The effective pumping factor for various Laser Diodes (LDs) is defined to explain the absorption characteristic of wavelength-stable LDs (WS-LDs). This factor is used to determine wavelength locking points and residual pump power. We evaluated the wavelength locking ability and residual pump power in quantity using two groups of comparable LDs and a narrow linewidth fiber laser. Prior to running LDs in a fully wavelength-locked state, the maximum residual pump power is observed at low pump power levels. This information can be useful in engineering applications for establishing appropriate cooling standards for CPS heat load. Additionally, it is valuable for WS-LD manufacturers to provide a universal evaluation factor for assessing different LDs and guiding wavelength locking quantity enhancement.
We report the effective suppression of stimulated Raman scattering (SRS) in a high-power fiber laser oscillator with 15 m long delivery fibers by the insertion of a single chirped and tilted fiber Bragg grating (CTFBG). Inscribed in 20/400 μm fiber, the single CTFBG has been carefully designed and fabricated with a filter bandwidth of 22.8 nm to match the Raman gain spectrum. A maximum suppression ratio over 30 dB is achieved when the oscillator operates at 2.7 kW, with beam quality factor M2 of ~1.4. Moreover, the signal insertion loss is measured to be around 0.2 dB and the maximum temperature is about 41.6 °C at a room temperature of 25 °C. By improving the performance of CTFBG, a further power scaling and longer delivery distance of high-power fiber lasers could be achieved.
The influence of mode instability (MI) on polarization extinction ratio (PER) has been investigated in a 2 kW level polarization-maintained (PM) fiber laser system with backward-pumped configuration, and the phenomena is different from the existing observation in forward-pumped PM fiber amplifiers. During the experiment, with the onset of MI, none decrease of PER has been observed, revealing that the MI effect has little impact on PER in backward PM fiber amplifiers. The discrepancy induced by the pump configuration has been theoretically analyzed, and the origin is attributed to the longitudinally-distribution difference of high order mode induced by the MI effect.
In a recent experimental study, we investigated a homemade distributed side-coupled cladding-pumped (DSCCP) fiber using a master oscillator power amplifier (MOPA) configuration and tandem-pumping technique. During the experiment, we observed an abnormal behavior of the residual pump power from the counter port of the pump core. This abnormal behavior exhibited a threshold-like characteristic, with both the residual power and corresponding power ratio to injected pump starting to increase exponentially above a specific value. Specifically, when the pump power injected into the system ranged from 7.2 kW to 11.77 kW, the corresponding residual power ratio increased from 4.05% to 11.01%. Simultaneously, the signal optical-to-optical conversion efficiency decreased from 84.11% to 75.33%. This sudden appearance of the phenomenon significantly limits the ability to further scale the power of the system. However, the underlying mechanism causing this abnormal behavior remains unclear and requires further investigation.
This paper proposes and verified a modified cavity configuration in oscillating-amplifying integrated fiber laser for stimulated Raman scattering suppression. A short segment of YDF and a long piece of GDF is used in the oscillating section, which can simultaneously suppress the onset of Raman component and avoid the self-pulsing operation caused by the extremely low gain in cavity. Experimental result shows a 31dB Raman suppression ratio at 1080nm when output power reaches 5kW without any other Raman suppression element.
In this work, a high-power polarization-maintained fiber laser with ultra-narrow linewidth and near diffraction limited beam quality is demonstrated. The stimulated Brillouin scattering is mitigated by optimizing phase modulation scheme, the mode instability is suppressed by coiling the active fiber, and the output power reaches to 3kW at the full width at half-maximum linewidth of 10.6GHz. At maximum output power, the stimulated Raman scattering suppression ratio reaches to 75dB, the polarization extinction ratio is 96%, the beam quality M2 is 1.156, and the further scaling of output power is limited by stimulated Brillouin scattering effect. To the best of our knowledge, this is the highest power for polarization-maintained fiber laser with about 10GHz linewidth ever reported so far.
This paper comprehensively considers the suppression effect of fiber parameter control on Stimulated Brillouin Scattering (SBS), and studies the influence of different phase modulation modes, different linewidths, different fiber Yb ion densities, and different fiber lengths on the SBS threshold of the whole system, The best system setting scheme for improving SBS threshold is also given.
The coherent beam combination of kW fiber lasers with a filling aperture has been in research. An experiment is set up to achieve the coherent beam combination. The coherent beam combination of two fiber lasers with a filling aperture is realized, and the phase bandwidth of the beam combination is measured and analyzed. The laser spots before and after the combination are detected respectively. The phase noise of high power laser is measured and the factors affecting the phase noise are analyzed.
In this paper, we report a high-robustness good-beam-quality 3×1 signal combiner that performs very well under high power. We use three 3.5kW fiber lasers to inject into the three ports of the combiner to achieve a high power output of 10.4kW, and the power transmission efficiency is 98.2%.The output beam quality M24σ and β factor are tested when the power reached the highest, which are (5.465,5.2) and 2.69 respectively. The temperature rising rate of housing is 0.9 °C/kW, is effectively controlled by water cooling package of the combiner. This 3×1 signal combiner has been used in 10-kW fiber laser product and it works well in harsh environment, demonstrating high robustness against unconventional conditions.
An all-fiber system based on the principle of optical waveguide self-imaging for coherent beam combination (CBC) system is designed. A square fiber is used as the beam combination device for all-fiber CBC system. The self-imaging characteristics of the square fiber are studied. The self-imaging effect of the square fiber is verified by theoretical simulation and experiment. The square fiber has good self-imaging characteristics and is suitable for CBC using its self-imaging characteristics.
We proposed a simple O-shaped cylinder all-fiber-integrated laser without inter-cladding-power-strippers (CPS) based on a quasi-bidirectional pumping scheme. The fiber grooves were inscribed on the outside of an O-shaped aluminium cylinder with both straight and curved tracks. The curved track with a diameter of 10 cm could suppress the high order modes and keep a stable beam quality with the increases of output power, while the straight parts improve the robustness for fusion points and unpackaged optical components. The simplified configuration of no CPS between the oscillator and the amplifier could also improve the total efficiency. The output power, the optical-to-optical efficiency, the beam quality, and the Raman suppression are systematically investigated. It is verified that this design introduces a practical way to simultaneously improve the transverse mode instability (TMI) and SRS thresholds in a high-power fiber laser system with a simple configuration and high efficiency.
In this paper, we present systematically theoretical analysis of the high order mode (HOM) oscillation in large mode area (LMA) fiber oscillator. A detailed numerical laser oscillator model based on LMA fiber has been built. Model analysis shows that the HOM begins oscillating in fiber laser oscillator when the fiber core diameter reaches 25μm. This HOM oscillation could seriously degrade the beam quality of laser output. In order to suppress the HOM oscillation in LMA fiber laser, we put forward two fiber device models which are partially inscribed fiber Bragg gratings (FBGs) and partially doped gain fiber. Simulation proves the inhibition ability against HOM oscillation of these two devices. Our work provides a complete and concise analysis on the mode characteristics in high power fiber laser oscillator.
We report a lossless all-fiber 7x1 signal combiner, which can be used to combine more than 10 kW laser power. The measured power transmission efficiency is larger than 98.1% and power handle capability is more than 2 kilowatt (kW) for each port. When the combiner is put on a 20°C water cool plate, the average temperature rise is less than 3°C/kW. Due to the nearly lossless efficiency and good thermal performance, we can conclude that this combiner is capable of more than 10 kW power.
Thermal effects are critical limit relevant to the power scaling of single crystalline fiber laser. In this paper, thermal effects in thin-rod single crystalline fiber are numerically researched. The simulation results show that thermal effects can be effectively reduced by enhancing the convective coefficient and decreasing the diameter of single crystalline fiber. For the most thin-rod single crystalline fibers utilized with diameter of 1 mm and length of 40 mm, the maximum heat load is only about 107 W due to the thermal rupture effect, which limits the laser output power to ~1 kW levels. The numerical results provide references for the developments and designing of thin-rod SCF laser
In this paper, the model to simulate the impact of bend induced mode distortion on the beam quality and mode instability (MI) threshold of fiber amplifier is established, and the results show that the bend induced mode distortion will degrade the beam quality of fiber laser and decrease the MI threshold. The bend induced mode distortion will change the gain of active fiber, then destroy the beam quality of fiber amplifier, and decrease the MI threshold, but the bend loss will suppress high order mode. Therefore, with the decreasing of fiber bend radius, the MI threshold will decrease firstly, and then increase.
In this paper, we demonstrated an all fiber laser passively Q-switched by black phosphorus saturable absorber with cylindrical vector beam output. A piece of few-mode fiber Bragg grating was used as the mode-selective output coupler. The repetition rate of the pulse trains increased from 16kHz to 24.9kHz when the pump power tuned from 364mW to 460mW. The maximum pulse energy was 305.2nJ with the shortest pulse duration of 7.5μs under the pump power of 460mW.Both radial polarized and azimuthal polarized pulse output could be achieved by adjusting the polarization controllers. The purity of the cylindrical vector beam output was estimated to be over 95%.
In this paper, we report the experimental study on stimulated Raman scattering (SRS) induced mode instabilities (MI) in large mode area step-index fiber in a counter-pumped all-fiberlized amplifier. When the output power is scaled to 1560W, the ratio of SRS is 2% and the beam quality factor M2 is about 1.4. With the further scaling of output power, the SRS power begins to increase nonlinearly, and then the beam quality degrades obviously when the ratio of SRS exceeds 3%, and the M2 is about 2.1 at 1910W, that is the SRS induced MI. The phenomenon is accompanied by the temperature increasing of output passive fiber, which is caused by heat deposition of quantum defect between signal light and Raman light. The temporal dynamics of SRS induced MI is studied in detail for the first time, which are characterized by using both camera measurement and photodiode traces. The experiment results express that both the output power and output beam profile remain stable before the MI occurs, and fluctuate obviously after the onset of SRS induced MI. The temporal frequency investigation indicates that the SRS induced MI is a dynamic process with slow speed fluctuation at second level, and the Fourier spectrum of time trace is within 10Hz, and the SRS induced mode degradation is different from the Yb-gain induced MI effect.
In this paper, a ring-doped fiber with Yb3+ is designed and fabricated. The intensity distribution of the fiber’s transverse modes is calculated and there are only four types of modes named as HEm1, TE01, TM01, and EHm1 in this fiber. Their near-field distribution is also investigated and it changes from thin-ring shape to thick–ring shape. A smart ring-doped fiber laser based on free space coupled optical oscillator is demonstrated firstly in experiment. It delivers 1038 W laser at 1071 nm with 1.6 nm 3dB bandwidth and the slope efficiency is 66.5%. The beam profile could be adjusted intelligently from ring-shape to flattop-shape which is benefit in industrial process.
The design of annular doping region located in the cladding can reduce signal overlap with the doped region in order to reduce saturation and minimize gain compression, which has important applications in EDFAs. Here, we present the design and power scaling characterization of a cladding-pumped amplifier with ytterbium dopant located in an annular region near the ultra low NA core in the cladding, which is found to be a promising way to achieve multi-kilowatt single mode fiber lasers. The ultra low NA ensures that the fiber amplifiers operate in single mode state, which results to that the fiber amplifiers are free of the limitation of the transverse mode instability, and that the mode field of the signal laser extends into the cladding to extract gain amplification. The annular ytterbium-doped region located in the cladding can overcome the contradiction between high doping concentration and ultra-low NA design, which can simultaneously obtain high pump absorption with ultra low NA. The size of annular ytterbium-doped region under different core NA has been studied for various core sizes, which shows that the optimal size of annular ytterbium-doped region is related to the core NA and the core size. Detail analysis of high power amplification of cladding-ring-up-doped ultra low NA single mode fiber amplifier has been presented, which includes various nonlinear effects and thermal effects. It shows that, due to the specific design, the single mode characterization of the fiber is less influenced by the detrimental thermo-optic effect, which means that the cladding-annular-doped ultra-low NA fiber has high mode instability threshold than the ultra-low NA fiber with the core being fully uniformly doped. The cladding-pumped fiber amplifiers based on cladding-annular-doped ultra low NA fiber has the capability to achieve >10kW single mode fiber lasers.
Detailed experimental investigations are presented on investigating mode instability and stimulated Raman scattering in 30μm core diameter fiber amplifier by varying the pumping light wavelength using 915/976nm LD sources. Results reveal that for 976nm pumping light, the highest power can reach to 3570W; and for 915nm pumping light, the highest power can reach to 3771W. Physical characteristics of mode instability do not perform in a traditional form both in output power and time domain traces, such as roll-over point in output power curve. However, the beam quality of output laser clearly reveals there are some more high-order modes which have an effect in mode instability characters. Compared to 976nm LD pumping source, laser of amplifier using 915nm LD pumping source shows stronger non-linear effect. When the output power is 3512W, the Raman light is 13dB below the signal light. By shortening the passive fiber before the endcap, when the output power is 3520W, the Raman light is 20dB.
Detailed experimental investigations are presented on suppressing mode instability and stimulated Raman scattering by varying the pumping power distribution in a large mode area all-fiber amplifier with fiber core diameter of 20μm using bi-directional configuration. Results reveal that compared to employing co-directional pumping scheme, the fiber amplifier employing counter-directional pumping scheme can enhance the MI threshold power from 1250W to 1447W. Optimizing the pumping power distribution can further strength the mitigation of mode instability, such as the ratio of 57% and 66%, the threshold power is 2176W (highest output power) and 2150W respectively. For the ratio of 49%, which means almost identical scale of co-pumped light and counter-pumped light, the threshold power is 1934W. On the other hand, raising the proportion of backward pumping power can also mitigate the stimulated Raman scattering. 66% of backward pumping power can acquire 2150W output power and 20dB signal-to-noise ratio of the Raman peak, which indicates that optimizing the pumping power ratio can suppress mode instability and stimulated Raman scattering simultaneously.
We report on the high power amplification of narrow-linewidth polarization-maintained (PM) laser in all-fiber PM MOPA configuration, which can operate with linewidth around 2GHz at above 1kW power level. Pump-limited maximum narrow-linewidth output power is 1.4kW without SBS effect, and the linearly-polarization single-mode output power was limited to ~550W by mode instabilities (MI). The beam quality of the fiber amplifier (M2 ) was measured under different power, which degrades gradually from ~1.4 to ~2.2 after the onset of MI. The polarization extinction rate is measured to be about 90% before the onset of MI and reduce dramatically after MI sets in. The experimental results are analyzed based on a novel semi-analytical model, which has taken the effects of gain saturation into account. The theoretical results agree well with the experimental results. Mitigating MI by coiling the gain fiber has been analyzed and demonstrated numerically. It shows that, by tight coiling of the gain fiber to a radius of 4cm, the MI threshold can increase to 2.5 times higher than that without coiling or loose coiling, which means that the amplifier have the potential to achieve MI-free 1.4kW output power.
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