Within the context of the E-TEST (Einstein Telescope EMR Site & Technology) project, Fraunhofer ILT develops thulium- and holmium-based seed sources and fiber lasers at app. 2 μm wavelength with highest demands on linewidth and stability for usage in a third-generation gravitational wave detector, the Einstein telescope. To fulfill the requirements, we develop a seed laser and a multi-stage fiber amplifier, consisting of holmium-doped fibers. Within this paper, we present our current laser concept and the first results of our dual-stage holmium-doped fiber amplifier stage. We achieve a low linewidth (< 2 MHz) output power of more than 5 W at a wavelength of 2095 nm. By using our in-house developed fiber laser simulation, we show that the efficiency of our amplifier is currently limited by pair induced quenching and the potential for further power scaling.
For specific indications in neurosurgery, such as the removal of brain tumors in eloquent locations and the deep brain stimulation, awake craniotomy offers multiple advantages. However, due to the severe discomfort experienced during burr-hole drilling, patients are hesitant to opt for awake craniotomy. Laser systems provide an alternative to surgical drills as a silent and vibration-free bone cutting method. Until now no laser system achieved adequate ablation rates (> 2:5mm3/s) at sufficiently high aspect ratio (>5) to fulfill the requirements for craniotomy. The aim of this study is to investigate the ablation of bone tissue under the needs for neurosurgery using three different Q-switched infrared laser sources assisted by a water spray system. One of the laser sources is a commercial Q-switched CO2 laser system operating at 10:6 μm with a pulse energy of 4 mJ. In addition, two in-house developed, short-pulsed IR-laser sources operating at 2:91 μm (Cr:ZnSe) with a pulse energy of 0:76mJ and at 1:9 μm (Tm:YLF) with a pulse energy of 2:2mJ are investigated. The results show that highly efficient bone ablation with the CO2 laser at rates of 6mm3/s is possible without carbonization. With an ablation strategy using the effect of multiple reflections inside the kerf an aspect ratio of 17 was achieved for narrow incisions at widths smaller than 100 μm. Another ablation strategy shows a twofold higher ablation depth shifting the laser focus stepwise into propagation direction. Even though a high ablation efficiency can be achieved with the Cr:ZnSe laser source and the CO2 laser source, the Cr:ZnSe laser cannot fulfill the required ablation rates. The CO2 laser shows a fast ablation in significant depth with a maximum depth of 7 mm. Further investigations will concentrate on increasing the ablation depth to 10 mm.
Based on our study of a high stability ultra-narrow linewidth fiber amplifier for the gravitational wave detector LISA, we have set up a hybrid fiber and Innoslab amplifier for further power scaling into <100 Watt regime. The fiber amplifier can provide a seed power of up to 8 W at 1064 nm and a linewidth <10 kHz. The booster stage consists of an in-band pumped Innoslab amplifier, which is pumped by stabilized laser diodes at 880 nm. An output power of 437 W has already been demonstrated with almost diffraction-limited beam quality. Further investigations of power stability and modal properties are currently ongoing.
We report an in band pumped single-stage 410 W INNOSLAB laser amplifier based on Nd:YVO4. VBG stabilized diodes are used to pump the crystal from both end faces at 880 nm wavelength. At 3 W@800KHz input power and a pulse duration of 300 ps an extraction efficiency of 44% is achieved. A beam quality of M2<1.5 (4-Sigma Method) and M2<1.3 (10/90 Knife Edge Method) is measured over the whole power range. The output beam is free of self-lasing or CW-background.
The spectral stability of a previously reported Ho:YLF single frequency pulsed laser oscillator emitting at 2051 nm is drastically improved by utilizing a narrow linewidth Optically Pumped Semiconductor Laser (OPSL) as a seed for the oscillator. The oscillator is pumped by a dedicated gain-switched Tm:YLF laser at 1890 nm. The ramp-and-fire method is employed for generating single frequency emission. The heterodyne technique is used to analyze the spectral properties. The laser is designed to meet a part of the specifications for future airborne or space borne LIDAR detection of CO2. Seeding with a DFB diode and with an OPSL are compared. With OPSL seeding an Allan deviation of the centroid of the spectral distribution of 38 kHz and 517 kHz over 10 seconds and 60 milliseconds of sampling time for single pulses is achieved. The spectral width is approximately 30 MHz. The oscillator emits 2 mJ pulse energy with 50 Hz pulse repetition frequency (PRF) and 20 ns pulse duration. The optical to optical efficiency of the Ho:YLF oscillator is 10 % and the beam quality is diffraction limited. To our knowledge this is the best spectral stability demonstrated to date for a Ho:YLF laser with millijoule pulse energy and nanosecond pulse duration.
A single-frequency q-switched Ho:YLF laser oscillator with a bow-tie ring resonator, specifically designed for highspectral stability, is reported. It is pumped with a dedicated Tm:YLF laser at 1.9 μm. The ramp-and-fire method with a DFB-diode laser as a reference is employed for generating single-frequency emission at 2051 nm. The laser is tested with different operating modes, including cw-pumping at different pulse repetition frequencies and gain-switched pumping. The standard deviation of the emission wavelength of the laser pulses is measured with the heterodyne technique at the different operating modes. Its dependence on the single-pass gain in the crystal and on the cavity finesse is investigated. At specific operating points the spectral stability of the laser pulses is 1.5 MHz (rms over 10 s). Under gain-switched pumping with 20% duty cycle and 2 W of average pump power, stable single-frequency pulse pairs with a temporal separation of 580 μs are produced at a repetition rate of 50 Hz. The measured pulse energy is 2 mJ (<2 % rms error on the pulse energy over 10 s) and the measured pulse duration is approx. 20 ns for each of the two pulses in the burst.
In the field of atmospheric research lidar is a powerful technology to measure remotely different parameters like gas or aerosol concentrations, wind speed or temperature profiles. For global coverage, spaceborne systems are advantageous. To achieve highly accurate measurements over long distances high pulse energies are required. A Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent InnoSlab-based amplifier stages was designed and built as a breadboard demonstrator. Overall, more than 500 mJ of pulse energy at 100 Hz pulse repetition frequency at about 30 ns pulse duration in single longitudinal mode were demonstrated. When seeded with 75 mJ pulses, the 2nd amplifier stage achieved an optical efficiency (pump energy to extracted energy) of more than 23 % at excellent beam quality. Recently, different MOPA systems comprising a single InnoSlab amplifier stage in the 100 mJ regime were designed and built for current and future airborne and spaceborne lidar missions. Amplification factors of about 10 at optical efficiencies of about 23 % were achieved. In order to address the 500 mJ regime the established InnoSlab design was scaled geometrically in a straight forward way. Hereby, the basic design properties like stored energy densities, fluences and thermal load densities were retained. The InnoSlab concept has demonstrated the potential to fulfill the strong requirements of spaceborne instruments concerning high efficiency at low optical loads, excellent beam quality at low system complexity. Therefore, it was chosen as baseline concept for the MERLIN mission, currently in phase B.
A test campaign for assessing the radiation hardness of different Erbium-doped garnet crystals including Er:YAG and a
compositionally tuned Er:YAG/Er:LuAG mixed garnet is reported. Tests with proton and gamma radiation have been
performed with parameters mimicking a 3-year low-earth-orbit satellite mission like MERLIN or ADM-Aeolus. For each
test sample broadband transmission spectra in the wavelength range of 500 nm – 1700 nm and characteristic laser curves
from a test laser oscillator have been measured. Radiation-induced losses have been calculated from the obtained data.
The results indicate that gamma radiation is the dominant loss source with about 0.5 %/cm radiation-induced losses for
the nominal dose of the chosen mission scenario.
Several laser systems in the infrared wavelength range, such as Nd:YAG, Er:YAG or CO2 lasers are used for efficient ablation of bone tissue. Here the application of short pulses in coaction with a thin water film results in reduced thermal side effects. Nonetheless up to now there is no laser-process for bone cutting in a clinical environment due to lack of ablation efficiency. Investigations of laser ablation rates of bone tissue using a rinsing system and concerning bleedings have not been reported yet. In our study we investigated the ablation rates of bovine cortical bone tissue, placed 1.5 cm deep in water under laminar flow conditions, using a short pulsed (25 ps), frequency doubled (532 nm) Nd:YVO4 laser with pulse energies of 1 mJ at 20 kHz repetition rate. The enhancement of the ablation rate due to debris removal by an additional water flow from a well-directed blast pipe as well as the negative effect of the admixture of bovine serum albumin to the water were examined. Optical Coherence Tomography (OCT) was used to measure the ablated volume. An experimental study of the depth dependence of the ablation rate confirms a simplified theoretical prediction regarding Beer-Lambert law, Fresnel reflection and a Gaussian beam profile. Conducting precise incisions with widths less than 1.5 mm the maximum ablation rate was found to be 0.2 mm3/s. At depths lower than 100 μm, while the maximum depth was 3.5 mm.
We report on a single-frequency laser oscillator based on a new Er:YLuAG laser crystal which is spectrally suitable for
application as a transmitter in differential absorption lidar measurements of atmospheric CH4. The laser emits singlefrequency laser pulses with 2.3 mJ of energy and 90 ns duration at a repetition rate of 100 Hz. It is resonantly pumped by two linearly polarized single-mode cw fiber lasers at 1532 nm. A scan of the CH4-absorption line at 1645.1 nm was performed and the shape of the line with its substructure was reproduced as theoretically predicted. A 2.5-dimensional performance model was developed, in which pump absorption saturation and laser reabsorption is included. Also the spectral output of the laser oscillator longitudinal multimode operation could be predicted by the laser model.
A Tm:YLF laser in INNOSLAB design is reported. It produces 200 W of output power at an optical efficiency of 24 % and a slope efficiency of 27 % with respect to incident pump power. The laser crystal is partially end-pumped in a tophat line focus with a width of 12 mm and a height of about 1 mm. It is placed in a stable, spherical laser resonator, which results in a highly elliptical output beam. The beam is near diffraction limit and Gaussian in shape in one axis and contains very high order transversal modes and is Top-Hat-like in shape in the other axis. The beam shape is ideal for pumping a Ho:YLF laser crystal in INNOSLAB design for a pulsed amplifier.
Rainer Lebert, Azadeh Farahzadi, Wolfgang Diete, David Schäfer, Christoph Phiesel, Thomas Wilhein, Stefan Herbert, Aleksey Maryasov, Larissa Juschkin, Dominik Esser, Marco Hoefer, Dieter Hoffmann
There is a strong demand for standalone actinic tools for mask blank and mask metrology. We expect to deliver
contributions to key issues for the infrastructure tools such as actinic reflectometer, actinic defect inspection and
components like high brightness sources together with our partners.
With our EUV-reflectometer EUV-MBR we are ready to fulfill HVM requirements in accurate and sensitive spectral
metrology. Migrating from mask blanks to masks is supported with integrated fiducial mark detection and small spot
sizes of down to < 0.03 mm2. Hence, the EUV-MBR is able to detect minimal variations on mask blank and can support
process monitoring for our partners in European EXEPT project.
For actinic blank inspection a proof of concept experiment based on an EUV microscope at BASC's EUV-Lamp allows
for comparing actinic signatures with AFM scans. Results allow for extrapolation to sub 30 nm sensitivity and fast full
blank scan.
For LPP sources we demonstrated a new concept utilizing a laser, with parameters optimized for high brightness EUV
generation and a new regenerative target concept for high position stability, gain, repetition rate operation and efficiency
in the first proof of concept experiment. Up to 350 W/(mm2 sr) from < 20 μm source size have been demonstrated.
An Innoslab based Nd:YV04 MOPA system with pulse energy of 7.25 mJ at 40 kHz repetition rate and pulse
duration of 11.4 ns has been used for third harmonics generation in Lithium Triborate (LBO) crystals. We report
UV pulses of 8.9 ns duration at pulse energy of 1.65 mJ, which means an average power of 66 W. We have been
able to show UV beam qualities (M2) of 1.7/2.4 (stable/instable direction with 90/10 knife edge method), while
IR beam quality is 1.8/5.2. A sinc2-shape transversal distribution of beam intensity has been used in instable
direction of the Innoslab MOPA system for conversion. Due to high average power and short pulse length at
355 nm the laser meets the demands for high-throughput micro material processing as stereolithography or edge
isolation of solar cells. The thermal dependence of the conversion efficiency (due to heating power of the beam)
has been investigated theoretically, using a time resolved numerical simulation model for the nonlinear process in
both LBO crystals. Scaling effects of the absorption coefficients of LBO and the pulse power on the conversion
efficiency are presented in this article.
For spaceborne lidar like the atmospheric backscatter lidar (e.g. ATLID on the ESA EarthCARE mission) highly reliable and efficient laser sources are needed. As pre-development model we realized a Nd:YAG MOPA diode pumped at 100 Hz. With more than 21 % optical-optical efficiency the amplifier based on the InnoSlab design raises the 8 mJ pulse energy from the single frequency rod oscillator to more than 70 mJ. Frequency-tripling leads to more than 25 mJ at 355 nm and a beam quality of M2 < 1.7. The total optical-optical efficiency of more than 7.5 % exceeds the efficiency of comparable current lidar transmitter systems at least by a factor of 2. The laser is designed to cope with diode degradation or failure. Moderate pulse intensities in the InnoSlab amplifier offer excellent possibilities to scale the pulse energy to several 100 mJ in a most reliable and efficient way.
We present a long term stable single frequency MOPA with 40 W output power and M2 < 1.2 consisting of an ultrastable
1 W non-planar ring cavity oscillator with outstanding single frequency characteristics and one Innoslab amplifier
stage. The partially end pumped Nd:YVO4 Innoslab amplifier is set up in a folded single pass configuration. A 30 GHz
tunable spectral bandwidth of 1kHz/100ms and a relative intensity noise (RIN) of < 1 10-5 Hz-1/2 (>10kHz) was measured
in free-running mode. Together with an active intensity noise suppression electronics a RIN of < 5 10-7 Hz-1/2 (>10kHz)
was achieved. A comprehensive discussion of experimental data is given.
Lidar Systems for the measurement of three-dimensional wind or cloud and aerosol formations in the earth atmosphere
require highly stable pulsed single frequency laser systems with a narrow line width. The lasers for ESAs ADM-Aeolus
and EarthCARE missions require frequency stabilities of 4 and 10 MHz rms at a wavelength of 355 nm and a line width
below 50 MHz at 30 ns pulse duration[1]. Transferred to the fundamental wavelength of the laser systems the stability
requirement is 1.3 and 3.3 MHz, respectively. In comparison to ground based lidar systems the vibrational load on the
laser system is much higher in airborne and spaceborne systems, especially at high frequencies of some hundred Hertz or
even some kHz. Suitable frequency stabilisation methods have therefore to be able to suppress these vibrations
sufficiently. The often used Pulse-Build-up method is not suitable, due to its very limited capability to suppress vibration
frequencies of the order of the pulse repetition frequency.
In this study the performance of three frequency stabilisation methods in principle capable to meet the requirements, the
cavity dither method, the modified Pound-Drever-Hall method and a modified Ramp-Fire method - named Ramp-Delay-
Fire - is theoretically and experimentally investigated and compared.
The investigation is performed on highly efficient, passively cooled, diode end-pumped q-switched Nd:YAG oscillators,
which are breadboard versions of the A2D (ADM-Aeolus) and possible ATLAS (EarthCARE) oscillators. They deliver
diffraction limited output pulses with up to 12 mJ pulse energy at a pulse duration of 30 ns and 100 Hz pulse repetition
rate.
Design and experimental characterization of a nonlinear optical converter module for the generation of widely tunable
UV radiation is presented. The module combines units for second, third and fourth harmonic generation of tunable
Ti:Sapphire lasers. A modified conversion scheme based on the combination of BIBO and BBO crystals reduces the
complexity of our former published UV setup - resulting in a significant increase of performance and long-term stability
of the system. Experimental characterization of the former and the improved UV setup are compared. The investigations
of the converter module are carried out with a widely tunable Ti:Sapphire laser with nanosecond pulses and a repetition
rate of 1 kHz. This laser provides a continuous tuning range of 690 nm to 1010 nm with pulse energies up to 2.0 mJ and
a spectral line width of less than 10 GHz resulting in an output power of the converter module of 1000 mW, 400 mW
and 200 mW respectively for the second, third and fourth harmonic generation. The new converter module is a decisive
step in the development of a hands-off solid-state laser system with a continuous tuning range from the UV to the NIR -
200 nm to 1000 nm.
A Master-Oscillator-Power-Amplifier (MOPA) design combining rod and slab laser technology for high pulse energy, high average power and near diffraction limited beam quality for industrial use has been developed. To achieve the good beam quality at high average and high pulse power, an advanced birefringence compensation scheme, which ensures a high mode overlap while simultaneously minimizing the power densities on optical surfaces, has been developed and applied. The prototypes deliver an average power of up to 860 W with M2 < 2 or 1.3 kW with M2 < 12 at 10 kHz repetition rate and 5-16 ns pulse duration. At 1 kHz up to 420 mJ pulse energy can be achieved. The prototypes are fully computer controlled and can be operated from 0 to 100 % output power and from single shot to 10 kHz. They are currently operated for plasma generation in a laboratory surrounding and have run for more than one thousand hours without failure up to now. An analytical solution of the thermally induced refractive index profile in dependency of a radially symmetric pump light distribution including the effect of thermally induced birefringence, temperature dependency of the thermal conductivity and the second derivative of the refractive index with the temperature (d2n/dT2) has been derived. This allows a fast calculation of thermally induced aberrations without the use of FEA. Experimental results are compared to predictions from analytical and FEA modelling. Based on experimental and theoretical results, scaling limits of rod based MOPAs are predicted.
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