The high-order Bessel beam (HOBB) is a special kind of vortex beam that carries orbital angular momentum (OAM) and shows non-diffractive and self-healing properties. Thus, HOBBs exhibit great application potential in laser micro/nanofabrication, microparticle manipulation, optical illumination, and nonlinear optics. So far, HOBBs have been generated by several methods, such as traditional optical systems, spatial light modulators, and metasurface. These methods, however, are incapable of achieving both high integrity and high laser-induced damage threshold (LIDT). Based on femtosecond laser-induced birefringent nanograting, a novel method is proposed for the fabrication of integrated optical modulation elements with high LIDT in this work. Multilayer structures with variable phase distribution embedded in silica glass enable the superposition of multiple optical modulation functions for the desired wavelength. For instance, an integrated HOBB generator is fabricated, and the optical modulation characteristics are demonstrated to be consistent with the simulation results. In addition, the LIDT of the fabricated HOBB generator is as high as 28.5 J/cm² (6 ns), which has considerable application potential for high-power laser modulation. Consequently, this method can be applied to the fabrication of further integrated optical elements with a high LIDT.

The laser-induced nanograting is an anisotropic structure with birefringent properties. First, the principle of geometric phase modulation is derived based on the birefringent nanograting. The birefringence characteristics of nanograting, including optical axis direction and phase retardance, can be employed to obtain the geometric phase modulation. Second, how the laser processing parameter affects the birefringence of the laser-induced nanograting in silica glass is explored. Then, two independent geometric phase optical elements (GPOEs), including a spiral phase plate (SPP) and a planar axicon, are fabricated and characterized individually. Within silica glass, a multilayer structure with spatially variable optical axis distribution is created by modifying the optical axis direction and phase retardance of nanograting. Finally, the phases of the two GPOEs are integrated. Two forms of nanograting (SPP and planar axicon) with different optical axis distributions are written in separate depths of silica glass. Thus, the incident Gaussian beam is transformed into a HOBB when passing through the multilayer structure. In order to evaluate the laser damage resistance of the prepared optical element, the LIDT is measured by the standard 1-on-1 method (based on the ISO 21254 LIDT test standard).

When the incident beam is circular polarization, and the retardance of nanograting is half-wave, the output cross-polarized beam will be encoded with a geometric phase change of twice the optical axis angle with high efficiency. The optical axis of the nanograting is perpendicular to the polarization direction of the femtosecond laser (Fig. 3). Therefore, by controlling the polarization direction of the laser, nanograting with spatially variable optical axis directions can be written inside the silica glass. The phase retardance of the nanograting is affected by laser processing parameters. When the laser pulse density is 100 pulse/μm, and the pulse energy is 0.3 μJ, the average retardance of the nanograting is 67.79 ± 1.47 nm at different focusing depths (Fig. 4). In this case, only four layers of the nanograting are required to provide a half-wave retardance of 532 nm. Then, it is demonstrated that the fabricated SPP and planar axicon have excellent optical modulation capability (Fig. 5 and Fig. 6) and that it is feasible to prepare other GPOEs by this method. The interference image shows that the SPP is capable of carrying the OAM of the target value. Over a transmission distance of 2 meters, the planar axicon maintains outstanding non-diffraction properties. After that, the HOBB generated by the integrated element is evaluated (Fig. 7). The light field is typically distributed as a black and hollow ring, with the central ring concentrating the majority of the light intensity. The HOBB carries a topological charge corresponding to its design value of 4. In the non-diffraction distance measurement, the HOBB maintains an almost constant spot size over a transmission distance of 4 meters (Fig. 8). It is noteworthy that the zero-probability LIDT of the prepared optical element is as high as 28.5 J/cm² (6 ns) (Fig. 9), which presents significant benefits in high-power beam shaping applications.

In the present study, a method for fabricating an integrated optical modulation element with a high LIDT is proposed and verified. The nanograting is a subwavelength birefringent structure whose optical axis direction and phase retardance can be modified by the laser polarization direction and processing parameters, respectively. The nanograting with spatially variable optical axis distribution can be written inside the silica glass at different depths. The multilayer cascade structure modulates the phase of the incident beam to generate the target beam. Based on the femtosecond laser-induced nanograting, an integrated HOBB generator with an operating wavelength of 532 nm and topological charge of 4 is fabricated. The test results indicate that the optical field modulation performance of the generator is satisfactory. The HOBB generated by the prepared element carries the specified topological charge and keeps spot size constant over a long non-diffraction transmission distance (4 meters). It is crucial that the LIDT of the prepared optical element is as high as 28.5 J/cm² (6 ns). This method enables the integration of optical elements with distinct functions, providing a novel concept for the integrated preparation of optical elements with high LIDT and complicated optical field modulation.