Thomas Schwarzbäck

Electrically driven quantum dot single-photon source at 2 GHz excitation repetition rate with ultra-low emission time jitter

The influence of the bias voltage on emission properties of a red emitting InP/GaInP quantum dot based single-photon source was investigated. Under pulsed electrical excitation, we can influence the band bending of the p-i-n diode with the applied bias voltage and thus the charge carrier escape by quantum tunneling. This leads to control over the non-radiative decay channel and allows carrier escape times as low as 40 ps, effectively reducing the time jitter of the photon emission. We realized high excitation repetition rates of up to 2 GHz while autocorrelation measurements with g(2)(0)-values of 0.27 attest dominant single-photon emission.

Mode-locked red-emitting semiconductor disk laser with sub-250 fs pulses

We report on passive mode locking of a semiconductor disk laser emitting pulses shorter than 250 fs at 664 nm with a repetition frequency of 836 MHz. A fast saturable absorber mirror fabricated by metal-organic vapor-phase epitaxy in a near-resonant design was used to enable the mode locking operation. It includes two GaInP quantum wells located close to the surface and an additional fused silica coating. The emission spectrum shows the superposition of a soliton-like part and a smaller “continuum” part.

High-power InP quantum dot based semiconductor disk laser exceeding 1.3 W

We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)0.52In0.48]0.5P0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of ∼654 nm with a slope efficiency of ηdiff=25.4 %.

High optical output power in the UVA range of a frequency-doubled, strain-compensated AlGaInP-VECSEL

We present a maximum continuous-wave optical output power of 260 mW from an optically pumped, frequency-doubled vertical-external-cavity surface-emitting laser with wavelengths ranging from 325 to 332 nm. The device consists of a GaInP/AlGaInP multi-quantum-well structure grown using metal–organic vapour-phase epitaxy. We use strain compensation in the active region of the laser chip to for better performance at short wavelengths. In addition to wavelength-tuning results in the UVA region, power transfer measurements of the fundamental mode exceeding 1.2 W are presented.

Wavelength tunable ultraviolet laser emission via intra-cavity frequency doubling of an AlGaInP vertical external-cavity surface-emitting laser down to 328 nm

We demonstrate an optically pumped vertical external-cavity surface-emitting laser in a compact v-shaped cavity configuration for frequency doubling to the ultraviolet (UV) spectral range at ∼330 nm. The fundamental red laser emission is realized with a metal-organic vapor-phase epitaxy grown (GaxIn1−x)0.5P0.5/[(AlxGa1−x)yIn1−y]0.5P0.5 multi-quantum-well structure. Second harmonic generation is accomplished by using a beta barium borate non-linear crystal to generate maximum UV output powers exceeding 100 mW. By using a birefringent filter, we are able to tune the fundamental laser resonance to realize a maximum tuning range of 7.5 nm of the second harmonic.

All quantum dot mode-locked semiconductor disk laser emitting at 655 nm

We present a semiconductor disk laser mode-locked by a semiconductor saturable absorber mirror (SESAM) with emission in the red spectral range. Both the gain and the absorber structure are fabricated by metal-organic vapor-phase epitaxy in an anti-resonant design using quantum dots as active material. A v-shaped cavity is used to tightly focus onto the SESAM, producing pulses with a duration of about 1 ps at a repetition rate of 852 MHz.

Intra-cavity frequency-doubled mode-locked semiconductor disk laser at 325 nm

We present a passively mode-locked semiconductor disk laser (SDL) emitting at 650nm with intra-cavity second harmonic generation to the ultraviolet (UV) spectral range. Both the gain and the absorber structure contain InP quantum dots (QDs) as active material. In a v-shaped cavity using the semiconductor samples as end mirrors, a beta barium borate (BBO) crystal is placed in front of the semiconductor saturable absorber mirror (SESAM) for pulsed UV laser emission in one of the two outcoupled beams. Autocorrelation (AC) measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps. With a repetition frequency of 836MHz, the average output power is 10mW per beam for the red emission and 0.5mW at 325nm.

Enhanced efficiency of AlGaInP disk laser by in-well pumping

The performance of a 665-nm GaInP disk laser operated continuous-wave at 15°C both in-well-pumped at 640 nm and barrier pumped at 532 nm is reported. The efficiency with respect to the absorbed power was enhanced by 3.5 times when using a 640-nm pump instead of a 532-nm pump. In-well pumping which is based on the absorption of the pump photons within the quantum-well heterostructures of the gain region instead of short-wavelength absorption in the barrier and spacer regions reduces the quantum defect between pump and laser photon and hence the heat generation. A slope efficiency of 60% with respect to the absorbed pump power was obtained by in-well pumping at 15°C. Continuous-wave laser operation was further demonstrated at heat sink temperatures of up to 55°C. Both the measurement of photoluminescence and COMSOL simulation show that the overall heat load in the in-well pumped laser is smaller than in the barrier-pumped laser. These results demonstrate the potential of optical in-well pumping for the operation of red AlGaInP disk lasers if combined with means for efficient pump-light absorption.

2.5 W continuous wave output at 665 nm from a multipass and quantum-well-pumped AlGaInP vertical-external-cavity surface-emitting laser

An output power of 2.5 W at a wavelength of 665 nm was obtained from a quantum-well (QW) and multipass-pumped AlGaInP-based vertical-external-cavity surface-emitting laser operated at a heat sink temperature of 10°C. Intracavity frequency doubling resulted in an output power of 820 mW at a wavelength of 333 nm. To the best of our knowledge, these are the highest continuous wave output powers from this type of laser both at the fundamental wavelength and in frequency-doubled operation. In fundamental wavelength operation, further power scaling by increasing the pump-spot size increased the output power to 3.3 W. However, at this power level, the laser was highly unstable. When the laser was operated at 50% pump duty cycle, a reproducible and stable peak output power of 3.6 W was obtained. These results demonstrate the potential of optical QW pumping combined with multipass pumping for the operation of AlGaInP-based semiconductor disk lasers.

Red AlGaInP-VECSEL emitting at around 665 nm: strain compensation and performance comparison of different epitaxial designs

We present a comparison of epitaxial designs for non-resonantly pumped vertical external cavity surface-emitting lasers for emission in the red spectral range around 665 nm. Here, the VECSEL chip is based on a metal-organic vapor-phase epitaxy grown (GaxIn1-x)0.5P0.5/[(AlxGa1-x)yIn1-y]0.5P0.5 multi-quantum-well structure with 20 compressively-strained quantum wells. The wells are placed in packages in a separate confinement heterostructure with quaternary AlGaInP barriers and cladding layers, respectively. The active region is fabricated on a 55 λ/4 pairs Al0.50Ga0.50As/AlAs distributed Bragg reflector. We compare two designs with different quantum well distributions in the chip: one design which includes 4 quantum wells in 5 packages whereas the other contains 10 quantum well pairs to have a larger absorption length. Laser parameters like output power, differential efficiency and threshold pump power of the different chip designs measured in a v-shaped cavity configuration are examined. By using the 10 × 2 quantum well distribution in the chip, we could improve the absorption efficiency by nearly 40% and output power by 25% compared to the 5 × 4 design. Additionally, by introducing tensile strained quaternary barriers and cladding layers in the 5 × 4 QW design, we could compensate for the compressive strain introduced by the quantum wells. Photoluminenscence measurements of structures with different numbers of quantum well packages reveal a more homogenous quantum well growth due to the strain-compensation technique. Furthermore, with the strain compensation technique, the output power could be increased over 30% compared to our conventional structures.