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.

The continuous improvement of interferometric gravitational-wave detectors (GWDs) and the preparations for next generation of GWDs set highly demanding requirements on their laser sources. A promising candidate to fulfill the challenging requirements of GWD laser sources is the hybrid master-oscillator power fiber amplifier (MOPFA) configuration. The implementation of a MOPFA relies principally on commercial silica glass-based optical fiber technology, which has been key in the successful development of high-power fiber amplifiers but that poses also a limitation to power scaling of these devices. It is well known that erbium (Er) ions tend to cluster in silica glass leading to ion-ion interactions and degradation of performance. The limited concentration of RE ions per unit length implies a limited optical gain per unit length and thus the requirement for long amplifying fiber lengths that enforce deleterious nonlinear effects, foremost stimulated Brillouin scattering (SBS). Numerous SBS suppression techniques have been proposed, alongside investigation of specialty optical fibers. One of the most promising solutions is the use of highly doped optical fibers based on multicomponent phosphate glass that allows the fabrication of ultra-compact active devices with minimized nonlinearities. To realize compact optical fiber amplifiers operating at 1.5 µm, a series of highly Er3+-doped custom phosphate glass compositions was designed and fabricated to be used as active materials for the core of the fiber amplifiers. Suitable cladding compositions were explored. Core and cladding glasses were synthesized by melt-quenching method. The core glass was cast into a cylindrical mold to form a rod, whereas the cladding tube was fabricated by extrusion technique. Phosphate fibers were then manufactured by drawing the preform assembled by rod-in-tube technique. Preliminary results of the application of the Er3+-doped phosphate fiber as laser active medium in a fully monolithic single-mode single-frequency core-pumped MOPFA setup resonantly pumped at around 1480 nm are presented.

During the last decade there has been a surge of interest in developing mid-infrared (mid-IR) fiber-based supercontinuum (SC) sources. Such broadband light sources take advantage of extreme spectral broadening of high-intensity laser pulses in infrared optical fibers usually made of soft glasses, such as chalcogenides that offer the widest transmission window and the highest nonlinearity. Beyond their spatial coherence and high brightness, mid-IR fiber SC sources are nowadays operating over some wavelength ranges of thermal sources with superior performances for spectroscopic applications. However, intrinsic limitations of current fibers or even integrated waveguides now appear to impose the long-wavelength edge of mid-IR SC sources around 13-15 μm. The current research has to focus now on extending the wavelength coverage over the entire mid-IR molecular fingerprint region (often defined as from 2 to 20 μm). We here overcome this limitation by the engineered nonlinear transformation of femtosecond pulses over the full transmission window of a step-index chalcogenide fiber. In contrast to previous works, we reach the long-wavelength transparency edge of Se-rich glass family near 18 μm, and without including arsenic and antimony compounds considered as toxic elements and pollutants. Our end-to-end control of both materials chemistry and nonlinear fiber optics, including glass synthesis and purification, fiber design and drawing, as well as engineering of SC generation, has allowed us to optimize each of these crucial steps in order to demonstrate coherent mid-IR SC generation spanning from 2 to 18 μm.

Here I will report on optical fibers that are interfaced with dielectric nanostructures and demonstrate their properties on the examples of optical trapping and tracking of single nano-objects. The first topic addresses the implementation of nanostructures on fibers by 3D nanoprinting. This allows microspheres and bacteria to be trapped with only a single-mode fiber by integrating meta-lenses on the fiber end faces, achieving numerical apertures of up to 0.88. Through modified electron-beam lithography, I will show that incoupling efficiencies into fiber can be boosted by orders of magnitude.

Controllable excitation of spatial and polarization modes is of high importance in numerous applications, such as nonlinear optics, mode division multiplexing, interferometric measurements or sensing. We propose an effective method for selective excitation of different combinations of modes from LP01 and LP11 groups in a birefringent fiber. In the proposed method, the mode selection is realized with only a Wollaston prism, a rotatable polarizer and a half-wave plate, which ensures the possibility of high-power operation, low wavelength dependence, and tunability. Our approach makes it possible to excite almost all possible combinations of the LP01 and LP11 polarization modes and to continuously tune the relative coupling efficiencies of different modes by transverse shifting of the Wollaston prism. We demonstrate experimentally that the suppression rate of the unwanted modes with respect to the targeted mode exceeds 20 dB, and discuss the system configurations ensuring the highest possible coupling efficiencies for specific modes combinations. As example applications we show direct soliton and supercontinuum generation in the LP11 mode, broadband conversion of a supercontinuum from the LP01 to LP11 mode, broadband generation of vortex beams, gain tunability of intermodal four-wave mixing and cross-polarization four-wave mixing.

We experimentally demonstrate the possibility of adiabatic conversion of LP11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the values of effective indices, the LP11 modes are selectively converted to right-/left-handed circularly polarized vortex modes HE21 with a total angular momentum of ±2 and to quasi-TE01/TM01 modes with a total angular momentum of 0. The proposed conversion method has a purely topological origin, therefore, it is broadband in nature, in contrast to the methods based on resonant effects, and can be applied as an all-fiber broadband source of vortex beams. Funding: Narodowe Centrum Nauki (DEC-2016/22/A/ST7/00089, Maestro 8).

Lensed optical fibers are required to collect, focus, or collimate light in many modern applications. This is the case for a wide range of applications, such as telecommunication, metrology, or material processing. To shape the fiber tip, many techniques of micro-lens fabrication exist. However, they all have limitations. The fiber tip can be melted using electric arc, or laser. It can also be mechanically polished or etched using dry or wet processes. Photo-polymerization, 3D-printing and lithography can also be used. Some of these techniques are quite easy to implement but allow a limited shape diversity to be realized. Some others are very accurate with a large variety of possible lens shapes. However, they are costly and require sophisticated equipment. The higher difficulty is undeniably for high curvature micro-lenses for fibers with small core diameters. Consequently, having the ability to simply fabricate a micro-lens for all kinds of fibers has been the focus of our newly developed technique. This technique is polymer based, and, using an optical self-alignment system, we are able to fabricate micro-lenses for multimode as well as for single mode fibers. This technique is not only compatible with all fiber types, but it also grants the ability to manipulate the size and geometry of the micro-lens, therefore giving control over the focalized spot size. Diffraction limited or even smaller spots can be easily obtained, especially by using high-curvature small-base micro lenses. Several examples will be presented: the fabricated micro-lenses, their theoretical behavior, their experimental characterizations, and their potential applications.

Increasing demand for high data transmission rates and bandwidth availability driven by the rapid development of the broadband services becomes challenging in the context of constraints imposed by nowadays exploited telecommunication optical fibers. Based on single-mode fibers and multiplexing in the wavelength domain (wavelength division multiplexing, WDM), the current technology seems to have reached its fundamental limits. At present, two promising technologies are a subject of intensive research. One of them is the increase of the data transmission through multiplying the number of fiber cores, thus implementing the concept of Multi-Core Fibers (MCF), where each core is used as a separate data transmission channel. The second widely investigated technique is based on the idea of mode division multiplexing (MDM), where different transverse modes of a Few-Mode Fiber (FMF) can be used as different carriers for data channels. In this work, we demonstrate the results of R&D works and the comprehensive tests of the few-mode fibers developed within the project NMKM+. Developed few-mode fibers (both passive and active) and their commercially available counterparts have been tested with respect of transmission parameters and applicability to the real telecom systems. In particular, guided mode profiles, numerical apertures, OTDR, and dispersion characteristics for the passive fibers have been recorded and compared. Also, methods of selective excitation and detection of singular modes have been discussed and partially verified. The results of data transmission experiments have been performed and discussed with respect to the quality of transmitted signals. These have been complemented by the amplification experiments with the use of erbium-doped few-mode fibers in “classical” and microstructural geometry. The results were studied and analyzed, showing the potential for future optimization. Acknowledgement: This work has received support from the National Centre for Research and Development through project NMKM+ (TECHMATSTRATEG1/348438/16/NCBR/2018).

We show that a non-Hermitian modulation of the potential along the nonlinear multimode fibers controls dynamics of propagating radiation. Specifically we consider simultaneous modulation of the refraction index and gain/loss profile. We predict and observe that the non-Hermitian modulation introduces a unidirectional and controllable coupling towards the lower/higher order transverse modes, depending on the potential parameters. Such effect may enhance the beam self-cleaning phenomena, i.e. improve the spatial structure of light in propagation. On the contrary, coupling towards higher order modes may enhance pulsing, turbulence and, eventually help in super-continuum generation.