Ing. Ailing Zhong

We propose an approach to interconnect a hollow-core fiber (HCF) of arbitrary core size with standard single-mode fiber with perfect mode-field size adaptation and experimentally achieve for the first time insertion loss agreeing with that predicted by simulations. We demonstrate this using three low-loss HCFs, including 1st window nested antiresonant nodeless fiber (NANF), 2nd window NANF and the state-of-the-art double NANF (DNANF). The connection with a minimum achieved insertion loss of 0.079 dB was permanently secured via gluing and did not degrade during 4 weeks of continuous measurement. To the best of our knowledge, this is the lowest reported value and is comparable to or lower than the connection between dissimilar single-mode fibers (e.g., standard single-mode fiber and dispersion-compensating fiber). We also show that such connection leads to excellent suppression of higher-order modes coupling, of importance to all applications sensitive to multi-path interference. Importantly, obtaining agreement between simulations and experiments validates for the first time the accuracy of the simulations and opens the door to further optimization via simulations with the ability to subsequently achieve the same result experimentally.

We propose a hollow-core fiber (HCF) end-cap that incorporates a short segment of a single-mode fiber (SMF) that serves as a modal filter. To adapt the end-cap input and output beam to the desired size, the SMF was fusion spliced with short segments of mode-field adapting graded index (GRIN) fibers on both sides. The end-cap is anti-reflective coated to minimize insertion loss and parasitic reflections. The presented proof-of-concept experiments show its ability to suppress coupling into HCFs' higher-order modes. For example, without any end-cap, the extinction ratio between the LP 11 and the fundamental mode was found to be as low as 9 dB when coupling light from a free-space beam that was misaligned by as little as 1.1 ∘ . This was improved to 23 dB when inserting the developed end-cap. Such small angle misalignment often exists when aligning the input beam with 3-axis (x,y,z) stages only (rather than 5-axis that also include pitch and yaw). Finally, we glued the end-cap with the HCF, providing hermetic sealing to the HCF input/output. This is of interest for stable operation in applications that use free-space light launch or require HCF output into free space.

The attenuation of hollow-core fibers (HCFs) is predicted to surpass the minimum intrinsic attenuation of standard single-mode fibers (SMFs) in the near future. Recent advances in HCF performance and drawing technology have motivated their application not only in telecommunications but also in sensing and high-power delivery. Among HCFs, nested antiresonant nodeless fibers (NANFs) have shown the lowest attenuation values with 0.28 dB/km at 1550 nm and 0.22 dB/km at 1625 nm. Furthermore, the latest generation of NANFs effectively mitigates higher-order modes, which in some applications introduces a significantly limiting factor. As HCFs are becoming more available, their incorporation into standard SMF-based systems needs to be efficiently addressed. Various solutions to the HCF-SMF interconnection have already been proposed, such as the commonly employed fusion splicing with bridge fibers, using tapers to match the mode-fields, employing micro-optics, or using the fiber-array approach. Based on the fiber-array approach we have recently demonstrated losses of only 0.16 dB per interconnection and back reflection below -60 dB. But what if the interconnection itself can provide some additional functionality beyond low loss and low back reflection? Such an approach was already proposed in the micro-optics interconnection providing a function as an optical isolator or a wavelength-division multiplexer. Still, the relatively high complexity of such a device might limit its wider application. In this talk, I will overview current trends in HCF-SMF interconnection techniques which are enabling their incorporation into current SMF-based fiber-optic systems. I will present a future outlook of providing additional functionality to the HCF-SMF interconnection. I will focus on an interconnection technique we developed, based on the fiber-array approach. I will show how components such as an optical filter, a gas cell, or a Fabry-Perot cavity can be easily formed by simple tailoring of the HCF-SMF interconnection.

We demonstrate design of fiber F abry-Perot c avities b ased o n large-core graded-index multimode fibers. Smallest full width at half maximum along with maximum transmission is reached for core diameters over 200mm core and reflectivity above 99%.

By modifying the design of the interconnection between standard and hollowcore fibers, we achieved low insertion loss while creating a gap in between. This will allow for insertion of thin optical elements such as filters.

We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below −60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to achieve the best combination of performance in terms of the coupling loss and back-reflection suppression. Furthermore, we examine parasitic cross-coupling to the higher-order modes and show that it does not degrade compared to the case of a flat interface, keeping it below −30 dB and below −20 dB for LP11 and LP02 modes, respectively.

With the onset of 5G networks and growing demands on data capacity, fiber-optics and fiber-optic systems are drawing more attention. Their fundamental element being the optical fiber. The conventional optical fiber is a result from decades of technological evolution. Up to now, optical fibers have been used not only in telcom but also for sensing, fiber lasers and high-power delivery. Still conventional optical fibers are made of glass, mostly silica, which limits their performance, e.g. in achieving zero attenuation. Using hollow-core optical fibers brings many advantages as light propagates in air. Currently we are on the verge of commercial application of hollow-core optical fibers which will have significant impact on telecommunications but also on sensory networks, interferometry and lasers from visible to mid-infrared regions.

We demonstrate a 3-way interconnection device (hollow-core fiber, standard single-mode fiber and gas inlet) that is compact, low-loss, and easy-to-use.We demonstrate its performance on fibre purging, observing water vapor via infrared spectroscopy

Low-loss hollow-core fiber interconnection to standard optical fibers paves the way for high-finesse resonators, low-noise sensors, high-power delivery and next-generation fiber-optic communications. We will present main interconnection principles, discuss how back-reflection and higher-order mode excitation can be mitigated, and provide application perspectives.

In this paper, we present results of long-term stability tests of a low-loss (<0.55 dB) hollow core fiber (HCF) to standard optical fiber interconnection prepared by modified gluing-based fiber-array technology. We measured insertion loss of three interconnected HCF samples over a period of 100 days at room temperature, observing a variation in insertion loss of less than 0.02 dB. Subsequently, we placed the HCF samples in a climatic chamber and heated to +85°C in four cycles. Maximum insertion loss variation of 0.10 dB was observed for HCF samples with angled 8° interconnections and only 0.02 dB for a HCF sample with a flat interconnection.

We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling into the higher-order modes of the NANF. We achieved cross-coupling as low as −35 dB into the LP11 mode (the lowest-loss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP11 mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP02 mode (>2000 dB/km in our fiber) was measured to be below −22 dB. Furthermore, we show experimentally that the anti-reflective coating applied to the interconnect interface reduces the insertion loss by 0.15 dB while simultaneously reducing the back-reflection below −40 dB over a 60 nm bandwidth. Finally, we also demonstrated an alternative mode-field adapter to adapt the mode-field size between SMF and NANF, based on thermally-expanded core fibers. This approach enabled us to achieve an interconnection loss of 0.21 dB and cross-coupling of −35 dB into the LP11 mode.