The “perfect” vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the Bessel-Gauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.3–1.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied non-collinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric down-conversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walk-off effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.
We report on the development of a high-power, high-repetition-rate, fiber laser based source of ultrafast ultraviolet (UV) radiation. Using single-pass second-harmonic generation and subsequent sum-frequency generation (SFG) of an ultrafast ytterbium fiber at 1064 nm in 1.2 and 5 mm long bismuth triborate (BIBO) crystals, respectively, we have generated UV output power as high as 1.06 W at 355 nm with single-pass near-infrared-to-UV conversion efficiency of ∼22%. The source has output pulses of temporal and spectral widths of ∼576 fs and 1.6 nm, respectively, at 78 MHz repetition rate. For given crystals and laser parameters, we have experimentally verified that the optimum conversion efficiency of the SFG process requires interacting pump beams to have the same confocal parameters. We also present a systematic study on the power ratio of pump beams influencing the overall conversion of the UV radiation. The UV source has a peak-to-peak short-term power fluctuation of <2.2%, with a power drift of 0.76%/h associated to different loss mechanisms of the BIBO crystal at UV wavelengths. At tight focusing, the BIBO crystal has a broad angular acceptance bandwidth of (∼2 mrad·cm) for SFG of the femtosecond laser.
We report on efficient nonlinear generation of ultrafast, higher order "perfect" vortices at the green wavelength. Based on Fourier transformation of the higher order Bessel-Gauss (BG) beam generated through the combination of the spiral phase plate and axicon, we have transformed the Gaussian beam of the ultrafast Yb-fiber laser at 1060 nm into perfect vortices of power 4.4 W and order up to 6. Using single-pass second-harmonic generation (SHG) of such vortices in 5 mm long chirped MgO-doped, periodically poled congruent LiNbO3 crystal, we have generated perfect vortices at green wavelength (530 nm) with output power of 1.2 W and vortex order up to 12 at a single-pass conversion efficiency of 27%, independent of the orders. This is the highest single-pass SHG efficiency of any optical beams other than Gaussian beams. Unlike the disintegration of higher order vortices due to spatial walk-off effect in birefringent crystals, here, the use of the quasi-phase-matching process enables generation of high-quality vortices, even at higher orders. The green perfect vortices of all orders have temporal and spectral widths of 507 fs and 1.9 nm, respectively, corresponding to a time-bandwidth product of 1.02.
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