Edge emitting lasers: Materials and

Applications in Optical Communications

Instructor: Dr. Dietmar KnippPresentation by: Nitin Mehta

Edge Emitting Lasers

� Definitions� Double Heterojunction� Fabry-Perot Lasers� DFB� DBR� Applications� References

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DefinitionsSemiconductor lasers can be grouped into two classes:� Edge-emitting lasers, where the laser light is

propagating parallel to the wafer surface of the semiconductor chip and is reflected or coupled out at a cleaved edge.

� Surface-emitting lasers, where the light propagates in the direction perpendicular to the semiconductor wafer surface.

(a) Surface emitting , (b) Edge-emitting . Ref: Saleh, Teich, Fundamentals of Photonics

Double Heterojunction� When a layer of material with a smaller bandgap

energy is sandwiched between layers of material with a higher energy bandgap a double heterojunction is formed. A double heterojunctionconsists of two heterojunctions. The recombination of carriers is restricted to the low bandgap region which is called „the active region“of the diode.

Double heterojunction. Ref.: H. J.R. Dutton, Understanding optical communications

Fabry-Perot laser

Schematic diagram of a Fabry-Perot laser.

Ref:http://britneyspears.ac/physics/fplasers/fplasers.htm

Fabry-Perot lasers exhibit significant problems if they are used for long distance communication. Wavelength Division Multiplexed (WDM) systems require the transmission of several channels in a single fiber. To do this it is important for each signal/channel to have as narrow a spectral width. The spectral width of Fabry-Perot lasers however is too high.

The distributed feedback laser (DFB) � They are unique in that they incorporate a diffraction

grating directly into the laser chip itself, usually along the length of the active layer (the gain medium)

Fujitsu device

Ref: http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb02/tunaf2.html

Bragg Diffraction Grating

Distributed Bragg Reflector laser (DBR)� A variation of the DFB laser

is the distributed Bragg reflector (DBR) laser. It operates in a similar manner except that the grating, instead of being etched into the gain medium, is positioned outside the active region of the cavity. Lasing occurs between two grating mirrors or between a grating mirror and a cleaved facet of the semiconductor.

Ref: http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb02/tunaf3.html

DFB vs. DBR

� DFB: easier to fabricate� DFB: less threshold current� DBR: tunable� Both are single mode

Applications

� Gigabyte/s parallel fiber-optic links based on edge emitting laser diode arraysNagarajan, R.; Sha, W.; Li, B.; Craig, R.;

� High-speed data and telecommunications fiberoptics

� LED’s

� Reading and writing on DVD’s

Conclusion

� Edge-emitting lasers are the original and still very widely used form of semiconductor lasers. They can generate very high output powers, although typically with poor beam quality.

� VCSELs are more ideal for optical interconnect applications as they have some clear advantages over edge emitting lasers.

References � Saleh, Teich, Fundamentals of Photonics� H. J.R. Dutton, Understanding optical communications� Web: http://britneyspears.ac/lasers.htm� Web:

http://www.spectrum.ieee.org/WEBONLY/publicfeature/feb02/tuna.html

� Web: http://ieee.ca/diglib/library/9705lear/lear-pp/sld048.htm

� Web: http://www.mtmi.vu.lt/pfk/funkc_dariniai/diod/led_laser.htm

� Dr. Dietmar Knipp