25 Nov 2008
Ursula Keller's group reveals details of a vertical external cavity surface emitting laser that offers a big leap in output power.
By carefully managing the thermal effects, researchers at ETH Zurich in Switzerland have created an optically pumped vertical external cavity surface emitting laser (OP-VECSEL) that emits more than 20 W of continuous-wave (CW) average output power in the fundamental mode at 960 nm. Previous OP-VECSELs have been restricted to around 4 W (Optics Letters 33 2719).
"In order to achieve fundamental transverse mode operation at high power levels, we have to control thermal effects and provide the appropriate mode dimensions for the laser and pump beams," ETH's Benjamin Rudin told optics.org. "The OP-VECSEL concept is scalable: if a one-dimensional heat flow is provided, the output power can be increased by increasing the mode areas and the pump power by the same factor."
Rudin explains that the 4 W output power in previous work was limited by the available pump power. However, the group's numerical simulations revealed that OP-VECSELs could be scaled up to several tens of watts for pump levels above 50 W, provided several conditions were met.
"We needed to use a high-quality gain structure mounted onto a heat sink with excellent thermal conductivity, apply sufficiently high pump power, and increase the mode area accordingly," commented Rudin.
The key to making this happen was to grow the VECSEL's semiconductor gain structure in reverse order, solder it directly onto a diamond heat spreader and remove the GaAs substrate using a wet-chemical etching procedure.
"We used chemical vapour deposition diamond with a thermal conductivity in excess of 1800 W/Km. Copper has a thermal conductivity of 400 W/Km in comparison," said Rudin. "With such a heat spreader, we can use larger mode sizes on the gain chip while at the same time keeping the absolute temperature inside the structure low."
Rudin and colleagues led by Ursula Keller placed their gain structure in a 50 mm straight cavity and pumped it with an 808 nm fibre-coupled diode array. Measurements revealed a slope efficiency of 49%, a pump threshold of 4.4 W, a maximum output power of 20.2 W at 960 nm and optical to optical efficiency of 43%.
The team is now looking to scale the average CW output power up to 100 W and modelock its devices. "We want to achieve high power levels in the pulsed regime, but the energy will be concentrated in pulses with a duration of a few picoseconds emitted at gigahertz repetition rates," said Rudin. "Ultimately, it would be a huge success if this technology was used in novel multi-core computers for optical clocking or optical interconnects."
Jacqueline Hewett is editor of Optics & Laser Europe magazine.