Low-noise lasers are invaluable tools in applications ranging from precision spectroscopy and displacement measurements to the development of advanced optical atomic clocks. Although all these applications benefit from reduced frequency noise, some also demand a low-cost and robust design. In this work, we introduce a novel and practical concept of a simple, single-frequency laser that utilizes a ring fiber cavity for self-injection locking of a standard semiconductor distributed feedback (DFB) laser. Contrary to previously reported solutions, our laser configuration is fully spliced from polarization-maintaining (PM) single-mode optical fiber components. This results in an adjustment- and maintenance-free narrow-band laser operation with significantly enhanced stability against environmental noise. Importantly, continuous-wave (CW) single-frequency laser operation is achieved through self-injection locking, while the low-bandwidth active optoelectronic feedback serves exclusively to maintain this regime. Operating with output powers of ∼8 mW, the proposed fiber configuration narrows the natural Lorentzian linewidth of the emitted light to ∼ 75 Hz and ensures phase and intensity noise levels of less than – 120 dBc/Hz (> 10 kHz) and – 140 dBc/Hz (> 30 kHz), respectively. Furthermore, the thermally stabilized laser exhibits a frequency drift of less than ∼ 0.5 MHz/min with a maximal frequency walk-off of < 8 MHz. We believe that translating this laser design to integrated photonics in the near future could dramatically lower costs and reduce the footprint in various applications, including ultra-high-capacity fiber and data center networks, atomic clocks, and microwave photonics.

We investigate the transmittivity spectra, fields, and energy distribution of the electromagnetic eigenwaves propagating in a one-dimensional (1D) dielectric photonic crystal [(TiO₂/SiO₂)^NAl₂O₃]^M with two periods formed by unit cells TiO₂/SiO₂ and (TiO₂/SiO₂)^NAl₂O₃. Spectra of TE- and TM-modes depend on the geometric parameters of the structure and undergo modifications with the change in the period numbers, layer thicknesses, and incidence angle. Special attention is paid to the applicability of the hybrid effective medium approximation comprising the long-wave approximation and two-dimensional (2 × 2) transfer matrix method. We demonstrate spectral peculiarities of the bi-periodic structure and also show the differences between the band gap spectra of the bi-periodic and ternary 1D dielectric photonic crystals. The presented photonic crystal structure can find its applications in optoelectronics and nanophotonics areas as omnidirectional reflectors, optical ultra-narrow bandpass filters, and antireflection coatings.

We present a theoretical study of the dispersion and energy properties of the eigenwaves (TE- and TM-modes) in a four-layer structure composed of a magneto-optical yttrium iron garnet guiding layer on a dielectric substrate covered by a planar nanocomposite guiding multilayer. The bigyrotropic properties of yttrium-iron garnet are taken into account for obtaining the dispersion equation and an original algorithm for the guided modes identification is proposed. We demonstrated the polarization switching of TE- and TM-modes dependent on the geometrical parameters of the guiding layers. The dispersion diagrams and field profiles are used to illustrate the change of propagation properties with variation of the multilayer thickness ratio of the nanocomposite’s layers. The energy flux distributions across the structure are calculated and the conditions of the optimal guiding regime are obtained. The power switching ratio in the waveguide layers of about 6 dB for the wavelength range of 100 nm is shown to be achieved.

This review summarizes recent progress and emerging trends in multiparameter optical fiber sensing, emphasizing techniques that enable the simultaneous measurement of temperature, strain, acoustic waves, pressure, and other environmental quantities within a single sensing network. Such capabilities are increasingly important for structural health monitoring, environmental surveillance, industrial diagnostics, and geophysical observation, where multiple stimuli act on the fiber simultaneously. The paper outlines the physical principles and architectures underlying these systems and focuses on strategies for compensating and decoupling cross-sensitivity among measured parameters. Special attention is devoted to advanced distributed sensing schemes based on coherent optical frequency-domain reflectometry (C-OFDR), coherent phase-sensitive time-domain reflectometry (Φ-OTDR), and Brillouin optical time-domain reflectometry (BOTDR). Their theoretical foundations, their signal-processing algorithms, and the design modifications that improve parameter discrimination and accuracy are analyzed and compared. The review also highlights the roles of polarization and mode diversity and the growing application of machine-learning techniques in the interpretation and calibration of data. Finally, current challenges and promising directions for the next generation of fiber-optic multiparameter sensors are outlined, with a view toward high-resolution, low-cost, and field-deployable solutions for real-world monitoring applications.

Spectral properties of surface TM-waves (polaritons) are considered in the guiding structure based on a narrow-band semiconductor (n-InSb) and a nanocomposite medium (layered metamaterial) in the mid-infrared region (5÷20 µm). The nanocomposite medium has a weak gyrotropy and contains layers of bismuth-doped iron garnet (BLIG, Lu3-xBixFe5-yGayO12) and gallium-gadolinium garnet (GGG, Gd3Ga5O12) layers magnetized to saturation. Solutions of the dispersion equations are investigated for the cases of linear and nonlinear response of the semiconductor. For an analytical description of the nonlinear response of the structure we use the «uniaxial» (longitudinal) and «biaxial» approximations. Dispersion spectra for the propagation constant, as well as field and energy characteristics of the surface polaritons are obtained and analyzed. Regions of the existence of soliton-like wave-fields of the surface polaritons are found for each of the description methods used. It is shown that the «uniaxial» model may be unsuitable for the calculation of the electromagnetic fields and energy flows in the structures based on cubic nonlinear media.

We study the transmittivity spectra of one-dimensional finite three-periodic photonic crystals of the structure [(ab)^N(cd)^M]^K composed of four different layers a, b, c, and d being dielectric oxides Al₂O₃, SiO₂, TiO₂, and ZrO₂, respectively. We analyze modification of the transmittivity spectra of near-infrared electromagnetic waves of the TE and TM polarizations in the vicinity of the photonic bandgaps with variations of the sub-cell numbers N and M and super-cell number K, the incidence angle, and the order of the layers in the structure. We propose the classification of three-periodic photonic crystals by the magnitude and sign of the optical contrast between the layers in pairs (ab) and (cd) forming the sub-cells and discuss common spectral properties as well as their differences in these structures.