Emmanuelle Daran

Spotted Custom Lenses to Tailor the Divergence of Vertical-Cavity Surface-Emitting Lasers

We demonstrate the output beam collimation of GaAs-based vertical-cavity surface-emitting lasers by means of self-aligned polymer microlenses deposited on SU-8 pedestals using a cantilever-based spotter. We show that this fabrication technique ensures a reproducible lens shape because the contact angle and the deposited volume are constant for a given surface. Device properties are presented and compared to optical beam propagation modeling results. We demonstrate that the final beam divergence is only controlled by pedestal design parameters and can be reduced with this method by a factor 10 (to 1.2° for single-mode devices) with no significant lasing performance modification.

VCSEL collimation using self-aligned integrated polymer microlenses

We report on the design and fabrication of polymer microlenses fabricated on patterned SU-8 layers in view of integrating microlenses on VCSEL arrays for laser beam shaping. For a standard top-emitting VCSEL, the lens has to be fabricated on a thick intermediate layer (pedestal) whose optimal thickness can be modelled as a function of the initial and of the aimed optical properties of the VCSEL beam. In this work, pedestals are fabricated with SU-8, which is a negative-tone photoresist transparent at the lasing wavelength. Lens deposition is realized using a robotized silicon microcantilever spotter technique after a simple SU-8 photolithography step in order to define high aspect ratio cylindrical pedestals with wide range diameters [30-140μm]. The effect of pedestal diameter on the final contact angle and curvature radius has been investigated using non contact optical profilometry and scanning electron microscopy. We show that this technique leads to a complete delimitation of the polymer droplets and to a better control of the final lens size. Moreover, lens positioning is fully ensured by the self-alignment of the droplet with the pillar center and consequently with the VCSEL source, and allows for meeting the stringent requirements on alignments.

Design of active lens for VCSEL collimation

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.

Detection of lateral spontaneous emission for VCSEL monitoring

VCSELs (Vertical Cavity Surface Emitting Lasers) are nowadays more and more exploited in optoelectronic applications, monitoring their lasing power in a compact and low cost manner becomes crucial. To collect and control the output light, an external photodetector associated with an optical microlens array can be used. Integrated solutions based on the use of a bulk or QW photodetection section added in single-or double-cavity structures have also been proposed. Here, we have investigated a simpler solution based on a standard VCSEL array. Light emitted by a VCSEL has been electrically detected by adjacent VCSELs located in the same array, using in plane optical waveguiding of spontaneous emission in the intrinsic central zone of the devices. We show that the detected photocurrent can be related to the power of the emitting VCSEL. Signal intensity has been studied as a function of VCSELs distance. This method could lead to a more efficient way to monitor VCSEL emission.

Polymer tunable microlens arrays suitable for VCSEL beam control

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 500x500μm2 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.

Uniform fabrication of thick SU-8 patterns on small-sized wafers for micro-optics applications

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.

VCSEL beam control with collective and self-aligned polymer technologies

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.

Advances in Polymer-Based Optical MEMS Fabrication for VCSEL Beam Shaping

In this paper, we discuss the design, fabrication, and characterization of electrothermally actuated polymer-based microoptical electromechanical microsystems (MOEMS) for active microoptics in vertical cavity surface emitting laser (VCSEL) devices. We describe in particular the principle of a SU8-based MOEMS designed for single-mode VCSEL beam active focusing. The ultimate objective is the realization of parallel compact optical scanners for sensing applications using collective and low-cost technologies. After discussing the advantages of the epoxy resist SU-8 for fabricating an integrated movable lens on active optical devices, we present our latest advances in technology for ensuring precise MOEMS fabrication on small III-V samples and for achieving accurate alignment of lenses on suspended circular membranes. Finally, we present our first results on the beam focusing of multimode VCSELs, which demonstrate the feasibility of our approach and could provide new insights in the MEMS-VCSEL field.

Polymer optical MEMS integrated on VCSELs for biosensing

We present our recent advances on design and fabrication of polymer optical MEMS that can be directly integrated on VCSELs arrays for dynamic beam focusing. These studies open new insights for the fabrication of compact optical sensors that require a real-time scan of laser beam position.

Design and fabrication of polymer microlenses arrays for VCSELs using a cantilever based microsystem

We report on the design and the fabrication of refractive microlenses using a polymer droplet deposition microsystem. The principle of this original technique consists in monomer droplets deposition using a robotized silicon-microcantilevers array. The advantages of this technique rely on the control of droplets dimensions and the positioning accuracy. Microlenses have been first modelled to optimize their geometrical parameters for VCSEL collimation. Results of lens optimization as well as the influence of the fabrication parameters fluctuations on the final divergence are detailed. First results on droplets deposition are presented, demonstrating the technique feasibility. Finally, the possibility of the modification of the surface energy to obtain the most suited contact angle before deposition is also discussed.