Title

Tunable Ultraviolet Dispersive-wave Emission Driven Directly by 40-fs Ti:sapphire Laser Pulses in a Hollow Capillary Fiber

Abstract

We demonstrate that by using a 1-m-long gas-filled hollow capillary fiber (HCF) with a core diameter of 100 μm, tunable ultraviolet (UV) dispersive-wave (DW) pulses can be generated in a compact, single-stage setup driven directly by 40-fs Ti:sapphire laser pulses. By adjusting the gas type and pressure inside the HCF, the central wavelength of the UV DW can be continuously tuned from 185 nm to ∼450 nm. In the experiment, we found that for ∼320–450 nm DW generation, Raman-active gas filled in the HCF can efficiently suppress the pulse fission effect of the high-order soliton due to the Raman-induced pulse energy dissipation, leading to the high-quality DW generation at these wavelengths with smooth, single-peak spectra. These results provide some useful insights for designing compact, wavelength-tunable ultrafast UV light sources with microjoule-level pulse energies.

DOI:10.1364/OL.553345 [Paper Links]

Corresponding authors

-Zhiyuan Huang, Email: huangzhiyuan@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Wavelength-tunable, Mid-infrared Ultrafast Pulse Generation Through Raman Self-frequency Shift in an All-solid Fluorotellurite Fiber

Abstract

We report a mid-infrared (mid-IR) fiber laser system that can deliver ultrafast soliton-like pulses with a wavelength tunable from 2.8 to 4.0 μm. The pump light source of the system is a mid-IR fiber laser mode-locked at 2.8 μm, which has an average output power of ∼500 mW, a repetition rate of ∼30 MHz, a pulse energy of ∼15 nJ, and a pulse duration of ∼150 fs. The pump light was then launched into a short (∼70 cm) sample of fluorotellurite (TeO2-BaF2-Y2O3) glass fiber as the high-nonlinearity waveguide with simultaneously good features of large Raman gain, tailored dispersion, and high-power capability. High-efficiency Raman soliton self-frequency-shift phenomenon can be obtained in this short fiber sample, leading to the generation of broadband tunable (3–4 μm), mid-IR pulses with hundreds-of-fs pulse duration and tens-of-mW average power, corresponding to a pulse energy level of ∼1 nJ. The frequency-conversion efficiency inside the nonlinear fiber was measured to be as high as ∼16%. The present system, combining the advanced techniques of fluorotellurite fiber fabrication and mid-IR ultrafast fiber laser, highlights its application potentials for generating low-noise, high-beam-quality, mid-IR ultrafast pulses with a compact fiber configuration.

DOI:10.1364/OL.559304 [Paper Links]

Corresponding authors

-Jiapeng Huang, Email: jiapenghuang@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Invited Commentary: Hollow Core Glass Fibre— the Perfect Pipe for Light

Abstract

DOI:10.1186/s43593-025-00086-w [Paper Links]

Corresponding authors

-Philip Russell, Email: philip.russell@r-cals.com

Title

Coherent Combination of Mid-IR Laser Beams Using a Sagnac-type Interferometer

Abstract

We report a high-efficiency coherent beam combination (CBC) system operating in the mid-infrared (2.8 µm) spectral region, implemented using two fluoride fiber amplifiers with a configuration of Sagnac-type interferometer. The system exhibits good performance at both continuous-wave (CW) and picosecond-pulse operation conditions. For the CW CBC, the system exhibits a high combination efficiency of >90%, delivering a maximum output power of 2.8 W with a beam quality factor (M2 ) of <1.2. When using a pulsed laser as the seed light, two beams of 1-ps pulses can be amplified and simultaneously temporally-compressed to ∼170 fs in the two fiber amplifiers. Then high-efficiency (∼84%) coherent combination of amplified ultrafast laser beams can be achieved in this CBC system, giving rise to a recombined output power of ∼1 W. These results highlight the application potential of this Sagnac-type CBC set-up for power scaling of both CW and ultrafast fiber lasers at mid-IR wavelengths.

DOI:10.1364/OE.558136 [Paper Links]

Corresponding authors

-Jiapeng Huang, Email: jiapenghuang@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Observation of Dynamics of Harmonic Order Self-adaptation in Optoacoustically Mode-locked Fiber Laser

Abstract

High-harmonic mode-locking based on strong optoacoustic interactions in solid-core photonic crystal fiber (PCF) has been an established mechanism to achieve a stable GHz repetition rate in a conventional soliton fiber laser, in which a uniform pulse sequence is self-locked to the acoustic core-resonance of the PCF with a specific harmonic order. However, due to the finite bandwidth of the acoustic core-resonance, there may be multiple choices of harmonic order within the resonance bandwidth that could lead to uncertainties in the resultant harmonic order. In this work, we report observations of dynamic self-adaptation of the pulses in the laser cavity towards a stable harmonic order when the cavity is initially set at a neighboring less stable harmonic order. We observed that self-adaptation can occur spontaneously towards either higher or lower harmonic order depending on the initial condition. We observed rich dynamics during the self-adaptation process namely transient pulse sliding, random pulse collisions, and pulse drop-outs. This work provides experimental insight into the robustness of the self-locked repetition rate in optoacoustic mode-locking, and we believe will be useful in understanding and controlling harmonically mode-locked lasers.

DOI:10.1364/OE.547755 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Retiming Dynamics of Harmonically Mode-locked Laser Solitons in a Self-driven Optomechanical Lattice

Abstract

Harmonic mode-locking, realized actively or passively, is an effective technique for increasing the repetition rate of ultrafast lasers. It is critically important to understand how a harmonically mode-locked pulse train responds to external perturbations and noise, so as to make sure that it is stable and resistant to noise. Here, in a series of carefully designed experiments, we elucidate the retiming dynamics of laser pulses generated in a soliton fiber laser harmonically mode-locked at GHz frequencies to the acoustic resonance in a photonic crystal fiber (PCF) core. We characterize the self-driven optomechanical lattice, which is distributed along the PCF and provides the structure that supports harmonic mode-locking, using a homodyne setup. We reveal that, after an abrupt perturbation, each soliton in the lattice undergoes damped oscillatory retiming within its trapping potential, while the retiming is strongly coupled to soliton dissipation. In addition, we show, through statistical analysis of the intra-cavity pulse spacing, how the trapping potentials are effective for suppressing timing jitter. The measurements and the theory developed in this work lay the groundwork for studies of the general stability and noise performance of harmonically mode-locked lasers as well as providing valuable insight into generic multi-pulse phenomena in mode-locked lasers.

DOI:10.1038/s41377-024-01736-3 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Understanding Low-threshold Mode-locking at Multi-GHz Repetition Rate (News & Views)

Abstract

Continuous-wave mode-locking at multi-GHz repetition rates is achieved in an ultrashort laser cavity at critical pulse energies 100 times lower than predicted by conventional theory. The authors reveal that dynamic gain depletion and recovery between consecutive round-trips is the key factor behind a low-pulse-energy transition from Q-switched mode-locking (QSML) to continuous-wave mode-locking (CWML). As well as providing new insight into gain dynamics, the results suggest a practical route to low-threshold lasing at very high-repetition rates.

DOI:10.1038/s41377-024-01682-0 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

Title

Ultraviolet-enhanced Flat Supercontinuum Light Generated in Cascaded Photonic Crystal Fiber

Abstract

We demonstrate that by using a cascaded configuration of five solid-core photonic crystal fiber (PCF) samples with progressively decreasing core diameters, ultraviolet light with wavelengths as short as ∼300 nm can be generated in a supercontinuum (SC) set-up. With a nanosecond laser as the pump light, the modulation instability effect leads to the generation of multiple optical solitons in the first PCF sample which has a close-to-zero dispersion value at the pump wavelength (∼1064 nm). The following PCF samples with decreasing core diameters enhance the waveguide nonlinearity, and at the same time provide continuously-shorter phase-matching wavelengths for dispersive-wave emission, thereby pushing the short-wavelength edge of the SC spectrum into the deep ultraviolet spectral region. While the PCF splicing technique ensures the compactness of this SC set-up, the generated SC spectrum, spanning ∼350 nm to ∼2000 nm with a flat spectral profile, may be applied in fluorescence microscopy and biochemical imaging.

DOI:10.1109/JPHOT.2024.3488039 [Paper Links]

Corresponding authors

-Zhiyuan Huang, Email: huangzhiyuan@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

-Xin Jiang, Email: xin.jiang@r-cals.com

Title

High Power Mid-infrared Side-pump Combiner with Good Thermal Stability Based on the Point-by-point Fusion Splicing Technique

Abstract

High-power mid-infrared fiber lasers, featuring superior beam quality and good power-scaling ability, have a few important applications in material processing, medical surgery, and molecule spectroscopy. The high-power pump light combiner, as one of the key elements for constructing a mid-infrared fiber laser, is crucial for the laser performance. While some advanced side-pump combiners based on fluoride fiber have been reported in recent literatures, the thermal stability of the fluoride fiber combiner, which is closely-related to its power-scaling capability, is a long-living challenge. In this work, we demonstrate a high-power mid-infrared side-pump combiner with improved thermal stability, realized using the point-by-point fusion-splicing technique between a silica fiber taper and a piece of Er-doped fluoride gain fiber. The developed combiner exhibits a high coupling efficiency of ∼90%, supporting highly-stable operation at an incident pump power of up to 60 W. Using this combiner, we constructed a continuous-wave mid-infrared fiber laser which can deliver stably 4 W output power at 2.8 µm without using active cooling system. At this lasing power, the maximum input pump power is limited to 20 W to prevent fiber end-facet degradation, which can be further improved with the use of endcaps. This remarkable thermal stability renders the combiner great application potentials in constructing compact, robust, high-power fiber lasers at mid-infrared wavelengths.

DOI:10.1364/OE.538468 [Paper Links]

Corresponding authors

-Jiapeng Huang, Email: jiapeng.huang@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Octave-wide Broadening of Ultraviolet Dispersive Wave Driven by Soliton-splitting Dynamics

Abstract

Coherent dispersive wave emission, as an important phenomenon of soliton dynamics, manifests itself in multiple platforms of nonlinear optics from fibre waveguides to integrated photonics. Limited by its resonance nature, efficient generation of coherent dispersive wave with ultra-broad bandwidth has, however, proved difficult to realize. Here, we unveil a new regime of soliton dynamics in which the dispersive wave emission process strongly couples with the splitting dynamics of the driving pulse. High-order dispersion and self-steepening effects, accumulated over soliton self-compression, break the system symmetry, giving rise to high-efficiency generation of coherent dispersive wave in the ultraviolet region. Simultaneously, asymmetric soliton splitting results in the appearance of a temporally-delayed ultrashort pulse with high intensity, overlapping and copropagating with the dispersive wave pulse. Intense cross-phase modulations lead to octave-wide broadening of the dispersive wave spectrum, covering 200–400 nm wavelengths. The highly-coherent, octave-wide ultraviolet spectrum, generated from the simple capillary fibre set-up, is in great demand for time-resolved spectroscopy, ultrafast electron microscopy and frequency metrology applications, and the critical role of the secondary pulse in this process reveals some new opportunities for all-optical control of versatile soliton dynamics.

DOI:10.1038/s41467-024-52955-6 [Paper Links]

Corresponding authors

-Zhiyuan Huang, Email: huangzhiyuan@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Critical-layered MoS₂ for the Enhancement of Supercontinuum Generation in Photonic Crystal Fibre

Abstract

Supercontinuum generation (SCG) from silica-based photonic crystal fibers (PCFs) is of highly technological significance from microscopy to metrology, but has been hindered by silica's relatively low intrinsic optical nonlinearity. The prevailing approaches of filling PCF with nonlinear gases or liquids can endow fibre with enhanced optical nonlinearity and boosted SCG efficiency, yet these hybrids are easily plagued by fusion complexity, environmental incompatibility or transmission mode instability. Here this work presents a strategy of embedding solid-state 2D MoS2 atomic layers into the air-holes of PCF to efficiently enhance SCG. This work demonstrates a 4.8 times enhancement of the nonlinear coefficient and a 70% reduction of the threshold power for SCG with one octave spanning in the MoS2-PCF hybrid. Furthermore, this work finds that the SCG enhancement is highly layer-dependent, which only manifests for a real 2D regime within the thickness of five atomic layers. Theoretical calculations reveal that the critical thickness arises from the trade-off among the layer-dependent enhancement of the nonlinear coefficient, leakage of fundamental mode and redshift of zero-dispersion wavelength. This work provides significant advances toward efficient SCG, and highlights the importance of matching an appropriate atomic layer number in the design of functional 2D material optical fibers.

DOI:10.1002/adma.202403696 [Paper Links]

Corresponding authors

Title

Spectral Bandwidth Tuning of Photoionization-induced Blue-shifted Solitons in Gas-filled Hollow-core Anti-resonant Fibers

Abstract

Frequency blue shift of optical solitons, due to light-plasma interactions is an important nonlinear process of laser frequency up-conversion in gas-filled hollow-core fibers, which can be applied for generating ultrafast pulses with tunable wavelengths covering infrared and visible regimes. While the mechanism of photoionization in this frequency up- conversion process has been well studied for a few years, the spectral width tunability of output ultrafast pulses generated in such a hollow-core fiber system has not been comprehensively investigated yet. Here, we demonstrate that the theory of adiabatic soliton propagation could be used to understand the spectral bandwidth evolution of photoionization-induced blue-shifted solitons in gas-filled hollow-core anti-resonant fibers. Experimentally, through varying the gas pressure in the hollow-core fiber and the input pulse energy, high-repetition-rate (10 kHz) tunable ultrafast pulses were obtained at the fiber output with a central wavelength tuning range from ∼900 nm to ∼650 nm and a bandwidth tuning range (at 700 nm) from ∼100 nm to ∼180 nm, corresponding to chirp-compensated pulse width tuning from 7.2 fs to 5.4 fs. These experimental results agree well with the theoretical predictions, and the femtosecond visible light source with high-flexible pulse parameters, demonstrated here, may have some potentials in ultrafast spectroscopy and nonlinear-optics applications.

DOI:10.1109/JSTQE.2024.3420428 [Paper Links]

Corresponding authors

-Zhiyuan Huang, Email: huangzhiyuan@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Broadband Dispersive-wave Emission Coupled with Two-stage Soliton Self-compression in Gas-filled Anti-resonant Hollow-core Fibers

Abstract

The underlying mechanism of broadband dispersive-wave emission within a resonance band of gas-filled anti-resonant hollow-core fiber is studied. Both numerical and experimental results unveiled that the pump pulse with a soliton order of ≈3, launched into the hollow-core fiber, experienced two stages of pulse compression, resulting in a multi-peak structure of the emitted dispersive-wave spectrum. Over the first-stage pulse compression, a sharp increase of the pulse peak power triggers the first time of dispersive-wave emission, and simultaneously causes the soliton frequency blue-shift due to soliton-plasma interactions. As the central frequency of the blue-shifting soliton approaches to a resonance band of the hollow-core fiber, it experiences a fast-decreasing dispersion value in the fiber waveguide, resulting in the second stage of pulse compression. The second-stage pulse compression triggers the second time of dispersive-wave emission with a phase-matched frequency slightly lower than that at the first stage. Multi-peak spectra of the output dispersive-waves and their formation dynamics can be understood using a delicate and unique coupling mechanism among three nonlinear effects including multi-stage soliton compression, soliton-plasma interaction, and phase-matched dispersive-wave emission. The output broadband dispersive-wave, exhibiting good coherence and stability, can be potentially compressed to sub-30 fs duration using a precise chirp-compensation technique.

DOI:10.1002/lpor.202400531 [Paper Links]

Corresponding authors

-Zhiyuan Huang, Email: huangzhiyuan@siom.ac.cn

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

GHz-rate 57-fs Optoacoustic Mode-locking Fiber Laser Based on Cascaded All-fiber Pulse Compression

Abstract

We demonstrate a compact ultrafast fiber laser system that can deliver 1.87 GHz pulse train at 1550 nm with a pulse energy of 52 pJ and an ultrashort pulse duration of 57 fs. While an acousto-optic mode-locking fiber laser was used as the seed light source at GHz rate, a stage of Er-doped fiber amplifier boosted the laser power to ∼320 mW, giving a pulse energy of ∼170 pJ. Then, a pulse compression setup was constructed, providing a high compression ratio of ∼10 with a total efficiency of ∼32%. In the cascaded compression configuration, multiple fiber samples with alternately normal and anomalous dispersion were fused together, providing efficient nonlinear spectral broadening while suppressing excessive pulse broadening over propagation. This GHz-rate ultrafast fiber laser, with compact configuration, broad optical spectrum, and high time-resolving ability could be used as the seed light source for constructing high-rate, high-power ultrafast laser systems and may find a few applications in optical measurements and microwave photonics.

DOI:10.1364/OL.520119 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Quadratic Phase Mismatch in Multisoliton Interferograms Based on Time-stretched Dispersive Fourier Transformation

Abstract

The time-stretched dispersive Fourier transform method (TS-DFT) has been widely used to analyze multisoliton structures in ultrafast lasers, featuring shot-by-shot access to their spectral interferograms. Most exemplary structures consist of two interacting solitons whose spacing and relative phase can be simply retrieved from their interferograms mapped in time domain by TS-DFT. However, such analysis has not been elaborated for structures with more solitons that could result in a complicated TS-DFT signal. Here we report our theoretical inference of a deterministic discrepancy when retrieving structures consisting of more than two solitons with their TS-DFT signal. We unveil the underlying source of these discrepancies as a result of the quadratic phase mismatch induced by TS-DFT and correspondingly propose an improved retrieval method. Our work may provide a theoretical guideline for conducting unambiguous analysis of complex multipulse structures using TS-DFT, particularly when the dispersive stretching ratio is limited.

DOI:10.1103/PhysRevA.109.053524 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Intrinsic Discrepancy of Multi-Soliton Interferogram in Dispersive Fourier Transform Measurements Due to Impulsive Response of Fast Electronics

Abstract

Time-stretched dispersive Fourier transform (TS-DFT) technique has been widely used for studying transient soliton dynamics due to its unique capability of direct time-frequency mapping of ultrafast pulses in the real time. Particularly, multi-soliton structures that are ubiquitous in ultrafast lasers have been routinely analyzed using TS-DFT technique, giving concurrently spacing and relative-phase information during soliton interactions. While the TS-DFT signal recorded by the fast electronics would inevitably deviate from the genuine spectral interferogram, the consequent discrepancies have not, however, been systematically interrogated yet. In this work, we performed detailed comparisons between the temporal TS-DFT signal of multi-soliton structures (with tunable spacings and phases) and their genuine spectral interferograms measured using optical spectrum analyzer. We revealed that due the impulsive response of the detection system, the retrieved relative-phases from the TS-DFT signal could have non-trivial discrepancies from their actual values, even though the bandwidth of the detection electronics is a few times broader than that of the TS-DFT signal. Our findings can help to provide clearer insights in multi-soliton dynamics using TS-DFT technique.

DOI:10.1109/JLT.2024.3399198 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Kilohertz-linewidth Brillouin Photonic Crystal Fiber Laser with Non-resonance Pumping Configuration

Abstract

Brillouin fiber laser with the non-resonance pumping configuration, has compact set-up, good wavelength tunability and high-power single-frequency-emission capability, being regarded as an important technique for highly-coherent light generation. Such Brillouin fiber lasers, using single-mode fiber (SMF) as the Brillouin gain medium, generally have high lasing thresholds and mode-hopping instabilities, resulting from the relatively-low Brillouin gain coefficient of the conventional SMF. Here, we report a compact, non-resonance-pumping Brillouin photonic crystal fiber laser that can generate kHz-level-linewidth single-frequency lasing with tens-of-mW lasing threshold. Mode-hopping instability of this laser is largely suppressed thanks to the short (∼20 m) cavity length, which gives rise to a free-spectral-range (FSR) value of ∼10 MHz properly matching the tens-of-MHz Brillouin gain bandwidth. Experimental results of both self-heterodyne and frequency-noise measurements indicate a laser linewidth of ∼1.4 kHz, corresponding to high linewidth-narrowing ratio of ∼18. No lasing line of high-order Stokes was observed in experiments as the laser pump power was gradually increased to ∼200 mW, giving a nearly-linear lasing slope.

DOI:10.1109/JPHOT.2024.3399030 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Non-markovian Doppler Velocimetry of Optically Propelled Microparticles in Hollow-core Photonic Crystal Fiber

Abstract

Doppler velocimetry has been widely used in many aspects of research and in applications. The conventional algorithm for tracking Doppler frequencies can induce large errors in the extracted particle velocity when the signal-to-noise ratio of the Doppler signal is low. Ambiguities in velocity identification are also present when multiple measured objects are moving at similar speeds. Here, we report non-Markovian Doppler velocimetry based on a time–frequency ridge extraction algorithm in which features of the historic trajectories are introduced to track the object’s instantaneous velocity. We demonstrate the technique on optically trapped dielectric microparticles in a hollow-core photonic crystal fiber. The technique can improve the accuracy of particle velocity tracking by more than 2 orders of magnitude in the low signal-to-noise regime and is capable of resolving the issue of multiple-particle velocity extraction. The proposed technique can improve the accuracy, sensitivity, and dynamic range of Doppler velocimetry related to vast numbers of applications.

DOI:10.1021/acsphotonics.3c01692 [Paper Links]

Corresponding authors

Title

Sub-ppm NO₂ Gas Sensing in CdTe Quantum Dots Functionalized Hollow-core Anti-resonant Fiber

Abstract

Recently emerged hollow-core fiber provides an excellent platform for various studies due to its remarkable features, such as broadband transmissions, enhanced light-matter overlapping, ultra-low nonlinearity, low latency and dispersion. At the same time, it is generally known that nitrogen dioxide (NO₂) is deemed a common air pollutant. Detecting its concentration is essential, covering environmental monitoring, industrial manufacturing, and human safety issues. Accurate detection of NO₂ in gaseous state, however, has been a long-existing challenge due to the requirements for high sensitivity, compatible equipment, short sampling duration, etc. Here, a novel approach is reported for measuring NO₂ gas by functionalizing a hollow-core anti-resonant fiber (HC-ARF) with CdTe-deposited quantum dots (QDs). The mechanism is based on fluorescence quenching of glutathione-capped CdTe QDs, triggered by NO2 gas. The hollow-core fiber works as a micro-reactor that enhances gas/light interactions. Consequently, the sensitivity can be effectively increased for detecting sub-ppm concentration (∼0.1 ppm) NO2 gas, with high efficiency, small sample volume, and short sampling durations of only a few minutes, much faster than that in many non-fiber-based systems. Such a rapid detection of low-concentration NO2 is applicable in most scenarios. These results provide a reliable, compatible, and promising solution for chemical and biological sensing of gas or liquid-phased substances in hollow-core fibers.

DOI:10.1016/j.snb.2024.135350 [Paper Links]

Corresponding authors

-Xin Jiang, Email: xin.jiang@r-cals.com

Title

Ultrahigh Transverse Mode Purity by Enhanced Modal Filtering in Double-clad Single-ring Hollow-core Photonic Crystal Fiber

Abstract

Maintaining high fundamental mode purity in low-loss single-ring hollow-core photonic crystal fiber remains a key challenge. Here a double-clad anti-resonant-reflecting design is reported that, through modal filtering, provides high suppression of higher-order modes while maintaining a low-loss fundamental mode. The structure consists of a ring of six thin-walled capillaries fused to the inner wall of a thicker-walled capillary that is in turn fused at a single point to the silica outer cladding. Greater than 40 dB m−1 suppression of higher order modes over a 6 nm wide window whose central wavelength can be tuned by adjusting the gas pressure is observed. The novel design paves the way to practical applications of such fibers in high-quality laser beam delivery and low-noise optical-signal transmission.

DOI:10.1002/lpor.202301111 [Paper Links]

Corresponding authors

-Jiapeng Huang, Email: jiapeng.huang@r-cals.com

-Xin Jiang, Email: xin.jiang@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Continuous Tuning of Pulse Parameters in a Soliton Fiber Laser by Adjusting the Effect of Nonlinear Polarization Rotation

Abstract

We demonstrate that through inserting a short length of highly birefringent small-core photonic crystal fiber (Hi-Bi SC-PCF) into a soliton fiber laser, the nonlinear polarization rotation effect in this laser can be manipulated, leading to continuous tuning of the output pulse parameters. In experiments, we observed that by adjusting the polarization state of light launched into the Hi-Bi SC-PCF and varying the cavity attenuation, the laser spectral width can be continuously tuned from ∼7.1 to ∼1.7 nm, corresponding to a pulse-width-tuning range from ∼350 fs to ∼1.56 ps. During the parameter tuning, the output pulses strictly follow the soliton area theory, giving an almost constant time–bandwidth-product of ∼0.31. This soliton fiber laser, being capable of continuous parameter tuning, could be applied as the seed source in ultrafast laser systems and may find some applications in nonlinear-optics and soliton-dynamics experiments.

DOI:10.1364/OL.509981 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Experimental Studies on the Core-structure Dependence of Backward Brillouin Gain in Solid-core Photonic Crystal Fibers

Abstract

Stimulated Brillouin scattering (SBS) in solid-core photonic crystal fibers (PCFs) differs significantly from that in standard optical fibers due to the tight confinement of both optical and acoustic fields in their µm-sized fiber cores, as resultantly evident in their Brillouin gain spectra. Despite many theoretical studies based on either simplified models or numerical simulations, the structural dependency of Brillouin gain spectra in small-core PCFs has not been characterized comprehensively using PCFs with elaborated parameter controls. In this work we report a comprehensive characterization on the core-structure dependences of backward SBS effects in solid-core PCFs that are drawn with systematically varied core-diameter, revealing several key trends of the fiber Brillouin spectrum in terms of its gain magnitude, Brillouin shift and multi-peak structure, which have not been reported in detail previously. Our work provides some practical guidance on PCF design for potential applications like Brillouin fiber lasers and Brillouin fiber sensing.

DOI:10.1364/OE.500633 [Paper Links]

Corresponding authors

-Wenbin He, Email: wenbin.he@r-cals.com

-Meng Pang, Email: pangmeng@siom.ac.cn

Title

Enhancement of Blue and Ultraviolet Components in PCF-based Supercontinuum Generation Through Intermodal Dispersive Wave Radiation

Abstract

Broadband supercontinuum (SC) light sources, generated through nonlinear effects in solid-core photonic crystal fibers (PCFs), have been widely used in spectroscopy, metrology and microscopy, leading to great application successes. The short-wavelength extension of such SC sources, being a longstanding challenge, is a subject of intense studies over the past two decades. However, the exact mechanism of the blue and ultraviolet light generation, especially for some resonance spectral peaks in the short-wavelength regime, has not yet been fully understood. Here, we demonstrate that the effect of intermodal dispersive-wave radiation, resulting from phase matching between pump pulses at fundamental optical mode and packets of linear waves at some high-order modes propagating in the PCF core, might be one of critical mechanisms that can result in some resonance spectral components with wavelengths much shorter than that of the pump light. We observed in the experiment that several spectral peaks resided in the blue and ultraviolet regimes of the SC spectrum, whose central wavelengths can be tuned by varying the PCF-core diameter. These experimental results can be well interpreted using the inter-modal phase-matching theory, providing some useful insights into the SC generation process.

DOI:10.1364/OL.488134 [Paper Links]

Corresponding authors

-Meng Pang, Email: meng.pang@r-cals.com

-Xin Jiang, Email: xin.jiang@r-cals.com

Title

Highly Stable, Flexible Delivery of Microjoule-level Ultrafast Pulses in Vacuumized Anti-resonant Hollow-core Fibers for Active Synchronization

Abstract

We demonstrate the stable and flexible light delivery of multi-microjoule, sub-200-fs pulses over a ~10-m-long vacuumized anti-resonant hollow-core fiber (AR-HCF), which was successfully used for high-performance pulse synchronization. Compared with the pulse train launched into the AR-HCF, the transmitted pulse train out of the fiber exhibits excellent stabilities in pulse power and spectrum, with pointing stability largely improved. The walk-off between the fiber-delivery and the other free-space-propagation pulse trains, in an open loop, was measured to be < 6 fs root mean square (rms) over 90 minutes, corresponding to a relative optical-path variation of < 2×10−7. This walk-off can be further suppressed to ~2 fs rms simply by using an active control loop, highlighting the great application potentials of this ARHCF setup in large-scale laser and accelerator facilities.

DOI:10.1364/OL.486899 [Paper Links]

Corresponding authors

-Meng Pang, Email: meng.pang@r-cals.com

-Xin Jiang, Email: xin.jiang@r-cals.com

Title

Characterization and Manipulation of Temporal Structures of Dispersive Waves in a Soliton Fiber Laser

Abstract

Dispersive waves (DWs) are ubiquitous in nonlinear optical systems that host optical solitons. In particular, DWs in soliton fiber lasers have attracted continuous interest over the past decades since they are closely related to laser soliton instabilities. Meanwhile, intra-cavity DWs also play important roles in solion interactions. While previous studies mostly focused on their spectral properties, characterizations and manipulations of the intra-cavity DWs in the time domain remain unexplored. Here, we report the experimental studies on temporal structures of intra-cavity DWs, unveiling their exponentially-decaying profiles which can be manipulated by varying cavity parameters, following a simple analytic model. We also studied the evolution dynamics of the intra-cavity DWs under abrupt cavity parameter variations. We observed that both the amplitude and the width of the DW waveform went through variations as the pump power was changed, while their evolution trajectories followed intimately that of the soliton energy. These results provide insights into the DW emission mechanism in mode-locked lasers, and the control methods of intra-cavity DWs may find potential applications in the fields of ultrafast pulse generation and soliton dynamics.

DOI:10.1109/JLT.2022.3224564 [Paper Links]

Corresponding authors

- Wenbin He, Email: wenbin.he@r-cals.com

- Meng Pang, Email: pangmeng@siom.ac.cn

- Yunxin Leng, Email: lengyuxin@siom.ac.cn