B. Kunert

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.

Laser operation of Ga(NAsP) lattice-matched to (001) silicon substrate

The lattice-matched growth of the direct band gap material Ga(NAsP) is a seminal concept for the monolithic integration of III/V laser on a silicon substrate. Here, we report on the growth, characterization, and lasing properties of Ga(NAsP)/(BGa)(AsP) multi quantum well heterostructures embedded in (BGa)P cladding layers which were deposited on an exactly oriented (001) Si substrate. Structural investigations confirm a high crystal quality without any indication for misfit or threading dislocation formation. Laser operation between 800 nm and 900 nm of these broad area device structures was achieved under optical pumping as well as electrical injection for temperatures up to 150 K. This “proof of principle” points to the enormous potential of Ga(NAsP) as an optical complement to Si microelectronics.

Direct-band-gap Ga(NAsP)-material system pseudomorphically grown on GaP substrate

Compressively strained Ga(NAsP) multi-quantum-well heterostructures with As concentration above 85% have been grown pseudomorphically on GaP substrates by metal organic vapor phase epitaxy. Detailed structural analysis applying high-resolution x-ray diffraction proves the high crystalline perfection of the samples. Optical spectroscopy appyling photoluminescence and excitation spectroscopy verify the direct-band-gap characteristic of this novel material system. The comparison of the experimental data with elemental calculations via the band anticrossing model demonstrates that the formation of direct band structure can be understood by the strong bowing of the band gap energy typical for diluted III-V nitrides.

High-Power Optically Pumped Semiconductor Laser at 1040 nm

We demonstrate near-diffraction limited (M 2 ¿ 1.5) output up to 23.8 W with optical-to-optical efficiency 27% and slope efficiency 32.4% and 40.7 W of multimode output from an optically pumped semiconductor laser at 1040 nm. Temperature-dependent photoluminescence measurements confirm accurate epitaxial growth according to the design thereby enhancing the effective gain.

In situ verification of single-domain III-V on Si(100) growth via metal-organic vapor phase epitaxy

Reflectance anisotropy spectroscopy (RAS) was used in situ for the quantification of antiphase domains on surfaces of thin GaP films deposited onto Si(100) by metal-organic vapor phase epitaxy (MOVPE). The preparation of a single-domain GaP∕Si(100) surface was determined via the analysis of RAS peak intensities in reference to the well-known P-rich surface reconstruction of homoepitaxially grown GaP(100). Both preprocessed Si(100) substrates and MOVPE as-grown GaP∕Si(100) films were also characterized ex situ by atomic force microscopy to identify the formation of mono- and diatomic surface steps and to analyze of the domain distribution, respectively.

15 W Single Frequency Optically Pumped Semiconductor Laser With Sub-Megahertz Linewidth

We report on a single frequency optically pumped semiconductor laser exhibiting an output power of 15 W in continuous wave operation. The GaAs-based structure presents an emission wavelength of 1020 nm and a tuning range , with a continuous tunability of 9 GHz. The TEM00 output beam exhibits very low transverse phase fluctuations across the entire mode, leading to a beam quality . A free-running laser linewidth of 21 kHz has been deduced from frequency noise measurements for a sampling time of 1 ms and increases to 995 kHz for a sampling time of 1 s.

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.

Heat Management in High-Power Vertical-External-Cavity Surface-Emitting Lasers

The thermal properties of a high-power vertical-external-cavity surface-emitting laser (VECSEL) are studied experimentally, focusing on the generation, distribution, and removal of excess heat under extreme pumping conditions. Different heat-spreading and heat-transfer approaches are analyzed. The performance of the device is optimized yielding a maximum emitted power beyond 70 W from a single spot. Finally, the potential for power-scaling in VECSELs and its restrictions are examined.

Temperature dependence and physical properties of Ga(NAsP)/GaP semiconductor lasers

We report on the properties of GaNAsP/GaP lasers which offer a potential route to producing lasers monolithically on silicon. Lasing has been observed over a wide temperature range with pulsed threshold current density of 2.5 kA/cm2 at 80 K (λ=890 nm). Temperature dependence measurements show that the radiative component of the threshold is relatively temperature stable while the overall threshold current is temperature sensitive. A sublinear variation of spontaneous emission versus current coupled with a decrease in external quantum efficiency with increasing temperature and an increase in threshold current with hydrostatic pressure implies that a carrier leakage path is the dominant carrier recombination mechanism.