Different applications require lasers of different wavelengths, and the Raman laser is one of the effective methods to expand spectral range of lasers. Raman lasers have advantages of high conversion efficiency, excellent beam quality, excellent scalability and wide range coverage etc. However, the cumbersome size of Raman cell (especially the long length of Raman cell) deteriorates the application of Raman laser. To reduce the length of Raman cell, a short-focus lens is required, and this would lead laser-induced breakdown (LIB).

To realize miniaturization of Raman laser devices while suppressing LIB, this work proposed a method to modulate the pump laser into a Bessel beam to achieve stimulated Raman frequency conversion using an axicon. The goal is to achieve high photon conversion efficiency (PCE) and beam quality in a compact system.

By comparing the intensity at focus and the depth of focus of an f = 0.5 m focal lens and an axicon, an axicon with a bottle angle of 2° could effectively reduce laser intensity at focus and increase the depth of focus. In this work, a pulsed 1064 nm laser was used as pump source, pressurized methane was used as Raman medium, and an axicon with a bottle angle of 2° was used to focus pump laser. Pressure of methane, pump laser divergence angles and diameter of pump beam were optimized to achieve the maximum conversion efficiency.

In 3.5 MPa methane and 366 mJ energy of 1064 nm pump laser, 128 mJ forward Raman laser at 1543 nm was generated; the corresponding photon conversion efficiency was 50.7%, and higher output energy and conversion efficiency were expected under higher pressure and at higher pump energy. By blocking the central rounded apex of the axicon, the Raman laser pulse energy of 97 mJ can still be retained with the beam quality β=2.19. An experiment verified that the Raman cell can be designed to be 0.4 m without damaging the window. Based on the results of multiple experiments, it can be inferred that the Raman cell can be further shortened to 0.3 m without sacrificing the conversion efficiency. By axially translating the axicon within an extended cell, the forward/backward Stokes light ratio became tunable.

This study demonstrates the viability of Bessel beams for compact, high-efficiency gaseous Raman lasers. The conical wavefront pumping strategy mitigates LIB risks and enables system miniaturization, offering a promising pathway for practical applications.