Uwe Brauch

Power-scalable system of phase-locked single-mode diode lasers

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.

Coherent coupling of vertical-cavity surface-emitting laser arrays and efficient beam combining by diffractive optical elements: concept and experimental verification

A phase-locked diode-laser system based on master-slave coupling of two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays by injection locking is presented. Frequencies and phases are adjusted by laser-trimmed microresistors. Additional beam-transformation optics consisting of two diffractive optical elements (DOEs) and a Fourier lens concentrates most of the far-field power in a nearly diffraction-limited beam. Both the VCSEL array and the microlens array are monolithically integrated and mounted in a compact module. With an array of 21 slave lasers a system coherence of 95% (for several hours) and of nearly 90% (for several months) has been demonstrated without any active phase control. The scalability of the output power has been verified by locking of an array of 77 slave lasers with a system coherence of 78%. The optical system efficiency is 20-23%; with beam-transformation optics this efficiency could be improved to 44%.

High-power laser float-zone crystal growth

A float zone crystal growth apparatus was developed. The heat source was a 3.5 kW CO2 Trumpf laser with a long term stability of 2 %. The original laser beam was divided, in the beam delivery system, into three equivalent focusable beams. The vacuum vessel is equipped with two equivalent rotation - translation systems with a maximum translational speed of 1 mm/minute. The vessel is also pierced with five windows, three of which serve the three incoming beams, one for observation with a television camera, one for an optical pyrometer or for direct observation. The three beam windows are protected by a laminar gas curtain. A heat shield system with an after heater is planned, in order to enable growth of high temperature oxides. The system was tested in the growth of small sapphire crystals (0 4 mm) and in the growth of silicon whose dimensions were 0 10 mm x 70 mm. * On leave from the Weizmann Institute, Rehovot, Israel