ns vertical cavity surface emitting laser · international university bremen 3/14 “ vcsel’s...

International University Bremen 1/14

VERTICAL CAVITY – SURFACE EMITTING LASER

Nandhavel


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.


“ 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


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 single-mode emission spectrum, and

• suitability for 2D-array integration.


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.


Distributed Bragg Reflector (DBR)

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

The basic requirements� High reflectivity� High refractive index

contrast� Good electrical and thermal

conductivity

n1λ/4n2λ/4


Working of a VCSEL

Contact Pad

Epitaxial Front Mirrot

Isolation

Burried Tunnel Junction

Dielectric Black Mirror

Contact & Integrated Heat Sink

oo Electron

oo HoleEmission


Types of VCSEL

Metallic Reflector Etched Well

Air Post Burried Regrowth


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


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 high-temperature and high-power operation;


Structure and index of refraction for various types of junction in the aluminium gallium arsenide system. Top: HomojunctionMiddle: Single heterojunction; Bottom: Double heterojunction


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 107 hours (high

reliability due to completely embedded active region).


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.


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

ns vertical cavity surface emitting laser · international university bremen 3/14 “ vcsel’s...

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