Christoph Möller

Diffuse Scattering From Rough Surfaces in THz Communication Channels

Recent years have seen a tremendous increase in the demand for wireless bandwidth. To support this demand by innovative and resourceful use of technology, future communication systems will have to shift towards higher carrier frequencies. Due to the tight regulatory situation, frequencies in the atmospheric attenuation window around 300 GHz appear very attractive to facilitate an indoor, short range, ultra high speed THz communication system. In this paper, we investigate the influence of diffuse scattering at such high frequencies on the characteristics of the communication channel and its implications on the non-line-of-sight propagation path. The Kirchhoff approach is verified by an experimental study of diffuse scattering from randomly rough surfaces commonly encountered in indoor environments using a fiber-coupled terahertz time-domain spectroscopy system to perform angle- and frequency-dependent measurements. Furthermore, we integrate the Kirchhoff approach into a self-developed ray tracing algorithm to model the signal coverage of a typical office scenario.

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.

Self-mode-locking semiconductor disk laser

The development of mode-locked semiconductor disk lasers received striking attention in the last 14 years and there is still a vast potential of such pulsed lasers to be explored and exploited. While for more than one decade pulsed operation was strongly linked to the employment of a saturable absorber, self-mode-locking emerged recently as an effective and novel technique in this field - giving prospect to a reduced complexity and improved cost-efficiency of such lasers. In this work, we highlight recent achievements regarding self-mode-locked semiconductor devices. It is worth to note, that although nonlinear effects in the active medium are expected to give rise to self-mode-locking, this has to be investigated with care in future experiments. However, there is a controversy whether results presented with respect to self-mode-locking truly show mode-locking. Such concerns are addressed in this work and we provide a clear evidence of mode-locking in a saturable-absorber-free device. By using a BBO crystal outside the cavity, green light originating from second-harmonic generation using the out-coupled laser beam is demonstrated. In addition, long-time-span pulse trains as well as radiofrequency-spectra measurements are presented for our sub-ps pulses at 500 MHz repetition rate which indicate the stable pulse operation of our device. Furthermore, a long-time-span autocorrelation trace is introduced which clearly shows absence of a pedestal or double pulses. Eventually, a beam-profile measurement reveals the excellent beam quality of our device with an M-square factor of less than 1.1 for both axes, showing that self-mode-locking can be achieved for the fundamental transverse mode.

Scaled Bistatic Radar Cross Section Measurements of Aircraft With a Fiber-Coupled THz Time-Domain Spectrometer

The knowledge of the radar cross section (RCS) of aircraft and other objects is of great interest both for civil and military applications. Scaled setups are often used in order to facilitate RCS measurements in a well-defined laboratory environment. As radar frequencies steadily increase, for high scaling factors these measurements have to be carried out in the THz regime. In this paper, we propose an experimental setup consisting of a fiber-coupled THz time-domain spectrometer integrated with a two circle goniometer, which enables bistatic scaled RCS measurements. To assess the accuracy of the setup, measurements on reference objects as well as on scale model aircraft are performed. The measured data of the reference objects is compared to the theoretical predictions. As for the aircraft, the comparison between a Panavia 200 Tornado and a Lockheed F117 Nighthawk is made and the influence of individual components like bombs on the overall RCS is evaluated.

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.

Type-II vertical-external-cavity surface-emitting laser with Watt level output powers at 1.2 μm

Semiconductor laser characteristics based on type-II band-aligned quantum well heterostructures for the emission at 1.2 μm are presented. Ten “W”-quantum wells consisting of GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs are arranged as resonant periodic gain in a vertical-external-cavity surface-emitting laser. Its structure is analyzed by X-ray diffraction, photoluminescence, and reflectance measurements. The lasers power curves and spectra are investigated. Output powers at Watt level are achieved, with a maximum output power of 4 W. It is confirmed that laser operation only involves the type-II transition. A blue shift of the material gain is observed while the modal gain exhibits a red shift.

Dual-Wavelength Emission From a Serially Connected Two-Chip VECSEL

We present a compact two-chip vertical-external-cavity surface-emitting laser design, which provides dual-wavelength emission with a wavelength separation of 10 nm. The design is ideal for type-I frequency conversion, since the two wavelengths exhibit the same polarization, and over 600-W intracavity power is generated. Dual-wavelength operation with other desirable wavelength difference can be achieved in this flexible cavity, by using different chip combinations and suitable filters.

Gain spectroscopy of a type-II VECSEL chip

Using optical pump–white light probe spectroscopy, the gain dynamics is investigated for a vertical-external-cavity surface-emitting laser chip, which is based on a type-II heterostructure. The active region of the chip consists of a GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs multiple quantum well. For this structure, a fully microscopic theory predicts a modal room temperature gain at a wavelength of 1170 nm, which is confirmed by the experimental spectra. The results show a gain buildup on the type-II chip that is delayed relative to that of a type-I chip. This slower gain dynamics is attributed to a diminished cooling rate arising from the reduced electron–hole scattering.

A serially-connected two-chip VECSEL for dual-wavelength emission with high intracavity power

We present a compact and flexible cavity design for high intracavity powers in dual-wavelength vertical-external-cavity surface-emitting lasers (VECSELs), by serially connecting two different gain chips in one cavity. Such device generates linearly polarized dual-wavelength emission with up to 640 W intracavity power at 10 nm wavelength spacing, which is tunable via a changing of the cavity angles on the chips. Furthermore, in this cavity, type-I second harmonic generation and sum-frequency generation have been performed in a LiNbO3 crystal.

High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well lasers emitting at 1.3 µm

Electrical injection lasers emitting in the 1.3 μm wavelength regime based on (GaIn)As/Ga(AsSb)/(GaIn)As type-II double “W”-quantum well heterostructures grown on GaAs substrate are demonstrated. The structure is designed by applying a fully microscopic theory and fabricated using metal organic vapor phase epitaxy. Temperature-dependent electroluminescence measurements as well as broad-area edge-emitting laser studies are carried out in order to characterize the resulting devices. Laser emission based on the fundamental type-II transition is demonstrated for a 975 μm long laser bar in the temperature range between 10 °C and 100 °C. The device exhibits a differential efficiency of 41 % and a threshold current density of 1.0 kA/cm2 at room temperature. Temperature-dependent laser studies reveal characteristic temperatures of T0 = (132 ± 3) K over the whole temperature range and T1 = (159 ± 13) K between 10 °C and 70 °C and T1 = (40 ± 1) K between 80 °C and 100 °C.