Alexander Hein

Optimization of a Broadband Gain Element for a Widely Tunable High-Power Semiconductor Disk Laser

The layer structure of the gain element in an optically pumped semiconductor disk laser was parametrically optimized with respect to a target function specifying a desired unsaturated reflectance over a desired wavelength range at a constant pump intensity. Spectral threshold pump intensity measurements confirmed the efficacy of the design, showing a much wider low-threshold regime than a conventional nonbroadband gain element, in good agreement with simulations. This evaluation avoids the possible influence of additional factors under high-power operation. Nonetheless, having a high and nearly constant broadband unsaturated reflectance at low pump intensity is a key to obtain good high-power performance, as evidenced by the obtained continuous tuning from 967 to 1010 nm with a maximum output power of 2.6 W.

Efficient 460-nm Second-Harmonic Generation With Optically Pumped Semiconductor Disk Lasers

We present a semiconductor disk laser with a fundamental emission at 920 nm which is frequency-doubled utilizing a bismuth triborate nonlinear crystal. The generated blue emission at 460 nm is the most suitable wavelength for display applications. The device layer design and the applied resonator geometry are discussed. Experimental results reveal overall optical-to-optical conversion efficiencies of up to 41.5% for the fundamental and up to 14.5% for the second-harmonic emission.

High-order dispersion in sub-200-fs pulsed VECSELs

We present a VECSEL based on a gain sample design which utilizes only a single-layer dielectric Al2O3 coating for dispersion management. The gain structure generated pulse durations down to 193 fs in combination with a surface-recombination SESAM, with an average power of 400 mW at 1.6 GHz setting a new peak power record for sub-200 fs mode-locked VECSELs. The pulses obtained were, however, 2x transform-limited and a further FROG measurement of a similar laser is presented revealing a linear chirp and cubic spectral phase.

Optically pumped high-power semiconductor disk laser with gain element engineered for wide tunability

The layer structure of the gain element in an optically pumped semiconductor disk laser (OP-SDL) was designed for wide tunability. This was achieved by a parametric optimization of the structure, which in effect balanced the spectrally varying influence of the gain of the quantum wells, the longitudinal distribution of the standing wave lasing field in the structure, and the degree of resonance in the subcavity formed between the distributed Bragg reflector at the bottom and the air-semiconductor interface at the top. The quality measure in the optimization was the spectral reflectance of the gain element for light incident from the external cavity at low power. This unsaturated reflectance was compared to its target function, which was constant at a specified value larger than unity over a wide, prescribed wavelength range. The fabricated gain element was used in a linear OP-SDL with a rotatable intra-cavity birefringent filter for wavelength tuning. The design principles for achieving wide tunability were experimentally validated by the strong agreement between measurements and simulations of the spectral threshold pump intensity. Furthermore, tuning experiments at high pump powers were performed showing that the lasing wavelength could be tuned from 967 nm to 1010 nm with a maximum output power of 2.6 W.

Optically quantum-well-pumped semiconductor disk lasers for single- and dual-wavelength emission

We present a design and output characteristics of an optically quantum-well-pumped semiconductor laser for single- and dual-wavelength emission which has a resonant disk structure for two wavelengths that lie 36.7nm apart. The smaller resonance wavelength is intended for the pump wavelength of 940 nm. Laser emission, however, can either take place at the short and/or at the long resonance wavelength. A switch between the emission wavelengths is performed by moving the gain peak towards the desired wavelength. For instance, while the heat-sink of the laser is kept at -15°C the laser will only emit a wavelength of 957.0 nm, a change of the heat-sink temperature to 50°C does result in an emission at the other resonance wavelength located at 997.5 nm. In both cases slope efficiencies above 50% and output powers beyond 10W are possible. Limiting factor is the available pump power. A simultaneous emission at 960.8 and 997.5nm is observed for heat-sink temperatures between 21.3 and 27.1°C. The intra-cavity performed sum-frequency generation (SFG) of the dual-wavelength laser leads to an emission at 489.7 nm.

Vertical-cavity surface-emitting laser arrays for miniaturized integrated optical lattice modules

Vertical-cavity surface-emitting lasers (VCSELs) can be readily arranged in two-dimensional arrays for diverse applications such as parallel fiber-optic data transmission or high-power generation for illumination, but also optical trapping in microfluidic channels. Optical trapping is a phenomenon where a focused laser beam attracts particles to the beam center [1]. These forces, which arise from momentum transfer, enable the contamination-free handling of micrometer-sized particles in various applications. So-called optical lattices are two-dimensional arrangements of optical traps and are an attractive tool for automated particle handling, e.g., deflection or sorting. Typical methods for the formation of the beam patterns such as interferometry [2] or holography [3] require the use of bulky external optics which need accurate alignment. By directly integrating VCSEL arrays with microfluidic channels, it is possible to minimize the distance between the lasers and the particles. External optics can thus be eliminated. Owing to the small beam divergence, optical manipulation is then performed without any focusing lenses.

High-power optically pumped green-emitting semiconductor disk lasers using second-harmonic generation

Summary form only given. Optically pumped semiconductor disk lasers (OPSDLs) combine the unique features of high output power, excellent beam quality, and wide wavelength coverage. In the fundamental regime, the wavelengths of these sources can be continuously addressed from 0.65 μm to 2.5 μm by choosing an appropriate material system. This wavelength band is essentially increased by generation of higher harmonics, thus, the shorter visible spectrum as well as the ultra-violet can be accessed. Additionally, the output power is in the watt-level for most of these wavelengths [1] at a rather high brilliance. Frequency down conversion (Raman shifting) can be employed to extend the range of well-established near infra-red semiconductor material systems such as GaAs-InGaAs [2].The presented devices are designed to generate high power both in the fundamental (1040 nm) and the second-harmonic (520 nm) regime. The target wavelength of around 520 nm is advantageous for a large colour gamut in projection displays. The devices are grown in a bottom-emitter fashion where the substrate is eventually removed, leaving a roughly 7 μm thick foil as the actual device. For more efficient heat dissipation, the devices were soldered/bonded to CVD diamond heat spreaders. At the fundamental wavelength of 1045 nm, output power in excess of 21 W was obtained. This optical output was extracted from pump spots of roughly 400 μm in diameter. In general, the excitation of larger areas and subsequent power scaling to the 100 W-level is possible [3]. The differential efficiency of the presented devices at room temperature operation exceeds 47 % as depicted in Fig. 1, and an overall efficiency of 38 % was achieved by sputtered dielectric coatings reducing the surface reflectivity. Moreover, watt-level operation of the devices with power exceeding 3.5 W was possible at heat sink temperatures of 90 C demonstrating the ability for applications in more severe environments.The second-harmonic output was generated in an intra-cavity fashion with a folded resonator setup. The cavity arrangement yields a large reduction of the second beam waist with a ratio of 6:1, thus, the increased intensity at the smaller waist allows a more efficient conversion. We applied a type-I critical phase matching in combination with a lithium triborate (LBO) nonlinear crystal. Slightly below room temperature spectrally narrow outputs of 8.2 W and 9.5 W were generated with an 11 mm long crystal as shown in Fig. 1. The beam quality at maximum optical outputs was approximately 1.2, conversion efficiencies for high pump levels reached values of 22 % and 20 %, respectively. Furthermore, it was possible to cover a wide spectral range of 22 nm (513-535 nm) in the second-harmonic regime by rotating the birefringent filter inside the laser cavity.

Frequency doubled high-power semiconductor disk lasers for stereo projection and ion traps

We present optically pumped semiconductor disk lasers (OPSDLs) emitting in the 900–1100nm band which are frequency doubled to access the visible spectrum. In particular, we focus on presenting the design, fabrication, and specific characteristics. Fundamental outputs exceeding 20W and visible radiation with powers of 5–10W are achieved. The blue and green emission at wavelengths around 450–470 and 520–540nm yield advantageous gamuts for display applications and can be utilized for stereo projection. Moreover, other accessible wavelengths in this spectral region, e.g. 493 nm, and the good beam quality of these devices enable optical ion trapping experiments.

Design of high-efficiency semiconductor disk lasers

The design of optically pumped semiconductor disk lasers is discussed with emphasis on the optimization for high power conversion e_ciencies. Main topics are the compensation of strain in the epitaxial layer sequence, the realization of a low-absorption Bragg reector which has a high reectivity for pump and laser wavelength and a low thermal resistance, and the e_ect of a surface coating reducing optical losses inside the semiconductor disk. As an alternative concept, quantum-well pumping may be more e_cient because of the reduced quantum defect. E_cient intra-cavity second-harmonic generation can be obtain in folded cavity setups.

VCSELs with two-sided beam emission for pressure sensor applications

A novel type of all-optical pressure sensor has been developed. In this context, a vertical-cavity surface-emitting laser (VCSEL) has been modified in its design to provide simultaneous light emission from both facets. One beam serves as measuring signal while the other establishes a reference; and both paths lie on the same optical axis. The VCSELs are based on active InGaAs quantum wells for laser output close to 960 nm wavelength where the GaAs substrate is transparent. From both top and bottom facet, single-polarization and single-mode beams are observed, having a power ratio of 1:2 to 1:4. In this paper we give insight into this new sensing application for VCSELs, describe the laser fabrication and the static operation characteristics as well as the noise properties which have paramount importance for high performance of the sensor. With regard to the sensor application in acoustics, the focus of the noise measurements is put on the low-frequency, i.e. kHz, regime. While laser diode noise performance is readily available for the MHz to GHz frequency range, only very limited data exists in the Hz to kHz domain. The relative intensity noise of both beams is measured and compared and the mutual correlation properties are investigated. The frequency noise is quantified.