Ultrafast laser inscription of 2D confinement MMI-based beam splitters with tunable splitting ratio in Nd:YAG crystal

The multimode-interference (MMI) devices based on the principle of self-imaging show great potential for applications in different scenarios. The research interest in two-dimensional confinement MMI devices has surged enormously due to the broader areas of applications compared with the one-dimensional confinement MMI devices. In this paper, we report on the characteristics of two-dimensional confinement 2 x 2 output MMI-based beam splitters which were achieved by femtosecond laser direct writing technology in a Nd:YAG crystal. The propagation features with different splitting ratios have been simulated by the beam propagation method. Two splitters with beam division performances of 2 x 2 output have been further demonstrated through an end-face coupling system and investigated with a laser beam at 532 nm wavelength, which have achieved two splitting ratios including 73 %: 12.5 %: 12.5 %: 2 % with a deviation less than 3 %, and 25 %: 25 %: 25 %: 25 % with a deviation less than 5 %. These performances with high consistency manifest that these devices can emerge into kinds of 3D optical applications with the ability of 3D spatial optical signal processing. Moreover, our work further proves that femtosecond laser direct writing technology presents absolute advantages in processing 3D micro-nano devices in transparent materials for on-chip integration.