Single-frequency and Dual-wavelength Operation of Vertical-external-cavity Surface-emitting Lasers
Vertical-external-cavity surface-emitting lasers (VECSELs), also referred to as semiconductor disk lasers (SDLs), were invented in the mid-1990s by combining the gain media of semiconductor lasers with the geometry concepts of solid-state disk lasers. After two decades of research and development, t...
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|Vertical-external-cavity surface-emitting lasers (VECSELs), also referred to as semiconductor disk lasers (SDLs), were invented in the mid-1990s by combining the gain media of semiconductor lasers with the geometry concepts of solid-state disk lasers. After two decades of research and development, this kind of laser offers a high-power output with excellent beam quality at a wavelength which can be tailored by semiconductor bandgap engineering. Moreover, the flexible external cavity of VECSELs allows for the utilization of intracavity elements and saturable absorbers. This feature provides the possibility to operate VECSELs under specific function modes such as single-frequency operation, dual-wavelength operation, and mode-locking.
This work focuses on the experimental study and development of single-frequency as well as dual-wavelength VECSELs. Additionally, an important factor, namely the spectral detuning of VECSELs is discussed and its impact on the performance of the device is experimentally demonstrated.
In order to achieve single-frequency operation, the VECSEL is allowed to operate only on fundamental transverse mode, single longitudinal mode, and single polarization mode. Therefore, to suppress the undesired modes, losses are required to a certain degree. However, this makes the realization of a high-power output challenging, where an overall high gain and a low loss level for the laser mode are favorable. In this work, a high-power single-frequency VECSEL is implemented by balancing the gain and the losses of the laser in combination with frequency stabilization methods. The maximum passively stabilized single-frequency output power from this device reaches 23.6 W, which is to this date the highest power among all single-frequency semiconductor lasers. The major noise sources are identified by analyzing the laser linewidth with respect to the sampling time. In order to further stabilize the single-frequency VECSEL, both passive and active frequency stabilization techniques are applied.
In contrast to a single-frequency laser, laser emission from dual-wavelength VECSELs contains two clusters of multiple longitudinal modes, which can be used for intracavity difference-frequency generation (DFG). For instance, Scheller et al. presented a room-temperature terahertz source based on a dual-wavelength VECSEL. However, both the achievable intracavity power and wavelength spacing of the two colors are limited by the single-chip design. Here, an alternative approach is demonstrated in which two different VECSEL chips are serially connected in one resonant cavity.In this way, the gain of both chips are combined, which enables dual-wavelength operation with over 600 W intracavity power at a wavelength spacing of 10 nm. The wavelength spacing can be flexibly altered by employing different chip sets and/or intracavity filters. Furthermore, to complement the characterization of the existing terahertz-emitting VECSELs, the beam quality of the terahertz signal is investigated. According to the ISO standard, the deduced M2-factors for the x- and y-axis amount to 1.41 and 1.72, respectively, which confirm the high quality of the terahertz beam emitted from this intracavity-DFG-based source.
In an additional effort to optimize the performance of VECSELs, which strongly depends on the thermal management, the chip design and its quality, the spectral detuning of VECSELs is targeted. The detuning of VECSELs is defined as the wavelength difference between the material gain and the longitudinal confinement factor at room temperature. Although the detuning is a key factor regarding the output power, threshold, and emission wavelength, it is difficult to conduct experimental studies while excluding the influence of other parameters. In this work, the cavity angle of a V-shaped cavity is varied to change the detuning of a VECSEL chip. Then the impact of different detunings on the performance of the device is demonstrated: By changing the detuning from -37 to -20 nm, an increment of the maximum output power by 70% is observed, while the threshold pump power is modified by a factor of four. Moreover, the wavelength tunability of the VECSEL can be greatly enhanced by the modification of the cavity angle, which is practical for applications that require additional wavelength accessibility.