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Combined Vertical Cavity - Distributed Feedback Laser Technology

IP.com Disclosure Number: IPCOM000117676D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 132K

Publishing Venue

IBM

Related People

Strite, S: AUTHOR

Abstract

Vertical Cavity Surface Emitting Lasers (VCSEL) and Distributed Feedback (DFB) lasers are highly desirable for numerous applications, but are difficult to design and build due to several constraints. VCSELs require an extremely short cavity length. DFBs must integrate a finely pitched grating along the laser active region to stabilize a single mode. An approach is presented herein which integrates VCSEL and DFB technology while relaxing the foremost technical challenges of each fabrication process.

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Combined Vertical Cavity - Distributed Feedback Laser Technology

      Vertical Cavity Surface Emitting Lasers (VCSEL) and Distributed
Feedback (DFB) lasers are highly desirable for numerous applications,
but are difficult to design and build due to several constraints.
VCSELs require an extremely short cavity length.  DFBs must integrate
a finely pitched grating along the laser active region to stabilize a
single mode.  An approach is presented herein which integrates VCSEL
and DFB technology while relaxing the foremost technical challenges
of each fabrication process.

      VCSELs, in order to realize single mode operation, must have a
sufficiently short cavity to insure large enough cavity mode spacings
such that no two modes lie within the mirror stopband.  This
restriction limits the total cavity length to 3-4 wavelengths, and
severely limits the laser design.  The short cavity constraint forces
current to be passed through the mirrors which increases device
series resistance, mirror design complexity, and lowers reliability.

      DFB lasers insure single mode operation by integrating a finely
pitched diffraction grating near the lasing region such that the
laser mode interacts with the grating.  The grating must be written
along 100 micron or more of the active region and demands expensive,
highly precise fabrication methods such as electron beam lithography.
Often, multiple growth steps are required to form the laser active
region, integrate the DFB grating, and then complete the laser
structure.

      The basic structure to be discussed is shown in the Figure.
This is a VCSEL in the sense that the light is emitted through one or
both surfaces of the vertical cavity.  However, the lower mirror is a
hybrid structure which integrates a conventional high reflectivity
Distributed Bragg Reflector (DBR) mirror with a second DFB grating
situated on top of it.  The formation of the DFB in the growth
direction is trivial using today's advanced deposition technologies.

The presence of the DFB introduces a selectivity between the vertical
cavity modes.  The VCSEL can, therefore, be designed with a long
cavity length without sacrificing mode stability.

      The first step in fabricating the structure shown is to grow
the hybrid DBR-DFB bottom mirror.  This can be done inexpensively
over large area substrates, e.g. glass, using SiNx/SiOx dielect...