A monolithic micro-optical interferometer deep etched into fused silica

•A monolithically integrated free-space-interferometer is demonstrated.•An optimized deep-reactive ion etching (RIE) process in fused silica shows optical quality.•The interferometer is able to measure path differences in the nanometer range.

For free-space micro-optical systems, the alignment of the components is still a challenging task in manufacturing. Alternatively, a monolithic integration can overcome this problem, but especially for in-plane optical elements in the visible wavelength range, the optical surfaces have to fulfill critical demands. Here, we show a fabrication process that allows the deep-reactive ion etching (RIE) of fused silica with high optical quality. We achieve vertical sidewalls with etch depths of about 100 μm with an arithmetic mean roughness of about 7.2 nm. By using this process, a new in-plane monolithic, free-space interferometer is demonstrated that reaches a resolution of 20 nm with our current setup.

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