Janus OPO generates high performance LG laser beams with color adjustment

The Laguerre-Goss (LG) mode is a light wave that can carry the external torque of a photon as it moves through space. They are useful in many fields, from optical communications to super-resolution imaging. Advanced developments in these and other applications require reliable and color-adjustable LG-mode laser sources that do not yet exist.

The optical parametric oscillator (OPO) is a device that can produce wavelength tunable laser beams, so it has been used to achieve color tunable LG laser sources - usually in one of two ways. One way is to change the regular beam to an LG beam using a phase component outside the OPO, but this makes the LG beam less pure. Another approach is to utilize the higher order resonator mode so that it can create the LG directly at the source, but this is always in the works.

According to Advanced Photonics Nexus, a team from Nanjing University and Sun Yat-sen University recently developed a double-sided "Janus" OPO scheme for generating a high-efficiency, high-purity wideband LG mode with adjustable topological charges. The Janus resonator consists of two cavity mirrors, a periodically polarized lithium niobate crystal, a Faraday rotator, a quarter wave plate, and a vector vortex wave plate. Unlike previous in-cavity mode conversion schemes based only on phase and polarization self-reproduction, the Janus OPO introduces an additional imaging system in the resonator to assist the self-reproduction of complex wavefronts, greatly improving the performance of the LG OPO.

The Janus cavity pattern consists of two different modes that intersect. The mode of the front end of the resonator is similar to the Gaussian mode, which can better match the Gaussian pump light to obtain high gain. At the output end, the cavity mode gradually and steadily evolves into the standard LG mode, which guarantees the high purity LG beam output and effectively reduces the diffraction loss. On the other hand, the reconstruction of intensity distribution during the imaging process actively forms a high purity LG mode instead of passive mode filtering, which further reduces the resonator loss.

According to corresponding author Yong Zhang, professor of physics at Nanjing University, "The Janus OPO greatly reduces the loss of the resonator and improves the efficiency and purity of the output LG beam through the imaging design. The output LG mode has a tunable wavelength between 1.5μm and 1.6μm, a conversion efficiency of more than 15%, a controllable topological charge of up to 4, and a mode purity of up to 97%. Zhang points out: "The efficiency of OPOs can be further improved by dual-channel pumping light, and the range of the output LG beam has the potential to be extended to visible and ultraviolet light, providing a powerful tool for exploring the interactions between LG beams and matter for potential applications such as ultra-resolution imaging based on stimulated emission loss (STED) microscopy and precise rotational sensing.

According to Dunzhao Wei, associate professor at the School of Physics at Sun Yat-sen University and lead author of the report, "Janus OPO's scheme can be further extended to vector beam output and entangled LG photon production, directions that will play an important role in fields such as atomic system spin-orbit interactions, laser manufacturing and highly entangled quantum sources.

Source: Laser Net