Bernd Heinen

4.35 kW peak power femtosecond pulse mode-locked VECSEL for supercontinuum generation

We report a passively mode-locked vertical external cavity surface emitting laser (VECSEL) producing 400 fs pulses with 4.35 kW peak power. The average output power was 3.3 W and the VECSEL had a repetition rate of 1.67 GHz at a center wavelength of 1013 nm. A near-antiresonant, substrate-removed, 10 quantum well (QW) gain structure designed to enable femtosecond pulse operation is used. A SESAM which uses fast carrier recombination at the semiconductor surface and the optical Stark effect enables passive mode-locking. When 1 W of the VECSEL output is launched into a 2 m long photonic crystal fiber (PCF) with a 2.2 µm core, a supercontinuum spanning 175 nm, with average power 0.5 W is produced.

A 23-watt single-frequency vertical-external-cavity surface-emitting laser

We report on a single-frequency semiconductor disk laser which generates 23.6 W output power in continuous wave operation, at a wavelength of 1013 nm. The high output power is a result of optimizing the chip design, thermal management and the cavity configuration. By applying passive stabilization techniques, the free-running linewidth is measured to be 407 kHz for a sampling time of 1 ms, while undercutting 100 kHz in the microsecond domain.

On the Measurement of the Thermal Resistance of Vertical-External-Cavity Surface-Emitting Lasers (VECSELs)

Heat management is a key concern in the development of vertical-external-cavity surface-emitting lasers. Especially, high power systems are mainly limited by their heat transfer capabilities. A commonly used quantitative measure for the heat flow in such systems is the thermal resistance. So far, the thermal resistance is usually determined by evaluating the shift rates of the emission spectrum induced by varying the heat sink temperature and the input power. Yet, in multimode operation, the shift rates at the lower and the upper wavelength limit of the emission spectrum differ. In this paper, we will investigate the connection between the emission wavelength and the temperature profile inside the gain medium. We will show that the thermal resistance corresponding to the maximum pump spot temperature can only be obtained, by considering the shift rates at the long wavelength limit of the emission spectrum. Furthermore, we will show that the roll-over temperature is independent of the heat sink temperature. Based on this finding we present a novel technique, which enables the determination of the thermal resistance without the need for spectrally resolved measurements. The new technique surpasses the wavelength shift-based method both in terms of accuracy and measurement speed.

Terahertz Radiation at 0.380 THz and 2.520 THz Does Not Lead to DNA Damage in Skin Cells In Vitro

The question whether nonionizing electromagnetic radiation of low intensity can cause functional effects in biological systems has been a subject of debate for a long time. Whereas the majority of the studies have not demonstrated these effects, some aspects still remain unclear, e.g., whether high-frequency radiation in the terahertz range affects biological systems. In particular for frequencies higher than 0.150 THz, investigations of the ability of radiation to cause genomic damage have not been performed. In the present study, human skin cells were exposed in vitro to terahertz radiation at two specific frequencies: 0.380 and 2.520 THz. Power intensities ranged from 0.03–0.9 mW/cm2 and the cells were exposed for 2 and 8 h. Our goal was to investigate whether the irradiation induced genomic damage in the cells. Chromosomal damage was not detected in the different cell types after exposure to radiation of both frequencies. In addition, cell proliferation was quantified and found to be unaffected by the exposure, and there was no increase in DNA damage measured in the comet assay for both frequencies. For all end points, cells treated with chemicals were included as positive controls. These positive control cells clearly showed decreased proliferation and increased genomic damage. The results of the present study are in agreement with findings from other studies investigating DNA damage as a consequence of exposure to the lower frequency range (<0.150 THz) and demonstrate for the first time that at higher frequencies (0.380 and 2.520 THz), nonionizing radiation does not induce genomic damage.

Evolution of multi-mode operation in vertical-external-cavity surface-emitting lasers

The longitudinal multi-mode emission in a vertical-external-cavity surface-emitting laser is investigated using both single shot streak camera measurements and interferometric measurement techniques. For this, the laser is operated in the single- and two-color emission regime using both an etalon and a free-running configuration without etalon, respectively. The laser emission is analyzed with respect to pump power and output coupling losses for a long and for a short resonator. We observe a steep increase of emission bandwidth close to the laser threshold and monitor the transition between longitudinal single- and multi-mode operation. Additionally, the results indicate that a stable two-color operation is related to a sufficiently high number of oscillating longitudinal modes within each color.

Field Exposure and Dosimetry in the THz Frequency Range

With a growing number of applications utilizing THz radiation appearing on the market the question of health protection against non-ionizing electromagnetic fields arises in this frequency range, as at lower frequencies before. To date, about 50 independent empirical studies on living organisms, model systems and cells have been performed to clarify bio-electromagnetic interaction in the THz frequency range. Many of these studies find behavioral effects or effects on the cellular level, even at non-thermal exposure levels, while others do not report effects other than thermally induced damage. We discuss the general challenges in performing reliable field exposure experiments in the THz frequency range and describe a methodology that was adopted in a large campaign searching for genotoxic effects of THz radiation in vitro.

Efficient terahertz en-face imaging

In this work, we develop a pulsed terahertz imaging system in reflection geometry, where due to scanning of the terahertz beam neither the sample nor the emitter and detector have to be moved. We use a two mirror galvanoscanner for deflecting the beam, in combination with a single rotationally symmetric focusing lens. In order to efficiently image planar structures, we develop an advanced scanning routine that resolves all bending effects of the imaging plane already during measurement. Thus, the measurement time is reduced, and efficient imaging of surfaces and interfaces becomes possible. We demonstrate the potential of this method in particular for a plastic-metal composite sample, for which non-destructive evaluation of an interface is performed.

The Thermal Resistance of High-Power Semiconductor Disk Lasers

We present a model for the simulation of the thermal resistance of flip-chip bonded vertical-external-cavity surface-emitting lasers based on the finite-element method. Therefore, we take on and deepen precedent models with regard to three modifications. Our model for the first time comprises the complete heat removal, incorporates temperature-dependent heat conductivity of the diamond heat spreader and features the consideration of the exact pump distribution. The simulations are accompanied by an extensive experimental investigation of four gain chips. Thereby, a high accuracy of our simulations is confirmed. In addition, we use our model in order to investigate the influence of a ternary distributed Bragg reflector, which lacks in pump light absorption and the subsequent additional heating. Recently, this model was used to push the output power of vertical-external-cavity surface-emitting lasers beyond 100 W.

High-Power Quantum-Dot Vertical-External-Cavity Surface-Emitting Laser Exceeding 8 W

We report on a record-high output power from an optically pumped quantum-dot vertical-external-cavity surface-emitting laser, optimized for high-power emission at 1040 nm. A maximum continuous-wave output power of 8.41 W is obtained at a heat sink temperature of 1.5°C. By inserting a birefringent filter inside the laser cavity, a wavelength tuning over a range of 45 nm is achieved.

In vitro field exposition of skin cells between 100 GHz and 2.52 THz

Initiated by the German Federal Office for Radiation Protection (BfS) field exposition experiments have been designed to examine genotoxic effects of THz radiation in vitro. Under defined environmental conditions, two different human skin cell types are exposed to continuous-wave radiation at six distinct frequencies between 100 GHz and 2.52 THz originating from different sources of THz radiation. The cell containers are irradiated with free space power densities between 0.1 mW/cm2 and 10 mW/cm2 measured traceable to the SI units.