Pulsed high power diode laser

A new concept for high power diode lasers with good focusability uses pulsed laser diodes with an opto-electronic guidance on a common optical path. High power diode lasers are efficient, small and simple in operation. But due to their very bad beam quality they are not useful for important industrial applications like cutting, drilling or deep penetration welding. The reason is the incoherent combination of the beams of many laser diodes in such a system. The suggested novel approach is to produce sequences of high power laser pulses with special laser diodes and to achieve the propagation of all pulses on a common optical path. This can be done with an inverse digital beam deflector that use the 90 rotation of the polarization of a laser beam by a so called Pockel`s cell, so that the laser beam can take two different ways in polarization selective optical elements depending on whether the polarization has been turned or not. With an appropiate combination of Pockel`s cells and polarization selective elements the pulses from 16 lasers will be guided on a common path in this project. A first realization will use laser diodes that reach a peak power of 75 W for 100 ns. It is assumed that 60W average optical power will be reached with such a good focusability that in the focus an intensity of ~1MW/cm will be reached. Herewith new fields of material processing can be made accessible for the small and efficient high power diode lasers.

Diode lasers are small efficient, cheap, easy to operate, available with highest power up to 6kW and hence the laser sources of the future. Their only and serious disadvantage is that due to the high number of installed laser diodes their laser beam quality offers very low beam quality such that only comparatively low focus densities are achieved. The project "Pulsed High Power Diode Laser" addressed the increase of beam quality with a new method, namely time-multiplexing. Here laser diodes are driven in a pulsed manner with the same average power as in continuous operation, such that one laser diode after another emits a laser pulse. These pulses are guided to an optical multiplexer, which connects with fast optical switches one input channel after another with one optical output channel such that correctly controlled input laser pulses will all leave on this output path. Herewith the power of several lasers is concentrated on one optical path and the resulting quasi continuous laser beam is as focusable as a single beam. The optical switches are realized with piezo-electric crystals, which are excited on a resonance frequency, like in a quartz clock, such that due to tension induced birefringence the polarization of correctly time-phased light pulses switches between horizontally and vertically polarized. In a subsequent polarizing cube beam splitter the laser pulse are reflected or transmitted such that indeed switching between two optical paths is realized. This technology will found many applications in optical metrology and laser technology and is the most important result of the project. Herewith time-multiplexing of four laser diodes was realized with an output power of 10.5W and an increase of focus intensity by ~75%. The size of this setup is ~12x12x6cm and hence comparable to a conventional 10W diode laser, such that it will be easy to combine several of such devices to obtain a more powerful laser.