B. Holzinger

Time multiplexing of high power laser diodes with single crystal photo-elastic modulators

Time-multiplexing is a method to increase the brilliance of diode lasers, i.e. a sequence of laser pulses emitted from different laser diodes at different times is guided onto a common optical path via a cascade of polarizing cube beam splitters and polarization switches. The latter are made of piezo-electric crystals oscillating in resonance and making use of the photo-elastic effect to obtain the desired modulation of polarization. We realized a demonstrator for time multiplexing of four laser diodes with such self-excited photo-elastic modulators. The latter is a new alternative to conventional photo-elastic modulators used in ellipsometers.

A single crystal photo-elastic modulator

We present theoretical and experimental data and possible applications of a photo-elastic-modulator (PEM) made of LiTaO3. The device with dimensions 13.2x7.1x5.5 mm in x-, y- and z-direction and electrodes on the zx-surfaces offers basic modulation frequencies at 199, 348 and 377 kHz corresponding to the longitudinal oscillations in x- and y-direction and to a yz-shear oscillation mode. The light travels along the optical axis. At the main resonance at 199 kHz the voltage amplitude to achieve a quarter wave retardation amplitude is only ~2.5 V, a very low value due to the strong piezo-electric response and the low loss of LiTaO3. Hence when compared to a conventional photo-elastic modulator, which is made out of at least two components, the device is extremely compact, cheap and easy to operate, especially when placed in a feedback loop of an amplifier such that it operates on one fixed frequency.

A Diode-Laser-System for Laser-Assisted Bending of Brittle Materials

We developed a small and compact system of diode lasers, which can be inserted into the lower tools of a bending press. The parts of the system allow easy plug and play operation and can be installed for any bending length. The diode laser, which is based on 200W laser bars on microchannel cooler, allows the heating of sheet metals in the forming zone shortly before and during the bending process. There is no unnecessary heating of other parts of the bending equipment, no wear of the tool, and, if properly done, no damage of the surface of the metal. The power per bending length is 16kW/m.

Forming of Brittle Materials—A New and Valuable Application of Diode Lasers

Laser assisted bending is a new and versatile method to allow simple bending of brittle materials. Laser technology is used to illuminate and heat the forming zone. Only a laser allows directing the power on a narrow area. Further there is no unnecessary heating of other parts of the bending equipment, no wear of the tool and, if properly done, no damage of the surface of the metal. We describe now the integration of 200 W‐diode‐laser‐bars on micro‐channel coolers that where installed into the lower tool of the bending press. The solution allows any required bending length by a combination of several bending tools with integrated lasers. The optical power of 16 kW per meter bending length allows achieving the temperature necessary to bend brittle sheet metals within seconds.

A new compact high brilliance diode laser type

Our work deals with a new approach to improve the beam quality of diode lasers, which is still insufficient for many applications. We propose time-multiplexing, where several pulsed laser diode beams are guided onto a common optical path. This allows to superpose the power of the diodes while maintaining the beam parameter product of a single laser diode. Pulsed operation of continuous wave laser diodes was shown to be possible up to pulse enhancement factors of ten provided that pulse duration is <300 ns. We use a fast digital optical multiplexer built up by a cascade of binary optical switches. For the latter we use a polarisation switch (voltage-driven LiNbO3-crystal) followed by a polarisation filter, which allows addressing of two optical paths. Instead of direct on/off-switching we drive the crystals with a harmonic voltage course to avoid ringing caused by piezo-electricity. Up to now an optical power of 10.5 W was generated, 13 W are expected with some improvements. With the use of new 8W laser diodes even the generation of 25 W will be possible.

A compact high brilliance diode laser

We explain some technical details regarding time-multiplexing of laser diodes, a method to improve the beam quality of diode lasers, which is still insufficient for many applications. Several pulsed laser diode beams are guided onto a common optical path to superpose the power of the laser diodes while maintaining the beam parameter product of a single laser diode. Pulsed operation of continuous wave laser diodes with average power equal to the specified cw-power of 4 W was tested for 150 hours without failure. We use a fast digital optical multiplexer built up by a cascade of binary optical switches. For the latter we use a Pockels cell followed by a polarization filter, which allows addressing of two optical paths. Instead of direct on/off-switching we drive the crystals with a harmonic voltage course to avoid ringing caused by piezo-electricity. Up to now an optical power of 10.5 W was generated, 13 W are expected with some improvements. Furthermore we discuss the use of new 8 W laser diodes and the involved implications on driver technology.

Realization of time-multiplexing for high-power diode lasers

We describe methods for time-multiplexing of high power diode lasers. This means that high power laser pulses emitted from a set of laser diodes are guided on a common optical path via an optical multiplexer with the aim to build a high beam quality diode laser. We examined pulsed operation of laser diodes and developed elements for a digital multiplexer suited for this special task. The technology developed until now will allow a device multiplexing 4 laser diodes with a power of 16W and a beam quality of one laser diode. Time-multiplexing of 8 laser diodes for a device with 30W should be feasible as well.

Time-multiplexing of high power laser diodes yields a high brilliance laser beam

Our work deals with a new approach to improve the beam quality of diode lasers, which is still insufficient for many applications. We propose time-multiplexing, where several pulsed laser diode beams are guided onto a common optical path. This allows to superpose the power of the diodes while maintaining the beam parameter product of a single laser diode. Pulsed operation of continuous wave laser diodes was shown to be possible up to pulse enhancement factors often provided that pulse duration is <300 ns. We use a fast digital optical multiplexer built up by a cascade of binary optical switches. For the latter we use a polarization switch (voltage-driven LiNbO/sub 3/-crystal) followed by a polarization filter, which allows addressing of two optical paths. Instead of direct on/off-switching we drive the crystals with a harmonic voltage course to avoid ringing caused by piezoelectricity. Up to now an optical power of 10.1 W was generated, 13 W are expected with some improvements. With the use of new 8 W laser diodes even the generation of 25 W will be possible.