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 M<sup>2</sup> 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 Yb<sup>3+</sup>-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%
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.