FBH research: 30.03.2020

Optimized buried Bragg gratings for high-power DFB-BA lasers

High-power, highly efficient broad area diode lasers that are wavelength-stabilized by integrated Bragg gratings (DFB-BA) are in high demand for example for pumping narrow absorption bands in solid-state lasers. FBH has been conducting intensive research in this field for many years, leading to continuous improvements of the institute’s DFB-BA lasers.

Record conversion efficiencies for DFB-BAs emitting at 975 nm were achieved by establishing a new Bragg grating manufacturing scheme [1]. This procedure uses a 2-step epitaxial growth process based on Metalorganic Vapor Phase Epitaxy (MOVPE) and holographic lithography for ex situ grating mask layer formation. At its core, the developed technology allows for floating GaAs/Ga0.5In0.5P grating stripes which are fully embedded into the p-type doped AlxGa1-xAs part of the waveguide core layers. This leads to residual oxygen peak concentrations at the regrowth interface as low as 1017 cm-3resulting in low absorption losses and high output powers (Fig. 1).

The approach makes use of an in situ etch back prior to regrowth, by which the GaAs mask layer pattern is transferred into the underlying Ga0.5In0.5P grating layer. This procedure uncovers the previously buried p-AlxGa1-xAs waveguide core layer between the grating stripes and removes the surface exposed to oxygen during processing. Furthermore, the absence of Ga0.5In0.5P/AlxGa1-xAs heterointerfaces removes the penalty of an increased series resistance, as these interfaces are known to add an additional barrier in the hole injection current path. This, in turn, helps maximizing the wall plug efficiency to a great extent.

These great improvements have recently been patented [2] by FBH. However, it turned out that the underlying technology is sensitive and not very stable. It is particularly difficult to reliably keep the oxygen peak concentration at the regrowth interface at the necessary low level to ensure low internal absorption and thus high conversion efficiency and output power.

In 2019, scientists at FBH demonstrated a solution to this problem by further optimizing the grating layer sequence and introducing wet-chemical treatment before regrowth combined with the in situ etching process. This combination yields low residual oxygen as proven on different reactors. The SIMS (Secondary Ion Mass Spectrometry) profiles in Fig. 2 reveal that the optimized grating layer sequence manufactured independently on the two machines shows oxygen peak concentrations in the low 1017 cm-3 range in both cases. The “old” grating layer sequence, in contrast, consistently exceeded 1019 cm-3 in these experiments.

This patterned regrowth technology will be beneficial not only for buried Bragg gratings but also for example for buried current confinement layers.

Publications

[1] C. M. Schultz, P. Crump, H. Wenzel, S. Knigge, O. Brox, A. Maaßdorf, F. Bugge, and G. Erbert, "Efficiency-optimized 973nm high power broad area DFB lasers with overgrown aluminium-free gratings for peak power conversion of 63%," in CLEO/Europe and EQEC 2011 Conference Digest, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CB7_3.

[2] O. Brox, F. Bugge, P. Crump, G. Erbert, A. Maaßdorf, C. M. Schultz, H. Wenzel, M. Weyers, patent on “Diode laser and method for producing a diode laser with high efficiency”, EP 2 595 259 B1, (2019).