Highly efficient detection of near-infrared optical vortex modes with frequency upconversion.

Vortex beams carrying orbital angular momentum (OAM) have been widely applied in optical manipulations, optical micromachining, and high-capacity optical communications. Vortex mode detection is very important in various applications. However, the detection of near-infrared vortex modes is still difficult because of the wavelength limitations of the detection device. Here, we present a study on measuring optical near-infrared vortex modes with frequency upconversion, which can convert a near-infrared beam into a visible beam. In our experiment, the optical near-infrared vortex modes can be measured by the number and orientation of the fringes of the second harmonic intensity patterns. The proposed method is a convenient and flexible way to measure the different OAM of vortex beams, which may have potential applications in all kinds of circumstances that vortex modes involve.