Optics : VCSEL-based MOEMS

 

 

 

Optical micro-analysis with near-infra-red VCSELs

 

Optical microsystems and VCSELs for biosensing and instrumentation

People involved : V. Bardinal, S.Abada, T. Camps (N2IS), E. Daran (TEAM), J.B. Doucet (TEAM), B. Reig (TEAM), I. Seguy, J. Launay.

 

Vertical-cavity surface-emitting lasers (VCSELs) have drained much attention as an attractive light sources for short-distance optical interconnects, optical storage, and laser printing. The proven potentials and flexibility of these optoelectronic devices have recently attracted much attention from novel fields of application such as microsensors and biomedical instrumentation. Due to this new interest, new functionalities have now to be developed to further extend the integration capabilities of the VCSELs in optical microsystems. Two aspects have to be addressed in this prospect: the adaptation of the laser source to the microsystem through integrated microoptics, and the diversification and/or association of other optical functionalities such as internal photodetection.

                                             

(left) schematic view of an oxide-confined VCSEL tested under probes (right) SEM view of a polymer MEMS for tunable lens integration on a VCSEL

 

Current topics:

  • VCSELs collimation with integrated microlenses

VCSELs are now strategic light sources for optical communications as well as for instrumentation or sensing applications. Despite a limited far-field beam divergence, these sources have more and more to be associated with micro-optical components to enhance their performances or to increase their integration in systems.

In order to integrate microlenses onto VCSEL devices, uniform thick SU-8 pedestals implementing various geometries and sizes were fabricated. Their influence on the deposited microlenses has been studied. A cylindrical shape was found to be the best suited to ensure a self-alignment of the polymer droplets on top of the SU-8 pedestal. The application of this method to the monolithic integration of self-aligned polymer refractive microlenses on VCSELs has led to a significant reduction of the initial beam divergence (in the range [1-2°] instead of 12.5°) °)[IEEE Phot. Technol. Lett. Vol 21, 1592-1594, 2010]. These results are in good agreement with optical modelling performed with ZEMAX-EE and with more complete simulation tools taking into account laser microcavity  (coll. IEIIT Politecnico Torino).

The thermal reliability of these microlenses has been also studied in collaboration with CIRIMAT laboratory and EADS Astrium (FIAB SU-8 project). [2014 Jpn. J. Appl. Phys. 53 08MC03].

                                    

(left) Optical images of SU-8 pedestals with various diameters [30-140µm] with self-positioned liquid droplets on the top  Insert : SEM image of a VCSEL with an integrated microlens  (Right) Beam divergence with and without lens Insert : corresponding L-I curves. [IEEE Phot. Technol. Lett. Vol 21, 1592-1594, 2010]        

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  • Near infra-red photopolymerization for micro-optics self-writting on a VCSEL

We have demonstrated a self-writing process for integrating self-aligned microtips on VCSELs in the frame of NIROPTICS ANR project (in collaboration with IS2M, Mulhouse and ICD LNIO, Troyes). Our single-step and room temperature technique is based on the use of novel NIR photopolymers [V. Bardinal et al., App. Phys. Lett., 2010]. These materials have been optimized to make possible a precise control of initial thickness prior to photo-fabrication. As a result, limitation of oxygen quenching effects is possible [I. Dika et al, Chem. Phys. Lett, 2011] as well as a collective fabrication at a wafer-scale [D. Barat et al, Micro. Eng., 2013]. Thanks to these improvements, a complete photo-chemical study has been led on different kinds of VCSEL sources. Suitable exposure doses and corresponding dimensions ranges have been determined. The focusing abilities of these tipped-VCSELs have been also evaluated both experimentally and theoretically in collaboration with Pr. Debernardi at IEIIT-CNR Italy, showing that our method can be exploited for the fabrication of compact optical micro-probes for scanning probe microscopy and for near-field optical microscopy  [D. Barat et al, Op. Express 2012].

The application of this technique for the fabrication of self-written optical links is currently under study in the frame of NIRConnect Inter-Carnot project (MICA/I2SM-LAAS) and of the Equipex project LEAF devoted to laser writing.

 

                                

Fabrication principle : (Right) SEM image of a polymer microtip self-written at the centre of a singlemode VCSEL by NIR self- photopolymerization for different exposure conditions [Appl. Phys. Lett. 96, 2010] [Optics Express, Vol. 20, Issue 20, pp. 22922-22933 (2012)]

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  • Polymer MOEMS for tunable lens integration on VCSELs

 

We have developed a low-cost polymer-based technique for the fabrication of a Micro-Optical-Electrical-Mechanical Systems (MOEMS) in view of dynamic focusing of VCSEL beam.

A simple method based on a SU-8 double exposure is proposed for the collective integration of small footprint transparent suspended membranes on vertical laser diodes arrays.  A complete optimization of the full circular membrane has been led thanks to a thermo-mechanical simulation tool (COMSOL). Our study shows that a 2.7 µm vertical displacement is possible with an applied thermal gradient of 100°C. [SPIE Proc. 771620, 2010]. This value is suited for a significant active beam vertical tuning given by optical modelling (coll. FOTON).  We recently demonstrated that this kind of MOEMS can be thermally actuated with a vertical displacement of ~0.2 µm/W over a maximal range of 8 µm. As a wide range of initial gaps between the membrane and the laser source can be chosen, this approach opens new insights for dynamic control of VCSEL beam waist position and for tunable VCSELs fabrication. [B. Reig et al, Journal of Micromechanics and Microengineering, 2012]  [Journal  of Micromechanics and Micro-Engineering, 22 065006, 2012]. Tri-dimensionnal electro-thermo-mechanical modelings recently confirmed these results [SPIE Proc. Paper 9130- (2014)].

Finally, first results on lens integration on multimode VCSELs show a beam focusing at a distance of 300µm suitable for further integration in a fluidic chamber [Invited talk MicroOptics  Conference MOC'13, Tokyo, Japon, 27-30 oct 2013] [Invited talk IEEE OMN'13 conference, Kanazawa, Japan, 18-22 aug 2013].

                                           

(left) VCSEL dynamic focusing principle (right) Polymer MOEMS arrays for VCSEL active focusing illustrating Journal of Micromechanics and Microengineering June cover.

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  • Transparent Conductive Electrodes for electrical injection improvement in optical devices

In the framework of previous FunFACS european project, the electrical properties of broad-area 850nm top emitting VCSELs have been investigated in order to improve carrier injection uniformity in their active zone. We have demonstrated using an electrical simulation tool that a multi-point localized injection design associated with a spreading layer at the top of the device (ITO) can lead to a significant improvement of carrier injection and on its spatial distribution. The electrical contrast achievable by applying this method with localized etchings has been experimentally measured. Stripe-shaped devices with output power up to 50mW in a continuous-wave operation at room temperature have been demonstrated. (E.Havard PhD thesis) [European Physical Journal D, 59, 1, 2010].

We are now studying the properties of alternative TCO layers (Transparent Conductive Oxides) such as ZnO:Al deposited by ALD in collaboration with NCKU Taïwan in the framework of AZOLED project. [SPIE proc. Paper 9137-49, 2014)]

 

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  • Integrated photodetection in VCSELs sources

 

We previously investigated the lateral waveguiding properties of a VCSEL for lateral detection application to integrated power monitoring. Spontaneous and Amplified Spontaneous emissions guiding in the plane of the cavity has been modelled and demonstrated experimentally in collaboration with Yann Boucher (ENIB-RESO-Brest). Thanks to the use of a Schottky contact, the threshold and the thermal extinction of the lasing emission can be clearly identified on the lateral signal. The monotonous increase of the photocurrent between threshold and extinction makes this lateral detection suitable for integrated VCSEL power monitoring with a higher level for the photodetected light. [Electron. Lett. Vol 41, 3,2005] [ Optical and Quantum Electronics, Vol 38, n°4-6,2006] . Current work focus now on a better correlation with the top-emitted light and optical feedback exploitation for velocimetry or distance measurement (coll. with OSE team).

 

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  • Collaboration projects

Current projects

  • FAST MOEMS : "Micro-optique polymère à actionnement ultra-rapide pour l'imagerie biomédicale" (AO1, 2012-2013)
  • AZOLED :"Transparent electrodes and reflective structures based on ZnO for high efficiency Light-Emitting Devices" (NSC NCKU 2012-2014)
  • NIR CONNECT (IS2M-LAAS) inter Carnot project (2013-2014)
  • HYPOCAMP (FOTON-LAAS-TB-LPN-IPR) "Hybrid Polarization Controlled and mOnolitic tunable vertical Cavity surface emitting lAsers" ANR project (2015-2018)

Previous projects

  • NIR OPTICS : "Micro-optiques sur VCSELs  par photopolymérisation auto-guidée dans le proche Infrarouge" (ANR blanc 2009-2012; coll. IS2M, LNIO)
  • FIAB SU-8 : "Tenue Thermique en atmosphère humide des propriétés optiques et mécaniques de la SU-8"  (projet région 2008-2012, labellisé Aerospace Valley,  coll.CIRIMAT, EADS-Astrium, NOVAMEMS)

(in PHOTO group):

  • MIOPY : MIcro-Optique intégrée à base de PolYmères (projet LAAS 2006-2009)
  • Optonanogen - Integrated OPTO-NANOmechanical Biosensor for functional GENomic Analysis - (IST-2002-2005) (coll. CNM Madrid, Séville et Barcelone, CRL Southampton, Genetrix)
  • FunFACS - Fundamentals, Functionalities and Applications of Cavity Solitons - (IST-FET-STREP 2004-2007) (coll. LPN, INLN, USTRAT, Univ. Bari, Univ. Como)
  • OSA 131 - Multi-standard transmitter micro-module working at 1.31 µm and 10 Gbit/s - (RMNT - French Research Network in Micro and Nano Technologies) (coll. Intexys, LETI, Infineon)

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