Mode-locked fiber lasers are extensively employed in a variety of fields, such as ultrafast spectroscopy, optical communication, precision machining, precision metrology, and biomedical science, because of their compact design, narrow pulse duration, high peak power, and excellent beam quality. Saturable absorbers with desirable nonlinear optical properties are essential to achieve stable mode-locking operation. Graphene oxide (GO) has several advantages, including a low saturation intensity, substantial modulation depth, high damage threshold, and rapid recovery time. Additionally, gold nanorods (GNRs) have a high third-order nonlinear coefficient and an ultrafast response time. Hence, they are exceptionally promising for applications in both saturable absorption and reverse saturable absorption. The synergistic integration of GO and GNRs can amalgamate the strengths of these two materials, producing state-of-the-art saturable absorbers with enhanced performance. To meet the experimental teaching requirements of the course “Nonlinear Optics: Principles and Applications,” with a focus on the nonlinear saturable absorption effects of materials and their applications in fiber lasers, we propose a comprehensive experiment on high-order harmonic mode-locked fiber lasers based on a graphene oxide–gold nanorod (GO-GNRs) composite material.

In this study, the GO was synthesized from natural graphite powder by a modified Hummers method, while the GNRs were fabricated through a seed-mediated growth technique. Subsequently, the GO-GNRs composite was fabricated by an ex situ hybridization method, ensuring a uniform and stable combination of these two components. The GO-GNRs were characterized and analyzed to evaluate their material properties and saturable absorption behavior. To fabricate a fiber device, the GO-GNRs were deposited onto the cross-section of a D-shaped optical fiber to serve as a saturable absorber (SA). Thereafter, the fabricated GO-GNRs were integrated into an Er³⁺-doped fiber laser cavity, enabling ultrashort pulse generation via their SA property.

By adjusting the pump power and polarization controller (PC), the fundamental mode-locked pulse was obtained with a central wavelength of 1559.65 nm and a repetition rate of approximately 20.5557 MHz. The mode-locking operation exhibited excellent long-term stability. When the pump power was increased to 600 mW, the 34^th-order harmonic mode-locked pulse was obtained by suitably adjusting the PC orientation, corresponding to a repetition frequency of 699.301 MHz and a signal-to-noise ratio (SNR) of 68.2 dB. The results demonstrated that GO-GNRs effectively enhanced nonlinear optical modulation, promoting the formation and stabilization of high-order harmonic mode-locking.

This comprehensive experiment effectively enhances students’ abilities to apply theoretical knowledge to practical analysis and operations. Through the synthesis and characterization of low-dimensional materials, students can gain practical experience in nanomaterial fabrication and optical measurement techniques. In the mode-locked Er³⁺-doped fiber laser experiment, students can understand the working principles, structural design, and output characteristics of fiber lasers. By combining theoretical teaching with experiment, this work stimulates students’ research interest, broadens their innovative thinking, and strengthens their ability to solve complex problems independently.