Structured optical field (SOF), which includes vortex beams, non-diffraction beams, cylindrical vector beams and so on, has been under intensive investigation theoretically and experimentally in recent years. Generally, current research focus on the extraordinary properties (non-diffraction propagation, helical wavefront, rotation of electrical field, et al), which can be widely applied in micro-particle manipulation, super-resolution imaging, free-space communication and so on. There are mainly two technical routes, that is, inner-cavity and outer-cavity (spatial light modulators, diffractive phase holograms, q-plates). To date, most of the SOFs generated from both technical routes involves with single monolithic beam. As a novel technical route, SOF based on array beams has the advantage in more flexible freedom degree and power scaling potential. In this paper, research achievements in SOF generation based on array beams are arranged and discussed in detail. Moreover, experiment of generating exotic beam by array beams is introduced, which illustrates that SOF generated from array beams is theoretically valid and experimentally feasible. SOF generated from array beams is also beneficial for capacity increasing and data receiving for free-space optical communication systems at long distance.
Beam combination of fiber laser array is an effective technique contributed to improve the brightness of fiber lasers. In order to realize high-efficiency CBC, challenges like phase distortion (mainly including piston and tilt phase aberrations) should be taken into consideration. Resent years, tilt phase aberrations control has been come true by adaptive fiber optics collimator using the stochastic parallel gradient descent (SPGD) algorithm. However, the convergence rate of tilt control system still cannot satisfy the needs of practical application. In order to increase the tilt control bandwidth, a new idea is put forward that applying the orthogonal single frequency dithering (OSFD) technique into tilt control, and numerical simulation has been completed. A hexagonal laser array with 7 elements has been simulated, and each element has a pair of initial tilt angles in horizontal and vertical direction. The initial tilt angles comply with normal distribution. In the same condition, tilt phase control has been realized through SPGD and OSFD individually, and the convergence steps (defined as the iteration steps that improve the normalized PIB above 0.9) with appropriate parameters are respectively about 20 (SPGD) and 7 (OSFD). Furthermore, tilt phase control of large number hexagonal array is simulated, and the results are as follows: for 19/37 elements, the least convergence steps are about 80/160(SPGD) and 19/55(OSFD). Comparing with SPGD algorithm, it is obvious that the OSFD has higher convergence rate and greater potential for tilt control application in large number coherent fiber laser array.
A focused conformal projection system (FCPS) has lots of advantages over parallel projection system, especially in near-field coherent beam combining situation. A home-made FCPS based on fiber laser array with adaptive fiber optics collimator is setup and its performances are investigated. First, the coherent beam combining based on the system is successfully achieved with simultaneous end-cap/tilt control and phase-locking control. Then performances of collimated conformal projection system (CCPS) are examined experimentally and consistent with the theoretical results with efficiencies above 80%. At last, we test the feasibility of FCPS in improving coherent combining efficiency. Results show that comparing with CCPS, the CBC efficiency of FCPS improves about 43.5%, from 59.0% to 84.7%, with perfect fitness to the ideal situation.
In this paper, we present an experimental comparison of coherent beam combining (CBC) effect with different truncation factors based on a triangle fiber laser array for the first time to our best knowledge. First, we fabricate a triangle fiber laser array based on adaptive fiber optics collimators with the fixed focusing length of 0.18m and clear aperture of 50mm. Two output fiber arrays (6/125 fiber array and 20/400 fiber array) with different numerical apertures (0.12 and 0.065, respectively) are used to generate different truncation factors. The direct measurement method is used to measure the intensity distribution of the two collimated beams with different sizes. Results show that the beam diameters are 14.5mm and 27.6mm for 20/400 output fiber and 6/125 output fiber, separately. This means that two fiber laser arrays with truncation factors of 0.29 and 0.55 are achieved. Then we numerically calculate the CBC efficiencies of two situations with different truncation factors. The analytical results show that the CBC efficiency improves from 0.144 with truncation factor of 0.29 to 0.413 with truncation factor of 0.55. At last, a CBC experiment platform is set up. Throughout the whole experiment, single frequency dithering algorithm and SPGD algorithm are separately used to perform the phase-locking control and the tilt control. Two CBC experiments of triangle fiber laser arrays are achieved successfully both with residual phase errors about λ/15. By analysis the experimental results, we get the CBC efficiencies are 0.099 (69% of 0.144) and 0.264 (64% of 0.413) for the two fiber arrays. The experimental results identify the importance of truncation factor on CBC efficiency and provide an important reference on the selection of fiber array parameters in order to achieve the largest energy proportion in the central lobe.
In this manuscript, we establish a model and theoretically investigate the novel structure of AFTP designed by ourselves. We analyze each sub-structure of the new type of AFTP and firstly use the software of ANSYS to simulate the deformation of the flexible hinge under the external force. The result shows that the deformation of the flexible hinge is mainly from and almost linear to the middle part. Further, after considering the influence of the levers and piezoelectric actuators, we setup the theoretical model in which the displacement is only relative to the ratio of the lever R. With the optimal value of R, we can get the relative largest displacement of the end cap when the other parameters are confirmed. As the maximal voltage applied on the piezoelectric stacks actuators (PSA) is finite, the largest displacement of the end cap is restricted. Neglecting the influence of the effective friction force (Ff) of inner-system, the relationship between the largest displacement of the end cap and the ratio (R) is derived numerically. From the calculated results, we get the largest displacement is about 67 μm with R of 6.9. This work provides a reference for structure optimization of AFTP based on flexible hinges and levers.
Beam combining (BC) of fiber lasers based on master oscillator power amplifier (MOPA) configuration has been
considered as a promising way to achieve high power laser output. In recent years, it has been demonstrated that tip-tilt phase errors impact the combining effect seriously, especially the beam quality in coherent beam combining even if all the beamlets are phase-locked. Adaptive fiber-optics collimator (AFOC) is an effective way to compensate the tip-tilt aberrations in fiber laser systems. As the piezoelectric bimorph actuators used in the AFOCs of traditional type provide very weak force (0.1~1N level), they can only actuate the naked fiber. So the application of traditional AFOCs in high power level is limited by the structure. When the output power is scaled up-to several kW, a coreless end cap is usually spliced on the output side as the end of fiber. Because of the end cap, the expansion of the beam reduces the extractable fluence and avoids fiber facet damage. Then the AFOCs that can be used in high-power situations become the direction of research and development.
In this paper, a new structure of AFOC based on flexible hinges is presented for the first time to our knowledge. It
utilizes two piezoelectric stacks actuators for X-Y displacement of the fiber end cap placed in the focal planes of the
collimating lens. Also the new type of AFOC based on flexible hinges has been developed and demonstrated
experimentally. The thrust of zero displacement of the piezoelectric stacks actuators is 330 N. The maximum tip/tilt
deviation angle of the collimated beam is 180μrad in X direction and 150 μrad in Y direction for a chosen focal length of 0.05m. The first resonance-frequency of this device is about 700 Hz and the bandwidth of this device is 500Hz. This work provides a reference for beam combing and beam pointing controlling in high power conditions.