Vortex beams carrying orbital angular momentum have been widely applied in optical manipulations, optical micromachining, and high-capacity optical communications. Since plenty of the useful applications of such OAM, measuring the OAM of vortex beams becomes a task of great significance. There have been many methods to measure the OAM of vortex beams based on interference method, diffraction method, geometric coordinate transformation. However, the near-infrared detectors based on InGaAs have lots of limitations such as low pixel sensitivity, high noise, and stringent cooling. On the contrary, silicon-based detectors have good performance in visible wavelength. Recently, researchers have reported a practical solution, which is online detection of near-infrared vortices using nonlinear photonic crystals (NPCs). However, this method is dependent on the structure of the NPCs, which needs complex fabrication. And it is phase-mismatched in the longitudinal direction based on the Raman-Nath diffraction. Hence, a laudable goal would be to develop a flexible and efficient method to measure the near-infrared vortex beams.

In this letter, we demonstrate a highly efficient detection method to realize measuring the near-infrared vortex modes including the sign and the magnitude through second harmonic generation (SHG). In our experiment, the hyperbolic gradually changing period pure-phase grating is imprinted into the homogeneous nonlinear medium by a spatial light modulator, whose function is to converse the mode between Laguerre-Gaussian mode and Hermite-Gaussian mode. The near-infrared OAM beam is incident on the nonlinear medium, respectively. The topological charge of the vortex beam can be judged from the second harmonic (SH) HG-like diffraction intensity patterns captured by silicon-based charge-coupled device in visible waveband. At last, we calculate and measure the nonlinear conversion efficiency of the SHG process. It has a relatively high conversion efficiency based on the phase-matched SHG process. The proposed method is a convenient and flexible way to measure the different OAM of vortex beams, which may feature potential applications in all kinds of circumstances that vortex modes involve.

The experimental results of the SH intensity distribution patterns. (a1)-(a8) and (b1)-(b8) are the measuring OAM results of the SH intensity distribution for FF OAM wave with TCs from l=+1 to l=+8 and l=-1 to l=-8, respectively.

This research is published by “Yujia Wu, Haigang Liu, and Xianfeng Chen, Highly efficient detection of near-infrared optical vortex modes with frequency upconversion, Optics Letters, 47(10), 2474-2477 (2022)”.

Link: https://doi.org/10.1364/OL.457462