## 1 Introduction  
Laser-induced periodic surface structures (LIPSS, which is referred to as ripples) have been extensively studied for various materials, including metals [1; 2; 3; 4; 5; 6; 7], semiconductors [8; 9], and dielectrics [10; 11], because of their promising applications in colorization [12], solar cells [13], surface wetting [14], and cell growth of implants [15]. For the aforementioned applications, uniform rippled structures with a well-defined large area are required. However, the efficient fabrication of uniform rippled structures with large areas is a challenge, and this limits its industrial applications. Compared with other physical and chemical methods for the fabrication of uniform rippled structures with large areas [16], directly inducing rippled structures by using a femtosecond laser is a relatively controllable approach. Ripples with periods close to the central wavelength of incident lasers are considered typical ripples, which are caused by the interference between incident laser waves and surface-scatteredwaves [17; 18]. Ripples with periods significantly smaller than the incident laser wavelengths are termed subwavelength ripples. Some explanations of formation mechanisms of subwavelength ripples have been proposed, including self-organization [19; 20], second harmonic generation [21], Coulomb explosion [22], and interaction between the incident laser and surface plasmon [11; 23; 24]. Furthermore, processing parameters considerably affect the formation and morphology of ripples, such as pulse durations [25; 26], polarization [27], pulse fluence [28], incidence angle [29], pulse number [30; 31], central wavelength [32], pulse delay [33], and processing environments [34; 35; 36]. For the fabrication of uniform rippled structures with a large area, the scanning speed and the scanning interval are crucial for the surface morphologies of ripple structures [16]. In addition, the morphologies of ripple structures of multiple-shot-ablated regions considerably affect the processing efficiency and precision of uniform rippled structures with large areas because incubation effects are crucial during multiple-shot ablation [37]. For metals, the incubation effect is attributed to the accumulation of laser-induced chemicals, structural changes in the material, and/or plastic deformation of the surface [37]. Such defects, which are generated by the first impinging pulses, facilitate the absorption of the subsequent laser pulses, thus enhancing the elimination of ablation and materials [37]. Therefore, during multiple-shot ablation, it is essential to study the effects of the pulse number and irradiation modes on the surface morphologies of rippled structures with the consideration of the incubation effect.  
In this study, the effects of the pulse number and irradiation modes on surface morphologies of rippled structures, including ablation areas and periods of subwavelength ripples, on the titanium surface are experimentally examined during the multishot femtosecond induced ripples. The evolution of ablation craters and subwavelength ripples with the increasing pulse number is investigated. The interference between a surface plasmon field and laser field plays an important role for the formation of subwavelength ripples. For investigating incubation effects during a multishot regime, different laser pulse irradiation modes are designed. The samples which are first irradiated using several shots and then irradiated using the remaining shots have larger ablation areas and finer rippled structures. Therefore, according to different requirements of rippled structure areas and periods, the uniform rippled structures with a large area can be efficiently fabricated and adjusted using the designed laser pulse modes and pulse number.