Vertical Cavity Surface Emitting Lasers (VCSELs)

1. Vertical Cavity Surface Emitting Lasers (VCSELs)

2. History • First invented in the mid- 1980’s • Became commercial available in 1996 and quickly over took the position of edge emitting lasers in many areas

3. Structure • As the name implies VCSELs produce a beam of light that is perpendicular to the layers of the device • This device also makes use of Distributed Bragg Reflectors (DBR) to produce the beam

4. Structure Metal Contact DBR Oxide Layer Active Region

5. Structure- Active Region • The active region is of a double hetero structure as we have seen before • The active region usually contains multiple quantum wells to produce a high volume of photons

6. Structure- Distributed Bragg Reflectors • DBRs allow for a high reflectivity, in the case of VCSELs this reflectivity is usually greater than 99.5 % • This high reflectivity allows for the a decrease in the size of the active layer

7. Structure- Oxide Layer • The oxide layer isolates the movement of the photons cross section of light that is desired by blocking the area that the carriers can travel through • Beam is now confined nearly identically on both sides

8. Structure • The cavity length is very small due to the High reflectivity of the DBRs meaning that the cavity resonances are spread far apart • Making VCSELs for the most part single frequency lasers

9. Growth of the VCSELs • The high degree of complexity of the epitaxial structure initially made the fabrication of the device nearly impossible • Device performance benchmarks are intimately related to the advances in epitaxial technology

10. Epitaxy Process Choice • Molecular Beam Epitaxy (MBE) was used early on over Metal Organic Vapor-Phase Epitaxy (MOVPE) despite being highly touted • This was due to the lack of an in situ probe to ensure usable wafers, not until recently has a useful probe been developed for MOVPEs

11. DBR Problems • While achieving high reflectivity was easy enough, there was a need for composition grading and dopant modulation at the DBR interfaces to reduce the resistance of the stack • A continuous parabolic grade offers the lowest resistance and is able to retain a reflectivity similar to that of an abrupt grade

13. Applications • What sets VCSELs apart includes surface emission, ease of array creation, testability, single mode, low temperature sensitivity, and a low threshold • These characteristics put the VCSEL in a great position to displace conventional diode lasers

14. Low Threshold • Important in low-power lasers for speed and efficiency • In applications such as smart pixels and cryogenic links it is desired to minimize power and on chip thermal dissipation

15. Parallel Fiber-Optic Data Communications • VCSELs fit into the parallelism trend due to their kinship with array fabrication, the ability to probe, temperature characteristics reduce cost, and the need for an 850 nm wavelength allows for the use of AlGaAs

16. Parallel Fiber-Optic Data Communications • With threshold currents below 200 micro amps it is possible to operate at Gbit/sec with low bias and modulation current

17. Long Distance Fiber-Optic Communications • Due to its circular beam and narrow emission angle VCSELs offer more efficient coupling to single mode fibers

18. Smart Pixels • Electronic processing cells with optical input and/or output • VCSELs are best suited for Fiber Data Links, Optical Back Planes, and Free-Space Optical Processing • Monolithic integration is required for use in these devices

19. Smart Pixels • Hybrid integration usually allows for more lee way when choosing components and materials • Can be bonded by three techniques Applique, Coplanar contacts, and Bonded wafer

20. End