Janus OPO enables color-tunable LG laser beams with superior performance.

Laguerre-Gaussian (LG) modes are a type of light wave that can carry the external torque of photons as they move through space. They are useful in many areas, from optical communications to ultra-high-resolution imaging. Advances in these and other applications require reliable, color-tunable LG-mode laser sources, which do not yet exist.

An optical parametric oscillator (OPO) is a device that can generate a wavelength-tunable laser beam, so it was used to achieve a colorimetrically tunable LG laser source – generally, in one of two ways. One way is to change a normal beam into an LG beam using an off OPO phase component, but this makes the LG beam less pure. The other way is to use a higher order resonator mode so it can generate the LG directly from the source, but that’s a work in progress.

As stated in Advanced Photonics NexusA team from Nanjing University and Sun Yat-Sen University recently developed a two-sided “Janus” OPO scheme for generating high-efficiency, high-purity broadband LG modes with tunable topological charge. A Janus resonator consists of two cavity mirrors, a lithium niobate crystal with a periodic pole, a Faraday rotor, a quarter wave plate, and a vector vortex wave plate.

Different from previous cavity mode conversion schemes that only rely on self-reproduction of phase and polarization, Janus OPO introduces an additional imaging system in the resonator to help self-reproduce the complex wave front, which greatly improves the performance of LG OPO.

The Janus cavity position consists of two different positions that intersect. The position at the front end of the resonator is a Gaussian-like pattern, which can better match the Gaussian pump light to obtain high gain. At the output end, the cavity mode gradually and smoothly develops into standard LG mode, which ensures high purity LG beam output and effectively reduces diffraction loss. On the other hand, the reconstruction of the intensity distribution during the imaging process forms a higher-purity LG mode—instead of passive mode filtering—which reduces resonator losses.

According to corresponding author Yong Zhang, Professor of Physics at Nanjing University, “Janus OPO greatly reduces resonator losses through imaging design and improves the efficiency and purity of the resulting LG beam.” The resulting LG mode features tunable wavelength between 1.5 μm and 1.6 μm, with conversion efficiency above 15%, topological charge controllable up to 4, and mode purity up to 97%.

Zhang notes, “OPO efficiency can be improved by passing the pump light twice, and the wavelength range of the resulting LG beam has the potential to expand into the visible and ultraviolet, providing a powerful tool for probing the interaction between LG beams and matter, for potential applications such as ultra-resolution imaging.” Based on stimulated emission depletion (STED) microscopy and precise spin sensing”.

According to Dunzhao Wei, assistant professor in the School of Physics at Sun Yat-Sen University and first author of the report, “The Janus OPO scheme can be further extended to vector beam output and entangled LG photon generation, trends that will play important roles in areas such as atomic group spin interaction, laser fabrication, and highly entangled quantum sources.”