Abstract

We study the relationship between the input phase delays and the output mode orders when using a pixel-array structure fed by multiple single-mode waveguides for tunable orbital-angular-momentum (OAM) beam generation. As an emitter of a free-space OAM beam, the designed structure introduces a transformation function that shapes and coherently combines multiple (e.g., four) equal-amplitude inputs, with the $k$th input carrying a phase delay of $({{k} - 1}){\Delta}{\varphi}$. The simulation results show that (1) the generated OAM order ℓ is dependent on the relative phase delay ${\Delta}{ \varphi}$; (2) the transformation function can be tailored by engineering the structure to support different tunable ranges (e.g., $l = \{- 1\}, \{- 1, + 1\}, \{- 1, 0, + 1\}$, or $\{{-}{2}, - {1}, + {1}, + {2}\}$); and (3) multiple independent coaxial OAM beams can be generated by simultaneously feeding the structure with multiple independent beams, such that each beam has its own ${\Delta}{ \varphi}$ value for the four inputs. Moreover, there is a trade-off between the tunable range and the mode purity, bandwidth, and crosstalk, such that the increase of the tunable range leads to (a) decreased mode purity (from 91% to 75% for $l = - {1}$), (b) decreased 3 dB bandwidth of emission efficiency (from 285 nm for $l = \{- {1}\}$ to 122 nm for $l = \{- {2},\;- {1},\;+ {1},\;+ {2}\}$), and (c) increased crosstalk within the C-band (from ${-}{23.7}$ to ${-}{13.2}\;{\rm dB}$ when the tunable range increases from 2 to 4).

© 2020 Optical Society of America

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