† Corresponding author. E-mail:
Project supported by the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. cstc2015jcyjBX0098), the National Natural Science Foundation of China (Grant No. 61575011), and the Foundation for the Creative Research Groups of Higher Education of Chongqing (Grant No. CXTDX201601016).
High power optically pumped vertical-external-cavity surface-emitting lasers with front and end pump are reported. The gain chip consists of 15 repeats of In0.26GaAs/GaAsP0.02 multiple quantum wells and 30 pairs of Al0.2GaAs/Al0.98GaAs distributed Bragg reflectors. The maximum output power of 3 W, optical-to-optical conversion efficiency of 22.4%, and slope efficiency of 29.8% are obtained with 5-°C heatsink temperature under the front pump, while the maximum output power of 1.1 W, optical-to-optical conversion efficiency of 23.2%, and slope efficiency of 30.8% are reached with 5-°C heatsink temperature under the end pump. Influences of thermal effects on the output power of the laser with front and end pump are discussed.
A new type of semiconductor lasers, optically pumped vertical-external-cavity surface-emitting lasers (OP-VECSELs), also known as semiconductor disk lasers (SDLs), have the unique feature of the combination of excellent beam quality, output power scalability, and emitting wavelength adjustability.[1,2] These features make VECSELs have a very wide range of applications in scientific research, biomedicine, industrial production, and military fields. At present, the emission wavelengths of VECSELs are mainly between 800 nm and 1200 nm, and the involved technologies are relatively mature.[3–6]
VECSELs can be pumped with front or end geometry according to the different demands of the laser. Generally, a VECSEL with a front pump is proper for high-power continuous-wave running, stable mode-locked working and second harmonic generation, while a VECSEL with an end pump is more suitable for a compact and miniature device in some special utility.[7–10]
By the use of a front pump, Leinonen et al. reported a VECSEL with the maximum output power of 33 W and emitting wavelength at 1275 nm under a heat-sink temperature of −5 °C.[11] The laser cavity was composed of a highly reflective mirror (i.e., the distributed Bragg reflector in the gain chip) and an output coupler with 2.5% transmittance. With similar laser cavity and pumping geometry, Heinen et al. presented a continuous-wave output power of 106 W at a heatsink temperature of 3 °C in 2012. The laser operated in transversal multimode, and the emitting wavelength was 1028 nm.[5] Kantola et al. used a V-shaped cavity and a nonlinear LBO crystal for second harmonic generation and got a 588-nm yellow VECSEL with 20-W output power in 2014. The maximum conversion efficiency from absorbed pump power to yellow output power was 28%, which is the highest output power in the visible waveband.[12] A front pump mode-locked VECSEL with 400-fs pulse width and 4.35-kW peak power were reported by Keith et al. in 2013, the average output power was 3.3 W and the repetition rate was 1.67 GHz at the center wavelength of 1013 nm, which is the highest peak power of mode locking VECSELs.[13]
Compared to a front pump, VECSELs with an end pump have better mode matching and are easier to arrange in the cavity structure for laser miniaturization, which is of high demand in various applications that require a compact module. In 2006, Lee et al. demonstrated an end pumped VECSEL with more than 9.1-W continuous-wave output power at a wavelength of 1079 nm.[10] Then, by combination of the end pump geometry and the efficient intra-cavity frequency doubling, Lee et al. reported a highly efficient continuous wave green light at 535 nm, where the laser output power of more than 7 W was obtained when the pump power was 26 W, and the optical-to-optical conversion efficiency of 27% was achieved.[14] In the same year, Kim et al. demonstrated a high power end pumped VECSEL emitting at 532 nm and 460 nm. 2.7-W green and 1.4-W blue output powers with good beam quality were achieved by intra-cavity frequency doubling.[15] In 2007, Cho et al. proposed a novel lens-less optical end pump scheme and compact green VECSEL. A maximum output power of 1.1 W and an optical-to-optical conversion efficiency of 15.7% were achieved. The end pump was performed by placing a single laser diode directly behind the gain chip, and the pump geometry were without any optical elements for beam focusing and shaping.[16]
So far, there has been no reported VECSEL with front and end pumps simultaneously, and the performances of VECSELs with different pumping have not yet been compared. We present high power VECSELs with front and end pump here. Furthermore, a stability of the VECSEL has been presented and the wavelength can be tuned from 1096 nm to 1106 nm. In the semiconductor gain wafer, In0.26GaAs/GaAsP0.02 multiple quantum wells (MQWs) are grown to obtain an emission wavelength of 1096 nm, and Al0.2GaAs / Al0.98GaAs distributed Bragg reflector (DBR) is grown to produce high reflectivity centering at 1096 nm with about 100-nm bandwidth. Under front pumping, a maximum output power of 3 W is achieved when the pump power reaches 12.8 W, the optical-to-optical conversion efficiency and the slope efficiency are 22.4% and 29.8%, respectively. For end pumping, the maximum output power is up to 1.1 W when the pump power is 5.5 W, the optical-to-optical conversion efficiency and the slope efficiency are 23.2% and 30.8%, respectively. Performances of VECSELs with different pumps are compared, and influences of the thermal effects on the output power of lasers with front and end pumps are discussed.
VECSELs with front and end pumps are illustrated in Fig.
Figure
Figure
Output powers of the front pumped VECSEL with different OC transmittance are plotted in Fig.
Output powers of the front pumped VECSEL under different heatsink temperatures are depicted in Fig.
![]() | Fig. 5. (color online) Output powers of the front pumped VECSEL under different heatsink temperatures. |
Figure
Output powers of the end pumped VECSEL with the different heatsink temperature are plotted in Fig.
![]() | Fig. 7. (color online) Output powers of the end pumped VECSEL under different heatsink temperatures. |
Comparison of the output powers of VECSELs with front and end pumps is shown in Fig.
On comparison, the end pumped VECSEL has a smaller laser threshold, higher optical-to-optical conversion efficiency, bigger SE, but significantly small maximum output power. This can be explained by the different way of mode-matching of the laser. In the end pumped VECSEL, the pump spot is an ideal circular light spot, just as the laser spot at the gain chip, so both of them can be matched very well, as can be seen from the inset in Fig.
Figure
The stability of the VECSEL is plotted in Fig.
In conclusion, we have demonstrated a high power VECSEL with front and end pump. The maximum output power of 3 W, optical-to-optical conversion efficiency of 22.4%, and slope efficiency of 29.8% are obtained with a 5-°C heatsink temperature for the front pumped VECSEL, while the maximum output power of 1.1 W, optical-to-optical conversion efficiency of 23.2%, and slope efficiency of 30.8% are reached with a 5-°C heatsink temperature for the end pumped VECSEL. Different performances of VECSELs with different pump geometry are determined by their way of mode matching and their ability of heat removal. Our experiments show that a front pumped VECSEL is suitable for high output power while an end pumped VECSEL is more appropriate for miniature laser application.
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