## 1 Introduction  
Surface morphology is a key factor associated with changes and the control of surface properties of a solid such as mechanical, optical, biological, and chemical properties. Fabricating micro-nano structures is one of the main methods for changing surface morphology [1]. Laser-induced periodic surface structures (LIPSS, also referred to as ripples) are a simple and rapid method of fabricating micro-nano surface structures, when the incident fluence is slightly higher than the ablation threshold of the material [2; 3; 4; 5; 6]. Due to the superior ability to control the surface properties, ripples have been widely used in many fields, including optics [7], optoelectronics [8], micro-nano fluids [9], biomedicine [10; 11; 12; 13] and biological implants [14; 15; 16; 17; 18; 19]. Regarding applications of biological implants, studies have shown that the growth of cells is selective to the surface morphology of materials [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Generally, cells tend to grow along grooves on the surface, and the degree of this trend depends on the depth and width of the groove on the surface. In addition, different cells have different requirements for the surface morphology of an implant. For example, fibroblasts are more oriented on the surface grooves. Hence, it is essential to adjust and control the periodic surface structure. When the incident fluence is slightly higher than the ablation threshold, ripple structures can be observed in various materials, including metals [6; 25], semiconductors [26; 27], and dielectrics [28; 29; 30]. The rippled structure can be divided into two types, i.e., typical ripples and subwavelength ripples. Due to interference between incident laser and surface-scattered waves [31; 32], ripples with the periods close to the wavelength of the incident laser (known as typical ripples) are induced by the laser. With the development of lasers, various new types of ripples have been observed experimentally, with the characteristic of being significantly smaller than the incident wavelength (known as subwavelength ripples). In order to better adjust subwavelength rippled structures, relevant theoretical and experimental studies have been carried out. Many studies have reported on the formation mechanisms of subwavelength ripples, including self-organization [33; 34], second harmonic generation [35], Coulomb explosion [36], and interaction between the incident laser and surface plasmon [37; 38; 39]. However, the interaction between the laser and material is a nonlinear and nonequilibrium dynamic process, which causes some differences between theoretical research and experimental results. In addition, many factors significantly affect the formation and morphology of ripples [30; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49], such as wavelength, pulse duration, number of pulses, pulse fluence, polarization, incidence angle, repetition rate, pulse delay, number of pulse trains, spatial distribution of laser energy, processing environment, processing mode, and processing materials. For the aforementioned application of ripple structures, it is still a significant challenge to efficiently fabricate uniform rippled structures with a large area. During the fabrication of uniform rippled structures with a large area, the scanning speed and the scanning interval are crucial for the surface morphologies of the rippled structures [50]. However, there is still a lack of methods for synthetically regulating the area and period of rippled structures fabricated using a femtosecond laser. In order to adjust the area and period of rippled structures induced by a femtosecond laser, the effects of the processing steps and laser polarization on the surface morphology of rippled structures on a titanium surface are experimentally investigated in this study. The processing sequence is designed, and two series of femtosecond laser pulses irradiate the titanium surface, for example, N = 50(0\({}^{\circ}\)) + 50(90\({}^{\circ}\)).