Vertical external cavity surface-emitting lasers (VECSELs) combine the advantages of both disc and semiconductor lasers, such as high beam quality, low cost, and compact packaging, making them suitable for a wide range of applications. The outer cavity structure of a VECSEL enables convenient control of the transverse mode size and optical gain region of the laser, resulting in a high-power Gaussian beam near the diffraction limit. This feature significantly improves the efficiency of applications in space laser communication, laser lighting, and beyond. Similar to traditional lasers, VECSELs can also employ acousto-optic or electro-optic modulation technology for space laser communication; however, achieving both high power and high speed simultaneously remains challenging, and the addition of a modulation system compromises compact packaging. Unlike solid gain media, which have excessively long fluorescence lifetimes, the VECSEL gain chip possesses a nanosecond-scale energy-level lifetime, supporting high-speed direct modulation within the cavity. Nevertheless, due to the limitations of current drive circuit performance, direct modulation of this type of laser has not yet been reported, highlighting the need to explore its high-speed modulation characteristics.

In this study, we design a high-current, high-frequency drive circuit to achieve high-power and high-speed direct modulation of high-power pumped laser diodes operating at several megahertz. The frequency response of electro-optical and opto-optical conversion in the modulation system is experimentally analyzed. Based on this, a directly modulated VECSEL system with a double-pump structure is constructed, and the influence of the VECSEL cavity structure on the high-speed direct modulation characteristics of this type of laser is further examined.

First, as shown in Figs. 3 and 4, optimizing the modulation drive circuit enables direct modulation of the high-power laser, enhancing the modulation amplitude of the output pump light signal. With this optimization, the 3 dB response bandwidth achieved by sine wave modulation is approximately 9.5 MHz. Building on this, the VECSEL system is directly modulated using a double-pump structure, and the modulation bandwidth is further improved by adjusting the VECSEL resonator length and the transmittance coefficient of the output mirror to increase cavity losses. As shown in Figs. 7 and 8, a VECSEL laser with an 80 mm cavity length and a 2% transmittance output mirror exhibits optimal modulation bandwidth, reaching approximately 6.3 MHz. With a pump power of 5.27 W and the removal of direct-current bias power, the output modulation laser power reaches approximately 2 W. Due to the low one-way gain of the VECSEL, increasing the output mirror transmittance leads to a rapid reduction in output optical power.

VECSEL technology, with its excellent beam quality and flexible wavelength design capability, holds promising applications in space optical communication. The VECSEL direct modulation system comprises a drive circuit, pump source, gain chip, and laser resonator, with its high-frequency characteristics determined by the module with the lowest upper-frequency response. To simplify the modulation drive circuit design, the laser diode (LD) pump source should operate in high-voltage, low-current mode, with further hardware optimizations achieved through a dual-pump structure to enhance laser modulation power. Addressing baseband signal modulation requirements, this study employs a high-power, high-frequency transistor as the core switching device, achieving an LD pump light modulation frequency of nearly 10 MHz with high modulation depth by optimizing circuit parameters. A VECSEL direct modulation system is developed based on these parameters, and the high-frequency modulation characteristics of the VECSEL laser are studied by adjusting the cavity length and end mirror transmittance. Results indicate that the VECSEL modulation bandwidth is significantly influenced by cavity losses. When the cavity length is 80 mm and the output mirror transmittance is 2%, the system achieves a 3 dB bandwidth of 6.3 MHz. With further advancements in drive circuit performance and selection of LD pump sources with lower capacitance effects, combined with resonator structure optimization, the modulation bandwidth of the VECSEL direct modulation system can potentially exceed 10 MHz. Additionally, integration with VECSEL frequency-doubling blue light technology can extend the system applications to underwater laser communication and other fields.