Mid-infrared Lasers for Medical Applications: introduction to the feature issue

Characterizing temporal stability of supercontinuum generation in higher-order modes supported by liquid-core fibers

The generation of tailored supercontinua is essential for studying ultrafast light-matter interactions and for a variety of practical applications requiring broadband light. Liquid-core fibers (LCFs) have emerged as an innovative nonlinear photonic platform, demonstrating high efficiency in nonlinear frequency conversion. In this study, we showcase that LCFs provide a stable platform for ultrafast supercontinuum generation in a selected higher-order vector mode at 1.55 μ m . Specifically, we demonstrate soliton fission and double-dispersive wave generation using a radially polarized mode in a CS 2 -silica liquid-core fiber. The experiments were performed in a temperature-controlled laboratory, showing excellent stability with no evidence of fiber degradation, material degradation, or drift-induced changes in mode excitation over extended periods under standard environmental conditions. Our results confirm that liquid-core fibers are a reliable platform for nonlinear photonics, suitable for applications such as computationally tailored supercontinuum generation, single pulse spxectroscopy, and tailored light sources, all of which rely on consistent and stable nonlinear frequency conversion.

Fast Viral Diagnostics: FTIR-Based Identification, Strain-Typing, and Structural Characterization of SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has triggered an ongoing global pandemic, necessitating rapid and accurate diagnostic tools to monitor emerging variants and preparedness for the next outbreak. This study introduces a multidisciplinary approach combining Fourier Transform Infrared (FTIR) microspectroscopy and Machine learning to comprehensively characterize and strain-type SARS-CoV-2 variants. FTIR analysis of pharyngeal swabs from different pandemic waves revealed distinct vibrational profiles, particularly in nucleic acid and protein vibrations. The spectral wavenumber range between 1150 and 1240 cm-1 was identified as the classification marker, distinguishing Healthy (noninfected) and infected samples. Machine learning algorithms, with neural networks exhibiting superior performance, successfully classified SARS-CoV-2 variants with a remarkable accuracy of 98.6%. Neural networks were also able to identify and differentiate a small cohort infected with influenza A variants, H1N1 and H3N2, from SARS-CoV-2-infected and Healthy samples. FTIR measurements further show distinct red shifts in vibrational energy and secondary structural alterations in the spike proteins of more transmissible forms of SARS-CoV-2 variants, providing experimental validation of the computational data. This integrated approach presents a promising avenue for rapid and reliable SARS-CoV-2 variant identification, enhancing our understanding of viral evolution and aiding in diagnostic advancements, particularly for an infectious disease with unknown etiology.

Coherent beam combining of two all-PM thulium-doped fiber chirped pulse amplifiers

In this paper, we report a coherent beam combining (CBC) system that involves two thulium-doped all-polarization maintaining (PM) fiber chirped pulse amplifiers. Through phase-locking the two channels via a fiber stretcher by using the stochastic parallel gradient descent (SPGD) algorithm, a maximum average power of 265 W is obtained, with a CBC efficiency of 81% and a residual phase error of λ/17. After de-chirping by a pair of diffraction gratings, the duration of the combined laser pulse is compressed to 690 fs. Taking into account the compression efficiency of 90% and the main peak energy proportion of 91%, the corresponding peak power is calculated to be 4 MW. The laser noise characteristics before and after CBC are examined, and the results indicate that the CBC would degrade the low frequency relative intensity noise (RIN), of which the integration is 1.74% in [100 Hz, 2 MHz] at the maximum combined output power. In addition, the effects of the nonlinear spectrum broadening during chirped pulse amplification on the CBC efficiency are also investigated, showing that a higher extent of pulse stretching is effective in alleviating the spectrum broadening and realizing a higher output power with decent combining efficiency.

Spectral narrowing and broadening of Cr:ZnS/Se laser oscillation due to mode competition and spatial hole burning in the gain element

In this paper, we demonstrate the laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective unpolarized, linearly polarized, and twisted mode cavities. Lasers were based on post-growth diffusion-doped, commercially available antireflective-coated Cr:ZnSe and Cr:ZnS polycrystals with a length of 9 mm. The spectral output of lasers based on these gain elements in non-selective unpolarized and linearly polarized cavities was measured to be broadened to ∼20-50 nm due to the spatial hole burning (SHB) effect. SHB alleviation in the same crystals was realized in the "twisted mode" cavity, with linewidth narrowing to ∼80-90 pm. Both broadened and narrow-line oscillations were captured by adjusting the orientation of intracavity waveplates with respect to facilitated polarization.

Self-Q-switched Er:Lu2O3 laser at 2.74 µm

A diode-pumped self-Q-switched 2.74 µm Er:Lu₂O₃ crystal solid-state laser has been experimentally and theoretically studied. Without any additional modulation elements, stable self-Q-switched pulses with a pulse width of 145.3 ns, a repetition rate of 227.8 kHz, and an average output power of 877 mW were generated. Considering the excited-state absorption on the laser photons of the Er:Lu₂O₃ crystal, we have simulated the dynamic process of self-pulsed generation by solving the rate equations numerically. The simulation results are consistent with the typical characteristics of a Q-switched laser.

Decaffeination and improvement of taste, flavor and health safety of coffee and tea using mid-infrared wavelength rays

Background

Coffee (Coffea arabica) and tea (Camellia sinensis) are beverages consumed widely across the globe. Flavor enhancement of beverages is the prime interest for consumers and industry, but it is still a major challenge for researchers.

Objectives

In this work, we aimed to enhance the sensory characteristics and lower the caffeine content of tea and coffee by applying 2-6 μm mid-infrared wavelengths emitted through our recently invented Mid-Infrared Generating Atomizer (MIRGA) without creating any adverse effects.

Methodology

Two methods were followed: Direct MIRGA spraying over the packaged coffee or tea powder packets, and direct MIRGA spraying over the liquid coffee or tea. Controls were maintained in both methods. The treated samples were subjected to organoleptic tests by an expert panel and consumers.

Results

This study is supported by comprehensive field trials, including sensory attributes evaluation and laboratory analyses. In coffee, spraying resulted in 8% decaffeination and increase in theobromine and theophylline by 40% and 10-20%, respectively. In tea, caffeine and theobromine increased by 20-25% and 30%, respectively in addition to a 0.6-1.2% increase in thearubigins. A 20-30% lower amount of sprayed coffee or tea powder was required to prepare beverages with regular sensory characteristics. We have proven that the MIRGA technology applied to the products reduced the caffeine content in coffee, rendered them safe to consume, improved the taste and flavor, and induced health benefits. In addition, as the MIRGA platform contributed toward improving the product characteristics, it can also positively impact their price and affordability.

Conclusion

Applications of MIRGA technique and its benefits can be potentially scaled up and utilized for a variety of products used in daily life.

Femtosecond tunable solitons up to 4.8 µm using soliton self-frequency shift in an InF3 fiber

A tunable ultrashort soliton pulse source reaching up to 4.8 µm is demonstrated based on a 2.8 µm femtosecond fiber laser coupled to a zirconium fluoride fiber amplifier followed by a small core indium fluoride fiber. This demonstration is extending by 300 nm the long wavelength limit previously reported with soliton self-frequency shift (SSFS) sources based on fluoride fibers. Our experimental and numerical investigation highlighted the spectral dynamics associated with the generation of highly redshifted pulses in the mid-infrared using SSFS enhanced by soliton fission. This study is intended at providing a better understanding of the potential and limitations of SSFS based tunable femtosecond fiber sources in the 3-5  µm spectral range.

Generation of 8–20 μm Mid-Infrared Ultrashort Femtosecond Laser Pulses via Difference Frequency Generation

Mid-infrared (MIR) ultrashort laser pulses have a wide range of applications in the fields of environmental monitoring, laser medicine, food quality control, strong-field physics, attosecond science, and some other aspects. Recent years have seen great developments in MIR laser technologies. Traditional solid-state and fiber lasers focus on the research of the short-wavelength MIR region. However, due to the limitation of the gain medium, they still cannot cover the long-wavelength region from 8 to 20 µm. This paper summarizes the developments of 8–20 μm MIR ultrafast laser generation via difference frequency generation (DFG) and reviews related theoretical models. Finally, the feasibility of MIR power scaling by nonlinear-amplification DFG and methods for measuring the power of DFG-based MIR are analyzed from the author’s perspective.

Surface phonon resonance enhanced Goos-Hänchen shift and its sensing application in the mid-infrared region

The effect of surface phonon resonance (SPhR) and long range SPhR (LRSPhR) on the Goos-Hänchen shift (GHS) in the mid-infrared wavelength region are investigated. The GHS is significantly enhanced around the resonant angles of SPhR and LRSPhR with the p-polarized incident light. A highly sensitive refractive index sensor based on the enhanced GHS is proposed. The LRSPhR shows higher GHS and sensitivity than those of SPhR. The GHS and refractive index sensitivity can be further enhanced by engineering the damping rate of the phononic material. These results provide a potential route toward the large GHS and high refractive index sensitivity, thus opening up new opportunities for high sensitivity optical sensors based on GHS at the mid-infrared wavelength range.

Fabrication of a Chalcogenide Glass Microlens Array for Infrared Laser Beam Homogenization

Infrared (IR) microlens arrays (MLA) have attracted increasing interest for use in infrared micro-optical devices and systems. However, the beam homogenization of IR laser light is relatively difficult to achieve because most materials absorb strongly in the IR wavelength band. In this paper, we present a new method for the application of double-sided quasi-periodic chalcogenide glass (ChG) MLAs to infrared laser homogenization systems. These are non-regular arrays of closely spaced MLAs. The double-sided MLAs were successfully prepared on the ChG surface using a single-pulse femtosecond laser-assisted chemical etching technique and a precision glass molding technique. More than two million close-packed microlenses on the ChG surface were successfully fabricated within 200 min. By taking advantage of ChG’s good optical performance and transmittance (60%) in the infrared wavelength band (1~11 μm), the homogenization of the IR beam was successfully achieved using the ChG quasi-periodic MLA.

Impact of third-order dispersion and three-photon absorption on mid-infrared time magnification via four-wave mixing in Si0.8Ge0.2 waveguides

We investigate the influence of third-order dispersion of dispersive elements, three-photon absorption and free-carrier effects on mid-infrared time magnification via four-wave mixing (FWM) in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si_0.8Ge_0.2 waveguides. It is found that the magnified waveform is seriously distorted by these factors, and conversion efficiency is decreased, mainly because of nonlinear absorption. A time lens based on FWM in ${{\rm Si}_{0.8}}{{\rm Ge}_{0.2}}$Si_0.8Ge_0.2 waveguides is proposed for time magnification of mid-infrared ultrashort pulses, in which the low-distortion, high-magnification in the time domain could be obtained by optimizing system parameters. These results make it possible to analyze the transient dynamic process through oscilloscopes and detectors with gigahertz bandwidth and have important applications in ultrafast process analysis, optical pulse sampling, and optical communications.