Design of straight and S-bend optical channel waveguides based on silver ion implantation in SiO2 substrates is
presented. 3D Beam Propagation Method (BPM) calculations are used for the design of the waveguides based on step
index profiles produced from a sequential multiple ion implantation process. An analysis of modal optical confinement
was done by means of the Effective Index Method (EIM) for selecting the right dimensions of the channel waveguides.
Core index values between 1.4623-1.4662 are obtained, depending on the fluence, are considered. Depth and width for
the waveguides were chosen to provide single mode operation. Bending losses are determined as function of bending
radius, refractive index change (Δn), and wavelength.
Integrated planar concave gratings are promising candidates for Wavelength Division Multiplexing (WDM) devices. However, device insertion losses reported to date are higher than other options such as Arrayed Waveguide Gratings (AWGs) mainly due to the strict requirement for a deep vertical etch of grating facets. We present a novel planar concave grating demultiplexer design based on total internal reflection and the focusing properties of ellipses which simplifies the fabrication of the device by avoiding grating facet metallization and improves grating efficiency by reducing its sensitivity to non-vertical etching of the facets. As a proof of concept of the grating design, a 4-channel coarse WDM demultiplexer working near 850nm with a channel wavelength spacing of 20nm has been fabricated in PMMA polymer using the LIGA process, a deep x-ray lithography technique using synchrotron radiation. Smooth and extremely vertical sidewalls 220 microns deep have been obtained. Test results for this preliminary device show a -7dB on-chip loss for the best channel. Based on the test data, a –3dB loss can be reasonably expected for a device with optimized material processing. In the future, the LIGA process can be used to fabricate a metal stamp that can massively produce devices in polymers with plastic embossing or molding techniques.
Periodically structured dielectric media with lattices on a sub-wavelength scale are receiving increased attention as they enable a variety of photonics applications. Fabrication, however, still imposes challenges to the scientific community. This article discusses process modifications in deep X-ray lithography to reduce minimum feature size and eventually allow the fabrication of high aspect ratio photonic crystal slabs with a moderate refractive index. Proximity printing requires an X-ray mask with high contrast and lateral resolution. Electron beam writing exposure doses were optimized to pattern feature sizes down to 400 nm in 3 μm thick resist. The voids were subsequently electroformed with 2 μm gold to generate the absorbers on a suspended silicon nitride membrane. The mask was copied into PMMA films of 5 μm thickness using X-ray lithography at about 0.4 nm. The yield of free standing smallest features is limited by adhesion of the resist to the substrate. Structures with aspect ratios as high as 8 to 12 tend to collapse after dip development. Periodic features are increased on the order of tens of nanometers compared to mask absorbers. Lattice constants need to be slightly reduced at fixed pore diameters before first photonic demonstrators made of PMMA can be fabricated.
A double mirror system has been studied and realized for purposes of Deep x-ray Lithography/LIGA. Because structuring high aspect ratio resist ask for high photon flux to maintain exposure time compatible with industrial process, hard x-ray synchrotron radiation is necessary and its broadband spectrum is normally used. However one would need to shape this spectrum on both low and high-energy parts. The former may lead to resist surface overdoes and is treated by means of appropriate filters. The later is prejudicial at resist-metallic substrate interface where high-energy photons are preferentially absorbed giving rise to extra-dose even behind absorber patterns. This may result in microstructures adhesion break down for positive resist and incomplete dissolution for negative tone ones. A mirror set up in the upstream beam-line is a correct answer to prevent these effects. The association of filters and mirror gives an adjustable band-pass filter to fit almost exposure requirements in deep x-ray lithography. Preliminary result of such a system are presented concerning its band-pass filter character, flux performances and first comparative effects on resist microstructures.
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