Hollow-Core Optical Fibers

3D-nanoprinted on-chip antiresonant waveguide with hollow core and microgaps for integrated optofluidic spectroscopy

Here, we unlock the properties of the recently introduced on-chip hollow-core microgap waveguide in the context of optofluidics which allows for intense light-water interaction over long lengths with fast response times. The nanoprinted waveguide operates by the anti-resonance effect in the visible and near-infrared domain and includes a hollow core with defined gaps every 176 µm. The spectroscopic capabilities are demonstrated by various absorption-related experiments, showing that the Beer-Lambert law can be applied without any modification. In addition to revealing key performance parameters, time-resolved experiments showed a decisive improvement in diffusion times resulting from the lateral access provided by the microgaps. Overall, the microgap waveguide represents a pathway for on-chip spectroscopy in aqueous environments.

Menezes L, Schmidt MA, Maier SA. 3D-Nanoprinted Antiresonant Hollow-Core Microgap Waveguide: An on-Chip Platform for Integrated Photonic Devices and Sensors

Due to their unique capabilities, hollow-core waveguides are playing an increasingly important role, especially in meeting the growing demand for integrated and low-cost photonic devices and sensors. Here, we present the antiresonant hollow-core microgap waveguide as a platform for the on-chip investigation of light-gas interaction over centimeter-long distances. The design consists of hollow-core segments separated by gaps that allow external access to the core region, while samples with lengths up to 5 cm were realized on silicon chips through 3D-nanoprinting using two-photon absorption based direct laser writing. The agreement of mathematical models, numerical simulations and experiments illustrates the importance of the antiresonance effect in that context. Our study shows the modal loss, the effect of gap size and the spectral tuning potential, with highlights including extremely broadband transmission windows (>200 nm), very high contrast resonance (>60 dB), exceptionally high structural openness factor (18%) and spectral control by nanoprinting (control over dimensions with step sizes (i.e., increments) of 60 nm). The application potential was demonstrated in the context of laser scanning absorption spectroscopy of ammonia, showing diffusion speeds comparable to bulk diffusion and a low detection limit. Due to these unique properties, application of this platform can be anticipated in a variety of spectroscopy-related fields, including bioanalytics, environmental sciences, and life sciences.

The Optofluidic Light Cage - On-Chip Integrated Spectroscopy Using an Antiresonance Hollow Core Waveguide

Emerging applications in spectroscopy-related bioanalytics demand for integrated devices with small geometric footprints and fast response times. While hollow core waveguides principally provide such conditions, currently used approaches include limitations such as long diffusion times, limited light-matter interaction, substantial implementation efforts, and difficult waveguide interfacing. Here, we introduce the concept of the optofluidic light cage that allows for fast and reliable integrated spectroscopy using a novel on-chip hollow core waveguide platform. The structure, implemented by 3D nanoprinting, consists of millimeter-long high-aspect-ratio strands surrounding a hollow core and includes the unique feature of open space between the strands, allowing analytes to sidewise enter the core region. Reliable, robust, and long-term stable light transmission via antiresonance guidance was observed while the light cages were immersed in an aqueous environment. The performance of the light cage related to absorption spectroscopy, refractive index sensitivity, and dye diffusion was experimentally determined, matching simulations and thus demonstrating the relevance of this approach with respect to chemistry and bioanalytics. The presented work features the optofluidic light cage as a novel on-chip sensing platform with unique properties, opening new avenues for highly integrated sensing devices with real-time responses. Application of this concept is not only limited to absorption spectroscopy but also includes Raman, photoluminescence, or fluorescence spectroscopy. Furthermore, more sophisticated applications are also conceivable in, e.g., nanoparticle tracking analysis or ultrafast nonlinear frequency conversion.

Hollow core fibers for optical amplification

Hollow core optical fibers are normally passive light transport components. In contrast, within this Letter, we numerically investigate the possibility of using them as optical amplifiers, through the adoption of a novel fiber structure. We show that optical amplification can be achieved in hollow core fibers, where the cladding region is partially doped and composed of both resonant and anti-resonant elements. A balance between loss and glass/optical mode overlap is obtained, which allows efficient amplification over a limited spectral bandwidth. We discuss the case of a thulium-doped optical amplifier based on this novel technological approach.

Borosilicate Based Hollow-Core Optical Fibers

We discuss the fabrication of hollow-core optical fibers made of borosilicate glass. We show that, despite the high attenuation of the glass relative to silica, the fiber optical losses can be of the same order of magnitude of those obtained by using ultrapure silica glass. Short lengths of the fabricated fibers, used in combination with incoherent optical sources, provide single-mode optical guidance in both near and mid-infrared spectral ranges without any additional optical components.