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Narrow Diode Beams Ease Fiber Coupling

A high-power GaAs laser diode that emits light with a very narrow vertical divergence offers unique advantages for beam shaping and coupling.

German researchers have succeeded in producing a laser diode that combines high optical output with narrow beam divergence, a difficult challenge for optoelectronic developers.

The Ferdinand-Braun Institute (FBH) team has made a GaAs diode that emits light above 1100 nm with a vertical divergence angle of just 13°.

The device, described in Optics Letters in October, also boasts cutting-edge optical power and efficiency, and the team consequently predicts high commercial interest levels.

“We have developed a diode laser that emits a beam with a very narrow vertical divergence angle," Agnieska Pietrzak, a researcher at FBH, told optics.org. “Such a beam is easier to shape in optical systems and is also easier to couple into an optical fibre. Earlier generations of this work are available through FBH spin-off Eagleyard Photonics and we expect these designs to follow."

Commercial laser users would like to combine maximum power with the highest quality beam. So far, researchers have fabricated laser diodes with record output powers and conversion efficiencies and have also succeeded in optimizing the beam quality. The difficulty is achieving both goals in the same structure.

The FBH team tackled this by increasing the thickness of the GaAs waveguide layers that surround the device s quantum well active region. Pietrzak and colleagues were then able to produce 200 µm stripe-width devices with an 8 mm long optical cavity. The devices produced output powers of 38 W under quasi-CW operation from a single emitter.

“We have designed a new thick waveguide structure, which allows robust single-mode vertical lasing, low optical losses and low series resistance," explained Pietrzak.

“Our diode lasers have a vertical divergence as low as 13° and 95 percent of the optical power is enclosed in an angle of 21 percent. Typically, commercial devices have an angle of 35° and 60° respectively."

Pietrzak and colleagues anticipate that their device will be useful for external cavity setups. "The semiconductor chip can be used as a common gain media, which can simply be pumped by current," she said. “Such lasers will address a lot of industrial requirements. The brightness could be especially useful in direct material processing."

The next step for the group is further optimisation of the design in order to reduce the vertical divergence angle to below 10° while still maintaining high optical power and efficiency.

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