Thomas Schumi

SCPEM-Q-switching of a fiber-rod-laser

We demonstrate high-frequency Q-switching of a fiber rod laser with a Single-Crystal Photo-Elastic Modulator (SCPEM) made of a LiTaO₃₋ crystal. This type of photo-elastic modulator can be driven simultaneously with two different eigenmodes to achieve a shorter rise time, which is essential for high-power operation. When operated in the laser cavity, a pulse repetition frequency of 183.6 kHz with an average power of 47 W, a pulse duration of 26 ns, and a peak power of 10.5 kW was achieved.

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 material for single crystal modulators: BBO

Single crystal photo-elastic modulators (SCPEM) are based on a single piezo-electric crystal which is electrically excited on a resonance frequency such that the resulting resonant oscillation causes a modulated artificial birefringence due to the photo-elastic effect. Polarized light experience in such a crystal a strong modulation of polarization, which, in connection with a polarizer, can be used for Q-switching of lasers with pulse repetition frequencies in the range of 100- 1000 kHz. A particularly advantageous configuration is possible with crystals from the symmetry class 3m. Besides LiTaO3 and LiNbO3, both already well explored as SCPEM-materials, we introduce now BBO, which offers a very low absorption in the near infrared region and is therefore particularly suited for Q-switching of solid state lasers. We demonstrate first results of such a BBO-modulator with the dimensions 8.6 x 4.05 x 4.5mm in x-, y-, z- direction, which offers a useful resonance and polarization modulation at 131.9 kHz. Since the piezo-electric effect is small, the voltage amplitude for achieving Q-switching for an Nd:YAG-laser is expected to be in the range of 100V. Nevertheless it is a simple and robust device to achieve Q-switching with a high fixed repetition rate for high power solid state lasers.

Hot bending with a fiber coupled solid state laser

For bending of brittle materials it is necessary to heat up the forming zone. This can be done with a fiber coupled solid state laser, whose beam is evenly distributed on the bending line with a beam splitter installed in the lower tool (die) of a bending press. With polarization optics the laser beam is divided there into partial beams that are evenly distributed on the bending line with lenses and prisms. A setup for a bending length of 200mm heated by a fiber-coupled 3kW Nd:YAG-laser shows the feasibility of the concept. Successful operation was shown for the Mg-alloy AZ31, which breaks during forming at room temperature, but can be well formed at temperatures in the range of 200-300°C. Other materials benefiting from this method are Ti-alloys, high-strength-Al-alloys, and high-strength-steels. Typical heating times are in the range of up to 5s and much of the heat input is generated during the bending operation where the laser continues to work. Laser Assisted Bending with a fiber coupled solid state laser is a straightforward way to perform the bending of brittle materials in a process as simple as cold bending.

Laser assisted bending based on macro-channel cooled diode lasers

Laser-assisted bending allows the bending of brittle materials by laser-heating the work-piece. We present a new development based on macro-channel cooled diode-lasers, initially developed for pumping of disc lasers. The new solution is more robust and reliable and will show the life-time necessary for an industrial application. The optical concept, however, shows problems with the uniformity of the intensity distribution.