In many coating-lines exact thickness-control is essential. We demonstrate some results with a stroboscopic imaging ellipsometer, which enables to acquire an image of the thickness distribution of a certain layer during or right after production making it an ideal tool for inline-monitoring. The prototype obtains a thickness-distribution with 10-100Hz acquisition rate. The accuracy is in the range 2-10% of the layer thickness. Since there are no moving parts and only monochromatic illumination with a standard laser-diode the solution is cost-effective and can be easily installed. We demonstrate three different applications, namely SiO2 on Si, oil on steel, and MgF2 on glass.
Nowadays high power lasers are mainly used for cutting of sheet metals, for welding, hardening and rapid prototyping. In the forming of sheet metals as bending or deep drawing lasers are not used. Nevertheless a few years ago a new application of high power lasers has been invented, where bending of materials that break at room temperature becomes possible by heating them along the bending edge with high power lasers thus allowing their treatment without cracks and rupture. For this purpose a large number of diode lasers are arranged in the bottom tool of a bending machine (a V-shaped die) which heat up the initially flat sheet metal during the bending process what is performed by pressing it into the die with a knife shaped upper tool where due to the laser heating the material is softened and thus cracks are avoided. For the technical realization of the new process of laser assisted die bending, modules equipped with numerous laser diodes and a total beam power of 2,5 kW are used. The light emitted by these modules enters a tool with a length of 15cm and is deflected towards the workpiece. By using ten of these modules with adjacent dies and by integrating those in a bending press a bending edge of sheet metals with a length of 1500mm can be realized. Such a bending press with laser assistance also needs energization with a power of practically 50kW, a respective water flow, a heat exchanger system and also a control for all functions of this system. Special measures have also been developed to avoid radiating of those tools that are not covered by a workpiece in the case of bending edges shorter than the full length of the bending tools whereas individual short circuiting of diode modules can be performed. Specific measures to ensure a safe operation without any harm to the operational person have been realized. Exploitation of the bending process has been carried out for titanium, where material thicknesses up to 3mm have been bent successfully.
First pulse of relaxation oscillations that appear after the start of the pumping can be used to realize an efficient pulsed laser based on gain switching. Because there is no need for any additional active optical element this can be very simple and robust technique to produce nanosecond pulses. Together with fiber technology it can produce compact and reliable lasers appropriate for industrial applications such as micro-processing. However, to produce pulses with appropriate peak power and duration, one must carefully design such systems. We report on a numerical model that describes time and spatial dependencies of photon and ion populations which was developed to enable design and optimization of a gainswitched fiber laser. The peak pump power influence on basic output laser pulse parameters is presented in this paper. To confirm theoretical result an experimental setup was built around double clad ytterbium doped fiber laser.
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.
We demonstrate a simple pulsed laser design with a potentially high efficiency for high harmonics generation. The basic
idea is to generate pulsed laser operation with a resonantly oscillating piezo-electric crystal made of LiTaO3 or LiNbO3,
i.e. Q-switching with a Single Crystal Photo-Elastic Modulators (SCPEM). The pulsed mode operation shows 30
(internal SHG) to 50 (external SHG) times higher green power than the cw-mode operation with a maximum green
output of 290mW.
For high speed quality control in production we had developed a novel approach for fast ellipsometric measurements.
Instead of a conventional setup that uses a standard photo-elastic modulator, we use a Single Crystal Photo-Elastic
Modulator (SCPEM), for which in this case a
LiTaO3-crystal is used. Instead of an analog Lock-In Amplifier, an
automated digital processing based on a fast analog to digital converter is used. This small, simple, and cost-effective
solution with its extremely compact and efficient polarization modulation allows fast ellipsometric testing where the
upper limit of measurement rates is only limited by the desired accuracy and repeatability of the measurements. Now we
present an extension of this measurement from 635nm in the VIS to 1064nm in the NIR and discuss the related problems
with signal measurement and retardation control. Further the system speed was enhanced by onboard processing, such
that now a sampling rate of 46 kHz is possible.
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.
An overview is given about experiments with a new method for Q-switching lasers at a constant pulse repetition
frequency. It uses inside the laser resonator a Single Crystal Photo-Elastic Modulator (SCPEM). This consists of one
piezo-electric crystal electrically excited on a mechanical resonance frequency. In resonance mechanical stresses are
induced that lead via the photo-elastic effect to a strongly modulated birefringence. Polarized light going through such an
oscillating crystal will experience a significant modulation of its polarization and of transmission through a polarizer.
Suitable materials should not be optically active, as it is for example the case for SiO2, and should allow the excitation of
a longitudinal oscillation with an electric field perpendicular to the travelling direction of the light. Crystals of the group
3m, like LiTaO3 and LiNbO3, proved to be ideally suited for SCPEMS for the NIR- and VIS-region. For the infrared
GaAs can be used.
We demonstrated SCPEM-Q-switching for a Nd:YAG-fiber, a Nd:YVO4-slab- and a Nd:YAG-rod-laser with typical
pulse repetition rates of 100-200kHz, pulse enhancement factors of ~100 and pulse durations ~1/100 of the period time.
Typically the average power during pulsed operation is nearly the same as the cw-power, when the modulator is switched
off. The most stable results were achieved up to now with the Nd:YVO4-slab-laser at 10W average power, 1.1 kW peak
power, 127 kHz pulse repetition rate, and 70ns pulse durations.
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.
We present a rod-Nd:YAG-Laser, side-pumped with eight 50W-laser diode bars at 808nm, and Q-switched with a Single
Crystal Photo-Elastic Modulator at 95.1 kHz. The latter is made of a z-cut LiNbO3-crystal, which is electrically y-excited
on the mechanical resonance frequency of the x-longitudinal oscillation. With a voltage amplitude of 3 V the crystal
shows a strong oscillation such that due to the photo-elastic effect a high polarization modulation is achieved, which,
together with a polarizer, can be used as a simple optical switch. With this inside the laser resonator the average power is
47.8W in cw-mode and 45.5W in pulsed mode, with pulse peak powers of 4 kW and pulse widths of 100ns. This kind of
operation is similar to cw-operation but offers due to the high peak powers different interaction physics with matter. The
source is therefore suited for micro-welding of metals, LIDAR, rapid prototyping of plastics, marking/engraving/cutting
of plastics, marking of glasses. In cases where high precision and a small heat affected zone are necessary this simple
kind of pulsed operation may be advantageous, when compared to cw-operation.
We present a study of a gain-switched end-pumped Yb-doped-fiber laser. The test laser that was mainly used in order to
verify the theoretical model consist of an 8.7 m long double clad active fiber with core and inner cladding diameter of
8 μm and 130 μm respectively, and absorption of 1.5 dB/m. An important part of the system is a control unit that
switches on the pumping diodes at a desired repetition rate and switches off at the moment when the first spike of the
transitional effect appears in order to suppress additional oscillations.
A simple rate equation model accurately predicts the main pulse parameters. It describes the population dynamics of the
photons and the laser levels, including the occupation by thermal effects. Further numerical simulations show that with
adequate active fiber geometry, active ion doping and sufficient pumping power, much shorter pulses in range of 50 ns
and peak power of several 100W can be achieved. Such a simple system with the potential addition of a one stage active
fiber amplifier can be interesting for some applications in micro-processing like scribing of solar cells, micro processing,
and thin film removal.
We propose a small and fast ellipsometer with a basic layout similar to that of conventional ellipsometers using photo-elastic
modulators (PEM) oscillating with 50 kHz. A conventional PEM is rather large, ~10×20×100mm, since it consists
of one piece of glass and an actuator. Both parts are carefully adjusted to the desired frequency and then glued together.
We replace such a standard modulator by a 127 kHz Single Crystal Photo-Elastic Modulator (SCPEM), a LiTaO3-crystal
with a size of 20.6×7.5×5mm. The polarization of light that travels through this crystal is strongly modulated. The
modulated light is reflected from the sample, passes a polarizer and hits a detector. Its signal is split into the dc-value and
the amplitudes of the 1st and 2nd harmonic of the modulation frequency. These values lead via simple formulas to the
ellipsometric parameters. Usually a Lock-In-Amplifier is used here, whereas we propose an automated digital processing
based on a fast analog to digital converter controlled by a highly flexible Field Programmable Gate Array (FPGA). This
and the extremely compact and efficient polarization modulation allow fast ellipsometric measurements as needed in
high volume manufacturing of optics.
We present experimental results with a 10W-Nd:YVO4-laser, which is Q-switched with a single crystal photo-elastic
modulator made of LiTaO3. This allows a simple setup driven by voltage amplitudes in the order of 10 V. We observed
stable and unstable pulse sequences. In stable operation a 127 kHz - pulse sequence with 70ns pulse width and 1100 W
peak power was achieved, while the average power remained constant at 10W.
We present a new optical device for pulse picking and Q-switching based on a LiTaO3-crystal together with polarizers.
LiTaO3 is piezoelectric, hence when a harmonic voltage course with a proper frequency is applied to the crystal it will
start to oscillate resonantly in a mechanical eigenmode. Due to photo-elasticity an artificial modulated birefringent is
induced by this oscillation such that the polarization of trough-going light is modulated. Together with polarizers the
transmission of the whole setup can oscillate between 0 and 100%. The applied voltage amplitude is usually in the order
of below 10 V. With a special choice of the crystal dimensions it is possible that the first shear eigenmode has exactly
three times the frequency of the first longitudinal eigenmode. Both modes have qualitatively the same influence on
polarization, such that with a proper superposition of these two modes a short opening time of the setup can be achieved,
which can be used to enforce pulsed laser operation. The latter was realized with a small end pumped fibre laser. A pulse
sequence with 127 kHz and a ratio of peak power to average power of ~30 was achieved.
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
We describe a new method for Q-switching. It uses a polarizer together with a photo-elastic modulator, i.e. a transmissive optical element, which oscillates in a longitudinal eigenmode. This induces due to the photo-elastic effect a modulated artificial birefringence which modulates the polarization of passing light and hence the transmission through the polarizer. With a proper choice of eigenmodes and corresponding phases and amplitudes the times of high transmission are short enough to act as a Q-switch in a laser cavity. The voltage amplitude of the excitation is in the order of 10 V. Hence a simple and robust pulsed laser with constant pulse repetition frequency can be built.
We introduce basic knowledge about diode lasers and point out some of the most important recent innovations in this field. Three key fields of progress are treated, namely laser diode material/structure, mounting/cooling/packaging-technology and optics/collimation/multiplexing. The most remarkable achievement is the continuous wave generation of 509 W by a single diode laser bar and 25 W by a single laser diode.
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.
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 Pockel's 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.
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.