Effective focusing optics for stack-bar laser diodes

In this paper, an effective optical focusing system for stack-bar laser diodes (LDs) has been proposed and characteristics of the system are described. Using the system, beams of an 8-bar stack LD whose total output power is 800 W, quasi-continuous wave (qcw), with 1.75 mm pitch along the stacking direction, were focused into an elliptical spot of 503 µm long and 340 µm wide. The distance from the final focusing lens to the converged spot was 80 mm. Two-stage fast-axis telescope-type compressors were used to reduce the size of the object for the fast-axis direction. Along the slow-axis direction, a beam divider was introduced between two fast-axis compressors reducing the size of the object. The divider consists of a pair of cylindrical lens arrays tilted by 45 degrees. It divides the beam at 1 mm pitch and rotates individually by 90 degrees. The overall transmission efficiency of the optical system was 81 %. Scaling characteristic of the acceptable number of the stacks was investigated by moving the LD head along the stacking direction. Transmission efficiency was kept flat up to 14 bars. This focusing optics is useful for the direct materials processing using the LDs.