Wavelength conversion of vortex laser beams using stimulated Raman scattering
Vortex laser beams have garnered significant interest in the last 20 years or so, due to their applications in optical trapping and tweezing,1, 2 microscopy,3 micro- and nanomachining,4 and most recently telecommunications.5 These lasers may be described as having an annular beam profile and orbital angular momentum,1–3 and are often produced by converting a Gaussian beam to a so-called Laguerre-Gaussian (LG) beam using extracavity methods.6–8 Another approach that has proven effective at generating high-beam-quality vortex laser output is to force the laser to oscillate on an LG mode instead of a Gaussian mode. This has been demonstrated using a range of methods, including pumping with an annular-shaped beam,9 employing thermal lensing,10 or using a defect spot on one of the resonator mirrors.11,12
It is interesting that for all the novel applications proposed for vortex lasers, the frequency conversion of their output remains relatively unexamined. The studies that do exist are limited to extracavity conversion methods such as second harmonic generation (SHG)13,14 and optical parametric conversion (optical parametric generators or optical parametric oscillators).15–17 Certainly, the ability to improve the wavelength versatility of these sources would go some way to further diversifying their range of applications. A well-demonstrated and efficient method of laser beam wavelength conversion that can be implemented both intracavity and extracavity is stimulated Raman scattering (SRS). SRS in gases18 and crystalline materials19 has been effective at improving the wavelength diversity of a range of lasers, primarily through very efficient conversion of low-order Gaussian TEM00 modes. One of the goals of our work is to apply the process of SRS to vortex lasers to further increase their wavelength versatility.
In our work, we have demonstrated direct conversion of a near-IR vortex laser beam oscillating at 1063 and 1173nm using intracavity SRS.20 The laser system comprises a self-Raman laser cavity using a 20mm-long, 0.3 atomic% Nd:GdVO4 (neodymium-doped gadolinium vanadate) a-cut crystal that has a high-reflecting layer (M1) applied to its input surface, and a 250mm-radius-of-curvature, high-reflecting (at 1063 and 1173nm) output coupler (M2): see Figure 1. The output coupler was laser-micromachined to remove a 40μm-diameter area of the high-reflecting coating to produce a region of low reflectivity (defect region). By positioning the output coupler such that the defect region is centered with the cavity mode, low-order Gaussian modes are suppressed and the cavity oscillates on an LG01 mode.
The power-scaling properties of the system are shown in Figure 2. We found that fundamental and Stokes field oscillation could be achieved for incident pump powers of 100mW and 2W, respectively. The maximum fundamental and Stokes field powers that could be achieved from the system were 400 and 380mW, respectively, for an incident diode pump power of 6.8W. The maximum output power was limited by deterioration of the spatial profile of the Stokes field, which evolved to resemble a higher-order LG mode or a combination of these modes. Figure 2 (inset) shows the spatial forms of the fundamental and Stokes fields, along with their interference patterns when operating just above each field's respective threshold.
The spatial forms of the fundamental and Stokes fields are very uniform, and the interferogram shows the presence of a phase singularity for each field. These results are the first demonstration of SRS conversion of a vortex laser beam, and show that the SRS process is one means by which a frequency-converted laser beam can retain the same topological charge as the original beam (conversion in an optical parametric oscillator being the only other demonstration of this effect). Because the size of the central dark spot in the annular profile of a vortex beam increases with its topological charge, retaining the charge under frequency conversion is important for applications such as super-resolution microscopy,3 where small central dark spots are desirable. Future studies will investigate the simultaneous application of intracavity SHG with intracavity SRS to generate vortex laser emission in the visible wavelength range from this all-solid-state laser platform.
Department of Physics and Astronomy