In this paper, the fast measurement of the coherence of the multi-transverse-mode optical field is realized by using the intensity type of liquid crystal spatial light modulator (LC-SLM). Using the characteristic and advantage of the LC-SLM which can change the location of the light hole quickly and easily, and basing on the method of dual-hole interference, we divided the 650nm multi-transverse-modes laser spot into 4×4 points, and performed a quick scan with pairs of double holes of diameter 150μm. The contrast ratio of the interference patterns is computed by a self-made image processing software, and the coherence degree between the two points can be obtained. The experimental results show that the measurement system can accurately measure the coherence of multi-transverse-mode optical field within 10 seconds, which providing guideline for the further mode decomposition and beam control of multi-transverse-mode optical field.
Quadriwave lateral shearing interferometry (QWLSI) could acquire the phase and intensity distribution of laser beam with remarkable resolution in only one measurement. It’s a potential method to measure the phase step of multimode laser. However, the discontinuity point of intensity will reduce the quality of interferogram and lead to wrong wavefront retrieve result. In this paper, analysis and simulations have been conducted on the measuring ability of the QWLSI for the higher-order modes (HOM) in large mode area (LMA) fiber. A set of interferogram image of ideal linearly polarized (LP) mode are calculated and analyzed by a commercial retrieve software. The results show that the proper ratio of the shearing distance to the gap size between HOM petals is the critical parameter to retrieve wavefront correctly. To study the influence of coherence on wavefront retrieving, we calculated the interferogram of multimode laser with different modal phase fluctuation. The results indicate that the partially coherent beam will introduce significant ambiguity into the retrieved wavefront. Finally, the feasibility of wavefront correction of multimode laser with LSI and spatial light modulator is demonstrated with simulations.
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