Objective The invention of semiconductor lasers has brought about a huge impetus to the laser industry. Traditional edge-emitting lasers can work with high efficiency and high average power, but the poor beam quality limits their widespread adoption. The invention of vertical cavity surface emitting lasers (VCSELs) improved the output of a semiconductor laser to a circular spot but with limited output power. Subsequently, vertical external cavity surface emitting lasers (VECSELs) emerged featuring a smaller divergence angle. Still, the power scaling capability and beam quality optimization could not be achieved at the same time due to the inhomogeneous electric current pump. Next came the optically pumped VECSELs. For conventional lasers, pumped laser crystals usually have a thermal lens effect. The range of focal length of thermal lens determines the scope of the stable area when the laser is operating. The gain chip of the vertical cavity surface emitting laser is a semiconductor material, so the focal length of the thermal lens cannot be directly calculated by the crystal thermal lens focal length formula. To evaluate the thermal lens effect of the gain chip, we estimate the thermal focal length of the gain chip. In addition, the tunable output range is an important parameter to characterize the output properties of the laser. In this article, we also propose a method for estimating the angle tuning range of the VECSEL by directly measuring the angle-dependent characteristics of the fluorescence spectrum of the gain chip. Furthermore, we conducted an angle tuning experiment to verify this method.
Methods To measure the thermal focal length of the gain chip, we use the stability conditions of the laser cavity to determine the thermal focal length of the gain chip. During the experiment, we ensure that the thermal rollover phenomenon does not occur and the length of the resonator is fixed. The thermal focal length of the gain chip gradually decreases with increasing pump power until the active resonator meets the stability conditions, and the output of VECSEL plunges. When the cavity length of the resonator is reduced, the output power of the shortened one does not decrease at the same pump power. The thermal focal length range of the gain chip at this pump power can be estimated later by simulation calculations. Additionally, the gain chip is pumped without an output coupler to investigate its angle-dependent characteristics of the fluorescence spectra and the angle tuning properties are further confirmed in a resonator.
Results and Discussions The VECSEL in our experiment employs a plane-concave resonator with a 45 mm cavity length. When the pump power is more than 28.36 W, the output power decreases rapidly with increase in pump power(Fig. 3). There are two possible reasons. The first is that the mode size on the gain chip becomes larger, and the laser output decreases rapidly with the increasing of diffraction loss. The second possible reason is the onset of rollover inside the semiconductor device as the pump power increases, which also leads to a rapid decrease in output power. Compared with another set of experiments, when the cavity length is 40 mm and the pump power is 31.3 W, the output power can reach 3.74 W. There is no similar phenomenon where the output power decreases with the increase of pump power(Fig. 4). Therefore, the possibility of thermal rollover causing the previous VECSEL output power to dip can be ruled out. This decline can be considered as the output power change caused by the variation of the thermal focal length under enhanced pump power. From the experimental and calculation results, it can be inferred that when the pump spot diameter is 380 μm and the pump power is 31.3 W, the thermal focal length of the gain chip used in the experiment is between 45.7 mm and 53.6 mm. Based on the observed results which show that the fluorescence wavelength of the gain chip changes with the observation angle, it is inferred that the tuning range of the VECSEL is about 35 nm(Fig. 6).
Conclusions To obtain high power and high beam quality laser through state-of-the-art semiconductor technology, VECSELs were developed. In this paper, the thermal focal length of a gain chip is estimated by the calculation of a laser resonator combined with experiments. The phenomenon of a fluorescence spectrum of a gain chip that varies with the observation angle is reported. A method is proposed to directly estimate the tuning range of a VECSEL by observing the fluorescence spectra of the gain chip at different angles.
