Introduction to vertical cavity surface emitting semiconductor laser (VCSEL)

Introduction to vertical cavity surface emitting semiconductor laser (VCSEL)
Vertical external cavity surface-emitting lasers were developed in the mid-1990s to overcome a key problem that has plagued the development of traditional semiconductor lasers: how to produce high-power laser outputs with high beam quality in fundamental transverse mode.
Vertical external cavity surface-emitting lasers (Vecsels), also known as semiconductor disc lasers (SDL), are a relatively new member of the laser family. It can design the emission wavelength by changing the material composition and thickness of the quantum well in the semiconductor gain medium, and combined with intracavity frequency doubling can cover a wide wavelength range from ultraviolet to far infrared, achieving high power output while maintaining a low divergence Angle circular symmetric laser beam. The laser resonator is composed of the bottom DBR structure of the gain chip and the external output coupling mirror. This unique external resonator structure allows optical elements to be inserted into the cavity for operations such as frequency doubling, frequency difference, and mode-locking, making VECSEL an ideal laser source for applications ranging from biophotonics, spectroscopy, laser medicine, and laser projection.
The resonator of the VC-surface emitting semiconductor laser is perpendicular to the plane where the active region is located, and its output light is perpendicular to the plane of the active region, as shown in the figure.VCSEL has unique advantages, such as small size, high frequency, good beam quality, large cavity surface damage threshold, and relatively simple production process. It shows excellent performance in the applications of laser display, optical communication and optical clock. However, VCsels cannot obtain high-power lasers above the watt level, so they cannot be used in fields with high power requirements.

The laser resonator of VCSEL is composed of a distributed Bragg reflector (DBR) composed of multi-layer epitaxial structure of semiconductor material on both the upper and lower sides of the active region, which is very different from the laser resonator composed of cleavage plane in EEL. The direction of the VCSEL optical resonator is perpendicular to the chip surface, the laser output is also perpendicular to the chip surface, and the reflectivity of both sides of the DBR is much higher than that of the EEL solution plane.
The length of the laser resonator of VCSEL is generally a few microns, which is much smaller than that of the millimeter resonator of EEL, and the one-way gain obtained by the optical field oscillation in the cavity is low. Although the fundamental transverse mode output can be achieved, the output power can only reach several milliwatts. The cross-section profile of the VCSEL output laser beam is circular, and the divergence Angle is much smaller than that of the edge-emitting laser beam. To achieve high power output of VCSEL, it is necessary to increase the luminous region to provide more gain, and the increase of the luminous region will cause the output laser to become a multi-mode output. At the same time, it is difficult to achieve uniform current injection in a large luminous region, and the uneven current injection will aggravate waste heat accumulation.In short, the VCSEL can output the basic mode circular symmetric spot through reasonable structural design, but the output power is low when the output is single mode.Therefore, multiple VCsels are often integrated into the output mode.