We describe a new coherent beam combining architecture based on passive phase-locking of two laser diodes in a Michelson external cavity on their rear facet, and their coherent combination on the front facet. As a proof-of-principle, two ridge lasers have been coherently combined with >90 % efficiency. The phase-locking range, and the resistance of the external cavity to perturbations have been thoroughly investigated. The combined power has been stabilized over more than 15 min with an optical feedback as well as with an automatic adjustment of the driving currents. Furthermore, two high-brightness high-power tapered laser diodes have been coherently combined in a similar arrangement; the combining efficiency is 70% and results in an output power of 4 W. We believe that this new configuration combines the simplicity of passive self-organizing architectures with the optical efficiency of master-oscillator power-amplifier ones.
Diode lasers are gaining importance, making their way to higher output powers along with improved BPP. The assembly of micro-optics for diode laser systems goes along with the highest requirements regarding assembly precision. Assembly costs for micro-optics are driven by the requirements regarding alignment in a submicron and the corresponding challenges induced by adhesive bonding. For micro-optic assembly tasks a major challenge in adhesive bonding at highest precision level is the fact, that the bonding process is irreversible. Accordingly, the first bonding attempt needs to be successful. Today’s UV-curing adhesives inherit shrinkage effects crucial for submicron tolerances of e.g. FACs. The impact of the shrinkage effects can be tackled by a suitable bonding area design, such as minimal adhesive gaps and an adapted shrinkage offset value for the specific assembly parameters. Compensating shrinkage effects is difficult, as the shrinkage of UV-curing adhesives is not constant between two different lots and varies even over the storage period even under ideal circumstances as first test results indicate. An up-to-date characterization of the adhesive appears necessary for maximum precision in optics assembly to reach highest output yields, minimal tolerances and ideal beamshaping results. Therefore, a measurement setup to precisely determine the up-to-date level of shrinkage has been setup. The goal is to provide necessary information on current shrinkage to the operator or assembly cell to adjust the compensation offset on a daily basis. Impacts of this information are expected to be an improved beam shaping result and a first-time-right production.
The conventional distributed feedback and distributed Bragg reflector edge-emitting lasers employ buried gratings,
which require two or more epitaxial growth steps. By using lateral corrugations of the ridge-waveguide as surface
gratings the epitaxial overgrowth is avoided, reducing the fabrication complexity, increasing the yield and reducing the
fabrication cost. The surface gratings are applicable to different materials, including Al-containing ones and can be easily
integrated in complex device structures and photonic circuits. Single-contact and multiple contact edge-emitting lasers
with laterally-corrugated ridge waveguide gratings have been developed both on GaAs and InP substrates with the aim to
exploit the photon-photon resonance in order to extend their direct modulation bandwidth. The paper reports on the
characteristics of such surface-grating-based lasers emitting both at 1.3 and 1.55 μm and presents the photon-photon
resonance extended small-signal modulation bandwidth (> 20 GHz) achieved with a 1.6 mm long single-contact device
under direct modulation. Similarly structured devices, with shorter lengths are expected to exceed 40 GHz small-signal
modulation bandwidth under direct modulation.
The 1550nm wavelength region is critical to the development of next generation eye safe military applications such as
range finding and friend or foe identification (FOE). So far the relatively low laser external efficiency was a strong
limiting factor favoring shorter wavelength diode lasers. We report on the development of a new monolithic multiple
junction pulsed laser diode offering an external efficiency of more than one Watt per Amp with high brightness. Peak
optical output power of more than 37 Watts has been achieved from a single multi-junction diode laser. Divergence is
narrow with less than 35 degrees (FWHM) in the fast axis direction. Starting from an AlGaInAs quantum well laser
structure, we show the criticality of the design of InP based tunnel junctions to the growth of the three layer epitaxial
monolithic laser. We then report on trenches employed to confine carriers under the contacting stripe and on growth
strategies used to decouple the multiple light sources resulting from the multi-junction design. A full set of
characterization data is presented concluding with a discussion on performance limitations and their potential causes.
Visible vertical-cavity surface-emitting lasers (VCSELs) are potential light sources for polymer optical fibre (POF) data transmission systems. Minimum attenuation of light in standard PMMA-POFs occurs at about 650 nm. For POFs of a few tens of meters in length VCSELs at slightly longer wavelengths (670 - 690 nm) are also acceptable. So far, the visible VCSELs have been grown by metal organic chemical vapour deposition (MOCVD). They may also be grown by a novel variant of molecular beam epitaxy (MBE), a so-called all-solid-source MBE or SSMBE. In this paper, we describe growth of the first visible-light VCSELs by SSMBE and present the main results obtained. In particular, we have achieved lasing action at a sub-milliamp cw drive current for a VCSEL having the emission window of 8um in diameter, while a 10um device exhibited an external quantum efficiency of 6.65% in CW operation at room temperature. The lasing action up to temperature of 45°C has been demonstrated.
Confocal photoluminescence imaging is an important tool in the investigation of recombination in semiconductors and in the characterization of material growth. This characterization is particularly important for II-VI wide band-gap semiconductors where the potential for blue-green lasers is being explored currently. To achieve room-temperature cw operation of these lasers over the multi-thousand hours necessary for commercialization, extremely low defect densities are required. The confocal microscope is used in this work to image photoluminescence from II-VI materials to characterize the defect formation and propagation within the quantum well region of the material. This imaging approach permits the degradation to be monitored in real time and over a large area in samples with low defect densities. The additional advantages of this set-up over a conventional microscope are, of course, the higher lateral resolution and narrow depth of field associated with a confocal microscope. While considerable effort has been focused on the degradation in these II-VI semiconductors, we have recently observed that annealing can occur simultaneously in the same sample when the material is exposed to intense optical excitation. Images of annealing and degradation of a range of II-VI samples will be presented to highlight these observations.
This paper presents the performance characteristics and reliability data of AlGaInP-based VISIBLE laser diodes emitting at the wavelengths from 630 to 670 nm. The lasers are grown by toxic gas free solid source molecular beam epitaxy.
ZnSe-based laser diodes have recently encountered strong competition from those grown from GaN related materials. These two material systems behave in a very different way as far as defect generation and propagation are concerned. For ZnSe-based materials the lifetime of a laser-diode is very sensitive to the density of pre-existing extended defects in the epitaxial material. Therefore, fabrication of a long- lived ZnSe-based laser diode requires an elimination of extended defects as well as making low-resistivity components in order to minimize device heating. We discuss the molecular beam epitaxy growth and characterization of ZnSe-based epitaxial structures on various III-V buffer layers lattice matched to GaAs. The status of our ZnSe-based laser diodes and microcavity LEDs will also be discussed.