2kW narrow linewidth all fiber laser

High power fiber laser systems have attracted extensive attention due to compactness, good beam quality, efficient heat dissipation and high conversion efficiency. They are widely used in industrial processing, military, medical treatment and other fields. Over the past two decades, owing to the development of double cladding fiber and high-brightness laser diodes, the output power of fiber lasers has been greatly improved. Unfortunately, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), restrict the further enhancement of the output power of fiber lasers. Apparently, increasing the core diameter is the most common way to suppress NLEs in the fiber, but this causes another limiting factor, i.e. mode instability (MI), resulting in the deterioration of the beam quality and in the limitation of the power scaling. Therefore, it is important and urgent to suppress the NLEs and MI simultaneously in fiber lasers. The M-type fiber, by designing refractive index profile, breaks through the stringent trade-off between mode area and numerical aperture (NA), so it possesses a larger mode area than the step index fiber, which helps to avoid NLEs and expand the power range. The M-type ytterbium doped double-clad fiber is fabricated by the modified chemical vapor deposition (MCVD) process with solution doping technology (SDT), the core/cladding diameter is 25/400 μm. The NA of high index ring and index dip in the core are 0.054 and 0.025, respectively. To test the performance of the M-type fiber during high-power operation, a 976 nm bidirectional pumped all-fiber amplifier is constructed. As a result, maximum output power of 2285 W is achieved with an optical-to-optical conversion efficiency of 66.5% under bidirectional pumping scheme, and the measured <i>M</i><sup> 2</sup> factor is 1.42, the central wavelength and 3 dB linewidth of output laser are 1080 nm and 3.01 nm, respectively. To the best of our knowledge, this is the highest output power in a continuous-wave fiber laser employing an M-type fiber at present. However, the MI effect is observed at the output power of 2252 W. The future work will focus on optimizing the structure of the M-type fiber to achieve a stabler higher-power and higher-efficiency laser output.

We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5 kW output power with an FWHM linewidth of 9.63 GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400 µm. The reflectivity of the main stage is 0.049‰ at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.

To improve the Raman scattering cross-section of trace gases, a Raman spectrum detection method based on the double-clad fiber laser is designed in this paper. The influence of double-cladding fiber laser as a light source on gas Raman detection is investigated. The higher the output power of the double-clad fiber laser, the larger the gas scattering cross-section and the stronger the Raman effect. Therefore, this paper first conducts a numerical analysis of the factors affecting the output power of the double-clad fiber laser and uses MATLAB to simulate and explore the influence of different pumping modes, pumping power, and rare earth doping concentration on the output power of the fiber laser. Then the gas detection system with ytterbium-doped fiber laser as the light source was used to analyze the material qualitatively and quantitatively. Fiber laser as a Raman detection source has the advantages of high linearity and good stability.

High-power narrow-linewidth continuous fiber lasers have wide applications in the fields of power synthesis, nonlinear frequency conversion, and industrial processing. The output power of kW or even MW can be achieved by adopting spectral synthesis. However, in the power amplification process, the nonlinear effect represented by the stimulated Brillouin scattering (SBS) effect limits the further increase of the laser power. And the accompanying self-pulsing effect poses a threat to the safety of the laser system. A high-order phase modulation scheme using a definite time sequence has been proposed to realize the controllable spectral broadening of the seed source, so as to suppress the SBS effect and improve the output power in the amplification stage. In this paper, we demonstrate a real-time definite time sequence generation scheme by using an integrated field-programmable gate array (FPGA) and digital-to-analog converter (DAC). Combined with a single-stage phase modulator, the broadened bandwidth up to 30 GHz and controllable spectral shape can be achieved by adjusting the output waveform of the DAC directly. The generation principles and waveform control methods of real-time sequence are introduced in detail. Proven by a master oscillator power amplifier (MOPA) structure, the laser output power has increased by 30% under the extended bandwidth of 10 GHz compared to the cascaded white noise modulation case. This scheme provides a compact real-time control module for spectral broadening, which lays the foundation for implementing low-cost, compact, and standardized high-power narrow-linewidth fiber lasers.

High-power narrow linewidth fiber lasers are extensively applied in coherent detection and power-spectrum beam combination etc. The suppressing method of stimulated Brillouin scattering is analyzed. And the theory of sinusoidal phase modulation is analyzed also. A single frequency laser is broadened to become a 2.9 GHz linewidth seed by sinusoidal phase modulation technology. The power of the seed is then boosted from 50 mW to 780 W through a three-stage power amplifiber configuration. Central wavelength and linewidth of the laser are 1064.34 nm and 2.9 GHz respectively, with an optical-optical efficiency being 79%. And the beam quality is Mx2 =1.44 and My2 =1.43. The output powers before and after phase modulation are compared with each other. And the reason why output power increases is analyzed. The stimulated Brillouin scattering threshold is promoted by added longitudinal mode. Finally, the output power is promoted after phase modulation, so that the output power of this laser is only limited by the pump power. If the pump power is increased, the higher output power of narrow linewidth fiber laser will be achieved.

Stimulated Brillouin scattering (SBS)-induced mode distortion (MD) in high power narrow linewidth fiber amplifiers has been implemented, and the origin has been investigated from the aspect of the evolution of the optical spectrum, spatial beam profiles, and temporal-frequency domain characteristics. It is shown that, following the onset of the backward giant pulses generated by SBS, forward giant pulses were generated, which reached multi-kilowatt level peak power and triggered the onset of stimulated Raman scattering (SRS). After the onset of SRS, the beam quality starts to degrade, and the beam profiles deteriorate obviously. It reveals that the SBS-induced MD is a two-stage physical process: SBS-induced forward giant pulses trigger the SRS effect, and then the SRS effect causes the beam deterioration of the signal laser, which means that SRS is the origin of the MD observed after the onset of SBS. To the best of our knowledge, this is the first revelation of SBS-induced mode distortion in high power narrow linewidth fiber amplifiers, which can facilitate the in-depth understanding and effective suppression of the complicated mode evolution phenomena.

In this study, the authors presented a distributed optical fibre temperature sensor whose performance is improved using the optimisation techniques and Fourier wavelet regularised deconvolution (ForWaRD) method. As the input power launched into the sensing fibre is critical, the authors have considered both conventional optimisation technique and evolutionary optimisation techniques namely: genetic algorithm, differential evolution algorithm and particle swarm optimisation algorithm to increase the stimulated Brillouin scattering (SBS) threshold power. Using the optimised value of the parameters and employing evolutionary computing techniques, the proposed 50 km long temperature sensing system offers a 4.7 dB improvement in SBS threshold power over the design of experiment based system. It is being verified that power of the backscattered signals approximately are the convolution of the input pulsed power and corresponding backscatter optical fibre impulse responses. The Fourier wavelet regularised deconvolution (ForWaRD) method is employed to improve the spatial resolution of the proposed sensing system without reducing the pulse width of the input pulse. Employing ForWaRD technique a 10 m better spatial resolution observed as compared with the Fourier deconvolution technique. The proposed 50 km long temperature sensing system exhibits a temperature resolution of 1.85 K because of suppression of SBS threshold and noise reduction.

A theoretical analysis of stimulated Brillouin scattering (SBS) in linear cavity Yb3+ doped dual-clad fiber lasers is presented by solving the steady-state rate equations with SBS included. Solutions to suppress the detrimental effect in the fiber laser are discussed. It is shown that the first-order Stokes initiated by forward and backward pump light changes exponentially along the fiber, and laser power reduces greatly in the presence of SBS under bidirectional end pump. Furthermore, it is found that the SBS threshold power can be improved effectively by using large mode area fiber and shortening the cavity length.

We report on a high peak power pulsed fiber laser emitting in the L band limited by stimulated Brillouin scattering (SBS). On-going developments target at building fiber lasers for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensing from space such as ASCENDS emitting at 1572nm. Unfortunately the relevant CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> lines compatible with Erbium amplification are located in the L amplification band, where the cross sections are small. Consequently, those fiber sources have low non-linear threshold and low efficiency. The European Space Agency is funding a project dedicated to the design of a high power pulsed fiber laser for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> DIAL monitoring with emission lines located closed to 1579 nm. The system requires 150 ns pulse duration at 4 kHz pulse repetition frequency (PRF). The high precision on the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration measurement requires narrow linewidth and the laser is primarily limited by Stimulated Brillouin Scattering (SBS). SBS threshold can be improved by: reducing fiber length, reducing effective Brillouin gain or increasing effective area. Due to the small cross-sections of Erbium in L band, large doping concentration or large fiber length are required. Large mode area phosphate Erbium-Ytterbium codoped fibers have been used to generate 84 μJ for 1 μs pulses with 1% efficiency which corresponds to ~84 W peak power.

Fiber lasers for flat bed laser cutting systems open new niche applications where users can take advantage of their high wall-plug to beam output energy efficiency, low maintenance burden, and a leaner manufacturing approach with the elimination of external beam alignment needs for set-up. The increase in output power of fiber lasers also calls for the right choice in methods and optics. This paper presents developments of special optics and methods at “extreme-speed-extreme-power” [1], for fiber laser cutting and welding applications as well as a case study for fiber laser-cutting of blanks components directly from coil strips.Fiber lasers for flat bed laser cutting systems open new niche applications where users can take advantage of their high wall-plug to beam output energy efficiency, low maintenance burden, and a leaner manufacturing approach with the elimination of external beam alignment needs for set-up. The increase in output power of fiber lasers also calls for the right choice in methods and optics. This paper presents developments of special optics and methods at “extreme-speed-extreme-power” [1], for fiber laser cutting and welding applications as well as a case study for fiber laser-cutting of blanks components directly from coil strips.

We investigate the soliton pattern formation in an erbium-doped figure-of-eight double-clad fiber laser. The mode-locking is realized with a nonlinear amplifying loop mirror. Different soliton complexes have been obtained similar to those obtained when the mode-locking is achieved through the nonlinear polarization rotation technique.

The power scalability of diffraction-limited Yb-doped fiber lasers is investigated with a new model which takes into consideration the spectrums of absorption and emission cross sections of active ions. It is found that the maximum output power of a fiber laser is not only dependent on the pump wavelength, but also depends on the core size and cladding-to-core area ratio of active fiber. To increase the pump wavelength is not sufficient to improve the maximum output power, where at the same time the large core, small cladding, high pump brightness and dopant concentration are simultaneously required. Further investigations show that to maximize the output power, the best choice of pump wavelength is highly dependent on parameters such as the pump brightness, dopant concentration, core size and cladding-to-core area ratio. It is found that the pump wavelength with the largest absorption cross section should be the best choice when the core size is small. Besides, the upper limitation of pump brightness for elevating the output power of fiber lasers is given.

A new fiber coupled AOM for high power Q-switched fiber laser is proposed and experimentally presented. Each end of the AOM has a fiber collimator. For its particular structure, amplified spontaneous emission (ASE) and other deleterious nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in high power pulsed fiber laser can be easily suppressed. Based on our proposed acoustic-optic modulator (AOM), the 3dB line width of the fiber laser can be narrowed to only 0.1nm, which made it suitable to be used as the seed in master-oscillator power amplifier (MOPA) configuration based all-fiber pulsed fiber laser. When the repetition rate is 20 kHz, 2W output from the fiber laser can be achieved. Full width at half maximum (FWHM) of the fiber laser's pulse is 56ns and the peak power of the fiber laser is nearly 1.8kW. When the fiber laser is used as the seed of a high power fiber laser, 20W with 20 kHz repetition rate pulsed fiber laser has been obtained. The pulse width is nearly 100ns and the peak power can reach as high as 10kW. Based on the proposed technique, mature product of the pulsed fiber laser with this configuration has been successfully implemented.

We report an all-fiberized SBS-based high repetition rate sub-nanosecond Yb fiber laser for supercontinuum generation. The high repetition rate ns laser pulses were produced from a fiber Bragg grating (FBG)-constructed fiber laser cavity consisting of a piece of double cladding Yb fiber as the gain medium and a short piece of Bi/Cr-doped fiber as a saturable absorber (SA). By optimizing the fiber length of the Bi/Cr-doped fiber and the reflectivity of the FBG, the Q-switching state of the fiber laser can be adjusted so that the energy storing condition within the fiber cavity can assure the start of stimulated Brillouin scattering (SBS) and as a result, compress the laser pulse duration. The fiber laser had an average laser power output of 1.2 W at 1064 nm with pulse repetition rate of about 80 kHz, almost four times the reported results. The pulse duration was about 1 ns with peak power of about 15 kW. After one stage of amplification, the laser power was raised to about 3 W and was used to pump a 20 m long photonic crystal fiber (PCF). Supercontiuum (SC) laser output was obtained with average power up to 1.24 W and spectrum spanning from 550 to 2200 nm.

One-stage master oscillator power amplification (MOPA) structure is a simple and robust way to acquire high-power narrow-linewidth fiber lasers (NLFLs), which have wide applications in spectral/coherent beam combination. In this work, an NLFL system was established based on a home-made fiber oscillator seed. By using this one-stage MOPA system, a 4 kW output power was achieved with 3 dB linewidth of about 0.53 nm and optical-to-optical conversion efficiency of 76.9%. The signal to Raman ratio was ∼22 dB. The measured beam quality (M2 factor) was about 1.6 at the output power of 3780 W then degraded to 2.8 at the maximum power of 4 kW. To the best of our knowledge, the 4 kW is the highest output power for such narrow linewidth one-stage MOPA fiber lasers.