Abubakar Isa Adamu

A critical component of optical communications is the availability of a suitable waveguide technology for the transport of electromagnetic waves with low loss over a broad spectral range. In the past four decades, despite extensive research, the attenuation and spectral bandwidth of silica-based optical fibres have remained relatively unchanged, with state-of-the-art fibres offering values of 0.14 dB km ⁻¹ and 26 THz below 0.2 dB km ⁻¹ , respectively. Here we report a microstructured optical waveguide with unprecedented transmission bandwidth and attenuation, with a measured loss of 0.091 dB km ⁻¹ at 1,550 nm that remains below 0.2 dB km ⁻¹ over a window of 66 THz. Instead of a traditional solid glass core, this innovative optical fibre features a core of air surrounded by a meticulously engineered glass microstructure to guide light. This approach not only reduces attenuation and other signal degradation phenomena, but it also increases transmission speeds by 45%. Furthermore, the approach theoretically supports further loss reductions and operation at wavelengths where broader bandwidth amplifiers exist, potentially heralding a new era in long-distance communications as well as remote delivery of laser beams.

The synthesis of multiple narrow optical spectral lines, precisely and independently tuned across the near- to mid-infrared region, is a pivotal research area that enables selective and real-time detection of trace gas species within complex gas mixtures. However, existing methods for developing such light sources suffer from limited flexibility and very low pulse energy, particularly in the mid-infrared domain. Here, we introduce a concept that is based on the combination of an appropriate design of near-infrared fiber laser pump and cascaded configuration of gas-filled anti-resonant hollow-core fiber technology. This concept enables the synthesis of multiple independently tunable spectral lines, with >1 μJ high pulse energies and a few nanoseconds pulse width in the near- and mid-infrared regions. The number and wavelengths of the generated spectral lines can be dynamically reconfigured. A proof-of-concept laser beam synthesized of two narrow spectral lines at 3.99 µm and 4.25 µm wavelengths is demonstrated and combined with photoacoustic modality for real-time SO₂ and CO₂ detection. The proposed concept also constitutes a promising way for infrared multispectral microscopic imaging.

A hybrid optical fiber comprising metal electrodes, high performance polymers, and a highly nonlinear glass core is presented in this work as a novel, to the best of our knowledge, platform for mid-infrared nonlinear devices. The fiber allows for electrical tuning of the temperature by joule heating using a set of embedded tungsten wires. Unlike temperature tuning by an external heater, this results in a strong modulation, which introduces alternating signs of its dispersion. Enhanced spectral broadening through supercontinuum generation in the mid-infrared due to this modulation is investigated numerically.

A hollow-core anti-resonant fiber (HC-ARF) design based on hybrid silica/silicon cladding is proposed for single-polarization, single-mode and high birefringence. We show that by adding silicon layers in a semi-nested HC-ARF, one of the polarization states can be strongly suppressed while simultaneously maintaining low propagation loss for other polarization states, single-mode and high birefiringence. The optimized HC-ARF design exhibits propagation loss, high birefringence, and polarization-extinction ratio of 0.05 dB/m, 0.5 × 10⁻⁴, >300 respectively for y-polarization while the loss of x-polarization is >5 dB/m at 1064 nm. The fiber also has low bend-loss and thus can be coiled to a small bend radii of 5 cm having ≈0.06 dB/m bend loss.

Stimulated Raman scattering (SRS) enabled by the emerging gas-filled low-loss anti-resonant hollow-core fiber (ARHCF) technology opens up a competitive way towards the development of novel lasers in the molecular fingerprint region. In this article, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging IR gas-filled ARHCF Raman laser technology.

In this work, we present a high-pulse-energy multi-wavelength Raman laser spanning from 1.53 µm up to 2.4 µm by employing the cascaded rotational stimulated Raman scattering effect in a 5 m hydrogen ( H 2 )-filled nested anti-resonant fiber, pumped by a linearly polarized Er/Yb fiber laser with a peak power of ∼ 13 k W and pulse duration of ∼ 7 n s in the C-band. The developed Raman laser has distinct lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm, with pulse energies as high as 18.25 µJ, 14.4 µJ, 14.1 µJ, and 8.2 µJ, respectively. We demonstrate how the energy in the Raman lines can be controlled by tuning the H 2 pressure from 1 bar to 20 bar.

In this work, we report the fabrication of an all-polymer multimaterial optical fiber based on two different grades of cyclo-olefin polymers (known as Zeonex) and the high-performance thermoplastic polysulfone (PSU) with glass transition temperature (T_g) of 189°C. The core/cladding structure using the Zeonex polymers (E48R/480R, respectively) was developed using a co-extrusion method followed by a rod-in-tube approach to form the final preform. The fiber materials were characterized in terms of their T_g, viscosity as well as refractive index profiles. The final preform was thermally drawn down to a fiber with ∼300 µm and ∼70 µm total and core diameter, respectively. We thermally characterized and compared our step-index fiber with a commercially available polymer (Cytop) as well as a purely Zeonex single-mode step-index fiber. The proposed multimaterial fiber exhibited stable operation at temperatures as high as 180°C being ∼35°C higher than any polymer fiber reported so far to the best of our knowledge. Therefore, we believe that our results constitute a significant step forward for the polymer optical fiber community making the proposed polymer multimaterial fiber an efficient route towards truly heat-resistant applications.

Hollow-core fiber (HCF) is a promising candidate for optofluidic applications because it can act as a gas-cell, permitting intense fluid-light interaction over extended lengths with low optical loss and inherent flexibility. Such a platform could pave the way for an all-fiberized, compact, robust and practical system for sensing applications. To facilitate this, we report a high-precision and repeatable micro-machining technique using focused ion beam (FIB) milling on a nodeless anti-resonant hollow-core fiber (ARHCF). Ga⁺ ions are bombarded on a 43 µm thick outer cladding of ARHCF for 30 minutes, to create a 50 µm deep fluidic channel, that has a negligible influence on the guiding properties of the fiber. The milled channel, followed by the 2.8 µm gap between adjacent 500 nm thin capillary tubes, provides direct access for liquid/gas to diffuse into the hollow-core region. The novel design presented here will allow ARHCFs to be spliced with solid-core fibers while preserving the fluidic channel. Corroborating results from simulation of such a structure are presented to demonstrate that no additional loss is induced by the milled hole.

UV supercontinuum laser sources based on resonant dispersive wave (RDW) generation in gas-filled hollow-core (HC) fibers offer an attractive architecture for numerous applications. However, the narrow UV spectral peak inherent to RDW generation limits the suitability for applications that require broad spectral coverage within the UV region such as spectroscopic scatterometry. In this Letter, we demonstrate how the UV spectrum can be shaped by modulating the peak power of the pump pulses driving the RDW generation, thereby creating a broadened and flattened UV spectrum. Using an argon-filled anti-resonant HC fiber, we generate a UV spectrum with a center wavelength of 323.6 nm with an FWHM of 51.7 nm, corresponding to a relative bandwidth of 16.1%.