Efficient fiber coupling in high-power diode lasers

Because of their high level of efficiency and compact dimensions, high-power diode lasers are increasingly being used throughout a wide range of industries. The demand for ever higher levels of performance coupled with smaller fiber diameters - either for direct application in industrial manufacture and medical technology or for the pumps used in solid state lasers - is resulting in a need not only for high-performance diodes but, above all, for efficient fiber coupling.

The main challenge in terms of fiber coupling of diode lasers, lies in the non-symmetrical beam quality of the emitters in the fast and slow directions. The ratio of the quality-defining beam parameter products M2 (Fast/Slow), for example, is approximately 1/30 or higher. This cannot be improved using passive replication systems.

The V-step module manufactured by INGENERIC GmbH, permits the diode lasers to be coupled easily and efficiently in fibers. The V-step module has the highest coupling efficiency of all optical systems currently commercially available. Further advantages of the V-step module lie in the high level of flexibility in terms of the diode laser used (no. of emitters, emitter width etc) the diameter of the fibers which can be coupled in and in the ease of assembly.

The concept underlying beam transformation is based on adapting the wide variations in the beam parameter products for the fast and the slow directions. The output beam is therefore divided into a number of partial beams which remain non-symmetrical. These are rotated by an angle of 90° around the axis of the direction of propagation and arranged to form a new beam with symmetrized beam quality within the two perpendicular axes.

The optical core element of the V-step module - the V-Step optic - links two mirror arrays in a monolithic element (c.f. diagram). The mirror array is arranged so as to ensure that the actual beam redirection in the glass, is achieved via total reflection. The use of high quality, highly refractive glass eliminates the need for an additional reflective coating on the mirror surfaces. The application of this principle ensures that the reflective surfaces are not aged by external influences and reduces the amount of time and work required in order to fit this module. The manufacturing technology specially developed by INGENERIC, highly transparent substrate materials and effective AR-coatings on the coupling in and out surfaces, guarantee a coupling efficiency in excess of 80 % for the V-Step module.

Dead zones and rounding of the boundary surfaces have been minimized to the extent that they now account for less than 3% of the surface, in order to meet the demand for the lowest possible optical losses. The geometric accuracy of the optical elements, which guarantees faultless replication/redirection of the partial beam is also very important. The evenness of the individual surfaces is therefore in the range < 250 nm. The positional accuracy of the surfaces in relation to one another is in the range < 0.05°.

The number of steps and/or the width of the steps is adapted to meet the requirements of the client or application concerned. The diameters of the fibers which can be coupled in, range from 1 mm to 50 µm in width, with an NA of 0.2. INGENERIC stocks optimized V-step modules for all types of diode laser in common use. In order to facilitate assembly, INGENERIC also supplies the V-step modules on individual base plates, optimized to suit the design of the laser for which it is intended. Our 100 % quality control policy for both individual optical elements and for the whole module, ensure that the user receives V-step modules with unrivalled coupling efficiency.