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.
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.
We report on the theoretical analysis and fabrication of a novel type of vertical-cavity surface-emitting laser
(VCSEL) that provides selection of a certain higher-order transverse mode. This selection is based on a spatial
variation of the threshold gain by adding an antiphase layer with an etched relief structure. The field intensity
profile emitted from this laser is calculated numerically as well as with an analytical approach. The main factors
that control the selected mode such as the threshold gain, the confinement factor, and the phase parameter are
calculated as a function of the active aperture, aiming to achieve single higher-order transverse mode emission.
For a given aspect ratio of a rectangular oxide aperture, the threshold gain difference between the selected
and neighboring modes is maximized via the relief diameter and the size of the aperture. The fabrication
process involves selective etching of the antiphase layer, passivation of the relief, oxidation of an AlAs layer to
the desired aperture after reaching this layer using wet-chemical etching. N- and p-metalization processes are
applied, followed by polyimide passivation. Finally, bondpad metalization is carried out for electrical contacting.
Mode selection is successfully achieved. Attractive applications for such devices are found in optical manipulation
of micro-particles such as sorting and separation.
The combination of microfluidics and optical manipulation offers new possibilities for particle handling and
sorting on a single-cell level in the field of biophotonics. We present particle manipulation in microfluidics based
on vertical-cavity surface-emitting lasers (VCSELs) which constitute a new low-cost, high beam quality nanostructured
laser source for optical trapping, additionally allowing easy formation of small-sized, two-dimensional
laser arrays. Single devices as well as densely packed linear VCSEL arrays with a pitch of only 24 μm are
fabricated. Microfluidic channels with widths of 50 to 150 μm forming T- and Y-junctions are made of PDMS
using common soft-lithography. With a single laser, selected polystyrene particles are trapped in the inlet
channel and transferred to the desired outlet branch by moving the chip relatively to the optical trap. In a
second approach, a tilted, linear laser array is introduced into the setup, effectively forming an optical lattice.
While passing the continuously operating tweezers array, particles are not fully trapped, but deflected by each
single laser beam. Therefore, non-mechanical particle separation in microfluidics is achieved. We also show the
route to ultra-miniaturization of the system avoiding any external optics. Simulations of an integrated particle
deflection and sorting scheme as well as first fabrication steps for the integrated optical trap are presented.
Vertical-cavity surface-emitting lasers (VCSELs) are used for oxygen monitoring via tunable diode laser spectroscopy
at 760nm wavelength. For the desired application, novel polarization-stable laser diodes based on
AlGaAs were developed.
We present measurements of the pressure-broadening coefficients of the electric dipole forbidden oxygen A-Band b1Σg+ -> X3Σg+ transition at 760nm. For the first time the pressure-broadening coefficients were determined
with a temperature tuned vertical-cavity surface-emitting laser. Because of special techniques of polarization
stabilization with a combination of a dielectric surface grating and a surface relief the VCSELs have a mode
hop-free tuning range of more than 7nm and a sidemode suppression of more than 30dB. This provides a low
cost laser diode system with a wide tuning range, which enables the possibility of simultaneous measurement of
temperature, pressure and oxygen concentration in air, high pressure measurements and also a higher accuracy
of oxygen concentration measurements due to averaging over 18 absorption lines.
The monolithic integration of 850 nm vertical-cavity surface-emitting laser diodes and GaAs-based metal-semiconductor-metal photodiodes is introduced as an approach to bidirectional optical data transmission in the Gbit/s
range of data rates. Polymer-clad silica fibers and graded-index fibers with core diameters of 200 and 100 &mgr;m, respectively, serve as the transmission medium, covering link lengths relevant
for in-car up to in-house communications and beyond.
High-performance vertical-cavity surface-emitting lasers (VCSELs) with an emission wavelength of approximately 764 nm are demonstrated. This wavelength is very attractive for oxygen sensing. Low threshold currents, high optical output power, single-mode operation, and stable polarization are obtained. Using the surface relief technique and in particular the grating relief technique, we have increased the single-mode output power to more than 2.5mW averaged over a large device quantity. The laser structure was grown by molecular beam epitaxy (MBE) on GaAs (100)-oriented substrates. The devices are entirely based on the AlGaAs mixed compound semiconductor material system. The growth process, the investigations of the epitaxial material together with the device fabrication and characterization are discussed in detail.
We report on the design, fabrication and test results of monolithically integrated transceiver chips consisting of GaAs metal-semiconductor-metal photodiodes and 850nm wavelength vertical-cavity surface-emitting lasers. These chips are well suited for low-cost and compact bidirectional optical interconnection at Gbit/s data rates in mobile systems and industrial or home networks employing large core size multimode fibers.
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.
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