In this work, we investigate the SBS mitigation of the pseudo random bit sequences (PRBS) modulation analytically and numerically. The lightwave is phase modulated by a Butterworth lowpass filtered and amplified PRBS signal, and the SBS suppressing capability versus the parameters such as the filter order, the filter cutoff frequency, the modulation depth, and the pattern length are illustrated. Therefore, we redefine the modulation depth by normalized RMS voltage to integrate binary sequences modulation signal and multi-value sequences modulation signal. On this basis, we demonstrate the impact of pattern length, modulation depth and modulation cutting rate on SBS mitigation and found the corresponding optimal component of n=9, 22.72 dBm and 0.54. On this situation, contrast to the unfiltered PRBS phase modulation scheme, a 17.7% enhancement of normalized SBS threshold can be obtained by using normalized power in the barrel to estimates the linewidth. This work may provide a new idea for SBS mitigation by lowpass filtered PRBS phase modulation in narrow linewidth fiber amplifiers.
In this paper, we investigated the association between beam quality and beam characteristic in theory in a spectral beam combining system and built a model to describe the dispersion. We analyzed that how the beam quality varies with different input beam parameters such as the waist radius 𝑤𝑤0 and the spectral width Δλ. There is a surprising result that the increased w0 and Δλ have a coefficient influence on the beam quality degradation, and the degradation is pretty sensitive to the increase of waist radius w0. The result gives us a guidance that besides using the narrow linewidth laser, we also need reduce the waist radius of the input beam in the SBC system appropriately to abate the beam quality degradation caused by dispersion.
Our study describes the development of coherent beam combining of an array of nine fiber lasers using an all-optical ring cavity feedback loop based on a diffractive optical element to achieve a single-aperture output. Nine 300-mW Yb-doped fiber amplifier beams arranged in a 1 × 9 end-cap array were combined to achieve a single-aperture beam with a power of 739 mW and a beam quality (M2) of 1.18 with 21.5% combining efficiency. The optical spectra, far-field distributions, and time-domain characteristics of the combined beams were investigated under open- and closed-loop conditions. Under open-loop conditions, the far-field coherent visibility changed constantly from 72.1% to 90.9% and the fluctuation intensity was strong. Under closed-loop conditions, the system achieved a steady state with a visibility of 98.6% and an average feedback intensity of 0.4 V, indicating the occurrence of phase locking. Furthermore, mode hopping was observed when there were more than four channels in a combination. However, the system interference pattern remained stable. Comprehensive research on the relevant literature indicated that novel filled-aperture CBC was achieved using an all-optical ring cavity feedback loop based on a DOE.
Diffractive optical element is used to realize single-aperture output for passive coherent combining of 8-channel fiber laser. Using this system, we demonstrated the far-field coherent visibility, output spectrum and beam quality after coherent combining. Experimental results show that phase noise will cause the mode frequency to change under multi-channel conditions. However, the beam quality and the coherent visibility after coherent combination do not change significantly. The far-field coherence visibility reached 98.6%, the beam quality M2=1.21, and the diffraction limit magnification factor β reached 1.98. This shows that in the presence of low phase noise, even if the number of channels is increased, a coherent combined output with high beam quality can be obtained.
Coherent polarization beam combining (CPBC) is an effective method to obtain a high-brightness laser with linear polarization state. In this paper, we propose a method to apply optical heterodyne detection to CPBC. An optical phase modulation is implemented to transform the phase difference of two beams to amplitude modulation for polarization state detection. The CPBC of two lasers with the same frequency is realized by linear proportional-integral-derivative (PID) phase-locked control. Based on this principle, a three-channel CPBC system is realized to obtain a linearly polarized output beam with a high PER. The PER obtained in the experiment reached 17.44 dB with the output power of 431 mW, while the control bandwidth is 66.1 kHz. The residual phase noise of the first-stage CPBC system achieved to 1×10-4 rad/ √Hz at 1 Hz and 4×10-6 rad/ √Hz for more than 100 Hz.Compared with other CPBC phase-locking methods, this method can effectively suppress phase noise and has a significant improvement in control bandwidth and polarization extinction ratio.
In this study, the total deviation of central diffracted beams caused by the periodic projection effect and refraction of zero-order diffracted beam of diffractive optical element was theoretically investigated. The theoretical model of the relationship between the total deviation of central diffracted beams and the combining angle of the ith order incident beam was developed. Inversely, the total deviation of central diffracted beams can be used to calibrate the actual combining angle of the ith order incident beam, further guide the correction of the actual combining angle in coherent beam combining.
We investigated the beam quality improvement of a tiled-aperture coherent beam combining by changing the intensity distribution of fiber beamlets array. An optimal gradient power distribution of the beam array is found. The beam quality is improved by 12.6% with a fill factor of 0.5 with gradient distribution architecture compared with the uniform distribution. With the expansion of the array scale, the improvement of beam propagation factor is becoming more obvious. In addition, the effects of phase error and beamlet arrangement layout are also researched, which shows the propagation factor of the hexagonal ring arrangement is improved by 16.14% compared with the ring arrangement under the gradient arrangement. The effect of phase error on the combined beam quality with respect to the gradient distribution is discussed.
Aimed to maintain excellent beam quality, the influence of pointing deviation on the beam quality is theoretically studied in the dual-grating spectral beam combination (SBC). The incident light field of the fiber laser array with the pointing deviation is built by the transformation of coordinates, and the variation rule of the combined beam quality with random perturbations is discussed by the principle of beam diffraction and the statistical analysis. As a result, the degradation of beam quality for the pointing deviation is respectively 0.31(±0.13) and 3.06(±1.27) for the standard deviation of 0.1 mrad and 0.5 mrad, spreading as a Normal distribution. It can be concluded that the pointing deviation of laser array will destroy the condition of the SBC of the common aperture output, resulting in the continuous growth of the M2 factor. These analyses provide a valid basis for setting up the experimental system of dual-grating SBC.
We have constructed an all-fiber single-frequency amplifier system and an additional all-fiber amplified spontaneous emission (ASE) source system to simulate the effect of generated backward ASE on stimulated Brillouin scattering (SBS) in master oscillator power amplification (MOPA) system. By injecting the artificial ASE source backward into the Yb-doped MOPA system directly and measuring the evolutions of SBS threshold power with the increasing ASE power level, we can get the changes of SBS process during the amplification. Effects of counterpropagating ASE on SBS threshold decreasing were obvious: When there was no counter-propagating ASE being injected, the backscattered power started to rise very slowly. However, with the added artificial ASE increasing, the backscattered power departed from linear quite early. And the SBS threshold power was reduced to different levels with different power level of additional ASE. As the artificial counter-propagating ASE power increased from 0 to 275.9mW, the SBS threshold power was correspondingly dropped down from 4.75W to 3.35W. The 30% reduction of the SBS threshold can be a siginificant influence in a single-frequency fiber amplifier.
Image segmentation is an important part of many computer vision tasks such as image recognition and image understanding. Traditional image segmentation algorithms are susceptible to the influence of complex backgrounds such as illumination, shading and occlusion, thus the application of convolution neural network to image segmentation becomes a hot spot of current research. But in the process of image convolution, as the convolution goes further, the image will lose some edge information, resulting in the blurring of the final partition edge. To overcome this problem, we propose an image segmentation algorithm combining the fully convolution neural network and K-means clustering algorithm. By conducting pixel matching between the coarse segmentation result obtained by using the convolution neural network and the segmentation results obtained by using K-means, the algorithm enhances the classification of pixels on the edge to improve segmentation accuracy. The proposed algorithm adopts two-stage training method to train and optimize the model. The experimental results on VOC2012 set validate the effectiveness of the proposed method.
To suppress high order modes and improve the beam quality, an active self-imaging mode filter based on multimode interference and self-imaging effect is proposed in large mode area (LMA) fiber amplifier. With this filter structure, transverse mode competition and individual transverse mode power distributions in strongly pumped fiber amplifiers are theoretically demonstrated. Employing this mode selection technique in 30/400 LMA strongly pumped fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 96.3%. By the modal power decomposition, the M2 parameter of beam quality decrease dramatically from 2.24 to 1.11 (0 relative phase) and from 3.01 to 1.24 (π/2 relative phase). This study provides a new method to achieve single mode in LMA fiber amplifier and this filter would be extended to larger mode area fiber amplifier to improve the beam quality.
We have demonstrated a kW continuous-wave ytterbium-doped all-fiber laser oscillator with 7×1 fused fiber bundle combiner, fiber Bragg grating (FBG) and double-clad gain fiber fabricated by corresponding technologies. The results of experiment that the oscillator had operated at 1079.48nm with 80.94% slope efficiency without the influence of temperature and non-linear effects indicate that fiber components and gain fiber were suitable to high power environment. No evidence of the signal power roll-over showed that this oscillator possess the capacity to highest output with available pump power.
We demonstrate a passive coherent beam combination of two nanosecond amplifiers by using an all-optical feedback loop. An electro-optic amplitude modulator is utilized to tune the pulse width and the pulse repetition frequency of combined laser pulse. The positive correlation between the visibility of far-field coherent patterns and the pulse duty ratio is found. The range of tunable pulse repetition frequency is from 2.023 MHz to 6.069 MHz, and the range of tunable pulse width is from 10 ns to 50 ns. The maximum visibility is up to 85%. This approach presented here provides a promising way for power scaling of high power nanosecond fiber laser and maintaining beam quality simultaneously.
We demonstrated a passive phase locking of a seven-element 352 W all-fiber polarization-maintaining amplifiers array using an all-optical feedback loop. Every single channel has four-stage amplifiers and is seeded by a broadband master oscillator for stimulated Brillouin scattering free. The seven laser beams are tiled side by side into a hexagonal laser array with a high space duty ratio of 65% in the near field. When system is in closed-loop, a visibility more than 90% of coherent pattern in the far field is obtained. By using the all-optical feedback loop and more pump power, higher power scaling with high beam quality appears to be achievable in a coherent beam combination system.
The inscription method of FBGs on large-mode-area double-clad fibers (LMA-DCFs) with phase mask technique was
described. A pair of LMA FBGs was prepared and the center wavelengths of them were both around 1076nm with 3 dB
bandwidths of about 0.5nm. The reflectivities of them are 99% and 10% in the fundamental mode, respectively. In order
to be protected and to withstand high-power laser, the FBGs were metally packaged, and then applied to an Yb3+-doped
LMA-DCF laser as the laser cavity. An output power of 314 W centered at 1075.71 nm with a slope efficiency of 60%
was achieved.
A master-oscillator fiber power amplifier system with a 4.5-m-long Yb3+-doped homemade large-mode-area double-clad fiber is reported. Up to 156 W average power of 1064 nm amplified pulse, corresponding to a slope efficiency of 64.9%, has been demonstrated. The amplified pulse possesses a pulse duration of 24 ns and a repetition rate of 50 kHz and demonstrates a good beam quality (M=2.81 and M=2.66), although no special transverse-mode-controlling techniques were adopted.
We simulated beam combination of Yb-Doped Double-Clad-Fiber(DCF) lasers theoretically by using a new external cavity and this system suits both of 1D array and 2D array. The numerical simulation result of 1D linear array shows that light energy from fiber lasers can be coupled into fiber cores through a single round trip. In comparison with several 2D arrays, we find that both the radius of the central spot shroten with the number of fibers increasing, on the contrary, the optical feedback through a single round trip coupled into cores increase gradually.
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