Cascade multi-dithering technique coherent beam combination for ultra-high power laser system

Abstract

A research of a novel active phase control technique called “Cascaded Multi-Dithering (CMD) technique” used in coherent beam combination for achieving high power is presented. In conventional active phase control methods, the maximum number of beam elements to be combined is limited up to 100~200 because of the physical bandwidth limit of modulators. Moreover, control bandwidth, having trade off relationship with the number of beam elements to be combined, is another cause of the limit of the maximum number of beam elements. In this thesis, the modified-LOCSET is proposed, which is a method to improve the control bandwidth by determining integration time as the least common multiple of modulating frequencies. The modified-LOCSET was demonstrated by four fiber beam combination with amplifier-like noises generated with computer, showed λ/24 of residual phase error and control bandwidth higher than 100 Hz. Based on the study, Cascaded Multi-Dithering (CMD) technique is proposed. In LOCSET, the maximum number of beam elements to be combined is restricted to 100~200 because of the limit of the number of modulating frequencies. However, in Cascaded Multi-Dithering technique, modulating frequencies used in LOCSET can be repeatedly used, allowing the maximum number of beam elements to be extended up to 2,500~10,000 in two dimensional Cascaded Multi-Dithering technique. It is firstly proved by simulations, comparing an error from 100 beam combination using conventional LOCSET with an error from 2,500 beam combination using Cascaded Multi-Dithering technique. As a result of simulations, the error from Cascaded Multi-Dithering technique is small enough to be controlled and is a similar degree of the error from conventional LOCSET. The feasibility of Cascaded Multi-Dithering technique was also shown by demonstrating sixteen beam combination. Sixteen beam combination using Cascaded Multi-Dithering technique was successfully performed with λ/31 of residual phase error, being in agreement the result of simulations.