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Formation of ultrashort pulses in a fiber laser cavity with a slow saturated absorber and spectral filter is studied. It is found that the finite relaxation time of the saturable absorber in a cavity with full normal chromatic dispersion leads to the spectral shift of generated pulses with respect to the central wavelength of the spectral filter. The numerical results are verified using two experimental laser sources: a fiber ring laser with a saturable semiconductor mirror and a fiber laser with a nonlinear amplifying loop mirror. The obtained results are topical for designing sources of ultrashort pulses in applications for which spectral properties of the radiation are essential. [ABSTRACT FROM AUTHOR]

Transverse mode instability is a key limit to power scaling of high-power fiber lasers. Accurate modeling efforts have, however, been hampered by a lack of experimental data to verify a model. Recently, there have been some good experimental studies, making it possible to validate a model. In this work, we developed a model by integrating a 3D fiber amplifier and stimulated thermal Rayleigh scattering. Since we are only interested in the regime where the fundamental mode dominates, our 3D amplifier divides the core into many cylindrical shells. This limits the model to situations where bend-induced mode distortion of the fundamental mode is negligible, but it is still applicable for most practical scenarios. The benefit of this model is high computational efficiency; it can run in minutes on a PC. This 3D amplifier model considers various pumping configurations and amplified spontaneous emission. It can simulate most experimental conditions. Excellent quantitative fit to experimental data was achieved. Additional studies were also conducted to show that gain saturation is a dominating effect in understanding the observed behaviors of transverse mode instability. [ABSTRACT FROM AUTHOR]

A detailed analysis of theexcited state absorption (ESA) transition 4I13/2→4I9/2 which occurs in erbium (Er) doped ZBLAN fiber lasers is performed through numerical modeling. It is found that the ESA plays a significant role in enhancing the 2.8 μm lasing of Er3+ ions. The ESA promotes the ions residing in 4I13/2 (the lower state of the 2.8 μm laser) to 4I9/2, where the ions subsequently decay to 4I11/2 (the upper state of the 2.8 μm laser) rapidly. An energy recycling is consequently realized via the ESA. The ESA can be activated in cascaded 2.8 μm/1.6 μm Er-doped ZBLAN fiber lasers by absorbing the 1.6 μm signal. Through numerical simulation it is found that in cascaded lasers the slope efficiency of the 2.8 μm laser can be elevated to ∼48% with the assistance of the ESA, a value that surpasses the Stokes limit (∼35%) substantially and agrees well with the experimental results previously reported. It is also found that the ESA-induced energy recycling is exothermic, an effect that is non-ignorable in the high-power circumstance. Moreover, the injection of an additional 1.6 μm pump for directly exploiting the 4I13/2→4I9/2 ESA is simulated using the model. It is found that as the second pump is introduced, the 2.8 μm lasing almost entirely relies on the ESA-induced energy recycling, and hence, the laser is capable of operating under very low 976 nm pump power. The simulation results indicate that the 2.8 μm lasing in Er-doped ZBLAN fiber lasers can even be realized using a single 1.6 μm pump, via a combination of the 4I15/2→4I13/2 and 4I13/2→4I9/2 transition. The outcomes of this work can inspire the development of novel, high-performance 2.8 μm Er-doped ZBLAN fiber lasers. [ABSTRACT FROM AUTHOR]

We developed a passive silica based large mode area fiber with the nanostructured core with a diameter of 30 μm devoted to fiber Bragg grating inscription for application in an all-fiber laser cavity. The fiber is perfectly matched to the commercial active fiber with NA = 0.06 and core/cladding diameters 30/250 μm. Measured modal characteristics and bending loss of the fiber confirm single-mode operation for bending diameter of 8 cm with bending losses as low as 0.15 dB/m. Although the nanostructured fiber core is low germanium doped, we fabricated high reflectivity (98.5%) UV induced fiber Bragg grating (FBG) and verified its performance in fiber laser setup. The laser cavity formed with highly reflective FBG inscribed in the fiber and Fresnel reflection on the free end of active fiber results in lasing efficiency of 65.5%. The flexibility of the nanostructuring approach gives prospects for the development of free-form large mode area fibers for passive fiber components and matched with any type of active fibers. [ABSTRACT FROM AUTHOR]

A dispersion-managed, figure-of-9, mode-locked ultrafast Yb-doped fiber laser is presented in this letter. The laser cavity employs a chirped fiber Bragg grating with a proper anomalous dispersion for intra-cavity dispersion compensation. The laser with large normal net dispersion delivers an output with maximum average power of 20 mW and pulse duration of 5.50 ps at the repetition rate of 39.1 MHz, and the pulse can be dechirped to 228 fs. While net dispersion of cavity is very close to zero, the laser delivers an output with an average power of 12 mW at the repetition rate of 47.3 MHz, and the pulse duration is 1.86 ps, which can be dechirped to 175 fs. The whole cavity is very compact and environmentally stable, for its all-polarization-maintaining and all-fiber structure. [ABSTRACT FROM AUTHOR]

We report a transition metal dichalcogenides-PtSe2 Q-switched single-mode fiber laser operating at 2783.2 nm with a nearly watt-level average output power. We made a PtSe2 coated highly reflective gold mirror to serve as a saturable absorber mirror. The modulation depth, saturable intensity and non-saturable loss of our PtSe2 sample were measured to be 10.2%, 0.093 GW/cm2 and 37.71%, respectively. By incorporating the as-prepared saturable absorber mirror in a single-mode Er3+-doped zirconium fluoride fiber laser, stable Q-switched laser pulses were obtained with a maximum average power of 932.7 mW. The pulse duration and repetition rate were $1.04 ~\mu \text{s}$ and 93.10 kHz, respectively. The corresponding pulse energy was $10.02 ~\mu \text{J}$ and the slope efficiency was 18.1%. By employing a plane ruled grating in the resonator, we also investigated the operating wavelength range of the PtSe2-coated mirror. A continuously wavelength-tunable Q-switched laser was obtained from 2733.0 nm to 2803.0 nm. To the best of our knowledge, this is the first demonstration of PtSe2 working as a saturable absorber for the 2.73- $2.80 ~\mu \text{m}$ waveband, and the Q-switched laser presents threefold and twofold increase in output power and pulse energy, respectively, over previously reported values achieved from ~3- $\mu \text{m}$ single-mode Q-switched fiber lasers enabled by two-dimensional materials. The results indicate that PtSe2 is an excellent wideband optical switch for high-power broadband mid-infrared laser pulses generation. [ABSTRACT FROM AUTHOR]

Swept source basedcoherent detection system (CDS) is a common method to measure weak signals in optical sensing and imaging systems. The nature of the phase noise of swept source based CDS however has attracted little attention. In this paper, the statistical properties of the phase noise in a CDS based on Fourier domain mode locked (FDML) laser are investigated theoretical and experimental. The fast spectral evolution of the FDML laser is characterized by using the time-varying phase. The probability density function (PDF) of the measured residual phase jitter (RPJ) is found to be close to the Gaussian distribution. The variance of the RPJ increases quadratically with the optical path difference of the CDS. The PDF of the RPJ and the variation of the variance of the RPJ and the frequency-noise power spectral density are studied numerically using stationary white noise. The experimental results agree well with simulations. The study provides insight into the noise property of FDML laser based CDS and evaluates the temporal coherence of swept lasers. [ABSTRACT FROM AUTHOR]

New light sources with low spatial coherence and high brightness are crucial for high‐quality imaging. In this paper, such a light source is reported, which is formed by combining random lasing used as seed light with a multimode fiber (MMF) mediated master oscillator power amplifier (MOPA) for power scaling and spatial decoherence. Experimental results show that the proposed multimode random fiber laser (MM‐RFL), with low noise, high spectral density, and high power efficiency, can achieve a maximum output power of 100.6 W and speckle contrast of as low as 0.037. Furthermore, MM‐RFL‐based speckle‐free imaging is demonstrated. The imaging quality can be improved by increasing the output power of the MM‐RFL as its spatial coherence decreases with an increasing number of effective modes excited in the MMF. Such a MM‐RFL provides a powerful light source for speckle‐free imaging applications where high power and low spatial coherence are essential for high‐quality imaging. [ABSTRACT FROM AUTHOR]

We demonstrated high power all-fiber $2\sim 3~\mu \text{m}$ supercontinuum (SC) generation from a short piece of germania-doped fiber (GDF) pumped by a thulium-doped fiber amplifier (TDFA). As 39 W 2 $\mu \text{m}$ picosecond pulse launched into GDF, a mid-infrared SC laser with maximum average power of 33.6 W and broadband spectrum of $1.8\sim 3.0~\mu \text{m}$ was achieved, which was the record SC power in $2\sim 3~\mu \text{m}$ region based on GDF to date, to the best of the authors’ knowledge. The spectral broadening mechanism of the high-power SC generation were theoretical investigated by solving the generalized nonlinear Schrödinger equation (GNLSE). [ABSTRACT FROM AUTHOR]

Thermal tunability for distributed feedback lasers is generally done at slow tuning speed. We study laser current boosting and implement a pre-equalization stage for short wavelength jump, improving the tuning time to the half. A wider jump is also done under a more practical context. [ABSTRACT FROM AUTHOR]

Stable output and beam profile at high power laser operation is a prime requirement for precision high-end manufacturing. The stability of laser charateristics during power scaling in a single-oscillator is limited by several unavoidable non-linear effects. Hence, kW-level power scaling through multiple amplification stages or using signal combination are feasible solutions. In a master oscillator power amplifier (MOPA) architecture, pump-signal combiner with high pump coupling efficiency and low loss signal transmission maintaining seed beam quality during coupling from a single mode fiber into a large mode area fiber, is an important component. In this respect, an effective method of fabricating mode-field matched pump-signal combiner has been presented employing a section of intermediate fiber, having adiabatically tapered core and uniform cladding, along with a conventional signal fiber (termed hybrid signal fiber) inside the input fiber bundle to obtain effective mode-field matching of signal light in between the input fiber bundle and the output fiber. Additionally a taper-less signal combiner design has been proposed for scaling laser power through incoherent combination. The reduced-clad signal fiber with low loss core guidance supports bundling of the input fibers inside an economic low-OH silica tube to obtain good transmission efficiency and beam profile. [ABSTRACT FROM AUTHOR]

Dual-wavelength mode-locked fiber lasers are considered as ideal solutions for fast, precise, and sensitive dual-comb spectroscopy. In this study, we present a self-started dual-wavelength fiber laser by combining a nonlinear amplifying loop mirror and a Lyot filter. Nonlinear phase accumulation, dual-wavelength competition, and crosstalk between the mode-locking mechanism and filtering effect are well addressed to realize the self-started dual-wavelength mode-locking. Furthermore, by temperature controlling the specific polarization-maintaining fiber, our dual-wavelength laser can be continuously tuned in a wavelength range of ∼6 nm, corresponding to a well-controlled repetition rates change of 80 Hz and their difference change of 30 Hz. Mutual coherence of the dual-wavelength pulses is demonstrated by detecting the multi-heterodyne beat notes and measuring the fluctuation of the repetition rate difference. Within 10 hours of measurement, the dual-wavelength repetition rates difference remains stable at 1180 Hz with an Allan deviation of ∼9 × 10−3 Hz @ 1s. By virtue of the all polarization-maintaining structure, our dual-wavelength laser shows improved long-term stability and repeatability, which will facilitate the turn-key, robust, and reproducible dual-comb spectroscopy for high-power or field applications. [ABSTRACT FROM AUTHOR]

Stimulated Raman scattering (SRS) enabled by the emerging gas-filled low-loss anti-resonant hollow-core fiber (ARHCF) technology opens up a competitive way towards the development of novel lasers in the molecular fingerprint region. In this article, the characteristics of noise and long-term stability of near- and mid-infrared (near-IR and mid-IR) gas-filled fiber Raman lasers have been investigated for the first time. The results reveal that an increase in Raman pulse energy is associated with a decrease in noise, and that the relative pulse peak intensity noise (RIN) is always lower than the relative pulse energy noise (REN). We also demonstrate that long-term drift of the pulse energy and peak power are directly linked with the high amount of heat release during the Raman Stokes generation. The demonstrated noise and long-term stability performance provide necessary references for potential spectroscopic applications as well as further improvements of the emerging IR gas-filled ARHCF Raman laser technology. [ABSTRACT FROM AUTHOR]

Nonlinear optical materials are key platforms in various electronic and photonic devices. This study investigated the feasibility of using bulk germanium monosulfide (GeS) particles to fabricate a saturable absorber (SA) for mode locking of fiber lasers. Bulk GeS particles were prepared via mechanical exfoliation without chemical etching. A theoretical investigation based on density functional theory (DFT) calculations was conducted to determine the origin of the saturable absorption of bulk GeS in the short-wavelength infrared (SWIR) wavelength region despite its large energy bandgap. Theoretically, the experimentally observed SWIR absorption of bulk GeS could be attributed to an energy bandgap reduction caused by strains on the bulk GeS crystal lattice structure. Using the bulk particles, an SA with a saturation power and modulation depth of approximately 38 W and 18 %, respectively, was realized at 1550 nm. The SA was then used in an erbium fiber ring cavity to produce stable optical pulses with a temporal width of ∼796 fs at ∼1556 nm. These results indicate that bulk GeS particles can be a low-cost, efficient, and nonlinear optical material for the realization of SAs in the SWIR region. • The feasibility of using bulk GeS particles to fabricate a saturable absorber was investigated. • A theoretical investigation based on DFT calculations was conducted to determine the bandgap of the bulk GeS. • The SWIR absorption of bulk GeS was theoretically found to be attributable to an energy bandgap reduction caused by strains. • Using an SA based on GeS bulk particles, 796-fs mode-locked pulses were readily produced at ∼1556 nm from a fiber laser. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the performance of two tantalum-based boride (TaB and TaB2) microparticles as potential saturable absorbers for high-power fiber lasers. Both materials are ultrahigh temperature ceramics with melting points above 3000 °C, but with different crystalline structures: TaB has an orthorhombic structure (nearly isotropic), whereas TaB2 has a hexagonal structure (uniaxial, anisotropic). Despite their different crystalline structures, the microparticles have a similar low fluence attenuation (between 2.3 and 2.60 dB/μm) and modulation depths (around 2.0 dB/μm), but remarkable different saturation fluences: TaB has a saturation fluence of 160 μJ/cm2, whereas TaB2 has a saturation fluence of 110 μJ/cm2. The measured damage thresholds are 112 and 106 mJ/cm2/pulse for TaB and TaB2, respectively. When incorporated to a fiber laser, the materials produce pulses with durations of 345 ns, lower than those reported by our group in previous papers. The results show that the materials can find potential applications in high-power Q-switched lasers.

We report a wavelength-tunable high-repetition-rate passively Q-switched fluoride fiber laser around 3 μm by using Ti3C2Tx MXene as saturable absorber (SA). The Ti3C2Tx MXene was synthesized by selectively etching aluminum layers in Ti3AlC2. The modulation depth, non-saturation loss and saturation fluence of the SA at 2866 nm were measured to be 43.10%, 25.16%, and 0.50 mJ /cm2, respectively. By introducing the Ti3C2Tx SA into a Ho3+/Pr3+-codoped fluoride fiber, stable Q-switched pulses with a continuously tuning range of 30.8 nm (2868.4 nm-2899.2 nm) were achieved. The repetition rate was as high as 215.3 kHz with an output power of 142 mW at the wavelength of 2879.0 nm. Such compact mid-infrared Q-switched laser source with a high repetition rate is of great importance in various applications such as medicine, high-resolution photoacoustic microscopy, and remote sensing. Our work indicates that the Ti3C2Tx MXene is a promising broadband light modulator for pulsed laser sources around 3 μm. [ABSTRACT FROM AUTHOR]

A broadband continuously tunable SESAM all-fiber laser based on optical parametric generation (OPG) for coherent anti-stokes Raman scattering (CARS) imaging applications is presented in this article. This laser structure is built on all-polarization maintaining fibers yielding high environmental stability. The OPG is based on PM photonic crystal fiber as parametric gain pumped by a wavelength-tunable (1025 to 1055 nm) and repetition rate-tunable (3 MHz to 500 KHz) Yb-doped pulsed fiber laser. The all-fiber structure features free polarization adjustment in slow-axis tunable operation with a wavelength range from 770 to 940 nm for signal radiation and 1225 to 1510 nm for idler radiation. The pump and signal pulses (CARS-Stokes and CARS-pump respectively) are intrinsically overlapped, both spatially and temporarily, due to the four-wave mixing (FWM) generation and emitted from a single fiber end. The CARS-Stokes and CARS-pump pulse duration vary between 25 to 32 ps and 21 to 24 ps, respectively in the whole band, ensuring a good overlap of the generated signals, and both exhibit a Gaussian profile with high spatial beam quality. The frequency conversion allows to obtain a frequency difference between the generated signals from 1100 to 3300 cm−1. The reported low-cost laser source is ideal for bio-imaging applications, especially in CARS imaging, due to its compactness, simplicity, robustness and ease of handling. A patent has been filed based on this technology. [ABSTRACT FROM AUTHOR]

Metal nanomaterials have attracted increasing attention due to their outstanding nonlinear optical and photonic properties, making them as potential saturable absorber (SA) candidates for realizing ultrafast photonic devices. In this article, we demonstrate the generation of mode-locked dual-wavelength pulse trains in an Ag nanoplates (AgNPTs)-based Yb-doped all-fiber laser for the first time to the best of our knowledge. The AgNPTs are synthesized by seed-mediated growth and then integrated into a fiber ferrule by optical deposition, which serve as SA in the ring laser cavity. The plasmonic properties of such nanoplates are measured by absorption spectrophotometry and their nonlinear optical properties are characterized by Z-scan. The measured nonlinear saturable absorption of the AgNPTs-based SA is 6.4%. In our laser, the dual-wavelength synchronous mode-locking is achieved at the center wavelengths of 1031.92 and 1033.24 nm with 3-dB spectral bandwidth of 0.52 and 0.46 nm, respectively, where 293-ps pulse trains with a repetition rate of 11.43 MHz are obtained at the pump power of 350 mW. The results demonstrate that the solution-processed AgNPTs are promising SA candidates for achieving stable and low-cost pulsed laser sources which could be used for spectroscopy and ultrafast photonics. [ABSTRACT FROM AUTHOR]

There have been many developments in nonlinear propagation models in the past few decades. Especially, a form of such model has been developed to allow a standard ordinary differential equation (ODE) solver to be directly used for its solution. But such a model currently does not consider gain saturation and wavelength-dependent gain, which are very important in high-pulse-energy lasers. In this work, the directly-ODE-integrable nonlinear propagation equation is extended to include gain saturation and gain dispersion, which is then used to study maximum pulse energy limited by amplified spontaneous emission and minimum pulse width limited by gain narrowing in ultrafast fiber lasers in order to demonstrate these new capabilities. [ABSTRACT FROM AUTHOR]

In this paper, a polarization-maintaining quad-wavelength (QW) single-frequency fiber laser (SFFL) at the C-band with the novel and compact structure is demonstrated. Applying the rate equations and multiple coupled nonlinear Schrödinger equations, the theoretical model of QW fiber lasers is established, and then the optical spectra, temporal evolution, and stability of the QW fiber laser are analyzed numerically. Based on the theoretical analysis, a QW-SFFL having an ultrashort linear distributed Bragg reflector cavity is proposed. Utilizing a wideband fiber Bragg grating coupled with a dual-channel polarization maintaining fiber Bragg grating as the wavelength selection and a 15-mm-long Er3+/Yb3+ co-doped phosphate fiber as the gain medium has realized a robust QW laser with the wavelength spacing of 0.4 nm. All the wavelengths maintain the single-frequency operation independently with the linewidths of 20 kHz. This type of compact SFFLs with quadruple wavelengths is widely adapted to lidar systems and fiber sensing.

We present optical frequency-domain reflectometry with high performance in terms of an ultralong range window, high spatial resolution, and precision enabled by a high-order optical phase-locked loop (OPLL)-assisted commercial fiber laser. Due to elaborate loop designs that allow matching with the nonlinear frequency modulation response of the laser, the potential tuning capability can be fully exploited, leading to efficient optimization of the frequency chirp range and rate in addition to chirp linearization and an enhancement of the dynamic coherence. A nearly Fourier-transform-limited spatial resolution is realized over multiples of the intrinsic coherence length. Practically, a cm-level spatial resolution is demonstrated along the entire fiber link. We achieved a spatial resolution of ∼3.2 cm and ∼4.3 cm for backscatter no more than 130 km and reflections at 242 km, respectively, corresponding to a high range resolution factor of up to 1.78 × 10−7 in the latter case. The precision of the backscatter measurement is verified to be at least 0.5 dB throughout the entire measurement range. Moreover, the limiting factors resulting in a degeneration of the spatial resolution are carefully investigated. The proposed approach can be readily applied to other lasers, offering a powerful tool for a variety of applications. [ABSTRACT FROM AUTHOR]

Distributed sensing plays an important role in many industrial applications. Sensor arrays consisting of identical ultraweak fiber Bragg gratings can be applied to such scenarios by monitoring the optical spectral response of the sensors. Yet, the spatial resolution, the sampling speed, and the supported number of sensors in the array are mostly determined by the interrogation scheme. While dual-optical frequency comb techniques had been successfully demonstrated as a high-resolution spectroscopic technique in metrology, it had been beyond the reach of industrial sensing applications because of its high complexity and costs. It is proposed and demonstrated here that, enabled by a simple fiber-optic, dual-comb spectroscopy setup consisting of a fiber laser without frequency stabilization, interrogation of an ultraweak fiber sensor array can be realized. High spatial and spectral resolution as well as kilohertz interrogation speed is achieved in both static and dynamic measurements. It is expected that our depth-resolved dual-comb scheme can measure hundreds of sensors response in less than 1 ms. It is also flexible to deal with targets closely spaced or farther apart from each other. This shows the potential of applying dual-comb spectroscopy to more and more industrial application areas. [ABSTRACT FROM AUTHOR]

We demonstrate an all-fiber ultra-low numerical aperture high power fiber laser system operating in the continuous-wave regime at a central wavelength of 1080 nm. A special Ytterbium-doped fiber preform has been designed and fabricated by using modified chemical vapor deposition technique with the deposition percentages of 0.03mol% of Yb2O3, 1.8mol% of Al2O3, and 2.1mol% of P2O5. Then it has been drawn to obtain an active fiber having core/cladding diameters of 26 μm/410 μm respectively. The numerical aperture of the fiber has been first predicted as 0.034 from refractive index profile of the preform. Afterward, this number has been verified with a simple test setup by altering the bending diameter of the fiber and the excitation conditions of it. To test the high power performance, a laser system has been constructed in a master oscillator power amplifier configuration, and ∼ 80 W seed signal power is amplified to 1.05 kW. The quality of the laser output beam has been measured in terms of M2 value along both x and y coordinates as 1.11 and 1.16 respectively. [ABSTRACT FROM AUTHOR]

Agile optical systems and elastic optical networks demand for flexible, high-performance laser sources. We demonstrate a hybrid silicon-fiber laser that can be largely tuned in wavelength, switched in frequency spacing and easily switched between multi- and single-wavelength operations. Single-mode laser with a fiber-coupled output power of 6 to 8 dBm was measured across the spectral range of 1545 to 1560 nm. No significant sign of power limitation from nonlinear absorption or free carrier generation in silicon was found. It is thus expected that a higher output can be obtained by improving the gain saturation performances as well as the fiber-to-chip coupling efficiency. For multiwavelength operation, we have achieved a frequency spacing switchable between 56 GHz, 75 GHz, and 225 GHz. For both the multi- and single-wavelength operations, a linewidth of less than 20 kHz was measured. All the tuning mechanisms have been realized on the silicon chip, providing a scalable solution for tunable fiber lasers with minimized cost and integration complexity. [ABSTRACT FROM AUTHOR]

We demonstrated a linear-cavity fiber laser using a fiber Bragg grating (FBG) fabricated by femtosecond laser and a Sagnac loop as cavity mirrors. The temperature sensing response of the fiber laser was characterized by placing the FBG in a high-temperature environment. The stability of the fiber laser at high temperature was improved after the FBG was annealed at 1100 °C. The fiber laser can work stably as a temperature sensor at 1000 °C and its temperature sensing sensitivity is approximately 15.9 pm/°C from 300 °C to 1000 °C. • A fiber laser based on an FBG fabricated by femtosecond laser and a Sagnac loop as cavity mirrors is demonstrated. • The fiber laser can work stably as a temperature sensor at 1000 °C after the FBG was annealed at 1100 °C. • The wavelength of the fiber laser had a quadratic dependence on temperature from room temperature to 1000 °C. • The temperature–wavelength had a good linear relationship from 300 °C to 1000 °C. Its sensing sensitivity was about 15.9 pm/°C. [ABSTRACT FROM AUTHOR]

The power scaling of single mode fiber lasers and amplifiers, due to the wide area of applications, has been the subject of great interest for many years. Increasing the mode area seems to be the obvious way to scale up the output power level from the single emitter if you consider well-known limitations like nonlinear effects, material damage threshold or thermal lensing. The nanostructurization of the fiber core is a method to control precisely optical properties of the active fiber. This method allows to design and develop the fiber with the core of any arbitrary defined refractive index distribution, with precision not available with other known fiber technology. The nanostructurization also open up an opportunity to incorporate simultaneously various active and non-active glasses into the fiber core. Those advantages can be used to fabricate the new class of fibers for laser applications. Here we show ytterbium doped phosphate single-mode fiber with nanostructured core, which is the first proof-of-concept of active fiber with entirely nanostructured core area. [ABSTRACT FROM AUTHOR]

We successfully demonstrate a blue diode-pumped green single-wavelength self-Q-switched Ho3+:ZrF4-BaF2-LaF3-AlF3-NaF (Ho:ZBLAN) all-fiber laser emitting at ∼550 nm. Such a green laser, being a very simple device, consists of a 15-cm-long Ho:ZBLAN fiber and a pair of fiber input/output mirrors. The Ho:ZBLAN fiber not only acts as a gain medium but also works as a saturable absorber (SA). The modulation mechanism is induced by the ground-state reabsorption effect in an ineffectively pumped section of the active fiber. Stable Q-switching pulses are characterized by a 889 ns of minimum pulse duration and a 264 nJ of maximum energy pulse at a repetition rate of 67.25 kHz. We believe that this work could provide an appreciable alternative for next generation of pulsed green laser source, which finds the applications in the medical diagnosis, optical imaging, and industrial manufacturing. [ABSTRACT FROM AUTHOR]

A passively Q-switched single-frequency fiber laser with stable multi-wavelength operation is presented in this report. The laser cavity is constructed by using a 13-mm Er3+/Yb3+ co-doped phosphate fiber with a multi-channel fiber Bragg grating and a semiconductor saturable absorber mirror attached to each fiber end, respectively. Quad-wavelength pulsed lasers are obtained and all of them maintain single-frequency operation. By controlling the pump power and the temperature of laser cavity, the Q-switched pulse operation is realized with repetition rates range of 126.6–350.9 kHz, a narrowest pulse duration of 104 ns, and a maximum pulse energy of 55.4 nJ. To the best of our knowledge, this is the first study to realize a Q-switched fiber laser operating at multiple wavelengths with single longitudinal mode. Furthermore, the evolutions of the pulse energy and repetition rates for different wavelength numbers are further analysis theoretically. This type of single-frequency pulsed fiber laser has extensive application prospect in fiber sensing and lidar systems. [ABSTRACT FROM AUTHOR]

We report a new method of generating continuous-wave (cw) radiation, Q-Switched as well as mode-locked laser pulses from a single linear fiber laser cavity. The three operating modes can be selected by the electric signal that is applied to an intracavity liquid crystal (LC) transducer cell. In addition, this is the first demonstration of using an LC modulator as an active mode-locking device in a fiber laser oscillator to date. In the proof-of-principle experiments presented here, the Q-Switched laser pulses with a duration of 1.3 $ {\mu }\text{s}$ , a maximum energy of 2.7 $ {\mu }\text{J}$ , and a peak power of 1.93 W were generated at low repetition rates ranging from 0.2 to 6 kHz. In mode-locked operation, pulse trains with repetition rates as high as 944.5 kHz and a shorter duration of 65 ns were achieved. Finally, a slight modification of the LC cell allows for a direct comparison between amplitude modulation (AM) and frequency modulation (FM) mode-locking. [ABSTRACT FROM AUTHOR]

We experimentally and theoretically investigate the power fluctuations of the temporal interference signal produced by up to 5 Stokes orders generated by cascaded stimulated Brillouin scattering in a multiwavelength Brillouin erbium-doped fiber laser system with a 2.5-km-long intracavity fiber and erbium-doped fiber amplifier. Power fluctuations in such a system are due to the existence of several modes within the SBS gain bandwidth with the possibility of random hopping resulting in a chaotic temporal evolution of the pump and Stokes-wave powers and consequently the output signal power. Our simulations and statistical analyses show that at the threshold power resulting in the initial Stokes wave generation over round trips, the output signals have the maximum correlation over replicas; however, by increasing the power and producing more stokes waves, the correlation fades out leading to replica symmetry breaking and the system emits in a disordered manner and shows the signatures of a random laser. [ABSTRACT FROM AUTHOR]

A general ${N}$ -by- ${N}$ coupling matrix that provides lossless uniform feedback to all laser array elements is demonstrated in the analysis of a passive coherent beam combining resonator. The resonator contains a spatial filter that sends rejected optical power from the combining optic back into the cavity. This recycling of otherwise unused power yields an increase in the average laser output power that scales with array size ${N}$ , even in the absence of passive phase effects that depend on the gain, intensity, and wavelength of the laser mode. To achieve this improvement, tailored path lengths and mirror reflectivities are required in the feedback arms, where the phase may be fixed, whereas in the gain arms, the path lengths vary randomly. [ABSTRACT FROM AUTHOR]

A self-started, switchable dual-wavelength all-polarization-maintaining mode-locked (ML) fiber laser for optical parametric generation (OPG) in photonic crystal fiber (PCF) applied to coherent anti-stokes Raman scattering (CARS) microscopy is reported in this paper. The complete laser setup is built in an all-fiber structure composed of commercial elements. It is based on a linear cavity with an active medium—namely, Ytterbium-doped fiber (YDF), a pair of fiber Bragg gratings as switchable partial mirrors between 1031.5 and 1049.7 nm, and a semiconductor saturable absorber mirror functioning also as the mode-locking device. The seed laser is amplified to 650 mW average power by two YDF amplifiers. The source delivers a pulse-train output of 30.9 ps at 1031.5 ps and 31.6 ps at 1049.7, a 3.1-MHz repetition rate, and over 200 nJ pulse energy and 6 kW of peak power at fundamental ML operation. The properties of this switchable dual-wavelength source ensures the correct OPG by degenerated four-wave mixing in PCF fulfilling the phase matching condition which corresponds to CARS resonance of 2850  $ {\mathbf{c}}{ {\mathbf{m}}^{ - 1}}$ (CH3-O molecular bonds) at 1031.5 nm and 1475 $ {\mathbf{c}}{ {\mathbf{m}}^{ - 1}}$ (CH2 δ molecular bonds) at 1049.7 nm. [ABSTRACT FROM AUTHOR]

This study reports the radical elimination of the photodarkening (PD)-effect in Yb-doped fiber through deuterium loading. With deuterium pre-loading, no PD-induced excess loss of the Yb-doped fiber was observed under 915-nm pumping conditions. Moreover, the existing loss induced by the PD effect was also eliminated via deuterium loading, and the excess loss declined to nearly 0  dB/m. The method was also validated in a KW-level fiber laser experimental setup. In the 20/400-μm deuterium pre-loaded Yb-doped fiber, an output laser power of 1630 W and beam quality factor (M2) of 1.69 were obtained. The two parameters remained stable during the entire continuous operation process of 40 min, and displayed very little power fluctuation of <0.5%. The results of our experiments confirmed that deuterium loading can protect Yb-doped fibers from the PD effect, thereby improving the power and mode stability, which is often deteriorated by PD. Notably, the results of this study are beneficial in terms of further development of high-power fiber lasers. [ABSTRACT FROM AUTHOR]

We report on, to the best of our knowledge, the first Q-switched bismuth-doped fiber laser operating at ${\sim }1.3~\mu \mathrm {m}$. An acousto-optic modulator (AOM)-based active Q-switching was realized in a ring cavity with a bismuth-doped phosphosilicate fiber as an active medium. Stable pulsing in the range from 1310 to 1350 nm was achieved using various fiber Bragg gratings. The dependence of the laser operation on the unsaturable loss of the active fiber was studied. Optimization of the scheme and selection of an appropriate bismuth-doped fiber provided maximum pulse energy of ${\sim }11.5~\mu \mathrm {J}$ at the output of the laser. Using the data obtained, the evaluation of the extractable energy and the concentration of the bismuth active centers in the fiber samples under investigation was performed. [ABSTRACT FROM AUTHOR]

We demonstrated a widely wavelength-tunable mid-infrared mode-locked Ho3+/Pr3+-codoped ZBLAN fiber laser by using single-wall carbon nanotubes (SWCNTs) with a wide diameter distribution as saturable absorber (SA). This laser emits stable mode-locked pulses in the water vapor transmission window with a maximum average output power of 126.6 mW at a repetition rate of 12.5 MHz. The central wavelength of the mode-locked laser can be continuously tuned from 2836.2 to 2906.2 nm. The 70-nm wavelength tuning range represents the widest tuning range ever achieved in mode-locked Ho3+-doped ZBLAN fiber laser oscillators. Our results also indicate that the SWCNTs can be promising broadband SAs for obtaining gain-bandwidth-limited tunable operation for 3- $\mu \text{m}$ Ho3+-doped ZBLAN fiber lasers. Such simple, reliable, wideband tunable, and low-cost mode-locked mid-infrared fiber laser source is highly attractive for molecular spectroscopy, breath analysis, and laser surgery of human tissue. [ABSTRACT FROM AUTHOR]

We report on our investigation of photo-darkening (PD)-induced thermal load in Yb-doped fiber lasers and the effective suppression by H2-loading. Compared with a quantum defect, the contribution of heat produced by PD was evaluated, indicating PD may become the primary heat source. To reduce the heat generation, the effect of H2-loading on thermal load was quantitatively analyzed. The experimental results showed that 86% of PD loss was mitigated and 82% of PD-induced thermal load was eliminated by H2-loading. In contrast, due to the presence of extra heat source, only 14% heat decreased in the Ce-doped fiber at similar PD loss level. The results indicate that H2-loading is promising for further improving the stable operation of fiber lasers. [ABSTRACT FROM AUTHOR]

Highlights • We demonstrate a dual-wavelength and dual-pulse Yb-doped fiber with a Tm-Ho co-doped fiber as saturable absorber. • The pulse at 1064 nm is generated via SA-induced self Q-switching. • The pulse at 2127.7 nm is generated by gain-switching with the laser at 1064 nm as the pump for the Tm-Ho co-doped fiber. • The pulse duration of the laser at 1064 nm can reach up to 1.4 μs. Abstract We demonstrate a dual-wavelength and dual-pulse Yb-doped fiber laser with a Tm-Ho co-doped fiber as saturable absorber (SA). The wavelength of the first pulse in the dual-pulsing output is at 1064 nm generated via SA-induced self Q-switching and the second pulse is at 2127.7 nm and generated by gain-switching with the laser at 1064 nm as the pump for the Tm ions in the Tm-Ho co-doped fiber and thus induced stimulated emission. With the increases of the pump power, the pulse duration of the two pulses is decreased depending on the level of the pump. The pulse duration of the laser at 1064 nm decreases from 1.4 μs to 296 ns when the pump power increases from 275 mW to 475 mW, while the pulse duration at 2127.7 nm is reduced from 440 ns to 246 ns when the pump increases from 300 mW to 475 mW. Accordingly, the repetition rate is decreased from 17.4 kHz to 6.8 kHz. The experimental result shows that Tm-Ho co-doped fiber is an excellent candidate for SA-induced Q-switching at 1064 nm. [ABSTRACT FROM AUTHOR]

We have experimentally demonstrated that soliton molecules can be achieved from an Er-doped fiber laser oscillator via polarization dispersion management in the anomalous dispersion regime. In the laser oscillator, selective high-birefringence (HIBI) fibers were cross-spliced twice to counteract the linear polarization dispersion, which could highlight the role of the accumulated nonlinear phase shift and thus ensure the stable generation of mode-locking pulses by means of nonlinear polarization rotation (NPR). Depending on the length of the HIBI fibers, the angle of waveplates, and the pumping power, different types of soliton, such as single-pulse soliton and soliton molecules in tightly and loosely bound state, are observed. [ABSTRACT FROM AUTHOR]

Highlights • Q-switched and mode-locked Tm-Ho laser with monolayer WS 2 SA by CVD method. • Mode-locked pulse-duration is up to 1.07 ns. • Q-switched pulse-duration is up to 6.2 μs. Abstract We demonstrate a passively Q-switched and mode-locked Tm-Ho co-doped fiber laser based on a WS 2 saturable absorber (SA) fabricated by chemical vapor deposition method. When the pump power increases from 0.32 W to 0.38 W, the repetition rate of the Q-switched laser pulse increases 47.1 kHz to 90.1 kHz and the pulse width decreases from 6.2 μs to 1.7 μs. In addition, the mode-locking operation can also be observed by adjusting the polarization controller (PC) and mode-locked laser pulses at 1883.2 nm can be achieved at a high pump power between 0.5 W and 0.64 W. The pulse duration is 1.07 ns and repetition rate is 23.8 MHz and the pulse is the longest mode-locked pulse with WS 2 -SA ever reported. [ABSTRACT FROM AUTHOR]

We report the direct generation of high-power and large-energy dissipative soliton resonance square pulses at 1940.8 nm from a compact mode-locked Tm-doped all-fiber laser. A fiber loop mirror of 10/90 splitting ratio and a fiber Bragg grating with high reflectivity of >99% constitute the cavity mirrors together. The fiber loop mirror not only acts as a broadband reflective mirror and output port but also initiates the mode-locked fiber laser. The strictly all-fiber structure and proper selection of fiber types allow the laser to work in a high pump power state. By increasing the pump power, pulse duration extends from 5.9 to 14.5 ns, while the pulse peak power and 3-dB spectrum bandwidth almost keep constant. At the maximum pump power of 13 W, we acquire maximum output average power of 1.98 W and single pulse energy of 684 nJ. To the best of our knowledge, this is the highest average output power and single pulse energy in an all-fiber Tm-doped mode-locked oscillator. At the same time, the output power is also a record value in all-fiber net anomalous dispersion oscillators. [ABSTRACT FROM AUTHOR]

The impact of thermo-optic refractive-index changes arising from the core heat load in a dual-core fiber amplifier is studied through finite-element-based numerical simulations. Trends in coupling lengths, effective area, core overlaps of fundamental and higher-order supermodes, effects of asymmetric heat loads, and thermo-optic mode coupling parameters are quantified. It is concluded that the coupling between cores is only moderately altered by the overall heat load, but can be strongly sensitive to asymmetric loads. The influence of the core coupling strength is demonstrated to be very important on the supermode effective area, besides the shrinking due to thermal effects. While the thermo-optic index perturbations can lead to guidance of higher-order supermodes, it seems realistic to maintain single-mode operation through gain suppression of the higher-order modes. On the other hand, thermo-optic couplings between fundamental supermodes are found to be quite strong. [ABSTRACT FROM AUTHOR]

The influence of the parameters of Q-switched thulium fiber laser at 1.94 μm on human calcium oxalate monohydrate (COM) urinary stone and soft tissues has been investigated to determine its efficiency on stone fragmentation and tissue ablation along with surrounding thermal injury. The designed Q-switched thulium fiber laser can be operated with peak powers varying from 93 to 493 W, pulse duration from 350 to 750 ns, and repetition rates from 55 to 135 kHz at 9.5 W average power. Comparative effects on COM stone fragmentation rate and soft tissue ablation by using Q-switched and continuous wave (CW) thulium fiber laser have been studied. The stable pulse from the Q-switched thulium fiber laser at high repetition rate is effective for COM stone fragmentation with very fine particle size, average in the range of 47.85 μm, called dusting, at the fragmentation rate of 12.75 mg/min. Soft tissue ablation employing the Q-switched thulium fiber laser is reasonably clean, leaving minimum residual carbonization of 170 μm and heat-affected zone of 0.76 mm. The Q-switched thulium fiber laser produces two times narrower adjacent tissue damage zone along with four times lower charring region compared to the equivalent CW thulium fiber laser exposure in soft tissue. [ABSTRACT FROM AUTHOR]

Light sources with high radiance are increasingly required for full-field real-time imaging. Conventional lasing sources are poorly suited for such imaging due to their high spatial or temporal coherence, which generates a speckle that deteriorates image quality. Here, a random fiber laser with multitransverse modes is used as an illumination light source to effectively reduce the speckle in imaging. Low spatial coherence and low temporal coherence of the random fiber laser give birth to significant reduction in the speckle. Under the power-limited condition, the multimode random fiber laser is verified to have a comparable or even better imaging quality compared to a multimode amplified spontaneous emission source. Furthermore, its potential to generate ultrahigh power of up to hundreds of Watts with extremely-high spectral density would make a breakthrough in the development of a new generation of high-power low-coherence light sources for many speckle-free imaging applications, where conventional light sources are not usable. As the multimode random fiber laser can naturally inherit all the advantages of single-mode random fiber lasers, including flexible wavelength, robust structure, and high power, this paper may provide a platform to develop powerful low-coherence light sources to meet wide range requirements of the full-field real-time speckle-free imaging. [ABSTRACT FROM AUTHOR]

Imaging of biological cells and tissues with subcellular spatial resolution is important in biology and medicine because it allows us to explore the dynamics of cells and to diagnose the structure of tissues. Among various optical imaging modalities, laser microscopy with fluorescent staining is a powerful method for this purpose. However, it still suffers from the limited applicability and cytotoxicity of a staining process. Stimulated Raman scattering (SRS) microscopy is an emerging technique of biological imaging based on molecular vibrational contrast, offering new opportunities of biomedical microscopy. In this paper, we introduce the principle and applications of SRS microscopy and discuss various methods of spectral imaging with SRS microscopy. Then, we present our multicolor SRS microscope, which employs a fast wavelength-tunable picosecond Yb fiber laser and a picosecond Ti:sapphire laser. In particular, we describe the technical details of the Yb fiber oscillator, synchronization electronics, high-speed wavelength-tunable filter, video-rate microscope setup and signal detection electronics including the photodetector and the lock-in amplifier. We hope that this paper will be informative for those interested in the application of ultrafast lasers to coherent Raman microscopy including SRS microscopy. [ABSTRACT FROM AUTHOR]

We present numerical studies on bend-induced effective single-mode large-mode area fibers with various refractive index profiles: step index, parabolic, and triangular and hyperbolic for laser applications. We report the fiber with effective mode area as large as 1530 μm2 for 110-μm all-solid core diameter using triangular refractive index profile, in single-mode regime. We also report the fiber with parabolic refractive index profile, 70-μm core diameter with effective single-mode area of 1170 μm2. Development of novel active very large nanostructured core fiber with dedicated gradient index profile is discussed. Owning to nanostructurization, the extremely large core in the all-solid fiber can be obtained and used in a laser system to diminish undesirable nonlinear effects and increase the output power level with diffraction-limited beam quality. The nanostructurization method allows us to break the limits of classical manufacturing methods and opens new opportunities in precise shaping of the refractive index distribution. [ABSTRACT FROM AUTHOR]

A fiber laser-pumped high-power burst-mode-operated picosecond mid-infrared (IR) laser at 3.8 μm is reported. A gain-switched distributed Bragg reflector laser diode with a pulse repetition rate (PRR) of 138 MHz and pulse duration around 200 ps was applied as the seed laser of a master oscillator power amplifier (MOPA)-structured Yb fiber laser. The PRR of the MOPA was increased to about 1.1 GHz through a pulse multiplier consisting of four cascaded 2 × 2 fiber couplers. A fiber-pigtailed acousto-optic modulator was used to carve the pulse train into pulse bursts so that the peak power of the final linearly polarized fiber laser output could be optimized by adjusting the duty cycle of the pulse bursts correspondingly. The output of the fiber laser was directed to pump a periodically poled magnesium-oxide-doped lithium niobate-based optical parametric oscillator through the quasi-synchronized pump scheme. Efficient parametric conversion was realized with a maximum average power output of 7.3 W at 3.8 μm under pump power of 45 W at 1.064 μm with pump-to-idler conversion efficiency exceeding 16%.

In the present work, the effect of gamma radiation on the performance of different types of erbium-doped fibers (EDFs) when they are used in a fiber ring cavity (FRC) configuration is studied. Several pieces of commercial EDF are gamma-ray irradiated with different doses to evaluate the output power variations over time. The influence of different doses, from 150 Gy to 1000 Gy, over the output power level measurement and their amplified spontaneous emission (ASE) are experimentally evaluated both in the C and L bands. By using an FRC configuration we can detect the presence of gamma radiation. We can also estimate the irradiation doses applied to EDFs by measuring the slope of the short-term emission power.

In this paper, a polarization-maintaining quad-wavelength (QW) single-frequency fiber laser (SFFL) at the C-band with the novel and compact structure is demonstrated. Applying the rate equations and multiple coupled nonlinear Schrödinger equations, the theoretical model of QW fiber lasers is established, and then the optical spectra, temporal evolution, and stability of the QW fiber laser are analyzed numerically. Based on the theoretical analysis, a QW-SFFL having an ultrashort linear distributed Bragg reflector cavity is proposed. Utilizing a wideband fiber Bragg grating coupled with a dual-channel polarization maintaining fiber Bragg grating as the wavelength selection and a 15-mm-long Er3+/Yb3+ co-doped phosphate fiber as the gain medium has realized a robust QW laser with the wavelength spacing of 0.4 nm. All the wavelengths maintain the single-frequency operation independently with the linewidths of 20 kHz. This type of compact SFFLs with quadruple wavelengths is widely adapted to lidar systems and fiber sensing. [ABSTRACT FROM AUTHOR]