Fabrication Of Optical Waveguides Research Articles

Vibrational Properties of the Potassium Titanyl Phosphate Crystal Family

The crystal family of potassium titanyl phosphate (KTiOPO4) is a promising material group for applications in quantum and nonlinear optics. The fabrication of low-loss optical waveguides, as well as high-grade periodically poled ferroelectric domain structures, requires a profound understanding of the material properties and crystal structure. In this regard, Raman spectroscopy offers the possibility to study and visualize domain structures, strain, defects, and the local stoichiometry, which are all factors impacting device performance. However, the accurate interpretation of Raman spectra and their changes with respect to extrinsic and intrinsic defects requires a thorough assignment of the Raman modes to their respective crystal features, which to date is only partly conducted based on phenomenological modelling. To address this issue, we calculated the phonon spectra of potassium titanyl phosphate and the related compounds rubidium titanyl phosphate (RbTiOPO4) and potassium titanyl arsenate (KTiOAsO4) based on density functional theory and compared them with experimental data. Overall, this allows us to assign various spectral features to eigenmodes of lattice substructures with improved detail compared to previous assignments. Nevertheless, the analysis also shows that not all features of the spectra can unambigiously be explained yet. A possible explanation might be that defects or long range fields not included in the modeling play a crucial rule for the resulting Raman spectrum. In conclusion, this work provides an improved foundation into the vibrational properties in the KTiOPO4 material family.

Optical imprinting subwavelength-period liquid crystal polarization gratings with dual-twist templates.

In this Letter, we report a dual-twist template imprinting method to fabricate subwavelength-period liquid crystal polarization gratings (LCPGs). In other words, the period of the template must be reduced to 800 nm-2 µm, or even smaller. To overcome the inherent problem that the diffraction efficiency shrinks as the period decreases, the dual-twist templates were optimized by rigorous coupled-wave analysis (RCWA). With the help of the rotating Jones matrix to measure the twist angle and thickness of the LC film, the optimized templates were fabricated eventually, and the diffraction efficiencies were up to 95%. Therefore, subwavelength-period LCPGs with a period of 400-800 nm were imprinted experimentally. Our proposed dual-twist template provides the possibility for fast, low-cost, and mass fabrication of large-angle deflectors and diffractive optical waveguides for near-eye displays.

Biocompatible Optical Fibers Made of Regenerated Cellulose and Recombinant Cellulose-Binding Spider Silk

The fabrication of green optical waveguides based on cellulose and spider silk might allow the processing of novel biocompatible materials. Regenerated cellulose fibers are used as the core and recombinantly produced spider silk proteins eADF4(C16) as the cladding material. A detected delamination between core and cladding could be circumvented by using a modified spider silk protein with a cellulose-binding domain-enduring permanent adhesion between the cellulose core and the spider silk cladding. The applied spider silk materials were characterized optically, and the theoretical maximum data rate was determined. The results show optical waveguide structures promising for medical applications, for example, in the future.

One-dimensional and two-dimensional Er3+-doped germanate glass waveguides by combination of He+ ion implantation and precise diamond blade dicing

Compact and miniaturized Er3+-doped waveguide structure is an important research direction in integrated optics. In this work, the Er3+-doped germanate glass was bombarded by a 400-keV He+ ion beam with a fluence of 6.0 × 1016 ions/cm2 for the formation of the one-dimensional waveguide in a vacuum. Subsequently, the precise diamond blade with a rotating velocity of 20,000 rpm and a cutting speed of 0.1 mm/s was applied to dice the surface of the one-dimensional waveguide for the fabrication of the two-dimensional structure. The formation mechanism and the refractive index profile of the one-dimensional waveguide were studied by the SRIM 2013 and the RCM, respectively. The electromagnetic field profiles for the fundamental modes of both one-dimensional and two-dimensional waveguides were measured by the end-face coupling method. It presents some useful information in the field of fabrication and characterization of optical waveguides for active photonic applications.

Ultrafast laser writing of arbitrary long low-loss waveguides in optical fibers.

We propose an innovative femtosecond laser writing approach, based on a reel-to-reel configuration, allowing the fabrication of arbitrary long optical waveguides in coreless optical fibers directly through the coating. We report few meters long waveguides operating in the near-infrared (near-IR) with propagation losses as low as 0.055 ± 0.004 dB/cm at 700 nm. The refractive index distribution is shown to be homogeneous with a quasi-circular cross section, its contrast being controllable via the writing velocity. Our work paves the way for the direct fabrication of complex arrangements of cores in standard and exotic optical fibers.

Single-mode SU-8 waveguide fabricated using ultrafast direct laser writing

In this work, we demonstrated the application of femtosecond laser direct writing using the two-photon polymerization (TPP) technique in the fabrication of single-mode optical waveguide in SU-8 photoresist. It was found that the polymerization of SU-8 using 780 nm femtosecond laser requires a fluence of 1.8–10 kJ/cm2, where 2.5 kJ/cm2 is sufficient to create a flat-top surface waveguide for a 1.8 µm thick SU-8 film. To reduce the coupling loss of the waveguide, the waveguide width was enlarged with double scan of the femtosecond laser by shifting the sample laterally with a center-to-center spacing of 0.5 µm between scans. The optimized waveguide has an insertion loss of 11 dB, with an approximated propagation loss of 1.27 dB/cm. Based on the information obtained, a directional coupler was fabricated. The resulting coupling ratio of the directional coupler is 47 % and 53 % at each port. To further demonstrate the viability of the proposed technique to fabricate complex waveguide structures, a vertically suspended waveguide has been designed and fabricated.

Development of High Refractive Index Polydimethylsiloxane Waveguides Doped with Benzophenone via Solvent-Free Fabrication for Biomedical Pressure Sensing

We present the fabrication and characterization of elastomeric optical waveguides, to be used for the manufacture of a conformable, water-resistant, and cost-effective pressure sensor that is amenable to the development of smart wearable health monitoring devices. To achieve this goal, high-sensitivity polydimethylsiloxane waveguides with a rectangular cross-section were fabricated. A new up-doping procedure, to tailor the refractive index of the ensuing waveguides, was experimentally developed using benzophenone additives. With this method we demonstrated a high refractive index change (up to +0.05) as a linear function of the benzophenone doping concentration. Propagation losses of about 0.37 dB/cm in the visible range and a high sensitivity to transverse compression of 0.10%/dB optical power loss were measured. It was also shown that one can further control the refractive index of the waveguide core and cladding regions through proper selection of the polydimethylsiloxane base to curing agent mixing ratio.

Near-infrared single-mode optical waveguide fabrication in LiF crystal by carbon ion irradiation

In this study, a carbon-ion beam irradiation method with energy of 6.0 MeV was employed to prepare single-mode optical waveguide structures in LiF single crystal. The results of this study reveal that the visible light and near-infrared laser beams can be well-confined in the irradiated region that is influenced by the irradiation fluences. An end-face coupling system was used to estimate the propagation loss (1.3 dB cm−1) after annealing treatment. The surface damage of the irradiated regions in LiF was analysed based on the displacements per atom calculation using the stopping and range of ions in matter code.

Elastomeric Optical Waveguides by Extrusion Printing

AbstractAdvances in optogenetics and the increasing use of implantable devices for therapies and health monitoring are driving demand for compliant, biocompatible optical waveguides and scalable methods for their manufacture. Molding, thermal drawing, and dip‐coating are the most prevalent approaches in recent literature. Here the authors demonstrate that extrusion printing at room temperature can be used for continuous fabrication of compliant optical waveguides with polydimethylsiloxane (PDMS) core and crosslinked Pluronic F127‐diacrylate (Pluronic‐DA) cladding. The optical fibers are printed from fluid precursor inks and stabilized by physical interactions and photoinitiated crosslinking in the Pluronic‐DA. The printed fibers show optical loss values of 0.13–0.34 dB cm–1 in air and tissue within the wavelength range of 405–520 nm. The fibers have a Young's Modulus (Pluronic cladding) of 150 kPa and can be stretched to more than 5 times their length. The optical loss of the fibers shows little variation with extension. This work demonstrates how printing can simplify the fabrication of compliant and stretchable devices from materials approved for clinical use. These can be of interest for optogenetic or photopharmacology applications in extensible tissues, like muscles or heart.

Controllable Dynamic Single‐ and Dual‐Channel Graphene Q‐Switching in a Beam‐Splitter‐Type Channel Waveguide Laser

AbstractDirect inscription of structures by femtosecond‐laser pulses facilitates the flexible fabrication of diverse optical channel waveguides in dielectric laser gain media. Among them, beam‐splitter‐type waveguide lasers have been recently studied; however, there is a lack of investigations on the effect of unique characteristics of such waveguides, such as controlled splitting ratios of output powers and selectable excitation of individual channels, for pulsed laser operation. Here, dynamic single‐ and dual‐channel graphene Q‐switching of an Yb:YAG waveguide consisting of one input and two output channels is demonstrated by controlling the power‐splitting ratio. Single‐channel Q‐switched operation, exhibiting typical Q‐switched pulses, is achieved by interaction between the graphene saturable absorber and the desired one of the two channels. When both channels exceed the Q‐switching threshold, dual‐channel Q‐switching generates a pulse train that simultaneously combines the separately Q‐switched pulses induced from each channel under excitation with a single pump source. At a fixed power‐splitting ratio, the pulsed mode can be dynamically switched between the single‐ and dual‐channel Q‐switched operations by varying the pump power. The beam‐splitter‐type waveguide laser demonstrated in this study can be further developed for multiple‐channel Q‐switching, high‐repetition‐rate mode‐locking, and on‐chip dual‐comb sources advantageous for diverse applications in optical communication, metrology, and sensing.

Quantum technologies in diamond enabled by laser processing

Integrated photonic circuits promise to be foundational for applications in quantum information and sensing technologies, through their ability to confine and manipulate light. A key role in such technologies may be played by spin-active quantum emitters, which can be used to store quantum information or as sensitive probes of the local environment. A leading candidate is the negatively charged nitrogen vacancy (NV−) diamond color center, whose ground spin state can be optically read out, exhibiting long (≈1 ms) coherence times at room temperature. These properties have driven research toward the integration of photonic circuits in the bulk of diamond with the development of techniques allowing fabrication of optical waveguides. In particular, femtosecond laser writing has emerged as a powerful technique, capable of writing light guiding structures with 3D configurations as well as creating NV complexes. In this Perspective, the physical mechanisms behind laser fabrication in diamond will be reviewed. The properties of waveguides, single- and ensemble-NV centers, will be analyzed, together with the possibility to combine such structures in integrated photonic devices, which can find direct application in quantum information and sensing.

Fabrication of Polymer Optical Waveguides by Digital Ultraviolet Lithography

We present the design, fabrication and characterization of polymer optical waveguides using a digital ultraviolet (UV) lithography. Grayscale optical exposure with a nonlinear compensation in quadratic form is applied to minimize the stitching loss of the transition zone between two adjacent subpatterns. Proximity effects in lithography process are compensated based on an exposure dose-map that is calculated under the approximation of scattered light pixel with a Gaussian distribution. The bending loss and propagation loss of the fabricated polymer waveguides are experimentally characterized to be around 0.1 dB/90°-bend and 0.238 dB/mm, respectively. Moreover, 1 × 2 multimode-interference power splitter, 1 × 2 Y-branch power splitter and microring resonator are demonstrated to show the feasibility of the lithography technology on rapid fabrication of waveguides. Such a UV lithography technology flexibly manipulates huge-number light pixels for all-digital grayscale and dynamic optical exposure and thus can enable the fabrication of novel optical waveguide-based devices and sensors, such as 2.5D waveguides for multimode crossing and polymer waveguide devices for biosensing.

BaF2 Ridge Waveguide Operating at Mid-Infrared Wavelength

We report on the fabrication of optical ridge waveguide in barium fluoride (BaF2) crystal by 15 MeV C5+ ions irradiation with femtosecond laser ablation. The near-field modal profile and propagation loss of the waveguide at mid-infrared wavelength 4 µm are investigated by using end-face coupling system. We implement a series of annealing treatment and it efficiently reduces the propagation loss of the waveguide. The confocal Raman spectra demonstrate that the lattice structure of BaF2 crystal does not change largely after C5+ ion irradiation.

Fabrication of ridge optical waveguide in thin film lithium niobate by proton exchange and wet etching

In this paper, a low-loss ridge waveguide based on the thin film lithium niobate (TFLN) is successfully fabricated using a proton-exchanged wet etching (PEWE) technique, which can result in a much better vertical sidewall angle and improve the surface roughness to reduce the scattering loss. The average etching rate is obviously improved by the structural defects and phase transition induced by our modified PE process, and the finally optimized etching rate is as fast as 13.5 nm/min. The propagation loss of the fabricated straight ridge waveguide is measured to be 4.3 dB/cm by using an 850 nm single mode laser as the light source. Meanwhile, a Y-branch waveguide is also measured with a uniform light splitting ratio of nearly 1:1. Furthermore, this work offers the route for the manufacturing of low-cost and high-efficiency photonic integration platform.

Fast fabrication and gas-sensing characteristics of petal-like Co-MOF membrane optical waveguide

In this work, metal organic frameworks (MOFs) were chosen as sensing materials for the development of a planar optical waveguide (POWG) gas sensor with improved selectivity. Petal-like Co-MOFs ([Co(TpA)dpNDI], where TpA = terephthalic acid and dpNDI = dipyridyl-naphthalenediimide) membranes were fabricated via UV light irradiation of the surface of an Sn-diffused glass optical waveguide slide as the substrate. The obtained film exhibited a high refractive index (1.873–1.875) and optimum thickness to support a strong single-mode (TE0) guided light signal, fulfilling the essential requirements of a POWG sensor. The [Co(TpA)dpNDI] membrane POWGs uniquely responded to ethylenediamine (EDA) coexisting with interfering gases such as methylamine, dimethylamine, and BTXs (benzene, toluene, xylene, and styrene). When the membrane was exposed to EDA gas, host–guest charge transfer and refractive index modulation were induced. The [Co(TpA)dpNDI] membrane POWG exhibited a fast and remarkable response to EDA (S/N = 3.15) even at low concentration (1 ppb), which suggests the capability of this membrane POWG for EDA sensing. The sensor also showed response repeatability in the detection range of 1000 ppm to 1 ppb, and a high adsorption capacity at ambient temperature (17.46 μg cm−2).

Cylindrical Confinement of Nanocolloidal Cholesteric Liquid Crystal.

The organization of nanocolloidal liquid crystals in constrained geometries has fundamental and practical importance, since under confinement, liquid crystals contain stable topological defects that can serve as templates for nanoparticle organization. Three-dimensional confinement of cholesteric (Ch) liquid crystals formed by cellulose nanocrystals (CNCs) have been extensively studied; however, their two-dimensional confinement remains under-investigated. Here, we report the results of systematic experimental studies of two-dimensional confinement of Ch-CNC liquid crystal in cylindrical capillaries with varying inner diameters. Confinement resulted in phase separation of the Ch-CNC liquid crystal into a Ch shell formed by concentric CNC pseudolayers with the helicoidal axis perpendicular to the inner surface of the capillary walls, and a micrometer-diameter isotropic core thread running parallel to the long axis of the capillary. The morphology of the confined Ch-CNC liquid crystal varied when progressively increasing the degree confinement. Finally, we show that phase separation of the Ch-CNC liquid crystal into a Ch shell and an isotropic core is preserved in flexible capillary tubing, suggesting the applicability of this system for the fabrication of flexible optical waveguides.

SiO2 Nanoparticles-Acrylate Formulations for Core and Cladding in Planar Optical Waveguides.

A combination of acrylate formulations and SiO2 nanoparticles is investigated with the aim to improve the optical properties of low-refractive index polymers that are used for the fabrication of planar optical waveguides. A decrease in refractive index and also in the thermo-optic coefficient of nanocomposite materials is clearly demonstrated, while some formulations exhibit an increase in the glass transition temperature. The possibility of using these nanocomposite materials to fabricate waveguiding layers with low optical propagation losses at telecommunication wavelengths around 1550 nm is also shown. The nanomaterials can be applied in optical microchips on polymer platforms.

Optical nanocomposites containing low refractive index MgF2 nanoparticles

Nanoparticles composed of magnesium fluoride were successfully introduced into polymer matrices used for the fabrication of planar optical waveguides. Optical layers without visible scattering were successfully prepared. The transparent nanocomposites were formulated by direct mixing of modified MgF2 nanoparticles with fluorinated co-polymer matrix or acrylate monomer mixture with a help of additional solvent. An addition of MgF2 nanoparticles results in decrease of refractive index and thermo-optic coefficient of the polymer but increases substantially the optical propagation losses for planar waveguide at 1547 nm.

The Reducing Approaches of Scattering Losses in Polymer Planar Optical Waveguides

The article presents the development prospects of planar optical waveguides for high-speed data transmission systems optoelectronic buses by polymer materials. The advantages and disadvantages of using non-specialized polymeric materials for general use are revealed. The polymer planar optical waveguides fabrication technologies are proposed. The main losses types in planar optical waveguides, the reasons for their occurrence, as well as approaches to their reduction are determined. Using the example of PDMS polymer and soft lithography technology, the technological process critical stages of polymer planar optical waveguides production are noted, which contribute to an scattering losses increase. For each stage, algorithms are proposed to prevent an scattering losses increase. These algorithms were implemented in practice in the manufacture of layouts of polymer planar optical waveguides of the optical-electronic data transmission bus.

Erbium Related Photoluminescence of Silicon: Influence of Co-doping with Oxygen and Hydrogenation

This experiment deals with the influence of co-doping with oxygen and hydrogen and also annealing parameters, temperature and excitation power on the Er related photoluminescence (PL) of silicon. The ultimate goal is to optimise the PL intensity of the Er3+ internal transition to make at room-temperature luminescence possible. Silicon is a very inefficient light emitter, because of the low radiative recombination rate due to the indirect band gap. However, by adding optically active impurities such as erbium, Si can be made luminescent. Silicon is an ideal material for the fabrication of optical waveguides that are compatible with optical telecommunication technology at 1.54µm, because of its high transparency and high refractive index at this wavelength. Co-doping with oxygen and hydrogen can enhance the initial Er luminescence. To investigate the influence of oxygen and hydrogen on the diffusion process and the luminescence intensity, samples were doped with erbium, oxygen and hydrogen at different concentrations that were implanted both on Float Zone (FZ) and Czochralski (CZ) silicon wafers. In FZ silicon, the samples implanted with both oxygen and hydrogen showed the highest luminescence yield with a six times higher peak intensity as compared to samples implanted with only erbium and a two times higher peak intensity as compared to samples with erbium and oxygen, in accordance with previous results. However, identified by the line position and contrary to previous results on CZ-Si, the luminescence stems from an erbium-oxygen impurity complex and not from the so-called cubic centre. In CZ samples doped with erbium, oxygen and hydrogen show a 3 times higher intensity as compared to samples doped only with erbium and oxygen. Although the PL lines of the cubic centre are visible, they do not exhibit the strongest luminescence. In samples doped only with erbium and hydrogen and annealed at 900°C, we observe luminescence from only the cubic centre. Although the appearance of the cubic centre in these samples is most likely due to out-diffusion of erbium in the absence of oxygen, hydrogen again enhances the luminescence intensity by a factor 2.