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Vertical cavity laser technology for interconnection and access links

Objective

Thanks to their intrinsic properties, Vertical Cavity Surface Emitting Lasers (VCSELs) have a wide variety of potential use and represent the new generation of laser sources for Tele or Data communication. The main objective of VERTICAL is to realise a Long Wavelength (LW) 1.3 / 1.55 um VCSELs operating CW at room temperature, through the optimisation of electrical, optical and thermal properties of the structure and to test them in a system environment.
The project addresses a very important issue : to succeed in the realisation of a 1.3 / 1.55 um VCSEL operating in continuous mode at room temperature. The intrinsic properties of VCSELs lead to specific characteristics of these devices.
The volume of the active layer is around 10 times lower than in edge-emitting lasers. A reduction in the power consumption is therefore expected as well as the possibility of operation over a wide range of temperature.
The output beam has intrinsically low and circular divergence, unlike in edge-emitters which have fan-shaped highly divergent astigmatic beams. In consequence, efficient coupling and simplified pig-tailing will contribute to the increase of positioning tolerances. This advantage, combined with the on-wafer testing capability, are key factors towards a sharp cost reduction of modules, since testing, mounting and assembly time represent more than half the cost of an optical source.
Expected Impact
All the arguments developed in the previous paragraph make VCSELs very good candidates for low-cost system in the access area. Such applications, which are generally Short Haul trunks (10 - 20 km) do not require high output power at the emission side. This relaxed tolerance is very well adapted to the limited output power we can expect for long wavelength VCSELs (10 to 100uW for current not exceeding 10 mA) as a consequence of their intrinsic low external efficiencies.
Thanks to the intrinsic single-mode operation of VCSELs, fabrication yields are expected to be high for single modules as well as for 1 and 2-D arrays. This property may be of major interest for parallel optical interconnection applications.
Main Achievements
mirror
- Wafer fusion demonstrated with low voltage drop at the p-GaAs / p-InP interface.
- Design and first experimental assessment of structured mirrors (mainly a grating etched at the top layer of a Bragg mirror) for the polarisation control of the optical beam.

VCSELs design and fabrication
- High performance In(Ga)AsP/InGaAsP strained compensated QWs demonstrated for both 1.3 & 1.5um active layers.
- Fully operational modelling tools available for calculation of electrical, optical and thermal VCSELs properties.
- Basic processing steps demonstrated (AlAs oxidation, dry etching of GaAs, selective chemical etching...)
- Fabrication of a shallow mesa structure including two different mirrors (InGaAsP/InP and Si/SiO2 on n and p side respectively). Pulsed operation up to + 50 C, a minimum threshold current less than 10 mA at -50 C and an output power of 100uW are the main features of the components. CW operation is observed up to -20 C.
- Fabrication of undercut ridge structure including two dielectric mirrors. The devices operate in pulsed regime at room temperature with threshold current less than 10 mA, a single mode operation and 100uW output power. CW operation is observed up to - 15 C.

L-I characteristics of an undercut ridgeVCSEL in CW regime at various temperatures. Inset : the CW spectrum at -25 C.

Main contributions to the programme objectives:
Main deliverables
Developed and tested a 1.3/1.5 µm vertical cavity surface emitting laser (VCSEL)
Contribution to the programme
New generation of lasers offering very low cost and high speed operation
Technical Approach
VCSELs differ from edge-emitting lasers by the position of the mirrors which form a cavity perpendicular to the substrate plane.
The active medium is the Double Heterostructure conventionally used for InP systems.
To compensate for short amplification lengths, the reflectance of the mirrors has to be as high as 99.5 % ; this is achieved with quarter-wavelength-thick layers featuring alternatively low and high refractive indices. For this, either semiconductor (GaInAsP or GaAlAsSb based) or dielectric (Si/Si02 or Si/Al2O3) mirrors are currently used. In addition, wafer fusion of GaAs/AlGaAs mirrors on InP is being investigated.
In parallel the stabilisation of the mode and the control of the polarisation of the output beam will be achieved thanks to the use of structured mirrors.
Uniform current injection is achieved with apertures defined by oxidised AlAs, undercut layers or cylindrical posts which enables current funnelling towards the middle of the cavity.

Structure of a Vertical Cavity Laser
Summary of Trial
There is no trial in VERTICAL project. However, based on the performances obtained we will develop, at the end of the project, an analysis on the feasibility of a LW VCSEL source fitting with the typical sources specifications for system applications.
The target values of different parameters like operating temperature, output power, differential efficiency, time bandwidth will be compared to the present ones and, if necessary, a way to reach them will be proposed, through eventually new conceptual approaches or enabling technologies.
As soon as cw operation is demonstrated at room temperature, system experiments will be initiated in collaboration with other ACTS projects and a conclusion on the viability of the VCSEL will be given.
Key Issues
high reflectivity with high thermal conductivity and low optical losses mirrors.
stabilisation of the output optical mode, fabrication of structured mirrors.
minimisation of leakage current and optical losses of the cavity.
minimisation of the series resistance as well as optimisation of the current injection.
adequate technological choices for integration of all these performances in a VCSEL.
System evaluation and reliability test of the devices.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Alcatel Alsthom Recherche
Address
Route De Nozay
91460 Marcoussis
France

Participants (7)

Ecole Polytechnique Fédérale de Lausanne
Switzerland
France Télécom
France
Address
38-40 Rue Du Général Leclerc
92131 Issy-les-moulineaux
ROYAL INSTITUTE OF TECHNOLOGY
Sweden
Address
Teknikringen, 56-58
UNIVERSITY OF ATHENS
Greece
UNIVERSITY OF ULM
Germany
Birkbeck College, University of London
United Kingdom
Address
Malet Street, Bloomsbury
WC1E 7HX London
University of Gent
Belgium