SUBTUNE (Widely Tuneable VCSEL using Sub-wavelength Gratings) is a multi-disciplinary European FP7 research project for three years (April 2008 to April 2011), to promote  development and implementation of technological innovation in optical components and subsystems.

The project involves eight partners from six European countries:

The Technische Universität Darmstadt, The Technische Universität München, Vertilas GmbH, Chalmers University of Technology (Sweden), Tyndall National Institute (Ireland), IR Microsystems SA (Switzerland), Commissariat à l’Energie Atomique (France), Consiglio Nazionale Delle Ricerche, IEIIT (Italy).

Total funding is around 2.7 millions Euros.

Wavelength-tunable lasers are among the key components to build future re-configurable optical networks and in demand of cost-effective and compact telecommunications infrastructures. Moreover, a broadband and continuously tunable laser with high purity emission spectrum is a versatile tool for many sensing applications. For instance, the emission of greenhouse gases can be controlled by laser absorption spectroscopy and deformations of buildings can be monitored by interrogating fibre Bragg grating sensors. The vertical-cavity surface-emitting laser (VCSEL) is the ideal candidate for its inherent longitudinal single-mode behaviour, low power consumption and compactness.

The project Subtune aims at the development of InP-based long-wavelength VCSELs ranging from 1.5 µm to 2.0 µm and GaAs-based VCSELs with wavelengths down to 800 nm, and thus introduces widely tuneable VCSELs in a broad range of the optical spectrum. In our actual approach the active component or so called half-VCSEL is combined with a bulk-micromachined moveable mirror membrane in hybrid two-chip assembly. The mirror can electrothermally be actuated to expand the air-gap and shift the cavity resonance towards longer wavelengths. Today, continuous tuning and mode-hop free tuning over 60 nm (Fig. 1) is already achieved at wavelengths around 1.55 µm.

Fig. 1 : Tunable VCSEL emission spectrum

We are developing a mature, integrated technology by means of inductive coupled, and thus low-temperature, plasma-enhanced chemical vapour deposition (IC-PECVD). The scanning electron microscope image (Fig. 2) shows a preliminary process result of a dielectric (Si3N4/SiO2) Bragg mirror realized by surface-micromachining using a sacrificial layer. The curvature of the mirror is essential to excite (only) the desired fundamental mode even from large aperture diameters. Thus high output power up to +5 dBm is expected. The polarisation stability of VCSELs is a critical issue because of the circular device structure. To lift the symmetry a polarisation selective sub-wavelength grating (SWG) will be incorporated in the device by means of e-beam writing. 

Fig. 2 : Micromachined moveable mirror membrane for hybrid integrated tuneable VCSELs

The optimization of the devices will be carried out in close cooperation between the universities and industrial partners, where already at the early stages of the project the first devices are investigated for gas detection, Fiber Bragg Grating sensing and optical communications :

• Multi species gas sensing
• Fiber Bragg Grating technology for Structural Health Monitoring
• Telecom application for long Wavelengths
• Telecom for the reconfigurable optical interconnections
• Ultra high resolution optical spectrum analysis