High speed industrial laser transfer printing requires high power lasers that can deliver pulses on demand and having arbitrary pulse duration in range of few nanoseconds to milliseconds or more. A special kind of MOPA fiber laser is presented using wavelength multiplexing to achieve pulses on demand with minimal transients. The system is further tested in printing application.
A gain switched pulsed laser based on ytterbium doped rod PCF type fiber is presented. The high performance pump
system was based on 976 nm laser diodes incorporating high speed and high current laser diode drivers with active
feedback loop based control that enable high pulse to pulse stability. Furthermore the temperature control ensure the
adequate output spectrum of the pump laser diodes in order to match maximum of the absorption peak of Yb doped
active medium. The pulses duration in range between 48 ns to 75 ns were achieved with peak powers up to 3.6 kW.
Further, the change of the laser output spectrum in regard to the pump pulse power is observed.
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.
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.
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 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 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.
A novel liquid crystal display (LCD) driving concept for light shutters is shown here, based on driving signal polarity reversal controlled by the driving voltage integral across the LCD switching element electrodes, as opposed to plain periodic polarity reversal. This driving scheme optimizes the DC driving signal voltage balancing and guarantees compensation of the long-term (DC) component of the driving voltage within one polarity reversal cycle, irrespectively of driving voltage amplitude variations. The possibility of using low driving frequencies reduces the LCD switching elements power consumption significantly compared to standard driving techniques, which is important for portable devices such as welding light shutters.