Fiber Oscillator Research Articles

Elevating the TMI threshold by incorporating QCW pumping in a high-power CW fiber oscillator

Transverse mode instability (TMI) significantly limits the power scaling of ytterbium-doped fiber lasers. In this Letter, what we believe to be a novel TMI mitigation strategy is proposed and demonstrated in a bidirectional output fiber laser. On the basis of the continuous wave (CW) pump, integrating a quasi-continuous wave (QCW) pump can effectively improve the TMI threshold of the system. In the experiment, a QCW pump with a repetition rate of 1 kHz and a duty cycle of 10% was injected into a CW-pumped fiber laser oscillator, which successfully broke the power limit and alleviated the beam quality degradation. Moreover, when applying a pump modulation depth of approximately 68%, the TMI threshold was notably increased from 624 W in the free-running condition to 1093 W at end B, which is a 469 W (75%) improvement. At this time, the beam quality factor M2 at end B is approximately 1.48, and the stimulated Raman scattering suppression ratio is about 28 dB. Compared to conventional TMI mitigation methods, this approach provides active control in the time domain of fiber lasers, without compromising the optical structures, thereby effectively mitigating TMI and offering broad application prospects.

Noise‐Like Pulse Seeded Supercontinuum Generation: An In‐Depth Review For High‐Energy Flat Broadband Sources

AbstractAs the need for compact, cost‐effective, and reliable laser sources continues to rise, fiber lasers have gained widespread interest in science and technology. In recent years, passively mode‐locked fiber lasers (PMLFLs) have emerged as pivotal tools for generating ultrashort pulses, propelling advancements across various domains including communication, manufacturing, medicine, defense, and security. Amongst the various types of lasing states supported by a PMFL, the emphasis in this review is on the noise‐like pulses (NLP) and their potential applications in supercontinuum generation (SCG). Interestingly, the quasi‐stationary operation of the NLP envelope containing numerous chaotic sub‐pulses has facilitated relatively high energy and broad bandwidth compared to standard mode‐locked laser pulses. Moreover, the NLP generation goes beyond a specific cavity arrangement, the nature of mode‐locking or cavity dispersion. Therefore, through this review, the foremost aim is to report the differences in NLPs across various experimental settings reported so far and highlight the strategies beneficial for high‐energy and broadband NLP development directly from a fiber oscillator. Secondly, the application of NLP as a seed laser is examined to stimulate SCG in different types of fibers, underlining the improved supercontinuum characteristics over the conventional ultrashort pulse pumping schemes. Finally, the benefit of NLP‐seeded SCG for various bio‐medical and industrial applications are highlighted, thanks to the broader and flatter continuum achievable through compact experimental settings.

915 nm pumping kilowatt fiber oscillator with high optical-to-optical efficiency

We demonstrate an all-fiber oscillator with high optical-to-optical efficiency using laser diodes working at 915nm as the pump sources to reduce the demand for thermal management. When the output power of the bidirectional pumped oscillator is 1.16kW, the optical-to-optical efficiency is as high as 75.4%. In this working state, the output characteristics of the oscillator are observed. The Raman suppression ratio is 41.23dB and the beam quality factor Mx2 = 1.14, My2 = 1.29. Analyzing the time trajectory of the highest power output, there are no significant characteristic peaks in the time domain signal and the frequency domain signal, which indicates that the oscillator has good stability.Improving the output characteristics of the 915nm pumping lasers has a positive significance for the application of non-strict ambient temperature control.

Spatiotemporal dissipation dynamics: a route to high-beam-quality and high-peak-power spatiotemporal mode-locked fiber lasers

Spatiotemporal mode-locking (STML) opens a new avenue for implementing high-energy, high-peak-power mode-locked fiber oscillators. However, the compromised beam quality poses a critical limitation to their broader applications. This study presents a method for enhancing the beam quality of STML fiber lasers by employing spatiotemporal dissipation involving the quenching and reabsorption effects of multimode erbium-doped fibers. The proposed technique introduces spatiotemporal saturable absorption, achieving high beam quality without the stringent conditions required for Kerr beam self-cleaning (BSC). Integrating spatiotemporal dissipation with Kerr BSC, we demonstrate an all-anomalous-dispersion Er-doped STML fiber laser, which produces solitons with 6.7 nJ pulse energy (the intracavity solitons with 25.8 nJ pulse energy and >52.8kW peak power), sub-500 fs pulse duration, and beam quality with M x 2/M y 2=1.23/1.20. To our knowledge, it is a record peak power for 1.5 µm band soliton lasers. Additionally, the approach enables the generation of noise-like pulses with M x 2/M y 2=1.04/1.13. This work not only advances our understanding of spatiotemporal dissipation dynamics in STML fiber lasers, but also paves the way toward high-performance STML fiber lasers, rendering them very attractive for applications.

Watt-level long-term stable ultra-narrow linewidth 1064 nm single-longitudinal-mode ring cavity fiber oscillator.

This study presents a high-power, single-longitudinal-mode (SLM) fiber oscillator with a ring cavity design, operating at 1064 nm. Utilizing a double-cladding ytterbium-doped fiber as the gain medium, the system incorporates a fiber Bragg grating Fabry-Perot cavity and a dual coupler ring for step-by-step filtering to achieve SLM operation. With a pump power of 4.19 W, the oscillator delivered an output power of 1.01 W and narrowed the linewidth to 147 Hz, with the potential for further power increase. The oscillator demonstrated excellent longitudinal-mode stability, maintaining mode-hop-free operation for 8.7 hours after temperature stabilization. To the best of our knowledge, this work represents the longest duration of mode-hop-free operation and the narrowest laser linewidth achieved by a free-running ring-cavity SLM fiber oscillator at watt-level output powers.

Energy-managed soliton fiber laser

Ultrafast fiber lasers constitute a flexible platform to investigate new solitary wave concepts. To surpass the low energy limitation of the conventional solitons generated in standard telecom fibers, successive breakthroughs have promoted the usage of an important frequency chirping within fiber oscillators. This lead to original solitary wave regimes such as stretched-pulse, all-normal-dispersion, and self-similar dynamics. We here revisit ultrafast fiber lasers built from standard optical fibers featuring solely anomalous dispersion. We propose a new cavity design enhancing key dissipative effects with contained frequency chirping and demonstrate the generation of high energy pulses in the few-picoseconds regime. The involved intracavity dynamics blends conventional and dissipative soliton features in an unseen way with low- and high-energy propagation regions, allowing an increased flexibility and novel scalability prospects.

44-fs, 1-MHz, 70-µJ Yb-doped fiber laser system for high harmonic generation.

We report the development of a robust Yb-doped fiber laser system based on chirped-pulse amplification (CPA), generating 44-fs laser pulses with up to 70-µJ pulse energy at a 1-MHz repetition rate. It consists of a Yb-doped nonlinear polarization evolution (NPE) mode-locked fiber oscillator, a chirped fiber Bragg grating (CFBG) stretcher, a wave-shaper for manipulating the spectrum of the signal, cascaded fiber amplifiers, and two compression units. The output pulse duration of 44 fs for efficient high harmonic generation (HHG) was achieved by a multi-pass multi-plate Herriott-type non-linear compression unit.

Femtosecond Mamyshev fiber oscillator started by ultra-low power microchip laser seeder at two different wavelengths: a comparison

Femtosecond Mamyshev fiber oscillator started by ultra-low power microchip laser seeder at two different wavelengths: a comparison

Figure-8 mode-locked Yb-doped fibre oscillator with a nonlinear amplifying loop mirror and power amplification

Abstract We report an all-polarization-maintaining Yb-doped fibre (YDF) femtosecond (fs) laser source at ∼1 µm comprising a figure-8 mode-locked oscillator and two YDF amplifier stages. By employing a nonlinear amplifying loop mirror, we could achieve dissipative soliton mode-locking with an output of 24.6 mW at a repetition rate of 6.2 MHz. In our experiments, a Raman signal of 16% in the pulse energy helps to obtain stable mode-locked operation in the figure-8 oscillator. The dissipative soliton pulse from the oscillator is stretched by a chirped fibre Bragg grating and is amplified by a two-stage YDF amplifier. After compression, the YDF master-oscillator power-amplifier system yields fs pulses with a pulse energy of 2.3 µJ and a pulse duration of 274.6 fs at a repetition rate of 0.52 MHz. The output pulse energy is limited by the third-order nonlinearity of the fibre amplifier chain. Prospects for further pulse energy scaling are discussed.

Performance studies on group-velocity-matched femtosecond optical parametric generation

We present a comparative study and detailed characterization of high-power femtosecond optical parametric generation (OPG) at 10 MHz repetition rate in the near and mid-infrared by exploiting near-zero group-velocity-mismatch (GVM) in the nonlinear crystals of PPLN and MgO:PPLN. Using a microchip-started amplified Mamyshev fiber oscillator delivering 198 fs pulses at 1064 nm as the pump source and deploying 19-mm-long PPLN and 42-mm-long MgO:PPLN as gain media, we study in detail the influence of crystal length and pump pulse duration on femtosecond group-velocity-matched interaction in the OPG process. The OPG source is tunable across 1445–1577 nm in the signal and 3318–4412 nm in the idler, and can provide average output powers of up to 439 mW in the signal at 1530 nm and 197 mW in the idler at 3550 nm, at slope efficiencies of ∼50% and ∼20%, respectively. Signal pulses as short as 275 fs are obtained using the shorter crystal, while longer signal pulses with similar output powers are generated with the longer crystal. Experimental results are supported by theoretical simulations, providing good agreement. The OPG source exhibits excellent power stability with high spatial quality of M2<1.5 in the signal and idler beams.

Dynamics of non-Newtonian fluid–fiber interactions and void formation mechanism based on fused filament fabrication

Due to the presence of internal defects such as voids in the composites manufactured by material extrusion additive manufacturing (MEX AM), the mechanical properties of the composites are significantly below the theoretical optimum. Under this circumstance, this paper is intended to investigate the effects of nozzle feeding angles and to analyze the void formation and evolution of multiphase flow, in which systematic computational fluid dynamics numerical simulations are conducted. The volume of fluid model and overset grid method are used to simulate the extrusion deposition process of carbon fiber-reinforced polymer composites, and the Carreau model is used to represent the shear-thinning behavior of molten polymer. The numerical method is validated against previously experimental and numerical simulation data. The numerical results show that the key factor leading to void formation is the vortices caused by fiber oscillation. The intense vortex at the front end of the fibers disrupts the interface between molten polymer and air, creating a beneficial condition for air to enter. Nozzle tilting can effectively reduce the probability of void formation by decreasing the fiber oscillation velocity. The results of the present study provide insights into the development and optimization of printer nozzles to enable the printing of longer fibers with less probability of potential void formation.

Development of an ultrafast laser system based on a Yb-doped fiber and a Yb:YAG thin-rod for the preclinical study of pigmented lesions treatment using a Hartley guinea pig.

In this research, we developed an ultrafast laser system based on a Yb-doped fiber oscillator and Yb:YAG thin-rod amplifier to investigate the efficacy of the laser for the treatment of pigmented lesions. The developed laser exhibited an output power of 22.7 W, center wavelength of 1030 nm, repetition rate of 495 kHz, pulse energy of 45.9 µJ, and pulse duration of 1.56 ps, respectively. For a compact and stable chirped pulse amplification system, a chirped fiber Bragg grating (CFBG) stretcher and a chirped volume Bragg grating (CVBG) compressor, both with fixed dispersion, were used. The dispersion of the total laser systems was precisely compensated by adjusting the length of the passive fiber and utilizing the self-phase modulation effect of the fiber amplifier. The developed ultrafast laser system was then applied in preclinical studies for the treatment of pigmented lesions in a guinea pig model. Three colored squares, each measuring approximately 15 × 15 mm, were treated by scanning a focused beam with varying laser fluences ranging from 0.5 to 2 J/cm2, using wavelengths of 515 nm and 1030 nm. The colorimeter measurements, which were performed 1-5 weeks after laser treatment, indicated that the laser was effective in reducing pigment, particularly black and blue pigments at higher fluences. This research represents the first trial of a preclinical study on pigmented lesions using an ultrafast laser system with a pulse duration below 10 ps, shorter than the stress relaxation time of 10 nm melanin granules. The results are meaningful as they offer valuable insights into the effectiveness of ultrafast laser therapy.

Dissipative spatiotemporal soliton in a driven waveguide laser

A distributed Kerr-lens mode locking regime can be realized in a waveguide laser by spatial profiling of the pump beam, thus creating a spatio-temporal soliton. Additional slow temporal modulation of the pump source stabilizes the spatio-temporal solution in a broad range of parameters defined by the dynamic gain saturation. We choose a Cr:ZnS waveguide laser as a practical example, but such a regime is feasible in various waveguide and fiber oscillators. A far-reaching analogy with Bose–Einstein condensates allows this approach to stabilize the weakly dissipative BECs.

Numerical modeling of an ASE suppression and power scalable approach in 1908 nm LMA-TDFO.

High-power Tm-doped fiber lasers operating at short wavelengths (< 1940 nm) are widely used in biomedical engineering, remote sensing, and pumping Ho-doped solid-state lasers. However, it is challenging to increase their output power. Amplified spontaneous emission (ASE) is a major bottleneck in scaling the power of Tm-doped fiber lasers operating at short wavelengths. This paper presents what we believe to be a novel approach for suppressing ASE and increasing output power in large-mode-area Tm-doped fiber oscillators (LMA-TDFOs) operating at short wavelengths. By designing a core with some particular doping profiles in LMA fibers, most ASE can be concentrated in the LP11 mode and suppressed by bending the fiber to a suitable radius. At the same time, the signal laser can be wholly coupled to the LP01 mode in the oscillator. A model describing the transverse mode competition and ASE is developed. A novel nested-ring doping scheme for LMA fibers is designed, and the output characteristics of a 1908 nm oscillator based on this scheme are numerically investigated. Theoretical analysis confirmed that this sophisticated doping design can effectively achieve the intended ASE suppression effect. Further simulations show that this method is expected to increase the output power of the 1908 nm LMA-TDFO to the kW level and has also demonstrated good practicability across a broader signal spectrum range (1848-1940 nm).

Super broadband light source: a promising master oscillator source for high brightness and high-power fiber amplifiers.

Stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects are crucial limitation factors for further power scaling of high-power fiber amplifiers with near-diffraction-limited beam quality. It is an important research direction to carry out laser system optimization from the perspective of seed construction. In this work, we experimentally investigated the impact of utilizing different seed lasers on the SRS and TMI characteristics in high power ytterbium-doped fiber amplifiers. Both the phase modulated single frequency fiber laser (PMSFL), fiber oscillator laser (FOL), and superfluorescent fiber source (SFS) as a typical type of temporally stable broadband light source (SBLS) are employed as seed lasers. The experimental results indicated that the SRS intensity in the fiber amplifier is influenced by the properties of the seed laser source. Notably, the SRS threshold of the SBLS could be comparable to that of a low-noise PMSFL source. There is a significant increase in TMI threshold when employing SBLS as a seed laser in high-power fiber amplifiers, which is 3.86times that of the injected seed laser with a 3dB narrow linewidth of 0.74nm. Considering the combined threshold characteristics of SRS and TMI, it can be found that SBLS is a promising master oscillator source for high brightness and high-power fiber amplifiers. Our work could provide a good reference for the selection of a master oscillator source, and we believe that it is an important direction to go beyond the power limitation of high-power fiber lasers by constructing a light source with broader spectral linewidth and temporal stability.

790–1000 nm continuously-tunable fiber-based femtosecond laser source

We report a 790–1000 nm tunable femtosecond laser source by second harmonic generation (SHG) of a 1580–2000 nm soliton self-frequency shift (SSFS) fiber laser. The SSFS fiber laser with an all polarization-maintaining fiber configuration consists of passively mode-locked figure-9 Er-doped fiber oscillator, two-stage fiber amplifiers and a nonlinear fiber, delivering femtosecond Raman solitons in a tunable range of 1580 to 2016 nm. The tunable Raman solitons are further injected into a specifically-designed chirped periodically-poled lithium niobite (CPPLN) crystal for efficient SHG, and therefore femtosecond pulses with wide wavelength tuning from 790 to 1000 nm are obtained. The shortest pulse duration is < 100 fs at 890 nm, and the average power is 7.55 mW with a repetition rate of 44.9 MHz. This is, to the best of our knowledge, the first demonstration of a < 1000 nm broadband tunable fiber-based femtosecond source almost covering the whole short-wavelength near-infrared spectrum, which may have important applications in the nonlinear microscopy, biomedical imaging, and material processing.

Transient behaviors in a spectrum-tailored all-PM NALM mode-locked fiber laser.

We demonstrate an all-polarization-maintaining (PM) dispersion-managed mode-locked ytterbium-doped fiber oscillator based on a nonlinear amplifying loop mirror (NALM). Experimentally, three mode-locking regimes with distinct tailored spectra are achieved with carefully designed cavity setup and adjusted pump power. Using the dispersive Fourier transformation (DFT) technique, the real-time transition dynamics among these regimes are observed as the pump power decreases. It is revealed that the intracavity gain does not decrease continuously during the relaxation period of the gain fiber. Instead, it decreases first and then increases, ultimately leading to the overdriving of NALM. This is attributed to the increase in the population inversion rate of gain fiber under the remaining pump. The dynamic evolution of intracavity gain and non-monotonic transmission properties of NALM collectively induce chaotic pulsation during the transformation. This work not only provides a new perspective for the design and development of novel spectrum-tailored laser sources, but also deepens the understanding of the transient dynamics of ultrafast fiber lasers.

Tunable high energy pulse generation in the range of 2.9-3.6 μm enabled by an actively Q switched Er3+/Dy3+ codoped fluoride fiber oscillator.

We demonstrate, for the first time, an actively Q switched red-diode-clad-pumped Er3+/Dy3+ codoped fluoride fiber oscillator. Its wavelength can be continuously tuned over the range of 2.906-3.604 μm (698 nm), representing the widest tuning span of pulsed fluoride fiber oscillators in the mid-infrared. In addition, the achieved pulse energy at each wavelength of >2.95 μm is also higher than that of a previously reported pulsed fluoride fiber oscillator at the corresponding wavelength, to the best of our knowledge. By tuning the wavelength to 3.204 μm, the highest pulse energy of 82 μJ has been gotten with a pulse width of 520 ns at a repetition rate of 500 Hz.

High-Power GHz Burst-Mode All-Fiber Laser System with Sub 300 fs Pulse Duration

An all-fiber low-repetition-rate SESAM mode-locked fiber oscillator combined with a dispersion-managed active fiber loop produces a flexible GHz burst-mode laser source. The high-power output is then produced by amplifying the GHz burst-mode laser source using an all-fiber chirped-pulse amplification system. Then, the laser is compressed using a grating pair compressor; a maximum amplified power of 97 W is obtained. This results in a compressed high power of 82.07 W with a power stability RMS of 0.09% and beam quality better than 1.2. Accurate dispersion control allows for the production of a high-quality pulse duration of 265 fs.

Mode-locked large-mode-area Er/Yb-doped fiber oscillator via nonlinear polarization evolution with enhanced mode suppression

We report a mode-locked Er/Yb-doped large-mode-area (LMA) fiber oscillator based on nonlinear polarization evolution (NPE), which utilizes a linear cavity primarily composed of polarization-maintaining (PM) fibers. The oscillator operates at 1.56 µm with a fundamental repetition rate of 34.47 MHz and has two output ports. One port can deliver high-quality soliton-like pulses with a pulse duration of 325 fs and an average power of 39.5 mW (corresponding to a pulse energy of 1.15 nJ). In contrast, the other port not only generates lower-quality complex pulses but also exhibits poorer short-term and long-term stability, likely due to cross-phase modulation effects. To the best of our knowledge, this is the first implementation of the NPE mode-locked technology in a PM-LMA Er/Yb-doped fiber oscillator at 1.55 µm which often suffers from poor self-starting mode-locking capabilities. This achievement is primarily attributed to the use of endlessly single-mode photonic crystal fibers, which effectively suppress higher-order modes in PM-LMA fibers.