Widely tunable femtosecond optical parametric oscillators (fs-OPOs) are attractive sources for ultrafast spectroscopy, nonlinear imaging, and nanophotonics, but their practical deployment is often limited by insufficient long-term stability. Here we present a high power, widely tunable femtosecond optical parametric oscillator with ultra-long-term stability over 12 h based on a model-free stochastic parallel gradient descent (SPGD) algorithm stabilization scheme. Pumped by the second harmonic of a Yb-fiber laser with a repetition rate of 49.22 MHz, the signal can be tuned from 660 to 1040 nm, with a maximum output power of 3.18 W at ∼ 800 nm. Unlike conventional feedback methods, the present approach enables stabilization both at the oscillation peak and at user-defined off-peak target-power operating points without either intentional modulation or complex adjustment of optical elements. RMS power fluctuations over 12 h of 0.31%, 0.99%, 0.29% are achieved at 770 nm, 800 nm, 830 nm, respectively. These results establish SPGD control as a practical route to robust long-term stabilization of widely tunable fs-OPOs for demanding applications in imaging, spectroscopy, and nonlinear photonics.

Carbon monoxide (CO) is a significant indicator gas with considerable application value in atmospheric monitoring, industrial production and medical diagnosis. Its fundamental vibrational band locates around 4.6 m and has larger absorption line strength than that of overtone band, which is more suitable for the precise identification and concentration detection of CO. In this paper, the up-conversion detection is employed to convert the mid-infrared absorption signal obtained by TDLAS to the visible light band, then a silicon-based detector is utilized for detection. By which, we can achieve the highest sensitivity of 79.6 ppb under the condition of cavity-free in-situ with an absorption range length of only 0.14 m. Furthermore, the single-photon level real-time detection of CO concentration after the diffuse reflection is realized by using SPAD. This work demonstrates the merits of the up-conversion detection in terms of its functionality at room temperature and capacity for sensitivity detection. Furthermore, it presents a design and optimization methodology that has the potential to underpin the advancement of the method towards more practical applications, like industrial process monitoring, medical diagnosis and so on.

In this work, we demonstrate a widely tunable fiber-based optical parametric oscillator (FOPO) synchronously pumped by a high-power tunable picosecond Yb-fiber laser. The laser system features a compact and environmental-stable all-polarization-maintaining configuration. The wavelength tuning and cavity matching of the tunable Yb-doped mode-locked fiber laser (TYMFL) rely on an intracavity diffractive grating mounted on a motorized stage. By tuning the TYMFL from 1032 to 1078 nm, the FOPO outputs from 737 to 1014 nm, which is the widest wavelength coverage in near-infrared FOPOs, to the best of our knowledge. All wavelengths can be tuned continuously and reversibly. The demonstrated FOPO is naturally synchronized with the TYMFL, enabling its application as a low-cost, highly flexible laser source in high-resolution coherent Raman scattering imaging.

The rising demand for AI training and inference, as well as scientific computing, combined with stringent latency and energy budgets, is driving the adoption of integrated photonics for computing, sensing, and communications. As active photonic integrated circuits (PICs) scale in device count and functional heterogeneity, physical implementation by manual scripting and ad-hoc edits is no longer tenable. This creates an immediate need for an electronic-photonic design automation (EPDA) stack in which physical design automation is a core capability. However, there is currently no end-to-end fully automated routing flow that coordinates photonic waveguides and on-chip metal interconnect. Critically, available digital VLSI and analog/custom routers are not directly applicable to PIC metal routing due to a lack of customization to handle constraints induced by photonic devices and waveguides. We present, to our knowledge, the first end-to-end routing framework LiDAR 3.0 for large-scale active PICs that addresses waveguides and metal wires within a unified flow. We introduce a physically-aware global planner that generates congestion- and crossing-aware routing guides while explicitly accounting for the region of photonic components and waveguides. We further propose a sequence-consistent track assignment and a soft guidance-assisted detailed routing to speed up the routing process with significantly optimized routability and via usage. Evaluated on various large PIC designs, our router delivers fast, high-quality active PIC routing solutions with fewer vias, lower congestion, and competitive runtime relative to manual and existing VLSI router baselines; on average it reduce via count by similar to 99%, user-specified design rule violation by similar to 98%, and runtime by 17x, establishing a practical foundation for EPDA at system scale.

This study investigates the influence of skin temperature fluctuations on near-infrared (NIR) diffuse reflectance spectra of human skin. To mitigate this influence, we applied a localized skin temperature control strategy and conducted a systematic analysis of spectral variations during the heating process from the initial skin temperature to 37 degrees C. Our results demonstrate that temperature-induced spectral changes can be categorized into two components: the direct temperature spectra (DTS) originating from changes in the absorption coefficient of water, changes in the refractive index (RI) of water, and changes in tissue volume and density, and the additional temperature spectra (ATS), arising from physiological responses including vascular reactivity, cutaneous blood perfusion, and sweat gland activity. Experimental investigations conducted on three tissue sites (forearm extension, dorsal hand and fingertip) revealed that DTS maintains a strong linear relationship with temperature when the temperature increase is limited to 2 degrees C. However, when the temperature rise exceeds 3 degrees C, ATS becomes increasingly prominent, exhibiting nonlinear behavior and temporal drift characteristics, particularly during temperature control at 37 degrees C. The experimental DTS showed preferable agreement with Monte Carlo (MC) simulation results, validating the capability of DTS to accurately represent tissue thermal characteristics. Furthermore, our analysis revealed significant inter-site variability in ATS, which we attribute to anatomical differences in vascular architecture, capillary density, and sweat gland distribution. This study provides references for spectral analysis of in vivo skins under temperature control, with the DTS and ATS classification method improving understanding of skin spectral changes with temperature.

Photon pairs play a vital role in modern science, driving extensive research into their generation. Yet, the narrow phase-matching bandwidth of conventional crystals has largely confined studies to specific wavelengths, leaving research on broadband tunable sources underexplored. Here, we employ a non-critical phase-matched lithium niobate (LN) crystal to generate widely tunable photon pairs. The generated near-infrared (NIR) photon pairs exhibit a high coincidence-to-accidental ratio (CAR > 20 dB) and are tunable across the 800-1600 nm range. We further showcase the utility of NIR photon pairs in spectroscopy by detecting carbon monoxide (CO) gas absorption. This approach will facilitate the design of advanced LN-based photonic experiments.

We extended self-similar amplification to a large-mode-area tapered Yb-doped fiber (LMA T-YDF) with longitudinally decreasing nonlinearity. The theoretical analysis and numerical simulation demonstrate that T-YDFs with different nonlinearity profiles can achieve self-similar evolution, which is confirmed by a self-similar amplifier that employs two kinds of T-YDFs. Further experimental study indicates that the T-YDF with a large core diameter at the thin end can achieve self-similar evolution across a wide range of pump powers and generate 51 W average power, 34 fs nearly transform-limited (TL) pulses with 32 dB gain. To the best of our knowledge, this is the first theoretical and experimental demonstration of self-similar amplification in T-YDFs. The high-gain feature of the T-YDF simplifies the laser system and can be used to build a compact all-fiber high-power femtosecond laser source.

This paper presents a compact, slow-light-enhanced segmented Mach-Zehnder modulator (MZM) designed for power-efficient photonic computing systems. By leveraging an apodized Bragg grating to induce slow-light effects, we achieve a total phase-shifter length of only 200 µm. The device operates as an electro-optic digital-to-analog converter (eoDAC) using two cascaded 3-bit segments to achieve 6-bit resolution, shifting bit weighting into the optical domain via the segment length ratio to bypass the exponential power scaling of high-resolution electronic DACs. The modulator achieves a normalized mean square error (NMSE) of <10-2 at clock rates up to 1 GHz and a power reduction of approximately 15%. This architecture provides a scalable, high-precision solution for dense, high-throughput silicon photonic accelerators in machine learning applications.

Single cavity dual-comb fiber lasers adopting different multiplexing configurations are benefited from the natures of common-mode noise suppression and superior coherence. Particularly, repetition-rate-tunable dual-combs enable non-ambiguous ranging and aliasing-free spectroscopy. However, their sampling rate and spectral resolution are severely restricted by the mechanical delay line (DLL). In a previous work, as rapid as 500 kHz/s tuning rate was realized to address this issue, while the minimum comb-frequency difference remained large under the inaccuracy of mechanical DLL. In this work, a polarization-multiplexing dual-comb fiber laser incorporated with a thermally controlled bidirectional Lyot filter is demonstrated with 870-times enhanced tuning precision compared with mechanical schemes. Linear correlation between temperature and repetition-rate-difference of this tuning mechanism is revealed. We achieve a tuning efficiency of 4.4 Hz/K and a control accuracy of 0.44 Hz, denoting a significant advance in operating Hz-scale differential comb lines. This design offers a robust platform for extending non-ambiguous distance in dead-zone-free dual-comb ranging and eliminating aliasing in spectroscopy.

We report, to our knowledge, a novel compact, thermally operated optical modulator with 6-bit resolution. The device comprises a slow-light photonic structure, a Bragg grating with a footprint of 10µm by 50µm. This single Bragg-grating modulator (SBG-M) operates based on thermally induced band edge shift of the photonic stop band and demonstrates almost constant normalized mean square error (NMSE) lower than 0.001 from room temperature to 40∘C, while maintaining thermal robustness up to 65∘C limited by the Johnson noise of the on-chip photodetector. The clock frequency, computing accuracy, power consumption, thermal stability, and fabrication-induced spectrum non-ideality are thoroughly studied. The SBG-M achieves a measured lowest NMSE of 5.0×10-4, with an average power consumption of 5.29 mW. Compared to other optical devices such as micro-ring resonators and slow-light incorporated Mach-Zehnder modulators, the reported device demonstrates significant advantages in compactness, lower power consumption, and thermal robustness.

In this paper, we studied the femtosecond laser welding of soda-lime-silicate glass and a pure aluminium film by employing a 100 kHz femtosecond laser. The morphology of the whole welding region is the same as that of the glass. The sizes of the defects in the welding regions must be very small, so that it looks like that the welding region integrates seamlessly with the glass. However, the real profile of the welding region can be still plotted after analyzing the SEM-EDS data and the Raman data. The welding region can be divided into three major parts: the area formed from the solidifying plasma of aluminium, from the solidifying mixed plasma of two materials, and where the silicate networks are partly destroyed, respectively. This means that the material properties in the welding region gradually transform from ductile to brittle towards the beam. We also studied two types of welding, continuous scanning welding and spot welding, and the lap shear strengths are compared in the different total areas of the welds. The data show that both types of the laser welding can provide strong strength. We attribute the high strength to the density of the welding region which results from a step welding dynamics caused by a relatively low repetition rate and a relatively high intensity.

In this paper, we successfully welded an ordinary glass and a single crystal graphite film without visible cracks by employing a high repetition femtosecond laser. The tensile strength of two welding samples exceeds those of the original films. Based on the SEM-EDS data and the Raman spectra data, two types of plasma welding regions can be clearly discriminated. The welding mechanism can be attributed to the mixture of graphite plasma and glass plasma and their resolidification. The energy density of single pulse at the interface is the most dominant factor because of this welding mechanism. From the Raman spectra data of the rear surface of the 20 mu m sample, how the shock wave influences the configurations of the C-C bonds in the graphite film can also be studied. Those results are helpful in understanding the dynamics of femtosecond laser welding and quickly optimizing laser parameters.

High repetition rate laser pulses are important for applications such as laser micromachining, bioimaging and optical frequency comb-based metrology. So far, no report on GHz-level fundamental repetition rate generation from an all-fiber ring laser. Here, we demonstrate the first all-fiber ring laser with GHz fundamental repetition rate. The laser produces ultrafast pulses at 1562 nm, with a pulse width of 682 fs, and a fundamental repetition rate of 1.028 GHz. The stable mode-locking is characterized by the low relative intensity noise and the high signal-to-noise ratio of the radiofrequency signal. The laser is mode-locked by carbon nanotubes (CNTs) with self-starting under a relatively low pump power. The proposed compact all-fiber ring laser could be used as the seed source where high repetition rate stable optical pulses are needed.

Spectral coherent combining (SCC) offers a powerful approach to increase output power and shorten pulse duration. Here, we comprehensively investigate SCC of two beams to achieve the high combining performance. The preliminary analysis indicates that incident spectra and the transition region of the combiner both affect the combining process. The simulation results show that optimizing the overlapping spectral range, the transition width and start wavelength of the combiner can achieve high combining efficiency and high pulse quality. Guided by the simulation results, we built a femtosecond laser system based on the SCC of two fiber amplifiers, achieving 96.9% combining efficiency and high-quality 42-fs pulses. To the best of our knowledge, this is the first time that high combining efficiency and high pulse quality have been achieved simultaneously in a fiber femtosecond laser system based on SCC. This study provides design guidelines for the high-performance combination of beams covering different spectral regions.

Repetition-rate-tunable fiber lasers are practical in addressing aliasing issue and exploring optimization parameters for dual-comb metrology and 3D surface profilometry. However, the tuning speed of typical delay lines is not very high, which limits the frame rate. Meanwhile, wide-range tunability is desired to extend nonambiguity distances and eliminate dead zones. Here, we developed a dual-comb fiber laser based on a resonate piezoelectric motorized delay line that shows a tuning speed up to 493.5 kHz/s over 329-kHz tuning range, which is the fastest to the best of our knowledge. Benefiting from the mechanically shared polarizationmaintaining oscillator, the proposed laser is stable with low crosstalk in full free-running, while this bidirectional polarization multiplexing configuration endows good flexibility and tunability. Consequently, the 1-hour variation of repetition rate difference is 0.52 Hz, with a standard deviation of 0.10 Hz. The dual combs are further pulled closer to 184.5 Hz by roughly compensating for fiber length. These results demonstrate a promising solution for developing accordion-comb seed light sources that can contribute to dual-comb metrology and 3D surface profilometry.

In this paper, the picosecond ultrasonic metal film thickness measurement based on asynchronous optical sampling with dual-comb is carried out to address the problems of the traditional picosecond ultrasonic technology based on mechanical delay lines, such as limited range and slow measurement speed. Based on two passively mode-locked ytterbium-doped fiber lasers working at 1030 nm waveband, a dual-comb picosecond ultrasonic measurement system is constructed. Based on asynchronous optical sampling, a time delay of 4 ns is generated, which is equivalent to a 0.6 m long mechanical delay line. Thickness measurement experiments are carried out on Au films with thickness of 43.67-165.33 nm, and the ultrasonic flight time is extracted by combining Gaussian filtering and multimodal Gaussian fitting to obtain a measurement resolution of similar to 2 nm. This device has potential application value for improving the film thickness detection

Comb-mode-resolvable fiber lasers output a set of evenly spaced and detectable frequencies, enabling precision applications in frequency metrology and high-resolution spectroscopy. Here, we report a narrow-bandwidth down to GHz level, polarization-maintaining mode-locked fiber laser with comb-mode resolvability. Under narrow-bandwidth filtering, the laser owns a minimum spectral width of 12.1 pm (1.49 GHz) at 1560 nm, with a pulse width of 437 ps and a comb spacing of 21 MHz. The comb spectrum is successfully resolved by using low-bandwidth electronics, owing to the high coherence of the combs. The output bandwidth (12.1 pm-3 nm) and center wavelength (1530 nm-1565 nm) can be easily tuned in a large range by setting the intracavity programmable pulse shaper. Therefore, the laser can operate at a broadband range despite its narrow bandwidth. The proposed widely tunable narrow-bandwidth laser has the potential for precise frequency comb metrology and spectroscopy with only a simple detection end.

A wavelet-based denoising method is proposed to improve the precision of group delay dispersion (GDD) measurements of chirped fiber Bragg gratings (CFBGs) in spectral interferometry. By analyzing the sparsity changes of wavelet detail coefficients, the optimal decomposition level and mother wavelet are determined. A hybrid thresholding strategy is introduced to effectively suppress noise. The proposed method achieves a reduction in GDD uncertainty and extends the available spectral range from 35 nm to 42 nm. It also maintains robust performance across a wide range of relative time delays, demonstrating strong potential for improving the accuracy and reliability of dispersion measurements in variable signal-to-noise ratio (SNR) conditions.

Congruent lithium niobate is a type of lithium niobate crystal with a mature growth process and is widely used in nonlinear optics research. Its refractive index accuracy will play a crucial role in the research and application of nonlinear optics. In this paper, we theoretically analyze the accuracy and reliability of nonlinear methods and experimentally measure the refractive index of ordinary light at different wavelengths and temperatures in a non-critical phase matching LN crystal by the sum-frequency generation and spontaneous parametric down- conversion processes, with the help of the existing accurate Sellmeier equation for the refractive index of extraordinary light. By fitting the refractive index of ordinary light, a new set of parameters for the Sellmeier equation is established. This equation shows excellent agreement with existing experimental results, and greatly extends the range of wavelengths and temperatures to 0.5- 10 mu m and 20- 250 degrees C. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement