Numerical simulation of ultrawideband supercontinuum generation covering a 2-20 µm waveband in cascaded all-soft-glass fiber.

We designed a cascaded all-soft-glass fiber structure and simulate midinfrared 2-20 µm ultrawideband supercontinuum (SC) generation numerically. The cascaded fiber structure consists of a 1.5 m I n F 3 fiber, a 0.2 m chalcogenide photonic crystal fiber, and a 0.2 m tellurium-based chalcogenide photonic crystal fiber. Using a 2 µm pulse pumping this cascaded structure, the generated SC covering the wavelengths longer than 20 µm has been demonstrated theoretically. The 30 dB bandwidth reaches 20.87 µm from 1.44 to 22.31 µm. The effect of different pulse widths on SC generation is considered. With the increase of peak power and the decrease of pulse width, the energy of SC in the 15-20 µm waveband increases gradually. The mechanism of SC broadening process has also been analyzed. The SC generation of more than 20 µm in this cascade structure is caused by the self-phase modulation, soliton effects, four-wave mixing, and redshifted dispersive wave. This method demonstrates the possibility of generating ultrawide bandwidth SCs up to a 20 µm waveband by a commercial 2 µm pump source and all-fiber structure.

Brillouin gain spectrum characterization in an acoustic anti-guided delivery fiber for high power narrow linewidth laser.

The Brillouin gain spectrum (BGS) provides key information for stimulated Brillouin scattering (SBS), such as the Brillouin frequency shift (BFS), Brillouin spontaneous linewidth, and Brillouin gain coefficient. In this study, we theoretically investigate the field distributions and BGS characterization of Ge-doped, Al-doped, and Al/Ge co-doped fibers. Additionally, we analyzed and compared the relationship between the BGS and acoustic refractive index. In particular, we demonstrate the crucial role played by acoustic modes in anti-waveguide structures. The simulation results show that the Brillouin gain coefficient decreases with a decreasing acoustic index in the fiber core region. Furthermore, we experimentally measure the SBS threshold and BGS of the Al/Ge co-doped fiber to examine the validity of the numerical model. The simulated and experimental results are consistent.

Decaying dynamics of harmonic mode-locking in a SESAM-based mode-locked fiber laser.

The entire decaying dynamics of harmonic mode-locking (HML) are studied utilizing the dispersive Fourier transform (DFT) technique in a SESAM-based mode-locked fiber laser. It is unveiled that the harmonic solitons do not disappear directly, but undergo transitional processes from the higher-order HML to the lower-order HML and then to the fundamental mode-locking (FML), and finally vanish. The "big corner" can also exist in the decaying process rather than just in the buildup process of HML, and there is at least one "big corner" during the decaying process between the consecutive multi-pulsing states. The energy stabilization phase (ESP) cannot be observed during every transitional process. A breathing behavior and a vibrating soliton molecule are observed in the decaying process from the 2nd HML to the FML and in the decaying process of the FML, respectively. Our work would enrich the understanding of HML behaviors and may contribute to the laser designs.

Transverse mode switchable fiber laser with a multimodal interference-based beam shaper.

We propose a Yb-doped fiber laser with an all-fiber beam shaper based on a single-mode-graded-index multimode-few-mode fiber (SMF-GIMF-FMF) structure. The excitation coefficients of the mode can be adjusted continuously by changing the GIMF length. Numerical simulations are performed to investigate the beam shaping dynamics in the fiber structure. Through adding the simple device geometry in the laser cavity, the switchable output between the fundamental transverse (LP01) mode and the second-order transverse (LP11) mode can be achieved. Cylindrical vector beams with high mode purity are also shown by removing the degeneracy of the LP11 mode.

Temporal coupled-mode theory for PT-symmetric chiral metasurfaces.

We have developed a temporal coupled-mode theory based on quasi-normal modes to investigate the chiroptical effects in parity-time (PT) symmetric metasurfaces. The PT symmetry enforces a different constraint for the direct scattering matrix and the coupling constants, which is verified by calculating the transmission spectra originating from the chiral quasi-bound states in the continuum. What's more, the scattering matrix can be analytically continued to the complex frequency plane. We find that the zero and pole singularities of the transmission coefficients and scattering matrix play an important role in the optical chirality. The pole singularities carry a quantized topological charge of -1. Our work paves the way for studying the enhanced optical chirality in non-Hermitian metasurfaces.

Pulse energy exchange of vector solitons in a fiber laser.

The energy exchange between orthogonal polarization components is crucial for the build-up of vector solitons (VSs). Unlike previous observations of energy exchange in the frequency domain, our experiments analyzed pulse energy flows in the time domain. We provide evidence to demonstrate the influence of the four-wave mixing (FWM) and cross-phase modulation (XPM) effect on VSs build-up in passive mode-locked fiber lasers through a perspective of pulse energy exchange. The results indicate that the energy exchange of PRVS caused by FWM and XPM is stronger than that of PLVS. The liner energy exchange caused by the birefringence of fiber and PC influences the period of energy exchange. After stabilization, the intra-cavity energy evolution period is one roundtrip for PLVS while serval roundtrips for PRVS. In the future, we can achieve PLVS by adjusting the linear energy exchange through cavity birefringence, thereby meeting the industrial demand for stable and uniform pulse trains.

High-efficiency mode-locked erbium-doped ZBLAN fiber laser around 2.8 µm by directly depositing Bi2S3 particles onto a cavity mirror.

Mid-infrared (MIR) pulsed lasers near a 3 µm waveband show great potential for the high absorption of water molecules and many important gas molecules. A passively Q-switched mode-locked (QSML) E r 3+-doped fluoride fiber laser with a low laser threshold and high slope efficiency around a 2.8 µm waveband is reported. The improvement is achieved by depositing bismuth sulfide (B i 2 S 3) particles onto the cavity mirror directly as a saturable absorber and using the cleaved end of the fluoride fiber as output directly. -QSML pulses begin to appear with the pump power of 280 mW. The repetition rate of the QSML pulses reaches a maximum of 33.59 kHz with the pump power of 540 mW. When the pump power is further increased, the output of the fiber laser switches from the QSML to the continuous-wave mode-locked operation with the repetition rate of 28.64 MHz and the slope efficiency of 12.2%. The results indicate that B i 2 S 3 is a promising modulator for the pulsed lasers near a 3 µm waveband, which paves the way for further development of various applications in MIR wavebands, including material processing, MIR frequency combs, and modern healthcare.

Spatial mode and wavelength switchable erbium-doped fiber laser based on a fiber beam shaper.

A fiber-based beam shaper to adjust the distribution of spatial modes in a few-mode fiber (FMF) is theoretically and experimentally investigated in this work. A compact and robust device, composed with a single mode fiber-graded index multimode fiber-few mode fiber (SMF-GIMF-FMF), is fabricated by simply fusion splicing of the fibers. Switchable spatial modes and multi-wavelength comb are obtained by the combination of the beam shaper and the few-mode fiber Bragg grating (FM-FBG). This combination acts as a filter to select the wavelength and spatial mode in the laser. A spatial mode switchable fiber laser with high mode purity is extended among LP01, LP11 and cylindrical vector beam (CVB) by adjusting the pressure applied on the beam shaper. Five-discrete wavelengths and their free combination wavelength comb are emitted with a slope efficiency higher than 10%. The fiber laser can be used in the spatial- and wavelength-division multiplexing (SWDM) fiber communication networks requiring particular structure light field.

2†µm sub-GHz harmonic mode-locked soliton generation based on a Bi2S3 saturable absorber.

Saturable absorber (SA) based harmonic mode-locking (HML) techniques at 2 µm waveband are much less reported than those at 1.5 µm waveband, the maximum repetition rate of the harmonic pulse generated by such techniques at 2 µm waveband is also much lower than those generated at 1.5 µm waveband. In this paper, the 39th harmonic with the repetition rate of 908.6 MHz is realized in a Bi2S3-based thulium-doped fiber laser. The fundamental mode-locked pulse has a central wavelength of 1954.2 nm and a 3-dB bandwidth of 5.1 nm. The repetition rate is 23.27 MHz and the pulse width is 902 fs. The characteristics of the material and harmonic mode-locked pulse are investigated. To the best of our knowledge, this is the highest and the closest resonance frequency to GHz among the reported SA-based harmonic mode-locked fiber lasers operating at 2 µm waveband.

Simulation and analysis of supercontinuum generation in the waveband up to 25 µm.

The ultra-wideband supercontinuum generation (SCG) in a Te-based chalcogenide (ChG) photonic crystal fiber (PCF) is simulated in the mid-infrared (MIR) waveband. The PCF core and cladding materials are Ge20As20Se15Te45 and Ge20As20Se17Te43, respectively. The supercontinuum (SC) broadening affected by the core diameter and fiber absorption is considered. The selected PCFs at different pumping wavelengths can demonstrate the generation of ultra-wideband MIR supercontinuum according to the simulated results. We consider SC broadening with and without fiber absorption. A SC range from 3 to 25 µm is demonstrated by simulation in a PCF with a core diameter of 8 µm and a pump wavelength of 6 µm considering the fiber absorption. With the increase of the peak power and the pulse width and the decrease of the core diameter, the degree of coherence gradually degraded. To the best of our knowledge, this is the first demonstration of the possibility of SCG up to the waveband of 25 µm in fiber. Our results highlight the potential of a novel Te-based chalcogenide multi-material PCF for SCG. We also provide a way to generate the SCs to longer wavebands than 20 µm in fiber, especially up to the far-infrared waveband.

Comparison of Different Linewidth Measuring Methods for Narrow Linewidth Laser.

We experimentally demonstrate a fiber laser with different linewidths based on self-injection locking (SIL) and the stimulated Brillouin scattering effect. Based on the homemade fiber laser, the error origin, resolution, and applicable range of delayed self-heterodyne interferometry (DSHI), self-correlation envelope linewidth detection (SCELD) and Voigt fitting are investigated numerically and experimentally. The selection of the linewidth measuring method should meet the following conclusions: an approximately Lorentzian self-heterodyne spectrum without the pedestal and high-intensity sinusoidal jitter is a prerequisite for DSHI; the SCELD needs a suitable length of delay fiber for eliminating flicker noise and dark noise of the electrical spectrum analyzer; a non-Lorentzian self-heterodyne spectrum without a pedestal is an indispensable element for Voigt fitting. According to the experimental results, the laser Lorentzian linewidth of SIL changes from 1.7 kHz to 587 Hz under different injection powers. When the Brillouin erbium fiber laser is utilized, the Lorentzian linewidth is measured to be 60 ± 5 Hz.

Polarization conversion in anisotropic dielectric metasurfaces originating from bound states in the continuum.

We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high-Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high-Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.

Micro-joule level visible supercontinuum generation in seven-core photonic crystal fibers pumped by a 515 nm laser.

A visible supercontinuum (SC) with high energy is of vital importance to applications in remote sensing and hyperspectral light detection and ranging. A fiber laser with a wavelength of 1030 nm is frequency doubled through a LBO (LiB3O5) crystal, and a high-energy 515 nm laser is obtained after wavelength conversion. Two kinds of seven-core photonic crystal fibers (PCFs) are used in this Letter. One is a uniform seven-core PCF (USC-PCF), and the other is a tapered seven-core PCF (TSC-PCF). Pumped by a 515 nm laser with a pulse width in nanosecond level, an SC covering 400 to 900 nm is efficiently generated in both PCFs. A maximum energy of 4.24 µJ is obtained in a USC-PCF. To prevent fiber damage of the coupling fiber end, the TSC-PCF which contains a transition fiber and a meters-long small core fiber is fabricated. One end of the transition fiber possesses a larger core diameter, and the pump laser can be coupled into the TSC-PCF without fiber damage. The meters-long small core fiber has the same core size with a USC-PCF and is utilized as the nonlinear medium to generate an SC. The dispersive wave in the short wavelength band is excited when more energy is shed into a fiber anomalous dispersion region. Up to 15th-order Raman peaks are observed during the SC evolution process.

Asterisk-shaped microstructured fiber for an octave coherent supercontinuum in a sub-picosecond region.

We selected two thermally matched silicate glasses with fair refractive index contrast and developed an asterisk-shaped all-solid microstructured optical fiber. The fiber presents a low, ultra-flat, and all-normal dispersion in a wide wavelength range, allowing for the generation of an octave-spanning coherent supercontinuum (SC) in a 20 dB dynamic range with 0.5 ps pump pulses at 1.55 µm. This result improves pump pulse duration that is only ∼100 fs, related to the broadband and highly coherent SC generation in fibers with all-normal dispersion. This enables broadband SC sources with all-fiber, high-power, and highly coherent properties.

Numerical study on supercontinuum generation by different optical modes in AsSe2-As2S5 chalcogenide microstructured fiber.

We investigate supercontinuum generation (SCG) in AsSe2-As2S5 chalcogenide microstructured optical fibers (MOFs) pumped by different optical modes. The influence on SCG by different optical modes including the fundamental and high-order modes is analyzed numerically. The evolution of the supercontinuum (SC) is investigated by changing the pump wavelength (2120, 2580, and 3280 nm) and peak power (from 200 to 1000 W) of each optical mode (LP01,LP11,LP31) in the MOFs with different fiber lengths. SCG in MOFs with different core diameters is also simulated. The different optical modes cause the variation of the chromatic dispersion profile and the effective nonlinearity, which induces different mechanisms of the SCG and changes the spectral range. The maximum SC spectral range covers 12.931 µm from 1.389 to 14.320 µm when pumped by the LP11 mode with the peak power of 1000 W at 3280 nm. The simulated results will be instructive for the experimental SCG up to the midinfrared waveband longer than 10 µm.

Theoretical investigation on mid-infrared cascaded Raman fiber laser based on tellurite fiber.

A numerical simulation of third-order cascaded Raman fiber laser based on tellurite fiber at the 2-5 µm waveband is presented. The Raman fiber laser can be optimized with the most suitable tellurite fiber length of 0.5-1.0 m and the most reasonable reflectivity of the third-order Stokes output FBG32 of 10%-20%. We demonstrate numerically that the third-order Stokes wave can reach the maximum average power of 45.2 W and the maximum optical conversion efficiency of 45.2%, corresponding to the FBG32 reflectivity of 10% and the tellurite fiber length of 0.3 m with the attenuation of 0.85 dB/m, when pumped by 2 µm light with the average power of 100 W. Our simulated results provide valuable theoretical guidance for the design and experiment of tellurite Raman fiber laser at a mid-infrared waveband.

Laser power density dependent energy transfer between Tm3+ and Tb3+: tunable upconversion emissions in NaYF4:Tm3+,Tb3+,Yb3+ microcrystals.

Energy transfer between Tm3+ and Tb3+ dependent on the power density of pump laser was investigated in NaYF4: Tb3+,Tm3+,Yb3+ microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb3+-Tm3+-Yb3+ doped samples exhibited intense visible emissions with tunable color between green and blue. The ratio of blue and green emission were determined by energy transfer between Tm3+ and Tb3+. When the power density of pump laser was low, the energy transfer process from Tm3+ (3F4) to Tb3+ (7F0) occurred efficiently. Upconversion processes in Tm3+ were inhibited, only visible emissions from Tb3+ with green color were observed. When the power density increased, energy transfer from the 3F4 (Tm3+) to 7F0 level (Tb3+) was restrained and population on high energy levels of Tm3+ was increased. Contribution of upconversion emissions from Tm3+ gradually became dominant. The emission color was tuned from green to blue with increasing the power density. Energy transfer processes between low-lying levels of activators, such as Tm3+ will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb3+-Tm3+-Yb3+ doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.

Stimulated Raman scattering in AsSe2-As2S5 chalcogenide microstructured optical fiber with all-solid core.

The effects of stimulated Raman scattering (SRS) is demonstrated in chalcogenide microstructured optical fiber (MOF) with all-solid AsSe2 core and As2S5 cladding. The first-order Raman Stokes wave is investigated in the MOFs with different core diameters pumped by the picosecond pulses at 1958 nm. The maximum conversion efficiency of -15.0 dB from the pump to first-order Raman Stokes wave is obtained in the MOF with the core diameter of 2.6 µm. The conversion efficiency decreases when the core diameter deviates from 2.6 µm. When the fiber core is larger, the effective nonlinearity is decreased. When the fiber core is smaller, the mode field is difficult to be confined in the core. The walk-off length between the pump and Stokes wave is crucial to the process of SRS according to the analysis of the experimental data. The Raman effects are simulated numerically. The simulated results can agree well with the experiments. It is the first time to demonstrate the Raman effect in AsSe2-As2S5 MOF, to the best of our knowledge.

Experimental investigation on supercontinuum generation by single, dual, and triple wavelength pumping in a silica photonic crystal fiber.

We investigate the supercontinuum (SC) generation in an 1 cm long silica photonic crystal fiber (PCF) pumped by the pulse sources with single, dual, and triple wavelengths, respectively. The silica PCF has two zero-dispersion wavelengths at 900 and 2620 nm, respectively. When pumped by a single wavelength, the SC spectral range covers about 1000 nm. When pumped by dual and triple wavelengths, the SC spectral range covers wider than 2000 nm. Both the SC spectral range and the flatness are improved obviously when pumped by triple wavelengths. The maximum SC spectral range is obtained when the silica PCF is pumped by the triple wavelengths at 800, 1450, and 1785 nm. The SC spectral range covers 2810 nm from 350 to 3160 nm wider than three octaves. The 10 dB bandwidth covers 2280 nm from 450 to 2730 nm wider than two octaves. This is the first investigation on comparison of the SCs generated by different pump wavelengths up to three experimentally. The generated SC spectra have covered the full transmission window of silica fiber.

Dispersion characterization of two orthogonal modes in a birefringence tellurite microstructured optical fiber.

An elliptical core tellurite microstructured optical fiber with high birefringence was demonstrated and the chromatic dispersion of the two orthogonal modes in this fiber was experimentally characterized by a white light spectral interferometric technique over a wide spectral range. A series of spectral interferograms as a function of the optical path difference were recorded in the Mach-Zehnder interferometer. The birefringence dependence of the wavelength in the fiber was determined by interferograms. The measured and calculated dispersion matched well within the whole spectrum range under test.