Extension of laser beam structures promises new laser appli­cations. Exploration of how beam structures change during nonlinear frequency conver­sion processes has drawn increasing interest in recent years. Nonlinear conversion is an excellent route for structured beam generation and represents a growing, hybrid field for researchers in nonlinear optics and laser techno­logy, as well as the emerging area of light-field regulation technology.

For structured beam genera­tion and nonlinear frequency conversion, researchers have considered both intra­cavity oscillation and external cavity spatial modulation. To achieve flexible outputs, spatial light modulators can be used to obtain structured beams both inside and outside the laser cavity. But this is an indirect, ineffi­cient method. Intracavity nonlinear frequency generation of structured beams offers a direct, efficient method that has only rarely been inves­tigated, until recently. Inside a laser cavity, transverse mode locking (TML) enables the direct generation of the vortex beams or optical vortices from a laser cavity.

It is known that both solid-state microchip lasers and VCSELs can produce quite similar outputs of TML beam patterns under large Fresnel number pumping conditions. The complex transverse patterns formed by the TML effect, commonly composed of different basic modes with different weight coeffi­cients and different locking phases, make for abundant spatial infor­mation in fundamental frequency modes. Nonlinear frequency conversion of these directly generated TML beams is of great interest, but not yet well studied. Now, researchers from the Beijing Institute of Techno­logy, Tsinghua University, and Arizona State University recently inves­tigated intra­cavity second harmonic generation (SHG) of various passively Q-switched laser beams in TML states.

They analyzed the electrical field trans­formation and propa­gation principle of the SHG of TML mode, precisely predicting the complex far-field beam patterns of the SHG beam, funda­mental frequency, and transverse modes. Structured TML beams and their SHG beams are generated simul­taneously by a sandwich-like microchip laser cavity that is passively Q-switched. The team observed many rare SHG far-field beam patterns and their experi­ments showed good agreement with the simu­lations.

The study shows that para­metric variation – especially the phase difference of funda­mental frequency modes – for the TML modes greatly changes the far-field beam patterns of the SHG beam. Generated SHG beam patterns vary with the propagation, from the beam waist to several times the Rayleigh length, and then they remain stable into the far field. The SHG beam patterns were observed to have more obvious structural charac­teristics than those of the fundamental frequency beam.

The generation of TML laser modes, especially the fre­quency-converted ones, opens intri­guing new avenues for obtaining various structured beams with a direct, intracavity method. This work will help advance future appli­cations of structured beams, parti­cularly in optical 3D printing, optical trapping of particles, and free-space optical communi­cation areas. (Source: SPIE)

Reference: Z. Zhang et al.: Second harmonic generation of laser beams in transverse mode locking states, Adv. Phot. 4, 026002 (2022); DOI: 10.1117/1.AP.4.2.026002

Link: School of Optics and Photonics, Beijing Institute of Technology, Beijing, China