Compact high-efficiency four-mode vortex beam generator within the telecom C-band

Vortex beams carrying orbital angular momentum have attracted a great deal of attention over the past few years. An integrated vortex beam generator with high efficiency is desirable for wide-ranging applications. Here we demonstrate a highly efficient silicon photonic integrated vortex beam generator based on superposed holographic fork gratings. A metal mirror is used to enhance emission efficiency by reflecting the power leaking down to the substrate back to air. Experimental characterization confirms that the emission efficiency of the generator increases by $\,\sim{\rm dB}$∼dB. Moreover, the present device shows preferable features of broadband, polarization diversity, and compact footprint.