Tilted Wave Lasers: A Way to High Brightness Sources of Light

Semiconductor laser diodes are conventionally based on a relatively thin waveguide structure grown epitaxially on a thick single crystalline substrate, wherein the latter serves as a medium for carrier flow and as mechanical support and plays no role in optics. Although earlier attempts to provide the outcoupling of light through a transparent substrate in leaky lasers realized a narrow leaky emission beam, either significant leakage losses led to the deterioration of the laser performance or/and a large fraction of the output optical power was concentrated in a co-existing angularly broad emission peak originating from the narrow active waveguide. Our solution, a tilted wave laser (TWL), includes polishing the back side of the substrate under the stripe providing mirror-like reflection for the leaky mode which can thus exhibit multiple reflection and amplification cycles before exiting the device from the substrate facet. Fulfillment of phase matching conditions allows wavelength-stabilized operation. At a wavelength of 1060 nm TWLs are shown to exhibit a very small thermal shift of the emission wavelength of 0.03 nm/K. A cw output power of 3.3 W for 2 mm long cavities with uncoated facets is obtained, wherein the entire power is concentrated in a single vertical lobe having a full width at half maximum of 0.8°. The scattering of the tilted optical wave by the back substrate surface roughness is modeled and found to be the main mechanism limiting the differential efficiency, wherein the scattering contributes up to 10 cm^-1 to the losses for a present roughness of ~30 nm. The free carrier absorption in the n-doped substrate ( ~3 cm^-1 for n ~10¹⁸ cm^-3) dominates for a roughness <; 10 nm.