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Optical Design & Engineering

Vertical-cavity-laser focusing using a self-written lens

Application of novel near-IR photopolymers enables fabrication of perfectly aligned microlenses.
18 May 2010, SPIE Newsroom. DOI: 10.1117/2.1201004.002918

Vertical-cavity surface-emitting lasers (VCSELs) are very attractive light sources for use in microsystems because of their highly parallel operation, low power consumption, and beneficial beam properties.1 As a result, they are increasingly exploited in various applications, including optical communications, sensing, and biomedical devices. However, to increase their coupling efficiency to optical fibers or meet collimation requirements in microsystem applications, the laser beams must be corrected using micro-optical elements.2–4 In addition, VCSEL microtip fabrication has become a key issue for novel optical-probe design for near-field scanning optical microscopy (NSOM) or high-density optical-data reading.5,6

To date, techniques employed to integrate microlenses or microtips with VCSELs use either a hybrid assembly method or photolithography. The latter approach enables increased precision of vertical and lateral positioning relative to the device.7,8 However, this precision is still not better than 1μm and depends on previous misalignments that may have occurred during device fabrication. To achieve perfect alignment with the emitted beam, the lens must ideally be created by the laser source itself. With this aim, we have developed a new method to integrate polymer optical micro-elements (micropillars and microlenses) with VCSEL diodes.9 Our approach is based on a simple and low-cost technique proposed in 2001 that aimed to implement a polymer microcomponent at the extremes of optical fibers.10,11 We rely on the near-IR laser source to trigger photopolymerization and use this to subsequently fabricate collectively self-aligned optical micro-elements. This process is simple, fast, and compatible with post-processing requirements. Moreover, it can result in a wide variety of geometries since the final shape of newly created micro-elements can be controlled by light exposure.12

We designed new chemical systems suitable for near-IR photopolymerization based on light-induced photofabrication using laser devices. Lens fabrication using VCSELs employs deposition of a photopolymer droplet on top of the VCSEL device. The laser is then electrically biased under probes, in turn leading to IR lasing. This light exposure induces local photopolymerization of the resist at room temperature. After rinsing the sample, a transparent micro-object that is perfectly aligned with the VCSEL's emission zone is observed at the device center (see Figure 1). Note that the photopolymerizable formulation is compatible with spin coating and, therefore, that collective fabrication will be possible by applying collective current injection to several devices simultaneously. The lens focuses the beam close to the tip surface (at a distance of less than 2μm), which is in good agreement with our optical modeling.9 These promising results represent a first demonstration of the potential of our technique.

Figure 1. Scanning-electron-microscope image of a self-written polymer microlens at the center of a 760nm single-mode vertical-cavity surface-emitting laser created using near-IR self-photopolymerization.

In summary, we have fabricated a micro-optical component that is perfectly aligned with the laser source with which it is integrated. Our novel method uses near-IR photopolymers that are sensitive at the relevant laser wavelength. The approach is fully compatible with the post-processing processes in use. Future work will involve detailed investigation of IR exposure effects on both lens geometry and the resulting optical properties. Potential applications include development of novel types of optical probes for near-field optics or high-density optical-data storage. Our other ongoing work focuses on extending this method to longer wavelengths.

This work was done in collaboration with B. Reig, T. Camps, E. Daran, D. Barat, J. B. Doucet, and C. Vergnenégre at the Laboratory for Analysis and Architecture of Systems-CNRS, and C. Turck, J. P. Malval, and D. J. Lougnot at the Institute of Materials Science Mulhouse-CNRS. The French National Research Agency (ANR) is gratefully acknowledged for financial support (ANR-09-BLAN-0168-01).

Véronique Bardinal
Laboratory for Analysis and Architecture of Systems-CNRS
University of Toulouse
Toulouse, France

Véronique Bardinal is a senior researcher in photonics. She has worked on semiconductor epitaxial-growth control, laser-device design and technology, and photodetection studies using vertical-cavity devices. Her current research interests include polymer micro-optics for VCSEL integration in optical microsystems.

Olivier Soppera
Institute of Materials Science Mulhouse (IS2M)-CNRS
Mulhouse, France

Olivier Soppera graduated from the Ecole Normale Supérieure in Cachan and received his PhD in polymer and photochemistry from the Université de Haute-Alsace in 2003. He is a CNRS researcher and co-author of 60 publications, one patent, and three book chapters.