Wavelength-tunable lasers have important applications, particularly in laser spectroscopy, life sciences, environmental monitoring, and information processing. Ultrashort laser pulses produced by mode-locking can be widely used in nonlinear frequency conversion, medical diagnosis, high-speed electro-optic sampling, and laser micro/nano fabrication. Wavelength-tunable mode-locked lasers combine the advantages of these two types of lasers and enable the development of multiple applications such as fluorescence excitation, laser spectroscopy, optical frequency combs, and super-resolution microscopic imaging. External-cavity surface-emitting lasers possess the advantages of both solid-state disk lasers and surface-emitting semiconductor lasers and can simultaneously produce high power with good beam quality. In this study, a wavelength-tunable Kerr mode-locked external-cavity surface-emitting laser is proposed. Wavelength-tuning and mode-locking are realized simultaneously in a linear cavity, and the output performance of the proposed approach is experimentally studied. In addition, relevant physical mechanisms involving wavelength tuning and Kerr mode locking are analyzed to understand the experimental results.

Using the nonlinear Kerr effect in a semiconductor gain medium and a soft aperture formed by the overlap between the pump and laser spots on the gain chip, an equivalent saturable absorber can be built into an optically pumped external-cavity surface-emitting laser. The Kerr effect forms an equivalent lens to focus the laser beam, and the soft aperture limits the transverse dimension of the laser. Their combined action, namely saturable absorption, involves selecting a pulse with sufficient strength from the intracavity noises, amplifying and shaping the selected pulse in the round-trip process, and finally maintaining a stable oscillation in the cavity and a steady output pulse train, thereby realizing continuous-wave mode-locking. Wavelength tuning of the laser was achieved by inserting a birefringent filter into the resonator. The birefringent filter was placed at the Brewster angle, and its optical axis was parallel to the surface of the filter. When the birefringent filter was rotated around the normal to the surface, the transmission wavelength allowed by the filter differed according to the tuning angle, thereby effectively realizing the tuning of the laser wavelength. A plane-concave reflector (radius of curvature: 150 mm) was used as the output coupler, and a 30 W fiber-coupled output semiconductor laser was employed as the pump source.

In the experiment, the length of the resonator is selected as approximately 135 mm for the purpose of mode-matching between the spots of the pump and laser. After the laser output stabilizes, continually increasing the pump power and carefully tuning the resonator can produce continuous-wave mode-locking. When the pump power is 5.5 W, steady Kerr mode-locking with a repetition rate of 1.1 GHz, pulse width of 4.3 ps (Fig. 4) and spectral width of 1.25 nm is achieved (Fig. 5). The time-bandwidth-product of the mode-locked pulse is 1.68, which is approximately 4 times that of the Fourier-transform limit pulse, indicating that the pulses contain an obvious chirp. We believe that this chirp is caused by the nonlinear refractive index in the semiconductor gain medium, because of which new frequency components are generated during the evolution of the pulse, which results in a large chirp. By inserting a 2 mm thick birefringent filter into the cavity, wavelength tuning from 950 to 979 nm can be achieved, and stable mode-locking can be maintained for a 15 nm wavelength range from 964 to 979 nm (Fig. 6). By changing the thickness of the inserted filter from 2 to 4 mm, the wavelength tuning range can be reduced to 22 nm, and the range for maintaining mode-locking is reduced to 12 nm (Fig. 7). The tuning range reduction of the birefringent filter with increased thickness is determined by its free spectral range.

In summary, we have demonstrated a wavelength-tunable Kerr mode-locked external-cavity surface-emitting laser using a simple linear cavity. The mode-locked pulse repetition rate is 1.1 GHz, and the pulse width is 4.3 ps. When a 2 mm thick birefringent filter is inserted into the cavity, a wavelength tuning range of 30 nm is achieved, the maximum average output power is 129 mW, and stable continuous-wave mode-locking can be maintained in the 15 nm wavelength range of 964-979 nm. When the thickness of the birefringent filter is increased to 4 mm, the wavelength-tuning range is reduced to 22 nm, the maximum output power is 166 mW, and the stable continuous-wave mode-locking range is 12 nm. Wavelength-tunable Kerr mode-locked VECSELs possess a compact structure and enable stable mode-locking with large wavelength tuning range. They have potential applications in laser spectroscopy, laser confocal microscopy, coherent optical communication, and multiple other fields.