VCSELs (Vertical-Cavity Surface-Emitting Lasers) emit circularly symmetric beams vertical to the substrate; the small footprint of the active area (around 400 um2) enables the simultaneous fabrication of several thousand devices on a single wafer. Micro-optical components can modify the free-space optical properties of VCSELs for applications such as fiber-coupling in transceiver modules, illumination purposes, or beam profiling in sensing applications. However, the alignment of a laser towards a lens, for example, is expensive when performed separately for each device. Here we demonstrate a wafer-scale replication process to realise microlenses directly on top of the undiced VCSEL wafers. The process combines uv-casting and lithography to achieve material-free bonding pads and dicing lines. Several examples of lenses and gratings are given. An organically modified sol-gel material (ORMOCER) has been used as lens material. The micro-optical components on the wafer show good stability while sawing and bonding, where temperatures up to 220°C may occur. We have compared refractive lenses on top of the VCSELs with lenses on glass substrates. The lenses on the glass wafers were illuminated from the back-side by a planar wave. Spot diameters around 1.2 um and focal lengths of 30 um to 100 um were measured depending on the radii of curvature. On the VCSELs the lenses showed a strong influence on the transversal mode behaviour.
A new wafer-scale replication process for fabricating buried ridge
waveguides for telecom/datacom applications using an uv-curable
sol-gel material is proposed. Spin coating of the core material on
the replication mould is used to form the waveguide cores with a
smooth thin layer. The spin parameters allow an accurate control
of the thickness and homogeneity. The bottom-cladding is uv-cast
between a substrate and the mould, which is covered by the spun
core layer. The ridge waveguide cores are demoulded and buried
under a top cladding. This process allows the stacking of several
layers of waveguides on top of each other to form two-dimensional
waveguide arrays. A specially adapted SUSS mask aligner is used to
control the cladding thickness between individual waveguide layers
and to align them. A waveguide loss comparable to lithographically
fabricated waveguides has been achieved.
We report on an angle-tunable oblique incidence resonant grating filter that can be used to drop individual channels from the C-band for incident TE-polarized light. For tuning purpose, the filter is glued onto a tiltable platform of a MEMS device. Continues scanning of the platform allows to monitor channel presence and power. The reflected wavelength is tuned by changing the angle of incidence of the resonant grating filter, which is composed of two thin films with a grating pattern on top of it. The first layer on a glass substrate acts as a waveguide, and the second layer separates the waveguide from the grating. The grating has been patterned by holographic recording and dry etching. The filter works over a wavelength range of 1520-1580 nm and its response has a Lorentian shape with 0.5 nm FWHM peak width. The MEMS part is based on SOI technology and is processed in only two DRIE steps. The platform measures 2 x 2 mm2 with a through-hole of 1.6 x 1.8 mm2 for light transmission. Two arrays of combs attached to the platform as well as a set of four static combs are used to electrostatically incline the platform by ± 4° with a driving voltage of about 60 V.
We designed a tunable, oblique incidence resonant grating filter covering the c-band as drop device. Our resonant grating filter consists of a planar waveguide on a glass substrate covered by low index medium that separates the waveguide from the grating on top of it. With these 3 layers we reach a finesse of more than 3000, which would require much more layers in traditional thin film technology. The drop filter can be tuned by tilting the MEMS platform on which the filter will be glued. Tuning over the c-band will require tilt angles of 3° of the MEMS platform in both directions. Measurements indicate a resonance peak shift of 1.2% and a negligible shape change of the resonance peak from 1526nm at 45° angle of incidence to 1573nm at 53° with a full width at half maximum of 0.4nm. In this range the peak wavelength shift is linear with respect to the change of the AOI.
Tandem chirped grating couplers for spectral measurement applications in optical communications are developed. The current devices are designed to monitor data/telecom dense wavelength-division multiplexing (DWDM) channels in the spectral range from 1528 to 1561 nm (C-Band). A replication process provides the diffractive structures, on the gratings a high-index waveguide material is deposited. Design parameters and fabrication tolerances are discussed in detail, and measurement results of the fabricated devices are presented.