The applications for optical resonators are manifold, ranging from laser resonators and filters to optical sensors based on resonators, such as optical gyroscopes. A concept of a miniaturized optical gyroscope has been developed. To realize a gyroscope of this sort, the assembly and precision alignment of the optical microcomponents to form a triangular cavity is crucial. In order to detect the rotation rate, the laser beam must circulate many times and not leave the resonator due to small misalignments. Therefore, the assembly of a miniaturized passive free space triangular ring resonator, in which the light can circulate by reflections by three mirrors, is investigated. To utilize the inherent alignments of crystal planes, two of the mirrors are realized by micromanufacturing within the same silicon crystal using wet etching, resulting in very perfect {111} facets. To further increase the reflectivity of the mirrors, different kinds of coatings are tested. With these two perfectly aligned mirrors, the resonator assembly challenge reduces to a three-degree of freedom alignment of a third mirror, in which a well-designed adjustable spacer is developed. Resonances with the etched {111} micromirrors in a linear cavity setup as well as in a triangular ring cavity setup have already been demonstrated.
For realization of a miniaturized optical gyroscope the precise assembly of optical micro components is crucial. To detect the rotation rate the laser beam must circulate many times and not leave the resonator due to small misalignments. The assembly of a passive free space triangular ring resonator in which the light can circulate by reflections at three mirrors is investigated. The beam path encloses an area of about 100 mm2. The resonator shall be activated by an external light source at 1550 nm, a wavelength at which silicon is translucent and allowing to couple light into the resonator through the silicon mirrors. To utilize an inherent mirror alignment two of the mirrors are fabricated with a micro manufacturing process within the same crystal by wet etching resulting in very perfect {111} facets. The etching solution was optimized with respect to process time and smoothness of the mirror surfaces. To further increase the reflectivity of the mirrors different kinds of coatings are tested. With these two perfectly positioned mirrors the assembly challenge reduces to 3DOF alignment of a third {100} mirror for which a well-designed adjustable spacer is developed. In first tests resonance in a linear cavity test setup is demonstrated already.
A new concept for the realization of a micro optical laser gyroscope was developed. This new concept involves a passive free space ring resonator in which the light is circulating by reflections at three double mirrors and an external light source to activate the resonator. To couple the light in and out of the resonator waveguide-couplers are employed. This paper reports on the simulation of waveguide-coupler structures and on experimental investigation of coupling efficiency using micro fabricated SU-8 coupler structures. The modeled coupler structures consist of two parallel waveguides. The waveguides with rectangular profile are in close proximity i.e. separated only by a narrow gap over a certain path length Waveguide-coupler structures with similar geometries have been micro fabricated and optically characterized. It has been found that as a consequence of the lithographic formation of SU-8 high aspect ratio waveguides residual SU- 8 material remains between the waveguides as the gaps become very small (below 5 μm). In these structures a parasitic connection between the two parallel waveguides could be identified. No coupling effect was observed in the micro fabricated devices with perfect gap separation. From comparison of simulations and experiments we can conclude that there is a coupling mechanism based on the residual SU-8 material bridging the separation gap. Bridging allows coupling light at gaps even larger than 1 μm. Such residual material coupling can be achieved with SU8 lithographic high aspect ratio structuring (height 30 μm x width 50 μm or height 30 μm x width 20 μm) in which sub μm-gaps are almost impossible to produce with standard technologies.
120° silicon double mirrors for use in a novel micro optical resonator laser gyroscope have been developed in order to maximize the robustness of the sensor design against alignment errors. The idea being pursued is that the angle between these two mirrors can be intrinsically defined by silicon crystallography which enables a well-defined resulting reflection angle that is robust against mirror misalignment. As a consequence, resonator losses resulting from misalignment can be minimized. This work describes the concept of a new type of optical micro-gyroscope with double mirrors and its design. The mirrors are made from silicon wafers which are slightly deviating from perfect (100) orientation and thus allow to create <111> facets oriented 60° against the surface instead of 54,7° by wet etching. Two likewise structured wafers are connected by silicon direct bonding and separated into double mirror elements. These elements are mounted on a test platform to assess the quality and to confirm the suitability of a gyroscope. In this setup various possible misalignments were simulated. The experiments confirm the predicted misalignment robustness.
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