Cost-Efficient Diode Lasers for Industrial Applications

DWDM prototype consisting of actively cooled DFB mini-bars (Source: Fh. ILT)

DWDM prototype consisting of actively cooled DFB mini-bars (Source: Fh. ILT)

The project Brilliant Industrial Diode Lasers, BRIDLE, has been finished successfully after 42 months of intense research activities, funded from the European Commission within the seventh framework program. The seven project partners finished their work at the end of February 2016. The project was coordinated by Dilas Dioden­laser, Germany. The other project partners are located in Germany, UK, Switzer­land, France and Finland. BRIDLE targeted a major increase in the achievable brilli­ance in direct diode laser systems, based on advances in diode laser and beam-combining technology. Throughout, the highest conversion was sought as was compati­bility with low cost, volume manufacture.

Design and technological development of high performance diode lasers was performed by three partners within BRIDLE. First, the Ferdinand-Braun-Institut, Leibniz-Institut für Höchst­frequenz­technik, 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 30 µm to operate with a brightness that is increased by at least a factor of two in compa­rison with state of the art chips with a 100 µm stripe width. Further­more, highly brilliant narrow-stripe DFB diode lasers with mono­lithi­cally-integrated surface gratings were developed and optimized to simulta­neously deliver narrow spectrum (< 1 nm), high power (5 W), high efficiency (50 %) within a low beam parameter product (< 2 mm mrad) for the first time. For coherent coupling experi­ments, mono­lithi­cally grating-stabilized tapered diode lasers were developed, with record 54-% conversion efficiency. Second, ridge waveguide diode lasers for coherent coupling experiments were devel­oped by Modulight Inc., which deliver an output power of 1 W per emitter. Finally, design optimization was supported through detailed simulation work performed by University of Nottingham (U-Nott).

Based on the high brightness diode laser mini bars developed within the project, Dilas was able to simplify its T-bar concept for 105 µm fiber coupling. Furthermore, Dilas could increase the optical output power up to 300 W ex 100 µm. The module’s wavelength can be stabilized 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 analyzed and compared different techniques for dense wavelength multi­plexing. These techniques include different approaches based on surface gratings, simulta­neous wavelength stabili­zation and multi­plexing by use of dielectric filters and VBGs as well as DWDM of wavelength chirped DFB diode lasers by dielectric filters. Filters from different inter­national manufac­turers were tested thoroughly. For the first time, ILT has developed concepts which can be used to implement and test compact modules in the medium power range of 10 to 100 W output power, with a fiber having a core diameter of 35 µm and a numerical aperture of 0.2. 46 W were realized experimentally. A 7:1 fiber combiner (35 / 105vµm) was developed for further power scaling.

Mounting of individually addressable diode lasers with rear and front facet access for CBC. (Source: CNRS-IO / Fh. ILT)

Mounting of individually addressable diode lasers with rear and front facet access for CBC. (Source: CNRS-IO / Fh. ILT)

The Centre National de la Recherche Scien­tifique’s 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 the 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 succes­sively a combined power up to 7.5 W 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 11 W with a combining efficiency of 76 %.

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 stabili­zation or parasitic back reflections. U-Nott developed a dynamic laser simulation tool for 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, U-Nott’s laser simulation tool Speclase was coupled to the external optical design software Zemax for external cavity simulations at the subsystem level.

Industrial applications of the developed prototypes are investigated by Bystronic Laser AG and Fraun­hofer ILT. For instance, lasers manufac­tured by Dilas are used for selective laser melting of metals at Fraunhofer ILT. (Source: Fh. ILT)

Link: Project website Brilliant Industrial Diode Lasers, BRIDLE

Further reading: J. Biesenbach & J. Neukum: Basic Concept of Tailored Bars – Results, applications and potential power scaling, Laser Tech. J. 13 (2), 60–63, April 2016; DOI: 10.1002/latj.201600011

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