M. Golling

Diode-pumped semiconductor disk laser with intracavity frequency doubling using lithium triborate (LBO)

Diode-pumped semiconductor disk lasers, also referred to as vertical external cavity surface-emitting lasers show excellent beam characteristics in combination with high output powers. In this letter, we present a diode-pumped semiconductor disk laser based on the InAlGaAs material system without any additional phosphorus containing layers for strain compensation. We have achieved a near-infrared /spl lambda//spl sim/972-nm laser output power of 0.25 W in the fundamental transverse mode. The disk laser beam has been coupled into a single-mode fiber with a coupling efficiency of >70%. Moreover, we have shown 490-nm blue-green light generation by intracavity frequency doubling using lithium triborate (LBO).

Photonic crystal surface-emitting lasers: tailoring waveguiding for single-mode emission

The concept of photonic crystal surface-emitting lasers (PCSELs) is introduced by etching a hexagonal pattern of small air holes into the top Bragg reflector of a conventional 980 nm-wavelength VCSEL structure. For certain current ranges, the resulting devices oscillate only on the fundamental transverse mode even for oxide aperture diameters up to 12 /spl mu/m, yielding device resistances in the 50 /spl Omega/-regime.

Current-spreading-induced bistability in bipolar cascade vertical-cavity surface-emitting lasers

We present experimental evidence that current spreading in the cavity of two-stage bipolar cascade vertical-cavity surface-emitting lasers is responsible for the formation of bistability loops in the light versus current characteristics. The bistable behavior strongly varies with the detuning between cavity resonance and gain maximum and is attributed to the wavelength and carrier density dependent absorption coefficient. In order to explain the measured dependencies, an analytical model is presented that very well matches the light output characteristics.

Scaling behavior of bipolar cascade VCSELs

We present theoretical and experimental results on the scaling properties of bipolar cascade vertical-cavity surface-emitting lasers with respect to active region stacking and out-coupling mirror reflectivity. The measurement results at around 980-nm wavelength confirm the theoretically expected behavior of an almost linear increase in slope efficiency and a reduction in threshold current density with the number of stages. A three-stage laser shows a differential quantum efficiency of 128% in continuous-wave operation at room temperature. For the threshold current density, we obtain a decrease from 3.75 kA/cm/sup 2/ for the conventional VCSEL to about 1.2 kA/cm/sup 2/ for a three-stage device. Such multi-diode cascade VCSELs are highly interesting for optical links, analog systems, and high-power applications.

Gas source MBE growth of 1.3 μm-InAsP/InGaAsP quantum wells GRINSCH laser showing low threshold current density and high output power

We report the successful fabrication of the first gas source MBE grown InAsP/InGaAsP multiple-quantum well lasers with compositional linearly graded InGaAsP confinement layers. The optical quality of the InAsP/InGaAsP quantum wells are investigated by room temperature photoluminesence (PL) spectroscopy; Intense PL-signal with small FWHM is observed in structures containing three quantum wells indicating that our structures are of high optical quality. Nonlinear temperature-ramps are developed and applied to the gallium effusion cell in order to grow lattice-match compositional linearly graded InGaAsP confinement layers. X-ray rocking curves show that the lattice-mismatches of the graded InGaAsP confinement layers. X-ray rocking curves show that the lattice-mismatches of the graded InGaAsP layers are well below 1 x 10 -3 . These results indicate that the control of material supply during growth of the InGaAsP layers is highly precise. Laser structures containing graded confinement layers and three quantum wells are grown and fabricated into broad-area laser diodes. Threshold current density of 160A cm - 2 was obtained for 1.5 mm long lasers. This value is among the lowest ever achieved for 1.3 μm lasers grown by any kind of MBE process.

Influence of design variations on bipolar cascade VCSEL performance

The basic idea of bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) is the monolithic stacking of active regions that are electrically coupled using Esaki tunnel junctions in between. Thus, carriers can be recycled for additional radiative recombination resulting in a higher roundtrip gain for cascade VCSELs compared to the conventional structure. This offers an additional design freedom to improve optical performance of VCSELs. Firstly, it is possible to scale down the threshold current density and increase the differential quantum efficiency simultaneously. Secondly, cascade VCSELs have the potential to boost optical output power. Such devices might be an interesting solution for high-power VCSEL applications, optical links with gain and analog systems. The scalability of device performance with respect to active layer stacking has been successfully demonstrated recently, even with a slope efficiency well exceeding unity in continuous wave operation at room temperature. However, to improve cascade laser performance further, we have investigated various designs to achieve a better understanding of device behavior. In this paper, we present the comparison of electrical and optical properties of three different two-stage diode cascade VCSEL designs. It is observed that current spreading in the cavity and device degradation are important issues that have to be considered in bipolar cascade VCSELs.

Optimization strategies for diode cascade InGaAs VCSELs

We present top surface emitting two- and three-stage bipolar cascade vertical cavity surface emitting lasers (VCSELs) for continuous wave (cw) operation at room temperature. The results are compared with a conventional VCSEL grown under identical growth conditions for reference. The improvement of both threshold current density and differential quantum efficiency can be clearly shown in structures with unchanged top mirror reflectivities (19 pairs). To further increase the differential quantum efficiency, a device with only 14 p-type top mirror pairs is fabricated, yielding an efficiency of 130% for cw room temperature operation. From noise measurements, we extract a modulation efficiency of 8.2 GHz/sqrt(mA}, indicating favorable dynamic properties of these VCSEL.