Single Fiber Laser Research Articles

Harnessing artificial intelligence for coherent beam combination

Coherent beam combining (CBC) is an effective scheme to surpass the physical power limits of single fiber lasers, achieving higher power and superior beam quality, with phase control being the critical factor. Active phase control compensates for phase noise-induced coherence degradation by directly or indirectly detecting phase differences among sub-beams. Traditional phase control algorithms face challenges in large-scale CBC systems due to low control bandwidth. With the rapid development of artificial intelligence (AI) technologies, integrating advanced intelligent algorithms into active phase control systems holds promise for enhancing the performance of CBC systems. This paper begins with a brief introduction to the principles of traditional phase control algorithms, such as Stochastic Parallel Gradient Descent (SPGD) and locking of optical coherence by single-detector electronic-frequency tagging (LOCSET), elucidating why AI can assist in active phase control systems. Subsequently, we review recent advancements in phase control based on deep learning and reinforcement learning, concluding with a summary and future outlook. As phase control technology advances, the integration of AI and traditional algorithms will play a pivotal role in achieving high-bandwidth and accurate phase control in large-scale CBC systems.

The energy synergistic mechanism of coaxial heterogeneous-wavelength hybrid laser beam and its effect on welding molten pool dynamics for aluminum alloy

The coaxial heterogeneous-wavelength hybrid laser beam (HW-HLB) heat source featuring a “Gauss + Flat top” energy distribution, which integrates the 1080 nm and 915 nm laser beam, was employed in this paper to achieve excellent stability in the welding of aluminum alloy. The interaction mechanism between aluminum alloy and laser beams with different wavelengths was taken as a point cut to explore the synergistic enhancement mechanism of coaxial HW-HLB . A thermal-fluid coupling model for HW-HLB welding, considering the synergistic effect of dual lasers, was established according to the welding experiment results. The influence of fiber laser power (PF) and diode laser power (PD) on the molten pool flow behavior during welding process was investigated. The dynamic of the molten pool and the maintenance mechanism of the keyhole were elucidated through in-situ monitoring experiments and thermal-flow simulations. The results indicated that, in contrast to the single fiber laser beam (FLB), the power density of HW-HLB presents a significant increase, which results in the “avalanche-like” augmentation of the plasma plume, as well as deeper depth of the keyhole. The energy synergistic enhancement effect of coaxial HW-HLB with PF=2800 W and PD=2500 W performs stronger and thorough changes in the morphology and flow behavior of the molten pool. The introduction of 915 nm laser beam improves the multi-reflection behavior in the inner keyhole. The findings of our research provide a theoretical framework for improving the stability of the laser weld molten pool and the quality of welds in aluminum alloys through the implementation of HW-HLB.

Yb-doped tapered double-clad fibers with polarization maintenance for half-kW power amplifiers.

Amplifying short pulses directly within a single fiber laser system has proven to be a challenging task, primarily due to thermally induced transverse mode instabilities and detrimental nonlinear effects. Another demanding aspect is preserving the linear polarization state at high power levels, which is even more pronounced for ultra-large-mode area fibers. This study demonstrates significant advancement in the direct amplification of narrow linewidth short pulses from tens of mW to several hundreds of Watts in a single-stage amplification, maintaining a high degree of linear polarization at the maximum output power. Through a comprehensive experimental investigation, two distinct types of Ytterbium-doped tapered double-clad fibers (T-DCFs), namely, PANDA (PT-DCF), with high built-in birefringence, and spun (sT-DCF), with ultra-low built-in birefringence, are examined. The unique geometrical architecture of the amplifiers is exploited for the realization of a compact and highly efficient picosecond fiber-based laser system, achieving more than 75% slope efficiency. In a single amplification stage, 50 ps pulses at a repetition rate of 20 MHz and an average power of 65 mW are amplified up to 457 W and 573 W of average power using PT-DCF and sT-DCF amplifiers, respectively. Both amplifiers exhibit near diffraction limited beam quality, M2 < 1.4 at the highest power level. At the maximum power levels, the system maintains a high degree of linear polarisation, achieving ∼ 90% and ∼ 94% for the sT-DCF and PT-DCF, respectively. These ultra-large mode area fiber amplifiers are verified as versatile solutions for direct amplification of short pulses up to half-kW level with excellent spectral, spatial, and polarization characteristics.

Switchable emissions of an Erbium-doped fiber laser using cascaded MZIs based on CHCF

In this work, a single and dual-wavelength erbium-doped fiber laser (EDFL) based on two Mach-Zehnder interferometers (MZIs) in cascade structure was experimentally validated. MZIs were assembled by joining a capillary hollow-core fiber (CHCF) piece between two multimode fibers (MMFs) sections. The switchable operation is reached by moving the spectrum of one MZI when the temperature is increased. The maximum measured signal noise to ratio (SNR) was more than 50 dB for the single and dual-wavelength laser lines. Besides, stable output is shown since no power and wavelength variations were noticed. It is important to mention that emissions are obtained at precise wavelength positions and not arbitrarily as described by other investigations. This EDFL can be used in applications of optical fiber communications systems and fiber sensing.

Unraveling microstructure evolution induced mechanical responses in coaxial fiber-diode laser hybrid welded Fe–36Ni Invar alloy

Invar alloys, renowned for its ultra-low coefficient of thermal expansion, are qualified for widespread use in aerospace applications. This study explores the utilization of fiber-diode laser hybrid welding in Fe–36Ni Invar alloy, specifically focusing on addressing the undesirable microstructure and properties arising from unstable molten pool and high-temperature gradient encountered during single fiber laser welding. Experimental and simulation analyses were conducted to systematically explore the influence of diode laser power on the microstructure evolution and mechanical responses of fiber-diode hybrid laser welded Invar alloy joints. Results indicate that the addition of diode laser could notably widen the keyhole opening and improve the molten pool stability. An acceleration of dynamic recrystallization is observed in the weld seam, with a significant increase in high-angle grain boundaries, which account for over 90 % of the total. Moreover, with increasing diode laser power, temperature gradient decreases, leading to the formation of numerous equiaxed grains. Electron backscatter diffraction (EBSD) results demonstrate a considerable enhancement in the intensity of the γ texture, particularly the {111}<110> texture. Comparatively, fiber-diode laser hybrid welding exhibits superior plasticity and tensile strength over single fiber laser welding, with a tensile strength reaching 493.1 MPa. The research findings presented here provide essential theoretical foundations and technical guidance for enhancing the quality of Invar alloy weld joints and facilitating the broader application of fiber-diode laser hybrid welding in the future.

Ultrafast laser state active controlling based on anisotropic quasi-1D material

Laser state active controlling is challenging under the influence of inherent loss and other nonlinear effects in ultrafast systems. Seeking an extension of degree of freedom in optical devices based on low-dimensional materials may be a way forward. Herein, the anisotropic quasi-one-dimensional layered material Ta2PdS6 was utilized as a saturable absorber to modulate the nonlinear parameters effectively in an ultrafast system by polarization-dependent absorption. The polarization-sensitive nonlinear optical response facilitates the Ta2PdS6-based mode-lock laser to sustain two types of laser states, i.e., conventional soliton and noise-like pulse. The laser state was switchable in the single fiber laser with a mechanism revealed by numerical simulation. Digital coding was further demonstrated in this platform by employing the laser as a codable light source. This work proposed an approach for ultrafast laser state active controlling with low-dimensional material, which offers a new avenue for constructing tunable on-fiber devices.

Linewidth compression of a single longitudinal mode ytterbium-doped fiber laser based on femtosecond laser fabricated fiber Bragg gratings.

We demonstrate a single longitudinal mode distributed Bragg reflection (DBR) fiber laser by directly fabricating fiber Bragg gratings (FBGs) on an ytterbium-doped fiber (YDF) using a femtosecond laser. A simple optical self-injection feedback method was used to effectively compress the linewidth and reduce relative intensity noise (RIN) of a single longitudinal mode DBR fiber laser. Further, we investigated the effect of self-injection feedback cavity length and reflectivity on linewidth compression and determined that the linewidth tends to decrease with the increase of the external cavity photon lifetime. By a self-injection feedback, the laser linewidth was compressed from 31.8kHz to 1.4kHz. Meanwhile, the relaxation oscillation peak from -103.2d B/H z at 1.51MHz was suppressed to -122.3d B/H z at 0.16MHz. This low-noise narrow linewidth single longitudinal mode fiber laser is expected to be a promising candidate for applications such as active detection of neutral atmosphere and distributed fiber sensing.

Compound four-cavity single-longitudinal mode fiber laser

A compound resonant cavity-type single longitudinal mode erbium-doped fiber laser is proposed and verified in this paper. We use a compound four-cavity to expand the longitudinal mode spacing of the laser and a homemade grating F-P filter and a saturable absorber to narrow the gain spectral bandwidth, enabling the laser to operate in a single-frequency state. Through theoretical analysis and experimental verification of the important roles of the main constructed fiber laser parts, the center wavelength of the laser output of 1550.06 nm is obtained, and the optical signal-to-noise ratio is 65 dB. In a 50 minute period, the wavelength fluctuation is exhibited along with the maximum power fluctuation of 0.28 dB. The value of the laser output linewidth is ∼250 Hz, measured by using the delayed self-heterodyne method.

Tunable single frequency Hz-magnitude narrow linewidth Brillouin fiber laser based on parity-time symmetry.

An Hz-magnitude ultra-narrow linewidth single-frequency Brillouin fiber laser (BFL) is proposed and experimentally demonstrated. The single frequency of the laser is selected by parity-time (PT) symmetry, which consists of a stimulated Brillouin scatter (SBS) gain path excited by a 24 km single-mode fiber (SMF) and an approximately equal length loss path tuned with a variable optical attenuator (VOA). These paths are coupled through a fiber Bragg grating (FBG) into a wavelength space. Accomplishing single-frequency oscillation involves the precise adjustment of polarization control (PC) and VOA to attain the PT broken phase. In the experiment, the linewidth of the proposed BFL is 9.58 Hz. The optical signal-to-noise ratio (OSNR) reached 78.89 dB, with wavelength and power fluctuations of less than 1pm and 0.02 dB within one hour. Furthermore, the wavelength can be tuned from 1549.9321 nm to 1550.2575 nm, with a linewidth fluctuation of 1.81 Hz. The relative intensity noise (RIN) is below -74 dB/Hz. The proposed ultra-narrow single-frequency BFL offers advantages such as cost-effectiveness, ease of control, high stability and excellent output characteristics, making it highly promising for the applications in the coherent detection.

Controlling intracavity dual-comb soliton motion in a single-fiber laser.

Ultrafast science builds on dynamic compositions of precisely timed light pulses, and evolving groups of pulses are observed in almost every mode-locked laser. However, the underlying physics has rarely been controlled or used until now. Here, we demonstrate a general approach to control soliton motion inside a dual-comb laser and the programmable synthesis of ultrashort pulse patterns. Introducing single-pulse modulation inside an Er:fiber laser, we rapidly shift the timing between two temporally separated soliton combs. Their superposition outside the cavity yields ultrashort soliton sequences. On the basis of real-time spectral interferometry, we observe the deterministic switching of intersoliton separation arising from the interplay of attracting and repulsing forces via ultrafast nonlinearity and laser gain dynamics. Harnessing these insights, we demonstrate the high-speed all-optical synthesis of nano- to picosecond pump-probe delays and programmable free-form soliton trajectories. This concept may pave the way to a new class of all-optical delay generators for ultrafast measurements at unprecedented high tuning, cycling, and acquisition speeds.

Precise Gas Temperature Measurement Using a Single Dual-Wavelength Mode-Locked Fiber Laser

Precise Gas Temperature Measurement Using a Single Dual-Wavelength Mode-Locked Fiber Laser

1.7 μm LD Resonantly Pumped Tm∶YAG Single Crystal Fiber Laser

1.7 μm LD Resonantly Pumped Tm∶YAG Single Crystal Fiber Laser

High-power and high-energy coherent sources at 1.1-μm based on compact diode-end-pumped Nd:YAG crystal fiber lasers

In this work, we have reported the first Nd:YAG single crystal fiber lasers at 1.1 μm spectral region operating in continuous-wave and passively Q-switched modes. A maximum continuous-wave output power of 34.52 W is achieved and the corresponding slope efficiency with respect to pump power is 39.9 %. Using Cr4+:YAG as saturable absorber, a maximum average output power of 10.37 W is obtained during passively Q-switched laser operation. The maximum pulse peak power and single pulse energy reach approximately 103.7 kW and 1.1 mJ, respectively. In terms of diode end pumping geometry, these reported results represent the best achievements of 1.1-μm lasers, which indicate that crystal fiber has great potential in improving laser power and energy.

Fabrication of Integrated Single Frequency DBR Fiber Laser Directly on YDF by Femtosecond Laser

Fabrication of Integrated Single Frequency DBR Fiber Laser Directly on YDF by Femtosecond Laser

Intra-cavity photoacoustic spectroscopy dual-gas detection system utilizing dual wavelength fiber laser

An intra-cavity photoacoustic spectroscopy dual-gas detection system using dual wavelength fiber laser has been developed and experimentally demonstrated. The output wavelength of the fiber laser can be tuned within the spectral range of 1520–1600 nm. Using a single fiber laser, the detection of different gases can be easily achieved by changing the wide fiber Bragg grating (FBG) and seed laser. Photoacoustic spectroscopy with the laser intra-cavity technique is employed to achieve high detection sensitivity. The mixture of carbon dioxide (CO2) and acetylene (C2H2) is selected as target gas for evaluating the detection characteristics of the dual-gas detection system. The minimum detection limits (MDLs) of ∼71.82 ppm for monitoring CO2 and ∼924.24 ppb for monitoring C2H2 are realized. The concentrations of CO2 and C2H2 are linearly fitted with the photoacoustic signal, with R-squared values of 0.99859 and 0.99732, respectively, proving the accuracy and robustness of the dual-gas detection system.

Switchable and tunable orthogonal single-polarization dual wavelength SLM fiber laser based on parity-time symmetric system and PM-FBG

A dual-wavelength narrow linewidth orthogonal single polarization fiber laser based on a parity-time (PT) symmetric system is proposed and demonstrated, which achieves the function of tunable and switchable wavelength, and single longitudinal mode (SLM) operation. The PT symmetric system composes of a single physical loop, which includes two polarization controllers (PCs) and a polarization beam splitter (PBS). By modulating PCs to match gain, loss and coupling coefficient, the switchable wavelength and stable SLM laser output have been achieved. Through stretching the polarization maintaining fiber Bragg grating (PM-FBG), the wavelength adjustment range from 1549 nm to 1551 nm is obtained. In the process of tuning, the full width at half maxima (FWHM) linewidths and polarization states at each wavelength have been measured and keep basically stable. The measured FWHM linewidths are less than 13.66 kHz and 11.89 kHz. Further, the stable orthogonal single polarization state has been realized. The degree of polarization (DOP) at each wavelength exceeds 97% and basically remains unchanged during 20 min.

Tunable Single and Multi-Wavelength Er-Doped Mode-Locked Fiber Laser Based on GIMF-PCF-GIMF

Tunable Single and Multi-Wavelength Er-Doped Mode-Locked Fiber Laser Based on GIMF-PCF-GIMF

High-efficiency Ho:YAG single-crystal fiber laser in-band pumped by a 1.9 μm laser diode

We report on a 10-watt-level, efficient 2 μm Ho:YAG (0.6 at.% Ho3+ doped Y3Al5O12) single crystal fiber (SCF) laser in-band pumped by a 1.9 μm laser diode. Benefiting from the high gain of the SCF and the unique propagation features of the pump and laser beams, a maximum output power of 10.77 W is achieved with T OC = 20%, corresponding to a slope efficiency of 61.8%. The higher efficiency of 64.8% and the output power of 9.8 W are obtained with T OC = 50%. Compared to the traditional bulk crystal, wave-guiding of the pump light in the SCF can extend the gain region and thus improve the laser efficiency, which provides a simple and effective method to improve the utilization rate of the pump light even for the low-brightness laser diodes.

Real-time evolution dynamics during transitions between different dissipative soliton states in a single fiber laser.

Various dissipative soliton solutions exist in the parameter space of mode-locked fiber lasers, including both coherent and incoherent pulses. Novel ultrafast laser designs can lead to distinctive dissipative soliton solutions formed by unique pulse shaping dynamics in the same cavity. However, transitionary states in between steady-state mode-locked regimes remain largely unexplored. Here, we investigate the intermediate transition dynamics in a versatile Tm-doped fiber laser capable of emitting both dissipative solitons with anomalous-dispersion and normal-dispersion pulse-shaping mechanisms by adjusting an intracavity polarization controller. Real-time pulse dynamics during mode-locking transitions are analyzed with a modified dispersive Fourier transform setup, illustrating characteristic pulse shaping mechanisms typically reserved for different dispersion regimes. Combined with a spectral intensity correlation analysis, the coherence evolution between two distinct mode-locked states is fully resolved for the first time.

Single longitudinal mode narrow linewidth thulium-doped fiber laser based on an eye-shaped dual-ring filter

A single-longitudinal-mode (SLM) thulium-doped fiber laser, based on an eye-shaped passive dual-ring filter, is designed and constructed. The eye-shaped passive compound cavity consisting of four couplers is used to increase the longitudinal mode spacing, and its performance is numerically analyzed in detail. A homemade uniform fiber Bragg grating serves as a wavelength selection device and a saturable absorber is used to further suppress the intense longitudinal mode competition in the laser cavity, ensuring the single-longitudinal-mode output. The experimental results demonstrate a laser output with a center wavelength of 2,049.85 nm and an optical signal-to-noise ratio of 63 dB. Moreover, the power fluctuation is less than 0.6 dB, and the center wavelength fluctuation is less than 0.03 nm over a continuous measurement period of 60 min, demonstrating an excellent stability. The laser linewidth is measured using an unbalanced Michelson interferometer and β-separation line method, resulting in a linewidth of 11.22 kHz.