This work presents a systematic study on the integration of highly nonlinear fiber (HNLF) into an actively mode-locked nanosecond erbium-doped fiber laser (EDFL) cavity, while assessing the performance and stability of high order harmonics (up to the 105th order) via comparative analysis of setups with a standard single-mode fiber and their respective HNLF-integrated setups, specifically characterizing the impact of nonlinearity on peak power, full-width-at-half-maximum (FWHM), and pulse stability. The results indicate that adding HNLF enhances peak power, particularly at lower harmonic frequencies, while improving the FWHM values. Peak power and FWHM were found to be dependent on both the modulator driving signal's frequency and its peak-to-peak voltage. In the non-HNLF setup, the peak power varied from 3.8 dBm to -6.19 dBm, and the FWHM ranged from 12.5 ns to 7.5 ns. In contrast, in the HNLF-integrated setup, the peak power ranged from 4.62 dBm to -3.37 dBm, and the FWHM varied from 10.5 ns to 9.5 ns as the modulator driving frequency was adjusted. The configuration with HNLF integration presents additional measurable improvements not only in pulse power and FWHM but also in several measures of spectral quality. The integrated configuration achieved an optical signal-to-noise ratio (OSNR) of 69.89 dB as compared to the length-matched standard SMF-28 configuration's OSNR of 61.56 dB; and a side mode suppression ratio (SMSR) of 68.7 dB at a modulation frequency of 9.78 MHz, which was 6.6 dB greater than that of the corresponding non-HNLF configuration. Thus, the contribution of HNLF integration not only extends to pulse shaping but also encompasses simultaneous enhancements in optical signal quality and pulse train coherence, both of which are directly applicable to optical communications and sensing applications.
Phase-sensitive optical time-domain reflectometry (Phi-OTDR) is widely used in distributed acoustic sensing (DAS), but its performance is often degraded by gauge-length mismatch and low-frequency artifacts introduced during phase unwrapping. In this work, a robust phase unwrapping method is proposed to suppress drift-induced errors and improve signal stability. The method prevents cumulative error propagation by applying a conditional update mechanism to consecutive phase differences. Experimental validation using real Phi-OTDR measurements demonstrates effective suppression of low-frequency noise and an improvement of approximately 4-5 dB in signal-to-noise ratio. The proposed approach enhances the reliability of phase reconstruction and can be readily implemented in practical fiber-optic sensing systems.
This paper investigates the performance of a drone-to-drone free-space optical (FSO) communication system using polarization division multiplexing (PDM) and spectral encoding with eight Gaussian optical filters at 1550 nm and 10 Gbps. The proposed system is investigated under three channel impairment such as transmission distance, atmospheric attenuation, and pointing error arising from UAV mechanical instability. The system performance is evaluated using BER, Q-factor and spectrum analyzer. The crosstalk is maintained below -30 dB using channel spacing 0.8 nm. From the result it is seen that the reliable communication is achieved within 2-3 km, with BER below 10-9 and Q-factor above 6. The pointing error is modeled through a variable optical attenuator where BER increases from 10-12 to 10-6 when beam is misaligned from 0 to 10 & micro;rad, respectively. The analysis is extended to 20 & micro;rad and investigates the behavior of the system setup in terms of BER, which exceeds 10-3 beyond 15 & micro;rad. Under combined impairments at 4-5 km, hard-decision forward error correction (FEC, ITU-T G.975) restores error-free operation. A parametric study identifies pointing error as the dominant impairment, degrading BER by 1-2 orders of magnitude per 5 & micro;rad, compared to approximately 1 order for a 0.2 dB/km increase in attenuation. A preliminary proof-of-concept experiment was conducted to validate the feasibility of the 1550 nm drone-to-drone optical link using a SIMTRUM STFL1550 CW laser, with the received spectrum confirmed by a Yokogawa AQ6370D optical spectrum analyzer.
Conventional ion traps rely on bulky free-space optics or integration of grating couplers under the electrodes. The grating couplers are narrow-band components, necessitating four different couplers, each individually aligned, focused, and routed. This work presents a single waveguide with a broadband 45 degrees angled mirror acting as an antenna. This structure demonstrated optical coupling from 422 nm to 1092 nm, suitable for Sr+ ion trap systems, with a gain of up to 15.51 to 16.59 dB. The broadband response demonstrated here helps to replace the commonly used four grating couplers and feed waveguides with a single set.
This study presents the development and optimization of a multi-D-shaped optical fiber sensor designed for enhanced refractive index (RI) and ethanol detection. The sensor probes were fabricated using a precise side-polishing technique on standard single-mode fiber (SMF-28) to maximize the evanescent field interaction with the surrounding medium. We systematically investigated the influence of the number of D-shaped sensing zones ($N$N = 1 to 5) on sensor performance using glycerin-water solutions (RI 1.357-1.428). The experimental results demonstrate a significant sensitivity enhancement as the number of sensing zones increases, achieving a peak sensitivity of 12.12 dB/RIU with a 4-point ($N$N = 4) configuration. A subsequent performance decline at $N$N = 5 indicated a saturation limit dominated by fundamental mode field distortion and cumulative insertion loss. When applied to ethanol detection (RI 1.333-1.365), the optimized 4-point sensor exhibited a highly linear response (R2 = 0.9828) with a sensitivity of 3.866 dB/RIU. Notably, the sensor demonstrated high wavelength stability with negligible spectral shift across the tested range, confirming its operation as a robust, intensity-modulated device suitable for cost-effective biochemical sensing applications.
Non-contact real-time monitoring of cardiopulmonary signals can achieve early warning and diagnosis and treatment of cardiovascular diseases. However, the signal transmitted from the human body lying on the bed to the sensor through the monitoring pad will produce loss, resulting in a weak cardiopulmonary signal monitored by the sensor. Therefore, it is of great significance to study the propagation mechanism of cardiopulmonary vibration signals. This paper proposes the Kelvin-Voigt model and derives the wave equation to determine the attenuation coefficient of cardiopulmonary physiological vibration signals propagating through viscoelastic materials to FBG sensors. Based on viscoelastic structures and the principle of energy conservation, the propagation mechanism of cardiopulmonary vibration signals is investigated. A physical model is established for the transmission of these signals through monitoring pads to FBG sensors, alongside a strain transfer model for the sensors. This reveals the strain transfer patterns of cardiopulmonary dynamic signals through the sensors, with finite element simulation analysis employed to evaluate stress-strain values.
Ultra-Dense Wavelength Division Multiplexing (UDWDM) Free Space Optical/Fiber-to-the-& times; (FSO/FTTx) hybrid networks suffer from drastic changes in communication system performance due to dynamic atmospheric changes, thus affecting the dependability of 5 G/6 G communication systems. In this paper, we propose a novel machine learning-driven multi-objective optimization framework to realize weather-resilient resource allocation. Our approach leveraged two-phase co-design: first, by ensembling pre-trained Extreme Gradient Boosting (XGBoost) and Random Forest (RF) models, to reduce the prediction variance, serving as high-fidelity surrogates for critical cross-layer performance metrics bit error rate (BER), optical signal-to-noise ratio (OSNR), and quality factor (QF), yielding exceptional accuracy; second, the obtained surrogates were integrated within an Non-dominated Sorting Genetic Algorithm II (NSGA-II) optimizer by replacing computationally intensive simulations, such that the multi-objective problem can be efficiently solved in this stage. In this stage, we perform simultaneous minimization of BER while maximizing OSNR and QF, to establish trade-offs among reliability, signal integrity, and noise resilience. This approach yields Pareto-optimal solutions, enabling dynamic power control along with modulation switching over various atmospheric conditions and link lengths, 0.5-12.5 km. The validation confirmed resiliency, while maintaining BER <= 3.21 & times; 10-6, QF stabilized at 24-30.61 dB, OSNR sustained at 48.23-57.69 dB.
This paper investigates optimized fusion splicing techniques for connecting single-mode fiber (SMF) and hollow-core fiber (HCF) with the aim of minimizing insertion loss and back-reflection. Several approaches were evaluated, including direct fusion splicing, fiber tapering, angled cleaving prior to fusion, and the insertion of a graded-index (GRIN) fiber. It was observed that conventional automatic fusion settings designed for SMF result in significant end-face deformation and high splice loss. By carefully adjusting arc power, discharge position, and fiber overlap, splice performance was substantially improved. Fiber tapering enhanced mode-field alignment but required precise and repeatable control. Angled cleaving effectively reduced back-reflection, albeit at the expense of increased insertion loss. The use of a GRIN fiber provided the most effective mode-field matching, achieving the lowest insertion loss and back-reflection among the tested methods. The results demonstrate that reliable SMF-HCF splicing is feasible, however, further optimization is required to ensure long-term robustness and reproducibility.
This study presents the development of a 1064 nm pulsed fiber laser system based on a master oscillator power amplifier (MOPA) architecture. The system employs a ring laser master oscillator with polarization additive pulse mode-locking (PAPM) to generate stable pulses, while precise control to gain fiber length and cavity length ensures efficient pumping to the target 1064 nm band. Through optimized amplifier design, the system achieves 1.8 W output power with excellent spectral properties (1065.06 nm center wavelength, 0.071 nm 3-dB bandwidth). The repetition rate is tunable from 21.37 MHz to 88.11 kHz, producing adjustable pulse widths, making it suitable for high-power ultrafast applications.
Conventional monitoring devices for breath pattern analyses such as spirometers and thermistors remain costly, cumbersome to use and susceptible to interference. Fiber-optic sensors (FOS) offer immunity to such interference, but its designs in sensitivity and usability need to be validated. We developed a reliable, easy-to-use, and rapid diagnosis device for real-time assessment of breath pattern analysis, which could potentially be used for sleep apnea screening and other clinical applications such as ICU, sleep clinics, and Magnetic Resonance Imaging (MRI) suites which require continuous respiratory monitoring. Methods: We present a W-bent FOS with multiple bend zones embedded in the nasal prongs to enhance light attenuation in the presence of moisture from exhaled breath. The W-bent FOS prototype accurately captured breathing waveforms and rates (15.8 +/- 2.4 breaths/min) during normal respiration, exhibited distinctive signal patterns during simulated apnea (30 s breath-holds with post-apnea hyperventilation), and functioned reliably in a mock MRI setup. This pilot study demonstrates the feasibility and evaluation of a device which could potentially be used in sleep clinics and monitors warning signs such as impending claustrophobia in an MRI console.
The growing demand for broadband services has led to the widespread deployment of optical access networks (OANs). However, as these networks expand, energy consumption is becoming an increasingly important economic and environmental issue. In this paper, we analyze the energy savings of the transition from a point-to-point (P2P) optical architecture to a gigabit-capable passive optical network (GPON) with a point-to-multipoint (P2MP) architecture. We present a comparative analysis of the energy requirements of both architectures, focusing on active and passive components, and evaluate their impact on overall energy consumption. Using a case study of the migration of a real optical network operator's network, we show that power consumption decreased significantly after the introduction of GPON.
Iron phosphorus triselenide (FePSe3), a 2D ternary layered material, is proposed as a saturable absorber (SA) to induce mode locking in 1.5 mu m erbium-doped fiber laser (EDFL) and 2 mu m thulium/holmium doped fiber laser (THDFL). Stable mode-locked lasers are generated in both regions, with the center wavelengths of 1561.30 nm and 1907.84 nm, respectively. The repetition rate and pulse width for EDFL are 17.86 MHz and 0.88 ps, while for THDFL they are 12.32 MHz and 1.44 ps. FePSe3 as SA offers various opportunities for further exploration of future photonics applications for 2D ternary layered materials.
This paper proposes a low-loss, high single-polarization hollow-core anti-resonant fiber (HC-ARF) with double-layer nested tubes and two resonant tubes. After structural parameter optimization, the fiber achieves a confinement loss of 6.63 dB/km and a polarization extinction ratio of 8414 in the 1.986-2.002 mu m band. Regarding bending resistance, when bent along the x-direction with a bending radius larger than 4.5 cm, the y-pol mode maintains a loss below 10 dB/km and a polarization extinction ratio (PER) over 100; when bent along the y-direction with a radius exceeding 4 cm, the PER remains above 100, confirming good bending tolerance.
In this study, we present the design and analysis of a mode filter based on the use of a 3-waveguide coupler. The coupler is assumed to have a multimode central guide surrounded by two similar single mode waveguides. A theoretical model is developed based on the coupled mode theory and using the coherent tunneling adiabatic passage approach. The model shows that the coupler can be characterized by an effective coupling length that accounts for the coupling of each of the outer guides with the central guide. The estimated coupling length is in good agreement with the numerically calculated one using the Semivectorial Beam Propagation Method SV-BPM. The filter function is also well explained by the supermode interference in the structure. Based on this model, the design of a 3-waveguide mode filter built on SiN multilevel technology is introduced. An optimized design of the filter showing a crosstalk better than -40 dB at a wavelength of 1550 nm is demonstrated. This opens the door for new 3D integrated optical circuits in which the transition from single-mode to multi-mode in the vertical direction can be handled.
The studies made in this article are directed toward the calculations accomplished for the evaluation and improvement of the performance of type-II GaAsSb/InGaAs DQW (double quantum well) heterostructure for detection of weak and low energetic radiations. The k.p technique considering the intraband transitions was used to investigate the performance of the heterostructure in context of dipole and momentum matrix elements followed by partial and total optical absorption spectra. The calculation results suggest that the radiations having energy similar to 150 meV can easily be detected by photodetectors comprising such QW heterostructures. It has also been proved that the performance in terms of optical absorption is improved with the red shift when the applied DC field on the heterostructure is increased. In the optical spectra, the two separate absorption peaks were evident: the first, centered around similar to 85 meV, corresponds to transitions between quantized hole subbands (h1 -> h2) in the valence band, indicating strong intraband coupling among hole states. While, the second peak, appearing near similar to 145 meV, is attributed to transitions among electron subbands (e1 -> e2 and e1 -> e3) in the conduction band. The clear separation between these two distinct peaks signifies well-defined energy level spacing in both bands, a characteristic feature of optimized QW heterostructure enhancing intersubband transition efficiency. Thus, it can be concluded that the designed InGaAs/GaAsSb QW heterostructure can detect weak and low energy radiations in the range of (similar to 75 meV to 150 meV) via capturing the intraband transitions with the enhanced optical absorption characteristics. These findings demonstrate the potential of InGaAs/GaAsSb QW heterostructures for mid-infrared photodetection applications.
This paper analyzes external forces shaping the Croatian telecommunications market using Porter's Five Forces complemented with PESTLE analysis, emphasizing network infrastructure and services. The threat of new entry remains limited by high investment and regulatory barriers, though low-earth orbit (LEO) satellites may disrupt future infrastructure competition. Supplier power is high due to reliance on few global vendors, while buyer power reflects strong price sensitivity and demand for bundled services. Substitution pressures arise from over-the-top (OTT) platforms and satellite broadband. Rivalry is intense among three incumbents. Strategic implications highlight vendor diversification, digital service innovation, and sustainable infrastructure investment.
In this work, a novel set of reconfigurable all-optical logic gates is designed based on two-dimensional (2D) photonic crystal (PC) with a high contrast ratio, fast response, and ultra-compact size. To implement high-tunability logic operations, we integrate a hybrid silicon (Si) and Ge2Sb2Te5 (GST) filled nano resonator within the photonic crystal at the telecommunication wavelength of 1550 nm. By transitioning the GST from amorphous phase to crystalline phase, we realize various logic functions, including OR to XOR gate and XNOR to AND gate, with high contrast ratios and minimal footprint (69.46 mu m2). Interestingly high contrast ratios (CR) such as 19.72 dB, 19.68 dB and 11.68 dB are obtained in XOR, XNOR and AND gate, respectively and the response times for OR, XOR, XNOR and AND are 0.0821 ps, 0.0777 ps, 0.0915 ps, and 0.0802 ps, respectively. The bit rate of the designed structure is 12.07 Tbits/sec. Plane Wave Expansion (PWE) method is carried out to determine the band gap diagram for the proposed gates, and the Finite-Difference Time-Domain (FDTD) methodology is used to analyze their performances. The result confirms that these reconfigurable structures by combing Si and Ge2Sb2Te5 (GST) are well-suited for optical interconnects and all-optical integrated circuits, offering promising advancements in high-speed quantum computing and signal processing.
Erbium-Ytterbium:co-doped Lithium Niobate on insulator (Er-Yb:LNOI) is a promising platform for implementation of photonic integrated circuits (PICs), offering optical gain to the LNOI system, and enabling on-chip lasers and amplifiers. However, a key challenge for Er-Yb:LNOI lasers and amplifiers lies in achieving higher output power and efficiency while preserving single-longitudinal mode operation. We propose for the first time, to the best of our knowledge, an on-chip integrated full C-band tunable continuous wave-master oscillator power amplifier (CW-MOPA) based on Er-Yb:co-doped waveguides (EYCDWs) in a single Lithium Niobate on insulator platform. The EYCDWs in master oscillator (MO) and power amplifier (PA) stages are pumped using 980 nm laser diodes in forward and backward configurations, respectively. The performance of the proposed tunable on-chip integrated CW-MOPA is analyzed over full C-band (1530-1565 nm). Record high on-chip power of 500 mW is achieved at 1545 nm for 50% of the coupling ratio. Besides, highest slope efficiency (SE) and side-mode suppression ratio (SMSR) of 67.4% and 70.7 dB are achieved at 1545 nm for 50% of coupling ratio, respectively. Similarly, minimum linewidth (LW) of 124 MHz is observed for lasing wavelength of 1555 nm considering 60% of the coupling ratio. Finally, the effect of energy transfer upconversion (UC) on the performance of on-chip CW-MOPA is also observed. A power penalty of approximately 140 mW has been observed in on-chip laser power. The demonstrated high-efficiency, tunable CW-MOPA system enables compact and scalable photonic solutions for next-generation applications like 6 G networks, underwater optical communications, LiDAR, and resilient free-space optical links, offering robust performance in challenging environments while outperforming bulk optics in size, power efficiency, and operational reliability.
This paper presents a mini-two-path Mach-Zehnder interferometer (MTP-MZI) curvature sensor based on a seven-core fiber (SCF). The sensor consists of a reference path using a single-mode fiber (SMF) and a sensing path with a SMF - SCF - SMF structure. Employing electric-arc discharge technology, two fusion regions act as a beam splitter and a coupler, integrating both paths into the MTP-MZI. When light enters the MTP-MZI, it splits at the first coupling region, where one beam travels through the reference path and the other propagates along the sensing path. Due to the mode-field mismatch between the SMF and SCF, higher-order modes are excited and propagated in the SCF cladding. However, the weak coupling in the sensing path prevents some of these cladding modes from coupling back into the output SMF core. Advantageously, the strong coupling at the second region recaptures these residual modes into the final output SMF, significantly enhancing curvature sensitivity. Experimental results demonstrate a maximum sensitivity of 100.98 dB/m-1 within the 0-0.09143 m-1 range, the highest reported value to date for intensity-modulated SCF-based curvature sensors to the best of our knowledge. Additionally, the sensor exhibits low-temperature sensitivity (62.2 pm/degrees C from 30 to 70 degrees C). This compact, high-sensitivity MTP-SCF structure offers a promising approach for adaptable fiber-optic sensor design.
