Abstract
The methods of near diffraction free (ndf) light beams generation in lasers are paid attention in a view of many interesting applications. An annular mirror [1], an intracavity axicon [2], or a diffractive mirror [3] were used for ndf Bessel-Gauss beams formation in argon and Nd:YAG lasers [1–3]. The present work offers a scheme where a simple plano-convex intracavity lens with a strong spherical aberration was used in Yb:YAG laser for the generation of ndf beams. A 9.8% at. Yb:YAG ceramics plane-parallel plate (9×11×1.1 mm) mounted on a copper cooler was end-pumped by a cw fiber-coupled laser diode at room temperature. The pump radiation (power 0.1–1.5W) was focused into the plate to a circular spot with a bell-shaped intensity profile of ≈100μm halfwidth One of the planar surfaces of the Yb-doped plate was high-reflection (HR) coated for the wavelength λ ≈1030nm and anti-reflection (AR) - for the pump light. The other surface was AR coated for λ ≈1030nm. The resonator was formed by the HR surface of the plate and the plane output coupler with the reflectance ≈ 95%. The length of the resonator, L was varied from 26 to 66cm. A glass lens of 25mm diameter without any coating and with the focus (for paraxial rays) f ≈ 25mm was placed inside the resonator with its spherical surface being turned towards the Yb-doped plate. The spacing between the lens apex and HR mirror was d ≤ f. The lens could be moved along the resonator axis. The Yb-laser operated in a cw regime (power 1–30mW) at the transition ≈ 1030nm and emitted multiringed beams with diameters up to 11mm. Different beam intensity profiles (depending on d and L) were registered. At d ≈ f the generation of several concentric rings with a deep and narrow (200–400 μm) minimum at the beam axis was observed. Under shifting the lens towards the HR mirror, one could observe other multiringed patterns alternating from a central minimum to a narrow central maximum. The comparison of the images obtained with the data taken from literature [1–4] allows one to classify the beams as Bessel-Gauss modes of zero and higher orders. Modes of Laguerre-Gauss (LGp1) type [4–6] were observed at d ≈ f as well. The angular divergence of radiation estimated using the central peak or minimum widths in the far field has yielded 2–4 × 10−4 rad, while the diffraction limit, associated with these widths is 3–6 × 10−3 rad. Fig. 1 illustrates output beam images monitored by a CCD-camera.
© 2009 IEEE
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