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This paper presents the first systematic study on second harmonics generation based on a supercontinuum source. We analyzed the effect of the supercontinuum beam diameter and the crystal length to generate second harmonics. The walk-off effect on the second harmonics was also investigated; the efficiency was limited by the walk-off, and the beam of the second harmonic was out of shape. A tunable ultraviolet in the range of 285–340 nm is obtained with the pump of a visible supercontinuum, which is generated by a laser with an all-fiber device. The results provide a theoretical basis for the improvement of the second harmonic power and optimization of the crystal and beam diameters. A potentially tunable ultraviolet with a power exceeding 10 mW can be obtained by a high-power supercontinuum pump.

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 G e 20 A s 20 S e 15 T e 45 and G e 20 A s 20 S e 17 T e 43 , 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.

Supercontinuum covering the ultraviolet-blue region is highly useful for fluorescence microscopy. Four zero-dispersion wavelength decreasing photonic crystal fibers with different fiber cross structures and taper profiles are fabricated to extend the short wavelength edge of supercontinuum. Both nanosecond and picosecond pump pulses at 1 μm are used to generate supercontinuum. With a 3 ns pump pulse, the short wavelength edge of supercontinuum is extended to below 400 nm in a fiber with high air-hole ratio (named T3). The underlying mechanism of supercontinuum generation is explored. The short and long wavelength edges of supercontinuum are highly related with the phase-matching condition which decided by the group velocity curve of fiber small core end. With a 10 ps pump pulse, the spectral intensity around ~800 nm increases in all four fibers. However, the intensity in shorter wavelength band decreased in fibers with a high air-hole ratio (named T3, T4). The experimental results imply that a zero-dispersion wavelength decreasing photonic crystal fiber suitable for nanosecond pulse pumping is not necessarily suitable for picosecond pulse pumping, especially for fibers with high air-hole ratio.

The spectral bandwidth of Er-doped fibers limits their lasing wavelength at longer wave band. Here, to the best of our knowledge, we report a broad emission band (1420‒1680 nm) of Er3+ and demonstrate for the first time an Er-phosphate fiber, which supports laser oscillation at the extended wavelengths of 1627 nm and 1630 nm, with the output powers and slope efficiencies of 44 mW/12.5% and 16.5 mW/5.6%, respectively, pumped at 1480 nm. To the best of our knowledge, these are the highest output powers and slope efficiencies at 1627 nm and 1630 nm from an Er3+-doped all-fiber configuration.

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.

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.

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.

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 Bi

In this work we report on Wavelength Modulation Spectroscopy of ethane with sub parts-per-billion detection limit in the mid-IR spectral band aided with a 30-meters long self-fabricated all-silica Antiresonant Hollow-Core Fiber-based absorption cell.

Perfluorinate glass ceramics with ultra-low phonon energy are very important optical and photonic materials. Unfortunately, there is no suitable method to obtain transparent perfluorinate glass ceramics due to poor thermal stability of fluoride glass. As a result, wide applications of glass ceramics in advanced infrared systems are restricted. Here, an effective method based on phase-separation engineering is used to develop transparent perfluorinate glass ceramics. In this article, a novel transparent Er3+-doped ZnZrF6-Ba6Zn7F26 perfluorinate glass ceramic was designed and fabricated by phase-separation engineering. The sample exhibits low phonon energy (564 cm−1), ultra-wide transmission range (0.33–8.2 μm, T ≥ 50 %), and strong infrared emission, which is better than that of ZBLAN glass, oxide-, and oxyfluoride-glass ceramics. These good properties of the perfluorinate glass ceramic demonstrate that phase-separation engineering not only offers an effective approach to obtain perfluorinate glass ceramics but also provides wide-ranging opportunities for advanced infrared optical and photonic materials.

Anti-resonant hollow-core fiber is featured by the broadband and low-loss transmission of light in the hollow core thanks to the optical property of anti-resonance cladding design. In this paper, we numerically explore the antiresonance of cladding from a new angle and confirm unique and important roles of refractive index of cladding material played in the confinement, bend and material losses of modes in anti-resonant hollow-core fibers. The numerical simulations demonstrate the dependence of confinement loss in anti-resonant hollow-core fiber on the cladding material refractive index which differs from the capillary waveguide and 1.5 refractive index is found favorable to achieve the lowest confinement loss ever in anti-resonant hollow-core fiber. Meanwhile, higher material refractive index can more effectively mitigate the material absorption and consequent contribution to the overall fiber loss in the anti-resonant hollow-core fiber than in the tube waveguide, by reducing the modal overlap factor. Besides, to the best of our knowledge, it is for the first time to discover that the modal dispersion of anti-resonant hollow-core fiber can be effectively altered by the cladding material refractive index, which was usually regarded to be determined by the core dimension and filled-in gas only. The refractive index around 1.8 is preferred to achieve a flattened profile of group dispersion over the transmission spectral window in anti-resonant hollow-core fibers. Our results bring in a new perspective to investigate the guidance mechanism of anti-resonant hollow-core fibers and provides guidance for material selection in the fiber design for optimization of waveguide properties.

In this work, we present femtosecond laser cutting of microchannels in a nodeless antiresonant hollow-core fiber (ARHCF). Due to its ability to guide light in an air core combined with exceptional light-guiding properties, an ARHCF with a relatively non-complex structure has a high application potential for laser-based gas detection. To improve the gas flow into the fiber core, a series of 250 × 30 µm microchannels were reproducibly fabricated in the outer cladding of the ARHCF directly above the gap between the cladding capillaries using a femtosecond laser. The execution time of a single lateral cut for optimal process parameters was 7 min. It has been experimentally shown that the implementation of 25 microchannels introduces low transmission losses of 0.17 dB (&lt, 0.01 dB per single microchannel). The flexibility of the process in terms of the length of the performed microchannel was experimentally demonstrated, which confirms the usefulness of the proposed method. Furthermore, the performed experiments have indicated that the maximum bending radius for the ARHCF, with the processed 100 µm long microchannel that did not introduce its breaking, is 15 cm.

In this work, we present simultaneous and sensitive detection of methane and ethane at ~3.34 µm using a 15-meteres long self-fabricated silica ARHCF and Wavelength Modulation Spectroscopy technique. The ARHCF was filled with a mixture of 10 ppmv and 20 ppmv ethane and methane, respectively via air-tight housings placed at both fiber end-facets. The gas molecules were excited using a self-built continuous wave Difference Frequency Generation source which radiation was coupled into the gas-filled ARHCF. The ARHCF-aided gas sensor reached a minimum detection limit at parts-per-billion by volume level, confirming the suitability of the proposed approach for trace-gas sensing.

One of the most unique features of the hollow-core fibers (HCF) is the possibility to fill the fiber core with desirable gas, which can be useful, especially in laser absorption spectroscopy (LAS) of gases. Sensitivity of LAS-based gas sensors can be easily and efficiently enhanced by increasing the gas molecules-laser light interaction path within the sensor’s setup. In commonly used multi-pass cells-based sensors, the increase in optical path length comes at the cost of the increased sensor’s complexity, leading to the reduced detection capability. HCFs, that are long, flexible, and devoid of additional optics, allows for constructing gas absorption cells that give the possibility to design less-complex sensor's. One of the main problems in developing HCFs-based sensors is the filling time of the fiber core with the target gas. The core diameter of an Anti-Resonant Hollow Core Fiber (ARHCF) is typically on the order of tens of micrometers, leading to a low volumetric flow rate. It was reported in [1] that the time required to fill a 1 meter-long ARHCF with a core diameter of ~84 μm with the target gas using relatively low overpressure (800 Torr) is about 20 seconds. Additionally, the nodeless type of ARHCF is made of separated capillaries, forming its cladding ( Fig. 1a ). The velocity of gas flow in the gaps between the cladding capillaries is far lower than velocity in the core. We believe that this has a significant impact on the time of pressure-driven gas-filling of such fibers, hence it is necessary to perform simulations on more complex geometric models than for the gap-less ARHCFs, where the core can be approximated by a simple 2D channel.

One of the main challenges of laser-based gas sensingis the development of sensors delivering simultaneously high sensitivity, selectivity, fast-response time and non-complex design. Mostly, the detection capability of such sensors depends on the interaction path length between the laser light and the measured gas. Hence, long optical paths are highly desired for e.g. low-concentration gas sensing. Our proposal is to use Antiresonant Hollow-Core Fibers (ARHCFs), which filled with the target gas mixture form absorption cells with potentially any length, delivering low-volume, long and versatile optical paths within the sensor configuration. Currently, the ARHCF core is filled with the target gas via specially designed bulk-optics-based cells placed at the fiber’s ends. This solution provides relatively fast fiber core filling time, however being only efficient while an overpressure is used to force the gas flow through the core, not the diffusion. Therefore, searching for alternative ways of fiber filling with the target gas is necessary. We propose a method of processing the fiber structure using a femtosecond laser allowing for non-invasive accessing the fiber core for more efficient and faster gas diffusion into it through the fabricated microchannels. The fiber structure modification was optimized in a way that does not introduce any unwanted damage of the fiber e.g. cracks on the glass parts or cladding capillaries. The performed experiments have indicated that the laser-processing of the ARHCF structure introduces negligible transmission loss regardless of the number of fabricated microchannels and their length (0.2dB loss for 25 microchannels), confirming the proposed method suitability.

In this work, we demonstrate a wide-spectrum multi-wavelength fiber laser generated by Brillouin-comb assisted four-wave mixing (FWM). The mechanism of the laser is based on the combination of stimulated Brillouin scattering in cavity and multiple FWM process outside cavity. The experiment results demonstrate that Brillouin-comb assisting is an effective method to improve the FWM process and increase the line number. More than 110 lines are achieved in 10 nm spectral range with the wavelength spacing of ∼ 0.09 nm. The multi-wavelength fiber laser exhibits good stability on both the operating wavelengths and output power. The tunability of this multi-wavelength fiber laser is improved significantly by inserting a tunable filter in the cavity. The total line number is kept more than 110 in the whole tunable range of 30 nm from 1540 to 1570 nm.

This work reports the plasmon enhanced near-infrared emission from Er3+-doped Bi2O3-B2O3-SiO2 glass prepared by conventional melt-quenching method. The silver nanoparticles (NPs) were introduced into glass host as starting constituent not in the form of silver univalent ions, avoiding the complex energy transfer between Ag+ and Er3+. TEM images analysis reveal the homogeneous distribution of almost spherical Ag NPs with average diameter ~6 nm in amorphous glass. Surface plasmon resonance (SPR) band was observed around 495 nm. Interestingly, the near-infrared emission intensity assigned to Er3+:4I13/2→4I15/2 transition undergoes significant enhancements up to 2 times. The peak emission cross section at 1.5 µm is estimated to be 7.3 × 10–21 cm2 on the basis of Fuchbauer–Ladenburger equation. Furthermore, the measured lifetime of Er3+:4I13/2 energy state is prolonged from 1.72 ms to 1.85 ms with the gradual increasing Ag NPs. The silver nanoparticles enhanced near infrared luminescence from bismuth glasses have potential application for optical fiber amplifier.

In this paper, the forward Brillouin scattering frequency shift of step-index fibers and photonic crystal fibers (PCFs) with different core diameters is investigated by numerical simulation. The result shows that, multiple Brillouin scattering peaks with the shift range from 20 to 800 MHz are generated in the step-index fibers. With the core diameter changing, the Brillouin scattering frequency shift has no change, but the scattering efficiency is different. In the PCFs with periodic air holes, there are a few dominant peaks. The frequency shifts vary with the core diameter changing. For the core diameter of 11.6 μm, there are three peaks at 327.39, 556.48 and 824.14 MHz, respectively. When the core diameter decreases to 2.32 μm, the frequency shifts of forward Brilloun scattering are 1.619, 2.779 and 4.123 GHz, respectively. The forward Brillouin frequency shifts increase with the core diameter decreasing. The results will be useful for the research on the mechanism of forward Brillouin scattering and have potential applications on the field of nonlinear fiber optics.

A tunable passively Q-switched fiber laser is fabricated using a graphene saturable absorber. The graphene saturable absorber is composed of a tapered fiber coated with graphene nanosheets. It combines the abilities of a saturable absorber and power-dependent tunable filter, thereby simplifying the structure of the tunable passively Q-switched fiber laser. The transmission peak of the graphene saturable absorber is linearly dependent on the input power and temperature, with slopes of approximately -0.35 nm/mW and -3.63 nm/°C, respectively. The Q-switched fiber laser exhibits a tunable wavelength range of 7 nm (from 1558 to 1565 nm). The output pulse offers the shortest pulse duration of 2.22 μs with a repetition rate of 39.1 kHz and the widest pulse duration is 5.9 μs that corresponds to a repetition rate of 21.3 kHz. The maximum pulse energy is 251.9 nJ, corresponding to an average output power of 8.6 mW.