Effective alloptical selfswitching of Cband femtosecond pulses, driven by pulse energy in a nonlinear fiber coupler, was experimentally investigated. A key contribution of this work is the detailed analysis of optical asymmetry between the two fiber cores influencing the switching mechanism. The asymmetry was examined using a multicore, highly nonlinear softglass fiber containing five noninteracting dualcore units with distinct propagationconstant mismatches. The study systematically evaluated the fiber length and input polarization effects, tracking changes in the output intensity distribution and extending the analysis to the spectral domain. Different regimes of highcontrast switching based on soliton selftrapping were observed and correlated with the degree of optical asymmetry. Beyond its fundamental insights, this work advances the state of the art by reducing the required switching energy, shortening the fiber length, and increasing the number of achievable switching steps.

We study nonlinear dual-wavelength switching in fibers with multiple dual-core units. The control of 1550/1700 nm femtosecond signal pulses by THz-rate 1030 nm pulse sequence demonstrates promising applications in ultrafast signal processing and spectroscopy.

Cyclic arrayed waveguide gratings (CAWGs) are widely used as multi-band wavelength routers in integrated photonic systems, where accurate alignment to the frequency-defined ITU-T DWDM grid is essential. In this work, we present a calibration-oriented analysis of ITU-T grid alignment in athermal CAWGs, focusing on practical design and optimisation aspects. An equation-level linear–quadratic output-plane mapping model is introduced, enabling independent control of channel spacing, dispersion-induced curvature, and absolute spectral positioning without modifying the intrinsic CAWG core geometry. Based on this model, a systematic four-stage calibration workflow is developed, allowing near-ITU conformity to be achieved within the selected design cycle C1. The results further demonstrate that simultaneous alignment across multiple cyclic replicas remains fundamentally constrained by order-dependent free spectral range behaviour. This provides a clear distinction between correctable mapping artefacts and intrinsic device limitations. In addition, temperature robustness is evaluated using the normalised temperature-dependent centre-wavelength shift over a wide temperature range, confirming stable performance of the athermal design and revealing cycle-dependent behaviour. The presented approach provides a practical framework for calibration-ready CAWG design and offers insight into the inherent trade-offs in multi-cycle wavelength routing systems.

A complex experimental study of dual-wavelength switching of ultrafast pulses in soft glass fiber with five noninteracting dual-core units was conducted. Two units with different dual-core asymmetries demonstrated distinct switching of 1550 nm and 1700 nm signal pulses governed nonlinearly by 1030 nm control pulses. The differences in switching performance between the two dual-core units indicate the possibility of their utilization for different signal processing tasks. Particularly, the side unit expressed significantly reduced asymmetry, enabling efficient all-optical cross-switching at 10.5 mm fiber length for a 1700 nm signal wavelength. Systematic optimization of the experimental conditions resulted in a nearly linear dependence of dual-core extinction ratio on control pulse energy at both signal wavelengths, with consistent spectral results. Additionally, THz-frequency control pulse trains and their sequential overlap with signal pulses confirmed the potential of this approach for ultrafast signal processing and time-resolved spectroscopy applications.

This study optimizes a dual-core fiber (DCF) design for femtosecond nonlinear dual-wavelength switching applications at 1030 nm (control pulse, CP) and 2000 nm (signal pulse, SP). Building on prior works in the C-band, we address CP-SP temporal synchronization, fabrication challenges, and functional enhancements. The DCF composes lead-silicate PBG-08 glass for the cores and borosilicate UV-710 glass for the cladding, for the purpose of high refractive index contrast, strong nonlinear interaction, and extended transmission beyond the near infrared. Core diameter (2.4 mu m) and spacing (3.9 mu m) were tuned for improved dispersion, coupling, and synchronization within a compact 1 cm length. We developed a novel fabrication approach to maintain circular core shapes and minimize asymmetry, improving consistency and applicability of previous DCF designs. We analyzed effects of +/- 0.2 mu m core ellipticity, finding minimal impact on group velocity and coupling length. The key importance characteristics of the optimized design promises higher switching contrast, reduced signal pulse distortions and extended wavelength range toward the mid-infrared. These predictions enhance the application potential of the analyzed specialty DCF in telecommunications, imaging, and sensing applications.

In this study, we investigate the impact of asymmetry and excitation wavelength on the coupling properties of soft glass dual-core optical fibers. Recent technological advancements have led to the development of high index contrast two-component soft glass dual-core fibers with sufficient symmetry, creating a new platform for nonlinear all-optical switching tasks. However, the nonlinear propagation of femtosecond pulses, which has been experimentally demonstrated, is significantly influenced by the linear optical properties of the fibers, including spectral profiles of dispersion, coupling length, and coupling efficiency. Our contribution extends beyond the novel non-destructive experimental investigation of these properties. We also conduct extensive numerical studies on the linear transmission characteristics. The findings we obtained not only deepen the understanding of the nonlinear experimental results but also provide valuable insights for development of the next generation of dual-core fibers, which will be more suitable for all-optical data processing applications.

This paper presents a simulation-based investigation of the dispersion characteristics of a 20 -channel, 100 GHz cyclic arrayed waveguide grating (CAWG) with a 2.2 THz free spectral range, evaluated across four free spectral range bands within the C and L telecommunications bands for Super PON applications. The CAWG is designed using an athermal waveguide structure comprising a silicon oxynitride (SiOxNy) core layer, a polymethyl methacrylate (PMMA) cladding layer, and a silicon substrate with a thermally grown silicon dioxide buffer layer. This configuration effectively mitigates temperature-induced shifts in the central wavelength. In addition to the athermal design, this work introduces a novel analysis of both waveguide and material dispersion effects in the cyclic AWG structure. The dispersion behavior is evaluated across four spectral periods, providing a comprehensive understanding of the transmission characteristics and spectral alignment of the CAWG under varying conditions. The device with central wavelength 1555.35 nm, is modeled using the Synopsys BeamPROP AWG utility. The results offer new insights into the interplay between thermal stability and chromatic dispersion in cyclic AWG devices for next-generation optical access networks.

We conducted a comprehensive experimental investigation of dual-wavelength switching of 1560 nm, 75 fs pulses (referred to as signal) driven by 1030 nm, 270 fs pulses (referred to as control) using two dual-core fibers with high refractive index contrast and different levels of asymmetry. The study explores the influence of fiber length, control pulse energy, and control-signal pulse delay on switching performance. For the fiber with higher dual-core asymmetry, we achieved an exceptional switching contrast of 41.6 dB at a 14 mm fiber length, exhibiting a homogeneous character within the spectral range of 1450–1650 nm. In contrast, the study of the weaker dual-core asymmetry fiber revealed a maximum switching contrast of 10.7 dB at a 22 mm fiber length, albeit under lower control pulse energy. These observations confirm that the switching mechanism is based on the nonlinear balancing of dual-core asymmetry, wherein the control pulse induces an enhancement of the effective refractive index in the fast fiber core, facilitating the switching of the signal pulse. This work demonstrates high switching contrasts with only a 0.4–0.6 nJ control pulse energy requirement, providing experimental confirmation of a previously reported theoretical model. For the first time, the dual-wavelength switching performance of dual-core fibers with varying levels of asymmetry is compared. The results reveal key directions for the further development of dual-core fibers in view of their potential applications.

One of the ways to realize all-optical switching of ultrafast signals is via nonlinear interaction in dual-core fibers (DCF). A novel technique leading to effective switching of 1.55 μm femtosecond (fs) pulses based on their cross-interaction with 1.03 μm control pulses in DCFs has been demonstrated previously [1]. We developed a second-generation DCF, which is a highly nonlinear, high-index contrast fiber made of thermally matched soft glasses PBG-08 (lead silicate) and UV-710 (borosilicate), comprising five dual-core units as shown in Fig. 1a [2]. The dual-wavelength cross-switching principle utilizes the nonlinear balancing of the dual-core asymmetry by the control pulse, which leads to the switching of the signal pulse when the fiber length is close to the coupling length [2]. In this contribution, we present notable improvements of the dual-wavelength switching approach in multiple key aspects, advancing its feasibility for single-shot time-resolved spectroscopy tasks.

Coupled nonlinear waveguides represent a promising approach for all-optical signal processing tasks with useful applications in telecom, quantum information, and spectroscopy. This contribution focuses on a study of a multicore, highly nonlinear soft glass fiber with advantageous properties allowing self-switching [1] or cross-switching [2] of sub-100 fs pulses at low switching energies. Following our preliminary achievements, we performed an in-depth analysis of the dual-wavelength nonlinear switching potential of the fiber containing five separated dual-core units (DCUs) with different degrees of asymmetry (Fig. 1a). A complex study was performed, sequentially addressing all 5 DCUs under fiber length optimization and various control-signal beam input combinations. The experimental setup has been detailed in [2]. It consists of two synchronized fs sources operating at a 1 kHz repetition rate: 1) a 1030 nm, 250 fs control source from a commercial ultrafast Yb:CaF2 amplifier, and 2) a 1550 nm, 80 fs signal source from an optical parametric amplifier, pumped by the second harmonic of the basic amplifier. The two pulse sequences were combined into a single beam with a dichroic mirror, temporally synchronized, and the energy of the signal pulses was set to 100 pJ to ensure their linear propagation. The nonlinear switching was controlled by varying the control pulse energy in the range of 1–10 nJ, and the combined beams were coupled into the fiber using a 50x microobjective. The fiber's output facet was imaged onto the chip of an IR camera by another 40x microobjective. Independent alignment of the signal beam was ensured by a tunable telescope and a mirror situated before the dichroic mirror. Selective launching of the control and signal beams into one of the two fiber cores was realized in a way that allowed four combinations for each DCU. Series of signal field output spatial distributions were registered by the camera as a function of control pulse energy for each combination. After processing the results, graphs of dual-core signal extinction ratio (ER $=10\log(E_{1}/E_{2})$ in dB, where $E_{1,2}$ is the output energy in the corresponding core) vs. input control pulse energy were created, depending on DCU selection, control/signal input combination, and fiber length.

The concept of all-optical switching, based on a nonlinear dual-core fiber coupler, was introduced theoretically in the early 1980s. After decades of imperfect demonstrations, our group has achieved high-contrast, low-energy, ultrafast switching using a specially designed dual-core fiber (DCF). Sub-100 fs C-band pulses were switched in both self-switching and cross-switching regimes at sub-nJ energies [1]. Following these promising approaches, a new fiber sample comprising five dual-core units with smaller intercore distances and various dual-core asymmetries was self-fabricated (Fig. 1a). By independently analyzing the nonlinear response of all five units, we could differentiate between them and identify the best one, as the switching performance is highly sensitive to dual-core asymmetry, which varies among central and side units. A Menlo C Fiber fs laser was used as the source, generating 4.5 nJ pulses at 1560 nm with a duration of 75 fs at a repetition rate of 100 MHz. By launching the laser beam separately into one fiber core via a microobjective, the nonlinear response of the respective unit was studied as a function of pulse energy. The pulse energy was controlled by a half-wave plate and polarizer system, ranging from 10 pJ to 4.4 nJ. The basis of the experimental study was fiber length optimization, accomplished by repeating the entire nonlinear propagation analysis for various lengths. The cores of the DCF were monitored by an IR camera and optical spectrum analyzer. Cutting back the fiber, all ten cores were subsequently excited, and the input energy dependence of the dual-core extinction ratio $(\text{ER}=10 \log (E_{\text {exc }} / E_{\text {non-exc }})$ in dB, where $E_{\mathrm{x}}$ is the output energy in the corresponding core) was measured.

We present a novel fiber optics sensor approach using an intentionally designed silica fiber that requires no additional processing after fabrication. The fiber, drawn from a D-shaped preform, maintains a D-shape crosssection along its entire length, ensuring high uniformity of geometrical parameters such as core diameter and the flattened surface distance from the core. The primary advantage of our approach is its unique ability to sense analytes at arbitrary distances from the fiber end and at any interaction length using the evanescent wave effect. We demonstrate the sensing performance by exposing a short fiber section (4 cm) to a liquid analyte in its pure form without using any signal-enhancing interlayer. Importantly, the output signal remains unchanged when water is applied as the medium, while exposure to an alcoholic medium significantly increases transmission. We further analyze the refractive index sensitivity of the technique by varying the water-isopropyl alcohol mixture ratio at a 1030 nm wavelength in a few-mode propagation regime. The results, interpreted in terms of the elimination of Rayleigh scattering losses of the higher-order modes by index matching between the fiber and the sensing medium, underscore the potential of our approach for water contamination sensing in both biological and environmental applications with distributed sensing capability, thereby addressing a critical need in the field.

Combining Stimulated Raman Scattering and spectral focusing, we generate narrowband tunable pulses for selective excitation of electric or magnetic dipole transitions in FIB-fabricated Eu 3 +Y 2 O 3 nanostructures. Spatial electivity is enhanced by implementing azimuthally polarized beams.

Implementing Stimulated Raman Scattering in a hollow-core fiber and spectral focusing in a long nonlinear crystal, we generate narrowband tunable ultrafast pulses for selective excitation of electric or magnetic dipole transitions in a FIB-fabricated Eu 3+ Y 2 O 3 nanostructure.

We present a comprehensive experimental investigation of dual-wavelength switching involving 1560 nm, 75 fs pulses (referred to as signal) driven by 1030 nm, 270 fs pulses ( referred to as control) using two high index contrast dual-core fibers with different measure of dual-core asymmetry. Beside the asymmetry, the study explores the influence of fiber length and control pulse energy on the switching performance. In the case of fiber with higher asymmetry, we achieved an exceptional switching contrast of 41.6 dB at 14 mm fiber length, exhibiting a broadband character within the spectral range of 1450-1650 nm. In contrast, the fiber with lower asymmetry enabled a maximum switching contrast of 10.7 dB at 22 mm fiber length. According to our understanding the switching mechanism is driven by nonlinear balancing of dual-core asymmetry, therefore it took place at lower control pulse energy in the latter case. Our findings underscore the critical role of proper dual-core asymmetry in achieving efficient switching performance and have significant potential applications.

We systematically present experimental and theoretical results for the dual-wavelength switching of 1560 nm, 75 fs signal pulses (SPs) driven by 1030 nm, and 270 fs control pulses (CPs) in a dual-core fiber (DCF). We demonstrate a switching contrast of 31.9 dB, corresponding to a propagation distance of 14 mm, achieved by launching temporally synchronized SP-CP pairs into the fast core of the DCF with moderate inter-core asymmetry. Our analysis employs a system of three coupled propagation equations to identify the compensation of the asymmetry by nonlinearity as the physical mechanism behind the efficient switching performance.

We investigate experimentally and theoretically effects of the inter-core propagation mismatch on nonlinear switching in dual-core high-index-contrast soft-glass optical fibers. Incident femtosecond pulses of various energy are fed into a single ("straight") core, to identify transitions between different dynamical regimes, viz., inter-core oscillations, self-trapping in the cross core, and retaining the pulse in the straight core. The transfer between channels, which has solitonic character, is controlled by the pulse's energy. A model based on the system of coupled nonlinear Schrödinger equations reveals the effect of the mismatch parameter and pulse duration on the diagram of the various energy dependent dynamical regimes. Optimal values of the mismatch and pulse width, which ensure stable performance of the nonlinear switching, are identified. The theoretical predictions are in agreement with experimental findings.

Effective all-optical switching using dual-core fibers is studied both experimentally and theoretically. C-band femtosecond pulses are switched by synchronized shorter wavelength ones and interpreted in terms of nonlinear balancing of the couplers mismatch.

The basic medium for implementing nonlinear switching of ultrafast optical signals is provided by dual-core optical fibers (DCF). Recently we have demonstrated that high switching contrasts at low pulse energies are offered by the new generation of all-solid highly nonlinear DCFs made of two soft glasses with sufficiently large index difference. The experiments have confirmed general predictions of previous theoretical works, and simulations of an appropriate model of the DCF have produced results which accurately agree with the experimental findings [1]. Following the successful analysis of the system with symmetric cores, here we extend the work, taking into account the propagation-constant mismatch between the cores [2]. The experimental and numerical results reveal that the asymmetry improves the switching performance of the nonlinear couplers based on DCFs, in the cases of both self-switching of near-IR femtosecond pulses and cross-switching controlled by the pulses at a shorter wavelength.

A novel fiber optics sensor approach was studied utilizing a new type of silica D-shaped self-made optical fiber. The fiber was applied as refractive index sensor for liquid medium exhibiting evanescent wave based sensing due to an asymmetric cross section. The tested liquid medium were isopropanol, distilled water and their mixture applied along 1-4 cm sensing region. The experiments were performed using a femtosecond Ytterbium doped fiber laser working at 1030 nm and effects such as analyte type, concentration and length of sensing region were examined. The sensing performance expressed polarization dependence with highest sensitivity in the case of perpendicular polarization to the flat fiber. Despite the small index difference between the two types of analytes, significant decrease of transmission was observed in the case of distilled water compared to isopropanol. Such effect was utilized for study for transmission dependence on water concentration in isopropanol solvent. The obtained results were explained in terms of scattering based losses of higher order modes causing elliptical output beam in the case of decreasing transmission.