Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2003-01-08
2004-11-23
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06822993
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor lasers. More particularly, the present invention relates to vertical cavity surface emitting lasers (VCSELs).
BACKGROUND OF THE INVENTION
Semiconductor lasers have become more important. One of the more important applications of semiconductor lasers is in communication systems where fiber optic communication media is employed. With growth in electronic communication, communication speed has become more important in order to increase data bandwidth in electronic communication systems. Improved semiconductor lasers can play a vital roll in increasing data bandwidth in communication systems using fiber optic communication media.
The operation of basic semiconductor lasers is well known. Semiconductor lasers can be categorized as surface emitting or edge emitting depending upon where laser light is emitted. They may also be classified by the type of semiconductor junctions used such as heterojunction or homojunction Referring to
FIG. 1A
, a prior art vertical cavity surface emitting laser (VCSEL)
100
is illustrated. VCSEL
100
is cylindrical in shape and includes heterojunctions. When VCSEL
100
is lasing, the laser light is emitted from the top surface in a region defined by the optical confinement region
103
. VCSEL
100
includes a first terminal
101
and a second terminal
102
coupled respectively to the top and bottom surfaces of the VCSEL to provide current and power. VCSEL
100
includes distributed Bragg reflector (DBR) layers
104
A and
104
B defining the optical confinement region
103
. The optical confinement region
103
provides optical confinement such that the light can be reflected between the DBR layers
104
A and
104
B in a reinforcing manner to provide light amplification. VCSEL
100
includes heterojunction layers
105
which forms an active region
106
with the optical confinement region
103
. The active region
106
provides current confinement so as to provide lasing when a threshold current is supplied to the VCSEL
100
. Threshold current is the current level required for injecting enough carriers (electrons and holes) for lasing to occur. When lasing, the VCSEL
100
has a transverse mode field
108
and a longitudinal mode field
109
. To improve optical confinement index guiding may be used. Index guiding uses layers of different compounds and structures to provide a real refractive index profile to waveguide the light. Alternatively a VCSEL may be gain guided. In gain guiding, the carriers induce a refractive index difference which is a function of the laser current level and output power.
There are three types of prior art VCSEL devices that are of interest. These are planar proton implanted VCSELs, ridge waveguide VCSELs (RWG) and oxide confined VCSELs. Referring now to
FIG. 1B
, a planar proton implanted VCSEL
110
is illustrated. Planar proton implanted VCSELs are relatively easy to fabricate and have a planar top surface that allows easy contact metalization and metal interconnect. As a result, a large contact area can be manufactured with low resistance. However, planar proton implanted VCSELs lack sufficient index refraction difference in the lateral direction to provide good optical confinement. Optical confinement of planar proton implanted VCSELs is generated by gain guiding and thermal lensing effect caused by heating. The thermal lensing effect (diverging/converging) provides a change in the index of refraction as a proportion of temperature due to the heating of the junctions. These methods of optical confinement provide poor performance and result in planar proton implanted VCSELs having a high threshold current, large turn-on delay and large timing jitter. The turn on delay and timing jitter of a VCSEL are functions of the threshold current. The lower the threshold current the easier it is to turn on a VCSEL and the less is the turn on delay time needed to generate the appropriate amount of current with the VCSEL for lasing. The higher the turn on threshold the greater is the timing jitter in turning on and off a VCSEL. The high threshold current additionally implies a higher operation current and thus a shorter lifetime in the operation of the planar proton implanted VCSEL. The planar proton implanted VCSEL having a large turn-on delay and a large timing jitter makes it unsuitable for high speed applications beyond 2.5 Gbps.
Referring now to
FIG. 1C
, an improvement over planar proton implanted VCSELs is the prior art ridge waveguide proton implanted VCSEL
120
. Ridge waveguide VCSELs have stronger optical mode confinement provided by the large index refraction difference between semiconductor and air. This large index refraction is similar to what is provided by edge emitting semiconductor lasers. The ridge waveguide proton implanted VCSEL provides a lower threshold current than a planar proton implanted VCSEL and thus potentially longer operational lifetime. The thermal lensing effect is minimal in ridge waveguide proton implanted VCSELs, and thus have fast turn-on and turn-off times. The timing jitter in ridge waveguide proton implanted VCSELs is much smaller than that of planar VCSELs. The ridge waveguide proton implanted VCSELs can potentially operate up to 5 Gbps, beyond which, they are limited by an RC time constant—the resistance being that of the device. However, ridge waveguide VCSELS are difficult to manufacture because of their nonplanar surface. The top surface metalization of a ridge waveguide VSCEL is particularly difficult to manufacture. A disadvantage to ridge waveguide VCSELs is that the heat dissipation is poor resulting in a thermal resistance typically 50% greater than that of planar proton implanted VCSELs. Thermal resistance causes a temperature rise in the active region as a function of the dissipated power therein. Heat dissipation is a very important factor in improving semiconductor device reliability. Large device resistances result in large RC time constants, ultimately limiting the device from high speed applications.
Referring now to FIG.
1
D and
FIG. 1E
, an improvement over ridge waveguide VCSELs is the prior art oxide confined VCSELs
130
and
140
. Oxide confined VCSELs utilize a partially oxidized AlAs layer to provide current blocking for current confinement. Oxide confined VCSELs have lower threshold currents due to good current confinement and lower resistance that allows for high speed operation. Depending on where the oxidized current blocking layer is manufactured in an oxide confined VCSEL, optical confinement for the optical mode of the semiconductor laser can be provided by the index refraction difference between the oxidized portion of the AlAs layer and the non-oxidized portion of the AlAs layer. Typically, Al
2
O
3
has an index of refraction of about 1.5 and AlAs has an index of refraction of 2.9. Disadvantages associated with the oxidized VCSEL technology are difficult manufacturability (i.e., low yield) and poor uniformity, consistency, and reliability. Generally, it is desirable to avoid oxidizing a material within a VSCEL because it creates lattice defects that will grow and eventually degrades VCSEL device performance. In addition, stresses caused by the volume change after material oxidation accelerates VCSEL device degradation.
BRIEF SUMMARY OF THE INVENTION
Briefly, the present invention includes a method, apparatus and system for planar index guided vertical cavity surface emitting lasers as described in the claims.
The apparatus and manufacture of a planar index guided vertical cavity surface emitting laser (VCSEL) is provided. Planar index guided vertical cavity surface emitting laser (PIG VCSEL) utilizes index guiding to provide improved optical confinement and proton implantation to improve current confinement. Index guiding is achieved by etching a plurality of index guide openings (holes, partial ridges or other shapes) into a p-DBR mirror around the desired optical confinement region. The index guiding may be adjusted by varying the etched volume and depth of index guide o
Chui Liew-Chuang
Lee Hsing-Chung
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
JDS Uniphase Corporation
Leung Quyen
LandOfFree
Index guided vertical cavity surface emitting lasers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Index guided vertical cavity surface emitting lasers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Index guided vertical cavity surface emitting lasers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3331387