The huge increase of datacom capacities requires lasers sources with more and more bandwidth performances. Vertical-Cavity Surface-Emitting Lasers (VCSEL) in direct modulation is a good candidate, already widely used for short communication links such as in datacenters. Recently several different approaches have been proposed to further extend the direct modulation bandwidth of these devices, by improving the VCSEL structure, or by combining the VCSEL with another high speed element such as lateral slow light modulator or transistor/laser based structure (TVCSEL). <p> </p>We propose to increase the modulation bandwidth by vertically integrating a continuous-wave VCSEL with a high-speed electro-modulator. This vertical structure implies multiple electrodes with sufficiently good electrical separation between the different input electrical signals. This high frequency modulation requires both good electrical insulation between metal electrodes and an optimized design of the coplanar lines. BenzoCyclobutene (BCB) thanks to its low dielectric constant, low losses, low moisture absorption and good thermal stability, is often used as insulating layer. Also, BCB planarization offers the advantages of simpler and more reliable technological process flow in such integrated VCSEL/modulator structures with important reliefs. As described by Burdeaux et al. a degree of planarization (DOP) of about 95% can be achieved by simple spin coating whatever the device thickness. In most of the cases, the BCB planarization process requires an additional photolithography step in order to open an access to the mesa surface, thus involving a tight mask alignment and resulting in a degraded planarization. <p> </p>In this paper, we propose a self-aligned process with improved BCB planarization by combining a hot isostatic pressing derived from nanoimprint techniques with a dry plasma etching step.
We report on the optimized design of a polymer-based actuator that can be directly integrated on a VCSEL for vertical beam scanning. Its operation principle is based on the vertical displacement of a SU-8 membrane including a polymer microlens. Under an applied thermal gradient, the membrane is shifted vertically due to thermal expansion in the actuation arms induced by Joule effect. This leads to a modification of microlens position and thus to a vertical scan of the laser beam. Membrane vertical displacements as high as 8μm for only 3V applied were recently experimentally obtained. To explain these performances, we developed a comprehensive tri-dimensional thermo-mechanical model that takes into account SU-8 material properties and precise MOEMS geometry. Out-of-plane mechanical coefficients and thermal conductivity were thus integrated in our 3D model (COMSOL Multiphysics). Vertical displacements extracted from these data for different actuation powers were successfully compared to experimental values, validating this modelling tool. Thereby, it was exploited to increase MOEMS electrothermal performance by a factor higher than 5.
This paper reports on an alternative method for precise and uniform fabrication of 100μm-thick SU-8 microstructures on small-sized or non-circular samples. Standard spin-coating of high-viscosity resists is indeed known to induce large edge beads, leading to an air gap between the mask and the SU-8 photo-resist surface during UV photolithography. This results in a non uniform thickness deposition and in a poor pattern definition. This problem becomes highly critical in the case of small-sized samples. To overcome it, we have developed a soft thermal imprint method based on the use of a nano-imprint equipment and applicable whatever sample fragility, shape and size (from 2cm to 6 inches). After final photolithography, the SU8 pattern thickness variation profile is measured. Thickness uniformity is improved from 30% to 5% with a 5μm maximal deviation to the target value over 2cm-long samples.
We report on a simple method for the collective fabrication of polymer tunable microlens arrays suitable for
VCSEL active beam shaping. Its principle is based on a SU-8 suspended membrane, surmounted by a polymer
microlens, and thermally actuated to achieve a vertical displacement of lens plane. SU-8 resist presents many advantages
for MOEMS fabrication, as this resist allows for high aspect ratio patterns and high transparency. In addition, it exhibits
a thermal expansion coefficient suitable for thermal actuation. Moreover, this kind of polymer MOEMS can be
fabricated on VCSEL arrays with footprints as low as 500x500μm<sup>2</sup> enabling a rapid, low cost and wafer-scale integration
technology. We have successfully fabricated this MOEMS on a glass substrate by means of a SU-8 double exposure
method and we report on a vertical displacement of 8μm under an applied power of 43mW (3V). A good agreement with
the theoretical thermo-mechanical behavior is found. Moreover, optical measurements of microlens focus displacement
under actuation are presented. We evaluate analytically the focus properties of the system under coherent laser
illumination, using the classical ABCD matrix formalism of Gaussian transformation optics. The same approach enables
one to assess its tolerance to opto-geometrical parameters, such as refractive index or dioptre curvature. As a wide range
of initial gaps between the membrane and the substrate can be chosen, this MOEMS technology opens new insights for
dynamic control of VCSEL beam or for tunable VCSELs fabrication.
We present recent results on the integration of polymer microlenses on single mode Vertical-Cavity
Surface-Emitting Lasers (VCSELs) to achieve output beam control. We describe in particular low
cost and collective fabrication methods developed to allow for a self-alignment of the lens with the
laser source. These approaches are based either on surface tension effects or on a self-writing
process using novel Near Infra-Red (NIR) photopolymers. Results on beam collimation at 850nm are
presented and compared to a fully vectorial and three-dimensional optical model that takes into
account the complete geometry of laser resonator is used. Results on short distance focusing using
self-aligned microtips are presented. Considerations to achieve an active beam control by means of
polymer-based MEMS (Micro-electro-mechanical System) are also discussed. Potential applications
may concern the improvement of VCSEL insertion in optical interconnects or sensing systems, as
well as the fabrication of optical micro-probes for near-field microscopy.
Active control of VCSELs beam properties is a key issue to improve their integration in microsystems. We have
designed a micro-optical system that allows for a dynamic displacement of the VCSEL beam. It consists of a polymer
microlens associated to a SU-8 membrane vertically moved by means of a thermal actuator. This approach is suitable
with laser sources arrays. We present results on optical design demonstrating that a small deflection of the membrane
(2μm) could lead to a large displacement of the beam waist vertical position (in the millimetric range). Thermomechanical
modelling is performed to evaluate the maximum membrane displacement achievable with this system.
Finally, first feasibility results are presented.
Laser beam shaping is a key issue for the photonic integration of VCSEL sources. Most of the techniques
proposed to integrate micro-optics elements onto VCSEL devices imply either a hybrid assembly or a photolithography
step, whose precision limits the accuracy of lens alignment relatively to the VCSEL source. We present here a new
method for self-fabrication of microtips on Vertical-Cavity Surface-Emitting Lasers (VCSELs) by means of Near Infra-
Red (NIR) photo-polymerization. This approach is based on a single fabrication step, implementing novel
photopolymers sensitive at the lasing wavelength. Consequently the process is triggered by the laser source itself and can
be applied easily to VCSEL devices during their electro-optic characterization. The method we have developed for tips
fabrication is detailed as well as corresponding optical properties. The applications of this new and simple method
concern laser light focusing and collimation for integrated micro-systems, coupling to fibers for optical communications
as well as novel micro-probes fabrication for near-field optical microscopy.