A frequency-domain demodulation method for fiber loop ring-down (FLRD) systems using multi-harmonic spectral analysis is proposed. By applying Fourier transform to the time-domain ring-down signals, ring-down cavity loss is extracted from the peak–valley differences of multiple harmonics in the amplitude spectrum, eliminating exponential fitting for ring-down time extraction. To mitigate the frequency-dependent amplitude attenuation induced by finite pulse width and time-window effects, a quadratic envelope compensation method is established to flatten the spectral envelope and enhance the utilization of multi-harmonic information. Numerical simulations based on an exponentially decaying pulse model show that, after envelope compensation, a strong linear relationship is obtained between the averaged peak–valley differences and the ring-down time constant, with a coefficient of determination of 0.9998. An experimental FLRD platform is further constructed to evaluate the proposed approach under bending losses ranging from 0 to 2.0 dB. The compensated peak–valley differences of the first seven harmonics exhibit a clear linear correlation with the applied loss, yielding a coefficient of determination of 0.994, in good agreement with conventional time-domain measurements. The proposed method provides a feasible approach for loss characterization in FLRD-based sensing systems.
High-quality dicing of sapphire with ultrafast laser Bessel beams relies on achieving continuous crack propagation along the dicing path. However, the energy threshold for continuous crack propagation under varying pulse widths and pulse spacings has not been systematically quantified, which limits the selection of suitable pulse energies and further process optimization. The energy threshold for continuous crack propagation in sapphire was investigated for different pulse widths and pulse spacings. The results reveal a distinct nonmonotonic dependence of the threshold on pulse width at each pulse spacing: as the pulse width increases from 640 fs to 9.8 ps, the threshold first decreases and then increases. This behavior may be attributed to a transition in the dominant laser-sapphire interaction mechanism. For shorter pulse widths, plasma shielding limits the effective energy coupling. As the pulse width increases to ∼1.6 ps, plasma shielding weakens and localized energy deposition improves, facilitating crack propagation. When the pulse width is further increased from 1.6 to 9.8 ps, thermal melting and heat diffusion become dominant, weakening the residual stress field and thus raising the required energy for continuous crack propagation. These findings suggest a possible pulse width-dependent crack propagation mechanism in Bessel beam sapphire dicing and may provide guidance for reducing the processing energy and improving the dicing quality.
This work enhances refractive index fitting of optical materials by utilizing minimum deviation refractometry with near-normal reflectance. Minimum deviation refractometry is advanced through an experimental methodology to characterize and propagate the associated error and uncertainty from the equipment and samples. Sources of error such as the mechanical limitations, sample geometry, beam shape restrictions, lateral beam sensitivity, the sample material’s internal absorption, and the sample surface figure and roughness are presented. The mechanical limitation is eased due to fitting the data to within the mechanical step. All other sources of error may be reduced to measurement offsets with additional sample information. The error in near-normal reflectance measurements used for refractive index data to increase the fitting physical meaning is alleviated by pinning the data to the minimum deviation data. A weighted specular reflectance model is presented to account for the wavelength dependence effect caused by the roughness of a sample’s surface. Part II will contain the experimental verification of this work.
Perfect light absorption is critical for optoelectronics, but MoS2’s wide bandgap limits its infrared absorption. We demonstrate a tunable near-perfect sub-bandgap absorber based on an Au/MoS2/W (Au: gold; MoS2: molybdenum disulfide; W: tungsten) Fabry-Pérot (F-P) cavity. The design was optimized by FDTD (finite-difference time-domain) simulations and subsequently fabricated via magnetron sputtering. By tuning the MoS2 thickness (20 to 160 nm) and adjusting the Ar (argon) flow rate to control the Mo:S stoichiometry, we achieved a continuous resonance redshift from visible (554 nm) to the NIR (near-infrared) (1755 nm). Peak absorption reaches 98% at 715 nm and 97% at 854 nm for 40- and 60-nm MoS2 cavities, respectively. Experiment and simulation show a strong linear correlation in thickness-dependent resonance positions (R2=0.996), despite a systematic experimental blue shift of 19% to 36%. The absorption discrepancy increases for thicker MoS2 films because of stoichiometric deviations, defects, grain boundaries, and other material nonidealities. FDTD field distributions indicate that the absorption enhancement originates from strong optical-field confinement and constructive interference within the F-P cavity. This work provides a practical and scalable route for engineering light–matter interactions in two-dimensional materials, with promising applications in infrared photodetectors and multispectral sensing systems.
Michelson interferometry, as a high-precision and noncontact optical measurement technique, can determine wavelength by analyzing interferometric ring fringes. However, manual fringe reading is susceptible to subjective operation, fringe contrast, illumination variation, and noise, whereas the conventional fractional Fourier transform (FRFT) method contains redundant transform terms and is limited by nonlinear wavelength-to-coefficient mapping and finite search spacing. To address these issues, we propose an improved FRFT-based wavelength measurement method for Michelson interferometric fringes. The original FRFT is first simplified into a concise FRFT (CFRFT) search to reduce redundant computation; then, endpoint-constrained linearization is introduced to make the wavelength-to-coefficient search more uniform; finally, seven-point Sinc reconstruction is used to refine the fractional-domain peak. Numerical simulations and real-image experiments demonstrate that the proposed method improves wavelength-estimation accuracy while maintaining the low computational cost introduced by CFRFT. In the search-step experiment, the CFRFT simplification reduces the average runtime by 16.4% compared with the original FRFT method, and the proposed method reduces the MRE from 0.2437% to 0.0697%. For real Michelson images acquired at air-gap thicknesses of 1 to 4 mm, the overall MRE is reduced from 0.6857% to 0.5901%, confirming the effectiveness and practicality of the proposed method.
