Milestones |
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Six milestones have been defined for the project, five of which are control points at which decisions are needed, and one of which is concerned with the basic technology breakthrough of this project, namely, the availability of an application suited TO-packed VCSEL with a wavelength beyond 2 µm. Here, application suitability means electric pumping and operation at room-temperature.
M1: Decision on preliminary designs of 2.3 µm, 2.7 µm and 3.3 µm VCSEL Delivery date: 02/07 Objective: While the basic VCSEL designs such as the BTJ-structure or the insulator defined structures are quite fixed, the optimum design of the electrical (resistance, voltage drops at heterointerfaces, current confinement), the thermal (thermal resistance, thermal gradients, thermal effects on carrier distribution and waveguiding), and the optical (waveguiding, threshold gain/loss, beam parameters, sidemode suppression) VCSEL parameters requires extensive and sophisticated modelling techniques. At this milestone, with the material data available at this time, the most promising designs for VCSELs at 2.3, 2.7 and 3.3 µm are identified for further design optimizations through simulations and experimental implementations. Status: M1 has been achieved. Abstract: Walter Schottky Institut developed designs for Sb-based VCSELs with a buried-tunnel junction for carrier conversion and current confinement, one semiconductor DBR (GaSb/AlAsSb) and one dielectric DBR (a-Si/CaF2)/gold mirror. The active region employs GaInAsSb/AlGaAsSb quantum wells. Using BCB, devices are separated on wafer and the substrate is completely removed, leaving the bottom gold heat sink as the only support for the VCSEL device. Low resistance InAsSb/GaSb tunnel junctions were developed. Mirrors were designed using a transfer matrix method and gradings were introduced in the semiconductor DBR to lower the potential barriers. The optical design of the resonator was optimized, using the transfer matrix method, with the active region positioned at an anti-node of the optical field for maximum gain and the tunnel junction positioned at a node for minimum loss due to free carrier absorption. Design work at Université Montpellier 2 was devoted to both the development of particular VCSELs designs (monolithic and hybrid) planned to be realized in Montpellier and the determination of corresponding VCSELs specifications. Bibliographical research and numerical simulations were performed to calculate these specifications. In particular, for the hybrid technology the design of the adapted dielectric Bragg mirrors requested the choice of the dielectric materials, and of the mirror structure, calculations of the refractive index and reflectivity simulations.
M2: Decision on test plan and test bench Delivery date: 02/07 Objective: Testing and characterisation of the laser diodes to be developed in this project is an extensive work, and the relevance of the various measurements (threshold current/pumping, efficiency, power, wavelength, spectrum) and measurement conditions (pulsed, continuous wave, on-wafer, packaged devices, cooling) as well as the temporal priority during the project requires an early plan. Status: M2 has been achieved. Abstract: This
milestone discusses and sets the methods for the characterization and the
testing the VCSELs and VCSEL structures. The document includes the following
issues:
M3: Decision about design of sensors Delivery date: 08/07 Objective: Up to now, laser diode-based gas-sensors usually employ the edge-emitting (DFB-type) lasers. Using the novel Sb-based VCSELs, preferentially mounted on TO-headers and equipped with a TE cooler, and the targeted novel applications makes major redesigns of the optical sensors necessary. With the gaining knowledge of the VCSEL parameters an optimum design of the corresponding sensors will be completed at this milestone. Status: M3 has been achieved. Abstract: M3
describes the new sensor design specifically adapted to the VCSEL based gas
sensing devices. It includes both the contribution of Siemens (wavelength
modulation based spectroscopy: WMS) and of Omnisens (photo-acoustic based
spectroscopy: PAS).
M4: TO-packaged, electrically pumped VCSELs with wavelength > 2 µm developed: Delivery date: 11/07 Objective: The availability of an application-suited VCSEL with a wavelength above 2.2 µm is believed as a basic breakthrough within the NEMIS project. The application-suitability particularly means electrical pumping and room-temperature operation of the (TO-)packaged devices. Milestone 4 therefore closely relates to deliverable D7, where 10 TO-packaged devices are delivered to the sensor development. At this milestone a ‘go/no go’ decision is introduced. If no electrically pumped VCSEL with wavelength above 2.2µm has been realized at this time, an intermediate review will be held at month 18 (midterm) to redefine the further path to follow.
M5: Decision on device concept for VCSELs at 2.7 µm wavelength Delivery date: 05/08 Objective: While the 2.3 µm wavelength is already a challenging goal for the VCSEL development, the longer wavelengths become even more difficult to access. Since the electrical pumping is preferred for a simple device employment, optical pumping exhibits the advantage that no doping is required so that neither optical losses nor ohmic heating affect the lasers and laser operation is usually easier achievable. At this milestone, therefore, a decision will be made whether to further pursue electrically or opticall pumped devices for the 2.7 µm applications.
M6: Decision on device concept for VCSELs at wavelength > 3 µm: Delivery date: 11/08 Objective: This milestone is concerned with the device concept and technology for the most advanced VCSELs in this project emitting above 3 µm. Its motivation and impact on the further work is analogue to the previous milestone M5. |