In this paper we will present two technological processes necessary to experimentally obtain diffractive optical elements (DOE) operating in reflection which generate optical beams presenting orbital angular momentum. This class of DOE, also known as spiral phase plates (SPP), are three-dimensional structures consisting in disks where the variation of their thickness is directly proportional with the azimuthal angle Φ. In this case a beam of light applied on the SPP surface acquires an angular orbital momentum (OAM) and a vortex – like configuration. The first technique is the 3D electron beam lithography (EBL). The materials utilized in this case are the positive electronresist PMMA 35 K which has a deposition thickness of 500 nm and the negative electronresist SU8. To obtain the three dimensional structure, the PMMA and SU-8 films were configured by different exposure doses such that after development process each exposure corresponds to a defined thickness of the electronresist. The calibration curve between exposure dose and the height of the structure was determined. The second technique investigated here is photolithography. In this case a photoresist layer was exposed through a mask presenting regions with various levels of transmittance in order to obtain different levels of heights.
The use of photonic integrated circuits made of polymer materials represents a solution for obtaining low-cost
immunosensors for fast clinical diagnosis. In this paper are presented the simulation studies of a photonic integrated
sensor on silicon substrate based on the configuration of Young interferometer. The core and cladding materials of the
photonic sensor are polymeric materials. This sensor works for the detection of the surrounding medium refractive index
variation and also for the detection of a thin adsorbed layer on the sensor surface. Simulations are performed using the
Beam Propagation Method and 2D mode solvers for obtaining the relation between the variation of the surrounding
refractive index or the presence of an adsorbed layer and the displacement of the interference fringe position. From this
dependence one can calculate the sensor sensitivity and also one can estimate the detection limit. In order to obtain
reliable results it is necessary to have waveguides which presents single mode operation regime both on the horizontal
and vertical direction. Rib waveguides which are more prone for satisfying single mode condition were considered. The
suppression of the higher order modes on the vertical direction by leakage in the silicon substrate is made by adjusting
the thickness of the silicon dioxide buffer layer.
This paper reports fabrication methods of polymer-based micro/nano optical structures based on replica molding.
Various molds have been used: polymer, silicon, and SiO2-based. Different types of treatment and release agent were
investigated in order to achieve an optimum demolding. Diffractive optical elements, micro-lenses and antireflective
layers have been obtained in commercial or doped polymers with controlled refractive index. The quality of the
replication was investigated using optical microscopy, SEM, profilometry and functional tests. The micro-optical
components can be transferred onto silicon silicon chip with photodetectors, or photonic integrated circuits using
We summarize the results of a European Project entitled WAPITI (Waferbonding and Active Passive Integration Technology and Implementation) dealing with the fabrication and investigation of active/passive vertically coupled ring resonators, wafer bonded on GaAs, and based on full wafer technology. The concept allows for the integration of an active ring laser vertically coupled to a transparent bus waveguide. All necessary layers are grown in a single epitaxial run so that the critical coupling gap can be precisely controlled with the high degree of accuracy of epitaxial growth. One key challenge of the project was to establish a reliable wafer bonding technique using BCB as an intermediate layer. In intensive tests we investigated and quantified the effect of unavoidable shrinkage of the BCB on the overall device performance. Results on cw-operation, low threshold currents of about 8 mA, high side-mode suppression ratios in the range of 40 dB and large signal modulation bandwidths of up to 5 GHz for a radius of 40 μm shows the viability of the integration process.
Theoretical and numerical studies regarding the possibility of using simple laterally coupled microring resonators as
refractometers are presented in this work. We have considered a core waveguide layer with its refractive index varying in the range 1.5...2 deposed on the silicon dioxide thermal grown layer. The waveguide width is set for achieving
single-mode condition at 1.55 &mgr;m radiation wavelength. The aim of this work is finding the optimum configurations for
microring resonators based refractive index sensors.
Poly (vinyl alcohol) [PVA] is a photo-induced cross-linking polymer, water-soluble,
biocompatible, used in holography, nonlinear optics, as tissue engineering scaffolds and as polymer
matrices for enzymes immobilization. PVA has been investigated for use as binder polymer in optical
waveguides for sensor applications. The Y-shaped waveguides is composed of a buffer layer (lower
refractive index) - SiO2, a core layer (higher refractive index) - PVA doped for the refractive index and
sensibility increasing and a cladding layer (lower refractive index) - an other polymer. The light
propagation in doped PVA waveguides represents the sensing element of the sensor. The preliminary
results suggest that doped PVA polymers are promising for optical (bio)chemical sensors; the processes
used to make them, represent environmentally friendly technology.
We present preliminary experiments for an integrated optical sensor based on a Mach-Zehnder interferometer for
biological applications. The sensor is sensitive to refractive index change produced by the presence of a biological
species in the cladding of the optical waveguide. A "window" can be patterned in the upper cladding, so that the
evanescent wave can be in direct contact with the environmental (the sensitive layer). We investigated as optical
waveguides a new material, SU-8, a negative photoresist well known from the development of 3D micromachmed
structures. We structured, by photolithographic techniques, rib and channel optical waveguides. We studied the influence
of the silicon substrate on propagation losses and the possibility to use these losses for the selective attenuation of the
higher order modes on the vertical direction. As biological materials we experimented collagen, which is a bio-polymer
which can bind different enzymes or antibodies.
Microring resonators will be one of the most important components of the next generation of optical communications. In this work, we have analyzed from theoretical perspective a new proposed microring resonator structure based on the wafer-bonding technique which implies the vertical coupling between the passive bus waveguide and the active ring resonator. We have investigated the possibility to obtain the monomode operation of the active ring waveguide for certain ring radius values by the selective attenuations of the higher order modes and the obtaining of the desired coupling efficiency by varying the technological parameters like the layers thickness, etching depth, bus waveguide width and the offset (misalignment between the ring and the bus waveguide). Depending on the fabrication method, the misalignment between the ring resonator and the bus waveguide may vary within a significant range. Therefore, we considered a much wider bus waveguide in the coupling region in order to minimise the effects of misalignment.
We report an analytical model for calculation of reflection and transmission coefficients of a Bragg reflector with periodic structure using transfer matrix method. Using explicit expressions for these coefficients, the reflectivity of the periodic structures for different pairs of layers and layer thickness was simulated. We investigate the reflectivity of the periodic structures consisting of following pairs of successive layers: SiO2 /Si3N4 (low ratio of refractive indexes); poly-Si/ SiO2 (high ratio), Si/air-gap (high contrast). The theoretical and experimental investigations of a particular periodic structure consisting of SiO2/Au are also presented. Our method allows the rapid evaluation of reflectance of Bragg reflector with periodic structure.
This paper presents the experiments we have performed to obtain freestanding SiO2 and c-Si based microphotonic components by anisotropic wet etching of silicon (111) wafers. The process is simpler than surface micromachining. It requires only one grown or deposited layer and one mask for SiO2 structure, or two masks for c-Si structures. Moreover, the technique provides plan-parallel microstructure with very flat (111) surfaces, useful for photonic components like micromirrors and waveguides. Movable SiO2 and silicon-based micromirrors and waveguides with very smooth surfaces were obtained by anisotropic etching in a KOH solution combined with plasma etching. The possible applications of SiO2 and silicon based freestanding structures include devices for optical communications and bio- or chemo-optical sensors.
In this paper we present the design and the experiments performed to obtain a micromechanical voltage tunable Fabry-Perot interferometer integrated with a p-n photodiode on a silicon substrate. It can be used as a voltage tunable filter for the input radiation or as a voltage controlled attenuator to regulate the light from a monochromatic source. Different solution have been analyzed and experimented. The top mirror of the Fabry-Perot cavity is a doped poly-Si or Au/SiO2 movable membrane, electrostatically actuated, obtained using Si micromachining. A complex design process was performed: optical, electomechanical and technological. All these phases were performed interactively. Different materials were considered in order to perform an optimum design. Experimental micromachined interferometers were obtained using two techniques: (1) surface micromachining, and (2) anisotropic etching of (111)-oriented Si wafers, combined with an isotropic pre-etching step. These processes were optimized and matched to the photodiode fabrication process. Monolithic integrated interferometers coupled to p-n photodiodes were obtained.
The development of MEMS microsystems can be increased by integration of optically active parts. Micromachining techniques allow the fabrication of monolithically integrated Fabry-Perot microcavities, avoiding hybrid assembly technique, which is a combination of etching and wafer bonding. These microcavities can be used as sensors, as modulators or as tunable optical filters. We investigated different mirror materials: silicon nitride and polysilicon and different sacrificial layers: polysilicon and phosphorus doped silicon dioxide (PSG), using LPCVD and CVD techniques. Different arrays and shapes for the top mirror, which is movable, were analyzed in order to establish a structural material with low tensile stress. The optical constants were determined by spectrophotometric methods. Experimental data and simulations were compared.
The fabrication process of c-Si waveguides based on the anisotropic etching of Si<111> oriented wafers is described. To obtain c-Si waveguides, the anisotropic etching was combined with an isotropic pre-etch step to a depth equal to the thickness of the final c-Si freestanding structure, followed by side-wall passivation. In addition, a second pre-etching step was performed to establish the depth of the air gap that acts as the bottom cladding of the waveguide. Freestanding c-Si waveguides with very smooth surfaces were obtained by anisotropic etching in a KOH solution. By using a Si3N4/SiO2 mask layer, double waveguides were obtained. The possible applications of c-Si based free standing structures include devices for optical communications and evanescent-wave bio- or chemical sensors.
In this work a few configurations of waveguide reflective modulators are studied theoretically. The waveguides considered here are made from epitaxial silicon layer grown on heavy doped silicon substrate. The working wavelength is 1.3 µm. The optimal configuration of the proposed devices was obtained using the optical analysis.