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In this study, a kilowatt-level high-efficiency narrow-linewidth all-fiber Tm3+-doped continuous-wave laser operating at 1.95 μm is demonstrated. Benefitting from an advanced boost design of a two-stage main amplifier, it not only effectively manages heat dissipation resulting from the high pump-induced quantum defect but also realizes the controlled extraction of optical gain and improves the optical conversion efficiency. Finally, this laser system has realized an output power of 1018 W, a linewidth of 3.8 GHz, and a slope efficiency of 60.0% simultaneously. Moreover, a high optical signal-to-noise ratio of over 45 dB and excellent beam quality of M2 factors 1.19 are obtained. To the best of our knowledge, this represents the narrowest linewidth and highest slope efficiency achieved in a kilowatt-level Tm³⁺-doped fiber laser. Such a high-performance laser is ideally suited for mid-infrared generation and remote sensing applications.

A 1.5 μm ultralow‐noise single‐frequency fiber laser (SFFL) is realized based on a delay fiber phase‐locked method and the nonlinear amplification effect of a semiconductor optical amplifier (SOA). Via feedback locking of the phase jitter signal, the phase noise achieves the minimum value of −160 dB rad Hz−1/2 m−1 with a maximum reduction of 50 dB. Meanwhile, SOA reduces the intensity noise by a maximum of 65 dB, making it near the shot‐noise limit for frequencies above 40 kHz. To demonstrate this state‐of‐art SFFL with ultralow‐noise property, a high‐resolution fiber‐optic dynamic strain sensing based on a fiber‐based Michelson interferometer (FMI) is performed. As a result, the strain resolution reaches 26 fε Hz−1/2 in the 30–200 Hz band, which is improved by almost three orders of magnitude than that realized by a free‐running SFFL. To the best of the knowledge, this is the first time to achieve such a high‐resolution fiber‐optic dynamic strain sensor in the low‐frequency range utilizing the FMI system. This high‐resolution fiber‐optic strain sensing has promising applications in geophysics and biophotonics. Moreover, this ultralow‐noise SFFL is competitive for sophisticated applications, such as precise absolute distance measurement, quantum key distribution, and generating squeezed light.

Yellow lasers are of great interest in biology, medicine and display technology. However, nonlinear emission of near-infrared lasers at yellow still presents particularly complex optical alignment to date. Here, to the best of our knowledge, we demonstrate the fabrication of a NaLa(WO4)2: Dy3+ glass-ceramic fiber (GCF) for the first time. More importantly, the emission band of the GCF, which is around 575 nm, has a wide full-width half maximum (FWHM) of 18~22 nm, which is remarkably larger than that of the Dy3+-doped YAG crystal (

Since high-resolution structure imaging of active faults within urban areas is vital for earthquake hazard mitigation, we perform a seismic survey line crossing the Pearl River Estuary Fault (PREF) in Guangzhou, China. First, ten shots of a new and environmentally friendly gas explosion source are excited with about 1 km spacing and recorded by 241 nodal short-period seismometers with an average spacing of 60 m. Then, based on these acquisition data, we adopt waveform inversion to explore the kinematic and dynamic information of early arrival wavefields to recover the subsurface structures. The inversion results indicate that while the low-velocity zone (LVZ) in depth surrounding the PREF is 2.5 km in width and extended to 0.7 km, another LVZ of 1.5 km in width and extended to 0.7 km in depth is surrounded by the Beiting–Nancun fault. We observe that the analysis of evolution and activities of the fault systems reveal no historical earthquakes in our study area; we interpret that the two LVZs controlled by the faults are probably attributed to the fluid dynamics, sediment source, and fault motion at different geological times, rather than fault-related damage zones. The results can provide significant basis for earthquake prevention and hazard assessment in Guangzhou. The finding also shows that the waveform inversion can effectively explore the fine structure of active faults in urban area with dense linear array and spare active source excitations.

In our work, based on chemical vapor deposition method and a newly designed glass-material-glass sandwich structure, MoS2 saturable absorber (SA) with a damage threshold of higher than 0.26 J/cm2 was fabricated and employed for demonstrating high-power, large-energy rectangular mode-locked Er-doped laser operation. Stable mode-locked operation with maximum output power of 122.77 mW and largest pulse energy of 130.49 nJ were successively achieved along with power scaling process. In comparison with previous works, our results show remarkable breakthroughs both in the aspects of average output power and pulse energy. Our experimental design and results will open a new avenue for generating high-power, large-energy mode-locked fiber lasers based on two-dimensional materials as SAs.

Based on hybrid wavelength pumping and tapered Yb-doped fibers (T-YDFs), a 650 W all-fiber single-frequency polarization-maintaining fiber amplifier was demonstrated experimentally at 1030 nm. Different pump power ratios in the T-YDF-based power-amplifier stage were proposed to investigate their influence on the transverse mode instability (TMI) effect. The highest TMI threshold was obtained when the pump power ratio of 940 nm to 976 nm was 1:4.4. A measured M2 factor of 1.7 and a polarization extinction ratio of 14 dB at the maximum output power were obtained. To the best of our knowledge, these results exhibit the highest output power of any all-fiber single-frequency polarization-maintaining fiber amplifiers created up to now.

A 1560 nm high-power linearly-polarized all-fiber single-frequency narrow-linewidth laser with near diffraction-limited beam quality is demonstrated. The Yb–Er energy transfer efficiency and the ability of the signal laser to capture pump light have been improved by specifically choosing the pumping wavelength and the input signal power in the final power amplifier stage of this laser system. Under the off-peak absorption pumping wavelength of 940 nm, along with the maximum input signal power of 6 W, a maximum output power of 102 W with a slope efficiency of 40.5% is acquired. At the highest output power status, a polarization extinction ratio (PER) of 15.5 dB, a linewidth of 3.05 kHz, and a beam quality of Mx2 = 1.14, My2 = 1.06 are obtained, respectively. This advanced single-frequency fiber laser has great potential for the long-range coherent Doppler lidar and the next generation of gravitational wave detection.

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.

Quantum defect originating from the stimulated Raman scattering (SRS) effect is a heat source that induces refractive index grating and, thus, significantly decreases the threshold of transverse mode instability (TMI), which is the main limiting factor for the power scaling of high-power fiber lasers. A semi-analytical model that takes the TMI and SRS effects into account was developed, and the impact of the Raman content variation due to changes in the injected seed and fiber length on the TMI threshold was investigated. Experimental demonstrations of the impact of the SRS effect on the TMI threshold are presented. We proved that TMI affects the beam quality of the signal light rather than the Raman light. Taking these facts into account, the suppression of SRS could be an effective way to counter the TMI of fiber lasers.

Based on a temperature-dependent longitudinal-mode-selection model, a broad temperature range for single-longitudinal-mode (SLM) operation of a short DBR phosphate fiber laser has been demonstrated. In order to make the variety in cavity resonance wavelength match the reflection peak changing in fiber Bragg grating (FBG) when the ambient temperature changes, the packaging materials with different coefficient of linear expansion are used for the active fiber and the FBG, respectively. The temperature variation range without mode hopping of the SLM fiber laser has been increased from about 2 °C to over 20 °C, while the continuous tunable wavelength range broadened to be > 149 pm.

Bi2Te3-based materials have been reported to be one of the best room-temperature thermoelectric materials, and it is a challenge to substantially improve their thermoelectric properties. Here novel Bi2Te3 core fibers with borosilicate glass cladding were fabricated utilizing a modified molten core drawing method. The Bi2Te3 core of the fiber was found to consist of hexagonal polycrystalline nanosheets, and polycrystalline nanosheets had a preferential orientation; in other words, the hexagonal Bi2Te3 lamellar cleavage more tended to be parallel to the symmetry axis of the fibers. Compared with a homemade 3-mm-diameter Bi2Te3 rod, the polycrystalline nanosheets’ preferential orientation in the 89-μm-diameter Bi2Te3 core increased its electrical conductivity, but deduced its Seebeck coefficient. The Bi2Te3 core exhibits an ultrahigh ZT of 0.73 at 300 K, which is 232% higher than that of the Bi2Te3 rod. The demonstration of fibers with oriented nano-polycrystalline core and the integration with an efficient fabrication technique will pave the way for the fabrication of high-performance thermoelectric fibers.

In our work, based on a high-damage-threshold MoS2 saturable absorber (SA), high-power intra-cavity Raman solitons within a passively mode-locked Yb-doped fiber laser were demonstrated successfully for the first time. The damage threshold of the MoS2 SA was as high as ~0.48 J/cm2. By adjusting the polarization states, stable single- or dual-pulse Raman soliton operations were obtained. The maximum average output power for single-pulse and dual-pulse Raman soliton operations was 80.11 and 89.33 mW, respectively. Our experiment results show significant enhancement in comparison with previous works, which provides fundamental guidance for future designs of high-power, large-energy, intra-cavity Raman soliton generations based on two-dimensional materials as SAs.

Narrow linewidth microwave signals generated from a heterodyne detection configuration of a dual-wavelength (DW) single-frequency highly Er3+/Yb3+ co-doped phosphate fiber laser is presented. The oscillating cavity of the fiber laser consists of a dual-channel narrow-band fiber-Bragg-grating (DC-NB-FBG), a 0.7-cm-long Er3+/ Yb3+ co-doped phosphate fiber and a wideband FBG (WB-FBG). The wavelength selecting gratings are spatially separated to create partially separated resonant cavities. Highly Er3+/Yb3+ co-doped phosphate fiber ensures that the mode competition is relatively weak under low pump power. DW single-frequency lasing with laser linewidths of 3 kHz is achieved. A 12.014-GHz microwave signal with a 3-dB linewidth of 3 kHz is obtained from the heterodyne detection of the DW fiber laser.

Transverse mode instability (TMI) limits power scaling of fiber lasers. A semianalytical model is established to calculate the TMI threshold in high-power fiber laser systems of the multi-kW-class. A linear inner-cladding fiber can mitigate the TMI effect by smoothing the heat profile and decreasing the nonlinear coupling coefficient along the fiber. We investigate strong pump absorption of a linear inner-cladding fiber, which leads to shorter fiber length. Utilizing a 915-nm copumping scheme, the linear inner-cladding fiber can realize 10-kW output power in single-mode regime theoretically. [ABSTRACT FROM AUTHOR]

Frequency noise in single-frequency high-gain phosphate fiber lasers is investigated theoretically and experimentally. An analytical expression relating the pump fluctuations to the laser frequency noise is established and verified by a homemade single-frequency phosphate fiber laser. The theoretical and experimental results indicate that the frequency noise of the output laser can be reduced by lowering the intensity noise of pump source. This will help improving the laser performance and progress the wide application of this type of fiber laser. [ABSTRACT FROM AUTHOR]

This paper reports on the spectroscopic properties and energy transfer analysis of Tm3+-doped BaF2-Ga2O3-GeO2-La2O3 glasses with different Tm2O3 doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm3+, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm2O3 doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm2O3-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm3+ in this sample is about 0.48 × 10−20 cm2 at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm3+-doped BaF2-Ga2O3-GeO2- La2O3 glass for 2.0-μm optical fiber laser. [ABSTRACT FROM AUTHOR]

With the rapid development of precision measurements relying on atom absorption such as atomic inertial sensors and optical magnetometry, advanced lasers are urgently required especially ultralow-noise lasers corresponding to the atom absorption line. For the D2 line of rubidium atoms, an ultralow-intensity-noise 780 nm single-frequency fiber-based laser with an output power of 340 mW is demonstrated. By eliminating polarization sensitivity in the noise suppression process based on the gain saturation of a semiconductor optical amplifier, a relative intensity noise of −151 dB Hz−1 in the frequency range from 0.1 to 50 MHz is achieved, which approaches the shot noise limit. [ABSTRACT FROM AUTHOR]

By optimizing the pump intensity noise, simultaneous improvements in the linewidth and the low-frequency (<20 kHz) relative intensity noise (RIN) of a 1550 nm single-frequency fiber laser (SFFL) are demonstrated. A lower-RIN distributed feedback laser diode with optoelectronic feedback replaces the fiber Bragg grating laser diode to pump the SFFL. As a result, the linewidth of the SFFL is narrowed from 1.3 to 0.65 kHz and the RIN is decreased by about 7–11.5 dB from 0.2 to 5 kHz, and by about 3–7 dB from 5 to 10 kHz. The improved SFFL is an ideal laser source for fiber-optic sensing applications. [ABSTRACT FROM AUTHOR]

Due to large coherence length, high detection sensitivity and feasible design, a narrow-linewidth single-frequency laser in the blue spectral range is essential for underwater communication and high-resolution atomic/molecular spectroscopy. By exploiting the efficient frequency doubling of a 978 nm Yb-doped phosphate fiber laser in a fiber-coupled 10 mm long periodically poled LiNbO3, an all-fiber single-frequency blue laser at 489 nm with an ultra-compact and practical design is demonstrated. Over 68 mW of stable narrow-linewidth blue radiation is generated with a conversion efficiency of 30.3%. The estimated laser linewidth is less than 15 kHz, while the measured relative intensity noise is less than –139 dB Hz−1 at frequencies above 20 MHz. The obtained blue laser output is naturally stable with a power stability of ±1.5% over a period of 120 min. [ABSTRACT FROM AUTHOR]

An iterative method based on coupled mode theory and the steepest descent principle in a microwave waveguide is proposed in the optical waveguide for the design of a LP0m mode converter. After iterative design, a LP02-LP01 mode converter and a LP03-LP02 mode converter can achieve 99% conversion efficiency in our model. The 1dB bandwidth of a LP02-LP01 mode converter is measured as more than 130 nm. These optimum waveguides are simulated by RSoft software and indicate high conversion efficiency as well. Furthermore, we investigate the compatibility of the iterative method for mode converter design with initial waveguides of multiple taper shapes. The proposed method is promising for applications in mode division multiplexing systems. [ABSTRACT FROM AUTHOR]

The high order vector mode (HOVM) coupling mechanism is investigated based on the mode matching method (MMM). In the case of strong HOVM coupling where the weakly guiding approximation fails, conventional coupling analysis methods become invalid due to the asynchronous coupling feature of the horizontal and vertical polarization components of HOVM. The MMM, which uses the interference of the local eigenmodes instead of the assumptive modes to simulate the light propagation, is adopted as a more efficient analysis method for investigating HOVM coupling processes, especially for strong coupling situations. The rules of the optimal coupling length, coupling efficiency, and mode purity in microfiber directional coupler are firstly quantitatively analyzed and summarized. Different from the specific input modes, some special new modes would be excited at the output through the strong HOVM coupling process. The analysis of HOVM coupling mechanism based on MMM could provide precise and accurate design guidance for HOVM directional coupler and mode converter, which are believed to be fundamental devices for multi-mode communication applications. [ABSTRACT FROM AUTHOR]

An efficient low-noise, single-frequency 1033 nm master oscillator power amplifier (MOPA) Yb3+-doped phosphate fiber (YPF) laser system is demonstrated. A maximal output power of 612 mW with a laser linewidth of <1.2 kHz is obtained from a 3.7 cm long YPF. By suppressing the intensity noise of the short-cavity phosphate fiber oscillator with a semiconductor optical amplifier, a measured signal-to-noise ratio from the MOPA system of >65 dB and relative intensity noise (RIN) of <−120 dB Hz−1 at the relaxation oscillation frequency are achieved. Moreover, an RIN of <−152 dB Hz−1 is achieved for frequencies above 15.0 MHz, which approaches the shot noise limit. This laser system has applications in the fields of optical frequency standards and beam combining. [ABSTRACT FROM AUTHOR]

We present a compact passively Q-switched single-frequency fiber laser based on a 12-mm-long laboratory-built highly Er3+/Yb3+ codoped phosphate fiber (EYDPF) and a semiconductor saturable absorber mirror (SESAM). An effective cavity length of less than 20 mm ensures the stable single-frequency operation of the Q-switched fiber laser. By employing a SESAM for Q-switching, a single-pulse energy of more than 34.4 nJ is realized with the narrowest pulse duration of 95 ns, and the repetition rate of the Q-switched fiber laser reaches over 600 kHz. In addition, the optical signal-to-noise ratio of the output laser is as high as 68.0 dB. [ABSTRACT FROM AUTHOR]

We report an all-fiber high power nanosecond pulsed laser at a center wavelength of 1064 nm. Optimizing the coiling diameter of the active fiber, 188 W average power is achieved at a repetition rate of 40 kHz. The pulse width is measured as 101 ns, while the peak power can be estimated to 46.5 kW. [ABSTRACT FROM AUTHOR]

An all-fiber Yb-doped kHz-linewidth low-noise linearly polarized single-frequency master-oscillator power-amplifier (MOPA) laser with a stable CW output power of >52 W is demonstrated. By suppressing the intensity noise of the DBR phosphate fiber oscillator, the linewidth of MOPA laser is not noticeably broadened, and an ultra-narrow linewidth of <3 kHz is obtained. Furthermore, the low-noise behavior of MOPA lasers is investigated. A measured relative intensity noise of < −130 dB Hz−1 at frequencies of over 2 MHz, a phase noise above 1 kHz of <5 μrad/Hz1/2, and a signal-to-noise ratio of >63 dB are achieved. [ABSTRACT FROM AUTHOR]

A compact frequency-modulation Q-switched single-frequency fiber laser is demonstrated at 1083 nm. The short linear resonant cavity consists of a 12 mm long homemade Yb3+-doped phosphate fiber and a pair of fiber Bragg gratings (FBGs) in which the Q-switching and the frequency excursion is achieved by a tensile-induced period modulation. Over 375 MHz frequency-tuning range is achieved with a modulation frequency varying from tens to hundreds of kilohertz. The highest peak power of the output pulse reaching 6.93 W at the repetition rate of 10 kHz is obtained. [ABSTRACT FROM AUTHOR]

A compact several-hundred-Hz-linewidth single-frequency fiber laser based on the slow-light effect induced by a fiber-Bragg-grating Fabry–Perot (FBG-FP) filter is demonstrated. An FBG-FP filter is utilized for optical time delay, extending the efficient cavity length of the resonator. As a result, a slow-light short-linear-cavity fiber laser at 1550 nm with a linewidth less than 780 Hz is obtained. The relaxation oscillation frequency is around 234 kHz owing to the slow-light effect, and the relative intensity noise is less than −150 dB/Hz at 25 kHz. [ABSTRACT FROM AUTHOR]

We demonstrate an optoelectronic feedback method to simultaneously reduce the intensity and frequency noise of a single-frequency phosphate fiber laser for the first time. Based on the fact that the intensity and frequency noise in this laser mainly originate from the pump power fluctuations, we implement a simple and compact feedback loop and achieve a maximum noise reduction of over 20 dB for the intensity noise at frequencies of 0.3–3 kHz and 10 dB for the frequency noise at around 1 kHz. In addition, the laser instability is improved from ±1.7% to ±0.1%, and the laser line shape is observed to be compressed at the central part. [ABSTRACT FROM AUTHOR]

We demonstrate an experimental investigation on the linewidth of a single-frequency Yb-doped phosphate fiber laser through a loss-compensated recirculating delayed self-heterodyne (LC-RDSH) method at 1.0 μm. A resolution of 3 kHz is achieved by only using a 6 km fiber delay line in the linewidth measurement system. With optimal parameters of the LC-RDSH system, more than 16 orders of beat note are detected. By analyzing the behaviors of different orders of beat note, a broadening effect on the linewidth is observed with an increasing pump strength of the under-tested fiber laser. Associated with theoretical analysis, the frequency noise incorporated with the linewidth broadening is shown to be the attribution of the Lorentzian noise. Frequency noise of the laser is also measured, to verify the observed linewidth characteristics. To the best of our knowledge, this is the first time that the LC-RDSH has been applied to the 1.0 µm window. [ABSTRACT FROM AUTHOR]

High-efficiency single-frequency compact master-oscillator power amplifiers (MOPAs) based on core-pumped highly Yb-doped phosphate fibers (YPFs), several centimeters long, are investigated theoretically and experimentally. By using numerical modeling and testing different YPF lengths to optimize the amplifier configuration, a stable output of more than 1.06 W, at 1014 nm, was measured experimentally from a MOPA laser, with a 4.0 cm long YPF. An optical-to-optical conversion efficiency of 81.4%, a typical net gain of more than 20.5 dB, and an output power per unit length of up to 265 mW/cm were obtained with this laser system. The measured optical signal-to-noise ratio of the MOPA laser is higher than 62 dB and the estimated laser linewidth is less than 20 kHz, without obvious broadening or degradation after amplification. There is excellent agreement between the results of the simulations and the experiments. [ABSTRACT FROM AUTHOR]

The article present a novel real-time frequency-encoded spatiotemporal (FEST) focusing technology. It mentions that FEST focusing uses a novel programmable two-dimensional optical frequency comb with an ultrafine line width to perform single-shot wavefront measurements, with a fast single-pixel detector; and a new avenue for high-speed wavefront shaping in the field of photonics.