  A new method to generate and control the amplitude and phase distributions of
a optical vortex beam is proposed. By introducing a holographic grating on top
of the dielectric waveguide, the free space vortex beam and the in-plane
guiding wave can be converted to each other. This microscale holographic
grating is very robust against the variation of geometry parameters. The
designed vortex beam generator can produce the target beam with a fidelity up
to 0.93, and the working bandwidth is about 175 nm with the fidelity larger
than 0.80. In addition, a multiple generator composed of two holographic
gratings on two parallel waveguides are studied, which can perform an effective
and flexible modulation on the vortex beam by controlling the phase of the
input light. Our work opens a new avenue towards the integrated OAM devices
with multiple degrees of optical freedom, which can be used for optical
tweezers, micronano imaging, information processing, and so on.
