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We report on a simple experimental scheme to generate and control the orbital angular momentum (OAM) spectrum of the asymmetric vortex beams in a nonlinear frequency conversion process. Using a spiral phase plate (SPP) and adjusting the transverse shift of the SPP with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order l into an asymmetric vortex beam of measured broad spectrum of OAM modes of orders l, l − 1, l − 2, …, 0 (Gaussian mode). While the position of the SPP determines the distribution of the OAM modes, we have also observed that the modal distribution of the vortex beam changes with the shift of the SPP of all orders and finally results in a Gaussian beam (l =0). Using single-pass frequency doubling of the asymmetric vortices, we have transferred the pump OAM spectra, l, l − 1, l − 2, …, 0, into the broad spectra of higher order OAM modes, 2l, 2l − 1, 2l − 2, …, 0 at green wavelength, owing to OAM conservation in nonlinear processes. We also observed an increase in single-pass conversion efficiency with the increase in asymmetry of the pump vortices producing a higher power vortex beam of mixed OAM modes at a new wavelength than that of the pure OAM mode. [ABSTRACT FROM AUTHOR]

We report on linear and nonlinear generation of ultrafast hollow Gaussian beams (HGBs). Using only two spiral phase plates (SPPs) having phase variation corresponding to vortex orders, l=1 and 2, and an experimental scheme, we have generated high power, ultrafast HGBs of orders up to 3 at 1064 nm. Based on single-pass, frequency doubling of the HGBs in a 5mm long, MgO doped, periodically poled LiNbO3 (MgO:PPLN) crystal, we have produced HGBs of average output power in excess of 250 mW at 532 nm and order as high as 6. Experimentally, we verified that the frequency doubled HGBs have orders twice those of the pump HGBs. Like the Gaussian beams, the HGBs of all orders have an optimum focusing condition for the highest conversion efficiency. On the contrary to previous reports, we observed that the propagation of the vortex beam of order, l, through a SPP corresponding to the vortex order of, -l, results in HGBs of the same order, jlj. [ABSTRACT FROM AUTHOR]

We report on a simple and compact experimental scheme to generate high-power, ultrafast, higher-order vortex-array beams. Simply by using a dielectric microlens-array (MLA) and a plano-convex lens, we have generated array-beams carrying the spatial property of the input beam. Considering the MLA as a 2D sinusoidal phase-grating, we have numerically calculated the intensity pattern of the array-beams in close agreement with the experimental results. Using vortex beams of order as high as l = 6, we have generated vortex array-beam with individual vortices of orders up to l = 6. We have also theoretically derived the parameters controlling the intensity pattern, size, and the array-pitch and verified experimentally. The single-pass frequency-doubling of vortex-array at 1064 nm in a 1.2 mm long BiBO crystal produced green vortex-array of order, lsh = 12, twice the order of pump beam. Using lenses of different focal lengths, we have observed the vortex-arrays of all orders to follow a focusing dependent conversion similar to the Gaussian beam. The maximum power of the green vortex-array is measured to be 138 mW at a single-pass efficiency as high as ~3.65%. This generic experimental scheme can be used to generate the array beams of desired spatial intensity profile across a wide wavelength range by simply changing the spatial profile of the input beam. [ABSTRACT FROM AUTHOR]