 0.0136nm/°C. For the quasi-distributed scenario, a noteworthy total isolation of 91dB is achieved without temperature, and an isolation of 4.83dB is achieved while applying temperature of 110°C for a five-stage temperature-sensing network. Further investigation is made proving that consistency in choosing the apodization profile in the quasi-distributed network is mandatory. If the consistency condition is violated, the proposed profile still survives with a casualty of side lobe level rise of -73.2070 dB when adding uniform apodization and -46.4823 dB when adding Gaussian apodization to the five-stage modified-Nuttall temperature-sensing network.

Method for precise evaluation of refractive index modulation amplitude inside the volume Bragg grating recorded in photo-thermo-refractive glass.

In this paper, we present a method for achieving precise evaluation of amplitude of refractive index modulation (RIM) inside the volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glasses. The Gaussian divergence characteristics of the incident beam is theoretically considered when calculating the angular selectivity of VBG, and the profiles of experimental angular selectivity curves are utilized to determine the value of RIM with one step. The effectiveness of our proposed method is experimentally verified. This method is applicable even if the full width at half maximum (FWHM) of VBG's angular selectivity curve has the same order of magnitude as or is less than the beam divergence.

Single- and dual-band dispersion compensation unit using apodized chirped fiber Bragg grating

In this study, a wideband dispersion compensation (WBDC) profile that effectively covers the entire C- and/or part of the L-band is designed and evaluated. Several apodizations with different apodization strengths applied to a chirp fiber Bragg grating (CFBG), different CFBG lengths (L), and different refractive index modulation amplitudes ( $$\Delta n$$ ) are investigated during the design and evaluation process. The design and optimization processes target parameters including a maximum full width at half maximum (FWHM), minimum group delay ripples (GDR) and an acceptable reflectivity and sidelobe suppression ratio (SLSR). A wavelength shift of no more than 2 nm is observed as a result of investigating the effect of temperature in the WBDC scenario. During single-stage operation, the results shows that a hyper-tanh with L = 15 cm and $$\Delta n$$ = 4e−4 is the optimum design choice that provides an FWHM of 36.9378 nm, a GDR of 0.85 ps, a reflectivity of $$-4.46706$$ dB and an SLSR of 42.08 dB. Optimization indicates that a tanh apodization with $$L =$$ 15 cm and $$\Delta n=$$ 4e−4 is the optimum choice for dual-stage operation that achieves an FWHM of 37.2244 nm, a GDR of 0.85 ps, a reflectivity of −5.36151 dB and an SLSR of 43.59 dB. Small variations in the SLSR level (e.g., 0.8 dB) are observed while investigating the effect of temperature on the dual-stage operation even in the worst-case operating scenario.

Research on transmission characteristics of phase-shifted CFBG in 2 μm band

Abstract Principles of single- and multi-phase-shifted chirped fiber Bragg grating (CFBG) in 2 μm band are analyzed with transmission matrix method, and transmission characteristics, including transmissivity, wavelength and bandwidth performance, of single- and multi-phase-shifted CFBG in 2 μm band with different grating parameters are discussed by simulation. It is found that phase-shifted position, grating length, chirped coefficient and refractive index modulation amplitude all have impacts on bandwidth of phase-shifted transmission peak. The transmission peak bandwidths of single-, dual- and tri-phase-shifted CFBG can reach to 0.0270, 0.0172 and 0.0112 nm respectively. The experiment of phase-shifted CFBG verifies the correctness of the simulation, and the bandwidths of 0.0901 and 0.0480 nm are acquired for single- and dual-phase-shifted CFBG respectively, which are narrower than previous filters based on fiber Bragg grating in 2 μm band multi-wavelength fiber laser system.

Single-frequency Yb-doped fiber laser with distributed feedback based on a random FBG

Single-frequency operation of a 1.03 μm fiber laser with random distributed feedback (RDFB) is demonstrated. The laser cavity is based on a 4 cm long fiber Bragg grating (FBG) consisting of 10 homogeneous subgratings with random phase and amplitude of refractive index modulation inscribed in a polarization maintaining (PM) Yb-doped fiber. Such RDFB laser generates single longitudinal mode with output power up to 25 mW, which is 3.5 times higher than that for a DFB laser based on regular π-shifted FBG of the same length in the same fiber. The single-frequency linewidth is measured to be <100 kHz in both cases. The observed difference of the DFB and RDFB lasers is confirmed by numerical simulation showing different longitudinal distribution of intra-cavity radiation in these cases, analogous to those in the experiment.

Nanoparticle-polymer composite volume holographic gratings dispersed with ultrahigh-refractive-index hyperbranched polymer as organic nanoparticles.

We report on volume holographic recording at a wavelength of 532 nm in photopolymerizable polymer nanocomposites that are incorporated with new hyperbranched polymers (HBPs) acting as transporting organic nanoparticles. Since HBPs possess an ultrahigh index of refraction of 1.82 due to the inclusion of triazine and aromatic ring units, high-contrast transmission volume holographic gratings with refractive index modulation amplitudes as large as 2.2×10(-2) are recorded. This value enables us to realize a 10 μm thick transmission volume grating with the diffraction efficiency near 100% in the green.

Novel Effective Approach for the Fabrication of PDMS‐Based Elastic Volume Gratings

All‐optical fabrication of elastic volume diffraction gratings in polydimethylsiloxane (PDMS) is presented. Novel material based on the commercially available PDMS with incorporated benzophenone (BPh) photoactive molecules is developed. The gratings are formed by a holographic technique and UV irradiation through an amplitude mask. New material permits to obtain efficient volume gratings with periods ranging from hundreds nanometers to dozens of micrometers in elastic films of different thicknesses. Besides symmetric transmission 1D gratings, slanted and 2D gratings have been fabricated as well. Photoattachment of BPh molecules and their real‐time diffusion within elastic PDMS matrix in accordance with the spatially modulated light is considered as a mechanism of the gratings formation. The refractive index modulation amplitude of about 7.0 × 10−4 is achieved. It is shown that mechanical strain of the elastic volume gratings allows fully reproducible alteration of the gratings parameters. In order to tune the diffracted wavelength over the whole visible spectral range (from 410 up to 700 nm), a strain of about 75% is required. New volume diffraction PDMS gratings represent very successful combination of cheap and widely known materials with one‐step optical fabrication techniques providing high‐performance tunable diffraction elements that can be applied in photonics, sensing, and spectroscopy.

Tunable dynamic Fano resonances in coupled-resonator optical waveguides

A route toward line-shape engineering of Fano resonances in photonic structures is theoretically proposed, which uses dynamic modulation of the refractive index of a microcavity. The method is exemplified by considering coupled-resonator optical waveguide systems. An exact Floquet analysis, based on coupled-mode theory, is presented. Two distinct kinds of resonances can be dynamically created, depending on whether or not the static structure sustains a localized mode. In the former case, a single Fano resonance arises, which can be tuned in both frequency and linewidth by varying the refractive index modulation amplitude and frequency. In the latter case, two resonances, in the form of narrow asymmetric dips in the transmittance, are found, which can overlap resulting in an electromagnetically induced transparency effect.