BRIDLE project sees advances in high brilliance diode lasers

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A project to increase the brilliance of direct diode laser systems, BRIDLE (Brilliant Industrial Diode Lasers), has been completed after 42 months. The project, coordinated by Dilas Diodenlaser, sought to build high brilliance diode lasers based on advances in diode laser and beam-combining technology.

The progress could pave the way to more efficient laser processing in areas like selective laser melting of metals.

Design and technological development was performed by three partners within BRIDLE. First, the Ferdinand-Braun-Institut für Höchstfrequenztechnik im Forschungsverbund Berlin (FBH) developed novel epitaxial designs and process technology. Those developments enabled the use of broad area mini bars with a narrow stripe width of only 30µm to operate with a brightness that is increased by at least a factor of two in comparison with chips with a 100µm stripe width.

Furthermore, highly brilliant narrow-stripe distributed feedback (DFB) diode lasers with monolithically-integrated surface gratings were developed and optimised to deliver narrow spectrum (<1nm), high power (5W), high efficiency (50 per cent) within a low beam parameter product (<2mm-mrad) for the first time.

Monolithically grating-stabilised tapered diode lasers were developed for coherent coupling experiments, with record (54 per cent) conversion efficiency. Second, ridge waveguide diode lasers for coherent coupling experiments were developed by Modulight, which deliver an output power of 1W per emitter. Finally, design optimisation was supported through detailed simulation work performed by University of Nottingham.

Based on the high brightness diode laser mini bars developed within the BRIDLE project, Dilas was able to simplify its T-bar concept for 105µm fibre coupling. Furthermore Dilas could increase the optical output power up to 300W ex 100µm. The modules wavelength’ can be stabilised and used for dense wavelength multiplexing to further increase output power and brightness. The assembly process is fully automated.

The Fraunhofer Institute for Laser Technology ILT analysed and compared different techniques for dense wavelength multiplexing. These techniques include different approaches based on surface gratings, simultaneous wavelength stabilisation and multiplexing by use of dielectric filters and volume Bragg gratings (VBGs), as well as dense wavelength division multiplexing (DWDM) of wavelength chirped DFB diode lasers by dielectric filters.

Filters from different international manufacturers were tested thoroughly. For the first time, Fraunhofer ILT has developed concepts which can be used to implement and test compact modules in the medium power range of 10W to 100W output power, with a fibre having a core diameter of 35µm and a numerical aperture of 0.2. Fraunhofer ILT produced 46W experimentally. A 7:1 fibre combiner (35/105µm) was developed for further power scaling.

Centre National de la Recherche Scientifique/Institut d’Optique (CNRS-IO) demonstrated a new architecture for passive coherent combining of diode laser with ridge lasers (delivered by Modulight) and tapered lasers (delivered by FBH). The set-up is based on the separation of the phase-locking stage, which takes place in an external cavity on the rear side of the lasers, and the beam combining stage, which is achieved outside the cavity on their front side. This configuration demonstrates a combined power up to 7.5W in a single beam from a bar of five high-brightness emitters, using a specifically designed diffractive combiner. Furthermore, the active coherent combining of five tapered amplifiers achieved a power of more than 11W with a combining efficiency of 76 per cent.

The University of Nottingham developed software tools that enable the investigation of coupling between external optics and the diode laser itself. These tools can be used to better understand coherent coupling, wavelength stabilisation or parasitic back reflections.

The University of Nottingham developed a dynamic laser simulation tool for coherent beam combining (CBC) diode laser systems. This tool is used in conjunction with external cavity models developed at CNRS-IO to investigate the nature and dynamics of the phase locking mechanisms in CBC laser systems. Furthermore, the University’s laser simulation tool Speclase was coupled to external optical design software (Zemax) for external cavity simulations at the subsystem level.

Industrial applications of the developed prototypes are investigated by Bystronic Laser and Fraunhofer ILT. For instance, lasers manufactured by Dilas are used for Selective Laser Melting of metals at Fraunhofer ILT.

The project was funded by the European Commission under its seventh framework programme.