Fernando Rinaldi

Efficient Gallium–Arsenide Disk Laser

Means that may optimize the efficiency of semiconductor disk lasers (optically pumped vertical external-cavity surface-emitting lasers) are discussed: the direct pumping of the quantum wells (QWs), the optimization of the interaction of the QWs with pump and laser field and the reduction of resonator losses by employing a dielectric/semiconductor Bragg reflector on the front face of the disk. GaAs-AlGaAs-quantum-well disk lasers designed accordingly achieved slope efficiencies of 67% and optical efficiencies of 55% (based on the absorbed pump power) with output powers well above 1 W and a pump absorption efficiency of 80%. With an additional simple pump re-imaging an output power of 1.6 W was realized with an optical efficiency of 50% related to the incident pump power.

VCSELs with enhanced single-mode power and stabilized polarization for oxygen sensing

Vertical-cavity surface-emitting lasers (VCSELs) with a single-mode single-polarization emission at a wavelength of approximately 763 nm have become attractive for oxygen sensing. Up to now, VCSELs used for this application are single-mode because of a small active diameter which correspondingly leads to small optical output power. Employing the surface relief technique and in particular the surface grating relief technique, we have increased the single-mode output to more than 2.5 mW averaged over a large device quantity. To the best of our knowledge, this is the highest single-mode power ever reported for VCSELs in this wavelength range. Through the grating relief simultaneously, we were able to stabilize the light polarization.

Surface Relief Versus Standard VCSELs: A Comparison Between Experimental and Hot-Cavity Model Results

We present a detailed experimental as well as theoretical study of vertical-cavity surface-emitting lasers (VCSELs) with and without etched surface modifications. The so-called inverted surface relief leads to a suppression of higher-order transverse modes, where measurements of output power and optical spectra show a maximum single-mode output power of 6.1 mW. For simulations, a hot-cavity model is applied, which can handle the complex electrical, thermal, and electromagnetic problems in a VCSEL structure in a fully 3-D manner. The optical characteristics of both structures, including current-dependent output power and spectral properties up to thermal rollover, are very well reproduced by the simulations. Furthermore, the evolution of the beam profile is investigated by simulations as well as spectrally resolved near-field measurements at various distances to the laser surface. Here, the simulations confirm the significantly stronger thermal guiding in the relief device indicated in the measurements.

Bidirectional Optical Interconnection at Gb/s Data Rates With Monolithically Integrated VCSEL-MSM Transceiver Chips

We present the operation characteristics of 850-nm wavelength GaAs-based monolithically integrated transceiver chips designed for low-cost short-distance bidirectional optical data transmission over a butt-coupled 200-mum core diameter polymer-clad silica fiber. The chips containing a vertical-cavity surface-emitting laser and a large-area metal-semiconductor-metal photodiode can well handle data rates of 2.5Gb/s in back-to-back mode and 0.5 Gb/s for 10-m fiber length

Small-Pitch Flip-Chip-Bonded VCSEL Arrays Enabling Transmitter Redundancy and Monitoring in 2-D 10-Gbit/s Space-Parallel Fiber Transmission

We demonstrate novel pixel architectures in 2-D vertical-cavity surface-emitting laser (VCSEL) arrays offering additional functionality without sacrificing efficient fabrication, compactness, and low module cost. Very high flip chip VCSEL packing densities enable both a built-in 3-per-channel VCSEL redundancy as well as simple intracell VCSEL monitoring. Each pixel has three individually addressable oxide-confined and substrate- removed 850-nm-wavelength VCSELs directly flip bonded to the mesas that are extremely close-spaced to permit equal butt coupling to 50-mum-core-diameter multimode fibers (MMFs) without the need for external coupling optics. Built-in radial VCSEL-to- fiber launch offsets as small as 7.8 mum were reached, leading to offset coupling penalties of only 0.4 dB. Quasi-error-free transmission of 10 Gbit/s signals is achieved over 500-m-long MMFs even under offset launch conditions. New opportunities for a range of further applications offered by high-density VCSEL arrays are discussed.

True bidirectional optical interconnects over multimode fiber

We report the fabrication and properties of 850nm wavelength AlGaAs/GaAs-based transceiver chips, in which vertical-cavity surface-emitting lasers (VCSELs) and photodiodes are monolithically integrated. Various types of devices allow half- and full-duplex bidirectional optical interconnection at multiple Gbit/s data rates over a single butt-coupled glass or polymer-clad silica optical fiber with core diameters of 100 or 200 μm. Whereas metal-semiconductor-metal (MSM) photodiodes are employed for these large-area fibers, we also investigate the integration of PIN-type photodiodes which appear more promising in combination with standard 62.5 or 50 μm core diameter graded-index multimode fibers. This interconnect solution based on two identical chips is attractive owing to lower volume, weight, and cost. Applications will be found in home, in-building, industrial, or automotive networks and potentially within computer clusters or central offices.

High-Power Single Transverse Mode Vertical-Cavity Surface-Emitting Lasers With Monolithically Integrated Curved Dielectric Mirrors

We report the monolithic integration of vertical-cavity surface-emitting lasers (VCSELs) with curved dielectric mirrors. The laser structure has an InGaAs/GaAs gain region and incorporates a photoresist pattern fabricated on the substrate side. A dielectric distributed Bragg reflector is applied on the curved photoresist surface which provides sufficient optical feedback for laser operation. On-wafer measurements at room temperature show single transverse mode output powers up to 15.0 mW under continuous wave operation.

VCSEL-based optical trapping for microparticle manipulation

In recent years, research into microfluidic devices has attracted much interest in the fields of biology and medicine, since they promise cheap and fast sample analysis with drastically reduced volume requirements. The combination of various analysis steps on one chip forms a small-sized biomedical system, where handling, fixing, and sorting of particles are major components. Here, it was demonstrated that optical manipulation is an efficient tool; in particular it is accurate, contactless, and biocompatible. However, the commonly required extensive optical setup contradicts the concept of a miniaturized system. We present a novel particle manipulation concept based on vertical-cavity surface-emitting lasers (VCSELs) as light sources. The small dimensions and the low power consumption of these devices enable a direct integration with microfluidic systems. The symmetric geometry of VCSELs leads to a high-quality, circular output beam, which we additionally shape by an etched surface relief in the laser output facet and an integrated photoresist microlens. Thus, a weakly focused output beam with a beam waist of some micrometers is generated in the microfluidic channel. With this configuration we were able to demonstrate particle deflection, trapping, and sorting with a solitary VCSEL with output powers of only 5mW. Furthermore, the surface emission of VCSELs allows a comparatively easy fabrication of two-dimensional laser arrays with arbitrary arrangement of pixels. Smart particle sorting and switching schemes can thus be realized. We have fabricated densely packed VCSEL arrays with center-to-center spacings of only 24 μm. Equipped with integrated microlenses, these arrays are integrated with microfluidic chips based on polydimethylsiloxane (PDMS), enabling ultra-compact particle sorting and fractionation.

Design of Highly Efficient High-Power Optically Pumped Semiconductor Disk Lasers

In this paper, we present a carefully elaborated layer design for semiconductor disk lasers. Experimental results of devices mounted on simple copper heat spreaders reveal a conversion efficiency of 54% at 13.2 W for 970-nm wavelength laser emission and a differential quantum efficiency of 73%.

Novel concepts of vertical-cavity laser-based optical traps for biomedical applications

Using vertical-cavity surface-emitting lasers (VCSELs) as light sources in optical traps offers various advantages compared to the common approaches. In particular, these are small dimensions, a circularly symmetric output beam, and the simple fabrication of two-dimensional laser arrays. We investigate the application of VCSELs in a standard tweezers setup, where trapping forces of up to 4.4 pN are achieved with 15 μm polystyrene particles and a transverse multi-mode VCSEL. The latter has improved trapping characteristics compared to a single-mode device. By introducing a small-spaced array of three VCSELs in the setup, non-mechanical movement with average velocities of up to 3 μm/s is demonstrated with 10 μm particles. Furthermore, the novel concept of the integrated optical trap is presented. By integrating a microlens directly on the VCSEL output facet, two-dimensional optical trapping is achieved in a small-sized system without any external optics. Elevation and trapping of 10 μm polystyrene particles is demonstrated at optical output powers of about 5 mW. In order to improve the beam quality of the lasers, the inverted surface relief technique is applied, which eliminates a previously observed offset between laser center and trapped particle.