VERTICAL CAVITY SURFACE EMITTING LASER Nandhavel International University Bremen 1/14
Outline Laser action, optical cavity (Fabry Perot, DBR and DBF) What is VCSEL? How does VCSEL work? How is it different from Edge emitting laser? Advantages and applications of VCSEL in Optical Communication. International University Bremen 2/14
VCSEL s create the photons that optical fibre loves It takes more than fibre to make a link; the photon source is critical to speed, distance, and efficiency - Bill Schweber, Executive Editor -- EDN, 2/15/2001 International University Bremen 3/14
What is VCSEL Initial motivation of SEL - full monolithic fabrication of laser cavity. low electric power consumption, capability of on-wafer testing, simplified fibre coupling and packaging overall cost efficiency longitudinal singlemode emission spectrum, and suitability for 2D-array integration. International University Bremen 4/14
Structure of a VCSEL Vertical Cavity is formed by surfaces of epitaxial layers and light is taken out from one of mirror surfaces. The VCSEL's emitted beam is round and has low divergence and is easier to couple into a single-mode fiber than the beam of other lasers. International University Bremen 5/14
Distributed Bragg Reflector (DBR) The basic requirements High reflectivity High refractive index contrast Good electrical and thermal conductivity Design Stack of 2 dielectric material junctions Each layer thickness is nλ/4 No. of layers for a particular λ determined by difference in refractive indices of the dielectric materials n 1 λ/4 n 2 λ/4 International University Bremen 6/14
Working of a VCSEL o Electron o Hole Emission Contact Pad Epitaxial Front Mirrot Isolation Burried Tunnel Junction Dielectric Black Mirror Contact & Integrated Heat Sink International University Bremen 7/14
Types of VCSEL Metallic Reflector Etched Well Air Post Burried Regrowth International University Bremen 8/14
How the properties differ between a Edge-emitting laser and VCSEL Parameter Edge emitting laser Surface emitting laser Active layer area 3 x 300 µm2 5 x 5 µm2 Active volume 60 µm3 0.07µm3 Cavity length 300 µm ~1 µm Reflectivity of facets 0.3 0.99 0.999 Relaxation Frequency < 5 GHz > 10 GHz International University Bremen 9/14
Materials design of resonant cavity and mode-gain matching; multilayered DBRs to realize high reflective mirrors; optical losses such as Auger recombination, intervalence band absorption, scattering loss, and diffraction loss; p-type doping to reduce the resistivity in p-type materials for CW and high efficiency operation (if we wish to form multilayer DBRs, this will become much more severe); heat sinking for hightemperature and highpower operation; International University Bremen 10/14
Structure and index of refraction for various types of junction in the aluminium gallium arsenide system. Top: Homojunction Middle: Single heterojunction; Bottom: Double heterojunction International University Bremen 11/14
Advantages offered Ultra-low threshold operation is expected from its small cavity volume. Wavelength and thresholds are relatively insensitive to temperature variation. Dynamic single mode operation. Wide and continuous wavelength tuning. Large relaxation frequency. Easy coupling to optical fibres. Monolithic fabrication and easy device separation without perfect cleaving requirement, wafer-level testability of the VCSEL allows designers to put VCSEL-support circuitry, such as drivers and control circuitry, on the die along with the VCSEL itself (low cost chip production) Vertical stack integration by MEMS (micro-machining) technology. Estimated operation time at room temperature is 10 7 hours (high reliability due to completely embedded active region). International University Bremen 12/14
How is it advantageous in communications Single mode emission is required both longitudinally and transversally for long-distance fiber-optical communication and gas monitoring applications. International University Bremen 13/14
References Prof. Dr. Dietmar Knipp, Notes on VCSEL and Laser Sources http://www.wsi.tum.de/e26/en/research/vcsel http://www.mtmi.vu.lt/pfk/funkc_dariniai/ Surface-Emitting Laser Its Birth and Generation of New Optoelectronics Field, Kenichi Iga, Fellow, IEEE Journal On Selected Topics In Quantum Electronics, Vol. 6, NO. 6, November/December 2000 International University Bremen 14/14