Tapered lasers for high power and high beam quality

PICWave is a 3D circuit simulator, solving the optical modes of the waveguide cross sections in its devices. This plays a pivotal role in simulations where spatial hole burning must be considered or, as is explored here, the effect of tapering gain sections used in high power lasers.

Taper laser graphic showing broad waveguide output (with simulated optical mode), taper, and straight waveguide ‘tail’ to ensure single mode operation.

The goals of creating lasers with a high power and good beam quality are seemingly in opposition:

• Large area waveguides will produce a large gain but also support multiple modes; if multiple modes are amplified the beam quality will be poor.
• A narrow waveguide width will ensure a single mode is supported but will ultimately provide a small gain.

LI curves of 2.5um and 25um ridge width FP lasers (current density ~ equal)
Left: 25um ridge width laser with many lasing modes but at higher power.
Right: 2.5um ridge width lases only in the fundamental mode.

A tapered waveguide design achieves both of the benefits.

A wide waveguide output enables a large gain area for high power. This waveguide tapers down to a narrow straight waveguide section; as this supports only the fundamental mode, the higher order modes are suppressed after propagating through this narrow length.

The straight waveguide section at the narrow end of the taper (the 'tail') plays a key role in suppressing higher order modes. It must be sufficiently long for higher order modes to be suppressed as shown by the simulation of a 550um taper laser (25um to 2.5um)